Small carboranes

CHAPTER

4

Small carboranes: Four- to six-vertex clusters 4.1 OVERVIEW

Readers of this book whose contact with carborane chemistry has been through applications in biomedical materials or other areas may have the impression that the field is limited to the icosahedral C2B10H12 clusters and their derivatives. In fact, it is not uncommon to see the term “carborane” employed narrowly as a synonym for the C2B10 systems. While it is true that most current carborane applications are based on the icosahedral compounds, the non-icosahedral systems are important for at least two reasons. First, their syntheses and properties lend perspective to an understanding of carborane chemistry as a whole. Second, there are major structural and electronic differences in the various carborane cage systems that can be exploited for specific purposes, and that expand the range of potential applications well beyond what can be achieved with C2B10H12 derivatives alone. A matter of definition requires comment here. What exactly is a carborane? While it is not unreasonable to view twodimensional C2BHn (borirenes and boriranes) and similar three-membered ring systems [1–4], as carboranes (even CBHn species have been so labeled [5,6]), these small molecules are more commonly described as classical organoboranes. For our purposes, we define carboranes as carbon-boron clusters of four or more vertexes in which nonclassical (electrondelocalized multicenter) bonding plays a significant role. In all but a few cases, this definition clearly separates the carboranes from conventional organoboranes whose hydrocarbon-like structures can be described entirely in terms of 2c2e bonds. As will be seen, some 4- and 5-vertex cages straddle the borderline between classical and nonclassical systems, but otherwise there is no difficulty in distinguishing carboranes from other compound types.

4.2 4-VERTEX OPEN CLUSTERS 4.2.1 CB3Hx and C2B2Hx Two nonclassical CB3 derivatives, 4-1 and 4-2, have been isolated and characterized, and several such systems have been explored theoretically (Table 4-1) [7–10]. The nido-carbatetraborane 4-1 has been prepared by reaction of the tetraborane anion B4H3(CMe3)4
with iodomethane [10]. Compound 4-2 was generated from an organotriborane precursor at
80  C and stabilized as the dianion, which was characterized crystallographically and shown to have a classical s-bonded framework [7]. In contrast, multinuclear NMR data on neutral 4-2 support its description as an aromatic, delocalized, and hence nonclassical structure [7]. CMe3 H

4-1

H

C

B

B B CMe3

R H

H

H

CMe3

4-2 R

C

B B

R

B CH2R R = 2,3,5,6-C6Me4H

Carboranes. DOI: 10.1016/B978-0-12-374170-7.00014-8 © 2011 Elsevier Inc. All rights reserved.

27

28

CHAPTER 4 Small carboranes: Four- to six-vertex clusters

TABLE 4-1 CB3Hx and C2B2Hx Derivatives Synthesis and Characterization Compound

Information

References

nido-H2CB3H2(CMe3)3 RHCB3(CH2R)R0 2 (R ¼ SiMe3; R0 ¼ 2,3,5,6-C6H2Me4) nonclassical triboracyclobutane RHCB3(CH2R)R0 2 (R ¼ SiMe3; R0 ¼ 2,3,5,6-C6H2Me4) classical

S, H, B S

[10] [7]

S, X, H, C, B

[7]

DFT GIAO ab initio ab initio ab initio ab initio, classical and nonclassical potential energy surface comparison to Si2C2H2

[7] [10] [9] [8] [11,14, 316–318] [319]

B, C (calculated IGLO) B (calculated IGLO)

[11] [12]

Theoretical Studies Molecular and electronic structure calculations CB3H5 nonclassical triboracyclobutane nido-H2CB3H2(CMe3)3 CB3H7 CB3H4
C2B2H4

NMR calculations C2B2H4 1,3-C2B2H4 X, X-ray diffraction; H, 1H NMR; B,

11

B NMR; C,

13

C NMR.

Although no parent C2B2Hx species has been experimentally characterized at this writing, C2B2H4 has attracted attention from theoreticians (Table 4-1). IGLO, GIAO-SCF, and GIAO-MP2 calculations [11,12] predict a stable existence for the nonclassical boracycloproplylidenediborane structure 4-3 that has a “bare” carbon atom, which is in agreement with X-ray diffraction data for the substituted derivatives (Me3Si)2C2B2(CMe3)2 and (Me3Ge)2C2B2(2,3,5,6-C6Ph4H)2 [13]. Schleyer and coworkers found a somewhat different structure 4-4 as the global energy minimum [14]. H H

4-3

H

C

H H

B B

C

H

4-4

C

B

C

H

B H

As the experimental data on 4-2 [7] illustrate, these small 4-vertex boron-carbon cyclic species occupy a border region between classical and nonclassical structures and properties and, in some cases, can fit into either category, depending on the solvent, temperature, or other factors. A similar situation arises in some 5- and 6-vertex systems, as will be seen later.

4.3 5-VERTEX OPEN CLUSTERS 4.3.1 Nido- and arachno-CB4 systems Nido-CB4H8 and arachno-CB4H10 (analogues of B5H9 and B5H11, respectively), whose 1-isomers are depicted as 4-5 and 4-6, have not been isolated although evidence for CB4H10 in mass spectra of product mixtures has been cited [15,16]. The synthesis of a pentaethyl derivative of nido-2-CB4H8 (with the skeletal carbon occupying a basal vertex and having a bridging CHEt group) was reported [17], but later GIAO-NMR calculations [18] indicate that this species is actually a

4.3 5-Vertex open clusters

29

nido-2,4-C2B4H8 derivative. At present, no compound having a confirmed nido-CB4 cage framework has been characterized. However, several arachno-CB4 clusters are known to be prepared, interestingly, by very different routes. Allene and B4H10 react in the gas phase at 70  C to give 1-MeCB4H9 and 1-MeCB4H8
2-Me (both having the cage structure of 4-6) [19] that can be isolated when the products are quenched in liquid nitrogen. Gas phase reactions of B4H10 with alkynes at the same temperature [20,21] afford 2-methyl and 2-ethyl derivatives of 4-6, as well as the bridged products 4-7, in which R and/or R0 are H, methyl, ethyl, n-propyl, or tert-butyl, as characterized by multinuclear NMR and mass spectroscopy, and supported by ab initio GIAO-NMR analysis [20,21]. A compound having the composition C3B4H12 that was initially claimed to be the first hypho-carborane [22,23] was later shown to be nido-1-MeCB4H7-m-CH2, a derivative of 4-7 with R ¼ Me and R, R0 ¼ H [20]. H

C

C

H H

H

B

B B

H

H

R

H

H H B

H

B H

H

H B

H

4-5

H

H

B

B B

H

C

H

H

B

H

H

R⬘

H

C

H

H

R⬙

4-6

H B

B

4-7

Structurally similar compounds, arachno-1-(MeR2C)CB4H7-m(2,3)-C6H4 (R ¼ Me, CH2CH2) (4-8) with a benzene ring occupying a dihapto-bridging position between two basal borons, have been isolated from reactions of diborapentafulvenes with excess LiBH4 followed by protonation with HBF4 [24].

B Me B

Me Me

H

H

Me

(1) LiBH4 (2) H+

C

H

B

B

H

B

H

H H B H

4-8

H

H

Still another route to arachno-CB4H10 derivatives involves the treatment of alkynylsilanes with a large excess of diethylborane (“hydride bath”), as in the reaction CMeÞ2 þ Et2 BH !! arachno-1-EtCB4 Et4 H3 -mð2; 3Þ-CEtðSiHMe2 Þ Me2 SiðC which is believed to involve initial Si-C bond cleavage followed by hydroboration of the triple bond [25,26]. Finally, the perethyl arachno-CB4 species 4-9 is reported to be an isolable intermediate in the conversion of diethyl(prop-1-ynyl)borane to closo-1,5-Et2C2B3Et3 (4-10), in the presence of tetraethyldiborane(6) (Figure 4-1) [27]. X-ray data have not been reported for the arachno-CB4 compounds 4-7 – 4-9, and the structures shown are based on spectroscopic evidence and theoretical calculations.

4.3.2 Nido-1,2-C2B3H7 Nido-1,2-dicarbapentaborane (Figure 1-3, top left, and Table 4-2), a structural and electronic analogue of B5H9, is an unusual compound. Known only as the parent species for decades after its discovery until the recent isolation of several derivatives (see below), it has some odd properties and its molecular structure is something of a rule-breaker. The compound was first isolated in 1969 as the main product of the gas phase reaction of B4H10 and acetylene at 25-70  C after quenching in liquid nitrogen, and was characterized from multinuclear NMR and mass spectra [21,28]. This synthesis was independently repeated 30 years later [29] in a study that also reexamined the reaction mechanism and concluded, in accord with an earlier,

30

CHAPTER 4 Small carboranes: Four- to six-vertex clusters Me

Et2B

Et4B2H2

Et4B2H2

(Et2B)3C–Et

Et2B–C≡CMe Et2B

Me Et4B2H2

Et

Et

C Et

B

B

C

Et Et4B2H2

Et

B

B Et Et

C Et Et

C

Et B H

B

H B

Et

H

Et2B

4-10

4-9

FIGURE 4-1 Synthesis of 1,5-Et2C2B3Et3 (4-10) via arachno-1-EtCB4Et4H3-m(2,3)-CEt(BEt2) (4-9).

TABLE 4-2 Open-Cage C2B3Hx Derivatives Synthesis and Characterization Compound

Information

References

nido-1,2-C2B3H7

S, S, S, S,

H, H, H, H,

B, B, B, B,

[21,28] [29] [33] [33]

S, S, S, S,

H, H, H, H,

B, C, MS B, C, MS B, C, MS C, B

[33] [33] [33] [43]

Detailed NMR Studies nido-1,2-C2B3H7

B(detailed), H(detailed)

[21]

Other Experimental Studies nido-1,2-C2B3H7

C2H2 insertion

[38]

Theoretical Studies Molecular and electronic structure calculations nido-C2B3H7, nido-C2B3H6
, nido -C2B3H52
nido-C2B3H52
cyclo-C2B3H52


Extended Hu¨ckel Geometry and cage isomerization Geometry

[34] [41] [43]

nido-1/2-R-1,2-C2B3H6 R ¼ Me, Et, n-C4H7, CMe3, Me3Si nido-1-Et-2-Me-1,2-C2B3H5 nido-1-Me-2-Et-1,2-C2B3H5 nido-1,2-Et2C2B3H5 cyclo-(SiMe3)2C2B3R(C6Me4H)22
[R ¼ N(SiMe3)2, OMe]

IR, MS C, ED MS C, MS

Continued

4.3 5-Vertex open clusters

31

TABLE 4-2 Open-Cage C2B3Hx Derivatives—Cont’d Synthesis and Characterization Compound

Information

References

nido-1,2-C2B3H7

Geometry (ab initio), IR Geometry (ab initio) Analogies to P*tBuCB3H5, (tBu)2P*3C2þ, etc.

[11] [35] [320]

B, C, H (IGLO) B (IGLO) C (IGLO) B, C (IGLO) H, B, C, spin-spin coupling (DFT)

[29] [12] [11] [36] [37]

Reaction with NH3 Mechanism of formation from B3H7þ C2H2 Mechanism of formation from B4H8þ C2H2

[42] [31]

NMR calculations nido-1,2-C2B3H7

Reactivity calculations nido-1,2-C2B3H7

X, X-ray diffraction; H, 1H NMR; B,

11

B NMR; C,

[32]

13

C NMR; IR, infrared data; MS, mass spectroscopic data; ED, electron diffraction data.

detailed kinetic investigation [30], that an important early step in the formation of C2B3H7 and other carboranes is the loss of H2 from B4H10 to form B4H8, which in turn reacts with C2H2. A possible competing pathway involving loss of BH3 to give B3H7, suggested by theoretical analysis [31,32], is less consistent with the experimental data [29]. Other products generated from B4H10 and C2H2 in the vapor phase include alkyl derivatives of the known 6-vertex nido-carboranes 1-CB5H9, 2,3C2B4H8, 2,3,4-C3B3H7, and 2,3,4,5-C4B2H6 [29]. Quenched gas-phase reactions of higher alkynes with B4H10 at 70  C generate a series of C(1)-, C(2)-, and C(1,2)substituted derivatives of nido-C2B3H7, listed in Table 4-2 [33], which are the first isolable nido-C2B3 carboranes to be reported other than the parent compound. The square pyramidal structure of 1,2-C2B3H7 shown in Figure 1-3, originally assigned from experimental and calculated spectroscopic data [21,28], is supported by later electron-diffraction [29] and molecular orbital studies [11,34,35], as well as IGLO calculations of NMR shifts [11,12,29,36,37]. The occupancy of an apical vertex by one of the carbon atoms violates the general observation that carbons prefer low-coordinate vertexes (see Chapter 1), which in this case is evidently superseded by the need to avoid C2 2H2 2B bridge-bonding. 1,2-C2B3H7 is a colorless liquid, more volatile than B5H9 and stable as a gas at 50  C, but it decomposes at 110  C to give tan solids, H2, and B2H6. The compound is unchanged over extended periods in dilute hydrocarbon solutions, but in concentrated solutions, or as a pure liquid, it polymerizes irreversibly to a structurally undefined white solid with no evolution of H2 or other side products; polymerization is also induced by HCl, ethers, and other compounds [28]. The compound reacts readily with alkynes [38], undergoing a two-carbon insertion to generate nido-2,3,4,5-C4B2H6 (Figure 1-3, second row center), a previously known carborane that is discussed in detail below. Reaction with propyne affords mainly 3-MeC4B2H5 with some 2-MeC4B2H5, and the interaction with 2-butyne forms 2,3-Me2C4B2H6, accompanied by a smaller amount of the 3,4-Me2 isomer (Figure 4-2). These findings, together with labeling experiments using 13CH, reveal that alkyne insertion into 1,2-C2B3H7 occurs mainly at the C2 CD and H13C 2B bonds, and that there DC is some incorporation into the carborane C2 2C bond as well [38].

32

CHAPTER 4 Small carboranes: Four- to six-vertex clusters

H

C R1–C≡C–R2

H H

B

C H

H

B

H

H

B

B

1

B

H

R1, R2 = H, Me

R1 R2

2

H

B6

C C

C

5

C4

3

H

H

+ R1

C C

B

C

H

C R2

H

FIGURE 4-2 Conversion of nido-C2B3 to nido-C4B2 carboranes via alkyne insertion.

Other chemistry of 1,2-C2B3H7 has been explored [28]. The compound combines with NEt3 to form a stable 1:1 adduct, and exposure to a limited amount of bromine in solution or in the gas phase results in apparent addition to the cage C2 2C bond, 2C bond, but without release of H2 or HBr. At the time, this finding suggested [28] possible double-bond character for the C2 ˚ determined later from electron diffraction data [29]. Excess Br2 this idea seems inconsistent with the C2 2C distance of 1.626 A attacks and destroys the carborane cage. Reaction with Fe(CO)5 under ultraviolet light [28] affords the nido-ferracarborane 1,2,3-(CO)3Fe(C2B3H7), a known [39,40] complex that contains a planar C2B3 ring ligand. Ethylene combines with 1,2-C2B3H7 in the gas phase at 90  C to give tetraethyldiborane, triethylboron, and unidentified products [28]. Treatment with NaH in THF in an attempt at bridge-deprotonation to form the C2B3H6
ion results in decomposition, while exposure to NaH in mineral oil gives no reaction. Molecular orbital calculations on the experimentally unknown C2B3H52
dianion and its hypothesized cage rearrangement show open pyramidal geometry for the various possible isomers [41]. The reaction of 1,2-C2B3H7 with NH3 has been explored computationally and the C2B3H7NH3 adduct is found to be less stable than the uncomplexed carborane [42].

4.3.3 Other open-cage C2B3 clusters Reductive cage-opening of closo-1,5-C2B3H5 derivatives 4-11 with potassium metal in diethyl ether gives the ring compounds 4-12 whose structures have been established by NMR and X-ray crystallography [43]. The cyclic planar products, which show antiaromatic properties including paramagnetic ring currents, are structurally different from the square pyramidal (nido) species that would be anticipated from a 2-electron reduction of closo-C2B3 clusters. While this might be attributed to substituent effects, ab initio calculations on the parent C2B3H52
anion show that the planar ring geometry has lower energy than any pyramidal structure [43].

R

SiMe3

R⬘

C

B

2 K+

2–

C

B

B

R

2K

Me3Si

C

SiMe3

B

4-11

C R⬘ SiMe3

R = 2,3,5,6-C6Me4H R⬘ = N(SiMe3)2, OMe

B R

B

4-12 R

4.3.4 Nido-C3B2 clusters DFT calculations [44] on the nido-C3B2H5
anion, a currently unknown species, predict three stable square pyramidal isomers, of which the most stable is the 2,3,4 system 4-13 that has all three carbons in the base; however, energy differences between them are small so that thermal interconversion may be possible. In the case of the hypothetical

4.4 5-Vertex closo clusters

33

neutral molecule nido-C3B2H6, extended Hu¨ckel calculations [34] favor the 1,2,3 isomer 4-14 with one apex carbon and a B-H-B bridging hydrogen on the pyramidal base. As in the case of nido-C2B3H7, discussed earlier, it appears that placement of a carbon atom in a normally less favored high-coordinate vertex is favored over alternative structures having C2 2H2 2B or C2 2H2 2C bridges. H

H

−

C

B

4-13

H

B

C C

H

H C

C

C

H B

H

H

H

4-14 B H

H

4.4 5-VERTEX CLOSO CLUSTERS 4.4.1 CB4Hx The closo-carborane anion CB4H5
and its protonated form CB4H6 are 5-vertex cages, with 12 skeletal electrons that are isoelectronic with closo-C2B3H5 and have been studied computationally [45–47]; however, they are currently known only as alkyl or silyl derivatives (Table 4-3). Reduction of the organoborane 4-15 with lithium in diethyl ether forms the aromatic closo-carborane anion, 4-16 which in turn can be protonated to afford neutral 4-17 along with, surprisingly, a bridge-protonated derivative of 4-15 (not shown) that lacks the three-dimensional aromaticity of 4-15 itself [48]. All these molecules have been structurally characterized by X-ray crystallography. The protonated species 4-17 is sufficiently stabilized in the crystal to retain aromaticity although DFT calculations indicate that this is not the case with the protonated parent cluster CB4H6 [48].

R

SiMe3

SiMe3

C

C

B

B

R

B Me3SiCH2

B

Li 2e

B

CH2SiMe3 R = 2,3,5,6-C6Me4H

R

H+

R

B

B B

CH2SiMe3

CH2SiMe3

4-16

SiMe3 C

B

B B

Cl

4-15

R

−

H

R

CH2SiMe3

B CH2SiMe3

4-17

4.4.2 1,5-C2B3H5 4.4.2.1 Synthesis Closo-1,5-dicarbapentaborane (Figure 1-1, top left and Table 4-4) is the smallest member of the closed polyhedral C2Bn
2Hn series, and was one of the first carboranes to be prepared, having been isolated in low yield from the products of the gas-phase reaction of B5H9 and acetylene in a glow discharge [49]. Similar results have been obtained in B2H6-acetylene electric discharge experiments [50] and in flash reactions of acetylene with B2H6 or B4H10 [51–53]. A more efficient process involves the pyrolysis of B5H9 and acetylene at 490  C in a stream of H2, which affords a mixture of 1,5-C2B3H5, 1,6-C2B4H6, and 2,4-C2B5H7 in 70% overall yield [54]. However, the best available synthetic routes to the parent compound involve dehydrogenation of nido-2,3-C2B4H8, which is itself obtainable in low-energy

34

CHAPTER 4 Small carboranes: Four- to six-vertex clusters

TABLE 4-3 CB4Hx Derivatives Synthesis and Characterization Compound


closo-1-(H3Si)CB4(C6Me4H)2(CH2SiMe3)2 (aromatic) closo-1-(H3Si)CB4(C6Me4H)2(CH2SiMe3)2H (non-aromatic) arachno-1-RCB4H7-m-CR0 R00 (R ¼ Me, Et, H; R0 ¼ H, Me; R00 ¼ H, Me, Et) arachno-1-MeR2C-CB4H7-m-C6H4 (R ¼ Me, (CH2)2) arachno-1-RCB4H7-m(2,5)-CHR0 (R ¼ H, Me, Et, n-C3H7; R0 ¼ H, Me, Et, n-C3H7, CMe3) arachno-1-RCB4H8
2-Me (R ¼ H, Me, Et, n-C3H7) arachno-1-RCB4H8
2-Et (R ¼ Me, Et) arachno-1-EtCB4Et4H3-m-CH(SiMe2) (exo, endo isomers) arachno-m-Et(BEt2)C-1-EtCB4Et4H4 arachno-1-EtCB4Et4H3-m-CEBEt2) “nido-2-EtCB4Et4H-m-CHEt” (later calculated to be a nido-C2B4 derivative; see Ref. [18]) arachno-1-MeCB4H8
3-Me, -2-Me (fluxional) arachno-syn-1-EtCB4Et4H3-m(2, 5)-CEtSiMe3 arachno-syn, anti-1-EtCB4Et4H3-m(2, 5)-CHMe arachno-anti-1-EtCB4Et4H3-m(2, 5)-CHMe arachno-syn-1-EtCB4Et4H3-m(2, 5)-CHBEt “hypho-C3B4H12” (actually arachno-CB4; see Ref. [20]) “hypho-C3B4H12” (actually arachno-CB4; see Ref. [20])

Information

References

S, S, S, S, S,

X, H, B, C X H, B, C, MS H, B, C, IR, MS H, B, C, MS

[48] [48] [20] [24] [33]

S, S, S, S, S, S,

H, H, H, H, H, H,

IR IR IR

[33] [33] [25] [27] [17] [17]

S, S, S, S, S, S, S,

B, MS H, C, Si, B H, C, Si, B H, C, Si, B H, B, C H, B, MS B(2d), H(2d), C, MS

[19] [26] [26] [26] [26] [23] [22]

B, B, B, B, B, B,

C, C, C C, C, C,

MS MS

Theoretical Studies Molecular and electronic structure calculations CB4H5
(all isomers) CB4H5n
(n ¼ 0, 1) arachno-/closo-CB4H6 closo-1-(H3Si)CB4(C6Me4H)2(CH2SiMe3)2
(aromatic) closo-1-(H3Si)CB4(C6Me4H)2(CH2SiMe3)2H (non-aromatic) nido-CB4H8 nido-CB4H53
arachno-1-CB4H10 arachno-1-MeCB4H8-Me (isomers) arachno-1-MeCB4H8
3-Me, -2-Me (fluxional) arachno-1-MeCB4H8
3-Me

DFT DFT Extended Hu¨ckel

NMR calculations closo-CB4H5


11

ab initio Energies, ionization potential

ab initio, IGLO ab initio, IGLO ab initio, IGLO DFT

nido-2-CB4H6-m(4, 5)-CH2 arachno-1-RCB4H7-m-CR0 R00 (R ¼ Me, Et, H; R0 ¼ H, Me; R00 ¼ H, Me, Et) arachno-1-MeR2C-CB4H7-m-C6H4 R ¼ Me, (CH2)2 S, synthesis; X, X-ray diffraction; H, 1H NMR; B, IR, infrared data; MS, mass spectroscopic data.

11

B NMR; C,

13

C NMR; Si,

29

B-11B, 11B-13C, 13C-13C spin-spin coupling (DFT) GIAO-NMR, B(calc) ab initio, IGLO ab initio, IGLO

Si NMR; 2d, two-dimensional (COSY) NMR;

[47] [46] [45] [48] [48] [34] [8] [19] [19] [19] [321]

[93] [18] [20] [24]

4.4 5-Vertex closo clusters

35

TABLE 4-4 Closo-C2B3H5 Derivativesa Synthesis and Characterization Derivative

Information

References

Parent

S (high yield, from nido-2,3-C2B4H8) S (B5H9þ C2H2 flow system) S (photolysis of nido-2,3-C2B4H8) S (B5H9þ C2H2 electric discharge), H, B, MS, IR, vapor pressure S (B2H6þ C2H2 electric discharge) H, B, MS, IR S (from nido-2,3-C2B4H8), H, B S (B4H10/B2H6þ C2H2 flash reactions) S (from alkylboron hydrides) S, MS S, H, B

[56] [54] [55] [49] [50] [57] [51,52] [123] [133] [53]

S (B4H10/B2H6þ C2H2 flash reactions) S (B2H6þ C2H2 electric discharge), H, B, MS, IR S, H, B, MS, Raman S (parent þ BMe3), H, B, MS S (parent þ BMe3), H, B, MS S (pyrolysis of BMe3), H, IR,MS S (thermolysis of nido-2-MeCB5H3Et5), MS S (hydroboration of alkylalkynylboranes) X H, B, C CPh), H, B, C, s X (Cl, C

[51,52] [50] [60] [67] [67] [64,65] [61] [62] [27,74] [27] [66]

S [from 1,2-B5H7(SiMe3)2], H, B, IR, MS S [from 1,2-B5H7(SiMe3)2], H, B, IR, MS S [from 1,2-B5H7(SiMe3)2], H, B, IR, MS S (parent þ Cl2), H, B, IR, MS S (parent þ BMe3), H, B, MS S [from nido-2,3-(Me2ClSiCH2)C2B4H8], H, B, MS S (parent þ alkyne, metal-catalyzed), H, B, IR,MS S, H, B S (parent þ atomic S), H, B, IR, MS X, H, B, C,MS S (hydroboration of alkylalkynylboranes)

[70] [70] [70] [67] [67] [71] [69] [109] [68] [72] [63]

S (parent þ BMe3), H, B, MS

[109]

S (PtBr2-catalyzed coupling) S (pyrolysis of parent), H, B, IR, C,MS

[113] [110]

Parent, Me, Me2 derivatives (gas phase flash photolysis) 2-Me 1,5-Me2
2,3,4-Et3 2,3-Me2 2,3,4-Me3 2,3,4-Me3 1,2,3,4,5-Me5 1,2,3,4,5-Et5

1,5-(Me3C)2
2,3,4-R3 (R ¼ Cl, CPh, NMe2) CCMe3, C Me, C 1-SiMeH2 1-SiH3 1-SiH3
2-Me 2-Cl 2-CH2SiMe2Cl 5CHCH2Me 2-CH5 5CH2, 2-CH5 “2-CH5 5CHMe” 2-SH 2,3,4-[N(CHMe2)2]3 1,5-(R0 CH2)2
2,3,4-R3 (R0 ¼ Et, cyclohexylCH2; R ¼ Me, Et, Pr) 2,20 -(C2B3H4)2 and Me derivatives 2,20 -(C2B3H4)2 (1,5-C2B3H4)2 and (C2B3H3) (C2B3H4)2 isomers

Continued

36

CHAPTER 4 Small carboranes: Four- to six-vertex clusters

TABLE 4-4 Closo-C2B3H5 Derivativesa—Cont’d Synthesis and Characterization Derivative

Information

References

(C2B3H4)(1,6-C2B4H5) isomers 2,2-(1,5-C2B3H4)-(1,6-C2B4H5) 2,20 -(C2B3H4)2 2,20 -3,20 -(C2B3H4)2-C2B3H3

S (copyrolysis of parent þ 1,6-C2B4H6) S (cophotolysis of parent), H, B, IR,MS S(pyrolysis of parent), H, B, IR, UV, Raman, MS S(pyrolysis of parent), H, B, IR, UV, MS

[110] [112] [111] [111]

Raman, IR, B (NMR-IR and Raman-IR correlations) B (NMR-IR correlations) H, C (trans-cage coupling) H(coupling) C C, B (11B, 10B; B2 2C coupling) 2B coupling) B (11B, 10B; B2

[103] [103] [322] [94,95] [96,97] [98] [99]

Detailed NMR Studies Parent

1,5-R2
2,3,4-R0 3 (R,R0 ¼ Me, Et) 2:20 ,3,100 -(C2B3H4)(10 ,50 -C2B3H3) (100 ,500 -C2B3H4) 2,20 -(C2B3H4)(1,6-C2B4H5) Other Experimental Studies Parent

1,5-Et2

B (11B,

B; B2 2B coupling)

10

Raman, IR UV photoelectron spectra He photoelectron spectra MS (negative ion) ED CH bond polarity, IR, C(d and JCH); comparison with other carboranes XPS: binding energies Metal insertion and polyhedral expansion Reaction with atomic S Reactions with Lewis bases Hydrolysis in CH3OH Reaction with B2H6 Catalytic reactions with alkynes Polymerization reaction of B2H6 þ PtBr2 ! C2B6H12 XPS: binding energies

Theoretical Studies Molecular and electronic structure calculations Parent (all isomers) Isomer stabilities, dipole moments (MNDO) Energy indexes, stabilities Isomer stabilities (ab initio, SCF, DFT) Isomer stabilities, charge distribution (CNDO)

[99]

[103,104] [105] [106] [102] [73] [108] [107] [117,118] [68] [120] [114] [115] [69] [103] [116] [107]

[323–325] [326] [92,327–329] [330] Continued

4.4 5-Vertex closo clusters

37

TABLE 4-4 Closo-C2B3H5 Derivativesa—Cont’d Synthesis and Characterization Derivative

1,2 and 1,5 isomers

C2B3H5 classical structures 1,2-, 1,5- and classical C2B3H5 B–Me, B–F isomers Parent and B–Xm derivatives (X ¼ Li, F, Cl NH2) C2B3Hn (n ¼ 3,5) Parent isomers and B-Me, B-F derivatives 2-m/terminal-B2Hx or -BH4 1,5-Et2 3,4,5-Y3 (Y ¼ NH2, Me, H) 1-CH-1,2-C2B3H4 Parent and 1,5-C2B3H3 2,20 -(C2B3H4)2 2,20 -3,20 -(C2B3H4)2-C2B3H3 Isomerization calculations Parent

C2B3H52


Information

References

Decisive evidence for nonclassical bonding Electronic structure (ab initio) Population density (SCF) Energies, geometries (SCF) Cage structure (ab initio, SCF) Cage structure, dipole moment, ionization potential, heat of formation (AM1) Localized MOs Binding energies (CNDO) Comparison with (CO)9Ru3C2H2 C2 2H bond length compared with halomethanes Vibrations; structure Vibrational spectra Population analysis Stabilities, three-dimensional aromaticity (ab initio) BH and CH capping; isomer stability Geometry, optimized (ab initio) Electron density distribution (ab initio) Second-order NLO properties

[86] [75–77,331] [87] [78] [11,332] [333]

Isomerization energies (DFT) Relative energies Cage structure (SCF, DFT, MP2, CCSD, CCSD[T])

[79,334] [107] [335] [336] [104] [92] [337] [47] [338] [80] [81] [328] [82] [83] [84] [90]

Localized MOs, BCB 3-center bond Relative energies

[89] [84]

Borane-carborane cage coupling (ab initio) Binding energies (CNDO) Bond orders, diamagnetic susceptibility (NLMO) Carboranyl carbenes Structural analogy with hydrocarbons; CBC 3-center bond B2 2B bond rotational barrier) B2 2B bond rotational barrier)

[304] [107] [91] [339] [88] [111] [111]

Cage rearrangement Cage rearrangement, diamond-square-diamond (EHMO) Cage rearrangement of B5H52
(tensor surface harmonic theory) Cage rearrangement

[296–298,340,341] [342] [343] [297] Continued

38

CHAPTER 4 Small carboranes: Four- to six-vertex clusters

TABLE 4-4 Closo-C2B3H5 Derivativesa—Cont’d Synthesis and Characterization Derivative NMR calculations Parent

1,2-MeC2B3H3
3-Me 1-SiMeH2, 1-SiH3, 1-SiH3
2-Me Reactivity calculations Parent 1,2- C2B3H5

Information

References

B, C, H spin-spin coupling (DFT) 11 B-11B, 11B-13C, 13C-13C spin-spin coupling (DFT) 11 B-1H coupling correlation with structure 11 B shifts (IGLO) 11 B, 13C shifts (IGLO) 11 B, 13C NMR (GIAO-MP2) C (IGLO) Aromatic solvent-induced 1H NMR shifts: correlation with Hþ charges (PRDDO) 11 B NMR shifts (IGLO, GIAO-MP2) Substituent 1H NMR shift effects

[37] [93] [100] [12] [11] [36] [96] [101]

Protonation to form C2B3Hþ 6 Reaction with NH3 Mechanism of formation from B3H7 and C2H2

[119] [42] [31]

S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, a 1,5-C2B3 cage system except where otherwise noted

[59] [70]

13

C NMR; IR, infrared data; MS, mass spectroscopic data.

reactions of B5H9 and C2H2 described later in this chapter. This has been accomplished via photolysis [55], thermolysis [56,57], and electric discharge [57]. At present, 1,5-C2B3H5 is the only definitively characterized closo-C2B3 carborane; theoretical analyses of the hypothetical 1,2 and 2,3 isomers and their interconversions, cited in Table 4-4, show them to be considerably less stable than the 1,5 system. Early reports of the isolation of alkyl derivatives of the 1,2 isomer from electric discharge or flash reactions of alkynes and boranes [50,52,58] appear to be incorrect based on calculated 11B NMR shifts, obtained via the GIAO-MP2 method, showing that the experimentally obtained NMR data are not consistent with a closo-1,2-C2B3 cage structure [59]. An attempted repetition [59] of the original experimental work was inconclusive as the conditions employed in the electric discharge experiments, as well as the product separation techniques, differed from those of the original study [50,52]. The nature of the compounds originally assigned as 1,2-C2B3H5 alkyl derivatives on the basis of mass spectra and 11B and 1H NMR data, which suggested three nonequivalent boron atoms in a C2B3 cluster [50], has not been determined. Alkyl derivatives of 1,5-C2B3H5 have been prepared from organoboranes by various methods (see Table 4-4), including thermolysis [60,61] and reactions with alkylboranes or alkynylboranes [62,63]; thus, pyrolysis of BMe3 at 475-520  C gives 1,5-H2C2B3Me3 in low yield, along with other products [64,65]. The synthesis of 1,5-Et2C2B3Et3 (Figure 4-1, 4-10) from diethyl(prop-1-ynyl)borane and B2H2Et4 has been cited earlier. These routes take advantage of the relative accessibility of organoboranes, but the B-peralkylated products are not generally reactive and, hence, not particularly useful as synthons. On the other hand, 1,1,1-tris(trichloroboryl)-3,3-dimethylbutane generates the tri-B-chloro derivative 1,5(Me3C)2C2B3Cl3 on heating, which in turn is converted to other 1,5-(Me3C)2C2B3R3 compounds in which R is Me, CPh, or NMe2 [66]. B-alkyl derivatives have also been prepared via reaction of parent 1,5-C2B3H5 with CCMe3, C C BMe3 in a hot-cold reactor, which gave mono-, di-, and tri-B-methyl products [67].

4.4 5-Vertex closo clusters

39

Other B-substituted 1,5-C2B3H5 derivatives have been synthesized by various means. Treatment of the parent carborane with 1D atomic sulfur affords 1,5-C2B3H4
2-SH [68], and metal-catalyzed reactions with alkynes produce B-alkenyl 5CHCH2Me compounds [69]. C-silyl derivatives are obtained by flash derivatives, including the 2-CH5 5CH2 and 2-CH5 thermolysis of 1,2-(SiMe3)2B5H7, which yields, among other products, 1,5-(SiH3)C2B3H4, 1,5-(SiH2Me)C2B3H4, and 1,5-(SiH3)C2B3H3
2-Me [70], the carboranes evidently forming via a carbon insertion mechanism. The 2-CH2SiMe2Cl derivative is among the products obtained on heating nido-2,3-C2B4H6
4-CH2SiMe2Cl at 690  C [71]. Most of the reported thermolytic syntheses afford low yields of C2B3 carboranes and are inefficient, although in some cases they constitute the only known pathways to particular compounds. An unusual preparation of a 1,5C2B3H5 derivative involves a retro Diels-Alder reaction of the norbornenyl borane 4-18 to form the tris(diisopropylamino) derivative 4-19 [72]: (Me2CH)2N (Me2CH)2N

B

H

H

B

C H

(Me2CH)2N

B

B

N(CHMe2)2

B

B (Me2CH)2N

H H

H H

C N(CHMe2)2

H

4-18

4-19

H

4.4.2.2 Structure and bonding The trigonal pyramidal cluster geometry of 1,5-C2B3H5 is established from electron diffraction data on the parent compound [73] and from X-ray studies of substituted derivatives [27,66,72,74] (Table 4-4). This cage system is unique in the C2Bn
2Hn closo-carborane series, in that it can be represented both as a classical tricyclic organoborane lacking any direct B2 2B bonding interactions, or as a nonclassical electron-delocalized cage in common with ˚ ) and electron density maps showing no charge accuother carboranes. The relatively long B2 2B distance (>1.8 A mulation in the B2 2B vectors [74], together with a number of theoretical studies [75–85], appear to favor the classical organoborane model with an implied absence of B2 2B bonding. However, this conclusion is difficult to reconcile with the relative thermal and chemical stabilities of 1,5-C2B3H5 (for example, its ability to survive temperatures up to 150  C and its unreactivity toward water or air at room temperature), which are behaviors far more typical of closo-carboranes than classical organoboranes. Schleyer and coworkers concluded, on the basis of ab initio calculations of energies and magnetic properties [86], that there is, in fact, a significant B2 2B bonding interaction and hence the nonclassical description is valid for 1,5-C2B3H5 and other 1,5-X2B3H3 clusters in which X is N, P, SiH, or BH
. 2B2 2C [87,88] or B2 2C2 2B [89] bondOther studies of the 1,5-C2B3H5 system favor delocalized three-center C2 ing. The full story, however, is more complex. It turns out that the cluster bonding is strongly influenced by substituents attached to the boron atoms (more so than in larger carborane systems) [76,90,91]; 1,5-H2C2B3X3 species in which X can donate electron density via p-interaction with boron (e.g, NH2) are essentially classical, with minimal B2 2B interaction, in contrast to derivatives in which X ¼ H or alkyl, which feature electron-delocalized cluster bonding and are nonclassical [91]. This model is supported by X-ray structural data on 1,5-C2B3Et5 that suggest multicenter bonding in the triangular faces [27,74], reflecting indirect rather than direct B2 2B bonding [91]. A separate study using SCF, DFT, MP2, and other methods found that 1,5-C2B3H5 derivatives having F, Cl, or NH2 substituents on two or more boron atoms have classical structures, while the 2,3-Li2 and Li5 derivatives are nonclassical [90]. Further insight into the bonding in 1,5-C2B3H5 has been gained from ab initio self-consistent field and DFT treatment [92] and other studies (Table 4-4) of the parent molecule and its derivatives, together with experimentally determined NMR data and IGLO calculations on 11B, 13C, and/or 1H chemical shifts and spin-spin coupling constants

40

CHAPTER 4 Small carboranes: Four- to six-vertex clusters

[11,12,36,37,93–100], a study of aromatic solvent-induced proton NMR shifts [101], and analyses of negative-ion mass spectra [102], infrared and Raman spectra [92,103,104], photoelectron spectra [105,106], and ESCA spectra [107]. Sig21H coupling nificant bonding involving s orbitals in the center of the cage is suggested by unusually high antipodal 1H2
1
1 [93,94], and the uniquely high C2 2H stretching frequency at 3160 cm (vs. 2600 cm for other closo-carboranes) is attributed to the low (4) coordination number of carbon [92,108]. In summary, 1,5-C2B3H5 and its derivatives straddle the border between classical and nonclassical systems and can fall into either category depending on the nature of attached substituents. Again, this property is unique among closocarboranes, all of the larger members having unequivocally nonclassical cluster frameworks.

4.4.2.3 Physical properties and reactivity

1,5-C2B3H5 is a gas at room temperature which melts at –126.4  C and boils at –3.7  C, and is thermally stable below 150  C [57]. Experimental studies on parent 1,5-C2B3H5 have been hampered by the relative inaccessibility of this compound, but some information is available. Although somewhat more reactive than its higher congeners in the closo-C2Bn
2Hn series, the parent carborane is unaffected by O2, H2O, NEt3, CO2, or acetone at room temperature [49]. However, unlike the larger closo-carboranes, liquid 1,5-C2B3H5 slowly polymerizes on standing, forming a dimer and higher molecular weight products [109]. A 1:1 adduct forms with NMe3 at low temperatures, but on melting it converts to an intractable polymer [109]. 2B Pyrolysis of 1,5-C2B3H5 has been investigated by several groups, who found that the main product is the B2 bonded dimer 2,20 -(1,5-C2B3H4)2 [109–111]; by trapping products on a cold finger in the reactor, higher oligomers including the B2 2B bonded trimer 2,20 -3,20 -(1,5-C2B3H4)2
1,5-C2B3H3 have been isolated and characterized [110,111]. Under conditions more favorable to secondary reactions, other products have been obtained, including dimers and trimers linked by B2 2C (but not C2 2C) bonds as well as the tetracarbon carborane nido-C4B7H11 [110]. 2B linked mixed-cage species 2,20 -1,5Co-pyrolysis of 1,5-C2B3H5 and 1,6-C2B4H6 generates in 47% yield the B2 0 0 C2B3H4
1 ,6 -C2B4H5 [110], a compound also obtained in small quantity by Hg-sensitized co-photolysis of the two carboranes [112]. The dimer 2,20 -(1,5-C2B3H4)2 has also been produced, in quantitative yield, by PtBr2-catalyzed dehydrogenation and 2B intercage bonds in the B2 2B linkage at 25  C [113]. MNDO calculations indicate a large rotational barrier for the B2 dimer and trimer, suggesting p-interaction between the polyhedra. In the dimer, the most stable conformation has D2d symmetry with the B3 planes in the two cages mutually perpendicular [111]. This finding is consistent with an analysis of boron-boron spin coupling constants in linked-cage carboranes [99], which gave a calculated value of 39% s character 2B linkage in 2,20 -(1,5-C2B3H4)2 and other linked-cage carboranes, suggesting that (sp1.6 hybridization) for the B2 p-p p-bonding is a distinct possibility. Exposure of neat 1,5-C2B3H5 to Cl2 and NO2 leads to uncharacterizable products [109]; however, reaction with Cl2 in solution gives the 2-chloro derivative, and treatment with BMe3 in the gas phase affords a mixture of B-mono- di-, and trimethylcarboranes [67]. The parent carborane reacts vigorously with water and methanol at room temperature to form triborapentane products [114]: 2CH22 2BðORÞ2 2CH22 2BðORÞ2 þ H2 ðR ¼ H or MeÞ C2 B3 H5 þ 5ROH ! ðROÞ2 B2 The compound initially formed with water (R ¼ H) is hydrolyzed, generating BMe(OH)2 and B(OH)3. Comparative studies show that the larger closo-carboranes 1,6-C2B4H6 and 2,4-C2B5H7 are much less reactive toward these reagents [114]. As mentioned earlier, treatment with atomic (1D) sulfur affords the B-mercapto derivative [68]. Reaction of 1,5-C2B3H5 with B2H6 at 300  C in a flow reactor effects boron insertion to give a new carborane, nido-C2B6H10 (see Chapter 5) [115]. The same reactants combine at room temperature in the presence of a PtBr2 catalyst to afford another new carborane, arachno-5,6-C2B6H12, also discussed in Chapter 5 [116]. Cage expansion of 1,5-C2B3H5 via addition of transition metals to form 6- or 7-vertex metallacarborane clusters [117,118] is described in Chapter 13. The reactivity of 1,5-C2B3H5 toward NH3 and protonating agents has been explored theoretically. The B-NH3 complex is calculated to be less stable than the free carborane by 24 kcal/mole, so complexation with ammonia is not

4.5 6-Vertex open clusters

41

expected [42]. MNDO computations [119] suggest that protonation to form C2B3H6þ occurs preferentially at carbon. Reactions of the parent carborane toward Lewis bases in general give unstable products [120].

4.4.3 Closo-C3B2H5þ A theoretical analysis [121] of the unknown closo-C3B2H5þ cation found an energy minimum for trigonal bipyramidal geometry of C2v symmetry having carbon atoms occupying two apexes and one equatorial position. This structure was rationalized on the basis that CH rather than BH groups are preferred as capping units because of greater ring-cap orbital overlap [121]. At present, unsubstituted CxByHzþ carborane cations are very rare and no small-molecule species of this class have been isolated.

4.5 6-VERTEX OPEN CLUSTERS Successive formal replacement of BHbridge units in hexaborane(10) (B6H10) with C atoms (or of B with Cþ) generates the series of 6-vertex nido species depicted in Figure 4-3, all of which are known as parent compounds or substituted derivatives; in some cases, additional isomers are known. The carboranes shown have been prepared via a diverse range of synthetic routes as described below, none actually involving B6H10. The nonclassical C6H62þ dication has not been prepared per se, but has been characterized as the hexamethyl derivative [122].

H H H

B B

B

H

B

H

H

H

H B

H

C H B

B B

H

H

H

C C H

B B

H

H B H

H

H

B

B B

H B

H

C H

2,3,4-C3B3H7

B

H

C

C

H

C C H

H

2,3,4,5-C4B2H6

+

C

H B

H

C H

2+

H

C

H H

B

2,4-C2B4H8

B

H

C C H

C B H

H

H

H

H

H

H

2,3-C2B4H8

H

C C H

H

2-CB5H9

B6H10

H

H B

H

H

H

B

B

B

B

H

H

H

H

H H

C

C H

H

C H C

C

C H

C H

2,3,4,5,6-C5BH6

+

C6H62+

FIGURE 4-3 Isoelectronic and isostructural 6-vertex nido-CnB6
nH10
n clusters.

4.5.1 Nido-2-CB5H9 4.5.1.1 Synthesis There are currently no high-yield, efficient routes to nido-2-carbahexaborane(9), 2-CB5H9 (the only known isomer). The parent compound is generated together with other carborane products in the flow pyrolysis of 1-MeB5H8 or 1,2-Me2B5H7 [123]; when 1-ethylpentaborane is used as the reactant, nido-2-MeCB5H8 is obtained, together with methyl derivatives of

42

CHAPTER 4 Small carboranes: Four- to six-vertex clusters

other carboranes. Flash thermolysis of 2-[(ClMe2Si)CH2]B5H8 affords 2-CB5H9 and closo-CB5H7 [124]. In a very different procedure, degradation of closo-1,6-C2B6H8 with BH4
salts, followed by treatment with anhydrous HCl, affords 2-CB5H9, along with several of its methyl, dimethyl, and trimethyl derivatives [125]. Alkyl and other derivatives (Table 4-5) have been synthesized in a variety of ways. In the earliest synthesis of the 2-CB5H9 carborane system, the reaction of acetylene with B5H9 in the vapor phase at 215  C afforded MeCB5H8, CB5H8-3-Me, and CB5H8-4-Me [126]. Methyl derivatives have also been isolated from gas-phase interactions of acetylene and other alkynes with tetraborane(10) (B4H10) [127]. More efficiently, flow pyrolysis of alkenylpentaboranes through a heated tube at 355  C generates alkyl derivatives of CB5H9: for example, 2-trans-1-propenyl-B5H8 produces CB5H8-3-Et 5CH-B5H8 produces 2-, 3-, and 4-Me-2-CB5H8 in a combined yield of and CB5H8-4-Et in 42% total yield, while 2-CH25 51% [128,129].

TABLE 4-5 Nido-2-CB5H9 Derivatives Synthesis and Characterization Compound

Information

References

Parent

S, H, B, IR, MS S, H, B(2d), C S S, H, B B C UV-photoelectron spectra S, H, B, IR, MS H, B, MS IR H, B C MS (detailed) MS (calculated monoisotopic) S, H, B, IR, MS S, H, B, MS S S, H, B, MS S, H, B, IR, MS* B H, B C S, H, B, IR, MS S, H, B, IR, MS* S, B, IR, H, C, P S, H, B, IR, MS S, B, IR, MS S, B, MS, IR

[125] [344] [124] [123] [132] [96] [105] [125] [125,126] [125] [134] [96] [133] [345] [125] [126] [123] [126] [128] [135] [134] [96] [125,128] [128] [137] [130] [70] [127]

1-Me 2-Me

3-Me

4-Me B-R (R ¼ Me, Et; 4 isomers) n-R (n ¼ 1, 2, 3, 4) n-Me (n ¼ 3, 4) Me2 (5 isomers) MeEt (4 isomers) m(Et3P)2PtH 2-Et 2-Me2HSi 2-Me-B-Me

Continued

4.5 6-Vertex open clusters

43

TABLE 4-5 Nido-2-CB5H9 Derivatives—Cont’d Synthesis and Characterization Compound

Information

References

2-Me-n-Et (n ¼ 3,4) 1,2,3-Me3 MeCB5H4Et4?, MeCB5H3Et5? MeCB5H3Et5

S, S, S, S,

[128,129] [125] [15] [61]

Other Experimental Studies Parent

H, B, MS H, B, IR, MS MS H, B, IR, MS

Dipole moment, microwave spectrum Pyrolysis ! 1, 7-C2B10H12 (16%) þ 1, 7-C2B6H8 (5%) þ 1, 6-C2B8H10 (trace) þ arachno-1, 3-C2B7H13 (30%) Pyrolysis ! 1, 7-C2B8H8-B, B0 -Me2 (16%)

3-Me

[131] [136] [136]

Theoretical Studies Molecular and electronic structure calculations Parent Extended Hu¨ckel ab initio, energies of isomers Structure, dipole moment, ionization potential DFT: ionization potential, valence structure UV-photoelectron spectra SCF, MNDO CB5H63


[34] [35] [333] [346] [105] [132,323,347] [8]

Isomerization calculations CB5H8-n-R (n ¼ 1, 2, 3, 4)

Isomerization

[135]

Parent, MeCB5H8, CB5H8-n-Me (n ¼ 3, 4)

B, C, H, DFT spin-spin coupling B IGLO GIAO/NMR C IGLO

[37] [12] [344] [96]

Reactivity calculations Parent

Protonation

[119]

NMR calculations Parent

S, synthesis; X, X-ray diffraction; H, 1H NMR; B, spectroscopic data; UV, UV-visible data.

11

B NMR; C,

13

C NMR; 2d, two-dimensional (COSY) NMR; IR, infrared data; MS, mass

Other routes to 2-CB5H9 derivatives involve reactions conducted in solution. Lithium methyl acetylide and B5H9 in diglyme combine to give 2-EtCB5H8 in 5% yield [130], and dehalogenation of alkylhaloboranes also has been successfully employed [15,61]. Remarkably, the action of lithium metal on EtBF2 in THF affords a series of lower carboranes (a rare example of carborane synthesis directly from a monoboron substrate) from which nido-2-MeCB5Et5H3 is isolated in ca. 7 mole % yield together with small amounts of nido-2-MeCB5Et4H4 isomers and tetra- and pentaethyl derivatives of closo-2,4-C2B5H7 [61]. Further, 2-(Me2HSi)CB5H8 is a minor product in the flash thermolysis of 1,2-(Me2HSi)2B5H7 [70].

44

CHAPTER 4 Small carboranes: Four- to six-vertex clusters

4.5.1.2 Structure and properties There are no published crystal structure determinations on CB5H9 or any derivative. However, the pentagonal pyramidal cage geometry (Figure 1-3, top center) has been confirmed by a microwave structural analysis [131] and is further supported by NMR and other spectroscopic evidence in conjunction with ab initio calculations (Table 4-5). Detailed 11B, 1 H, and 13C NMR investigations, including measurements and calculations of chemical shifts [12,96] and nuclear spin-spin coupling constants [37,132], allow insight into the nature of the cluster framework bonding in the parent compound. The mass spectrum has been analyzed and compared with those of related compounds [133]. Several theoretical studies, listed in Table 4-5, have explored the bonding in parent CB5H9 and its isoelectronic nidocarborane relatives (Figure 4-3), all concluding, in line with experiment, that species having carbon in the base of the pyramidal cage are thermodynamically favored. Similar studies have appeared on B-methyl derivatives of 2-CB5H9 [134,135]. Exploration of the chemistry of nido-2-CB5H9 is surprisingly sparse for a molecule that has been known for nearly half a century and is accessible by several different routes. Pyrolysis of the parent carborane gives evidence of cage fusion, affording the closo-carboranes 1,7-C2B10H12 (16%), 1,7-C2B6H8 (5%), and 1,6-C2B8H10 (trace) together with arachno4,5-C2B7H13 (30%) [136]; similar treatment of 2-CB5H8-3-Me produces 1,7-C2B8H8-B, B0 -Me2 in 16% yield. Although MNDO calculations [119] predict that protonation of 2-CB5H9 will occur preferentially at carbon, this has not yet been confirmed experimentally. The platinum-bridged complex 4-20 was obtained in 23% yield from Pt2(m-C8H12)(PEt3)4 and 2-CB5H9 at room temperature in diethyl ether solution, and characterized from infrared and multinuclear NMR spectra [137]. Curiously, this compound is the only reported metal complex of CB5H9 or any of its derivatives (contrasting sharply with the hundreds of known metal complexes of the nido-2,3-C2B4 system). H

B H H H

C

B H

B H

B

H

4-20

B H

Et3P

PEt3 Pt H

Some chemistry of the peralkylated derivative 2-MeCB5Et5H3 has been investigated [61]. The compound reacts with 2H2 2B bridging proton) and forming a salt assumed to be NaþBHEt3
, releasing H2 (presumably via abstraction of a B2 NaþMeCB5Et5H2
, which has not been characterized; acidification of this salt with HCl or DCl regenerates the neutral carborane and its monodeuterated form, respectively. No deuterium exchange occurs on treatment with B2D2Et4 at 120  C. The compound 2-MeCB5Et5H3 resists oxidation, reacting with atmospheric O2 only very slowly (months) and showing little reactivity toward H2O2 in the presence of H2SO4. It is stable on pyrolysis up to 180  C; above that temperature, it decomposes to give alkyl derivatives of small closo-carboranes and other products [61]. Calculations [135] on the four possible monomethyl derivatives of nido-2-CB5H9 indicate that their relative stabilities are in the order 3-Me > 4-Me > 1-Me > 2-Me, and experimentally [135] it has been found that the 3-Me and 4-Me derivatives are in thermal equilibrium at 225  C.

4.5.2 Hypho-CB5H13 Stepwise degradation of arachno-6-(Me3N)CB9H13 in MeOH/KOH solution at 60  C gave a product characterized from NMR, infrared, and mass spectroscopy as hypho-6-(Me3N)CB5H11, a derivative of the as-yet unknown species hypho-6CB5H13 (“hypho” designates a closo cluster with three missing vertexes and n þ 4 skeletal electron pairs). This compound was isolated, remarkably, in 75% yield as a white crystalline solid that is moderately air-stable [138]. Its proposed structure

4.5 6-Vertex open clusters

45

4-21 is represented as a derivative of B5H11 in which one of the bridging hydrogens is replaced by a 2 2CHNMe32 2 bridge that increases the skeletal electron count from eight to nine pairs [138]. No other hypho-CB5 clusters are known at this time. H

B

H

B H H

H H

B H

H

B H B

H H

4-21

C H

NMe3

4.5.3 Nido-2,3-C2B4H8 Nido-2,3-dicarbahexaborane(8) (Figure 1-3, top right) and its derivatives, discovered by Onak, Williams, and their coworkers in 1962 [139,140], are the preparative starting point for a large area of metal sandwich chemistry involving 6- and 7-vertex nido-MC2B3, closo-MC2B4, and closo-M2C2B3 clusters and related systems, hundreds of which have been prepared and characterized (Chapter 13); among metallacarboranes, only the 12-vertex icosahedral complexes are more numerous.

4.5.3.1 Synthesis All preparative routes of any practical significance for nido-2,3-C2B4 carboranes (Table 4-6) involve the interaction of alkynes with boron hydrides, usually B5H9 (for a general discussion see Chapter 3). Alkene-borane reactions, in contrast,

TABLE 4-6 Nido-2,3-C2B4H8 Derivatives Synthesis and Characterization Compound Derivatives with Main Group Element Substituents No substituents on boron Parent

Information

References

S, H, B S, IR S (large scale) S [macro scale, from nido-2,3(SiMe3)2C2B4H6] S (from closo-2,3-C2B5H7) X X (refinement) H, B, IR, MS H (decoupled) H B C MS (calculated monoisotopic) XPS: binding energies

[57] [127] [348] [155] [349] [162] [163] [170] [171] [71] [172] [29] [345] [107] Continued

46

CHAPTER 4 Small carboranes: Four- to six-vertex clusters

TABLE 4-6 Nido-2,3-C2B4H8 Derivatives—Cont’d Synthesis and Characterization Compound

Information

References

2,3-C2B4H7
[Ph3PCH2]þ 2,3-C2B4H7


S, H, B S, B, H, P S, H, B S, H, B, C, MS S, IR S (large scale) CH/ S (from 1-MeB5H8þ MeC 2-pentyne), H, B S, H, B S, H S, H, B, C, MS S, H, B, UV S (large scale) S, H, B, C, MS S, H, B, IR, MS S, H, B S, H, B, IR, MS* S, IR, MS* S, H, B, IR, MS S, H, B, C, Si, MS S, X, H, B, C, Si, IR, MS S, IR S, H, B S, IR S (large scale) X B S, B S, B, H, IR S, B S (improved) S (detailed), IR, MS, VP S, H, B, C, UV, IR, MS S, H, B, C, UV, IR, MS S, B S, H, B, IR, MS IR, MS* S, H, B, C, Si, MS S, H, B, C, Si, MS S, H, B, C, IR, MS

[170,182] [184] [57,140] [33] [127] [145] [143]

2-Me

2-n-C3H7

2-Ph 2-R (R ¼ Et, CMe3) 2-CH2R (R ¼ fluorenyl, indenyl) 5CMe 2-CH25 2-(CH2)nPh (n ¼ 2, 3) 2-SiH3 2-SiMe3 [2,3-(SiMe3)C2B4H5
]2Ba(THF)22þ 2,3-D2 2,3-Me2

2,3-Me2C2B4H5
2,3-(n-C3H7)2 2,3-R2 (R ¼ Et, Ph) 2,3-Et2 2,3- (n-C6H13)2 2,3-Ph2 2,3-Ph2C2B4H5
[(CH2)5-nido-2,3-C2B4H7]2 2,3-(SiH3)2 2,3-(SiMe3)2 2-Me-3-SiMe3 2-R-3-SiMe3 (R ¼ Bu, CMe3)

[140] [139] [33] [57] [145] [33] [146] [57] [147] [141] [141] [154] [350] [197] [140] [127] [145] [162] [172] [170] [187] [172] [157] [148,156] [149] [149] [149] [150] [141] [154] [154] [152] Continued

4.5 6-Vertex open clusters

47

TABLE 4-6 Nido-2,3-C2B4H8 Derivatives—Cont’d Synthesis and Characterization Compound þ




(C4H8ONa )2 (SiMe3)2C2B4H5 Li[(H2N)2C2H4]2þ(SiMe3)2C2B4H5
2,3-(PhCH2)2 2,3-(n-C4H9)2 2,3-(i-C5H11)2 2,3-(CH2R)2 (R ¼ fluorenyl, indenyl) [(CH2)5C2B4H7]2 (2,3-Me2C2B4H5)2 2-Et-3-(CH2)n[3-(2-EtC2B4H6)] (n ¼ 4,6) [2-EtC2B4H6
3-(CH2)6]2Et2C4B8H8 [2-EtC2B4H6
3-(CH2)6Et2C4B8H8-]2 (L)Liþ [(SiMe3)RC2B4H4
-(m-H)2-exo-Li(L)] [L ¼ (Me2N)2C2H4, none; R ¼ SiMe3, Me, H) LNaþ [(SiMe3)RC2B4H5
] [L ¼ THF, (Me2N)2C2H4; R ¼ SiMe3, Me, H] (THF)Naþ [(SiMe3)C2B4H4
-exo-Li(THF)] (R ¼ SiMe3, Me, H)

Information

References

S, S, S, S, S, S, S, S, S, S, S, S,

[164] [165] [151] [149] [149] [146] [150] [112] [150] [150] [150] [166]

X, H, B, C, IR X, H, B, Li, C, IR H, B, IR, MS H, B, C, IR, UV, MS H, B, IR, UV, MS H, B, IR, MS H, B, IR, MS H, B, IR, MS H, B, C, MS, UV, IR H, B, MS, UV, IR H, B, MS, UV, IR X[SiMe3, (Me2N)2C2H4], H, B, C, Li, IR

S, X [(Me2N)2C2H4, SiMe3; THF, SiMe3], H, B, C, IR S, H, B, C, IR

D-, hydrocarbon-, B-, or Si-containing substituents on boron S, H, B C2B4H7D n-Me (n ¼ 1, 4, 5) S, ring currents n-Me (n ¼ 1, 2, 4, 5) S, H 4-Et S, H, B, IR, MS 5-Et S, H, B, MS S, H, B, IR, MS,UV 4-CH2Ph S, H, B, IR, MS 4-CH2C6H4Me S, H, B, IR, MS,UV 4-(CH2)3Ph n-(cis-2-but-2-enyl) (n ¼ 1, 4, 5) S, H, B, IR, MS S, H, B, MS 4/5-CH2SiMe2Cl S, H, B 5-SiMe2CH2Cl S, B, IR, MS, H n-SiH3 (n ¼ 1, 2, 4) S, B, IR, MS, H n-SiMe3 (n ¼ 2, 4) S, H, B,VP m(4,5)-CH2SiClMe2 S, B, IR, MS,H m,m0 (4,5)-SiH2(2,3-C2B4H7)2 S, B, IR, MS,H m(4,5)-(CH2)4SiCl S, B, IR, MS,H m(4,5)-SiMeH2 S, B, IR, MS, H m(4,5)-SiR3 R ¼ H, Me 2-Me-n-Et (n ¼ 4, 5) S, H, B, C S (1-MeB5H8þ MeCCH), H, B 2,B-Me2 S, H, B, MS 2,4-Me2 Liþ Naþ [(SiMe3)RC2B4H4]2
(R ¼ H, Me, SiMe3)

[166] [166]

[170,182] [144] [71] [188] [142] [188] [188] [188] [69] [71] [71] [189] [189] [71] [190] [190] [190] [189] [29] [143] [142] [167] Continued

48

CHAPTER 4 Small carboranes: Four- to six-vertex clusters

TABLE 4-6 Nido-2,3-C2B4H8 Derivatives—Cont’d Synthesis and Characterization Compound

Information

References

4,m(4,5)-(SiMe3)2 exo-“closo”-Me(SiMe3)C2B4H4
1-Na(TMEDA)m(4,5)-H2Na(TMEDA)2 4,5,6-D3 2,3,4-Me3

S, B, IR, MS, H S, H, B, C, IR

[190] [168]

S, H*, B, IR, MS S (from 1-MeB5H8þ MeCCH/2-pentyne), H, B S, H, B, MS S (1-MeB5H8þ MeCCH), H, B S, ring currents S, H, B, IR, MS S, H, B, IR, MS S, H, B, IR, MS, UV S, H, MS S, H, B, MS, UV, IR S, X, H, B, C, i S, H, B, C, IR S, H, B, C, IR

[170] [351] [142] [143] [144] [352] [188] [188] [353] [150] [195] [196] [196]

S, H, B, C, IR

[195]

S, H, B, C, IR S, S, H, B, IR, MS H, B, IR, MS H, B, IR, MS S, H, B, C, IR

[195] [352] [191] [191] [354]

F-, Cl-, Br-, or I-containing substituents on boron 2-Me-4-X (X ¼ Cl, Br, I) 2,3-Me2
4-X (X ¼ Cl, Br, I) 2,3-Me2
5-Br 4-X (X ¼ Cl, Br) n-Cl (n ¼ 4, 5) 4-I

S, S, S, S, S, S,

H, H, H, H, H, H,

[201] [201] [187] [200] [187] [201,307]

Main-group metal substituents on boron exo-Me(SiMe3)C2B4H5
4,5-m(H)2Na(TMEDA)2 (TMEDA)Mg-[(SiMe3)C2B4H5]2 m(4,5)-AlMe2 m(4,5)-Al(Ph3P)3 2,3-Et2-m(4,5)-AlH2(NEt3) 2,3-Et2-m(4,5)-Al(NEt3)(3,4-Et2C2B4H4) m(4,5)-GaMe2

S, S, S, S, S, S, S,

X, H, B, C, IR X, H, B, C, IR MS H, B, IR, MS H, B, IR H, B, IR, MS B, IR, MS

2-Et-3,4-Me2 2,3,n-Me3 (n ¼ 1, 4, 5) 2,3-Me2
5-Ph 2,3-(PhCH2)2
4-Me 2,3-(PhCH2)2
4-CH2Ph 1,3,5-(nido-2,3-Et2C2B4H5
1-)3C6H3 cyclo-[C2B4H6-(CH2)n-C2B4H6-(CH2)5-]- (n ¼ 4, 5, 6) Na(THF)þ 2,3-(SiMe3)2C2B4H4
5-iBu
2,3-(Me3Si)2
5-SiMe2CH2Cl 2,3-(Me3Si)2
5-SiMe2-(1–1,2-CB10H10C-R) (R ¼ Me, Ph) 2,3-(SiMe3)2
5-R (R ¼ Me, iBu, CH2CH2Cl, CH2CH2Br) 2,3-(SiMe3)2
4-Me-5-iBu 5,50 -(2,3-Me2C2B4H5)2 2,3-Me2-m(4,5)-SiMe2R (R ¼ Me, Cl) 2,3-Me2
4-SiMe3 2,3-(Me3Si)2
5-SiMe2NMeCMe3

B, B, B, B, B, B,

IR, MS, VP IR, MS,VP MS* IR, MS, VP MS* IR, MS, VP

[168] [355] [192] [192] [356] [356] [192] Continued

4.5 6-Vertex open clusters

49

TABLE 4-6 Nido-2,3-C2B4H8 Derivatives—Cont’d Synthesis and Characterization Compound

Information

References

4-GeMe3 m(4,5)-GeR3 (R ¼ H, Me) m,m0 (4,5)-GeMe2-(4-SiMe3-C2B4H6)2 m(4,5)-SnMe3 m(4,5)-PbMe3

S, S, S, S, S,

B, B, B, B, B,

[189] [189] [189] [190] [190]

S, S, S, S, S, S, S, S,

X, H, B, IR, MS MS H, B, IR, MS H, B, IR, MS B, MS X, H, B, IR, MS X H, B, IR, UV, MS

[357] [358] [357] [185,186] [146] [193] [359] [360]

S, S, S, S, S, S, S, S, S, S, S, X

B, H, C, MS H, B, IR H, B, IR, P H, B, IR, P H, B, IR, P H, B, IR B, H, C, P, IR X, H, B, C, P, IR B, MS H, B, IR, MS X, H, B

[361] [362] [194] [194] [194] [192] [137] [137] [192] [352] [363] [364]

Transition Metal s- and m-Complexes 2,3-[CH2PhCr(CO)3]2 2,3-[(CO)3CrPhCH2]2-m-D 2-CH2Ph-3-CH2PhCr(CO)3 m(4,5)-FeCp(CO)2 Fe[2,3-(indenyl-CH2)2C2B4H6]2 5CMe) 2,3-Et2-m(4,5)-CpFe(CO)(PPh3)-(m-MeC5 2,3-Me2C2B4H5-(2,3-Me2C2B4H3)CoCp 2,3-Me2
4-[40 -nido-1,2,3-CpCo(Me2C2B3H4)] (isomers) 2,3-Et2
1-C2RCo2(CO)6 (R ¼ Ph, SiMe3) m(4,5)-NiCl(Ph2PCH2)2 m(4,5)-Cu(Ph3P)2 m(4,5)-Cu(Ph2PCH2)2 2,3-Me2-m(4,5)-CuR2 (R ¼ Ph3P, Ph2PCH2) m(4,5)-Rh(PPh3)3 2,3 Me2-m(4,5)-Pt(Et3P)2H m(4,5)-(Et3P)2PtH m(4,5)-HgPh [2,3-Me2C2B4H5-m(4,5)]2Hg [2,3-(Me3Si)MeC2B4H5-m(4,5)]2Hg Nd3[(SiMe3)2C2B4H4]6[(O-CMe3)(THF)Li]3 Detailed NMR Studies Parent

Parent, 2-Me, 2,3-Et2 2-Me 2,3-Me2 2,3-Et2 2,3-Et2 C2B4H7
, parent Et2C2B4H5


MS, H IR, MS, H MS, H IR, MS, H IR, MS, H

B (line narrowing) H (B-H spin relaxation) C C (IGLO) B (line narrowing) B (line narrowing) B (solvent shifts) B (2d) B (paramagnetic effects of Sm2þ) B, C, H spin-spin coupling (DFT) B (paramagnetic effects of Sm2þ) B (2d)

[173] [365] [97] [96] [173] [173] [366] [174] [367] [37] [367] [174] Continued

50

CHAPTER 4 Small carboranes: Four- to six-vertex clusters

TABLE 4-6 Nido-2,3-C2B4H8 Derivatives—Cont’d Synthesis and Characterization Compound Other Experimental Studies Parent

C2B4H7
, 2-MeC2B4H6
2-Me

2-CH2-fluorenyl 2,3-Me2

2,3-Et2

2,3-R2 [R ¼ Me, Et, Ph, CH2Ph, indenyl, CH2PhCr(CO)3]

Information

References

MS MS (negative ion) UV photoelectron spectra Photolysis VP IR, Raman Reaction with BMe3 Catalytic reactions with alkynes Deuteration Deprotonation, deuteration High yield conversion to closo-carboranes Co-pyrolysis with BMe3 ! closo-2,4H2C2B5Me5 CMe Reaction with MeC Reaction with Fe(CO)3 Reaction with silanes Reaction with GaMe3, InMe3 Adsorption on silica, angle-resolved photoemission Reactions with CoCl2þ Cp
IR, Raman MS (negative ion) VP Kinetics of bridge deprotonation IR, Raman H, MS MS (negative ion) VP Photolysis Deuteration Photoemission spectra Inner shell electron energy-loss spectroscopy (SEELS) Conversion to closo-Et2C2B5H5 Adsorption on silica, angle-resolved photoemission Synchrotron radiation-induced deposition of boron carbide films APP Kinetics of bridge deprotonation

[133] [178] [105] [55,112] [201] [176] [204] [69] [197] [170] [56] [205] [208] [40] [141] [206,368] [202] [369,370] [176] [178] [201] [183] [176] [139] [178] [201] [112] [197] [177] [179,180] [203] [202] [371] [183] Continued

4.5 6-Vertex open clusters

51

TABLE 4-6 Nido-2,3-C2B4H8 Derivatives—Cont’d Synthesis and Characterization Compound

Information

References

2-SiMe3 2,3-(SiMe3)2 2-SiMe3
3-RC2B4H5
(R ¼ H, Me, SiMe3) 2,3-(Me3Si)2

ED Cage expansion reactions Conversion to closo-1,2-RR0 C2B4H6 alkylation Thermal fusion ! nido-(Me3Si)4C4B8H10 oxidative cage closure

[169] [207] [166] [195] [199] [195]

Valence structures Fractional three-center bonds Three-center bonds DFT: ionization potential, valence structure Localized MOs Localized MOs (SCF) Classical versus nonclassical structures (ab initio) MNDO, dipole moment Energies (ab initio) Electron population analysis Isomer energies (ab initio) XPS: binding energies (CNDO) Extended Hu¨ckel Charge distribution Geometry MNDO MNDO B3LYP optimized geometries

[297,347] [372] [373] [346] [374] [375] [181] [323] [304] [376] [35] [107] [34] [377] [41] [177,179,180] [202] [195]

B3LYP optimized geometry ab initio

[195] [35]

H (solvent shifts) B (IGLO) B (GIAO) B (GIAO) B (GIAO) B (IGLO), GIAO B

[175] [12] [143] [143] [143] [294] [355]

2,3-(SiMe3)2
5-iBu Theoretical Studies Molecular and electronic structure calculations Parent

Parent, C2B4H7
Parent, C2B4H62
C2B4H62
2,3-Et2 2,3-R2 (R ¼ H, Et) 2,3-(SiMe3)2
5-R (R ¼ Me, CMe3, CH2CH2Cl, CH2CH2Br) 2,3-(SiMe3)2
4-Me-5-iBu 2,3-C2B4H62
NMR calculations Parent 2-Me 2,B-Me2 2-Et-3,4-Me2 nido-H2C2B4Cl4 (H2NC2H4NH2)Mg(2,3-C2B4H6)2

Continued

52

CHAPTER 4 Small carboranes: Four- to six-vertex clusters

TABLE 4-6 Nido-2,3-C2B4H8 Derivatives—Cont’d Synthesis and Characterization Compound þ

]2 nido-2,3-(R3Si)2C2B4H42


[M R ¼ H, Me) Parent, 2-Me, 2,3-Et2

Reactivity calculations Parent Parent, 2-Et, 2,3-Me2

(M ¼ Li, Na;

Information

References

B (MP2) covalence or strong ion pairing between M and anions C (IGLO)

[378] [96]

Alkyne incorporation Mechanism of formation from B4H8þ C2H2

[230] [32]

S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; 2d, two-dimensional (COSY) NMR; IR, infrared data; MS, mass spectroscopic data; UV, UV-visible data; ED, electron diffraction; VP, vapor pressure.

yield organoboranes but not carboranes. Gas-phase thermolysis reactions of small alkynes with B4H10, B5H9, or B5H11 generate mixtures of lower nido-carboranes that invariably include 2,3-C2B4H8 or its derivatives [29,57,127,139, 141,142]; alkyne-alkylpentaborane reactions afford B-alkyl C2B4H8 derivatives [143,144]. However, yields are typically low, and owing to the limitations of scale (and safety) imposed by gas phase operations, only millimoles of pure carboranes are usually obtainable by such methods. An alternative approach utilizes Lewis base–promoted reactions at ambient temperature: the first reported nido-dicarbahexaborane compounds, that is, the 2-methyl, 2,3-diethyl, and 2-n-propyl derivatives, were prepared via liquid phase interaction of B5H9 with alkynes at room temperature promoted by 2,6dimethylpyridine, with separation of the products by vapor phase chromatography [57,140]. The reaction is driven by base-extraction of a BH3 unit from B5H9 to form an adduct: CR0 þ L ! 2;3-RR0 C2 B4 H6 þ L:BH3 B5 H9 þ RC R; R0 ¼ H; Me; Ph; CMe5 5CH2 This procedure was later improved by employing triethylamine as the Lewis base at 0  C, allowing separation by vacuum-line fractionation to afford a range of C-substituted and C,C0 -disubstituted derivatives, in moderate to good yields [145–151]; the reaction works well even with bulky alkynes such as bis(indenyl)- and bis(fluorenyl)acetylene [146,149]. Trimethylsilyl-substituted nido-2,3-dicarbahexaboranes, (Me3Si)RC2B4H6 (R ¼ H, alkyl, or SiMe3), useful synthons in CSiMe3 in a stainless steel cylinder without lower carborane research [152,153], are prepared from B5H9 and Me3SiC solvent or base reagent [154]. (Me3Si)2C2B4H6 generates parent 2,3-C2B4H8 on a multigram scale when heated at 160170  C with HCl gas for several days [155]. Alkyne-pentaborane reactions conducted in the absence of solvent are highly exothermic, and explosions have resulted. For the synthesis of the C,C0 -diethyl-, C-phenyl-, and C,C-diphenyl derivatives, this problem is circumvented by conducting the reaction in cold diethyl ether [156] or THF [157] solution, which moderates the rate of the alkyneborane interaction and takes advantage of the stability of solutions of B5H9 in organic media toward air oxidation [157]; the Et2C2B4H6 product is separated by vacuum-line fractionation from the more volatile solvent. In this manner, the diethylcarborane can be routinely and safely obtained on a scale of 100 grams or more [156]. A further issue remains. What had been the main supply of B5H9 in the world—a large reserve of ca. 100,000 kg originally produced at considerable cost for the borane fuels program described in Chapter 1, and maintained for decades by the U.S. Army—has regrettably been destroyed in a shortsighted decision. The loss of this irreplaceable resource complicates the continued study of R2C2B4H6-based chemistry and much other research on boron clusters [158], including medical applications. For laboratory-scale operations at least, this difficulty can be overcome. A useful route,

4.5 6-Vertex open clusters

53

originally demonstrated in 1979 [145], utilizes the conversion [159] of the inexpensive bulk chemical NaBH4 to NaB3H8, from which B5H9 is generated [160] in situ and reacted with alkynes to give the desired carborane products. NaB3H8 has also been prepared from boric acid and converted to B5H9 [161].

4.5.3.2 Structure and properties The pentagonal-pyramidal molecular geometry of the nido-C2B4 framework (Figure 1-3) is established from X-ray diffraction studies on the parent compound [162,163], Me2C2B4H6 [162], several C- and B-substituted derivatives (Table 4-6), and salts of C-SiMe3 substituted mono- and dianions [164–168], and from a gas phase electron diffraction investigation of (Me3Si)C2B4H7 [169]. Electronic structure and bonding in nido-2,3-R2C2B4H6 carboranes (R ¼ H or alkyl) has been examined in detail via 11B, 1H, and 13C NMR [57,170–175], infrared and Raman [176], ESCA [107], photoemission [177], and negative-ion mass spectroscopy [178], inner shell electron energy-loss spectroscopy (SEELS) [179,180], and in numerous theoretical investigations listed in Table 4-6. The compound 2,3-C2B4H8 is an isoelectronic and isostructural analogue of nido-CB5H9 and other 6-vertex CnB6
nH10
n clusters depicted in Figure 4-3, and can be described in both localized valence-bond and delocalized molecular orbital language, as discussed earlier in Section 2.3. An ab initio investigation of nonclassical and classical descriptions of the bonding in 2,3-C2B4H8 found the classical form to be less stable by 56.5 kcal than the nonclassical (delocalized) model [181]. The chemistry of 2,3-C2B4H8 and its derivatives is, in large part, based on the Brnsted acidity of its B-H-B bridging protons, one of which is easily removed by metal hydrides in ethereal solvents to form the monoanion [170]. Hydride ion does not attack the remaining bridge proton even at 200  C, but it can be removed by alkyllithium reagents to give the carborane dianion (Figure 4-4). Proton removal occurs exclusively at the B2 2H2 2B bridges, as shown by treatment of the monoanion with anhydrous DCl to generate R2C2B4H5(m-D) [170,182]. A kinetic study [183] of the deprotonation of a wide variety of C,C0 -disubstituted derivatives established that the reaction occurs at the metal hydride surface and is pseudo-first order, independent of the amount of metal hydride present; in general, the reaction rate decreases as the steric bulk of R and R0 are increased. However, in the case of 2,3-Ph2C2B4H6, the acidity of the B-H-B protons is greatly enhanced by electron-withdrawal by the phenyl subsitutuents, resulting in very fast deprotonation [183]. The ylide [Ph3PCH2]þ I
deprotonates the parent carborane (but not 2,3-Me2C2B4H6), forming a [Ph3PCH2]þ C2B4H7
salt [184]. The mono- and dianions are highly reactive toward metal ions and other electrophiles, readily forming B2 2M2 2B bridged derivatives in which M can be a transition metal or a main-group element such as Al, Si, Ge, or Sn (Table 4-6), as well as face-bonded complexes in which all five C2B3 ring atoms are coordinated to the metal (Chapters 12 and 13). An early example featuring both types is the reaction of C2B4H7
with CpFe(CO)2I to form yellow, air-stable solid C2B4H7-m(4,5)-Fe(CO)2Cp (4-22), which under UV radiation loses CO to form the ferracarborane sandwiches 4-23, a diamagnetic sublimable orange solid, and brown paramagnetic 4-24 [185,186]. This reaction sequence is unusual, as sandwich complexes such as 4-23 and 4-24 are normally prepared directly from carborane anions as described in Chapter 13. Compounds of this type are analogues of metallocenes in which a cyclopentadieneide (C5H5
or Cp
)

H

R⬘ C B

C B H

H

B H

NaH H

−H2

R H

R⬘ C

C B

B H

2−

B

B

B R

H

−

H

H

B

LiCMe3 H

−H2

R H

H

FIGURE 4-4 Bridge-deprotonation of nido-2,3-RR0 C2B4H6 carboranes (R, R0 ¼ H, hydrocarbon, or silyl).

R⬘ C B

C B H

B

H

54

CHAPTER 4 Small carboranes: Four- to six-vertex clusters

ring is replaced by C2B4H62
or another six-electron donor carborane ligand. Further discussion on the carboranearomatic hydrocarbon analogy can be found in later chapters. H

H

B H

H

C B

H

C

B H

hυ

C B

H

C

B

H

+

H

H

H

C B

C

B

H

B H

FeII

Fe

FeIII

C O C O

4-22

H

B H

H

B

H

H

B

H

H

B

4-24

4-23


Halogenation of the nido-2,3-Me2C2B4H5 anion has been examined in detail. Reaction with ICl with or without an AlCl3 catalyst yields the 3- and 4-Cl derivatives in an approximate 1:3 ratio, while treatment with Br2 affords the 3- and 4-Br products, with the former predominating by 9:1 [187]; B2 2X2 2B halogen-bridged intermediate species are believed to form initially and rearrange to the observed B2 2Xterminal products. A similar sequence is found in reactions of the 2,32M2 2B bridged C2B4H7
ion with halomethyl reagents [188] and with halides of the Group 14 elements, which afford B2 derivatives as shown in Figure 4-5 [71,189,190]. At 80-175  C, the Si- and Ge-bridged derivatives of parent 2,3-C2B4H8 rearrange irreversibly to their B(4)-substituted (and equivalent B(6)-substituted) isomers; at higher temperatures, 2,3C2B4H7
4-SiMe3 is converted to B(1)- and C(2)-SiMe3 derivatives and closo-carborane products [189]. Surprisingly, while 2,3-Me2C2B4H5-m-GeMe3 undergoes thermal isomerization to the 4-GeMe3 isomer, its m-SiMe3 counterpart does not [191]. The corresponding SnMe3-, PbMe3-, AlMe-, and GaMe-bridged derivatives of parent 2,3-C2B4H8 do not isomerize, but all except the m-AlMe derivative react with HCl to regenerate C2B4H8 [190,192]. Transition-metal bridged derivatives are also

R⬘

R

C

B H

B

H

–

H

R⬙CH2X –

H

H

R, R⬘ = Et, CH2Ph R⬙ = H, Me, Ph, p-C6H4Me, (CH2)2Ph X = Cl, Br

C H

C B H

B H –

R

− X−

H

X = Br, Cl

H

R⬘ C B

C B H

H

B

CH2R⬙

H

H

CH2R⬙ H

not isolated

B MR⬙3 = SiH3, SiMe3, GeH3, GeMe3, R, R⬘ = H

B MR⬙3X

R

B H

B

H

R⬘

H

C B

X–

B C B

R⬘

R

H

R

B

B

B C H B

H

H

H

H H

H

C B

C H

R⬘ C B

C H

H

R⬙

R⬙

M R⬙

R, R⬘ = H, Me MR⬙3 = SiH3, SiMe3, SiMe2CH2Cl, GeH3, GeMe3, SnMe3, PbMe3

FIGURE 4-5 Bridge-insertion into nido-2,3-RR0 C2B4H5
anions and rearrangement.

H H

B

MR⬙3

H

H H

MR⬙3 = SnMe3, PbMe3 R, R⬘ = H

B H

B

B

B H

C B

C H

B H

B H

MR⬙3

4.5 6-Vertex open clusters −

H

B Me3Si H

SiMe3 C B

C H

B

C

C B

H

B

H

B SiMe3

RX Me3Si

H

B

H

H

B H

B

H

HCl

H H

H

H

B SiMe3

C B

C

B

B

H

55

R

1) NaH

H

2) R⬘X

H

R⬘

H

SiMe3 C B

C B H

B

H

H

R

R

FIGURE 4-6 Mono- and dialkylation of the 2,3-(Me3Si)2C2B4H5
anion. R ¼ Me, i-C4H9; R0 ¼ Me; X ¼ Cl, Br.

known (an example is 4-22 shown above) [185,186,192–194]; further discussion of main group element- and transition element-bridged carboranes can be found in Chapters 12 and 13. Alkylation of the 2,3-(Me3Si)2C2B4H5
anion with alkyl halides leads not to the B(4/6)-substituted products as shown in Figure 4-5, instead giving exclusively the B(5)-alkyl derivatives (Figure 4-6), a finding that is attributed to the steric bulk of the trimethylsilyl groups on the cage carbon atoms [195]. Further alkylation is achieved as shown, by treatment with HCl to remove an SiMe3 group followed by deprotonation with NaH and reaction with R0 X [195]. The compound 2,3-(Me3Si)2C2B4H5
5-SiMe2CH2Cl is similarly prepared from 2,3-(Me3Si)2C2B4H5
and Me2SiCl (CH2Cl); reaction of this product with 1,2-LiRC2B10H10 gives the mixed-cage linked carborane 4-25 [196]. R H

C

B

B

B

B

H

Me3Si

C

4-25

C

B

B

H

C

B = BH C = CH B

B

B

B

Me3Si

B

Si

H

Me

H

Me

B

B B

Linked-carborane rings and chains such as 4-26 and 4-27 can be prepared via reactions of polyalkynes with B5H9, in C unit in the chain is converted to a nido-C2B4 cage [150]. which each C H

H

B

B

H H

4-26

C

B

B

H H

C

B

C

C

B

H

(CH2)n

B

(CH2)n

Et

H

4-27

H

H

H

B

B H

C

C B

H

H

H

Et

(CH2)5

(CH2)5

B

n = 4,5,6

H

H

H

C

B

H

B H

H

C

C

B

B H

B H

H H

H

B

B H

B H

C B H

H

56

CHAPTER 4 Small carboranes: Four- to six-vertex clusters

While the chemistry of the anions is predominant, reactions involving neutral 2,3-C2B4H8 and its derivatives have been explored to some extent. Photolysis of the vapor generates the lower closo-carboranes 1,5-C2B3H5 and 1,2- and 1,6-C2B4H6 in substantial yields [55]. The parent compound undergoes deuterium exchange with B2D6 at 100  C, with all three basal BH (but not B2 2H2 2B) groups participating [170]; in contrast, D2 exchanges with all terminal and bridging hydrogens [197]. The compound 2,3-(Me3Si)C2B4H7 reduces aldehydes and ketones to the alcohols [198]. On heating at 210  C for 3 days, 2,3-(Me3Si)2C2B4H6 undergoes cage fusion with loss of two SiMe3 groups, to form (Me3Si)2C4B8H10 in good yield [199]. Halogenation in the presence of aluminum halide catalysts occurs with Cl2 or Br2 to afford the 2,3-C2B4H7
4-X product [200], while iodine monochloride and C2B4H8 combine over AlCl3 to give 2,3-C2B4H7
4-I [201]. Similarly, the 4-chloro, 4-bromo, and 4-iodo derivatives are obtained via reactions of 2,3-MeC2B4H7 or 2,3-Me2C2B4H6 with Cl2, Br2, and ICl, respectively, under electrophilic conditions [201]. In contrast to the electrophilic halogenation of the 2,3-Me2C2B4H7
anion discussed above, the neutral carboranes afford only the asymmetric B(4)or equivalent B(6)-halogenated product, reflecting the higher negative charge on the borons closest to carbon and demonstrating that there are significant differences in charge distribution between the neutral carborane and its anion. Very few studied reactions of 2,3-C2B4H8 or its derivatives directly involve the cage carbon atoms. In the parent carborane, this reflects the relative inertness of the C2 2H bonds, which, unlike those in closo-carboranes, have low polarity and essentially no acidic character. The C2 2Si bond in silyl derivatives can be cleaved: 2,3-(Me3Si)2C2B4H6 reacts with NaHF2 or HCl at elevated temperatures to lose an SiMe3 group, forming 2,3-(Me3Si)C2B4H7 and Me3SiX (X ¼ F or Cl) quantitatively [169]. Interestingly, an angle-resolved photoemission study of the adsorption of 2,3-Et2C2B4H6 on Si(111) surfaces at 90 K revealed that the ethyl groups dissociate from the molecule in the process [202]. Thermally induced insertion of boron and other elements into the open face of 2,3-C2B4 clusters to create 7-vertex closo polyhedra has been demonstrated with several reagents, including Et3NBH3 [203] and BMe3 [204,205], which afford derivatives of closo-2,4-C2B5H7 and closo-1,7-C2B6H8; GaMe3 and InMe3, which form 1,2,3-MeGa(C2B4H6) and 1,2,3-MeIn(C2B4H6), respectively [206]; and Fe(CO)5, which produces 1,2,3-(CO)3Fe(C2B4H6) and nido-1,2,3-Fe (C2B3H7) [185], the latter compound having an open C2B3 face. A remarkable 4-boron insertion occurs upon reaction of 2,3-(Me3Si)2C2B4H6 with B5H9 to generate 10-vertex arachno-6,9-(Me3Si)C2B8H13 in 21% yield [207]. Another noteworthy cage-expansion is the NiCl2-catalyzed insertion of 2-butyne into 2,3-Et2C2B4H6 to form 8-vertex nido-Me2Et2C4B4H4 (Chapter 5) [208]. However, insertion into the bridge-deprotonated C2B4H7
and C2B4H62
anions and their substituted derivatives is a more versatile approach to the synthesis of closo-MC2B4 clusters in which M is a main-group or transition element (Chapters 12 and 13). A wholly unexpected reaction of bis(2,3-dicarbahexaboranyl) metal complexes was discovered in 1974 [209,210] in studies of HnM(2,3-R2C2B4H4)2 sandwiches, in which M is a transition metal (usually Fe or Co) and R is a hydrocarbon group. As described in Chapter 11, such complexes undergo face-to face oxidative fusion of two nido-C2B4 dianionic ligands to form neutral 12-vertex C4B8 carboranes. Remarkably, this reaction occurs under very mild conditions with a wide variety of R groups, to give typically quantitative yields of fused products [211]. Subsequent investigation [212] has shown that nido-1,2,3-(ligand)M(R2C2B3H4)
metallacarboranes exhibit similar metal-promoted oxidative fusion to give 12-vertex M2C4B6 cages. The fused carboranes and metallacarborane products, in turn, have given rise to a subfield centered on large four-carbon carboranes, previously unknown. This chemistry and recent developments are discussed further in Chapters 11 and 13.

4.5.4 Nido-2,4-C2B4H8 4.5.4.1 Synthesis Nido-2,4-dicarbahexaborane(8), with nonadjacent carbons in the cage framework, has not been isolated as a neutral parent species but is known in the form of the 2,4-C2B4H7
ion and as a substituted derivative (Table 4-7). Most known preparative routes to nido-2,4-C2B4 clusters are based on rearrangement or degradation of other carboranes as in the cage-opening of closo-1,6-C2B4H6 with fluoride that affords the nido-2,4-C2B4H6-5-F
ion [213] and reaction of closo-1,2-(Me3Si)2C2B4H4 with alkali metals to generate the carborane dianion [214] (Figure 4-7). Closo-to-nido

4.5 6-Vertex open clusters

57

TABLE 4-7 Nido-2,4-C2B4H8 Derivatives Synthesis and Characterization Compound

Information

References

S, B, H S (degradation of C2B5H7) S, B, H, F S, B, H S, B S, B, H S, X, B, H, C, IR S, X, B, H, C, Li, IR, MS S, X, IR S, X, H, B, IR, MS S, H, B, C, MS S S, H, B, C S, H, B, C S, H, B, C X, IR

[182] [215] [213] [120] [120] [182] [219] [214] [220] [224] [17] [217,218] [218] [218] [218] [221]

S, H, B, C, Li S, X

[222] [222]

Detailed NMR Studies 2,4-C2B4H7
, 2,4-C2B4H6-3-NMe3

C

[96]

Theoretical Studies Molecular and electronic structure calculations 2,4-C2B4H62
2,4-R2C2B4R42
(R ¼ H, Me) 2,4-R2C2B4R4H
(R ¼ H, Me) 2,4-R2C2B4R4H2 (R ¼ H, Me)

ab initio DFT DFT DFT

[35] [218] [218] [218]

NMR calculations 2,4-C2B4H7
, 2,4-C2B4H6
3-NMe3 2,4-R2C2B4R4Hnn
2 (R ¼ H, Me) m(5,6)
R2NBH-2,4-C2B4H6
(R ¼ H, Me) 2,4-R2C2B4R4H
(R ¼ H, Me) [1,2,4-(THF)2M]þ [2,3-(SiMe3)2C2B4H5
] (M ¼ Na, Li)

C (IGLO) B (DFT) B (IGLO) B (GIAO) GIAO

[96] [218] [216] [18] [222]




2,4-C2B4H7 2,4-C2B4H7
, m-R2NBH-2,4-C2B4H6
(R ¼ Me, Et) 2,4-C2B4H6
5-F
2,4-C2B4H6
-n-PMe3þ (n ¼ 3, 5) 2,4-C2B4H6
-3-NMe3þ (n ¼ 3, 5) 2,4-C2B4H6-n-NMe3 (n ¼ 3, 5) [2,4-(SiMe3)2C2B4H4
]2(Naþ)2(THF)4 2,4-(SiMe3)2C2B4H4 2
LiLþ (L ¼ THF, TMEDA) 2,4-(SiMe3)2-m-5,6-H2-Ln[(SiMe3)2C2B4H4]2 (Ln ¼ Er, Dy) (classical open) H2C2B4[NH(CHMe2)2]4 2,4-Et2C2B4Et4H2 (reported as nido-2-EtCB4Et4H-m-CHEt)

2,4-Et2C2B4Et4H
2,4-Et2C2B4Et42
Naþ3[(2,4-(SiMe3)2C2B4H4]2Ln[m-5,6-nido-2,4-(SiMe3)2C2B4H4] Ln ¼ Dy, Er 1,2,4-(THF)2Mþ [2,3-(SiMe3)2C2B4H5
] (M ¼ Na, Li) 1,2,4-(THF)(TMEDA)Naþ [(SiMe3)2C2B4H5
]

3


S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; F, 19F NMR; Li, 7Li NMR; IR, infrared data; MS, mass spectroscopic data.

cage-opening can also be achieved with other Lewis bases such as amines and phosphines [120,182]; for example, reaction of 1,6-C2B4H6 with triethylamine gives nido-2,4-C2B4H6þ-5-NEt3
,which can be converted to Naþ 2,4-C2B4H7
via reaction with NaH [182]. Both 2,4-C2B4H6þ-5-NEt3
and 2,4-C2B4H6þ-5-PEt3
undergo thermal rearrangement to the 3-amino or 3-phosphino substituted isomer, the reaction in the latter case accompanied by extensive decomposition [120,182].

58

CHAPTER 4 Small carboranes: Four- to six-vertex clusters H

C B H

−

H

H

H

B

B

F−

B

B

Na+

H H

H

H

B

C B

C

H

C

H

B F

H H

B H

Me3Si

2−

B

2 LiL+ H

Me3Si

B

C H B

L

C

C

Li0, naphthalene

H

B

B

H

SiMe3 L = THF or TMEDA

B

C

B

SiMe3

H

H

FIGURE 4-7 Conversions of closo-1,6-C2B4H6 to nido-2,4-C2B4H6-5-F
and of closo-1,2-(Me3Si)2C2B4H4 to nido-2, 4-(Me3Si)2C2B4H42
.

Nido-2,4-C2B4H7
can also be obtained from the 7-vertex closo-carborane 2,4-C2B5H7, via treatment with LiNR2 reagents (R ¼ Me, Et, CHMe2) in acetonitrile at room temperature [215]. The conversion is nearly quantitative and proceeds through a B(5,6)-bridged nido-2,4-C2B4H6-m-BHNR2 intermediate 4-28, as determined by IGLO-NMR studies [216]. H

B H

4-28

H

H

B C

C

H

B

B H

B NR2

H

The hexaethyl derivative 2,4-Et2C2B4Et4H2 (4-29), in which all of the terminal hydrogens are replaced by ethyl groups, can be obtained by thermolysis of the carbon-bridged arachno-CB4 carborane 4-12 described earlier (Figure 4-8) Et C

Et B Et Et

Et

C

B

Et B H

H B

B H

Et

> 100 °C

Et

− Et2BH

Et

Et B C

H

C B Et

Et2B

4-12

4-29

FIGURE 4-8 Conversion of arachno-1-EtCB4Et4H3-m(2,3)-CEt(BEt2) (4-12) to 2,4-Et2C2B4Et4H2 (4-29).

B H

Et

4.5 6-Vertex open clusters

59

[17,27,217]. The structure of 4-29 was originally proposed [17] as a novel carbon-bridged nido-CB4 cluster, but was later identified by GIAO-NMR studies as a nido-2,4-C2B4 carborane [18]. This route avoids the problem of B5H9 availability mentioned earlier, and the protective sheath of relatively unreactive alkyl groups lends stability to the molecule; consequently, the chemistry of this derivative is effectively limited to bridge-deprotonation and metal sandwich formation [218].

4.5.4.2 Structure and properties The pentagonal-pyramidal 2,4-C2B4 geometry with non-vicinal cage carbon atoms is supported by X-ray diffraction data on several anionic derivatives [214,219–222] and metal sandwich complexes (Chapters 12 and 13); a detailed analysis of the 13C NMR spectra of 2,4-C2B4H7
and 2,4-C2B4H6
3-NMe3 [96]; and calculations of electronic and molecular structure and NMR chemical shifts (Table 4-7). As is often the case with open-cage carboranes, separation of the skeletal carbons does not necessarily lead to greater stability as it typically does in closo systems (Section 2.7): the 2,4C2B4H8 isomer is calculated [18] to be 4.6 kcal mol
1 higher in energy than the 2,3 isomer. However, for the C2B4H7
anions, this order is reversed, the 2,4 isomer being more stable by 15.4 kcal mol
1. These findings underline the importance of hydrogen placement, as the relative instability of neutral 2,4-C2B4H8 can be attributed to its having only one B2 2B edge on the open face that can accommodate a B2 2H2 2B bridge, forcing one of the carbon atoms to adopt a CH2 group mode; in the anion, with only one “extra” hydrogen, this problem disappears and the tendency of carbons to separate from each other reasserts itself. As with 2,3-C2B4H8 and its derivatives, one or both bridging protons in nido-2,4-C2B4 carboranes can be removed by metal hydrides, amines, or other nucleophiles [182,214,215,218]. Treatment of [Na(TMEDA)þ]2[2,4-(Me3Si)2C2B4H42
] with PbCl2 in cold benzene forms the closo-plumbacarborane 1,2,4-PbII[(Me3Si)2C2B4H4] [223], while complexation of the 2,4-Et2C2B4Et4H
anion [218] with Fe2þ affords the sandwich H2Fe(2,4-Et2C2B4Et4)2 [217]. The air-stable iron compound, unlike its “carbons-adjacent” analogue H2Fe(2,3-Et2C2B4H4)2 mentioned earlier, shows no tendency to undergo oxidative ligand fusion to form C4B8 carboranes. Most of the known chemistry of nido-2,4-C2B4 clusters centers on the synthesis of metal sandwich complexes, as described in Chapters 12 and 13.

4.5.5 Arachno- and hypho-C2B4Hx clusters Like some molecules described earlier in this chapter, several open-cage 6-vertex carbon-boron species have “borderline” structures that can be described as organoboranes and/or as open-cage carboranes (Table 4-8). An example of strongly electrondonating groups exerting major influence on cage structure is found in the species H2C2B4[N(CHMe2)2]4 (4-30), which

TABLE 4-8 Arachno- and Hypho-C2B4Hx Derivatives Synthesis and Characterization Compound

Information

References

hypho-C2B4H12

B, H B, H, MS ED IR, vapor pressure S, X, H, B, IR, MS

[227] [228] [229] [226] [224]

B2 2C bond energies B2 2C bond energies, ab initio, IGLO 1H, 11B,

[230] [230] [229]

(classical open) H2C2B4[NH(ipr)2]4 Theoretical Studies Molecular and electronic structure calculations arachno-C2B4H10 hypho-C2B4H12 1

H, H NMR; B,

11

13

C NMR

B NMR; IR, infrared data; MS, mass spectroscopic data; ED, electron diffraction.

60

CHAPTER 4 Small carboranes: Four- to six-vertex clusters

was prepared in 73% yield via dehalogenation of a tetrakis(dichloroboryl)ethane derivative, H2C2{B[N(CHMe2)]Cl}4, by Na/K in refluxing benzene [224]. Rather than a nido-carborane structure, X-ray diffraction data revealed this molecule to have, at least in the solid state, the classical geometry shown, described as a 1,3-diboretane having a 2,4˚ , indicating a very 2B distance in the diboretane ring is quite long at 1.93 A bridging B2[N(CHMe2)2]2 group. The B2 weak boron-boron interaction [224].

[CHMe2]2N H

N[CHMe2]2

H

B

B H

H

B

C

B H

B

H

H

B

C

B

C

H

N[CHMe2]2

C

H

H

B

H H

4-30

C

H

C B

H

N[CHMe2]2

4-31

B B

B

4-32

The skeletal framework in 4-30 has been calculated [82,225] as a stable intermediate in the rearrangement of closo-1,2C2B4H6 to the 1,6 isomer, as described below. The 2,4-dimethylenetetraborane 4-31, obtained via reaction of B4H10 [tetraborane(10)] with ethylene [226], has been structurally characterized by 11B, 13C, and 1H NMR [226–229] and gas-phase electron diffraction [229] and can be viewed as a 6-vertex hypho-C2B4H12 carborane cluster. Its 20 skeletal electrons (six and eight from the two CH and four BH units, respectively, plus six electrons from the six “extra” hydrogens) constitute 2n þ 8 electrons or n þ 4 pairs, corresponding to a closo 9-vertex polyhedron with three missing vertexes, according to the electron-counting rules laid out in Chapter 2. With a certain amount of imagination, the geometry of 4-31 fits this description as suggested in 4-32, keeping in mind that this is considered a borderline case. A proposed arachno-C2B4H10 molecule has been explored computationally [230] but has not yet been reported as an isolated species.

4.5.6 Nido-2,3,4-C3B3H7 4.5.6.1 Synthesis The first nido-tricarbahexaborane, indeed the first three-carbon carborane of any type, to be isolated and characterized was 2,3,4-MeC3B3H6, obtained in 1966 as one of several carboranes formed in the gas-phase reaction of B4H10 with acetylene at 25-70  C (whose major product when the reaction is quenched is the previously described nido-1,2C2B3H7) [51,127,231]. Two other products of this reaction, originally thought to be 2,3,4-Me2C3B3H5 isomers, were later identified as the nido-2,3-dicarbahexaboranes 2,3-MeC2B4H6
4-Me and 2,3-C2B4H7
5-Et [29]. Alkyl derivatives of nido-2,3,4-tricarbahexaborane (Table 4-9) have been obtained from the reaction of 2-methylbut-1-ene-3-yne, CH, with B4H10 at 70  C, which also affords parent 2,3,4-C3B3H7 [232]. 5CMeC H2C5 A different route to 2,3,4-tricarbahexaboranes utilizes the hydroboration of diethyl(1-propynyl)borane, Et2BC-Me, with tetraethyldiborane, Et4B2H2, catalyzed by trialkyltin chorides, to form the carbon-bridged derivatives C 4-33 and 4-34 [233]. Also obtained in this reaction are alkyl derivatives of 2,3,5-C3B3H7, as discussed below. Et B

4-33 R = Me 4-34 R = Et

H

Et

R C B

C

C

B Et

Et

Me

4.5 6-Vertex open clusters

61

TABLE 4-9 Nido-C3B3H7 Derivatives Synthesis and Characterization Compound

Information

References

2,3,4-C3B3H7 2,3,4-MeC3B3H6

S, S, S, S, S, S, S, S, S, S,

H, B, MS B, MS IR, MS* H H, B, C H, B, C H, B H, B IR, MS H, B, MS

[232] [231] [51] [127] [29] [29] [234] [234] [379] [232]

S, S, S, S, S, S, S, S, S, S,

H, B, C H, B H, B, C H, B, C, MS H, B, C, MS H, B, C B(Et), C(Et), H(Et) B H, B, C H, B, C

[233] [236] [236] [236] [236] [240] [240] [233] [241] [241]

2,3,4-C3B3H6
1-Me 2,3,4-MeC3B3H3
2,3,4-MeC3B3H3-m(5,6)-D 2,3,4-MeC3B3H3D3, 2-2,3,4-MeC3B3H2D4 2,3,4-RC3B3H5-n-R0 (n ¼ 1, 5; R ¼ H, Me; R0 ¼ H, CH2Me) 2,3,4-HMeEtC3B3Et3-m(5,6)-CH2Me (2 isomers) 2,3,5-Et2(CHMe2)C3B3EtMe2H 2,3,5-Et2(CHMe2)C3B3Et3H 2,3,5-Me(CHMe2)2C3B3EtMe2H 2,3,5-Me(CHMe2)2C3B3Et3H 2,3,5-Et2RC3B3Et3H (R ¼ Me, Et) 2,3,5-Et2RC3B3Et3 (R ¼ Me, Et) 2,3,5-HMeEtC3B3Et3H 2,3,5-(spiro-H2CBEt)H2C3B3Et3 2,3,5-(spiro-H2CBEtR)H2C3B3Et3 (R ¼ NC5H4Me, Bu) 2,3,5-(CMe3)Et2C3B3EtMe2 Other Experimental Studies 2,3,4-MeC3B3H5 Theoretical Studies Molecular and electronic structure calculations C3B3H7 C3B3H7 2,3,4-C3B3H7 C3B3H6
isomers C3B3H6þ isomers

NMR calculations 2,3,4-C3B3H7 2,3,5-R3C3B3R3H (R ¼ H, Me) 2,3,4-HMeEtC3B3Et3-m-5,6)-CH2Me (2 isomers)

S, H, B, MS

[237]

kinetics of formation from B4H10þ C2H2

[30]

MNDO extended Hu¨ckel SCF DFT: ionization potential, valence structure ab initio; energies of isomers Hartree-Fock and B3LYP; isomer stabilities

[323] [34] [347] [346] [35] [121]

GIAO/IGLO GIAO/IGLO GIAO/IGLO

11 11

B shifts B shifts

[12] [18] [233] Continued

62

CHAPTER 4 Small carboranes: Four- to six-vertex clusters

TABLE 4-9 Nido-C3B3H7 Derivatives—Cont’d Synthesis and Characterization Compound

Information

References

2,3,5-(spiro-H2CBEt)H2C3B3Et3 2,3,5-(spiro-H2CBEtR)H2C3B3Et3 (R ¼ NC5H4Me, Bu)

GIAO/IGLO GIAO/IGLO

[241] [241]

Reactivity calculations 2,3,4-MeC3B3H6

mechanism of formation from B4H8 þ C2H2

[32]

S, synthesis; X, X-ray diffraction; H, 1H NMR; B, spectroscopic data; UV, UV-visible data.

11

B NMR; C,

13

C NMR; 2d, two-dimensional (COSY) NMR; IR, infrared data; MS, mass

4.5.6.2 Structure and properties Although no X-ray diffraction or electron diffraction studies of this carborane or any derivative have been reported, the pentagonal pyramidal cage structure with three contiguous carbon atoms in the base (Figure 1-3, second row left) is established with reasonable certainty from multinuclear NMR, infrared, and theoretical investigations (Table 4-9). Nido-2,3,4-C3B3H7 and its 2,3,5 isomer, as discussed below, are members of the family of isoelectronic CnB6
nH10
n clusters shown in Figure 4-3. The lone B-H-B bridging proton in 2,3,4-MeC3B3H6 can be removed by reaction with NaH in THF or diglyme to form the 2,3,4-MeC3B3H5
anion, which on treatment with DCl generates the bridge-deuterated species 2,3,4-MeC3B3H5-m (5,6)-D [234]. The sodium salt of the anion reacts with BrMn(CO)3 without loss of CO to afford NaBr and a red complex presumed to be 2,3,4-MeC3B3H5-m(5,6)-Mn(CO)3, which at 100  C loses CO to form the manganacarborane sandwich complex 4-35, a thermally stable yellow liquid [234]. This compound, the first reported metallacarborane having more than two skeletal carbon atoms, can also be prepared by reaction of neutral 2,3,4-MeC3B3H6 with Mn2(CO)10 [234,235]. H

B H H

H

C B

4-35

C

Mn C O

C

Me

B

C O

H

C O

4.5.7 Nido-2,3,5-C3B3H7 4.5.7.1 Synthesis Parent nido-2,3,5-tricarbahexaborane, in which one cage carbon atom is separated from the other two, has not been prepared, but alkyl derivatives 4-38 have been obtained by hydroboration of 1,3-dihydro-1,3-diborafulvalenes (4-36) or 4,5-diisopropylidine-1,3-diborolanes (4-37) followed by loss of BEt3 (Figure 4-9) [236–239]. CMe) or bis(diethylboryl)alkenes [R(Et2B)C5 5CEt(BEt2)] can be treated Alternatively, 1-propynyldiethylborane (Et2BC with excess tetraethyldiborane(6) to generate the hexaalkyl compounds nido-2,3,5-Et2RC3B3Et3 (R ¼ Me or Et), accompanied by the 2,3,4-C3B3 derivatives 4-33 and 4-34 discussed above [233,240]. Both the 2,3,4 and 2,3,5 isomers are proposed to form via EtBH2 loss from an unisolated C2-bridged arachno-carborane common intermediate, 4-39 [233]. This process appears

4.5 6-Vertex open clusters

63

R Et

B

Et

R⬘

(HBEt2)2

R⬙

⎯ BEt3 R⬘

B R

B

4-36

Et R

Me

Et

C B

C C

R

B

R

H

R⬙ B

Me

R, R⬘ = Me, Et R⬙ = H, CHMe2, CMe3

⎯ BEt3

H

Me

4-38

(HBEt2)2

Me B R

Me

4-37

FIGURE 4-9 Synthesis of nido-2,3,5-tricarbahexaboranes.

similar to the conversion of arachno-1-EtCB4Et4H3-m(2,3)-CEt(BEt2) (4-12) to nido-2,4-Et2C2B4Et4H2 (4-29) (Figure 4-8), the difference being that 4-12, with only one bridging carbon, gives rise to dicarbon carborane products, while 4-39, with a dicarbon bridge, generates tricarbaboranes. Et C

Et B

4-39

H

H

C

Et

B B

H B

Et

H

C Et

Me

A structurally unique 2,3,5-tricarbahexaborane spiro derivative, 4-40 (Figure 4-10), was isolated from the hydroboration of bis(diethylboryl)acetylene with tetraethyldiborane, along with several other remarkable products including C4B4 − Li+

Et

Et B

B H H

Et C C

Et

B B Et

4-42

C

n-C4H7Li

CH2

H H

B Et

C C

B

n-C4H7

C

C

B

B Et

H

Et

H

NC6H4Me

H C H C

Et B B B

Et Et

N

4-41 Me

FIGURE 4-10 Reactions of the spiro-carborane 4-40 with g-picoline and n-butyllithium.

H H

B

4-40

C

C

Et

64

CHAPTER 4 Small carboranes: Four- to six-vertex clusters

and C6B6 carboranes (Chapters 5 and 11, respectively) [241]. The structure assigned to 4-40 is based on experimental NMR data and ab initio/GIAO-IGLO calculations, and is further supported by its reaction chemistry. The compound is extremely O2-sensitive, but treatment with g-picoline results in simple base-substitution on the exo-polyhedral boron to give 4-41 (Figure 4-10), while n-butyllithium opens the ring as shown, to afford the salt 4-42 [241].

4.5.7.2 Structure and properties X-ray studies of 2,3,5-tricarbahexaboranes are not available, but the nido-C3B3 cluster geometry is confirmed by multinuclear NMR spectroscopy and high-level computation (Table 4-9), supported indirectly by X-ray diffraction data on metal sandwich compounds bearing nido-C3B3 ligands [242–245]. In the uncomplexed neutral carboranes (e.g., 4-38), the salient feature is an unusual C2 2HB bridging proton, a consequence of the fact that there is no available B2 2B edge on the open face to accommodate a B2 2H2 2B group. The known chemistry of 2,3,5-tricarbahexaboranes is largely based on removal of this very acidic proton with nucleophiles to generate the carborane monoanion [236,240]. Reactions of both neutral and anionic 2,3,5-tricarbahexaboranes with metal reagents form MC3B3 closometallacarborane clusters and sandwich complexes (Chapter 13), as illustrated by the synthesis of red 4-43 (Figure 4-11) and the related dark yellow cobalt complex (CO)2Co[(Me3C)Et2C3B3Et3), prepared from the neutral carborane and Co2(CO)8 [236,237]. Et B B Et Et

Ni(C3H5)2

Me C C

B B

C

− C3H6 CMe3

H

Et Et

B

R C C

B

C

B Ni

−

Et

Et

R

[(C3H5)NiBr]2 CMe3

4-43

Me

− Br−

Et Et

Et C C

B B

C

CMe3

Et

R = Me, Et

FIGURE 4-11 Formation of 1,2,3,5-NiC3B3 metallacarborane clusters.

4.5.8 Nido-2,3,4,5-C4B2H6 4.5.8.1 Synthesis Nido-2,3,4,5-tetracarbahexaborane(6) (Figure 1-3, second row right), one of the most carbon-rich carborane systems known, is intimately related to classical organoboranes and, indeed, is often obtained as a product of organoborane rearrangements. Peralkyl derivatives of C4B2H6 were obtained by Binger as early as 1966 from cis-bis(dialkylboryl)-1,2dialkylethylenes [(R2B)2C ¼ CR0 2, R, R0 ¼ Me, Et, n-C3H7] by pyrolysis at 150-180  C [246,247], or alternatively, by addition of a catalytic quantity of Et2BCl at 40  C [248]. Parent C4B2H6 was first synthesized by Onak and Wong in 1970 by passing 1,2-tetramethylenediborane [m(1,2)-(CH2)4B2H2] through a flow system at 550  C [123,249]; it was also obtained by Miller and Grimes via insertion of acetylene into nido-1,2-C2B3H7, as discussed earlier in this chapter (see Figure 4-2) [38]. Alkyne-B4H10 flash reactions generate both parent and alkylated C4B2 carboranes [53], CH with B4H10 at 70  C affords HMe3C4B2H2, along with C3B3 carboranes, as 5CMeC while the reaction of H2C5 mentioned earlier [232]. Substituted C4B2 carborane derivatives (Table 4-10) have been prepared from organic or organoborane precursors via a remarkable variety of synthetic approaches, as illustrated in Figures 4-12 and 4-13. Clearly, an important driving force in these reactions is the thermodynamic stability of the nido-C4B2 cluster framework—a consequence of the delocalized framework bonding—which tends to remain intact once formed. However, the substituents bound to boron also

4.5 6-Vertex open clusters

65

TABLE 4-10 Nido-2,3,4,5-C4B2H6 Derivatives Synthesis and Characterization Compound

Information

References

Parent

S, H, B, IR, MS S Microwave structure C ED S(gas phase flash photolysis), H, B S, MS S, H, B, C, MS S, H, B, IR S, H, B, MS C S, H, B, MS S, H, B S, H, B S, H, B S, B, IR, MS S, H, B S, H, B, MS S, X(Et), H, B, C, MS B, C S, H, B S, H, B, C, MS S, H, B, C, MS S, X (C6Me3H3),H, C S, H, B, MS S, H, B, C S, H, B, C, MS S, X, H, B, C, MS S, H, B, C, MS S, H, B, C, MS S, H, B, C, MS Reactions with BX3 and LiEt3BH S, H, B, C S, H, B, C, Sn, MS S, H, B, C, Sn, MS S, H, B, C, Sn, MS S, B, C, P, Se S, B, C, P, IR* S, X(Me), H, B, IR, MS S, H, IR*, Raman* S, H, IR*, Raman*

[38,123,249] [28] [260] [96] [261] [53] [38] [254] [38] [232] [96] [38] [255] [255] [255] [248] [256] [262] [313] [380] [258] [259] [259] [315] [237] [257] [250] [269] [269] [259] [259] [262] [262] [263] [263] [263] [265] [265] [267] [246] [246]

Me6 Parent þ Me and Me2 derivatives 13 12 C2 C2B2H6 1-Me-6-N(CHMe2)2 2/3-Me 3-Me 6-Me 2,3/3,4-R2 (R ¼ D, Me) 3,4-Me2C4B2H2Me2 3,4-Et2C4B2H2-1-Me-6-Et 3,4-Et2C4B2H2-1-Et-6-Me Me2Et2C4B2Et2 isomers Et2H2C4B2Ph2 Et2H2C4B2EtBr R4C4B2I2 (R ¼ Et, Me, Ph) R4C4B2R0 2 (R, R0 ¼ alkyl) 5CH2)2C4B2R2 (R ¼ Me, Cl) (Me2HC)2(MeC5 REt2RC4B2X2 (X ¼ Br, I; R ¼ Et, CH2Et, CHMe2, Bu) (CH2Et)Et2(CH2Et)C4B2MeR (R ¼ Et, Me) 2,3-C4H4-4-SiMe3-C4B2R2 (R ¼ CMe3, 2,4,6-C6Me3H3) H(CMe3)Et2C4B2Me2 (2 isomers) (i-pr) 2Me2C4B2RR0 (R, R0 ¼ Me, CHMe2, Ph) 3-(i-C3H5)-4-(Me2CH)-C4B2(Me2CH)2 new synthesis Cp*Co(H4C4)[(SiMe3)HC4B2(CMe3)2] L(H4C4)[(SiMe3)HC4B2(CMe3)2] L ¼ CpRh, (CO)3Fe BuEt2BuC4B2ClX (X ¼ Cl, Et) REt2RC4B2XH (X ¼ Br, H; R ¼ Et, Bu) MeEtRR0 C4B2-1-R00 -6-Et (R, R, R00 ¼ Me, Et) MeEtRR0 C4B2-1-R00 -6-X (R, R, R00 ¼ Me, Et; X ¼ Br, I, H) Me2Et2C4B2-1-Et-6-SnR3 isomers (R ¼ Me, Ph) Me2Et2C4B2-1-Et-6-SnPh2X isomers (X ¼ Cl, Br) Me2Et2C4B2-1-Et-6-SnX3 isomers (X ¼ Cl, Br) Me4C4B2-1-Me-6-L (L ¼ P(O)Ph2, P(S)Ph2, P(Se)Ph2) Me2Et2C4B2-1-Et-6-L (L ¼ Ir, P(BH3)Ph2, P[W(CO)5]Ph2) Me4C4B2R-6-NSFe2(CO)6 (R ¼ Me, Br) Me4C4B2Et2 Me4C4B2-1-Me-6-Et

Continued

66

CHAPTER 4 Small carboranes: Four- to six-vertex clusters

TABLE 4-10 Nido-2,3,4,5-C4B2H6 Derivatives—Cont’d Synthesis and Characterization Compound

Information

References

Me6

S, H, IR*, Raman* S, MS S, B*, IR*, MS* S, X(Ph, tolyl), H, B, C, MS S, H, B, C, P, MS S, X, H, B, C, IR, MS S, H, B, C, MS S, H, B, C, MS S S, H, B, C, Sn, MS S S, X, H, B, C, Si, MS S, X, H, B, C, MS S, X, H, B, C, MS S, H, B, C, MS S, H, B, C, MS CPh), H, B, C, MS S, X(C S, H, B, C, MS S, H, B, C, MS S, H, B, C, MS halogen exchange S, H, B, C, MS S, H, B, C, MS S, X(Ph), H, B, MS

[246] [314] [248] [266] [266] [266] [266] [266] [266] [266] [266] [266] [266] [266] [266] [266] [264] [264] [264] [264] [264] [270] [270] [270]

S, B, MS

[270]

S, X, H, B, C, MS

[270]

S, H, F, IR, MS S, H, F, IR, MS S, H

[253] [268] [251]

Structure, dipole moment, ionization potential SCF Extended Hu¨ckel DFT: ionization potential, valence structure Classical structures

[333] [347] [34] [346] [82]

ab initio; isomer energies

[35]

Et6 CR (R ¼ Ph, CMe3, SiMe3, p-tolyl) Et4C4B2-1-I-6-C Et4C4B2-1-I-6-PPh2 Et4C4B2-1-I-6-FeCp(CO)2 Et4C4B2H-1-I Et4C4B2-1-I-6-Et Et4C4B2-1-Ph-6-Et Et4C4B2-1-I-6-SnMe3 Et4C4B2-1,6-(SnMe3)2 CSiMe3-6-C CPh Et4C4B2-1-C Et4C4B2-1-I-6-C2PhCo2(CO)6 Et4C4B2-1-I-6-[1-(1,2-C2B10H11)] (Et4C4B2-1-I-6-C6H4-)2 (Et4C4B2-1-I-6-)2O CPh (R¼Br, C CPh) Et4C4B2-1-R-6-C CCMe3 Et4C4B2-1-Br-6-C Et4C4B2-1-I-6-F C-p-CH2C6H4Me]2 1,10 -[Et4C4B2-6-C CPh Et4C4B2-1-I-6-C CR (R ¼ Me, SPh, PPh2) Et4C4B2-1-I-6-C CR-6-R (R ¼ n-C4H9, PPh2) Et4C4B2-1-C CpCo[cyclo-C4R2(Et4C4B2-1-I-6-)2] (R ¼ Ph, SPh) cyclobutadiene complexes CSiMe3-6-)2] CpCo[cyclo-C4R2(Et4C4B2-1-C cyclobutadiene complex CpCo[cyclo-C(O)C4Ph2(Et4C4B2-6-n-C4H9-1-)2] Other C4B2 systems “nido”,arac-R4C4B2F2 (planar) (R ¼ H, Me) “nido”,arac-Me4C4B2F2 (planar) [arac-1,4-H4C4B2(C5H4FeCp)2]2
Theoretical Studies Molecular and electronic structure calculations Parent

Isomerization calculations Parent

Continued

4.5 6-Vertex open clusters

67

TABLE 4-10 Nido-2,3,4,5-C4B2H6 Derivatives—Cont’d Synthesis and Characterization Compound

NMR calculations Parent, 6-Me Parent cyclic planar 1,4-H4C4B2F2

Information

References

Isomer stability; formation from dimerization of C2BH3 [borirene])

[2]

IGLO 13C shifts DFT (H, B, C spin-spin coupling) IGLO 11B shifts IGLO/GIAO, B, F

[96] [37] [12] [294]

S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; F, 19F NMR; P, 31P NMR; Li, 7Li NMR; Pt, dimensional (COSY) NMR; IR, infrared data; MS, mass spectroscopic data; ED, electron diffraction.

195

Me

I B

B Me Me

Me B

C C

Pt NMR; 2d, two-

C8K, MeBBr2

Me

C

C

R

C

BI3, NaK2.8

R

− KI

R

R

C

I B

C C

C

R

C

R = Me, Et, Ph R

Me

A

Me3C CMe3

Me2C Me

Li C

C

C

H

B

C

(Me3C)2B2Cl2

C

B

CMe2

Me3C

B

B

Cl

B

SiMe3

C

C C

Ph

H

CHMe2

SiMe3

B

B Me

C H

C

Me

H

D

B

R = CMe3, 2,4,6-C6Me3H3

CMe3 Et

C

C

R

Mg

C

Me Et

B

B

R

C

H2C=C Me

CMe3 C C

R

R Cl

H

C H

H C

Me

B 100 °C

B Et Et

Me C C

B

C H

C CMe3

FIGURE 4-12 Routes to nido-2,3,4,5-C4B2H6 derivatives from A alkynes [313,314], B allenes [250], C diboraalkenes [315], and D diborapentafulvene [237].

68

CHAPTER 4 Small carboranes: Four- to six-vertex clusters R⬘ Me

Me

R⬘

H

B

C C

R

B Me

Me

A

Δ

Br

Br B

B

R

Me

N(CHMe2)2

N(CHMe2)2

B

Me

B MeBBr2

H

−78 °C

H

C C

N(CHMe2)2

B

C

H

C

B

H

CHMe2

Me C R

C

R B

C Me

CHMe2

B

B C

Me2HC

C CHMe2

Me

R C

B

CHMe2

C Me

R B

B C

C

C

C

hν Me2HC

B

D

C C

R = Me, Ph, CHMe2

R

C C

B

R C

C

CHMe2 Me

R = Me, Cl

B

R R⬘ R⬘

C3H7

MeBBr2 BEt2

Et

R = Me, Et

Et

R

C C

B

Et2B

Et

R⬘ R⬘

R = Me, Et; R⬘ = Et, n-C3H7, CHMe2, n-C4H9

C3H7

Et

B

BX3 X = Cl, Br, I

C

C X

Sn

E

MeBBr2

H

Sn

2− 2 Li

C

C

R, R⬘ = Me, Et

Me

+

BEt2

Et

B

R⬘ Et

C C

B

X C

R⬘

C Et

FIGURE 4-13 Routes to nido-2,3,4,5-C4B2H6 derivatives from heterocyclic compounds.

play a major role [82,248,250–252]. For example, in contrast to nido-2,3,4,5-R4C4B2R2 carboranes (4-44, R ¼ H or alkyl), the isolated form of R4C4B2F2 (prepared from reactions of BF with alkynes)[253] is a planar heterocycle (4-45), stabilized by attachment of electron-rich fluorines to boron [82]. Similar effects on the C4B2 cluster structure have been observed with other nucleophiles such as amino and ferrocenyl [250–252]. The influence can be subtle: addition of an amino group to one boron in nido-2,3,4,5-H4C4B2H2 stabilizes the carborane, but placing amines on both borons favors a classical structure via electron donation to the cage—a very rare example of an apparent electronically driven nonclassical-to-classical transformation [254].

4.5 6-Vertex open clusters

69

F R R

B R R

R B

C C

C

R C

C

C

R = H, Me

R

C

4-44

B C

R

R

B

4-45

F

R

Figure 4-12 illustrates several synthetic pathways to nido-C4B2 carboranes based on unsaturated hydrocarbons. The bicyclic intermediate in the reaction in Figure 4-12B has been isolated and crystallographically characterized [250], affording remarkable insights into an organoborane ! carborane molecular rearrangement. Other synthetic pathways to nido-C4B2 clusters, involving heterocyclic precursors, are shown in Figures 4-13 A [255,256], B [254], C [257], D [258], and E [259]. The contrast between the various preparative approaches to this carborane system and those used for other small carboranes (most of which are based on boron hydrides) is quite striking, and makes C4B2 clusters somewhat more accessible to organic chemists than are the other lower carboranes.

4.5.8.2 Structure and properties The pentagonal-pyramidal geometry of the nido-2,3,4,5-C4B2 cage is supported by a microwave analysis of parent C4B2H6 [260], a gas-phase electron-diffraction study of Me4C4B2Me2 [261], and X-ray crystallographic and multinuclear NMR investigations of several derivatives (Table 4-10), in all of which boron occupies the high-coordinate apex position. Isomers with carbon in the apex, though possible, would be energetically unfavorable (see Chapter 2) and have not been observed. Most of the known chemistry of this carborane system involves highly C-alkylated derivatives, with B-halogenated compounds [e.g., Figure 4-13] playing a major role. Peralkylated nido-tetracarbahexaboranes react with BBr3 or BI3 to afford 2,3,4,5-R4C4B2R-6-X (R ¼ Me, Et; X ¼ Br, I), quantitatively replacing the B(6)-alkyl group with a halogen atom [262,263]. Treatment of the peralkyl derivatives with Liþ Et3BH
generates R4C4B2R-6-H, while excess BBr3 attacks the 5C(BBr2)2Et [262]. Palladium-catalyzed reaccage, degrading it to the tetrakis(dibromoboryl)diethylalkene, Et(BBr2)2C5 CPh with arylzinc reagents (generated in situ from aryl bromides) give 2,3,4,5-Et4C4B2-1-Br-6tions of Et4C4B2-1-I-6-C CCMe3 is treated CPh, which has been crystallographically characterized [264]. However, when Et4C4B2-1-I-6-C C CCMe3)2. The reaction of Et4C4B2-1with PhZnCl and Pd(PPh3)4, the main product is a dialkynyl species, Et4C4B2(C C-p-tolyl with Pd(PPh3)4 and a dialkylzinc reagent affords the apically-linked biscarborane 4-46 [264]. I-6-C Et

Et

4-46

Et

C B

Me-p-C6H4C≡C

C C B

C Et

Et

Et

C C B Et

C

C Et B C≡C-p-C6H4Me

Selective halogen exchange can also be accomplished at the basal boron, provided that the heavier halogen is the leaving atom (with the weaker B2 2X bond). Thus, the diiodo compound 2,3,4, 5-Et4C4B2-1,6-I2 on treatment with excess AgF undergoes selective replacement of the basal iodine with fluorine, forming 2,3,4,5-Et4C4B2
1-I-6-F in 56% yield; no apically fluorinated product is obtained [264]. The B(2)-brominated compound 2,3,4,5-Me2Et2C4B2Et-2-Br reacts with trimethyl- or triphenylstannyl lithium in THF to give B(2)-SnR3 products. The SnPh3 derivative interacts with electrophiles such as TiCl4 (slowly), SnCl4,

70

CHAPTER 4 Small carboranes: Four- to six-vertex clusters

Br2, and BBr3, progressively cleaving the Sn-C bonds to afford Me2Et2C4B2Et-2-SnPhnX3
n (X ¼ Cl, Br; n ¼ 0-3) and ultimately giving the SnX3 derivative [263]. Similarly, reactions of B(2)-Br derivatives with LiPPh2 yield the corresponding B(2)-PPh2 phosphanes, which can be oxidiatively converted to B(2)-PXPh2 products in which X is O, S, or Se [265]. Me2Et2C4B2Et-2-PPh2 reacts with MeI, removing the phosphane group to give the B(2)-I derivative; treatment with BH3THF or W(CO)5 generates Me2Et2C4B2Et-2-PPh2Y complexes [Y ¼ BH3 or W(CO)5] [265]. The B,B0 -diiodo species 2,3,4,5-Et4C4B2I2 is generally reactive toward nucleophiles, allowing the synthesis of B(2)-X CR, PPh2, FeCp(CO)2, and Et [266]. In most such reactions, the apical B(1)-I bond is inert, derivatives in which X is C CPh, it can be replaced by a C CSiMe3 group via Pd0-catalyzed Negishi cross-coupling but in 2,3,4,5-Et4C4B2
1-I-6-C CZnCl, affording 2,3,4,5-Et4C4B2(C CPh)2. with Me3SiC Nucleophilic substitution on C4B2 carboranes that have reactive groups on boron can be used to generate linked clusters such as 4-46, with further examples (all crystallographically characterized) shown in Figure 4-14 [266,267]. In sharp contrast to the isoelectronic and isostructural 2,3-C2B4H62
and 2,3,4-C3B3H6
anions and their derivatives, which readily form face-bonded (Z5-coordinated) sandwich complexes with metal and metalloid electron-acceptors, as described above, no such compounds featuring nido-C4B2 carborane ligands have been prepared (complexes of classical 1,4-diborabenzene C4B2 rings are known, however [251,268]). It is not clear to what extent this reflects intrinsic inertness of C4B2 ligands toward metal ions—which could be a consequence of depletion of electron density in the ring carbons—or whether it is simply a case of limited experimental study in this area having failed to turn up such species. However, the exo-polyhedral C4 chain in C4B2 benzocarborane derivatives (Figure 4-12C) does coordinate to metals, forming Z5-bound complexes, as depicted in Figure 4-15 [269]. Alkynyl-substituted C4B2 carboranes have been used to construct B(1)- and B(6)-bonded (Z4-cyclobutadiene)cobalt complexes, as illustrated in Figure 4-16 [270].

O C

O C

X

B

B R⬘ R⬘

Co

R C C

X

O C

C

R⬘ C

C

B

C

Ph R⬘

C O

C O

Co2(CO)8

C O

C

R⬘

C

B

C R

C R

B

B B B

Fe (CO)3

Fe (CO)3

X = Me, Br; R = Me, n-C4H9 R⬘ = Me, Et

B

Y

R

ClZnC6H4C6H4ZnCl X = Y = I; R = R⬘ = Et X

X B B

B C

S

Fe(CO)3

N

C

LiCB10H10CH X = Y = I; R = R⬘ = Et H

N

(CO)3 Fe

B

R C C

R⬘

X

−

X B

R⬘

C

C R

X = I; Y = C≡CPh; R = R⬘ = Et

R⬘ C

C

Co

R

S

R C C

B B

B B = BH

B

B R⬘ B R⬘

R C C

C

B

C R

FIGURE 4-14 Syntheses of linked clusters from B-functionalized C4B2 carboranes.

R B

C C R

C C

R⬘ R⬘

4.5 6-Vertex open clusters CMe3 B C C

OC

C O

SiMe3 C O CMe3

CMe3

B

B C C

C H

C

Fe Fe(CO)3(C8H14)2

CMe3

B

B

CMe3 C H

C C

Cp*Co(C2H4)2

C

B

CMe3 C H

C

Co

SiMe3

SiMe3 CpRh(C2H4)2

CMe3 B C C

B

CMe3 C H

C

Rh

SiMe3

FIGURE 4-15 Synthesis of benzo-tetracarbahexaborane metal sandwich complexes.

Et Et

I

C C

C B Et Et

Et C C

C

B

CpCo(C2H4)2 C

C

Co

R

C Et

R = CMe3, SiMe3

I B

Et C

Et

C C

B

C Et

Et

FIGURE 4-16 Synthesis of bis(6-tetracarbahexaborane)(Z4-cyclobutadiene)cobalt complexes.

B

C B Et

I

Et

71

72

CHAPTER 4 Small carboranes: Four- to six-vertex clusters

4.5.9 Nido-2,3,4,5,6-C5BH6þ 4.5.9.1 Synthesis The nido-pentacarbahexaborane(6) cation (Figure 4-3), a cluster type that is commonly designated as boranediyl or borylene but also qualifies as a carborane, is unknown in parent form but can be stabilized by coordination of boron to electron-acceptor groups. At present, all the characterized species are C-permethylated, and many have a halogen substituent on boron (Table 4-11). Several derivatives have been prepared from cyclopentadienyl precursors (Figure 4-17) [271–274], although other routes are known. For example, reaction of the nido-(C5Me5)Snþ cation with BI3 exchanges boron for tin and forms nido-(C5Me5)BIþ [275], while treatment of decamethylsilocene [(C5Me5)2Si] with BCl3 or BBr3 affords the nido-2,3,4,5,6-Me5C5B-SiX2Cp*þ cation, with a SiX2Cp* group coordinated to boron [276]. A side product obtained in the reaction with BBr3 is arachno2SiBr2Cp* (4-47), a neutral species whose X-ray diffraction analysis revealed an open structure, as (Br2Cp*Si)Me5C5B2 2BCl3 [271]. shown. Similarly, diboron tetrachloride (B2Cl4) reacts with (C5Me5)2Si or C5Me5SiMe3 to give neutral Me5C5B2 SiBr2Cp* B

4-47

Me Me

C C

C

Me Me

C

C SiBr2Cp*

Me

TABLE 4-11 C5BHxþ Derivatives Synthesis and Characterization Compound

Information

References

nido-2,3,4,5,6-Me5C5B-SiX2Cp*þ (X ¼ Cl, Br) nido-2,3,4,5,6-Me5C5B-Fe(CO)4

X S, S, S, S, S, S, S, S,

[278] [271] [272] [273] [273] [273,275] [274] [276] [381]

Arachno-C5BH8þ derivatives arachno-(Br2Cp*Si)Me5C5B-SiBr2Cp*þ

S, X, H, B, C, Si, MS

[276]

extended Hu¨ckel SCF: aromaticity; bending of CH hydrogens toward B DFT DFT: singlet ground state and lowest triplet excited state; bond distances and angles DFT: singlet ground state and lowest triplet excited state; bond distances and angles

[34] [277] [279] [280]

þ

Nido-C5BH6 derivatives nido-2,3,4,5,6-Me5C5B-Brþ nido-2,3,4,5,6-Me5C5B-BXCl2 (X ¼ SiCl3, Cl) nido-2,3,4,5,6-Me5C5B-C5Me5þ nido-2,3,4,5,6-Me5C5BClþ nido-2,3,4,5,6-Me5C5BBrþ nido-2,3,4,5,6-Me5C5BIþ

Theoretical Studies Molecular and electronic structure calculations nido-H5C5BHþ nido-2,3,4,5,6-R5C5B-1-BCl3 (R ¼ H, Me) nido-2,3,4,5,6-Me5C5B nido-2,3,4,5,6-Me5C5B-Fe(CO)4 X, X-ray diffraction; H, 1H NMR; B,

11

B NMR; C,

13

X(Cl), H, B, C, MS H, B B B B H, B, C, MS, conductivity X(Cl), H, B, C(Br), MS(Br) X, H, B, C, IR

C NMR; P,

31

P NMR; Si,

29

[280]

Si NMR; IR, infrared data; MS, mass spectroscopic data.

4.6 6-Vertex closo clusters I

Me Me

73

+

B Me GeMe3

BI3 −Me3GeI

Me Me

Me

Me C C

C

C

BI4− Me

C

Me

Me X

Me Me

+

B Me BX2

Me Me

BX3

Me

X = Cl, Br, I Me

Me C C

C

C

Me

BX4−

C Me

FIGURE 4-17 Synthesis of nido-Me5C5BXþ salts.

4.5.9.2 Structure and properties The nido-C5B cage geometry is confirmed by X-ray structures and multinuclear NMR data on several species (Table 4-11). In accord with prediction from theory [277], the methyl groups in the Me5C5BBrþ cation are bent out of the C5 plane toward the boron atom [278]. Stabilization of the nido-R5C5B unit by Lewis acids coordinated to boron has been explored in several theoretical studies [34,277,279,280]; DFT calculations show that the carborane functions as a two-electron s donor toward an Fe(CO)4 group [280]. Beyond their synthesis and characterization, the chemistry of nido-R5C5B clusters has been little studied.

4.6 6-VERTEX CLOSO CLUSTERS 4.6.1 1-CB5H7 and 1-CB5H6
4.6.1.1 Synthesis Parent monocarbahexaborane(7) has been obtained in a variety of high-energy reactions involving organoboranes or borane-hydrocarbon mixtures, including electric discharge or flow pyrolysis of 1-MeB5H8 vapor [123,130,281], pyrolysis of alkenylpentaboranes [128], flash thermolysis of 1,2-(Me3Si)2B5H7 [70], and the reaction of B5H9 with carbon vapor [282]. C- and B-methyl derivatives have been similarly prepared by pyrolyzing 1-MeB5H8 [123,128], 1,2(Me3Si)2B5H7 [70], or 1-(Me3Si)B5H7-m-BMe2 [283]. As in other lower carborane syntheses (Chapter 3), complex product mixtures are obtained with low yields of individual products, and efficient routes to monocarbahexaboranes are not yet available.

4.6.1.2 Structure and properties The octahedral CB5 cage geometry (Figure 1-3, top right), was recognized early on [281] from NMR and other spectroscopic data (Table 4-12) and by analogy with the isoelectronic 14-electron clusters C2B4H6 and B6H62
. Gas-phase electron diffraction [284] and microwave [285] studies of CB5H7 have confirmed this structure and locate the “extra” hydrogen over a triangular B3 face. Variable-temperature 11B NMR data reveal that this hydrogen atom tautomerizes rapidly around the molecule, presumably using the four equivalent triboron faces [286] (although one study concluded

74

CHAPTER 4 Small carboranes: Four- to six-vertex clusters

TABLE 4-12 Closo-1-CB5H7 Derivatives Synthesis and Characterization Compound

Information

References

Parent

S, H, B, IR, MS S, MS S, H, B, MS H (coupling) B (variable temp) ED Microwave S, B, IR, MS S, H, B, MS S, MS S, H, B, IR, MS

[281] [282] [123,130] [94,95] [286] [284] [285] [283] [123] [128] [70]

6-Me 1-Me 2(4)-Me (mixture)

Theoretical Studies Molecular and electronic structure calculations Parent Structure, dipole moment, ionization potential Isomerization calculations Parent n-Me (n ¼ 1, 2, 4, 6) CB5H6
CB5H6
(all isomers) NMR calculations Parent CB5H6
2-R (R ¼ H, Me) 1-R (R ¼ H, Me) B-Me (3 isomers) Reactivity calculations Parent CB5H7, CB5H6
CB5H6


[333]

Isomer stabilities Isomer stabilities EI [energy indexes]; stabilities ab initio, DFT ab initio

[135] [135] [326] [382] [47]

11

B, IGLO H, B, C spin-spin coupling (DFT) 11 B-11B, 11B-13C, 13C-13C spin-spin couling (DFT) 13 C, IGLO 11 B, GIAO, IGLO 11 B, GIAO, IGLO

[12] [37] [93] [96] [288] [288]

Protonation Bridge H tautomerism Face-capping Fluorination mechanism; formation of CB5H5F


[305] [287,333,383,384] [385] [289]

S ¼ synthesis; X ¼ X-ray diffraction; H ¼ 1H NMR; B ¼ ED ¼ electron diffraction.

B NMR; C ¼

11

C NMR; IR ¼ infrared data; MS ¼ mass spectroscopic data;

13

4.6 6-Vertex closo clusters

75

that the tautomerizing proton is mainly restricted to the equatorial B2 2B edges [287]). The weakly bound bridging proton is easily removed by NaH or Et3N to generate the CB5H6
anion [282,288], which is the smallest known member of the closo-CBn
1Hn
carborane anion family. As one of the smallest and structurally simplest carboranes, CB5H7 has been a subject of many theoretical investigations (Table 4-12) centered on proton scrambling, NMR shifts, and (in substituted derivatives) isomer stabilities. Theory is well ahead of experiment with respect to this carborane; for example, while the mechanism of fluorination of the anion has been explored computationally [289], there are as yet no reported experimental studies of fluorination (or halogenation of any type) on this system.

4.6.2 1,2- and 1,6-C2B4H6 Two isomers of closo-dicarbahexaborane(6) having an octahedral cage structure are possible, and both are known; 1,2and 1,6-C2B4H6 (Figure 1-3, top row) were among the first carboranes to be discovered in the early work on alkyneborane reactions in the late 1950s, as described in Chapter 3. As with all lower carboranes, there are no truly efficient, high-yield routes to the parent compounds, and the experimental generation and study of the unsubstituted species (Tables 4-13 and 4-14) have been confined to the very few laboratories having access to small boron hydrides such as B5H9. On the other hand, the C2B4H6 isomers are attractive to theoreticians owing to their simple compositions and small-cage structures.

TABLE 4-13 1,6-C2B4H6 Derivatives Synthesis and Characterization Compound

Information

References

Parent

S (B5H9þ C2H2 flow system) S (pyrolysis of alkylboron hydrides), H S (B2H6þ C2H2 electric discharge), MS, IR S (electric discharge of nido-2, 3-C2B4H8) S (photolysis of nido-2,3-C2B4H8) S (high yield, from nido-2,3-C2B4H8) S (gas phase flash photolysis of B4H10þ C2H2) S (B4H10/B2H6þ C2H2 flash reactions) S (electric discharge of B5H9þ C2H2), MS (detailed), H, B, IR H, B, MS S (B4H10/B2H6þ C2H2 flash reactions) S (gas phase flash photolysis) S (B2H6þ C2H2 electric discharge), H, B, IR, MS S (pyrolysis of nido-2,3-C2B4H8), H, B, MS S (pyrolysis of alkylboron hydrides), H S, B, IR, MS X, ED S (pyrolysis of nido-2,3-C2B4H8) H, B, MS S (oxidation of nido-2,4-Et2C2B4HEt4
), H, B, C S, H, B, IR, MS S, H, B, IR, MS

[54] [123] [50] [57] [55] [56] [53] [51,52] [290]

1-Me, 2-Me Me, Me2 derivatives 2-Me 1,6-Me2

1,6-(SiMe3)2 1-C3H7 1,2,3,4,5,6-Et6 2-(cis-2-but-2-enyl) 2,3/2, 4-(cis-2-but-2-enyl)2 isomers

[139] [51,52] [53] [50] [139] [123] [306] [295] [139] [217] [69] [69] Continued

76

CHAPTER 4 Small carboranes: Four- to six-vertex clusters

TABLE 4-13 1,6-C2B4H6 Derivatives—Cont’d Synthesis and Characterization Compound

Information

References

2,3,4-(cis-2-but-2-enyl)3 2,3,4, 5-(cis-2-but-2-enyl)4 1,6-Li2 1,6-(SiMe3)2 1-SiMe3
6-R (R ¼ n-C4H9, CMe3) 2-CH2SiMe2Cl 2-Cl

S, H, B, IR, MS S, H, B, IR, MS S S (rearrangement of 1,2 isomer), ED S, C, I, MS S, H, B, MS S (photolysis), H, IR, VP, MS S (AlCl3), H, B, MS microwave S, H, B, MS S, H, B, C, Cl, IR

[69] [69] [306] [295] [152] [71] [308] [309] [386] [309] [155]

XPS, B, IR, MS XPS, B, IR, MS S, B, MS S MS S, H, B S, B, VP, MS S, H, B, IR, MS S, H, B, MS S, H, B, IR, MS S (improved) S, IR, H, B, MS S (gas phase pyrolysis of C2B4H6)

[310] [310] [306] [306] [312] [307] [68] [116] [112] [110] [113] [387]

H (coupling) B (NMR-IR correlations) C C 11 B, 10B; B2 2B coupling

[94,95] [103] [96,97] [96] [99]

Raman, IR (detailed) Raman, IR MS MS (negative ion) ED UV photoelectron spectra ion cyclotron resonance, Hþ affinity CH bond polarity, i, C(d and JCH; comparison with other carboranes

[388] [103] [133] [102] [73,389] [105] [390] [108]

2,4-Cl2 H2C2B4Cl4 (probable classical structure [294]) 2-X (X ¼ Cl, Br, I) 2,4-X2 (X ¼ Cl, Br, I) 1-Br-6-Me 1-I-6-Me 2-BBr2 2-I 2-SH 2-(10 ,20 -B2H5) 1,5-C2B3H4
1,6-C2B4H5 isomers 2-(20 -1,5-C2B3H4) (improved) 2:20 -(1,6-C2B4H5)2 (C2B4H2)n nanoparticles Detailed NMR Studies Parent

2-Cl, 2,4-Cl2 2-(20 -1,5-C2B3H4) Other Experimental Studies Parent

Continued

4.6 6-Vertex closo clusters

77

TABLE 4-13 1,6-C2B4H6 Derivatives—Cont’d Synthesis and Characterization Compound

Information

References

2-X (X ¼ Cl, Br, I)

XPS: binding energies Metal insertion and polyhedral expansion Reaction with BMe3 Reaction with Me3N Reactions with Lewis bases Catalytic reactions with alkynes Competitive electrophilic halogenation, alkylation Ignition mechanism Ignition in H2O vapor—kinetics Ignition promoted by isopropyl nitrate: kinetics Pyrolysis, mechanism of clustering High temperature oxidation Cage-opening reaction with F
Hydrolysis in CH3OH Reaction with atomic S Hg-sensitized cophotolysis with 1, 5-C2B3H5 Competitive electrophilic halogenation, alkylation

[106,107] [118] [204] [182] [120] [69] [311] [391] [299] [300] [301] [302] [213] [114] [68] [112] [311]

Theoretical Studies Molecular and electronic structure calculations Parent Population analysis Isomer stabilities (SCF, DFT)

CSiMe3, C N, 1,6-R2 (R ¼ C O, N N) C 1,6-(SiMe3)2 (formation from 1,2 isomer)

Fractional 3-center bonds Electronic structure Localized MOs Geometry (ab initio) Cage structure, dipole moment, ionization potential, heat of formation (AM1) Charge distribution (CNDO) Charge distribution (EHMO) Heat of formation C–H bond length compared with halomethanes Energy indexes, isomer stabilities XPS: binding energies (CNDO) Vibrational modes and structure Second-order NLO properties Y Hybridization, bond order, comparison with X derivatives of other clusters; electronic cage-substituent interactions Geometry (ab initio)

[337,376] [78,92,324,325,327– 330,332,382,392] [372] [79,331] [75,79,334,374,393] [11,47,77,81] [333] [330,394] [395] [396] [336] [326] [107] [397] [328] [293]

[295] Continued

78

CHAPTER 4 Small carboranes: Four- to six-vertex clusters

TABLE 4-13 1,6-C2B4H6 Derivatives—Cont’d Synthesis and Characterization Compound

Information

References

classical C2B4H6 2(m, terminal)-BH4 Nitro- and amino-substituted derivatives

geometry ab initio, linked carboranes DFT, first hyperpolarizability (b)

[82] [304] [398]

Cage rearrangement from 1,2-C2B4 clusters Rearrangement mechanism

[82,225,296– 298,340,399,400] [296]

Isomerization energy (DFT)

[83]

B (IGLO) C (IGLO) B, C (IGLO) B, C, H spin-spin coupling (DFT) 11 B-11B, 11B-13C, 13C-13C spin-spin coupling (DFT) aromatic solvent-induced 1H NMR shifts: correlation with Hþ charges (PRDDO) H (solvent shifts) B-H coupling C (IGLO) 11 B shifts B (IGLO, GIAO)

[12] [96] [11] [37] [93] [101]

Alkyne incorporation Protonation to form 1, 6-C2B4H7þ Combustion mechanism

[230] [119,305] [303]

Isomerization calculations Parent

Parent and classical NMR calculations Parent

2-Cl, 2,4-Cl2 2-X (X ¼ H, F, Cl, Br, I, Me “classical” H2C2B4Cl4 Reactivity calculations Parent

[175] [100] [96] [401] [294]

S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; 2d, two-dimensional (COSY) NMR; IR, infrared data; MS, mass spectroscopic data; UV, UV-visible data; VP, vapor pressure; XPS, X-ray photoelectron spectra; ED, electron diffraction data.

4.6.2.1 Synthesis The parent 1,2 and/or 1,6 isomers have been obtained via three main routes: (1) reactions of small boranes (usually B2H6, B4H10, or B5H9) with alkynes under high-energy conditions such as electric discharge [50,290], flow pyrolysis [54], and flash photolysis [51–53]; (2) from alkyl derivatives of B5H9 [123]; or (3) from nido-2,3-C2B4H8 via electric discharge [57], pyrolysis [56,139], or photolysis [55]. Many of these reactions also yield C- and B-alkyl derivatives of 1,6-C2B4H6 (Table 4-14), but not the 1,2 isomer, as the latter species rearrange to thermodynamically favored 1,6C2B4 clusters under high-energy reaction conditions. For the synthesis of substituted derivatives, several relatively efficient approaches are available. 1,2-(Me3Si)2C2B4H4 has been prepared in 94% yield via the reaction of the 7-vertex stannacarborane closo-1,2,3-Sn[(Me3Si)2C2B4H4] with PtCl2, probably via formation of an intermediate closo-Pt[(Me3Si)2C2B4H4] cluster (not isolated), which decomposes

4.6 6-Vertex closo clusters

79

TABLE 4-14 1,2-C2B4H6 Derivatives Synthesis and Characterization Compound

Information

References

Parent

S (electric discharge of nido-2,3-C2B4H8), H, B, IR S (photolysis of nido-2,3-C2B4H8) S (pyrolysis of alkylboron hydrides), H S (electric discharge of B5H9þ C2H2 mixture), MS (detailed), H, B, IR S (closo-1,2,3-Sn[(Me3Si)2C2B4H4] þ PtCl2), H, B, C, Si, IR S [oxidation of nido-2,3-(Me3Si)RC2B4H42
], H, B, C, IR

[57] [55] [123] [290]

S [oxidation of nido-2,3-(Me3Si)RC2B4H42
], H, B, C, IR, MS

[152]

S [oxidation of nido-2,3-(Me3Si)2C2B4H3(i-C4H9)2
], H, B, C, IR X, ED

[195] [295]

H(coupling) C, IGLO

[94,95] [96]

microwave (cage structure) ED MS MS (negative ion) He photoelectron spectra ED, thermal rearrangement K þ ErCl3 ! (THF)2Kþ Er[(SiMe3)2C2B4H4]2
reductive cage-opening MS (detailed)

[402] [284] [133] [102] [106] [295] [403]

1-SiMe3
2-R (R ¼ SiMe3, Me, H) 1-SiMe3
2-R (R ¼ n-C4H9, CMe3) 1,2-(SiMe3)2
3-i-C4H9 1,2-(SiMe3)2 Detailed NMR Studies Parent

Other Experimental Studies Parent

1,2-(SiMe3)2

1-CR5 5CH2 (R ¼ H, Me)

Theoretical Studies Molecular and electronic structure calculations Parent isomers EI (energy indexes), stabilities classical versus nonclassical C2B4H6 geometry Parent isomer stabilities (DFT) ab initio SCF C–H bond length compared with halomethanes Charge distribution (CNDO) Cage structure, dipole moment, ionization potential, heat of formation (AM1) Dipole moment, ionization potential, heat of formation Electronic structure

[291] [166]

[404]

[326] [82] [329,382] [47,77,327] [78,332,392] [336] [330] [333] [333] [331,337,376,405] Continued

80

CHAPTER 4 Small carboranes: Four- to six-vertex clusters

TABLE 4-14 1,2-C2B4H6 Derivatives—Cont’d Synthesis and Characterization Compound

Information

References

1,2-R2 (R ¼ H, CH3, NH2, OH, F, SiH3, PH3, SH, Cl) 1,2-(R
)2 (R ¼ H, CH3, NH2, OH, F, SiH3, PH3, SH, Cl) 1-CH 1,2-R2 (R ¼ H, OH, SH, NH2, PH2, CH3, SiH3) “classical” H2C2B4Cl4 1,2-(SiMe3)2

Geometry, ab initio electron correlation effects Isomer stabilities Localized MOs DFT; unusually long C2 2C bond distances; influence of substituents on C2 2C length DFT; unusually long C2 2C bond distances; influence of substituents on C2 2C length Carbene DFT optimized structure of singlet and triplet states of neutral species and dianions formed by Hþ removal from R groups IGLO, GIAO,B(Cl) ab initio

[11] [324,325,330] [334,373,374,393] [406]

Isomerization calculations Parent

Cage rearrangement

Parent and classical

3d Hu¨ckel; isomers; rearrangement mechanism Isomerization (DFT)

[82,225,296– 298,340,399,400] [407] [83]

B, C, H spin-spin coupling (DFT) B2 211B, 11B2 213C, 13C2 213C spin-spin coupling (DFT) B2 2H coupling Calculated 11B and 13C NMR shifts (IGLO)

[37] [93] [100] [11,12]

Protonation to form 1,2-C2B4H7þ Alkyne incorporation

[305] [230]

NMR calculations Parent

11

Reactivity calculations Parent

[406] [339] [292] [294] [295]

S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; IR, infrared data; MS, mass spectroscopic data; UV, UV-visible data; ED, electron diffraction.

immediately to give the bis(trimethylsilyl)carborane and platinum metal [291]. Metal-promoted oxidation of dilithium salts of nido-2,3-(Me3Si)RC2B4H42
anions yields closo-1,2-(Me3Si)RC2B4H4 products (Figure 4-18) [152,166,195], together with smaller amounts of (Me3Si)2R2C4B8H8 oxidative fusion products (Chapter 11). The nido-to-closo conversion of a 6-vertex cluster via removal of two electrons nicely illustrates the prediction from theory (Chapter 2), as does the oxidation of Naþ nido-2,4-Et2C2B4Et4H
with I2 to afford 1,6-Et2C2B4Et4 [217]. The introduction of substituents to 1,6-C2B4 clusters, and related chemistry, are described below.

4.6.2.2 Structure and physical properties The octahedral cluster geometry for 1,2- and 1,6-C2B4H6 is well established from microwave [402] and electron diffraction [73,284,389] studies of the parent compounds and 1,6-C2B4H5-2-Cl [386], supported by X-ray crystallographic and electron diffraction analyses of 1,2- and 1,6-(Me3Si)2C2B4H4 [295]. The C2B4 carboranes have been thoroughly

4.6 6-Vertex closo clusters

Me3Si R

C C

SiMe3

H

H

2−

B

2 Li+ B

B

Me3Si

n-C6H12 0 °C

H

B

C

B NiCl2

H

B

C

H

B

C

R

H

sealed tube H

250-255 °C

B

B H

R = Me, n-C4H9, CMe3, SiMe3

H

B

B

H

C

B

H

81

R

H

FIGURE 4-18 Synthesis of closo-1,2-C2B4 clusters via oxidative closure of nido-2,3-C2B4 dianions.

investigated via mass spectrometric, NMR, IR, UV, Raman, X-ray photoelectron, and other studies on numerous derivatives (Tables 4-13 and 4-14). The tables also list extensive theoretical work on these systems, the results of which correlate closely with experimental findings on electronic structure and charge distribution, proton affinity and other reactivity, cage rearrangement, NMR chemical shifts and coupling constants, and vibrational modes. For both isomers, the observation of high antipodal 1H-1H coupling implies significant bonding involving s orbitals in the center of the cages [94]. Of particular interest is a DFT study of 1,2-R2C2B4H6 derivatives in which R is H, OH, NH2, PH3, SH, CH3, or SiH3; it was found that triplet-state dianions obtained by deprotonation of the R groups undergo major structural change, adopting a nido-C2B4 pyramidal geometry that has an apex BH group [292]. The parent C2B4H6 clusters with their five-coordinate carbon atoms are clearly nonclassical molecules, in contrast to 1,5-C2B3H5, which (as discussed earlier) has been variously treated as classical, nonclassical, or fluctuating between the two. However, electronically active substituents can profoundly influence the cage geometry [293]; an example is the species H2C2B4Cl4 [155] for which NMR evidence and theory point to an open classical structure [294]. The thermal rearrangement of 1,2- to 1,6-C2B4H6, which occurs quantitatively at 250  C [57], has been explored computationally by many workers (see Table 4-13) and is driven by mutual repulsion of the negatively charged carbon atoms. The conversion of 1,2- to 1,6-(Me3Si)RC2B4H4 (R ¼ SiMe3, n-C4H9, CMe3) also requires temperatures of at least 250  C [152,295], which is surprising because the presence of bulky groups on the cage carbon atoms would normally be expected to lower the activation energy for skeletal rearrangement, as is found, for example, in the icosahedral carboranes (Section 10.3). As is generally the case with carborane cage isomerizations (particularly the icosahedral systems, discussed in Chapter 10), it is probable that more than one mechanism is operative although a particular pathway may dominate [296]. For the C2B4H6 system, PRDDO (partial retention of diatomic differential overlap) calculations by Lipscomb and coworkers [82] and a later ab initio study by McKee [225] suggest a mechanism involving a classical benzvalene-like intermediate (Figure 4-19), which invokes a modification of the diamond-square-diamond (dsd) sequence originally proposed by C

B

C

B

B

B

B

B B

C

C

B

B C

B

B

B C

FIGURE 4-19 Simplified representation of the calculated rearrangement of 1,2- to 1,6- C2B4H6 (hydrogens omitted).

82

CHAPTER 4 Small carboranes: Four- to six-vertex clusters

Hoffmann and Lipscomb [297,298]. Indirect experimental support for such an intermediate is given by the synthesis and isolation of a stable B4-tetrakis(triisopropylamino) derivative 4-30, mentioned earlier in this chapter.

4.6.2.3 Reactions of closo-1,2-C2B4 clusters Aside from its thermally induced conversion to the 1,6 system, reports of the chemical reactivity of the 1,2 isomer are sparse. Cage-opening of 1,2-(Me3Si)2C2B4H4 via treatment with alkali metals, as described earlier in Section 4.5, followed by reactions with transition metal or lanthanide salts, generate 1-M[2,4-(Me3Si)2C2B4H4]n2 or 1,2,4-LM[(Me3Si)2C2B4H4]n- “carbons-separated” sandwich complexes, in which L is a donor ligand such as TMEDA (Chapter 13). Calculations on a carbene-substituted species, 1,2-(HC)C2B4H5, show that it is an energy minimum, but full incorporation into the cage to form a 7-vertex C3B4 cluster is favored [339].

4.6.2.4 Reactions of closo-1,6-C2B4 clusters: Hydrolysis, methanolysis, and oxidation The parent carborane is significantly more stable and less reactive than 1,5-C2B3H5, but is degraded by water and methanol to form monoboron products such as B(OR)3 and MeB(OR)2 [114]. The processes occurring during pyrolysis and hightemperature oxidation and combustion have been investigated in detail [299–303], driven in part by interest in using volatile boranes and carboranes as fuels (see Chapter 1).

4.6.2.5 Polyhedral expansion Reductive cage-opening of closo-1,6-C2B4 carboranes to afford nido-2,4-C2B4 clusters—the reverse of oxidative closure—can be achieved with electron-donors, such as fluoride, amines, and phosphines, as described earlier in Section 4.5 and Figure 4-7 [120,182]. Thermal insertion of boron into parent 1,6-C2B4H6 is accomplished by heating with BMe3 at 550-600  C to form a dicarbaheptaborane, 2,4-C2B5H5Me2 [204]. Polyhedral cage expansion of parent 1,6C2B4H6 via direct reaction with transition metal reagents to afford 7-vertex 1,2,4-MC2B4 metallacarborane clusters [118] is discussed in Chapter 13.

4.6.2.6 Cage linkage As mentioned earlier in this chapter, co-pyrolysis of 1,6-C2B4H6 with 1,5-C2B3H5 [110], or Hg-sensitized co-photolysis of the same carboranes [112], yields the B
B linked dicluster 2,20 -1,5-C2B3H4
10 ,60 -C2B4H5. Cage linkage can also be achieved via transition metal catalysis, as in the formation of the dimer 2,20 -(1,6-C2B4H5)2 in quantitative yield by 2B linkage at 25  C [113]. The same platinum reagent promotes the reaction PtBr2-assisted dehydrogenation and B2 of B2H6 with 1,6-C2B4H6 to form the C2B4H5-B2H5 linked species 4-48 [116]. This behavior contrasts with that of 1,5-C2B3H5, which on reaction with B2H6 undergoes cage expansion to form the 8-vertex carborane arachnoC2B6H12; the difference has been explained by ab initio calculations that show a lower activation energy for boron incorporation into the cage in C2B3H5 compared to C2B4H6 [304]. H

C H H

4-48 H

B

B C H

4.6.2.7 Introduction of substituents

H

B

B

H

B H H

B H

Protonation of the C2B4H6 cage to form closo-C2B4H7þ has been explored theoretically [119,305] and the calculated proton affinities are in agreement with experiment; for the 1,6-isomer, protonation is favored at the B2 2B edges [119]. Various approaches have been explored for attaching substituents to the polyhedral framework. As in the icosahedral C2B10H12

4.6 6-Vertex closo clusters

83

carboranes, the CH hydrogens in 1,6-C2B4H6 have protonic character and can be removed with n-butyllithium in ether/hexane to afford the C-mono- or C,C0 -dilithio clusters, from which the C,C0 -dimethyl and C-methyl-C0 -bromo derivatives can be prepared [306]. In comparison with 2,4-C2B5H7 and 1,2-C2B10H12 (Chapters 5 and 9, respectively), the reaction with n-butyllithium is slow, requiring 48 h for completion at room temperature. There is evidence that the monolithio compound in solution exists in equilibrium with the dilithio and parent carboranes [306]. 2C4 H9 Li

MeI

C2 B4 H6




! Li2 C2 B4 H4


! Me2 C2 B4 H4 C4 H9 Li

Br2

C2 B4 H6




! LiC2 B4 H5


! MeBrC2 B4 H4 Substitution at the boron vertexes in 1,6-C2B4 carboranes has been explored in several studies. Halogenation under Friedel-Crafts conditions yields the B-chloro, B-bromo, or B-iodo derivative [306–310], and has also been used to prepare the corresponding 2,4-dihalo compounds [310]. The B-chloro derivative can also be obtained by photolysis of nido-2,3C2B4H7
4-Cl or via the photolytic reaction of Cl2 with 1,6-C2B4H6 [308]. In Friedel-Crafts chlorination, there is some decomposition of the cage, apparently owing to reaction with HCl [309]. In the B-monochloro compound, the chlorine substituent enhances the reactivity of the opposite (antipodal) boron [B(4)] toward electrophiles [311], which explains, for example, why dichlorination occurs preferentially at the 2,4 boron positions (this effect is notably absent in 2,4C2B5H7, probably for reasons having to do with cluster symmetry). Competitive reactions of 1,6-C2B4H6 and 2,4C2B5H7 with Cl2/AlCl3 show that the larger carborane is the more reactive [311], apparently reflecting the more varied charge distribution in 2,4-C2B5H7 (which has three different types of BH units) compared to 1,6-C2B4H6, which has identical BH vertexes and, hence, no preferred site for initial attack. X-ray photoelectron spectra reveal strong p-interaction between the halogen substituents and the cluster molecular orbitals [310], exerting a strong influence on the reaction chemistry and NMR chemical shifts. Reaction of 1,6-C2B4H6 with BBr3 at 265  C gives the 2-BBr2 derivative [312]. Other approaches to cage substitution on 1,6-C2B4H6 include reaction with atomic sulfur to generate the 2-mercapto derivative [68] and the use of dicobalt catalysts of the type (R2CR0 )Co2(CO)6 (R,R0 ¼ H, Me, or Et) to prepare alkenylcarboranes via carborane-alkyne interactions [69]: ðMeCCMeÞCo2 ðCOÞ2

CR0













! C2 B4 H5 -2-ðRC5 C2 B4 H6 þ RC 5CHR0 Þ 5CHR0 Þ2 þ other alkenylcarboranes þ C2 B4 H4 -2; 3=2; 4-ðRC5

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

Budzelaar, P. H. M.; Kos, A. J.; Clark, T.; Schleyer, P. v. R. Organometallics 1985, 4, 429. Budzelaar, P. H. M.; van der Kerk, S. M.; Krogh-Jespersen, K.; Schleyer, P. v. R. J. Am. Chem. Soc. 1986, 108, 3960. Rasul, G.; Surya Prakash, G. K.; Field, L. D.; Olah, G. A.; Williams, R. E. J. Organomet. Chem. 2000, 614–615, 195. Williams, R. E.; Prakash G. K. S.; Field, L. D.; Olah, G. A. In: Liebman, J. F., Greenberg, A., Williams, R. E., Eds.; Advances in Boron and the Boranes; New York: VCH Publishers, Inc: 1988, pp. 191–224 [Mol. Struct. Energ. Vol. 5]. McDonald, L. E.; Gellene, G. I. Mol. Phys. 2001, 99, 377. Minyaev, R. M. Russ. J. Inorg. Chem. 2000, 45, 1069. Amseis, P.; Mesbah, W.; Pra¨sang, C.; Hofmann, M.; Geiseler, G.; Massa, W.; et al. Organometallics 2003, 22, 1594. Jemmis, E. D.; Pavankumar, P. N. V. Proc. Ind. Acad. Sci. 1984, 93, 479. Jemmis, E. D.; Subramanian, G.; Naga Srinivas, G. Inorg. Chem. 1994, 33, 2317. Neu, A.; Radacki, K.; Paetzold, P. Angew. Chem. Int. Edit. 1999, 38, 1281. Bu¨hl, M.; Gauss, J.; Hofmann, M.; Schleyer, P. v. R. J. Am. Chem. Soc. 1993, 115, 12385. Bu¨hl, M.; Schleyer, P. v. R. J. Am. Chem. Soc. 1992, 114, 477. (a) Klusik, H.; Berndt, A. Angew. Chem. Int. Ed. Engl. 1983, 22, 877. (b) Wieczorek, C.; Allwohn, J.; Schmidt-Lukasch, G.; Hunold, R.; et al. Angew. Chem. Int. Ed. Engl. 1990, 29, 398.

84 [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61] [62] [63] [64]

CHAPTER 4 Small carboranes: Four- to six-vertex clusters Budzelaar, P. H. M.; Krogh-Jespersen, K.; Clark, T.; Schleyer, P. v. R. J. Am. Chem. Soc. 1985, 107, 2773. Ko¨ster, R.; Benedikt, G.; Grassberger, M. A. Justus Liebigs Ann. Chem. 1968, 719, 187. Matteson, D. S.; Mattschei, P. K. Inorg. Chem. 1973, 12, 2472. Wrackmeyer, B.; Schanz, H.-J.; Milius, W. Angew. Chem. Int. Edit. Engl. 1997, 36, 75. Hofmann, M.; Fox, M. A.; Greatrex, R.; Williams, R. E.; Schleyer, P. v. R. J. Organomet. Chem. 1998, 550, 331. Fox, M. A.; Greatrex, R.; Hofmann, M.; Schleyer, P. v. R.; Williams, R. E. Angew. Chem. Int. Edit. Engl. 1997, 36, 1498. Fox, M. A.; Greatrex, R.; Hofmann, M.; Schleyer, P. v. R. Angew. Chem. Int. Edit. Engl. 1994, 33, 2298. Franz, D. A.; Grimes, R. N. J. Am. Chem. Soc. 1970, 92, 1438. Fox, M. A.; Greatrex, R.; Greenwood, N. N.; Kirk, M. Polyhedron 1993, 12, 1849. Greatrex, R.; Greenwood, N. N.; Kirk, M. J. Chem. Soc. Chem. Commun. 1991, 1510. Gangnus, B.; Stock, H.; Siebert, W.; Hofmann, M.; Schleyer, P. v. R. Angew. Chem. Int. Edit. Engl. 1994, 33, 2296. Ko¨ster, R.; Seidel, G.; Wrackmeyer, B. Angew. Chem. Int. Edit. Engl. 1994, 33, 2294. Wrackmeyer, B.; Schanz, H.-J. Collect. Czech. Chem. Commun. 1997, 62, 1254. Ko¨ster, R.; Boese, R.; Wrackmeyer, B.; Schanz, H.-J. J. Chem. Soc. Chem. Commun. 1995, 1691. Franz, D. A.; Miller, V. R.; Grimes, R. N. J. Am. Chem. Soc. 1972, 94, 412. Fox, M. A.; Greatrex, R.; Nikrahi, A.; Brain, P. T.; Picton, M. J.; Rankin, D. W. H.; et al. Inorg. Chem. 1998, 37, 2166. Franz, D. A.; Grimes, R. N. J. Am. Chem. Soc. 1971, 93, 387. McKee, M. L. J. Am. Chem. Soc. 1995, 117, 8001. McKee, M. L. J. Am. Chem. Soc. 1996, 118, 421. Fox, M. A.; Greatrex, R.; Nikrahi, A. Dalton. Trans. 2008, 676. Gill, W. R.; Jones, M. E.; Wade, K.; Porterfield, W. W.; Wong, E. H. J. Mol. Struct. (Theochem) 1992, 261, 161. Hofmann, M.; Fox, M. A.; Greatrex, R.; Schleyer, P. v. R.; Williams, R. E. Inorg. Chem. 2001, 40, 1790. Schleyer, P. v. R.; Gauss, J.; Bu¨hl, M.; Greatrex, R.; Fox, M. A. J. Chem. Soc. Chem. Commun. 1993, 1766. Onak, T.; Jaballas, J.; Barfield, M. J. Am. Chem. Soc. 1999, 121, 2850. Miller, V. R.; Grimes, R. N. Inorg. Chem. 1972, 11, 862. Brennan, J. P.; Grimes, R. N.; Schaeffer, R.; Sneddon, L. G. Inorg. Chem. 1973, 12, 2266. Grimes, R. N. J. Am. Chem. Soc. 1971, 93, 261. Meneghelli, B. J.; Rudolph, R. W. Inorg. Chem. 1975, 14, 1429. McKee, M. L. Inorg. Chem. 1988, 27, 4241. Unverzagt, M.; Winkler, H.-J.; Brock, M.; Hofmann, M.; Schleyer, P. v. R.; Massa, W.; et al. Angew. Chem. Int. Edit. Engl. 1997, 36, 853. Lokbani-Assouz, N. S.; Boucekkine, A.; Saillard, J.-Y. J. Mol. Struct. (Theochem) 2003, 664–665, 183. Housecroft, C. E. J. Organomet. Chem. 1984, 276, 297. McKee, M. L. J. Am. Chem. Soc. 1997, 119, 4220. Schleyer, P. v. R.; Najafian, K. Inorg. Chem. 1998, 37, 3454. Sahin, Y.; Pra¨sang, C.; Hofmann, M.; Geiseler, G.; Massa, W.; Berndt, A. Angew. Chem. Int. Edit. 2005, 44, 1643. Shapiro, I.; Good, C. D.; Williams, R. E. J. Am. Chem. Soc. 1962, 84, 3837. Grimes, R. N. J. Am. Chem. Soc. 1966, 88, 1895. Grimes, R. N.; Bramlett, C. L. J. Am. Chem. Soc. 1967, 89, 2557. Grimes, R. N.; Bramlett, C. L.; Vance, R. L. Inorg. Chem. 1969, 8, 55. Fox, M. A.; Greatrex, R.; Greenwood, N. N.; Kirk, M. Collect. Czech. Chem. Commun. 1999, 64, 806. Ditter, J. F.; Klusmann, E. B.; Oakes, J. D.; Williams, R. E. Inorg. Chem. 1970, 9, 889. Spielman, J. R.; Scott, J. E. J. Am. Chem. Soc. 1965, 87, 3512. Ditter, J. F. Inorg. Chem. 1968, 7, 1748. Onak, T. P.; Drake, R. P.; Dunks, G. B. Inorg. Chem. 1964, 3, 1686. Grimes, R. N. J. Am. Chem. Soc. 1966, 88, 1070. Hofmann, M.; Fox, M. A.; Greatrex, R.; Schleyer, P. v. R.; Bausch, J. W.; Williams, R. E. Inorg. Chem. 1996, 35, 6170. Ko¨ster, R.; Rotermund, G. W. Tetrahedron Lett. 1964, 1667. Grassberger, M. A.; Hoffmann, E. G.; Schomberg, G.; Ko¨ster, R. J. Am. Chem. Soc. 1968, 90, 56. Ko¨ster, R.; Horstscha¨fer, H. J.; Binger, P. Angew. Chem. Int. Edit. Engl. 1966, 5, 730. Ko¨ster, R.; Horstscha¨fer, H.; Binger, P.; Mattschei, P. K. Justus Liebigs Ann. Chem. 1975, 1339. Brown, M. P.; Holliday, A. K.; Way, G. M. J. Chem. Soc. Chem. Commun. 1972, 850.

4.6 6-Vertex closo clusters [65] [66] [67] [68] [69] [70] [71] [72] [73] [74] [75] [76] [77] [78] [79] [80] [81] [82] [83] [84] [85] [86] [87] [88] [89] [90] [91] [92] [93] [94] [95] [96] [97] [98] [99] [100] [101] [102] [103] [104] [105] [106] [107] [108] [109] [110] [111] [112] [113] [114] [115]

85

Brown, M. P.; Holliday, A. K.; Way, G. M.; Whittle, R. B.; Woodard, C. M. J. Chem. Soc., Dalton. Trans. 1977, 1862. Bayer, M. J.; Pritzkow, H.; Siebert, W. Eur. J. Inorg. Chem. 2002, 1293. Dobbie, R. C.; Distefano, E. W.; Black, M.; Leach, J. B.; Onak, T. J. Organomet. Chem. 1976, 114, 233. Plotkin, J. S.; Sneddon, L. G. J. Am. Chem. Soc. 1977, 99, 3011. Wilczynski, R.; Sneddon, L. G. Inorg. Chem. 1982, 21, 506. Leach, J. B.; Oates, G.; Tang, S.; Onak, T. J. Chem. Soc., Dalton. Trans. 1975, 1018. Ungermann, C. B.; Onak, T. Inorg. Chem. 1977, 16, 1428. Bromm, D.; Seebold, U.; Noltemeyer, M.; Meller, A. Chem. Ber. 1991, 124, 2645. McNeill, E. A.; Gallaher, K. L.; Scholer, F. R.; Bauer, S. H. Inorg. Chem. 1973, 12, 2108. Antipin, M.; Boese, R.; Bla¨ser, D.; Maulitz, A. J. Am. Chem. Soc. 1997, 119, 326. Armstrong, D. R. Rev. Roum. Chim. 1975, 20, 883 [Chem Abstr 205587g (1975)]. Burdett, J. K.; Eisenstein, O. J. Am. Chem. Soc. 1995, 117, 11939. Jemmis, E. D.; Subramanian, G. J. Phys. Chem. 1994, 98, 9222. Bader, R. F. W.; Legare, D. A. Can. J. Chem. 1992, 70, 657. Semenov, S. G. Teor. Eksp. Khim. 1987, 23, 451 [Chem. Abstr. 107:183891]. Fitzpatrick, N. J.; Fanning, M. O. J. Mol. Struct. (Theochem) 1977, 40, 271. Takano, K.; Izuho, M.; Hosoya, H. J. Phys. Chem. 1992, 96, 6962. Camp, R. N.; Marynick, D. S.; Graham, G. D.; Lipscomb, W. N. J. Am. Chem. Soc. 1978, 100, 6781. Mire, L. W.; Wheeler, S. D.; Wagenseller, E.; Marynick, D. S. Inorg. Chem. 1998, 37, 3099. Graham, G. D.; Marynick, D. S.; Lipscomb, W. N. J. Am. Chem. Soc. 1980, 102, 2939. Jemmis, E. D.; Subramanian, G.; Srivastava, I. H.; Gadre, S. R. J. Phys. Chem. 1994, 98, 644. Schleyer, P. v. R.; Subramanian, G.; Dransfeld, A. J. Am. Chem. Soc. 1996, 118, 9988. Torre, A.; Lain, L.; Bochicchio, R.; Ponec, R. J. Comput. Chem. 1999, 20, 1085. Reed, L. H.; Allen, L. C. Int. J. Quantum. Chem. Quantum. Chem. Symp. 1991, 25, 489. Kar, T.; Jug, K. Int. J. Quantum. Chem. 1995, 53, 407. Dyczmons, V.; Horn, M.; Botschwina, P.; Meller, A. J. Mol. Struct. (Theochem) 1998, 431, 137. Subramanian, G.; Schleyer, P. v. R.; Dransfeld, A. Organometallics 1998, 17, 1634. Salem, A.; Deleuze, M. S.; Francois, J. P. Chem. Phys. 2001, 271, 17. Wrackmeyer, B. Z. Naturforsch. B 2005, 60, 955. Onak, T.; Jarvis, W. J. Magn. Reson. 1979, 33, 649. Onak, T.; Wan, E. J. Chem. Soc., Dalton. Trans. 1974, 665. Diaz, M.; Jaballas, J.; Arias, J.; Lee, H.; Onak, T. J. Am. Chem. Soc. 1996, 118, 4405. Olah, G. A.; Prakash, G. K. S.; Liang, G.; Henold, K. L.; Haigh, G. B. Proc. Natl. Acad. Sci. USA 1977, 74, 5217. Ko¨ster, R.; Wrackmeyer, B. Z. Naturforsch. B 1981, 36, 704. Anderson, J. A.; Astheimer, R. J.; Odom, J. D.; Sneddon, L. G. J. Am. Chem. Soc. 1984, 106, 2275. Jarvis, W.; Abdou, Z. J.; Onak, T. Polyhedron 1983, 2, 1067. Jarvis, W.; Inman, W.; Powell, B.; Distefano, E. W.; Onak, T. J. Magn. Reson. 1981, 43, 302. Onak, T.; Howard, J.; Brown, C. J. Chem. Soc., Dalton. Trans. 1973, 76. Jotham, R. W.; Reynolds, D. J. J. Chem. Soc. A 1971, 3181. Jensen, J. O. Spectrochim. Acta. A Mol. Biomol. Spectros. 2003, 59, 2049. Ulman, J. A.; Fehlner, T. P. J. Am. Chem. Soc. 1978, 100, 449. Fehlner, T. P. Inorg. Chem. 1975, 14, 934. Allison, D. A.; Johansson, G.; Allan, C. J.; Gelius, U.; Siegbahn, H.; Allison, J.; et al. J. Electron. Spectros. Relat. Phenomena 1973, 1, 269. Leites, L. A.; Vinogradova, L. E. J. Organomet. Chem. 1977, 125, 37. Burg, A. B.; Reilly, T. J. Inorg. Chem. 1972, 11, 1962. Astheimer, R. J.; Sneddon, L. G. Inorg. Chem. 1983, 22, 1928. Anderson, E. L.; DeKock, R. L.; Fehlner, T. P. J. Am. Chem. Soc. 1980, 102, 2644. Plotkin, J. S.; Astheimer, R. J.; Sneddon, L. G. J. Am. Chem. Soc. 1979, 101, 4155. Corcoran, E. W., Jr; Sneddon, L. G. J. Am. Chem. Soc. 1984, 106, 7793. Dobbie, R. C.; Wan, E.; Onak, T. J. Chem. Soc., Dalton. Trans. 1975, 2603. Gotcher, A. J.; Ditter, J. F.; Williams, R. E. J. Am. Chem. Soc. 1973, 95, 7514.

86 [116] [117] [118] [119] [120] [121] [122] [123] [124] [125] [126] [127] [128] [129] [130] [131] [132] [133] [134] [135] [136] [137] [138] [139] [140] [141] [142] [143] [144] [145] [146] [147] [148] [149] [150] [151] [152] [153] [154] [155] [156] [157] [158] [159] [160] [161] [162] [163] [164] [165] [166]

CHAPTER 4 Small carboranes: Four- to six-vertex clusters Corcoran, E. W., Jr; Sneddon, L. G. J. Am. Chem. Soc. 1985, 107, 7446. Barker, G. K.; Green, M.; Garcia, M. P.; Stone, F. G. A.; Bassett, J. M.; Welch, A. J. J. Chem. Soc. Chem. Commun. 1980, 1266. Miller, V. R.; Sneddon, L. G.; Beer, D. C.; Grimes, R. N. J. Am. Chem. Soc. 1974, 96, 3090. Dekock, R. L.; Jasperse, C. P. Inorg. Chem. 1983, 22, 3843. Lew, L.; Haran, G.; Dobbie, R.; Black, M.; Onak, T. J. Organomet. Chem. 1976, 111, 123. Jemmis, E. D.; Ramalingam, M.; Jayasree, E. G. J. Comput. Chem. 2001, 22, 1542. Hogoveen, H.; Kwant, P. W. Accounts Chem. Res. 1975, 8, 413. Groszek, E.; Leach, J. B.; Wong, G. T. F.; Ungermann, C.; Onak, T. Inorg. Chem. 1971, 10, 2770. Leach, J. B.; Oates, G.; Handley, J. B.; Fung, A. P.; Onak, T. J. Chem. Soc., Dalton. Trans. 1977, 819. Dunks, G. B.; Hawthorne, M. F. Inorg. Chem. 1969, 8, 2667. Onak, T.; Dunks, G. B.; Spielman, F. R.; Gerhart, F. J.; Williams, R. E. J. Am. Chem. Soc. 1966, 88, 2061. Grimes, R. N.; Bramlett, C. L.; Vance, R. L. Inorg. Chem. 1968, 7, 1066. Wilczynski, R.; Sneddon, L. G. Inorg. Chem. 1981, 20, 3955. Wilczynski, R.; Sneddon, L. G. J. Am. Chem. Soc. 1980, 102, 2857. Onak, T.; Mattschei, P.; Groszek, E. J. Chem. Soc. A 1969, 1990. Cheung, C- CS; Beaudet, R. A. Inorg. Chem. 1971, 10, 1144. Onak, T.; Leach, J. B.; Anderson, S.; Frisch, M. J.; Marynick, D. J. Magn. Reson. 1976, 23, 237. Ditter, J. F.; Gerhart, F. J.; Williams, R. E. Adv. Chem. Ser. 1968, 72, 191–210. Onak, T. P.; Spielman, J. J. Magn. Reson. 1970, 3, 122. Onak, T.; Tseng, J.; Tran, D.; Correa, M.; Herrera, S.; Arias, J. Inorg. Chem. 1992, 31, 2161. Rietz, R. R.; Hawthorne, M. F. Inorg. Chem. 1974, 13, 755. Barker, G. K.; Green, M.; Stone, F. G. A.; Welch, A. J.; Onak, T. P.; Siwapinyoyos, G. J. Chem. Soc., Dalton. Trans. 1979, 1687. Duben, J.; Hermanek, S.; Sˇtı´br, B. J. Chem. Soc. Chem. Commun. 1978, 287. Onak, T. P.; Gerhart, F. J.; Williams, R. E. J. Am. Chem. Soc. 1963, 85, 3378. Onak, T. P.; Williams, R. E.; Weiss, H. G. J. Am. Chem. Soc. 1962, 84, 2830. Ledoux, W. A.; Grimes, R. N. J. Organomet. Chem. 1971, 28, 37. Fox, M. A.; Greatrex, R. J. Chem. Soc. Chem. Commun. 1995, 667. Fox, M. A.; Greatrex, R.; Hofmann, M.; Schleyer, P. v. R. J. Organomet. Chem. 2000, 614–615, 262. Onak, T.; Marynick, D.; Mattschei, P.; Dunks, G. Inorg. Chem. 1968, 7, 1754. Hosmane, N. S.; Grimes, R. N. Inorg. Chem. 1979, 18, 3294. Fessler, M. E.; Spencer, J. T.; Lomax, J. F.; Grimes, R. N. Inorg. Chem. 1988, 27, 3069. Swisher, R. G.; Sinn, E.; Grimes, R. N. Organometallics 1985, 4, 890. Maynard, R. B.; Borodinsky, L.; Grimes, R. N. Inorg. Synth. 1983, 22, 211. Boyter, H. A., Jr; Grimes, R. N. Inorg. Chem. 1988, 27, 3075. Boyter, H. A., Jr; Grimes, R. N. Inorg. Chem. 1988, 27, 3080. Spencer, J. T.; Grimes, R. N. Organometallics 1987, 6, 328. Hosmane, N. S.; Colacot, T. J.; Zhang, H.; Yang, J.; Maguire, J. A.; Wang, Y.; et al. Organometallics 1998, 17, 5294. Hosmane, N. S.; Maguire, J. A. ACS Symp. Ser. 2002, 827, 46–66. Hosmane, N. S.; Sirmokadam, N. N.; Mollenhauer, M. N. J. Organomet. Chem. 1985, 279, 359. Hosmane, N. S.; Islam, M. S.; Burns, E. G. Inorg. Chem. 1987, 26, 3236. Stockman, K. E. Ph.D. thesis, University of Virginia, 1995. Cendrowski-Guillame, S. M.; Spencer, J. T. Organometallics 1992, 11, 969. Hosmane, N. S.; Maguire, J. A. J. Organomet. Chem. 2000, 614–615, 10. Nainan, K. C.; Ryschewitsch, G. E. Inorg. Nucl. Chem. Lett. 1970, 6, 765. Ryschewitsch, G. E.; Miller, V. R. J. Am. Chem. Soc. 1975, 97, 6258. Adams, L.; Hosmane, S. N.; Eklund, J. E.; Wang, J.; Hosmane, N. S. J. Am. Chem. Soc. 2002, 124, 7292. Boer, F. P.; Streib, W. E.; Lipscomb, W. N. Inorg. Chem. 1964, 3, 1666. Pawley, G. S. Acta Cryst. 1966, 20, 631. Hosmane, N. S.; Siriwardane, U.; Zhang, G.; Zhu, H.; Maguire, J. A. J. Chem. Soc. Chem. Commun. 1989, 1128. Wang, Y.; Zhang, H.; Maguire, J. A.; Hosmane, N. S. Organometallics 1993, 12, 3781. Hosmane, N. S.; Saxena, A. K.; Barreto, R. D.; Zhang, H.; Maguire, J. A.; Jia, L.; et al. Organometallics 1993, 12, 3001.

4.6 6-Vertex closo clusters [167] [168] [169] [170] [171] [172] [173] [174] [175] [176] [177] [178] [179] [180] [181] [182] [183] [184] [185] [186] [187] [188] [189] [190] [191] [192] [193] [194] [195] [196] [197] [198] [199] [200] [201] [202] [203] [204] [205] [206] [207] [208] [209] [210] [211] [212] [213] [214] [215] [216] [217]

87

Siriwardane, U.; Islam, M. S.; West, T. A.; Hosmane, N. S.; Maguire, J. A.; Cowley, A. H. J. Am. Chem. Soc. 1987, 109, 4600. Hosmane, N. S.; Jia, L.; Wang, Y.; Saxena, A. K.; Zhang, H.; Maguire, J. A. Organometallics 1994, 13, 4113. Hosmane, N. S.; Maldar, N. M.; Potts, S. B.; Rankin, D. W. H.; Robertson, H. E. Inorg. Chem. 1986, 25, 1561. Onak, T.; Dunks, G. B. Inorg. Chem. 1966, 5, 439. Onak, T. Inorg. Chem. 1968, 7, 1043. Williams, R. E.; Onak, T. P. J. Am. Chem. Soc. 1964, 86, 3159. Akitt, J. W.; Savory, C. G. J. Magn. Reson. 1975, 17, 122. Venable, T. L.; Hutton, W. C.; Grimes, R. N. J. Am. Chem. Soc. 1984, 106, 29. Onak, T.; Inman, W.; Rosendo, H.; Distefano, E. W.; Nurse, J. J. Am. Chem. Soc. 1977, 99, 6488. Jotham, R. W.; McAvoy, J. S.; Reynolds, D. J. J. Chem. Soc., Dalton. Trans. 1972, 473. Byun, D.; Lee, S.; Hwang, S. D.; Hu, Y. F.; Bancroft, G. M.; Glass, J. A., Jr; et al. J. Electron. Spectros. Relat. Phenomena. 1994, 69, 111. Brown, C. L.; Gross, K. P.; Onak, T. J. Am. Chem. Soc. 1972, 94, 8055. Hitchcock, A. P.; Wen, A. T.; Lee, S.; Glass, J. A., Jr; Spencer, J. T.; Dowben, P. A. J. Phys. Chem. 1993, 97, 8171. Lee, S.; Dowben, P. A.; Wen, A. T.; Hitchcock, A. P.; Glass, J. A., Jr; Spencer, J. T. J. Vac. Sci. Technol. 1992, 10, 881. McKee, M. L. J. Phys. Chem. 1989, 93, 1265. Onak, T.; Lockman, B.; Haran, G. J. Chem. Soc., Dalton. Trans. 1973, 2115. Fessler, M. E.; Whelan, T.; Spencer, J. T.; Grimes, R. N. J. Am. Chem. Soc. 1987, 109, 7416. Barton, L.; Rush, P. K.; Zhu, T.; Nevels, P.; Owens, M. H. Inorg. Chem. 1989, 28, 381. Sneddon, L. G.; Beer, D. C.; Grimes, R. N. J. Am. Chem. Soc. 1973, 95, 6623. Sneddon, L. G.; Grimes, R. N. J. Am. Chem. Soc. 1972, 94, 7161. Savory, C. G.; Wallbridge, M. G. H. J. Chem. Soc., Dalton. Trans. 1974, 880. Davis, J. H., Jr; Grimes, R. N. Inorg. Chem. 1988, 27, 4213. Thompson, M. L.; Grimes, R. N. Inorg. Chem. 1972, 11, 1925. Tabereaux, A.; Grimes, R. N. Inorg. Chem. 1973, 12, 792. Savory, C. G.; Wallbridge, M. G. H. J. Chem. Soc., Dalton. Trans. 1972, 918. Magee, C. P.; Sneddon, L. G.; Beer, D. C.; Grimes, R. N. J. Organomet. Chem. 1975, 86, 159. Mirabelli, M. G. L.; Carroll, P. J.; Sneddon, L. G. J. Am. Chem. Soc. 1989, 111, 592. Barton, L.; Rush, P. K. Inorg. Chem. 1985, 24, 3413. Maguire, J. A.; Wang, J.-Q.; Zheng, C.; Li, C.; Hosmane, N. S. Inorg. Chim. Acta. 2002, 334, 91. Zhu, Y.; Maguire, J. A.; Hosmane, N. S. Inorg. Chem. Commun. 2003, 6, 1344. Spielman, J. R.; Warren, R.; Dunks, G. B.; Scott, J. E.; Onak, T. Inorg. Chem. 1968, 7, 216. Von Arx, U.; Pradan, P. R.; Keese, R. Chimia 1986, 40, 13. Hosmane, N. S.; Dehghan, M.; Davies, S. J. Am. Chem. Soc. 1984, 106, 6435. Spielman, J. R.; Dunks, G. B.; Warren, R. Inorg. Chem. 1969, 8, 2172. McAvoy, J. S.; Savory, C. G.; Wallbridge, M. G. H. J. Chem. Soc. A 1971, 3038. Lee, S.; Li, D.; Dowben, P. A.; Perkins, F. K.; Onellion, M.; Spencer, J. T. J. Am. Chem. Soc. 1991, 113, 8444. Beck, J. S.; Kahn, A. P.; Sneddon, L. G. Organometallics 1986, 5, 2552. Fung, A. P.; Distefano, E. W.; Fuller, K.; Siwapinyoyos, G.; Onak, T.; Williams, R. E. Inorg. Chem. 1979, 18, 372. Seklemian, H. V.; Williams, R. E. Inorg. Nucl. Chem. Lett. 1967, 3, 289. Grimes, R. N.; Rademaker, W. J.; Denniston, M. L.; Bryan, R. F.; Greene, P. T. J. Am. Chem. Soc. 1972, 94, 1865. Wermer, J. R.; Hosmane, N. S.; Alexander, J. J.; Siriwardane, U.; Shore, S. G. Inorg. Chem. 1986, 25, 4351. Mirabelli, M. G. L.; Sneddon, L. G. Organometallics 1986, 5, 1510. Maxwell, W. M.; Miller, V. R.; Grimes, R. N. Inorg. Chem. 1976, 15, 1343. Maxwell, W. M.; Miller, V. R.; Grimes, R. N. J. Am. Chem. Soc. 1974, 96, 7116. Grimes, R. N. Adv. Inorg. Chem. Radiochem. 1983, 26, 55. Grimes, R. N. Coord. Chem. Rev. 1995, 143, 71 [review]. Tomita, H.; Luu, H.; Onak, T. Inorg. Chem. 1991, 30, 812. Zhang, H.; Wang, Y.; Saxena, A. K.; Oki, A. R.; Maguire, J. A.; Hosmane, N. S. Organometallics 1993, 12, 3933. Abdou, Z. J.; Gomez, G.; Abdou, G.; Onak, T. Inorg. Chem. 1988, 27, 3679. Onak, T.; Tseng, J.; Diaz, M.; Tran, D.; Arias, J.; Herrera, S.; et al. Inorg. Chem. 1993, 32, 487. Wrackmeyer, B.; Schanz, H.-J.; Milius, W.; McCammon, C. Collect. Czech. Chem. Commun. 1999, 64, 977.

88 [218] [219] [220] [221] [222] [223] [224] [225] [226] [227] [228] [229] [230] [231] [232] [233] [234] [235] [236] [237] [238] [239] [240] [241] [242] [243] [244] [245] [246] [247] [248] [249] [250] [251] [252] [253] [254] [255] [256] [257] [258] [259] [260] [261] [262] [263] [264] [265] [266] [267] [268]

CHAPTER 4 Small carboranes: Four- to six-vertex clusters Wrackmeyer, B.; Schanz, H. J. Z. Naturforsch. B 2004, 59, 685. Hosmane, N. S.; Jia, L.; Zhang, H.; Bausch, J. W.; Prakash, G. K. S.; Williams, R. E.; et al. Inorg. Chem. 1991, 30, 3793. Wang, J.; Li, S.; Zheng, C.; Maguire, J. A.; Hosmane, N. S. Organometallics 2002, 21, 3314. Wang, J.; Li, S.; Zheng, C.; Maguire, J. A.; Sarkar, B.; Kaim, W.; et al. Organometallics 2003, 22, 4334. Ezhova, M. B.; Zhang, H.; Maguire, J. A.; Hosmane, N. S. J. Organomet. Chem. 1998, 550, 409. Hosmane, N. S.; Zhang, H.; Maguire, J. A.; Demissie, T.; Oki, A. R.; Saxena, A.; et al. Main Group Met. Chem. 2001, 24, 589. Kra¨mer, A.; Pritzkow, H.; Siebert, W. Angew. Chem. Int. Edit. Engl. 1990, 29, 292. McKee, M. L. J. Am. Chem. Soc. 1992, 114, 879. Harrison, B. C.; Solomon, I. J.; Hites, R. D.; Klein, M. J. J. Inorg. Nucl. Chem. 1960, 14, 195. Onak, T.; Gross, K.; Tse, J.; Howard, J. J. Chem. Soc., Dalton. Trans. 1973, 2633. Shapiro, I.; Williams, R. E.; Gibbins, S. G. J. Phys. Chem. 1961, 65, 1061. Hnyk, D.; Brain, P. T.; Rankin, D. W. H.; Robertson, H. E.; Greatrex, R.; Greenwood, N. N.; et al. Inorg. Chem. 1994, 33, 2572. DeKock, R. L.; Fehlner, T. P.; Housecroft, C. E.; Lubben, T. V.; Wade, K. Inorg. Chem. 1982, 21, 25. Bramlett, C. L.; Grimes, R. N. J. Am. Chem. Soc. 1966, 88, 4269. Fox, M. A.; Greatrex, R.; Nikrahi, A. J. Chem. Soc. Chem. Commun. 1996, 175. Wrackmeyer, B.; Schanz, H.-J.; Hofmann, M.; Schleyer, P. v. R. Eur. J. Inorg. Chem. 1998, 633. Howard, J. W.; Grimes, R. N. Inorg. Chem. 1972, 11, 263. Howard, J. W.; Grimes, R. N. J. Am. Chem. Soc. 1969, 91, 6499. Fessenbecker, A.; Hergel, A.; Hettrich, R.; Scha¨fer, V.; Siebert, W. Chem. Ber. 1993, 126, 2205. Bayer, M. J.; Siebert, W. Z. Naturforsch. B 2002, 57, 1125. Siebert, W.; Greiwe, P.; Beez, V.; Lo¨sslein, W.; Mu¨ller, T.; Hettrich, R.; et al. In: Davidson, M.; Hughes, A. K.; Marder, T. B.; Wade, K. editors, Contemporary Boron Chemistry, Cambridge: Royal Society of Chemistry; 2000. p. 345–352. Siebert, W.; Hettrich, R.; Pritzkow, H. Angew. Chem. Int. Edit. Engl. 1994, 33, 203. Wrackmeyer, B.; Schanz, H.-J. Main Group Met. Chem. 1998, 21, 29. Wrackmeyer, B.; Schanz, H.-J.; Hofmann, M.; Schleyer, P. v. R.; Boese, R. Eur. J. Inorg. Chem. 1999, 533. Deforth, T.; Pritzkow, H.; Siebert, W. Angew. Chem. Int. Edit. Engl. 1995, 34, 681. Kuhlmann, T.; Pritzkow, H.; Zenneck, U.; Siebert, W. Angew. Chem. Int. Edit. Engl. 1984, 23, 965. Zwecker, J.; Kuhlmann, T.; Pritzkow, H.; Siebert, W.; Zenneck, U. Organometallics 1988, 7, 2316. Zwecker, J.; Pritzkow, H.; Zenneck, U.; Siebert, W. Angew. Chem. Int. Edit. Engl. 1986, 25, 1099. Binger, P. Angew. Chem. Int. Edit. Engl. 1968, 7, 286. Ko¨ster, R.; Grassberger, M. A. Angew. Chem. Int. Edit. Engl. 1967, 6, 218 [review]. Binger, P. Tetrahedron Lett. 1966, 2675. Onak, T. P.; Wong, G. T. F. J. Am. Chem. Soc. 1970, 92, 5226. Enders, M.; Pritzkow, H.; Siebert, W. Angew. Chem. Int. Edit. Engl. 1992, 31, 606. Herberich, G. E.; Hessner, B. Chem. Ber. 1982, 115, 3115. Herberich, G. E.; Hessner, B. J. Organomet. Chem. 1978, 161, C36. Timms, P. L. J. Am. Chem. Soc. 1968, 90, 4585. Herberich, G. E.; Ohst, H.; Mayer, H. Angew. Chem. Int. Edit. Engl. 1984, 23, 969. Berger, H.; No¨th, H.; Wrackmeyer, B. Chem. Ber. 1979, 112, 2884. Killian, L.; Wrackmeyer, B. J. Organomet. Chem. 1977, 132, 213. Wrackmeyer, B.; Kehr, G. Polyhedron 1991, 10, 1497. Hauss, J.; Kra¨mer, A.; Pritzkow, H.; Siebert, W. Z. Naturforsch. B 1994, 49, 1677. Wrackmeyer, B.; Kehr, G. J. Organomet. Chem. 1995, 501, 87. Pasinski, J. P.; Beaudet, R. A. J. Chem. Phys. 1974, 61, 683. Haase, J. Z. Naturforsch. A 1973, 28, 785. Wrackmeyer, B.; Glo¨ckle, A. Z. Naturforsch. B 1996, 51, 859. Wrackmeyer, B.; Glo¨ckle, A. Main Group Met. Chem. 1997, 20, 181. Nie, Y.; Pritzkow, H.; Wadepohl, H.; Siebert, W. J. Organomet. Chem. 2005, 690, 4531. Wrackmeyer, B.; Glo¨ckle, A.; Kehr, G. Phosphorus Sulfur Silicon 1997, 131, 25. Nie, Y.; Pritzkow, H.; Siebert, W. Eur. J. Inorg. Chem. 2004, 2425. Heberhold, M.; Bertholdt, U.; Milius, W.; Glo¨ckle, A.; Wrackmeyer, B. J. Chem. Soc. Chem. Commun. 1996, 1219. Maddren, P. S.; Modinos, A.; Timms, P. L.; Woodward, P. J. Chem. Soc., Dalton. Trans. 1975, 1272.

4.6 6-Vertex closo clusters [269] [270] [271] [272] [273] [274] [275] [276] [277] [278] [279] [280] [281] [282] [283] [284] [285] [286] [287] [288] [289] [290] [291] [292] [293] [294] [295] [296] [297] [298] [299] [300] [301] [302] [303] [304] [305] [306] [307] [308] [309] [310] [311] [312] [313] [314] [315] [316] [317] [318]

89

Deobald, B.; Hauss, J.; Pritzkow, H.; Steiner, D.; Berndt, A.; Siebert, W. J. Organomet. Chem. 1994, 481, 205. Goswami, A.; Nie, Y.; Oeser, T.; Siebert, W. Eur. J. Inorg. Chem. 2006, 566. Greiwe, P.; Betha¨user, A.; Pritzkow, H.; Ku¨hler, R.; Jutzi, P.; Siebert, W. Eur. J. Inorg. Chem. 2000, 1927. Jutzi, P.; Seufert, A. J. Organomet. Chem. 1978, 161, C5. Jutzi, P.; Seufert, A.; Buchner, W. Chem. Ber. 1979, 112, 2488. Jutzi, P.; Seufert, A. Angew. Chem. Int. Edit. Engl. 1977, 16, 330. Kohl, F.; Jutzi, P. Angew. Chem. 1983, 95, 55 (German Ed.). Holtmann, U.; Jutzi, P.; Ku¨hler, T.; Neumann, B.; Stammler, H.-G. Organometallics 1999, 18, 5531. Jemmis, E. D.; Schleyer, P. V. R. J. Am. Chem. Soc. 1982, 104, 4781. Dohmeier, C.; Ko¨ppe, R.; Robl, C.; Schno¨ckel, H. J. Organomet. Chem. 1995, 487, 127. Timoshkin, A. Y.; Frenking, G. J. Am. Chem. Soc. 2002, 124, 7240. Macdonald, C. L. B.; Cowley, A. H. J. Am. Chem. Soc. 1999, 121, 12113. Onak, T.; Drake, R.; Dunks, G. B. J. Am. Chem. Soc. 1965, 87, 2505. Prince, S. R.; Schaeffer, R. J. Chem. Soc. Chem. Commun. 1968, 451. Gaines, D. F.; Ulman, J. J. Organomet. Chem. 1975, 93, 281. McNeill, E. A.; Scholer, F. R. Inorg. Chem. 1975, 14, 1081. McKown, G. L.; Don, B. P.; Beaudet, R. A.; Vergamini, P. J.; Jones, L. H. J. Am. Chem. Soc. 1976, 98, 6909. Onak, T.; Leach, J. B. J. Chem. Soc. Chem. Commun. 1971, 76. Lambiris, S. K.; Marynick, D. S.; Lipscomb, W. N. Inorg. Chem. 1978, 17, 3706. Jaballas, J.; Onak, T. J. Organomet. Chem. 1998, 550, 101. McKee, M. L. Inorg. Chem. 2001, 40, 5612. Shapiro, I.; Keilin, B.; Williams, R. E.; Good, C. D. J. Am. Chem. Soc. 1963, 85, 3167. Hosmane, N. S.; Barreto, R. D.; Tolle, M. A.; Alexander, J. J.; Quintana, W.; Siriwardane, U.; et al. Inorg. Chem. 1990, 29, 2698. Oliva, J. M.; Serrano-Andre´s, L. J. Comput. Chem. 2006, 27, 524. Kaszynski, P.; Pakhomov, S.; Young, V. G. Collect. Czech. Chem. Commun. 2002, 67, 1061. Onak, T.; Diaz, M.; Barfield, M. J. Am. Chem. Soc. 1995, 117, 1403. Maguire, J. A.; Lu, K.-J.; Thomas, C. J.; Gray, T. G.; Wang, Y.; Eintracht, J. F.; et al. Chem. Eur. J. 1997, 3, 1059. Gimarc, B. M.; Ott, J. J. Main Group Met. Chem. 1989, 12, 77. Hoffmann, R.; Lipscomb, W. N. Inorg. Chem. 1963, 2, 231. Lipscomb, W. N. Science 1966, 153, 373. Slutsky, V. G.; Tsyganov, S. A.; Severin, E. S.; Kazakov, O. D. Chemical Physics Rep. 1998, 17, 1525. Slutsky, V. G.; Tsyganov, S. A.; Severin, E. S.; Kazakov, O. D. Chemical Physics Rep. 1998, 17, 739. Slutsky, V. G.; Tsyganov, S. A.; Severin, E. S.; Kazakov, O. D. Chemical Physics Rep. 1999, 17, 2263. Slutsky, V. G.; Tsyganov, S. A.; Severin, E. S.; Kazakov, O. D.; Polenov, L. A. Chemical Physics Rep. 2000, 18, 1073. Slutsky, V. G.; Hofmann, M.; Schleyer, P. v. R. Mendeleev Commun. 1994, 12. McKee, M. L. J. Am. Chem. Soc. 1988, 110, 4208. Whelan, T.; Brint, P. J. Chem. Soc. Faraday Trans. 2 1985, 81, 267. Olsen, R. R.; Grimes, R. N. Inorg. Chem. 1971, 10, 1103. Reilly, T. J.; Burg, A. B. Inorg. Chem. 1973, 12, 1450. Spielman, J. R.; Warren, R. G.; Bergquist, D. A.; Allen, J. K.; Marynick, D.; Onak, T. Synth. React. Inorg. Met. Org. Chem. 1975, 5, 347. Takimoto, C.; Siwapinyoyos, G.; Fuller, K.; Fung, A. P.; Liauw, L.; Jarvis, W.; et al. Inorg. Chem. 1980, 19, 107. Beltram, G. A.; Fehlner, T. P. J. Am. Chem. Soc. 1979, 101, 6237. Nam, W.; Onak, T. Inorg. Chem. 1987, 26, 1581. Nam, W.; Onak, T. Inorg. Chem. 1987, 26, 48. Nie, Y.; Schwiegk, S.; Pritzkow, H.; Siebert, W. Eur. J. Inorg. Chem. 2004, 1630. van der Kerk, S. M.; Budzelaar, P. H. M.; van Eekeren, A. L. M.; van der Kerk, G. J. M. Polyhedron 1984, 3, 271. Michel, M.; Steiner, D.; Wocadlo, S.; Allwohn, J.; Stamatis, N.; Massa, W.; et al. Angew. Chem. Int. Edit. Engl. 1992, 31, 607. McKee, M. L. Inorg. Chem. 2000, 39, 4206. Schleyer, P. v. R.; Cremer, D.; Kraka, E.; Budzelaar, P. H. M. Angew. Chem. Int. Edit. 1984, 96, 374. Krogh-Jespersen, K.; Cremer, D.; Dill, J. D.; Pople, J. A.; Schleyer, P. v. R. J. Am. Chem. Soc. 1981, 103, 2589.

90 [319] [320] [321] [322] [323] [324] [325] [326] [327] [328] [329] [330] [331] [332] [333] [334] [335] [336] [337] [338] [339] [340] [341] [342] [343] [344] [345] [346] [347] [348] [349] [350] [351] [352] [353] [354] [355] [356] [357] [358] [359] [360] [361] [362] [363] [364] [365] [366] [367] [368] [369]

CHAPTER 4 Small carboranes: Four- to six-vertex clusters Zdetsis, A. D. J. Phys. Chem. A 2008, 112, 5712. Canac, Y.; Bertrand, G. Angew. Chem. Int. Edit. 2003, 42, 3578. Sayin, H.; McKee, M. L. Inorg. Chem. 2007, 46, 2883. Onak, T.; Wan, E. U S Nat. Tech. Inform. Serv, AD Rep. 1973 No. 755464 [Chem. Abstr. 166641v (1973)]. Dewar, M. J. S.; McKee, M. L. Inorg. Chem. 1980, 19, 2662. Ott, J. J.; Gimarc, B. M. J. Am. Chem. Soc. 1986, 108, 4303. McKee, M. L. J. Phys. Chem. 1991, 95, 9273. Macchi, P.; Proserpio, D. M.; Sironi, A. Organometallics 1997, 16, 2101. Ott, J. J.; Gimarc, B. M. J. Comput. Chem. 1986, 7, 673. Qiu, Y.-Q.; Chen, H.; Sun, S.-L.; Fan, H.-L.; Su, Z.-M. Chin. Sci. Bull. 2007, 52, 2326 [English]. Williams, R. E.; Bausch, J. W. Appl. Organomet. Chem. 2003, 17, 429. Cheung, C- CS; Beaudet, R. A.; Segal, J. A. J. Am. Chem. Soc. 1970, 92, 4158. Guest, M. F.; Hillier, I. H. Mol. Phys. 1973, 26, 435. Koetzle, T. F.; Lipscomb, W. N. Inorg. Chem. 1970, 9, 2743. Dewar, M. J. S.; Jie, C.; Zoebisch, E. G. Organometallics 1988, 7, 513. Dixon, D. A.; Kleier, D. A.; Halgren, T. A.; Hall, J. H., Jr; Lipscomb, W. N. J. Am. Chem. Soc. 1977, 99, 6226. Song, J.; Hall, M. B. Inorg. Chim. Acta. 1993, 213, 75. Stanko, V. I.; Semin, G. K.; Babushkina, T. A. Dokl. Akad. Nauk. SSSR 1975, 221, 1129 [Russian]. Hoffmann, R.; Lipscomb, W. N. J. Chem. Phys. 1962, 36, 3489. Jemmis, E. D. J. Am. Chem. Soc. 1982, 104, 7017. McKee, M. L. J. Am. Chem. Soc. 1991, 113, 9448. Gimarc, B. M.; Ott, J. J. Stud. Phys. Theor. Chem. (Graph. Theor. Topol. Chem.) 1987, 51, 285. McKee, M. L. Theochem 1988, 45, 191. Gimarc, B. M.; Ott, J. J. Inorg. Chem. 1986, 25, 83. Wales, D. J.; Stone, A. J. Inorg. Chem. 1987, 26, 3845. Bakardjiev, M.; Holub, J.; Hnyk, D.; Sˇtı´br, B. Chem. Eur. J. 2008, 14, 6529. McLaughlin, E.; Rozett, R. W. J. Phys. Chem. 1972, 76, 1860. Tian, S. X. J. Phys. Chem. A 2005, 109, 6580. Epstein, I. R.; Tossell, J. A.; Switkes, E.; Stevens, R. M.; Lipscomb, W. N. Inorg. Chem. 1971, 10, 171. Williams, R. E.; Ditter, J.; Oakes, J. D.; Gerhart, F. J. Bench Scale Production of C2B4H8. U.S. Clearinghouse Fed. Sci. Tech. Inform A D 662029 1967 [Chem. Abstr. 87331j]. Bausch, J. W.; Matoka, D. J.; Carroll, P. J.; Sneddon, L. G. J. Am. Chem. Soc. 1996, 118, 11423. Westerhausen, M.; Gu¨ckel, C.; Schneiderbauer, S.; No¨th, H.; Hosmane, N. S. Angew. Chem. Int. Edit. 2001, 40, 1902. Rockwell, J. J.; Herzog, A.; Peymann, T.; Knobler, C. B.; Hawthorne, M. F. Curr. Sci. 2000, 78, 405. Hosmane, N. S.; Grimes, R. N. Inorg. Chem. 1979, 18, 2886. Bluhm, M.; Pritzkow, H.; Siebert, W.; Grimes, R. N. Angew. Chem. Int. Edit. Engl. 2000, 39, 4562. Wang, J.; Zhu, Y.; Li, S.; Zheng, C.; Maguire, J. A.; Hosmane, N. S. J. Organomet. Chem. 2003, 680, 173. Hosmane, N. S.; Zhu, H.; McDonald, J. E.; Zhang, H.; Maguire, J. A.; Gray, T. G.; et al. Organometallics 1998, 17, 1426. Beck, J. S.; Sneddon, L. G. J. Am. Chem. Soc. 1988, 110, 3467. Spencer, J. T.; Pourian, M. R.; Butcher, R. J.; Sinn, E.; Grimes, R. N. Organometallics 1987, 6, 335. Whelan, T.; Spencer, J. T.; Pourian, M. R.; Grimes, R. N. Inorg. Chem. 1987, 26, 3116. Borelli, A. J.; Plotkin, J. S.; Sneddon, L. G. Inorg. Chem. 1982, 21, 1328. Plotkin, J. S.; Sneddon, L. G. Inorg. Chem. 1979, 18, 2165. Nie, Y.; Goswami, A.; Siebert, W. Z. Naturforsch. B 2005, 60b, 597. Barton, L.; Rush, P. K. Inorg. Chem. 1986, 25, 91. Yang, J.; Zheng, C.; Maguire, J. A.; Hosmane, N. S. Inorg. Chem. Commun. 2004, 7, 111. Zheng, C.; Hosmane, N. S.; Zhang, H.; Zhu, D. M.; Maguire, J. A. Internet. J. Chem. 1999, 2, 1. Weiss, R.; Grimes, R. N. J. Am. Chem. Soc. 1977, 99, 1036. Venable, T. L.; Brewer, C. T.; Grimes, R. N. Inorg. Chem. 1985, 24, 4751. Cendrowski-Guillame, S. M.; Spencer, J. T. Main Group Met. Chem. 1996, 19, 791. Grimes, R. N.; Rademaker, W. J. J. Am. Chem. Soc. 1969, 91, 6498. Beer, D. C.; Miller, V. R.; Sneddon, L. G.; Grimes, R. N.; Mathew, M.; Palenik, G. J. J. Am. Chem. Soc. 1973, 95, 3046.

4.6 6-Vertex closo clusters [370] [371] [372] [373] [374] [375] [376] [377] [378] [379] [380] [381] [382] [383] [384] [385] [386] [387] [388] [389] [390] [391] [392] [393] [394] [395] [396] [397] [398] [399] [400] [401] [402] [403] [404] [405] [406] [407]

91

Grimes, R. N.; Beer, D. C.; Sneddon, L. G.; Miller, V. R.; Weiss, R. Inorg. Chem. 1974, 13, 1138. Perkins, F. K.; Onellion, M.; Lee, S. W.; Li, D. Q.; Mazurowski, J.; Dowben, P. A. Appl. Phys. A Solids Surf. 1992, 54, 442. Marynick, D. S.; Lipscomb, W. N. J. Am. Chem. Soc. 1972, 94, 1748. Lipscomb, W. N. Accounts Chem. Res. 1973, 6, 257. Kleier, D. A.; Halgren, T. A.; Hall, J. H., Jr; Lipscomb, W. N. J. Chem. Phys. 1974, 61, 3905. Marynick, D. S.; Lipscomb, W. N. J. Am. Chem. Soc. 1972, 94, 8699. Cruickshank, D. W. J.; Chablo, A.; Eisenstein, M.; Reidy, P. L. Acta. Chem. Scand. A 1988, A42, 530. Maguire, J. A.; Ford, G. P.; Hosmane, N. S. Inorg. Chem. 1988, 27, 3354. Fox, M. A.; Hughes, A. K.; Johnson, A. L.; Paterson, M. A. J. J. Chem. Soc., Dalton. Trans. 2002, 2009. Franz, D. A.; Howard, J. W.; Grimes, R. N. J. Am. Chem. Soc. 1969, 91, 4010. Wrackmeyer, B. Z. Naturforsch. B 1982, 37, 412. Cowley, A. H.; Lomeli, V.; Voigt, A. J. Am. Chem. Soc. 1998, 120, 6401. Minyaev, R. M.; Minkin, V. I.; Gribanova, T. N.; Starikov, A. G. Russ. Chem. Bull. 2004, 53, 1159. Dewar, M. J. S.; McKee, M. L. J. Am. Chem. Soc. 1977, 99, 5231. McKee, M. L. J. Phys. Chem. 1989, 93, 3426. McKee, M. L.; Bu¨hl, M.; Charkin, O. P.; Schleyer, P. v. R. Inorg. Chem. 1993, 32, 4549. McKown, G. L.; Beaudet, R. A. Inorg. Chem. 1971, 10, 1350. Slutsky, V. G.; Tsyganov, S. A.; Severin, E. S.; Polenov, L. A. Propellants Explos Pyrotechnics 2005, 30, 303. Bragin, J.; Urevig, D. S.; Diem, M., Jr J. Raman Spectrosc. 1982, 12, 86. Mastryukov, V. S.; Dorofeeva, O. V.; Vilkov, L. V.; Golubinskii, A. V.; Zhigach, A. F.; Laptev, V.; et al. Zh. Strukt. Khim. 1975, 16, 171 [Russian]. Dixon, D. A. Inorg. Chem. 1980, 19, 593. Kalinin, V. N.; Mukoseev, Yu. K.; Petrunin, A. B.; Severin, E. S.; Slutskii, V. G.; Tereza, A. M.; et al. Fiz Goreniya Vzryva 1989, 25, 16 [Chem. Abstr. 111:80908a] [Russian]. Epstein, I.; Koetzle, T. F.; Stevens, R. M.; Lipscomb, W. N. J. Am. Chem. Soc. 1970, 92, 7019. Epstein, I. R.; Marynick, D. S.; Lipscomb, W. N. J. Am. Chem. Soc. 1973, 95, 1760. Semenov, S. G. Zh. Strukt. Khim. 1981, 22, 164 [Russian]. Mageswaran, R.; Fitzpatrick, N. J. J. Natl. Sci. Counc. Sri Lanka 1987, 15, 47 [Chem. Abstr. 112:7670h]. Gal’chenko, G. L.; Tamm, N.; Brykina, E. P.; Bekker, D. B.; Petrunin, A. B.; Zhigach, A. F. Khim 1985, 59, 2689 [Russian]. Jensen, J. O. Spectrochim Acta A Mol. Biomol. Spectros. 2004, 60, 57. Suponitsky, K. Y.; Timofeeva, T. V. Cent. Eur. J. Chem. 2003, 1, 1. Halgren, T. A.; Pepperberg, I. M.; Lipscomb, W. N. J. Am. Chem. Soc. 1975, 97, 1248. McKee, M. L. J. Am. Chem. Soc. 1988, 110, 5317. Fehlner, T. P.; Czech, P. T.; Fenske, R. F. Inorg. Chem. 1990, 29, 3103. Beaudet, R. A.; Poynter, R. L. J. Chem. Phys. 1970, 53, 1899. Wang, J.; Li, S.; Zheng, C.; Maguire, J. A.; Hosmane, N. S. Organometallics 2002, 21, 5149. Zhigach, A. F.; Petrunin, A. B.; Bochkarev, V. N.; Siryatskaya, V. N. Zh. Obshch. Khim. 1974, 44, 2787 [Russian]. Semenov, S. G. Vestn Leningr Univ Ser 4: Fiz Khim 1987, 97 [Chem. Abstr. 107:161980n] [Russian]. Oliva, J. M.; Allan, N. L.; Schleyer, P. v. R.; Vin˜as, C.; Teixidor, F. J. Am. Chem. Soc. 2005, 127, 13538. Gimarc, B. M.; Zhao, M. Inorg. Chem. 1996, 35, 825.