Polypyrazolates of the heavier group 13 and 14 elements: A review

Inorganica Chimica Acta 362 (2009) 4328–4339

Contents lists available at ScienceDirect

Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica

Review

Polypyrazolates of the heavier group 13 and 14 elements: A review Miguel-Ángel Muñoz-Hernández *, Virginia Montiel-Palma Centro de Investigaciones Químicas, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001, Col. Chamilpa, Cuernavaca, Morelos 62209, Mexico

a r t i c l e

i n f o

Article history: Received 25 June 2009 Accepted 26 June 2009 Available online 3 July 2009 Dedicated to Prof. Swiatoslaw Trofimenko. Keywords: Group 13 and 14 polypyrazolates Aluminum complexes Gallium complexes Silicon complexes Germanium complexes Tin complexes

a b s t r a c t Heavy group 13 and 14 analogs of the poly(pyrazolyl)borates are the subject of this review. Within these, the most extensive research has been performed on the study of polypyrazolylgallates Me2Ga(R2pz)2] (R = H or Me) and [MeGa(pz)3], and their complexes with transition metals. In this review, the common features shared with the boron analogs are presented and contrasted. Other recently reported series of 0 group 13 analogs are the alkali polypyrazolylaluminates Na[(R2pz)3 AlR3n ] with R = Me, tBu; R0 = M. The complexes with one or two pyrazolyl ligands on aluminum display anagostic Al–CH3  Na interactions, these interactions are persistent even if complexes were obtained from solutions with THF that normally coordinate alkali ions. The polypyrazolylsilane ligands Me2Si(R2pz)2 and MeSi(R2pz)3 with R = H, Me are remarkably easy to obtain and isolate, in contrast is the fact that the carbon analogs are much harder to obtain and isolate in reasonable yields. Therefore it is surprising that the chemistry of the former ligands is not as developed as could be anticipated. Nevertheless there are examples of the use of these silanes as ligands with the following transition metals: Cr, Mn, Cu, Zn, Sc and Zr. The heavier group 14 analogs with Ge and Sn display coordination patterns with alkali and alkaline ions that resemble those observed with the borates and aluminates. The formation of cationic bimetallic cages of the type [E2(R2pz)3]+ and neutral complexes [E2(R2pz)4] has also been observed that can be consider formal isomers of the alkenes. The use of these compounds as ligands has been recently reported. Ó 2009 Elsevier B.V. All rights reserved.

Miguel-Ángel Muñoz-Hernández was born in 1968 in Mexico City, Mexico. He was an undergraduate and graduate student at the Universidad Nacional Autónoma de México, UNAM until 1997 when he obtained his Ph.D. degree with Prof. Raymundo Cea-Olivares. Just after graduated he moved to the US as a postdoctoral associate at North Dakota State University and then at the University of Kentucky with Prof. David Atwood from 1997 to 1999. Later in the fall of 1999 joined the faculty of the Center for Chemistry Research at UAEM where he holds a permanent position since 2005. His research interests are focused on the synthesis of organometallic main group complexes as catalysts for different organic transformations and as single-source precursors for the deposition of materials.

Virginia Montiel-Palma was born in 1973 in Mexico City, Mexico, and studied chemistry at the Universidad Nacional Autónoma de México, UNAM. After obtaining her Ph.D. from the University of York, England, under the supervision of Professor Robin Perutz, she moved to the Laboratoire de Chimie de Coordination du CNRS in Toulouse, France for a postdoctoral position with Dr. Sylviane Sabo-Etienne working on transition metal sigma–borane complexes. In 2004 she joined the faculty of UAEM where she now holds a permanent position. Her research interests are in organometallic synthesis with a current focus on transition metal complexes of group 13 elements.

* Corresponding author. Tel./fax: +52 777 3297997. E-mail address: [email protected] (M.-Á. Muñoz-Hernández). 0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2009.06.051

M.-Á. Muñoz-Hernández, V. Montiel-Palma / Inorganica Chimica Acta 362 (2009) 4328–4339

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aluminum polypyrazolates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Galliumpolypyrazolates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Siliconpolypyrazolates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Germanium and tinpolypyrazolates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Contents 1. 2. 3. 4. 5. 6.

1. Introduction Almost half a century has passed since the introduction of the poly(pyrazolyl)borate ligands by Swiatoslaw Trofimenko, and the number of publications that deals with this ligand system and their complexes continues to grow to date. There are compelling reasons for their enthusiastic use by the scientific community, for instance they are easy to prepare, are robust due to the presence of strong B–H(C), and B–N bonds, are electronically and sterically tunable at convenience with variations of the 3, 4 and 5 positions of the pyrazol rings, and are isolobal with cyclopentadienyl ligands. Analogs of this ligand system can be envisioned by replacing the pyrazolyl rings by other substituents or the boron atom for other main group element [1]. The latter variation is the subject of this review and is focused on the polypyrazolates of the heavier 13 and 14 elements expecting to draw attention to this area of chemistry, which as it will be evident from this contribution is still very undeveloped. 2. Aluminumpolypyrazolates Storr and coworkers reported for the first time the synthesis and isolation of polypyrazolylaluminates [Na{Al(pz)2Me2}] (1) and [Na{Al(pz)4}] (2) by the nucleophilic addition of [Na(pz)] to AlMe3 in THF and subsequent alkane elimination of one and three equivalents of pzH, respectively [2]. The report however, did not give any spectroscopic data for complex 1 and only aluminum analysis information was presented for complex 2. The first authenticated and structurally characterized example of a polypyrazolylaluminate [Na{Al(Me2pz)2Me2}]1 (3) was obtained from the reaction of [Na(Me2pz)(thf)]4 with the neutral bimetallic pyrazolyl aluminum complex [(Me2pz)FlMe2]2 in THF (Eq. (1)), followed by recrystallization in hexane [3]. Interestingly the crystal structure of complex 3 depicted in Fig. 1 shows a solvent free coordination polymer with a helical structure (space group P212121) in which the polymer grows with Na–CH3–Al bridges (Na–C and Al–C bond distances of 2.707(1) and 1.985(1) Å, respectively), g1 Na–N interactions (2.319(1), 2.370(1), 2.757 Å) and g5-coordination of one pyrazolyl ligand to sodium (Na–C bond lengths: 2.732(1)– 2.889(1) Å; Na–N bond lengths: 2.642(1) and 2.757(1) Å). Me Me

N

CH3

More recently a series of polypyrazolylaluminates have been prepared with 3,5-di-tert-butylpyrazole ligands that confer high solubility in hydrocarbon solvents to the aluminates [4]. The approach to synthesize these complexes is similar to the one reported by Storr to prepare polypyrazolylgallates, which consists of the reaction of a group 13 organyl with an alkali pyrazolate to obtain a monopyrazolylmetalate and further sequential reactions of this metalate with pyrazol ligand to access polypyrazolylmetalates. Thus, reaction of AlMe3 with [Na(tBu2pz)] in toluene/THF afforded the monopyrazolylaluminate [{Na{Al(tBu2pz)Me3}}3(THF)2]1 (4). Further reaction of complex 4 with 1 and 2 equivalents of tBu2pzH afforded the bis and tris(pyrazolyl)aluminates [Me2Al(tBu2pz) (NaTHF)] (5) and [Na{Al(tBu2pz)3Me}(THF)] (6), respectively. A general reaction scheme for the synthesis of complexes 4–6 is given in Scheme 1 and the molecular or crystal structures are shown in Fig. 2. Complexes 3–6 show Al–CH3  Na interactions (Na–C bond lengths 1.973(2)–2.707(1) Å; Na–H bond lengths 2.37(4)– 2.76(4) Å) that may be described as consisting of an electrostatic dþ interaction between the dipoles Al—CHd 3    Na , and a donor– acceptor interaction between the C–H bonding density and the metal cation as postulated by Klinkhammer and others for organyls of the alkali metals with a strong electrostatic component [5]. Although Klinkhammer has named these interactions as electrostatic agostic, recently Brookhart et al. [6] proposed the use of the term anagostic coined by Lippard and coworkers [7] to describe these interactions that should be distinguished from the M–H–C agostic interactions, which are 3-center-2-electron in nature. Clearly from the crystal structures of 3–6 the anagostic Al– CH3  Na interactions play an important role in the grow of the polymers or the oligomers and their mere existence is surprising if one considers that the complexes were prepared in the presence of a r donor solvent (THF). An attempt to prepare complex 6 by a different route using the neutral complex [(g1:g1-tBu2pz)(lAlMe2)]2 [8] with two equivalents of tBupzH and tBu2pzNa afforded complex [Me2Al(tBu2pz)2Na(tBu2pzH)] (7) (Eq. (2)), which can be considered a reaction intermediate of 6 starting from complex 5 upon addition of tBu2pzH. The molecular structure of 7 depicted in Fig. 3 gives a clue of how 7 produces 6 upon dissolution at room temperature. The structure features an anagostic Al–Me  Na interaction (Na(1)–C(35) 3.103(2) Å; Na(1)–H(35B) 2.55(2) Å) that favors the activation of the methyl group bridging the aluminum and sodium atoms affording unsolvated 6 in solution (Scheme 2).

N

Al [Na(Me2pz)(thf)]4 + [(Me2pz)AlMe2]2

CH3

THF Me

Na

N

ð2Þ

N

Me 3

ð1Þ

4

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Fig. 1. X-ray structure of complex 3 showing a section of the polymer with thermal ellipsoids at the 50% probability level.

Me

3 AlMe3 + 3 [ Na ( tBu2pz) ]

Me

Me

THF/ toluene

Me

Me

Na

Al N

Al

Me

N N

Me Al

Na

O Na Me O Me

N N

N

t t

Bu

t

Bu

t

Bu

Bu

4 THF / toluene t

t

t

Bu

Na

N

N

Me O

Na

N

N

Al t

Bu

2 tBu2pzH

t t

O

Bu

Bu

N N

Me Al N N Na t t Bu Bu N N

Me

N N t

THF / toluene

Bu

N N Me Al

Bu2pzH

t

Me

t

Bu

O

Bu

6

Bu

5 Scheme 1. Obtention of polypyrazolylaluminates by stepwise addition of tBu2pH to 4.

3. Galliumpolypyrazolates There is no doubt that the most developed chemistry of polypyrazolates involving heavy Group 13 elements is the one of the gallates informed in a number of publications authored by Storr and coworkers. The general methods to obtain these gallates are depicted in Scheme 3. The monopyrazylylgallate (Scheme 3a) is prepared by nucleophilic addition of sodium pyrazolate to trimeth-

ylgallium, then reacted in situ with pyrazole to obtain the bis(pyrazolyl)gallate by alkane elimination (Scheme 3b) [2]. A similar reaction scheme fails to lead to the tris(pyrazolyl)gallate analog, which alternatively is prepared through a straightforward pathway via a metathesis reaction of dimethylgalliumchloride and sodium pyrazolate (Scheme 3c) [9]. Any of these gallates as such were not characterized structurally by X-ray diffraction, although they were isolated and authenticated by analytical methods.

M.-Á. Muñoz-Hernández, V. Montiel-Palma / Inorganica Chimica Acta 362 (2009) 4328–4339

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4 (a)

5 (b)

6 (c)

Fig. 2. (a) Perspective view of the trimeric subunit of the polymeric structure of [{Na{Al(tBu2pz)Me3}}3(THF)2]1 (4) showing thermal ellipsoids at the 50% probability level. Most hydrogen atoms and tert-butyl groups have been omitted for clarity. (b) Perspective view of [Na{Al(tBu2pz)2Me2}(THF)]2 (5) showing thermal ellipsoids at the 50% probability level, (most hydrogen atoms and tert-butyl groups have been removed for clarity). Symmetry transformations used to generate equivalent atoms: (#1) x, y + 1, z; (#2) x + 2, y + 2, z + 1. (c) Perspective view of [Na{Al(tBu2pz)3Me}(THF)] (6) showing thermal ellipsoids at the 50% probability level.

Complexes of the two gallate systems [Me2Ga(R2pz)2] (R = H or Me) and [MeGa(pz)3] with several transition metals are known, they are listed in Table 1. Examples of crystal structures with these two ligand systems are presented in Fig. 5. From Table 1 it is evident that the number of complexes which incorporate bis(pyrazolyl)gallates [1] is considerably higher than with tris(pyrazolyl) gallates. It seems that steric crowding imposed by pyrazole rings on gallium could prevent the formation of tris(pyrazolyl)gallates even when small alkyl groups such as Me are used. Thus, the attempted synthesis of complexes incorporating the gallate [MeGa(Me2pz)3] into the Mo and W organometallic complexes (M = Mo, W), results [M(g3-C4H7)(CO)2{MeGa(Me2pz)2}3] in the elimination of dimethylpyrazolate and addition of hydroxide ion on gallium affording complexes [M(g3-C4H7)(CO)2{MeGa(Me2pz)2}2OH] [20]. In these hydroxy complexes the OH ligand bridges Ga and the respective transition metal center (see Fig. 4).

The VT solution 1H NMR spectra of complexes 8 and 9 give evidence of a fluxional process of the Ga–(N–N)2–M (M = Rh, Ni) six-membered ring. In both these species at higher temperatures a single resonance is observed for the GaMe2 moiety but on cooling the solution this signal broadens, coalesces, and is replaced by two singlets at lower temperatures consistent with static boat conformations for the Ga–(N–N)2–M six-membered rings at these temperatures. The proposed mechanism involves a boat conformation of the Ga–(N–N)2–M ring which rapidly inverts in solution at high temperature giving an average planar conformation of the ring (Scheme 4). The molecular structures of both complexes 8 and 9 shows approximate Cs symmetry with a pseudo square planar geometry about Rh for 8 and a pseudo trigonal geometry about Ni for 9. The Ga atom display in both cases a distorted tetrahedral geometry. Complex 8 has been compared with the boron analog B(pz)4Rh(COD) [27] that shows two pyrazolyl rings coordinated

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Fig. 3. Perspective view of the monomeric structure of [Me2Al(tBu2pz)2Na(tBu2pzH)] (7) showing thermal ellipsoids at the 50% probability level. Hydrogen atoms (except H1 and H35B) and toluene have been omitted for clarity.

t

t

Bu

Bu t

N N Me Al

Na

N

N

Me O

Na

N

2 tBupzH

Me

-2 THF N

Al

N N t

t

O

t

Bu

Me

t

Bu

Bu

t

N N

2 Me Al t

Me N N

Na N t

Bu

Bu

t

Bu 2 THF N

t

Bu

-2 CH4

Bu H

Bu

N N

2 Me Al N N Na O t t Bu Bu N N t

7

Bu

t

Bu

Bu

6

5 Scheme 2. Isolation of the reaction intermediate 7 from 5 which gives 6 by activation of the methyl group bridging the aluminum and sodium atoms.

to Rh in a g1 fashion and the remaining two pyrazolyl rings occupying remaining tetrahedral positions about the B atom [14]. In solution all pz groups are equivalent for the boron complex, but at lower temperatures two distinct pz groups are resolved. Therefore it has been proposed that a rapid exchange of the four pz groups occur at room temperature via a pentacoordinate tris (pyrazolyl)borate rhodium intermediate. In the solid state the folding of the central six-membered ring in [Me2Ga(pz)2]Rh(COD) is greater than that observed in compound B(pz)4Rh(COD). Complex 10 [26] was prepared in the context of the isolation of related species to group 13 metallocenium cations [(g5-C5R5)2M]+ (M = Al, Ga, In), yet unknown in 1992 [28], with basis in the formal analogy of [HB(pz)3] and [MeGa(pz)3] to the cyclopentadienide ion (C5R5). As expected from this analogy, complex 10 and others related with polypyrazolylborates [(HBpz3)2M][MCl4] (M = Al, Ga) [29] and [(HBpz3)2In][InI4] [26] show the planes formed by the coordinated nitrogen atoms on each (HBpz3) or [MeGa(pz)3] ligand parallel. The reactivity of the polypyrazolylgallate complexes has also been investigated to some extent. For example, the reaction of the tris(pyrazolyl)gallate anion [Mo(CO)3{MeGa(pz)3}] towards MeI and EtBr resulted in the isolation of the r-alkyl complexes Mo(CO)3Me{MeGa(pz)3} and Mo(CO)3Et{MeGa(pz)3}, respectively. Complex [Mo(CO)3Me{MeGa(pz)3}] was identified in solution by

IR spectroscopy and demonstrated by IR and NMR spectroscopy to give rise to the g2-acyl complex [Mo(CO)2(g2-COMe){MeGa(pz)3}] after workup. Interestingly, a similar CO insertion occurs for the related tris(pyrazolyl)borate anion [Mo(CO)3{HB(pz)3}] with no detectable formation of the intermediate r-methyl complex [24]. 4. Siliconpolypyrazolates The chemistry of siliconpolypyrazolates RnSi(RR0 pz)3n is even more undeveloped than that of the polypyrazolylmethane ligand systems RnC(RR0 pz)3n, which is unforeseen if considered the better yields of the silicon ligands which are up to 90% in the case of the bis(pyrazolyl)silanes Me2Si(R2pz)2 (R = H 11, Me 12) [30] and the tris(pyrazolyl)silane MeSi(Me2pz)3 [31,32] (13) prepared by simple metathesis reaction of RnSiCl3n with ½MðR02 pzÞ (M = Li or Na) in a hydrocarbon solvent (Eq. (3))

Rn SiCl3n þ n½MðR02 pzÞ pantane

ƒƒƒ! RSiðR02 pzÞn nMCl

M ¼ Li; Na; R ¼ Me; R0 ¼ H; Me:

ð3Þ

The first complexes reported in the literature with these ligands pioneered by Rabinovich were the Cu(I) complexes

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R GaMe3 + [Na(R2pz)]

THF

Me3Ga N

N

Na

R

(a)

Gallate ligand system 

[Me2Ga(R2pz)2] R = H, Me, Cy

Me3Ga N

N

R

Na + R2pzH

-RH

N

Na

(b)

R

N

R R

Cl2GaMe + 3[Na(R2pz)]

THF -2NaCl

N N

R R

MeGa

N

Na N

N N R

(c)

R

R

R = H, Me Scheme 3. General reaction scheme for the synthesis of polypyrazolylgallates.

[Cu{MeSi(Me2pz)3}(NCMe)]+ and [Cu{MeSi(3-tBupz)3}(NCMe)]+ characterized by spectroscopic methods. Although neither the yield nor spectroscopic data for the ligands were given, the tris(pyrazoly) silane MeSi(3-tBupz)3 was characterized by singlecrystal X-ray diffraction [32]. Group 6 tricarbonyl complexes were prepared in 60–75% yield from the reactions of equimolar amounts of the labile nitrile adducts M(CO)3(NCMe)3 (M = Cr (14), Mo (15)) or W(CO)3(NCEt)3 (16) and 13 in acetonitrile or tetrahydrofuran according to Eq. (4) [31].

3RCN

¼ Cr ð14Þ;

M

Mnð15Þ; W ð16Þ; R ¼ Me; Et:

[2]

ð4Þ

These carbonyl complexes were characterized by analytical and spectroscopic methods including X-ray diffraction. The crystal structures are isostructural with differences in the packing of the molecules resulting in the case of 14 crystals in the monoclinic space group P21/m with two molecules in the unit cell and the isomorphous 15 and 16 analogs with crystals in the orthorhombic space group Pnma with four molecules per unit cell. The molecular structure of 14 is shown in Fig. 5. The main features of the crystal structures of 14–16 are the octahedral environment about the metal center with the silane ligand bound to the metal atom in a tridentate face capping fashion, and the large cone angle of the ligand. In comparison, the ligand cone angle in 14–16 is moderately larger than that of HB(Me2pz)3 but similar to well-known ‘‘tetrahedral enforcer” ligands such as HB(3-tBupz)3 [33]. A synthetic strategy to obtain complexes that incorporate lantanides and polypyrazolylsilanes has been explored using complexes of the type [M(CH2SiMe3)3(THF)2] and 13. However the isolation of a sole product was not possible. A more convenient synthetic route to prepare group 3 complexes is the one depicted in Eq. (5), reacting the Sc or Y chlorides MCl3(THF)3 and 13 in toluene solutions to yield [M{MeSi(Me2pz)3}Cl3] M = Sc (17), Y (18). Characterization was accomplished for 17 and 18 using analytical and spectroscopic methods. 18 was structurally characterized by single-crystal X-ray diffraction [34], its molecular structure is in

NEtþ 4

[Mo(g3-C3H5)(CO)2{MeGa(pz)3}2] [M(CO)2NO{MeGa(pz)3}2] M = Mo, W [Mn(CO)3{MeGa(pz)3}2] [Ni{MeGa(pz)3}2] [Rh2(l-CO)3{MeGa(pz)3}2] [Mo(CO)3L{MeGa(pz)3}2] L = SnMe3, SnPh3, SnMe2Cl A[Mo(CO)3{MeGa(pz)3}] A = Na+, NEtþ 4, HAsPh3+ [Mo(CO)3L{MeGa(pz)3}] L = H, (g2-COMe), Et [Re(CO)3{MeGa(pz)3}] [In{MeGa(pz)3}2][InI4]

[MeGa(pz)3]

MðCOÞ3 ðNCRÞ3 þ MeSiðMe2 pzÞ3  ƒƒ! MeSiðMe2 pzÞ3 MðCOÞ3

[Na{Me2Ga(pz)2}] [A{Me2Ga(pz)2}] A = [M{Me2Ga(pz)2}2] M = Ni, Cu, Co [M{Me2Ga(Me2pz)2}2] M = Cu, Co [M(g3-C4H7)(CO)2{MeGa(Me2pz)2}2OH] M = Mo, W [Mo(g3-C3H5)(CO)2{MeGa(Me2pz)2}2OH] [M(g3-C3H5){Me2Ga(Me2pz)2}] M = Ni, Pd [Mn(NC3H4)(CO)3{Me2Ga(Me2pz)2}2] [(NEt4){M(CO)4{Me2Ga(Me2pz)2}2}] M = Mo, W [(NEt4){M(NC3H4)(CO)3{Me2Ga(Me2pz)2}2}] M = Mo, W [M(g3C3H5)(NC3H4)(CO)2{Me2Ga(Me2pz)2}2] M = Mo, W [M(g3C7H7)(NC3H4)(CO)2{Me2Ga(Me2pz)2}2] M = Mo, W [Rh(L)2{Me2Ga(pz)2}] L = CO, PPh3, COD [Rh(CO)(PPh3){Me2Ga(pz)2}] [Rh(CO)(PPh3){Me2Ga(Me2pz)2}] [PtMeL{Me2Ga(pz)2}] L = PPh3, CO, (PhC)2, (MeOCOC)2 [Ir(COD){Me2Ga(pz)2}] [Ir(CO)2{Me2Ga(pz)2}] [Ir(CO)(PPh3){Me2Ga(pz)2}] [Ir(CO)(PPh3){Me2Ga(Me2pz)2}] [Re(CO)3(PPh3){Me2Ga(pz)2}] [MgCl(THF)2{Cy2Ga(pz)2}]

R

N N Me2Ga

THF

Reference

NMeþ 4,

R R

Complex

[2] [2,10,11] [2] [13] [13] [12] [14] [13] [13] [13]

[13]

[14] [14] [15] [16] [17] [17] [17] [17] [18] [19] [13] [13] [13] [20] [21] [22] [23] [23] [24] [25]

general very similar to the group 6 carbonyl complexes discussed above, see Fig. 5.

MCl3 ðTHFÞ3 þ MeSiðMe2 pzÞ3 Toluene

ƒƒƒ! MfMeSiðMe2 pzÞ3 gCl3

M ¼ Sc ð17Þ; ð18Þ:

ð5Þ

Interestingly, the reaction of 13 with the dimeric zirconium imido complex [Zr2(l-NAr)2Cl4(THF)4] (Ar = 2,6-C6H3iPr2) gives via Si–N activation the novel group 4 heteroscorpionate complex [Zr{(Me2pz)2Si(Me)NAr}Cl3] (19), Eq. (6) [35]. Me Si Cl THF Zr THF

Ar N

Cl Zr

N Cl

Ar

Cl

Ar = 2,6-C6H3iPr

N

N

THF

MeSi(Me2pz)3

THF

CH2Cl2

N N

Cl Zr

ð6Þ

Cl

N Cl (19)

Complex 19 was characterized by analytical and spectroscopic methods including single-crystal X-ray diffraction (Fig. 6). The

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Fig. 4. Representative molecular structures from X-ray diffraction of transition metal complexes which incorporate bis(pyrazolyl)gallates and tris(pyrazolyl)gallates. (a) [Rh(COD){Me2Ga(pz)2}] (8) [14]. (b) [Ni(g3-C3H5){Me2Ga(Me2pz)2}] (9) [12]. (c) [In{MeGa(pz)3}2][InI4] (10) [26].

Ga

N N

N N

M

N

N M

Ga

Ga

N N

N N

M

N N

Scheme 4. Fluxional process of the six membered ring Ga–(N–N)2–M (M = Rh, Ni) in complexes 8 and 9.

molecular structure shows the ligand bound to zirconium in a facial fashion in a very distorted octahedral geometry. It was found that 19 by activation with MAO is very highly active as olefin polymerization catalyst and forms polyethylene with a productivity in excess of 3120 kg(PE) mol1 h1 bar1 (Mw = 432,500; Mw/Mn = 2.5) accompanied by a very rapid exotherm. The productivity of 19 is better to other related heteroscorpionates and comparable to MAO-activated tris(pyrazolyl)borates [36]. The synthesis and chemistry of complexes with bis(pyrzolyl)silane ligands is very undeveloped compared to related tris(pyr-

azolyl)silanes. So far the only report is related to the isolation of 11 and 12 from the reaction of Me2SiCl2 and the alkali metal pyrazolates M(R2pz) (M = Li or Na), see Eq. (3) [37]. Both ligand systems were used to prepare a series of zinc halide complexes LZnX2 (L = 11, 12; X = Cl, Br, I), all of which have been characterized by a combination of analytical and spectroscopic techniques and, in the case of [(Me2Si(Me2pz)2)ZnI2] (20), also by single-crystal Xray diffraction. The molecular structure of 20 depicted in Fig. 7 shows a six membered inorganic carbon-free ring with an envelope conformation in which the silicon moiety Me2Si is above the plane formed by the four nitrogen atoms and ZnI2 moiety.

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Fig. 5. Molecular structure from X-ray diffraction of [MeSi(Me2pz)3Cr(CO)3] (14). Fig. 7. Molecular structure from X-ray diffraction of [(Me2Si(Me2pz)2)ZnI2] (20) without hydrogen atoms for clarity.

5. Germanium and tinpolypyrazolates

Fig. 6. Molecular structure from X-ray diffraction of [Zr{(Me2pz)2Si(Me)NAr}Cl3] (19) without hydrogen atoms for clarity.

GeCl2 ·dioxane SnCl2

THF 3Napz

THF 3Napz

Alkali and alkaline germanium and tin(II) tris-pyrazolates have been prepared by reaction of the pyrazolates E(R2pzn) (E = Na, Ba; R = H, Me) with the group 14 dichlorides MCl2 (M = Ge, Sn) in THF in the appropriate molar ratio as depicted in Scheme 5 [38,39]. For [(THF)3Na[l-pz)3Ge] (21) (Eq (a), Scheme 5), the [pz3Ge] anion acts as a tridentate ligand towards the sodium cation as observed for similar group 13 tris-pyrazolates. Interestingly, in the case of the tin complex [(THF)2(pzH)Na{l-pz)2Sn(pz)]2 (22) (Eq (b), Scheme 5), the pyrazolate rings turn around the Sn–N bond to coordinate two different sodium ions in a dimmer. The resulting complex 22 contains a central 12-membered ring as revealed by Xray crystallography, Fig. 8 [39]. Compared to 21, complex 22 exhibits a more pronounced s-character of the lone pair, reflected in more acute N–E–N angles (N–Ge–N average 93.7° and N–Sn–N average 87.3°). Therefore repulsions of the lone pairs of the nitrogen atoms are increased in 22 causing the pyrazolyl rings to twist to avoid repulsion. The solid state structure of Ba[(Me2pz)3Ge]21/2dioxane (23) (Eq (c), Scheme 5) obtained by means of X-ray crystallography (Fig. 9), seems to be partially intact in the gas phase (peak at m/z 854). The molecular structure of 23 shows the barium atom encapsulated by two (Me2pz)3Ge ligands each binding the metal center via N(r) donation of two pyrazoly rings and one side-on in a p fashion. Complex 23 was the first example of p bonding between a pyrazol ligand and an alkaline ion [38].

[(THF)3Na[µ-pz)3Ge] + 2NaCl

[(THF)2(pzH)Na{µ-pz)2Sn(pz)]2+ 2NaCl

3Ba(Me2pz)2 + GeCl2 ·dioxane

THF

Ba[(Me2pz)3Ge]2 ·1/2dioxane + 2BaCl 2

Scheme 5. Preparation of sodium and barium germanium and tin tris-pyrazolates.

(a) (b) (c)

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Fig. 8. Molecular structure from X-ray diffraction of [(THF)2(pzH)Na{l-pz)2Sn(pz)]2 (22) without hydrogen atoms for clarity.

Fig. 9. Molecular structure from X-ray diffraction of Ba[(Me2pz)3Ge]21/2dioxane (23) without dioxane and hydrogen atoms for clarity.

Cationic cage complexes [Ge2(Me2pz)3][GeC13]1/2dioxane (24) and [Sn2(Me2pz3)][SnCl3] (25) are obtained by the reaction of GeCl21/2dioxane or SnCl2 with Ba(Me2pz)2 in THF in the molar ration 1:2. In solution the 1H NMR of 24 and 25 show one resonance for the methyl groups attached to the pyrazolyl ligands, and consistently, the 119Sn NMR of 25 shows two distinct resonances at d 337.0 and 497.8 ppm as expected for the two different tin environments [38].

The structural characterization of 24 and 25 by X-ray diffraction reveal single charged cations [E2(Me2pz)3]+ with nearly local D3h symmetry and [ECl3] anions with C3v symmetry, the structure of the germanium ion pair is giving in Fig. 10. The [E2(Me2pz)3]+ cations can be described for both complexes as a ‘‘paddle-wheel” with a Ge(II) or Sn(II) shaft, due to the exo-bidentate coordination of the three Me2pz ligands bridging the two metallic centers. The coordination polyhedron of the cations is better described as a dicapped

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Fig. 10. Molecular structure from X-ray diffraction of [Ge2(Me2pz)3][GeC13]1/2dioxane (24) without hydrogen atoms and dioxane for clarity.

H

E

H

E

H

E

H

H

H

A H E

E

H H

H

D

E

E

R

R F

E C

H H

H E

E E

n

R

E

B

H H

E

H

H H

H

H

R R E

E

R

R G

M = In, Tl and n = 0 M =Snandn=1 Fig. 11. Possible isomers A–E for the model compounds E2H4 (E = Group 14 element). Triply R-bridged derivatives (F), and the quadruply R-bridged derivative (G). Fig. 12. Molecular structure from X-ray diffraction of [{Sn(CF3)2pz)2}2] (26) without hydrogen and fluor atoms for clarity.

trigonal prism. The six nitrogen atoms occupy the corners of the prism, while the two metal centers are located above the triangular faces of the prism [38]. Bis(stannylenes) or distannenes ‘‘turned inside out” [{Sn(R2pz)2}2] with R = CF3 (26), CMe3 (27) have been prepared by reaction of Lapert’s stannylene [Sn{N(SiMe3)2}2] and steric demanding substituted pyrazoles, R2pzH (R = CF3, tBu) in hexane, Eq. (7) [40].



SnfNðSiMe3 Þ2 g2 2R2 pzH

t

ð26Þ; CMe3 ð27Þ:

ð7Þ

Ge

Bu tBu

Complexes 26 and 27 are formal analogs of the quadruply bridged olefin isomer G in Fig. 11. The other possible isomers A– F in Fig. 11 represent the homo- and heteroleptic heavier Group 14 analogs of alkenes calculated [41] and experimentally obtained by a number of research groups [42–45]. The molecular structure of 26 shown in Fig. 12 features a Sn  Sn intermolecular distance of 3.851 Å that strongly suggests a nonbonding interaction, though the distance is shorter than the

Bu

Bu

N N N N

t

Bu

N N

N N t

3"[Ge(tBu2pz)2]" (28)

-6(Me3Si)2NH t

t

Bu



ƒƒƒƒƒƒƒ! 1=2½fSnR2 pzÞ2 g R ¼ CF3

2ðMe3 SiÞ2 NH

3[Ge{N(SiMe3)2}2] + 6H(tBu2pz)

Ge Bu

Ge

t

t

Bu

tBu

tBu

N N

N N

t

Bu

tBu

28 Scheme 6. Preparation of complex 28 and its structure.

sum of the van der Waals radii of tin (4.4 Å). The solution 119Sn NMR of 26 shows a single resonance at d 720 ppm with no sizable tin isotopes coupling detected, confirming the absence of any significant Sn  Sn interaction.

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[{Ge(tBu2pz)2}2]

[Ge(µ2-tBu2pz)3Ge] + [tBu2pz] t

1/2[{Ge(tBu2pz)2}2] + [ Bu2pz]

(a)

[Ge(tBu2pz)3]

(b)

Scheme 7. Proposed exchange of pyrazolyl ligands of 28 in solution.

2"[Ge(tBu2pz)2]" 28

[Sn(tBu2pz)2]2 26

CF3SO3H

[Ge2(tBu2pz)3][CF3SO3]

Et2O

(a)

29

H(tBu2pz) CF3SO3H

[Sn2(tBu2pz)3][CF3SO3]

Et2O

(b)

30

H(tBu2pz)

Scheme 8. Preparation of cationic cages 29+ and 30+.

The attempted preparation of a digermene via a transamination reaction used to obtain distannenes 26 and 27 (Eq. (7)), gives in the solid sate the singly charged dinuclear cage cation [Ge(l2-tBu2pz)3Ge]+ and a tris(pyrazolyl)germanide counter anion [Ge(tBu2pz)3] (28), Scheme 6. It has been proposed that the resulting structural motif observed for 28 can formally be rationalized as consisting of three neutral germylene fragments of the composition ‘‘[Ge(tBu2pz)2]” (Figure in Scheme 6). A combination of high steric demand of the pyrazolyl ligands bound to the Ge(II) atom in cationic 28+, and the smaller ionic radius of the Ge(II) ion compared to Sn(II) ion, seems to cause the formation of smaller aggregates that contain a Ge(II) atom coordinated to three pyrazolyl ligands. Consequently, solution 1H NMR in d6-benzene or d8-toluene suggests a rapid exchange of pyrazolyl ligands in 28, Scheme 7 shows the plausible implicated equilibria. The germanium and tin complexes 28 and 26 have been used to prepare cationic cage species [E(l2-tBu2pz)3E]+[OTf] (E = Ge (29), Sn (30)) by reaction with triflic acid in diethyl ether according to Scheme 8. The X-ray structural characterization of 29 and 30 reveal cage germanium and tin cations with an overall structure very similar to 24 and 25 (the structure of 30 is shown in Fig. 13). As a result of steric interactions between tert-butyl groups of pyrazol ligands in 29 and 30, the pyrazol rings are tilted in comparisont to 24 and 25 with mean dihedral angles E–N–N–E of 33.6° for 29 and

Fig. 13. Molecular structure from X-ray diffraction of [Sn(l2-tBu2pz)3E]+[OTf] (30) without hydrogen atoms for clarity.

52.5° for 30; hence the intramolecular Ge  Ge distance of 3.549 Å in 29+ is shorter than in 24+ (3.63 Å). A similar trend is observed for the cationic part 30+; the intramolecular Sn  Sn distance of 3.843 Å is shorter than those observed for 25 (3.994 and 4.009 Å). The triflate anion for the tin complex 30 is in close proximity to the tin cage cation (O  Sn contacts 2.57 and 2.46 Å), which somehow precludes a closer Sn  Sn contact as may be expected from the Sn–N–N–Sn diehedral angle. Based on quantum chemical calculations at the DFT level of theory it was confirmed that the intramolecular E  E distances of 29+ and 30+ are affected by the tilting of the pyrazolyl ligands. In the context of the use of the cage tin and germanium complexes previously described as linkers to redox-active transition metals, it was investigated the reaction of the protic molybdenum hydride [Mo(H)Cp(CO)3] with the germanium(II) complex [(CF3)2pz}2Ge]2 (31) in diethylether which afforded CF3

2

CF3

F3C

CF3

Ge

CF3

N

N

N

N

N

N

N

N

CF3

Ge

31

CF3

CF3

(H) 2 Mo

O 3) Cp(C

OC OC

Mo

N

N

N

N

Ge

CO

CF3

CF3

H

Et 2O

CF3

32

2M

CF3

o(H) C Tolu p(CO ene 3 ) -(CF 3 )2 pzH

CO OC OC

Mo

Mo CO N

N

Ge

CO F3C CO Ge CF3

H

N

N

CF3

N N

CF3

33

CF3

Scheme 9. Preparation of complexes 32 and 33 from 31 by appropriate use of solvent.

CF3

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[Cp(CO)3Mo{HGe{(CF3)2pz}2] (32). Interestingly the same reaction in toluene gave the bis-metal substituted digermane complex [{Cp(CO)3Mo}(H)- {(CF3)2pz}Ge–Ge{(CF3)2pz}2{MoCp(CO)3}] (33), Scheme 9. A divalent germylene derived from 31 inserts into the MoH r-bond of the Mo hydride to afford 32. Complex 33 has been proposed as the product of the elimination of pyrazol (CF3)2pzH upon addition of the Mo hydride to 31 as the first step, the second step involves subsequent addition of one more molar equivalent of Mo hydride to the product of the presiding reaction that triggers a intramolecular reorganization with formation of a Ge–Ge bond to give 33. Both complexes have been spectroscopically characterized and by single-crystal X ray diffraction [46]. 6. Summary A limited number of complexes in which the boron atom has been replaced for aluminum have been prepared of general formulae [Na{Al(R2pz)nMe3n}] (R = Me, tBu). As the number of pyrazolyl rings decrease in these complexes there is a tendency to olygomerization. The olygomers grow at the expense of pyrazolyl binding to sodium ions with different hapticity and Al–CH3  Na interactions. Polypyrazolylgallates and their transition metal complexes are clearly the most extensive family of group 13 and 14 heavy analogs of the poly(pyrazolyl)borates developed so far. The complexes with these gallates include Ni, Cu, Co, Mo, W, Pt, Pd, Rh, Ir, Re and I. There is a tendency for this complexes to not accept bulky tris(pyrazolyl)gallates such as [MeGa(Me2pz)3]. Thus, the attempted preparation of complexes [M(g3-C4H7)(CO)2{MeGa(Me2pz)2}3] (M = Mo, W), results in the elimination of dimethylpyrazolate and addition of hydroxide ion on gallium affording complexes [M(g3-C4H7)(CO)2{MeGa(Me2pz)2}2OH]. The development of the coordination chemistry of the neutral polypyrazolylsilane ligands RnSi(RR0 pz)3n has been much more slow if compared to that of the polypyrazolylmethane ligands RnC(RR0 pz)3n, even though the straightforward preparation and high yields of the formers compared to the carbon counterparts. The germanium and tin analogs include monomers such as [(THF)3Na[l-pz)3Ge] (21), dimmers such as [(THF)2(pzH)Na{lpz)2Sn(pz)]2 (22), and cage cations of general formulae [E(l2-tBu2pz)3E]+ with E = Ge, Sn. The bis(stannylenes) [{Sn(R2pz)2}2] with R = CF3 (26), CMe3 (27) are formal analogs of olefins with a configuration ‘‘turned inside out”, and serve as reagents to prepare cationic tin cages[Sn(l2-tBu2pz)3- Sn]+ [OTf]. Acknowledgments The authors gratefully acknowledge CONACyT (Grant U154151) for financial support during the preparation of this manuscript.

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[28]

[29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41]

[42]

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