Dehydrocoupling of Organosilanes with a Dinuclear Nickel Hydride

Organometallics 2010, 29, 6527–6533 DOI: 10.1021/om100887v

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Dehydrocoupling of Organosilanes with a Dinuclear Nickel Hydride Catalyst and Isolation of a Nickel Silyl Complex Erin E. Smith, Guodong Du,† Phillip E. Fanwick, and Mahdi M. Abu-Omar* Brown Laboratory, Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States . †Current address: Department of Chemistry, University of North Dakota, Grand Forks, ND 58202. Received September 14, 2010

The dehydrocoupling of organosilanes is an efficient method for producing polysilanes. Although this is traditionally done with group 4 metallocene catalysts, there are a few examples of nickel catalysts that are effective for this reaction. We report the dehydrocoupling of phenylsilane and phenylmethylsilane with [(dippe)Ni(μ-H)]2 (1) (dippe=1,2-bis(diisopropylphosphino)ethane). As expected from thermodynamic and steric evidence, the primary silane is more active toward dehydrocoupling than the secondary silane. This catalyst compares favorably in required reaction conditions, molecular weight of polysilane product, and selectivity for linear oligomers. Possible mechanisms for the dehydrocoupling of silane are discussed. The hypothesized intermediate is a hydrido silyl nickel complex. We report the isolation and single-crystal X-ray structure of a stable analogue of the proposed catalytic intermediate, (dippe)Ni(SiCl3)Cl (2).

1. Introduction Polysilanes are of interest for their conductivity and optical properties. Traditional industrial preparation of these materials is done by the W€ urtz-Fittig coupling reaction. This method involves the coupling of diorganodihalosilanes using molten sodium.1 The functional groups that are tolerated by this method are limited, and despite high molecular weights, the percent cyclic oligomers is also high. Other methods that can be utilized to produce polysilanes include polymerization

of masked disilenes2 and electroreduction of dichlorosilanes.3 The most efficient alternative is dehydrocoupling of organosilanes with transition metal catalysts (eq 1). This method was first studied by Harrod and co-workers using derivatives of titanocene and zirconocene.4 Since then, much more work has been done with group 4 catalyst systems.5 Other metal complexes used for dehydrocoupling include groups 5-12 and the lanthanide series.6-14

*Corresponding author. E-mail: [email protected]. (1) (a) Corey, J. Y. The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; John Wiley and Sons: New York, 1989; pp 1-56. (b) West, R. The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; John Wiley and Sons: New York, 1989; pp 1207-1240. (2) Sakamoto, K.; Obata, K.; Hirata, H.; Nakajima, M.; Sakurai, H. J. Am. Chem. Soc. 1989, 111, 7641. (3) Shono, T.; Kashimura, S.; Ishifune, M.; Nishida, R. J. Chem. Soc., Chem. Commun. 1990, 1160. (4) (a) Aitken, C.; Harrod, J. F.; Samuel, E. J. Organomet. Chem. 1985, 279, C11. (b) Aitken, C.; Harrod, J. F.; Samuel, E. J. Am. Chem. Soc. 1986, 108, 4059–4066. (c) Aitken, C.; Harrod, J. F.; Samuel, E. Can. J. Chem. 1986, 64, 1677–1679. (d) Aitken, C.; Harrod, J. F.; Gill, U. S. Can. J. Chem. 1987, 65, 1804–1809. (5) (a) Mu, Y.; Aitken, C.; Cote, B.; Harrod, J. F. Can. J. Chem. 1991, 264, 164. (b) Campbell, W. H.; Hilty, T. K. Organometallics 1989, 8, 2615– 2618. (c) Woo, H. G.; Tilley, T. D. J. Am. Chem. Soc. 1989, 111, 3757–3758. (d) Woo, H. G.; Tilley, T. D. J. Am. Chem. Soc. 1989, 111, 8043–8044. (e) Woo, H. G.; Walzer, J. F.; Tilley, T. D. J. Am. Chem. Soc. 1992, 114, 7047– 7055. (f) Tilley, T. D. Acc. Chem. Res. 1993, 26, 22–29. (g) Bourg, S.; Corriu, J. P.; Enders, M.; Moreau, J. J. E. Organometallics 1995, 14, 564–566. (h) Peulecke, N.; Thomas, D.; Bauman, W.; Fischer, C.; Rosenthal, U. Tetrahedron Lett. 1997, 38, 6655–6656. (i) Hengge, E.; Gspaltl, P.; Pinter, E. J. Organomet. Chem. 1996, 521, 145–155. (j) Corey, J. Y.; Zhu, X. H.; Bedard, T. C.; Lange, L. D. Organometallics 1991, 10, 924–930. (k) Corey, J. Y.; Huhmann, J. L.; Zhu, X. H. Organometallics 1993, 12, 1121–1130. (l) Wang, Q.; Corey, J. Y. Can. J. Chem. 2000, 78, 1434–1440. (m) Banvetz, J. P.; Stein, K. M.; Waymouth, R. M. Organometallics 1991, 10, 3430–3432. (n) Dioumaev, V. K.; Harrod, J. F. J. Organomet. Chem. 1996, 521, 133–143. (o) Choi, N.; Onozawa, S.; Sakakura, T.; Tanaka, M. Organometallics 1997, 16, 2765–2767. (p) Obora, Y.; Tanaka, M. J. Organomet. Chem. 2000, 595, 1–11.

(6) (a) Corey, J. Y. Adv. Organomet. Chem. 2004, 51I, 1–52. (b) Gauvin, F.; Harrod, J. F.; Woo, H. G. Adv. Organomet. Chem. 1998, 42, 363–405. (c) Tilley, T. D. Comments Inorg. Chem. 1990, 10, 37–51. (7) Aitken, C.; Barry, J. P.; Gauvin, F.; Harrod, J. F.; Malek, A.; Rousseau, D. Organometallics 1989, 8, 1732–1736. (8) Itazaki, M.; Ueda, K.; Nakazawa, H. Angew. Chem., Int. Ed. 2009, 48, 3313–3316. (9) (a) Ojima, I.; Kogure, T.; Nagai, Y. J. Organomet. Chem. 1973, 55, C7. (b) Chang, L. S.; Corey, J. Organometallics 1989, 8, 1885–1893. (c) Rosenberg, L.; Davis, C.; Yao, J. J. Am. Chem. Soc. 2001, 123, 5120– 5121. (d) Rosenberg, L.; Kobus, D. J. Organomet. Chem. 2003, 685, 107– 112. (e) Fryzuk, M. D.; Rosenberg, L.; Rettig, S. J. Organometallics 1991, 10, 2537–2539. (f) Fryzuk, M. D.; Rosenberg, L.; Rettig, S. J. Inorg. Chim. Acta 1994, 222, 345–364. (10) Diversi, P..; Marchetti, F.; Ermini, V.; Metteoni, S. J. Organomet. Chem. 2000, 593-594, 154–160. (11) (a) Boudjouk, P.; Rajkumar, A. B.; Parker, W. L. J. Chem. Soc., Chem. Commun. 1991, 245–246. (b) Fontaine, F. G.; Kadkhodazadeh, T.; Zargarian, D. J. Chem. Soc., Chem. Commun. 1998, 1253–1254. (c) Groux, L. F.; Zargarian, D. Organometallics 2001, 20, 3811–3817. (d) Fontaine, F. G.; Zargarian, D. Organometallics 2002, 21, 401–408. (e) Fontaine, F. G.; Zargarian, D. J. Am. Chem. Soc. 2004, 126, 8786–8794. (12) (a) Brown-Wensley, K. Organometallics 1987, 6, 1590–1591. (b) Tanaka, M.; Kobayashi, T.; Hayashi, T.; Sakakura, T. Appl. Organomet. Chem. 1988, 2, 91–92. (c) Chauhan, B.; Shimizu, T.; Tanaka, M. Chem. Lett. 1997, 785–786.

r 2010 American Chemical Society

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Smith et al.

Figure 1. (a) Plot of [PhSiH3] versus time. (b) Log-scale. Conditions: 3.72 mM (1.0 mol %) 1 and 0.372 M phenylsilane in toluene at 25 °C.

Some interesting work has also been done on the catalytic dehydrocoupling of organophosphines15-20 and heterodehydrocoupling of organophosphines with other compounds such as boranes19-21 and the elements of group 14.19,21,22 Oligomeric phosphines have been prepared using catalysts such as Cp*2Zr(H)3-,16 Cp*Rh(allyl)2,17 (dippe)Rh(allyl),18 and (N 3 N)Zr(PHR) (N 3N = N(CH2 CH2NSiMe 3 )3 3-.17 Harrod and co-workers studied the dehydrocoupling of organophosphines with [(EBTHI)Ti(μ-H)]2 (EBTHI = ethylene1,2-bis(η5-4,5,6,7-tetrahydro-1-indenyl), which is a structural analogue of the catalyst system discussed in this report, as they both feature a bridging hydride.21 Nickel catalysts are of interest due to their low cost when compared with precious metals. Examples of dehydrocoupling of organosilanes with nickel are rare and have some limitations. Nickel indenyl complex precursors require activators such as AgBF4, MAO, or LiAlH4.11b,c It is believed that activation of the catalyst precursor gives a cationic nickel species and, in the case of LiAlH4, a neutral nickel hydride complex, which is presumed to be the active catalyst. Mechanisms proposed for the dehydrocoupling of organosilanes with titanium and zirconium metallocene complexes involve an intermediate complex containing a hydride and silyl group.5c-f,23,24c The oligomerization of organosilanes with late transition metals such as nickel likely begins with oxidative addition of the silane to give a metal silyl complex. Examples of nickel silyl complexes have been isolated and char(13) (a) Sakakura, T.; Lautenschlager, H. J.; Nakajia, M.; Tanaka, M. Chem. Lett. 1991, 913–916. (b) Forsyth, C. M.; Nolan, S. P.; Marks, T. J. Organometallics 1991, 10, 2543–2545. (14) Li, Z.; Iida, K.; Tomisaka, Y.; Yoshimura, A.; Hirao, T.; Nomoto, A.; Ogawa, A. Organometallics 2003, 26, 1212–1216. (15) Waterman, R. Organometallics 2007, 26, 2492. (16) Fermin, M. C.; Stephan, D. W. J. Am. Chem. Soc. 1995, 117, 12645. (17) Bohm, V. P. W.; Brookhart, M. Angew. Chem., Int. Ed. 2001, 40, 4694. (18) Han, L.-B.; Tilley, T. D. J. Am. Chem. Soc. 2006, 128, 13698. (19) (a) Waterman, R. Curr. Org. Chem. 2008, 12, 1322. (b) Waterman, R. Dalton Trans. 2009, 18. (20) Roering, A. J.; MacMillan, S. N.; Tanski, J. M.; Waterman, R. Inorg. Chem. 2007, 46, 6855. (21) (a) Xin, S.; Woo, H. G.; Harrod, J. F.; Samuel, E.; Lebuis, A.-M. J. Am. Chem. Soc. 1997, 119, 5307. (b) Xin, S.; Harrod, J. F.; Samuel, E. J. Am. Chem. Soc. 1994, 116, 11562. (22) Dorn, H.; Singh, R. A.; Massey, J. A.; Nelson, J. M.; Jaska, C. A.; Lough., A. J.; Manners, I. J. Am. Chem. Soc. 2000, 122, 6669. (23) Spaltenstein, E.; Palma, P.; Kreutzer, K.; Willoughby, C. A.; Davis, W. M.; Buchwald, S. L. J. Am. Chem. Soc. 1994, 116, 10308– 10309.

Figure 2. Plot of volume of dihydrogen versus time. Conditions: 16 mM (1.0 mol %) 1 and 1.6 M phenylsilane in toluene at 25 °C.

acterized.24 We report in this contribution dehydrocoupling of PhSiH3 and PhMeSiH2 catalyzed by [(dippe)Ni(μ-H)]2 (1) (dippe = 1,2-bis(diisopropylphosphino)ethane). The molecular weight distributions (by GPC) of the polysilane products, the catalytic activity of 1, and selectivity toward production of linear oligomers in comparison to conventional group 4 metallocene catalysts are also reported. A mechanistic proposal is advanced in which a nickel silyl hydride is the active dehydrocoupling catalyst. In support of our mechanistic hypothesis, we report the preparation, isolation, and X-ray molecular structure of (dippe)Ni(SiCl3)Cl (2).

2. Results and Discussion 2.1. Oligomerization of Phenylsilane with [(dippe)NiH]2 (1). Oligomerization of a primary silane, PhSiH3, was carried out in toluene under ambient conditions with 1 as the precatalyst at 1.0 mol % loading (eq 1). The consumption of PhSiH3 was followed by 1H NMR. Figure 1 shows a typical kinetic profile. The log-scale plot (Figure 1b) deviates from linearity, and the observed rate decreases as the reaction progresses. This deviation from pseudo-first-order kinetics is attributed to catalyst deactivation during the reaction. Deviation from linearity is also observed for the second-order plot of 1/[PhSiH3] versus (24) (a) Iluc, V. M.; Hillhouse, G. L. Tetrahedron 2006, 62, 7577– 7582. (b) Adhikari, D.; Pink, M.; Mindiola, D. J. Organometallics 2009, 28, 2072–2077. (c) Iluc, V. M.; Hillhouse, G. L. J. Am. Chem. Soc. 2010, 132, 11890–11892.

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Table 1. Summary of GPC Data for Dehydrocoupling of PhSiH3 and PhMeSiH2 Catalyzed by 1a peak # area (%) Mnb Mwb PDI

silane phenylsilane (PhSiH3) phenylmethylsilane (PhMeSiH2)

1 2 1 2 3

32 68 66 17 17

1750 3200 1.82 521 549 1.05 951 1160 1.22 475 479 1.01 320 330 1.03

a

[PhSiH3] = 2.78 M, [PhMeSiH2] = 2.27 M, and 2.0 mol % of 1 in toluene at 25 °C. b Determined by GPC with polystyrene standards.

Table 2. Performance of Selected Group 4 Dehydrocoupling Catalysts from the Literaturea catalyst

solvent

t

Cp2Zr(NMe2)2 CpCp*Zr[Si(SiMe3)3]Me Cp2ZrCl2/2nBuLi (EBI)ZrCl2/2nBuLi

neat neat toluene toluene

∼1 min 15 min 10 days 7h

Mwb PDI % cyclicc ref 2810 3100 2450 2963

1.44d 1.8 1.18 2.28

14d nd 45d 13.8

5l 5f 5n 5m

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Table 3. Direct Comparison the Dehydrocoupling of PhSiH3 with Nickel versus Zirconium Catalystsa catalystb

solvent t (h)

% yield GPC H2c peak #

Cp2ZrMe2 Cp2ZrMe2

toluene neat

2 1

79 98

Ind2ZrMe2

toluene 96 neat 24

25 78

[(dippe)NiH]2 toluene

1.5

81

1 2 1 2 1 2

Mw

PDI % area

nd 2390 580 nd 3200 1530 3200 549

nd 1.33 1.05 nd 1.14 1.05 1.83 1.05

nd 68 32 nd 31 69 32 68

a Dehydrocoupling of phenylsilane; all reactions at 21 °C. b Results from reactions with catalyst loading of 2.0 mol %. c Measured by volume displacement using a gas evolution apparatus.

Scheme 1. σ-Bond Metathesis Mechanism for Dehydrocoupling of Phenylsilane with 1

Dehydrocoupling of phenylsilane; all reactions at 20-25 °C. b High molecular weight peak only. c Determined by 1H NMR. d Calculated from reported data. a

time. The incomplete consumption of PhSiH3 is also consistent with catalyst death. The reaction was monitored as well by following dihydrogen evolution (Figure 2). The results are consistent with those obtained for PhSiH3 consumption, the observed rate decreases as the reaction progresses, and only 69% conversion is observed based on the volume of H2 produced. Some of the H2 evolved is from the dihydride catalyst, but since the catalyst concentration is low (1-2 mol %), this volume is negligible. The conditions between the silane consumption and dihydrogen evolution experiments differ, hence, the difference in reaction times and conversion between Figures 1 and 2. Some scatter is observed late in the reaction due to error in peak integration at low silane concentrations. Increasing the catalyst loading to 2.0 mol % resulted in improved conversion, 81% yield based on dihydrogen gas evolved. The resulting polysilane was analyzed by gel permeation chromatography (GPC) and showed two peaks (Figures S1 and S2 and Table 1). Peak 2 accounts for the majority of the product (68%) and corresponds to oligomers incorporating six monomers. The polydispersity index (PDI) is quite narrow (1.05). Peak 1 corresponds to longer oligomers/ polymer containing an average of 16 phenylsilane monomers. However, this peak is the minor product (32%) and exhibits a broad PDI (1.82). Zirconium catalysts have been used widely as dehydrocoupling catalysts. Table 2 shows representative examples of the more successful zirconium catalysts. The molecular weight distributions of the resulting silane polymer fall in a relatively narrow range, 2450-3100 g mol-1. Both time-scale and percentage of cyclic oligomer vary among different catalysts. To ascertain reliable comparisons, we prepared two zirconium catalysts (Cp2ZrMe2 and Ind2ZrMe2) and investigated their dehydrocoupling catalysis of phenylsilane. GPC of the resulting oligomers/polymers and percent yields for each of the catalysts are presented in Table 3. For the high molecular weight polyphenylsilane obtained with 1, Mw was considerably higher than that observed for Cp2ZrMe2 (3200 versus 2390 g mol-1). However, the distribution of high and low molecular weight oligomers is very similar for Ind2ZrMe2 and 1. It is worth noting that the zirconium catalysts required neat conditions to afford higher conversions, and in general the time of reaction

for the nickel catalyst is more favorable. Also, for Ind2ZrMe2, the “lower” molecular weight peak consists of much higher molecular weight oligomers than the other catalyst. The percentage of cyclic oligomers in the product from dehydrocoupling of PhSiH3 with catalyst 1 was determined using 1H NMR. The chemical shift of Si-H groups for cyclic phenylsilane oligomers lies between 4.8 and 5.3 ppm, while the chemical shift for linear oligomers lies between 4.4 and 4.8 ppm.5m The resulting oligomer/polymer mixture using catalyst 1 had 19% cyclic oligomers after 17 h. Time was allowed to reach thermodynamic equilibrium as linear oligomers are slowly converted to cyclic. Catalyst 1 gives relatively high linear/ cyclic selectivity in comparison with group 4 metallocene catalysts, which have cyclic percentages in the range 3-55%, as determined by 1H NMR.5l-p In one of the nickel systems investigated by Zargarian and co-workers, (1-Me-indenyl)Ni(PPh3)Me and Me2AlCH2PMe2 activator, mostly cyclic oligomers are generated with Mw = 470 and PDI = 1.03 in less than 15 min with >95% conversion.11e 2.2. Oligomerization of Phenylmethylsilane with 1. The dehydrocoupling reaction of phenylmethylsilane (PhMeSiH2) was investigated with 2.0 mol % loading of 1 in toluene under ambient temperature. Based on the amount of dihydrogen produced, the reaction proceeded to 76% conversion of PhMeSiH2. This conversion is high compared to only 16% for Ind2ZrMe2 under the same conditions and after 1 day. Gel permeation chromatography (GPC) analysis of the product showed three distinguishable peaks (Figure S2 and Table 1). Light scattering detection by GPC has been shown to underestimate the

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Scheme 2. Oxidative-Addition Mechanism for Dehydrocoupling of Phenylsilane with 1

molecular weight of polysilane by 20% when referenced versus polystyrene standards.25 Therefore, peak 1 accounts for the majority of the product (66%) and corresponds to an average oligomer length of ten monomers. Peaks 2 and 3 account for chains of three, four, or five monomers of phenylmethylsilane. The high molecular weight peaks obtained using catalyst 1 with phenylmethylsilane average 10 monomers. This is very high when compared with secondary silane oligomerization with other late transition metal catalysts. By comparison to results for phenylsilane, it can be concluded that phenylsilane is more active for dehydrocoupling than phenylmethylsilane. Zirconium catalysts also follow this trend. Oligomerization of phenylmethylsilane with the bis-indenyl zirconium catalyst [(Ind)2Zr(CH3)2] gave 16% yield compared with 25% for phenylsilane. The same reaction using the bis-cyclopentadienyl zirconium catalyst [(Cp)2Zr(CH3)2] gave a 25% yield compared with 98% for phenylsilane. The extent and rate of polymerization decrease moving from primary to secondary silane. This trend is rationalized by bond energies and steric hindrance. There is a difference of 5 kJ mol-1 for the silicon-hydrogen bond strength between phenylsilane (377 kJ mol-1) and phenylmethylsilane (382 kJ mol-1).26 Secondary silanes are also more crowded (sterically

demanding), which results in a higher activation barrier for silicon coupling at the metal center as the chain length increases. 2.3. Dehydrocoupling Mechanism. Two possible mechanisms for the catalytic dehydrocoupling of phenylsilane have been envisaged. The first is σ-bond metathesis (Scheme 1), which is similar to the mechanism proposed by Tilley and coworkers for the group 4 metallocene catalyst.5c-f The dimeric precatalyst dissociates to active monomeric nickel in solution. This step is induced by the presence of the organosilane as a substrate (Ia ligand-exchange mechanism). η2-Silane complexes of nickel are known. 24a The mononuclear nickel hydride undergoes σ-bond metathesis with silane to form a nickel silyl complex. Additional σ-bond metathesis with silane would regenerate nickel hydride and disilane. Repetition of this sequence results in growth to oligomers and polymers. An alternative mechanism (Scheme 2) is formation of a nickel(0) from H2 extrusion,27 followed by oxidative addition of phenylsilane to afford a silyl hydride complex. The latter initiates via a silylene intermediate, followed by propagation. Termination of the disilyl complex regenerates Ni(0) and yields cyclic oligomers. Chain termination of the silyl hydride affords linear polysilane. Recently, Hillhouse and co-workers reported a reaction of [(dtbpe)NiH]2 with Mes2Si(H)K where they were able to isolate both a nickel silyl complex and, with

(25) Devaux, J.; Daoust, D.; De Mahieu, A. F.; Strazielle, C. In Inorganic and Organometallic Oligomers and Polymers; Laine, R. M., Harrod, J. F., Eds.; Kluwer Publishers: Amsterdam, 1991; pp 49-60. (26) Brook, M. A. Silicon in Organic, Organometallic, and Polymer Chemistry; John Wiley and Sons: McMaster University, Hamilton, Ontario, Canada, 2000; p 172.

(27) (a) Vicic, D. A.; Jones, W. D. J. Am. Chem. Soc. 1997, 119, 10855–10856. (b) Garcia, J. J.; Jones, W. D. Organometallics 2000, 19, 5544–5545. (c) Vicic, D. A.; Jones, W. D. J. Am. Chem. Soc. 1999, 121, 4070–4071. (d) Tran, B. L.; Pink, M.; Mindiola, D. J. Organometallics 2009, 28, 2234–2243.

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Figure 3. ORTEP drawing of (dippe)Ni(SiCl3)(Cl) (2). Ellipsoids are shown at 50% probability. Hydrogens have been omitted for clarity. Selected bond distances (A˚) and angles (deg): Ni(1)-P(3) = 2.2171(8), Ni(1)-P(4) = 2.1433(8), Ni(1)-Cl(1) = 2.1851(8), Ni(1)-Si(2) = 2.2390(8), Si(2)-Cl(21) = 2.1050(10), Si(2)Cl(22) = 2.0861(11), Si(2)-Cl(23) = 2.0935(11); Cl(1)-Ni(1)P(3)=91.31(3), Cl(1)-Ni(1)-Si(2)=85.08(3), Cl(1)-Ni(1)-P(4) = 172.87(4), Si(2)-Ni(1)-P(4) = 95.74(3), P(3)-Ni(1)-P(4) = 88.81(3), Si(2)-Ni(1)-P(3) = 171.42(3), Cl(21)-Si(2)-Cl(22) = 98.65(4), Cl(22)-Si(2)-Cl(23) = 104.81(5), Cl(23)-Ni(1)Cl(21) = 98.96(5).

the addition of Cp2FeþB(ArF)4-, a silylene complex showing an agostic interaction with the silyl hydrogen.24c In an attempt to distinguish between the two mechanisms, we investigated the reaction of triethylsilane-d1 (Et3SiD) with precatalyst 1. We knew a priori that 1 does not catalyze disilane (Et3Si-SiEt3) formation from triethylsilane. We wanted to probe if triethylsilane would induce H2 or HD evolution and formation of Ni(0). Such a reaction would support the mechanism in Scheme 2. Neither H2 nor HD was formed. Instead scrambling of Et3SiD with nickel hydrides was observed, as evidenced by the appearance of Et3SiH in the 1H NMR spectra. The source of the proton was confirmed by the observation of Ni-D at -9.0 ppm by 2H NMR. This result indicates that triethylsilane interacts with the nickel center, but no hydrogen/deuterium atom from the silane is eliminated as H2 or HD gas. The lack of dihydrogen evolution and Si-D/H scambling can be taken in support of σ-bond metathesis but, of course, not a proof. The mechanistic evidence is not conclusive because Et3SiH is not coupled by 1 to give Et3Si-SiEt3. Furthermore, considering ample precedence for 1 acting as a source of Ni(0),27 the oxidativeaddition mechanism cannot be ruled out. 2.4. Synthesis and Characterization of (dippe)Ni(SiCl3)(Cl) (2). Attempts to isolate or detect a nickel silyl complex under catalytic conditions were not successful. Therefore, we reacted 1 with trichlorosilane (Cl3SiH), a tertiary silane that does not dimerize in the presence of 1. The reaction afforded (dippe)Ni(SiCl3)(Cl) (2) (eq 2) as the major product in good yields (61% isolated). The 31P spectrum is characteristic of two different phosphorus environments: two doublets with resonances in benzene-d6 at δ 76.1 ppm (JP-P = 25.0 Hz) and 89.7 ppm (JP-P = 25.0 Hz). Minor products of this reaction included (dippe)NiCl2 (31P NMR δ 87.2 ppm in benzene-d6). Residual gas analysis (RGA) was used to observe dihydrogen gas, which was evolved during the course of this reaction. RGA


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is an in-house built residual gas analyzer that consists of a single quadrupole electron impact mass spectrometer.28 The formation of 2 from 1 with trichlorosilane corroborates the oxidativeaddition mechanism outlined in Scheme 2. It is likely that a transient nickel-silyl hydride complex is formed in route to product 2 (eq 2). NMR provided evidence for the intermediacy of (dippe)Ni(SiCl3)H. The 1H NMR spectrum of the product mixture from reaction 2 shows two sets of CH peaks for the dippe ligand and a peak at -4.0 ppm, which is typical for a Ni-H. Furthermore, two species are observed in the 31P spectrum at δ 76.1 (d, JPP = 25.0 Hz) and 89.7 (d, JPP = 25.0 Hz) and a singlet at 59.9 ppm. The doublets are assigned to compound 2, and the singlet is assigned to the proposed intermediate, (dippe)Ni(SiCl3)H. The absence of splitting can be explained by dynamic behavior via η2 species as shown in eq 2. Two species are also observed by 29Si NMR: δ 16.5 (dd, JSiP = 79.4 and 303 Hz) and a triplet at 92.2 (t, JSiP = 20.9 Hz). The doublet of doublets is assigned to 2; the chemical shift and coupling constants compare well with similar compounds in the literature.24a The triplet at 92.2 ppm is assigned to the proposed intermediate, (dippe)Ni(SiCl3)H.

There is another synthetic route for the preparation of 2 that is better understood and easier to reconcile. The reaction of 1 with chlorobenzene affords (dippe)Ni(Ph)(Cl), 3, in 79% yield and releases dihydrogen (eq 3). This reaction has previously been studied by Jones and co-workers.27c The reaction of 3 with trichlorosilane yields 2 and PhSiCl3. Phenyltrichlorosilane was observed as the major siliconcontaining product by GC-MS with phenyldichlorosilane as the minor product. This observation supports a nickelsilyl hydride intermediate (eq 4). The formation of PhSiCl3 is accompanied by the formation of the intermediate, which reacts further with SiHCl3 to give 2. The minor products, (dippe)NiCl2 and PhSiHCl2, are formed by the direct substitution of a chloride for the phenyl group.

(28) Zdilla, M. J.; Lee, A. Q.; Abu-Omar, M. M. Inorg. Chem. 2009, 48, 2260.

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The molecular structure of complex 2 was solved by X-ray crystallography. An ORTEP rendition is shown in Figure 3 alongside selected bond distances and angles, and crystallographic data are tabulated in the Supporting Information. The structure features square-planar nickel(II) with the silicon of the silyl group in an approximate tetrahedral geometry. The Ni-Si bond of 2.24 A˚ is consistent with that observed for other nickel silyl complexes.24

3. Conclusions The dinuclear nickel hydride complex 1 produces polysilane with molecular weights and linear-to-cyclic ratios that are comparable to those obtained using group 4 metallocene catalysts. The dehydrocoupling reaction can proceed by either σ-bond metathesis or an oxidative-addition mechanism with a nickel chloro hydride intermediate. Compound 2, (dippe)Ni(SiCl3)(Cl), prepared and isolated via two pathways, represents a stable analogue of the hypothesized intermediate and provides support for the nickel silyl intermediate in catalytic dehydrocoupling of phenylsilane.

4. Experimental Methods 4.1. General Procedures and Methods. All reactions were carried out under a nitrogen atmosphere using standard Schlenk and drybox techniques. Solvents were distilled from sodium/ benzophenone ketyl and degassed. Deuterated NMR solvents (Cambridge Isotope Laboratories) were stored over molecular sieves. All glassware was dried in an oven at 120 °C. All reagents were used as received from commercial sources. 1H and 31P NMR spectra were collected using a Varian 300 MHz spectrometer. 29Si and 2H NMR spectra were collected using Bruker DRX 500 MHz spectrometers. A gas evolution apparatus purchased from ChemGlass was used for the measurement of the volume of evolved hydrogen gas. Gas analysis was done with an in-house built RGA equipped with a Varian model SH 100 vacuum pump and a Stanford Research Systems RGA 100 mass spectrometer with an Alcatel ATH131 Series turbopump.28 Complex 1, [(dippe)NiH]2, was prepared from the published procedure.27a Cp2Zr(CH3)2 was purchased from Strem. 4.2. Monitoring the Oligomerization of Phenylsilane. The kinetic data for the oligomerization of phenylsilane was obtained by two methods. The first method used (plotted in Figure 1) was NMR. Phenylsilane has a chemical shift of 4.2 ppm; the decrease in area of this peak was quantified by integration against a diphenylmethane internal standard (3.7 ppm). Peaks for oligomers and polymers were observed below 4.8 ppm, and cyclic oligomers were observed above 4.8 ppm.5m The second method used to monitor the kinetics was measuring the volume of dihydrogen gas evolved. This was done using a buret connected to an equalizing bulb filled with mineral oil. The displacement of oil with hydrogen gas was used to determine the volume evolved. The data from the gas evolution method are plotted in Figure 2. 4.3. Preparation of Samples for Gel Permeation Chromatography. Compound 1 (23.4 mg, 0.0363 mmol, 2.0%) was dissolved in benzene (0.4 mL). Phenylsilane (223.9 μL, 1.82 mmol) was also dissolved in toluene (0.176 mL) and added by syringe to the solution. The evolution of dihydrogen was monitored using a gas evolution apparatus. The yield of hydrogen gas was 81% after several hours. The remaining solution was red-brown and viscous. Compound 1 (17.9 mg, 0.0278 mmol, 2%) was dissolved in toluene (0.308 mL). This solution was added to phenylmethylsilane (192.8 μL, 1.31 mmol). The evolution of dihydrogen was monitored using a gas evolution apparatus. The yield of hydrogen gas was 76%. The remaining solution was red-brown and viscous.

Smith et al. Phenylsilane (666.6 μL, 5.408 mmol) was added by syringe directly to Cp2Zr(CH3)2 (27.2 mg, 0.108 mmol, 2%). The resulting solution was yellow. The evolution of dihydrogen was monitored using a gas evolution apparatus. The yield of hydrogen gas was 98%. The resulting product mixture was a reddish solid. Phenylsilane (534.6 μL, 4.337 mmol) was added by syringe directly to Ind2Zr(CH3)2 (30.5 mg, 0.0807 mmol, 2%). The resulting solution was colorless. The evolution of dihydrogen was monitored using a gas evolution apparatus. The yield of hydrogen gas was 78%. The resulting product mixture was orange and extremely viscous. 4.4. Gel Permeation Chromatography. Gel permeation chromatography was done using a Waters 2695 Separations Module, equipped with a vacuum degasser, and a Waters 2410 differential refractometer. Separation was done with Polymer Laboratories PLgel 5 μm Mixed-C columns preceded by a PLgel 5 μm guard column. Analyses were performed with certified grade THF as eluent with a flow rate of 1.0 mL/min. The column and detector were heated to 35 °C. Samples were prepared in THF at about 0.5% w/v solids and filtered through 0.45 μm PTFE syringe filters. An injection volume of 100 μL was used, and data were collected for 25 min. Data collection and analyses were performed using ThermoLabsystems Atlas chromatography software and Polymer Laboratories Cirrus GPC software. Molecular weight averages were determined relative to a third-order calibration curve created using polystyrene standards of molecular weight 580-2 300 000. The precision and accuracy of the analysis of these samples has not been established. In general, the repeatability of conventional GPC can be expected to be (5-10%. 4.5. Reaction of 1 with Triethylsilane-d1 by 1H NMR. A solution of triethylsilane-d1 (71.3 μL, 0.706 mmol) and diphenylmethane (3.7 μL, 0.023 mmol) internal standard was prepared in 500 μL of benzene-d6. A 1H NMR spectrum was collected, and no signal was observed at 3.8 ppm, the expected chemical shift for protio-triethylsilane. A solution of 1 (14.3 mg, 0.0222 mmol, 3.1 mol % loading) was prepared in 200 μL of benzene-d6. The solution of 1 was added to the solution of triethylsilane-d1. After 10 min, 1H and 31P NMR spectra were collected. The growth of a sextuplet at 3.8 ppm indicated scrambling of triethylsilane-d1 deuterium with a proton. The 31P NMR had only a singlet at 78 ppm, indicating that the catalyst was still in the form of 1. 4.6. Reaction of 1 with Triethylsilane-d1 by 2H NMR. A solution of 1 (14.3 mg, 0.0222 mmol, 3.1 mol % loading) was prepared in 700 μL of benzene. Triethylsilane-d1 (71.3 μL, 0.706 mmol) and benzene-d6 (10.5 μL, 1.18 mmol) were added to the solution. After 10 min, the 2H NMR spectrum was collected. The appearance of a peak at -9.0 ppm indicated that the deuterium from the triethylsilane-d1 was scrambling with the hydride from 1. 4.7. Synthesis of (dippe)Ni(SiCl3)(Cl) (2). Complex 2 was prepared using 1 (50.0 mg, 0.780 mmol) dissolved in benzene (50 mL) and adding trichlorosilane (156.0 μL, 1.544 mmol). The reaction was left to stir for two hours and then filtered through a frit of medium porosity. The filtrate was collected, and the benzene was removed under vacuum. The resulting yellow solid was dried overnight under vacuum. Yield: 61%. Hydrogen gas evolving from this reaction was detected by RGA-MS. NMR for 2 in benzene-d6: 1H, δ 0.67 (dd, JHH = 12.6, 6.9 Hz, 6H, CH3), 0.76 (dd, JHH = 13.5, 6.9 Hz, 6H, CH3), 0.92 (m, 2H, P(CH2)2P), 1.17 (dd, JHH = 16.4, 7.1 Hz, 6H, CH3), 1.27 (dd, JHH = 18.6, 7.2 Hz, 6H, CH3), 1.40 (m, 2H, P(CH2)2P), 2.03 (m, 2H, CH), 2.67 (m, 2H, CH); 31P{1H}, δ 76.1 (d, JPP = 25.0 Hz), 89.7 (d, JPP = 25.0 Hz); 29Si{1H}, δ 16.5 (dd, JSiP = 79.4 and 303 Hz, SiCl3). 4.8. Synthesis of (dippe)Ni(Ph)Cl (3). Complex 3 was prepared using 1 (50.0 mg, 0.78 mmol) dissolved in THF (30 mL) and adding chlorobenzene (198 μL, 1.98 mmol, 25 times excess). The solution was left to stir for 2 h and was reduced to 5 mL by vacuum. A 30 mL amount of hexane was added, and the mixture was filtered through a medium frit. An orange solid was isolated and dried overnight under vacuum. Yield: 79%. NMR spectra

Article for 3 in THF-d8: 1H, δ 1.04 (dd, JHH = 15.3, 7.2 Hz, 6H, CH3), 1.20 (dd, JHH = 14.0, 6.9 Hz, 6H, CH3), 1.29 (dd, JHH = 12.5, 7.1 Hz, 6H, CH3), 1.48 (dd, J = 15.5, 7.1 Hz, 6H, CH3), 1.57 (m, 2H, P(CH2)2P), 1.82 (m, 2H, P(CH2)2P), 2.16 (m, 2H, CH), 2.33 (m, 2H, CH), 6.64 (t, JHH = 7.5 Hz, 1H, p-H), 6.79 (dt, JHH = 1.5, 7.8 Hz, 2H, m-H), 7.40 (t, JHH = 6.9 Hz, 2H, o-H). 31P{1H}, δ 72.2 (d, JPP = 20.3 Hz), 75.4 (d, JPP = 20.4 Hz). This procedure was modified from the synthetic method for 3 published by Jones and co-workers.27 4.9. Preparation of 2 from 3. Complex 2 was prepared using 3 (15.0 mg, 0.03459 mmol) dissolved in benzene (20 mL). Trichlorosilane (5.2 μL, 0.05146 mmol) was added to the solution. The solution was left to stir for 2 h and was filtered through a frit of medium porosity. The filtrate was collected, and the benzene was removed under vacuum. The resulting yellow solid was dried overnight under vacuum. Yield: 61%. 5.0. Crystallographic Studies. Orange needle crystals of 2 were obtained by solvent diffusion in toluene with hexane for the diffusing solvent after several days at -35 °C in a glovebox. A crystal was mounted on a fiber in a random orientation. Preliminary examination and data collection were performed with Mo KR radiation (λ = 0.71073 A˚) on a Nonius KappaCCD equipped with a graphite crystal, incident beam monochromator. The orthorhombic cell parameters and calculated volume were (29) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307. (30) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112. (31) Burla, M. C.; Caliandro, R.; Camalli, M.; Carrozzini, B.; Cascarano, G. L.; De Caro, L.; Giacovazzo, C.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 2005, 38, 381. (32) International Tables for Crystallography; Kluwer Academic Publishers: Utrecht, The Netherlands, 1992; Vol. C, Tables 4.2.6.8 and 6.1.1.4. (33) Johnson, C. K. ORTEPII, Report ORNL-5138; Oak Ridge National Laboratory: Oak Ridge, TN, USA, 1976.


6533

a = 8.8414(4) A˚, b = 13.6132(5) A˚, c = 18.7005(8) A˚, V = 2250.79(16) A˚3. For Z = 4 and fw = 490.97 the calculated density is 1.45 g/cm3. The refined mosaicity from DENZO/ SCALEPACK29 was 0.40°, indicating good crystal quality. The space group was determined to be P212121 (#19) by the program XPREP.30 The data were collected at a temperature of 150(1) K to a maximum 2θ of 54.9°. Frames were integrated with DENZO-SMN.29 Lorentz and polarization corrections were applied to the data. The linear absorption coefficient is 15.3/mm for Mo KR radiation. An empirical absorption correction using SCALEPACK29 was applied. The crystallographic data are provided in the Supporting Information. The structure was solved by direct methods using SIR2004.31 The remaining atoms were located in successive difference Fourier syntheses. Scattering factors were taken from the International Tables for Crystallography.32 Refinement was performed on a LINUX PC using SHELX-97.30 Crystallographic drawings were done using the programs ORTEP33 and PLUTON.34

Acknowledgment. Thank you to the U.S. Department of Energy for financial support (grant no. DE-FG-0206ER15194) and to the Dow Corning Corporation for both GPC analysis and financial support. Supporting Information Available: Crystallographic data (CIF), table of crystallographic data, and GPC for nickel and zirconium catalysts. This material is available free of charge via the Internet at http://pubs.acs.org. (34) Spek, A. L. PLUTON, Molecular Graphics Program; University of Utrecht: The Netherland, 1991.

Dehydrocoupling of Organosilanes with a Dinuclear Nickel Hydride

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