Recent topics of the metallacycles composed of the heavier group 14 elements

Accepted Manuscript Digest paper Recent Topics of the Metallacycles Composed of the Heavier Group 14 Elements Makoto Tanabe PII: DOI: Reference:

S0040-4039(14)00789-8 http://dx.doi.org/10.1016/j.tetlet.2014.05.012 TETL 44602

To appear in:

Tetrahedron Letters

Received Date: Revised Date: Accepted Date:

20 February 2014 26 April 2014 1 May 2014

Please cite this article as: Tanabe, M., Recent Topics of the Metallacycles Composed of the Heavier Group 14 Elements, Tetrahedron Letters (2014), doi: http://dx.doi.org/10.1016/j.tetlet.2014.05.012

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Tetrahedron Digests

Recent Topics of the Metallacycles Composed of

the Heavier Group 14 Elements Makoto Tanabe

Chemical Resources Laboratory, Tokyo Institute of Technology, 4259-R1-3 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan

Received, 20 February 2014 To whom correspondence should be addressed. E-mail: [email protected]

Abstract This article reviews recent advance of metallacycles with chelating Si-, Ge-, and Sn-ligands. Dehydrogenative bond-forming reactions of organosilanes, -germanes, and -stannanes promoted by Pd and Pt complexes afford four- and five-membered metallacycles composed of heavier group 14 elements. It has a couple of advantage in easier preparation of the starting compounds and reaction procedure than the common metathesis reactions of dianions with transition metal dihalide complexes. These

metallacycles are regarded

as possible

intermediates

in catalytic dehydrocoupling

polymerizations or as convenient precursors to form the discrete oligomers.

Keywords: Metallacycles, Dehydrocoupling reactions, Cyclic polymers, Group 14 elements,

1. Transition metals in cyclic oligosilanes

Polymers composed of heavier group 14 elements have been known as the σ-conjugated polymers whose backbone containes E–E (E = Si, Ge, Sn) bonds.

The inherent σ-electron

delocalization along the polymer chain and the relatively small band gap provide the polymers unique optical, electronic, and chemical properties, which distinguishes these polymers from saturated carbon polymers.1 Dehydrocoupling polymerization catalyzed by transition metal complexes has attracted attention as a convenient method for preparation of these polymers. Since the first report of the dehydrocoupling polymerization of primary silanes,2 this catalytic polymerization has been intensively studied,3 and a number of early transition metal catalysts such as those containing Ti and Zr have been investigated. Dehydrocoupling polymerizations of primary and secondary germanes4 and secondary stannanes5 were also achieved in a manner analogous to the primary silanes. Early transition metal catalysts provide linear (acyclic) polymers as the major products and cyclic polysilanes with lower molecular weight as the by-products. The extended polymerization time or the elevated temperature leads to formation of thermodynamically preferred cyclic oligomers. Scheme 1 exhibits a plausible pathway for formation of cyclic polysilanes via a back-biting reaction of a terminal M–Si bond of the polymer with the H–Si bond at the other end group.3a,6

H R M Si

H

Si H

H R M Si H R

H

Si H R

Si Si R

R

H

+ M H

Scheme 1

Recently, Gilorami reported that titanium(II) alkyl complex with chelating phosphine ligands [TiMe2(dmpe)2] (dmpe = 1,2-bis(dimethylphosphino)ethane) was a catalyst for dehydrocoupling oligomerizations of phenylsilane and that titanium(0) trisilane complex with two Ti–H–Si agostic interaction, [Ti(Si3H5Ph3)(dmpe)2] (1, eq 1), was isolated at low temperature (–20 °C).7 This result

2

clearly showed that the metallacycles with chelating oligosilanyl ligands have been suggested as one of possible intermediates for the catalytic dehydrocoupling condensations.

Cyclic polysilanes possess some chemical properties which resemble those of the well-known aromatic hydrocarbons. For example, cyclic polysilanes showed strong absorption bands in the UV/vis spectra, formed charge-transfer complexes with a π-acceptor compound, tetracyanoethylene, and stabilized anion radicals upon reduction, in which the unpaired electron was delocalized over the polysilane ring.8 Incorporation of transition metals into cyclic oligosilanes is expected to exhibit unique electronic properties different from normal oligosilane rings. The first example of transition metal oligosilane complexes was reported by Schram in 1976.9 The octaphenyl-cyclo-tetrasilane (SiPh2)4 reacted with lithium to produce a 1,4-dilithio(octaphenyl)tetrasilane Li[SiPh2]4Li and subsequent addition of HCl gave 1,4-dihydrido(octaphenyl)tetrasilane H(SiPh2)4H. Double oxidative addition of the two Si–H bonds to a Pt(0) complex [Pt(η2-C2H4)(PPh3)2] afforded a five-membered tetrasilaplatinacyclopentane [Pt(SiPh2)4(PPh3)2] (2) (Scheme 2).

3

2. Preparation of the metallacycles using bifunctional oligomers

Scheme 3 summarizes general synthetic pathways for the metallacycles with chelating Si-, Ge-, and Sn-ligands. The metathesis reactions of oligosilyl, oligogermyl, and oligostannyl dianions with transition metal dihalide complexes (Scheme 3(i)) produce the corresponding metallacycles with two M–E bonds. This reaction is similar to preparation of the metallacycles with two M–C bonds from the reactions of alkylene or arylene dianions with metal dihalides.10 Oxidative addition of the oligomers with two E–H bonds to low valent metal complexes (Scheme 3(ii)) is much suitable for electron-rich late transition metals. The three-membered cyclic complexes with π-coordinated disilene (R2Si=SiR2) ligands are isolated in the case of Pt or Pd.11 The bond between the two Si atoms is easily activated upon addition of small molecules such O2 and S8 to produce the four-membered disilametallacycles containing heteratoms (Scheme 3(iii)). These synthetic schemes are also suitable for preparation of disila- and digermametallacycles, which were recently reviewed by us.12 (i) Li Li (ii)

R2 E X (ER2)n + LnM -2 LiX X E R2

R2 H E (ER2)n + H E R2

(iii)

R2 E

L M L

E R2

R2 E

L

E R2 R2 E

L MLn

-H2

"Z"

E R2 R2 E Z

M L

E = Si, Ge, Sn Z = O, S Scheme 3

(ER2)n

M L

L +

(ER2)n

M L

E R2

Incorporation of early transition metals into the oligosilane rings was performed by the metathesis reaction of [TiCl2(η5-C5H5)2] with Li[SiPh2]4Li, giving a tetrasilatitanacyclopentane [Ti(SiPh2)4(η5C5H5)2] (3) as green crystals (Scheme 4(i)).13 A six-membered pentasilatitanacycle [Ti(SiPh2)5(η5C5H5)2] (4) was obtained similarly and identified as the six-membered ring with a boat conformation by

4

X-ray crystallography (Scheme 4(ii)).14 When dissolved in CHCl3, 3 decomposed via radical pathways, as evidenced by a gradual color change from green to yellow. The UV/vis absorption spectrum of 3 exhibited two red-shifted peaks at λmax = 600 and 830 nm, compared to absorption peaks of [TiCl2(η5C5H5)2] (390 and 520 nm). Although the electrochemical measurement of cyclo-pentasilane (SiPh2)5 displayed irreversible reduction at –2.85 V due to the Si–Si bond cleavage, the reduction of tetrasilatitanacycle 3 was possible to undergo a quasi-reversible process at ambient temperature (E1/2 = – 1.54 V). Incorporation of transition metals into the cyclic oligosilanes resulted in stabilization of the radical anion due to electron delocalization in the oligosilametallacycles.

In general, preparation of the metallacycles requires the corresponding oligosilyl dianions as the precursors. Use of an alkyllithium in their preparation, however, may cleave the inner Si–Si bonds of linear oligosilanes.15,16

Marschner found simple preparation of branched oligosilyl dianion

K2[(Me3Si)2Si(SiMe2)2Si(SiMe3)2]

through

selective

terminal

Si–Si

bond

cleavage

of

(Me3Si)3Si(SiMe2)2Si(SiMe3)3 upon addition of potassium tert-butoxide (Scheme 5).17 The oligosilyl dianion precursor easily underwent the metathesis reaction with metal dihalides, [MCl2(η5-C5H5)2] (M = Zr,

Hf),

yielding

five-membered

zircona-

and

hafnacyclopentasilanes

[M{(Me3Si)2Si(SiMe2)2Si(SiMe3)2}(η5-C5H5)2] (5).18

5

Me3Si

SiMe3 Me2 Me3Si Si Si Si Si SiMe3 Me2 Me3Si SiMe3

+2 t BuOK

+

M

Cl Cl

Me3Si

SiMe3 Me2 K Si Si Si Si K Me2 Me3Si SiMe3 Me3Si

SiMe3 Si

SiMe2

Si

SiMe2

M

Scheme 5

Me3Si SiMe3 5, M = Zr, 89% = Hf, 85%

An analogous reaction with [TiCl2(η5-C5H5)2] did not form the corresponding titanacyclosilane [Ti{(Me3Si)2Si(SiMe2)2Si(SiMe3)2}(η5-C5H5)2] (6) but converted it into a four-membered 1,1,2,2tetrakis(trimethylsilyl)tetramethylcyclotetrasilane

(Me3Si)2Si(SiMe2)2Si(SiMe3)2

(7),

which

was

presumably formed via reductive elimination of the titanium(IV) tetrasilanediyl precursor (Scheme 6).19 In contrast, the Ti(III) complex [TiCl2(η5-C5H5)2][Li(NMe2(CH2)2NMe2)] also reacted with the same oligosilyl dianion to produce a similar tetrasilatitanacycle with the oxidation state +3, [Ti{(Me3Si)2Si(SiMe2)2Si(SiMe3)2}(η5-C5H5)2]K (8), which were much more stable than Ti(VI) complex 6.

The ESR spectroscopy of 8 with a d1 electron configuration was considered as

delocalization of the radical anion along the transition-metal-containing oligosilane ring.

6

The addition of H2 to the solution of the persilylated zirconacycle 5 led to the quantitative formation

of

1,4-dihydro-1,1,4,4-tetrakis(trimethylsilyl)tetramethyltetrasilane

(9)

and

the

dihydridometallocene [ZrH2(η5-C5H5)2] (Scheme 7).20 The zirconacycle 5 underwent clean insertion of one

isonitrile

into

the

Zr–Si

bond

to

give

the

ring-expanded

complex

[Zr{Si(SiMe3)2(SiMe2)2Si(SiMe3)2(C=NC6H3Me2-2,6)}(η5-C5H5)2] (10). Further insertion of isonitrile was prevented because the coordination site of zirconium was blocked by interaction with the nitrogen atom.

In contrast, carbon monoxide, similar to isonitrile, did not insert into the Zr–Si bonds of

tetrasilazirconacycle (5).

Marschner also studied the preparation of group 4 metallocene complexes π-coordinated by disilenes (R2Si=SiR2) and digermenes (R2Ge=GeR2). In the paper, their attempts to prepare distannene analogues [M{η2-(Me3Si)2Sn=Sn(SiMe3)2}(η5-C5H5)2] (11, M = Zr, Hf) were presented by using the metathesis reaction of K2[(Me3Si)2Sn–Sn(SiMe3)2] with [MCl2(η5-C5H5)2] in the presence of MgBr2 (Scheme 8).21

The reaction mixtures contained unexpected four-membered metallacycles,

[M{Sn(SiMe3)2}3(η5-C5H5)2] (12), which were characterized by X-ray crystallography. The reactions were considered that the initially formed distannene complexes 11 involved the structure 11’ with stannylene character (:SnR2) which was more stable compared to the distannene character (R2Sn=SnR2). The dissociated stannylene can insert into the Sn–Sn bond of the initial complexes 11, which formed thermally stable cyclic complexes 12.

7

SiMe3 1, MgBr2/Et2O K Sn SiMe3 2, Cp2MCl2 K Sn SiMe3 M = Zr, Hf SiMe3

Me3Si Cp2M

SiMe3 Sn

Cp2M

Sn Me3Si SiMe3 11

SiMe3 :Sn SiMe3 :Sn SiMe3 SiMe3 11'

Me3Si + :Sn(SiMe3)2

SiMe3 Sn SiMe3 M Sn SiMe3 Sn Me3Si SiMe3 12, M = Zr, 54% Hf, 54% Scheme 8

The reaction of a Pt(0) complex [Pt(η2-C2H4)(PPh3)2] with a 1,4-bis(hydridogermyl)disilane HGePh2(SiMe2)2GePh2H

underwent

oxidative

addition

of

the

H–Ge

bonds

to

form

a

hydrido(germyl)platinum complex cis-[Pt(H){GePh2(SiMe2)2GePh2H}(PPh3)2] (13) (Scheme 9).22 Subsequent intramolecular cyclization of 13 produced a five-membered digermadisilaplatinacycle Pt[GePh2(SiMe2)2GePh2](PPh3)2 (14).

8

3. Preparation via dehydrocoupling reactions

The persilylated and pergermylated metallacyclopentanes have been proposed as the intermediates in the catalytic reactions for the cyclic compounds containing group 14 elements. West et al. reported that Pd-catalyzed reactions of cyclotetrasilane (SiEt2)4 with alkynes produced cyclotetrasilahexenes (SiEt2)4(CR=CR).23 Mochida et al. studied a ring expansion of cyclotetragermane (GeiPr2)4 by insertion of alkynes into a Ge–Ge bond in the presence of Pd catalysts.24 They proposed a similar catalytic pathway (Scheme 10), on the basis of bisgermylation mechanism of alkynes. The catalytic cycle is composed of (i) oxidative addition of the E–E bonds to the palladium to form a palladacyclopentane intermediate (A), (ii) insertion of alkynes into the Pd–E bonds, forming a sevenmembered germaplatinacyclic structure (B), and (iii) reductive elimination from the insertion product to give the ring expansion product (C) and recover Pd(0) complex.

These proposed palladium

intermediates, however, have been not isolated from the reaction mixtures probably due to the rapid catalytic reactions.

R2E R2E

R2 E

R'

E R2 C

R2 R2 E E

PdLn

R'

(iii)

E E R2 R2

(i)

R' R' ER2

L Pd L

E R2 B

L Pd

ER2

E E R2 R2

L

E R2

R2 R E E2

A (ii) R'

R'

E = Si, Ge

Scheme 10

Tetragermaplatinacyclopentanes composed of dibutylgermylene or diphenylgermylene units, [Pt(GeR2)4(dmpe)] (17: R = Bu,25 18: R = Ph26), were prepared from dehydrocoupling reactions of excess

secondary

germanes

with

the

corresponding

bis(germyl)platinum

complexes

[Pt(GeHR2)2(dmpe)] (15: R = Bu, 16: R = Ph), as shown in Scheme 11. Equimolar reactions of H2GeR2

9

with the starting platinum complexes formed four-membered trigermaplatinacycles [Pt(GeR2)3(dmpe)] (19: R = Bu, 20: R = Ph) as the intermediates. Complex 19 was not isolated but confirmed by 31P{1H} NMR spectra of the reaction mixture, while the molecular structures of 18 and 20 with chelating (GePh2)n ligands were characterized by X-ray crystallography.

The Ge–Pt–Ge bond angle of 20

(83.14(3)°) was smaller than that of 18 (89.76(1)°) due to the ring strain of the four-membered structures. Further addition of excess H2GePh2 into the isolated products 20 caused ring enlargement to give the tetragermacyclopentane 18. The products were accompanied by the formation of oligogermane dihydrides, H(GeR2)nH (n = 2, 3). These oligogermametallacycles were air- and thermally stable and were purified by silica gel column chromatography.

Tetragermaplatinacycle 17 reacted with dimethyl acetylenedicarboxylate (DMAD) in 1:3 ratio at 90 °C to undergo insertion of DMAD into the Pt–Ge bond, forming a 4,5,6,7-tetragerma-3platinacycloheptene, [Pt{C(CO2Me)=C(CO2Me)(GeBu2)4}(dmpe)] (21, eq 2). The molecular structure of 21, determined by X-ray crystallography, had a seven-membered ring with a boat conformation and the square-planar Pt atom. Interestingly, the

13

C{1H} NMR spectrum of 21 in the aliphatic region

displayed 32 signals due to the eight butyl groups, indicating that the conformation of the sevenmembered ring was slow on the NMR time scale. In contrast, the GePh2 analogue 18 did not react with DMAD under the same conditions. Thus, insertion of alkynes into the tetragermaplatinacyclopentane took place in a mechanism similar to the proposed mechanism (Scheme 10). However, reductive

10

elimination from 21 did not proceed even at higher temperature or addition of excess amount of DMAD, DMPE, and H2GePh2 for promotion of the reductive elimination.

The reaction of bis(dibutylgermyl)platinum complex 15 with H2GePh2 in 1:3 ratio at room temperature caused an immediate exchange of GeHBu2 with GeHPh2 ligands to form bis(diphenylgermyl)platinum complex 16, accompanied with elimination of H2GeBu2 (Scheme 12(i)). In contrast, the reverse reaction of complex 16 with H2GeBu2 in 1:2 ratio proceeded slowly even at 90 °C to give a four-membered platinacycle [Pt(GeBu2)(GePh2)2(dmpe)] (22) as a major product, which contained two GePh2 and one GeBu2 units (Scheme 12(ii)).27

Recently, Mochida found that bis(germyl)platinum complexes with a six-membered chelating phosphine ligand, [Pt(GeArH2)2(dppp)] (2 23) (dppp = 1,3-bis(diphenylphosphino)propane, Ar = C6H2Me3-2,4,6, C6H2Et3-2,4,6), were converted to the digermyl(hydrido)platinum complexes [Pt(H)(GeArHGeArH2)(dppp)] (24), as shown in Scheme 13.28 Thermodynamic parameters about an equilibrium between complexes 23 and 24 were estimated. They suggested the intramolecular Ge–Ge

11

bond couplings at the Pt center through a Pt=Ge intermediate, which was formed by α-hydrogen elimination of the GeH2Ar ligand and migratory insertion of the GeH2Ar group from the Pt center to the =GeHAr ligand. Banaszak Holl29 and Ishii30 also reported similar Ge–Ge bond-forming reactions of bis(germyl)platinum complexes with primary or secondary germyl ligands to give (digermyl)platinum complexes .

Tobita reported that heating of the isolated iron complex with silylene and silyl ligands, [(η5C5Me5)(OC)Fe(=SiMes2)SiMe3] (25), in the presence of tBuNC caused 1,2- and 1,3-silyl migration accompanied by scrambling of the substituents, giving a disilanyl iron complex [(η5C5Me5)(OC)(tBuNC)Fe{Si(MesMe)SiMesMe2}] (26) (eq 3).31 A similar 1,2-migration mechanism involving the intermediates with silylene,32 germylene,33 and stannylene34 ligands has been discussed in the coupling reaction of heavier group 14 elements at the late transition metals, affording the oligomers or polymers released from the complexes.

Scheme 14 shows the plausible pathway to form four-membered germaplatinacycles with two different substituents starting from the digermyl(hydrido)platinum complex D. Excess H2GeBu2 can exchange the hydride ligand of D with the GeHBu2 ligand, together with elimination of H2, forming a tetraphenyldigermyl(dibutylgermyl) complex (E).

The 1,2-hydrogen shift from Ge to Pt and

12

dissociation of a phosphorus atoms of the dmpe ligand formed a Pt=Ge intermediate (F). Migratory insertion of the digermyl group into the Pt=GeBu2 bond of F gave a trigermyl(hydrido)platinum complex (G), similarly to Scheme 13.

Finally, intramolecular cyclization of G formed the

trigermaplatinacyclobutane 22.

Although the preparation and fundamental chemical properties of persilylated and pergermylated metallacycles have been established, there have been limited papers about perstannylated metallacycles. Tetrastannapalladacyclopentane [Pd(SnPh2)4(dmpe)] (27) was synthesized from the reaction of a Pd(0) complex [Pd(dmpe)2]n (n = 1 or 2) with H2SnPh2 in 1:4 ratio at room temperature (Scheme 15).35 The dehydrogenative Sn–Sn bond-forming reaction occurred rapidly to form complex 27 without formation of any intermediates such as [Pd(SnHPh2)2(dmpe)]. Thermal decomposition of 27 caused at 70 °C to give a bis(triphenylstannyl)palladium complex [Pd(SnPh3)2(dmpe)] (28). The thermolysis induced the Sn–Sn bond cleavage in the PdSn4 rings and unusual migration of the Ph groups from the internal Sn atoms to those bonded to the Pd center.

13

Preparation of discrete linear and branched oligogermanes was performed by stepwise Ge–Ge coupling reactions of R3Ge–H with R’3Ge–CH2CN in the absence of transition metal complexes.36 The reaction of five-membered tetragermaplatinacycle 17 with Ph2GeH2 in 1:2 ratio at room temperature produced a mixture of tetragermane dihydride H(GeBu2)4H (29) and a bis(diphenylgermyl)platinum complex 16, as shown in Scheme 16. A similar reaction of the platinacycle having GePh2 units 18 with excess PhGeH3 at 90 °C provided H(GePh2)4H (30) and unidentified platinum-germanium complexes due to lower solubility of the complex. These exchange reactions involve cleavage of the Pt–GeR2 bond of the germaplatinacycles and formation of the stable H–GeR2 bond of the tetragermane dihydrides. The results demonstrate a new route for the preparation of linear oligogermane dihydrides directly from the reaction of the arylgermanes and the Pt complexes 17 and 18.

(i)

Bu Bu Bu Me2 + Ph2GeH2 Bu Bu Bu Bu P Ge Ge Bu (2 eq.) Ge Ge H + Pt H rt Ge Ge P Ge Ge Bu Me2 Bu Bu Bu Bu Bu Bu Bu 17 29, 90%

(ii)

Ph Ph Ph + PhGeH Me2 3 P Ge Ge Ph (excess) Pt 90 oC P Ge Ge Ph Me2 Ph Ph Ph 18

Ph Ph Me2 P Ge H Pt P Ge H Me2 Ph Ph 16, 80%

Ph Ph Ph Ph H

Ge

Ge Ge

H + Pt-Ge complexes Ge

Ph Ph Ph Ph 30, 66%

Scheme 16

14

Incorporation of heteroatoms into the oligogermanium chain is expected to affect the conformation and electronic properties of the σ-conjugated polymers.

Transition-metal-catalyzed

dehydroheterocoupling reactions of thiols with group 14 element compounds led to the S–E bond formation (E = Si, Ge).37 A reaction of tetragermane dihydride 29 with 2-fold of HSC6H4tBu-4 in the presence of [RhCl(PPh3)3] catalyst provided complicated mixtures containing bis(thiolato)germane Bu2Ge(SC6H4tBu-4)2.

The reaction would involve Ge–Ge bond scission rather than direct

dehydrocouplings between the H–S and H–Ge bonds. Treatment of germametallacycles 17 with diaryl disulfide (SR)2 (R = C6H4tBu-4, C6H4Me-4, C6H5, C6H4-Cl, C6F5) at room temperature produced 1,4dithiolatotetragermanes, RS(GeBu2)4SR (31) in moderate yields, accompanied with formation of bis(thiolato)platinum complexes [Pt(SR)2(dmpe)] (32) (Scheme 17).38 The S–S bond activation of dibutyl disulfide (SBu)2 was performed by UV-photoirradiation, giving the tetragermane BuS(GeBu2)4SBu and [Pt(SBu)2(dmpe)]. observed during the reactions.

The Ge–Ge bond cleavage of the tetragermanes is not

Equimolar reaction of disulfides with 17 produced a ring-opened

platinum complexes [Pt(SR){(GeBu2)4SR}(dmpe)] (33, R = C6H4tBu-4, C6F5). Further addition of the same disulfides resulted in complete conversion to the dithiolatogermane 31 and complex 32. These results indicated that the reaction of 17 with disulfides proceeded smoothly to form thermally stable ring-opened complexes. In contrast, the corresponding intermediates formed by addition of H2GePh2 to 17 were not obtained in the exchange reactions of the Pt–Ge bonds. Bu Bu Bu Me2 P Ge Ge Bu Pt P Ge Ge Bu Me2 Bu Bu Bu 17 + (SR')2 r.t. (equimolar)

Bu Bu Bu Bu

+ (SR)2 (2 eq.)

RS

r.t.

Ge

Ge

Ge

Ge

Bu Bu Bu Bu 31, 61-91% r.t.

+ (SR)2 (equimolar)

Bu Bu SR' Bu Bu Ge Ge Me2 P Ge Ge Bu Bu Pt Bu Bu P SR' Me2 33 R' = C6H4t Bu-4, 11% C6F5, 66%

SR +

Me2 P SR Pt P SR Me2 32, 47-98%

R = C6H4t Bu-4, C6H4Me-4, C6H5, C6H4Cl-4, C6F5, Bu

Scheme 17

15

Braddock-Wilking conducted the reactions of sila- and germafluorenes H2ER2 (ER2 = SiC12H8 or GeC12H8) with a Pt(II) complex with a chelating dppe ligand, and isolated persilylated and pergermylated

platinacyclopentanes

bis(diphenylpylphosphino)ethane)

[Pt(ER2)4(dppe)] 18).39

(Scheme

(34)

Abu-Omar

(dppe reported

the

=

1,2-

Ni-catalyzed

oligomerization of phenylsilane to produce a mixture of cyclic and acyclic poly(phenylsilanes) and suggested

formation

of

silanickellacycles

[Ni(SiPhH)3(dippe)]

(35)

(dippe

=

1,2-

bis(diisopropylphosphino)ethane) as the precursor for cyclic polysilanes.40

Selective cyclopolymerization of primary silanes was achieved by a Ni complex with a chelating phosphine ligand.41 The cyclopolymerization of phenylsilane catalyzed by [Ni(dmpe)2] yielded the cyclic poly(phenylsilane) (poly-36c, Scheme 19). The GPC trace of the cyclic poly(phenylsilane) indicated an average molecular weight of Mn = 610 (Mw/Mn = 1.02). The polymerization using the catalyst prepared by [Ni(cod)2] and PMe3 produced acyclic poly(phenylsilane) (poly-36a). It’s average molecular weight (Mn = 1300 and Mw/Mn = 1.16) was larger than that of the cyclic polymer. The linear or cyclic structures of the obtained polysilanes were determined by comparison of the chemical shifts in the 1H and 29Si{1H} NMR spectra.42 Me2 Me2 P P Ni P P Me2 Me2 (1 mol%) Ph n H Si H

Ph Si H n poly-36c, 64%

-H2, rt

H

Ph Ni(cod)2 (1 mol%) PMe3 (0.5 mol%)

H

Si

H

H n poly-36a, 33%

Scheme 19

16

Scheme 20 depicts the plausible mechanism of the ring-forming step for cyclic polysilanes. The polymer (H) involved the Ni center located at one end group of the linear polysilanes. Oxidative addition of the H–Si bond at the other end group to a Ni(0) complex formed the dinickel intermediate (I). Intramolecular exchange of the Ni–Si bond with the Ni–Y bond yielded the cyclic intermediate (J), and the subsequent coupling of the silyl ligands afforded the cyclic polysilanes.

Another possible

mechanism involved intramolecular cyclization of H to give the Ni(IV) intermediate (J’) and formation of the same cyclic polymer (poly-36c). Y H Ph Ni Si

H

Si H

Y

+ Ni(0) Ph

Ni Ni Si Si H H Ph H Ph I

H (Y = H, SiH2R)

H Ph Si Ni Si Ph H J

- Ni(0)

H

Y

or Ni

+ Ni

Si Si Ph Ph poly-36c

H Ph Y Si

H

H Si Ph H J'

H

Scheme 20

4. Conclusions Metallacycles with chelating Si-, Ge-, and Sn- ligands were synthesized by several synthetic methods.

The most common preparation is the metathesis reaction of bifunctional oligomeric

compounds with metal dihalides. Double oxidative addition of the E–H (E = Si, Ge, Sn) bonds is suitable for low valent late transition metal complexes.

The metallacycles undergo insertion of

unsaturated compounds into the M–E bonds or exchange of the M–E bonds to give oligomers, but chemical properties of these metallacycles have not been widely investigated yet. More concern should be added to this area to understand the reactivity of the coordination bonds and new findings of the reactions of organic compounds having heavy elements promoted by transition metal complexes.

17

The research of the four- and five-membered metallacycles of late transition metals provided us their relevance with the catalytic dehydrocoupling polymerization of heavier group 14 elements, because these metallacyclic compounds imply it being as possible intermediates for formation of the cyclic polymers.

The Ni-catalyzed polymerizations of primary silanes achieved the controlled

polymerization to produce linear or cyclic polysilanes by the appropriate choice of catalysts. Further studies of the metallacycles are expected to lead to an understanding of the fundamental properties of the transition metal complexes and realize application to the dehydrogenative cyclopolymerization with controllable molecular weight.

Acknowledgments The authors are grateful to Prof. Kohtaro Osakada of Tokyo Institute of Technology, Prof. Akihiko Ishii and Dr. Norio Nakata of Saitama University for helpful discussion. The studies of our group cited in this article were done with help of Ms. Naoko Ishikawa, Mr. Masaya Hanzawa, Ms. Tomoko Fukuta, Mr. Takashi Deguchi, Mr. Atsushi Takahasi and with financial support by Grants-inAid for Scientific Research for Young Chemists (No. 23750059, 21750057), and, from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.

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Graphical Abstract

Recent Topics of the Metallacycles Composed of the Heavier Group 14 Elements

Leave this area blank for abstract info.

Makoto Tanabe

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