Novel Pt(II) Mono- and Biscarbene Complexes: Synthesis, Structural

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Novel Pt(II) Mono- and Biscarbene Complexes: Synthesis, Structural Characterization and Application in Hydrosilylation Catalysis Jian Jin Hu, Fuwei Li, and T. S. Andy Hor* Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, Kent Ridge, Singapore 117543 ReceiVed October 10, 2008

Reactions of imidazolium and benzimidazolium halides with PtBr2 and PtI2 in DMSO afford cis-Pt(II) heterocyclic carbene DMSO mixed ligand complexes 1-3 and biscarbene complexes 4-6. The reaction between 1,3-dibenzylbenzimidazolium bromide and PtBr2 in CH3CN gives no carbene complex but a benzimidazolium hexabromoplatinate(IV) salt 7. All the products have been fully characterized by NMR and ESI-MS spectroscopy as well as X-ray single-crystal diffraction study. Both intramolecular γ-hydride interactions with metal or its coordinated bromide and intermolecular H-bonding between heterocyclic proton with bromide are evident in some of these structures. These complexes are active in the hydrosilylation of terminal alkynes (phenylacetylene and trimethylsilylacetylene) with triethylsilane and bis(trimethylsiloxy)methylsilane. When phenylacetylene is used, the monocarbene complexes (1-3) show higher catalytic activity than chelating biscarbene analogues (5 and 6). Non-chelating biscarbene complex 4 gives a different selectivity pattern favoring the β(Z) and dehydrogenative silylation products when compared to 1-3, 5 and 6, and 7. In contrast, 4 and 5 are more active than 1 and 2 toward trimethylsilylacetylene. In general, monocarbene complexes give higher conversions over a short duration in the hydrosilylation of phenylacetylene with triethylsilane and bis(trimethylsiloxy)methylsilane, whereas in the hydrosilylation of trimethylsilylacetylene with triethylsilane, the chelating and biscarbene tend to be more active. Introduction The strongly σ-donating property of N-heterocyclic carbenes (NHCs) has put them closer to phosphines than the classical Fischer or Schrock carbenes.1 Driven primarily by their applications in catalysis, numerous NHC complexes have emerged in recent years. Typical examples are found in the Suzuki- and Heck-active Pd(II) complexes1 with NHC ligands derived from imidazole, benzimidazole, and imidazoline precursors. Comparatively, Pt(II) NHC complexes are less developed, probably due to their more limited catalytic uses.2 Recently, Marko` et al. reported some air- and moisture-stable Pt(0) NHC complexes which catalyze the hydrosilylation reaction of alkenes, as well * To whom correspondence should be addressed. E-mail: andyhor@ nus.edu.sg. Fax: (65) 6873 1324. (1) (a) Herrmann, W. A.; Ko¨cher, C. Angew. Chem., Int. Ed. Engl. 1997, 36, 2162. (b) Tamm, M.; Hahn, F. E. Coord. Chem. ReV. 1999, 182, 175. (c) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290. (d) Cavell, K. J.; McGuinness, D. S. Coord. Chem. ReV. 2004, 248, 671. (e) Ce´sar, V.; Bellemin-Laponnaz, S.; Gade, L. H. Chem. Soc. ReV. 2004, 33, 619. (f) Hahn, F. E. Angew. Chem., Int. Ed. 2006, 45, 1348. (g) Kantchev, E. A. B.; O’Brien, C. J.; Organ, M. G. Angew. Chem., Int. Ed. 2007, 46, 2768. (h) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47, 3122. (2) (a) Cardin, D. J.; Cetinkaya, B.; Cetinkaya, E.; Lappert, M. F J. Chem. Soc., Dalton Trans. 1973, 514. (b) Liu, S.-T.; Hsieh, T.-Y.; Lee, G.-H.; Peng, S.-M. Organometallics 1998, 17, 993. (c) McGuinness, D.; Clement, N. D.; Cavell, K. J.; Yates, B. F. Chem. Commun. 2003, 400. (d) Duin, M. A.; Clement, N. D.; Cavell, K. J.; Elsevier, C. J. Chem. Commun. 2003, 400. (e) Hahn, F. E.; Langenhahn, V.; Lugger, T.; Pape, T.; Van, D. L. Angew. Chem., Int. Ed. 2005, 44, 3759. (f) Hahn, F. E.; Jahnke, M. C.; Pape, T. Organometallics 2006, 25, 1927. (g) Boyeston, A. J.; Rice, J. D.; Sanderson, M. D.; Dykhno, O. L.; Bielawski, C. W. Organometallics 2006, 25, 6087. (h) Voutchkova, A. M.; Feliz, M.; Clot, E.; Eisenstein, Crabtree, R. H. J. Am. Chem. Soc. 2007, 129, 12834. (i) Unger, Y.; Zeller, A.; Ahrens, S.; Strassner, T. Chem. Commun. 2008, 3263.

as terminal and internal alkynes with remarkable efficiency and selectivity.3,4 However, preparation of these complexes necessitates dry, air-free conditions as it involves deprotonation of the NHC precursors by a strong base such as NaH in a dry solvent such as THF.2h,3d Hydrosilylation of alkynes is of great importance as it provides a direct access to synthetically significant vinylsilanes.5,6 It is somewhat surprising that very few Pt(II) NHC complexes have been applied in hydrosilylation even though they are generally more stable and, in principle, easier to prepare than their Pt(0) counterparts. Among the few known Pt(II) NHC systems are the bridged and chelating complexes7 prepared from in situ deprotonation of diimidazolium salts with NaOAc or with Pt(acac)2 (acac ) acetylacetonate) in wet DMSO without NaOAc. Use of Ag NHC complex as a transfer agent in the reaction of Ag2O with imidazolium or (3) (a) Marko´, I. E.; Ste´rin, S.; Busine, O.; Mignani, G.; Branlard, P.; Tinant, B.; Declercq, J. P. Science 2002, 298, 204. (b) Marko´, I. E.; Ste´rin, S.; Buisine, O.; Berthon, G.; Michaud, G.; Tinant, B.; Declercq, J. P. AdV. Synth. Catal. 2004, 346, 1429. (c) Buisine, O.; Berthon-Gelloz, G.; Brie`re, J. F.; Ste´rin, S.; Mignani, G.; Branlard, P.; Tinant, B.; Declerq, J. P.; Marko´, I. E. Chem. Commun. 2005, 3856. (d) Berthon-Gelloz, G.; Busine, O.; Brie`re, J. F.; Ste´rin, S.; Mignani, G.; Branlard, P.; Tinant, B; Declerq, J. P.; Chapon, D.; Marko´, I. E. J. Organomet. Chem. 2005, 690, 6156. (4) (a) Guillaume, D. B.; Berthon-Gelloz, G.; Tinant, B.; Marko´, I. E. Organometallics 2006, 25, 1881. (b) Berthon-Gelloz, G.; Schumers, J.; Guillaume, D. B.; Marko´, I. E. J. Org. Chem. 2008, 73, 4190. (5) (a) Tamao, K.; Kumada, M.; Maeda, K. Tetrahedron Lett. 1984, 25, 321. (b) Blumenkopf, T. A.; Overman, L. E. Chem. ReV. 1986, 86, 857. (c) Langkopf, E.; Schinzer, D. Chem. ReV. 1995, 95, 1375. (d) Bunlaksananusorn, T.; Rodriguez, A. L.; Knochel, P. Chem. Commun. 2001, 745. (6) (a) Fleming, I.; Dunogues, J.; Smithers, R. Org. React. 1989, 37, 57. (b) Hatanaka, Y.; Hiyama, T. Synlett 1991, 845. (c) Denmark, S. E.; Sweis, R. F. Acc. Chem. Res. 2002, 35, 835.

10.1021/om800978j CCC: $40.75  2009 American Chemical Society Publication on Web 01/29/2009

NoVel Pt(II) Mono- and Biscarbene Complexes Scheme 1. Synthesis of Imidazolium and Benzimidazolium cis-Pt(II) DMSO-Coordinated Monocarbene Complexes 1-3

Organometallics, Vol. 28, No. 4, 2009 1213 Scheme 2. Synthesis of Imidazolium cis-Pt(II) Biscarbene Complex 4

Scheme 3.

Synthesis of Chelating cis-Pt(II) Carbene Complexes 5 and 6

oxazole-functionalized imidazolium salts is also noted.8 As part of our current interest in carbene complexes,9 we herein report a series of novel Pt(II) NHC complexes and their use as hydrosilylation catalysts in terminal alkynes using triethylsilane and bis(trimethylsiloxy)methylsilane as substrates. Many of these species also exhibit weak intramolecular anagostic (C-H · · · M) interactions that have been found in related systems.10

Results and Discussion Synthesis of Platinum(II) Mono- and Biscarbene Complexes. Reaction of PtBr2 with N,N-dibenzylbenzimidazolium bromide and NaOAc in 1:2:2 molar ratio in DMSO at 120 °C does not give the stoichiometrically directed biscarbene but the hybrid Pt(II) monocarbene complex with coordinated DMSO [PtBr2(Bz2-Bimy)(DMSO)], 1 (Bz2-Bimy ) 1,3-dibenzylbenzimidazolin-2-ylidene) in 70% yield (Scheme 1). Analogous imidazolium-derived products [PtBr2(Bz2-Imy)(DMSO)], 2 (Bz2Imy ) 1,3-dibenzylimidazolin-2-ylidene), and [PtBr2(Me,BuImy)(DMSO)], 3 [Me,Bu-Imy ) 1-methyl-3-(3-methylbutyl)imidazolin-2-ylidene], are obtained from the use of N, N-dibenzylimidazolium or 1-methyl,3-(3-methylbutyl)imidazolium bromide. These complexes are presumably formed from the solvent (DMSO)-induced facile bridge cleavage reaction of the intermediate dinuclear Pt(II) NHC complex, which is not observed even under molar equivalence of PtBr2 and imidazolium salt. The presence of excess DMSO drives the formation of these mixed ligand compounds even though DMSO is generally regarded as a weaker σ-donor than carbene.10c Huynh et al.11a reported a similar product from the reaction of PtBr2 with diisopropylbenzimidazolium bromide and NaOAc with the trans-biscarbene as a byproduct (∼3%), which is not detected in our system. Replacement of PtBr2 by PtI2 in the synthesis results in the biscarbene complex [PtI2(Me2-Imy)], 4 (Me2-Imy ) 1,3dimethylimidazolin-2-ylidene), as the major product (69%) (Scheme 2). The cis product is isolated, which is less common (7) (a) Muehlhofer, M.; Herdtweck, E; Herrmann, W. A.; Strassner, T. J. Organomet. Chem. 2002, 660, 121. (b) Ahrens, S.; Herdtweck, E.; Goutal, S.; Strassner, T. Eur. J. Inorg. Chem. 2006, 1268. (c) Ahrens, S.; Strassner, T. Inorg. Chim. Acta 2006, 359, 4789. (d) Fantasia, S.; Petersen, J. L.; Jacobsen, H.; Cavallo, L.; Nolan, S. P. Organometallics 2007, 26, 5880. (8) (a) Poyatos, M.; Maisse-Franc¸ois, A.; Bellemin-Laponnaz, S.; Gade, L. H. Organometallics 2006, 25, 2634. (b) Newman, P. C.; Deeth, R. J.; Clarkson, G. J.; Rourke, J. P. Organometallics 2007, 26, 6225. (9) (a) Yen, S. K.; Koh, L. L.; Hahn, F. E.; Hor, T. S. A. Organometallics 2006, 25, 5105. (b) Yen, S. K.; Koh, L. L.; Huynh, H. V.; Hor, T. S. A. Dalton Trans 2007, 3952. (c) Yen, S. K.; Koh, L. L.; Huynh, H. V.; Hor, T. S. A. Dalton Trans 2008, 699. (d) Li, F. W.; Bai, S. Q.; Hor, T. S. A. Organometallics 2008, 27, 672. (10) (a) Huynh, H. V.; Wong, L. R.; Ng, P. S. Organometallics 2008, 27, 2231. (b) Yen, S. K.; Koh, L. L.; Huynh, H. V.; Hor, T. S. A. J. Organomet. Chem. 2008, in press (doi: 10.1016/j.jorganchem.2008.10.048). (c) Evans, D. R.; Huang, M.; Seganish, W. M.; Fettinger, J. C.; Williams, T. L. Inorg. Chem. Commun. 2003, 6, 462.

compared to its trans isomer.8b,11,12 Contrary to the bromo derivatives, the mixed ligand complex is not formed. This could suggest that the product is formed directly from the carbene addition upon HOAc elimination without going through a dinuclear iodo-bridged intermediate. As expected, use of a diimidazolium dication such as 1,1′dibenzyl-3,3′-methylene diimidazolium would yield the cischelating biscarbene complexes in either bromo (5) or iodo (6) form (Scheme 3). Their 1H NMR spectra show that the two protons on the methylene bridgehead are inequivalent (5: δ 6.09 and 6.16 ppm; 6: δ 6.09 and 6.16 ppm), which is probably attributed to the nonplanarity of the chelate ring of the NHC ligand upon complexation. The solvent DMSO plays a key role both in the formation and stabilization of the mono-NHC complexes 1-3. This is further exemplified in the reaction of PtBr2 with 1,3-dibenzylbenzimidazolium bromide, [Bzim]+[Br]-, and NaOAc in refluxing CH3CN, which does not result in any NHC complex but a redox reaction giving the [PtBr6]2- salt of 1,3-dibenzylbenzimidazolium 7 in the form of red residue and Pt black as byproduct (Scheme 4). Similar complexes with [PtCl6]2- anion have been reported, but they are generally obtained from ionic exchanges with K2[PtCl6] or addition of PtCl4.13 Complexes 1-4 show good solubility in chlorinated solvents such as CH2Cl2 and CHCl3, whereas 5 and 6 are soluble in CH3CN. They are air-stable and moisture-insensitive and can be stored for a prolonged period without apparent decomposition. The 1H NMR spectra of 1-6 are characterized by the absence of the NCHN protons which are fingerprints of the NHC precursors, thus suggestive of successful formation of the metal NHC moieties. The 13C NMR spectra of 1 and 2 and 4-6 show the Pt-Ccarbene signals in the range of 145.1-157.8 ppm. A similar resonance was not detected in 3. Presence of the imidazolium NCHN proton in 7 is supported by 1H (δH 10.0 ppm) and 13C NMR (δC 142.7 ppm) data. The negative mode of the ESI-MS spectra also gives major molecular ion peaks at 594.7 ([PtBr5]-) and 972.2 ({[PtBr6][Bzim]}-). (11) (a) Han, Y.; Tan, G. K.; Huynh, H. V. Organometallics 2007, 26, 4612. (b) Huynh, H. V.; Wong, L. R.; Ng, S. Organometallics 2007, 27, 2231. (c) Rack, J. J.; Gray, H. B. Inorg. Chem. 1999, 38, 2. (d) Brookhart, M.; Green, M. L. H.; Parkin, G. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 6909. (12) Liu, Q.; Song, H.; Xu, F.; Li, Q.; Zeng, X.; Leng, X.; Zhang, Z. Polyhedron 2003, 22, 1515. (13) Hasan, M.; Kozhevnikov, I. V.; Siddiqui, M.; Rafiq, H.; Femoni, C.; Steiner, A.; Winterton, N Inorg. Chem. 2001, 40, 795.

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Scheme 4. Preparation of 1,3-Dibenzylbenzimidazolium Bromoplatinate(IV) in CH3CN

All the complexes have been characterized by X-ray singlecrystal crystallographic analysis. Their structures are in general agreement with those proposed from the solution spectroscopic data. Complexes 1-6 show typical square planar geometry for Pt(II). Complexes 1-3 are isostructural with the S-coordinated DMSO (Pt-Sdmso 2.207(3)-2.215(4) Å), cis to the NHC ligand (Pt-Ccarbene 1.974(6)-2.008(14) Å). These lengths are similar to those in other hybrid Pt(II) monocarbene complexes with coordinated DMSO (Pt-Sdmso 2.196(2)-2.219(1) Å and Pt-Ccarbene 1.970(3)-1.993(4) Å).7d,8b,11a The solvate coordination shortens the SdO bonds (1.457(5)-1.483(9) Å) compared to free DMSO (1.492(1) Å).11b The Pt-Br bonds trans to the NHC ligand (2.480(1)-2.490(2) Å) are generally longer than those trans to DMSO (2.437(2)-2.448(1) Å), indicating a stronger trans influence of NHCs than the S-donating DMSO. Weak intramolecular interactions are seen in 1 (C8-H8B · · · Pt1 ) 2.879(2) Å) and 2 (C11-H11B · · · Pt1 ) 2.842(1) Å) between one of the benzylic protons and the metal (Figures 1 and 2a). The NHC plane is rotated 75.3° out of the Pt coordination plane (Pt1C1S1Br1Br2), which promotes γ-H · · · Pt interactions at 30.0° to the metal plane. This type of anagostic interaction, which is generally in the range of M · · · H ≈ 2.3-2.9 Å and ∠M-H-C ≈ 110-170 °C, is electrostatic in nature and has been discussed elsewhere.11c It is also retained in solution, as evident from the 1 H NMR spectra of 1 and 2 which show a downfield shift of the benzylic protons (δH 6.06 ppm for 1 and 5.65 ppm for 2) compared to that of the free NHC precursors (δH 5.87 ppm for 1 and 5.54 ppm for 2). In 2, the other type of γ-H within the imidazole ring is also involved in intermolecular H-bonding with thecoordinatedbromideoftheneighboringmolecule(C3A-H3A · · · Br1 2.790(1) Å), thereby giving a chain-like supramolecular polymer (Figure 2b). The presence of two types of γ-hydrogen in the form of intra- and intermolecular H-bonding is a unique

Figure 1. Molecular structure of complex 1 showing γ-hydrogen anagostic interaction with the metal. Selected bond lengths (Å) and angles (deg): Pt(1)-C(1) 2.009(1); Pt(1)-S(1) 2.215(4); Pt(1)-Br(1) 2.437(3); Pt(1)-Br(2) 2.490(2); S(1)-O(1) 1.479(1); C8-H8B · · · Pt1 ) 2.879(2); C(1)-Pt(1)-Br(1) 90.5(4); C(1)-Pt(1)-S(1) 88.3(4); Pt(1)-S(1)-O(1)116.1(5);S(1)-Pt(1)-Br(2)90.1(1);Pt(1)-H(8B)C(8) 111.12(1).

structural feature of 2. When the benzyl substituent is replaced by isopentyl, which also carries γ-H, as in 3, the interaction is weakened to become negligible (H5B · · · Pt1 ≈ 2.965 Å; Figure 3). Complex 4 is an unusual cis form of biscarbene (Figure 4) amidst the vast majority of trans- Pt(II) biscarbene complexes.8b,11a,12 The Pt-C bonds (1.978(5) and 1.988(5) Å) are shorter than those in Pt(0) complexes, viz. Pt(IMe)(dvtms) (IMe ) 1,3-dimethylimidazolin-2-ylidene; dvtms ) divinyltetramethylsiloxane) (2.050(11) Å)3d and other trans-Pt(II) biscarbene complexes (2.015(4)-2.044(7) Å).8,11,12 Complexes 5 and 6 show a common chelating carbene square planar structure. The presence of benzylic γ-hydrogen also results in H-bonding in 5 (Figure 5a,b). However, instead of interaction with the metal (as in 1 and 2), one of the benzylic protons interacts intramolecularly with the coordinated bromide (C8-H8A · · · Br1 ) 2.720(1) Å and C15-H15A · · · Br2 ) 2.815(1) Å) to give a pseudo-six-membered rings. As there are two benzyl substituents and both are involved, this effectively results in the fusion of three six-membered and two five-membered rings, thus lending much stability to the complex. The bromide further participates in intermolecular H-bonding with the hydrogen on the 3-position of the imidazole ring (Pt1-Br1 · · · H3A ) 2.757(1) Å) (Figure 5b). Contact between the second set of bromo and imidazole (H6A · · · Br2 ) 2.970(1) Å) is too long to justify any bonding interaction. These interactions help to align the molecules to give a supramolecular network similarly to that in 2. However, similar interactions are not observed in 6 (H15a · · · I2 ≈ 3.003 Å and H8b · · · I1 ≈ 3.034 Å; H3 with nearest neighboring I ∼3.393 Å), possibly a result of the lower electronegativity of iodide (Figure 6). Complex 7 is a 1,3-dibenzylbenzimidazolium salt of hexabromoplatinate. The analogous [PtCl6]2- has been reported.13 Significant cation-anion interaction is evident through Hbonding between imidazolium proton and bromide (C1-H1 · · · Br2 and C1-H1 · · · Br2A ) 2.867(1) Å; Figure 7). Catalytic Hydrosilylation of Terminal Alkynes with Complexes 1-6: Mono- versus Biscarbene Complexes. The isolation and characterization of Pt(II) NHC complexes 1-6 have presented a model system in which we can compare directly the catalytic activities of mono- with biscarbene and their chelating form. This also enables a comparison with the documented NHC Pt(0) activities in hydrosilylation.3,4 The hydrosilylation of alkynes generally yields three regioisomers: the R,β(E) and β(Z) isomers (Scheme 5). The thermodynamically more stable β(E) vinylsilane is usually the major product in platinum-catalyzed reactions.4,8a,14 The R isomer can be obtained as the major product using a [Cp*Ru]-based catalyst,15 while rhodium16 and iridium17 complexes can lead to the β(Z) vinylsilane as the major product (Scheme 5). Hydrosilylation of phenylacetylene with triethylsilane was carried out at 100 °C in toluene using 1 mol % of catalysts (14) (a) Green, M.; Spencer, J. L.; Stone, F. G. A. Dalton Trans. 1977, 1525. (b) Sy, K. G.; Bryant, G. L.; Lewis, L. N. Organometallics 1991, 10, 3750. (c) Aneetha, H.; Wu, W.; Verkade, J. G. Organometallics 2005, 24, 2590. (d) Roy, A. K.; Taylor, R. B. J. Am. Chem. Soc. 2002, 124, 9510.

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Figure 2. (a) Molecular structure of complex 2 showing the anagostic interactions with Pt(II) and γ-hydrogen. Selected bond lengths (Å) and angles (deg): Pt(1)-C(1) 1.974(6); Pt(1)-S(1) 2.215(2); Pt(1)-Br(1) 2.481(2); Pt(1)-Br(2) 2.448(1); S(1)-O(1) 1.457(5); C11-H11B · · · Pt1 ) 2.843(1); C(1)-Pt(1)-Br(2) 86.3(2); C(1)-Pt(1)-S(1) 90.5(2); S(1)-Pt(1)-Br(1) 92.45(4); Pt(1)-S(1)-O(1) 116.4(2); Pt(1)-H(11B)-C(11) 114.63(1). (b) Crystal packing diagram of complex 2 showing intramolecular C11-H11B · · · Pt1 (2.842(1) Å) and intermolecular C3A-H3A · · · Br1 (2.790(1) Å) interactions to give a supramolecular chain.

Figure 3. Crystal structure of complex 3; hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Pt(1)-C(1) 2.01(1); Pt(1)-S(1) 2.207(3); Pt(1)-Br(1) 2.482(2); Pt(1)-Br(2) 2.440(2); S(1)-O(1), 1.483(9); C(1)-Pt(1)-Br(2) 88.7(3); C(1)-Pt(1)-S(1) 88.8(3); S(1)-Pt(1)-Br(1) 90.94(7); Pt(1)-S(1)-O(1) 118.1(3).

Figure 4. Crystal structure of complex 4; hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Pt(1)-C(1) 1.978(5); Pt(1)-C(6) 1.988(5); Pt(1)-I(1) 2.659(1); Pt(1)-I(2) 2.665(1); C(1)-Pt(1)-C(6) 91.4(2); I(1)-Pt(1)-I(2) 92.08(2); C(1)-Pt(1)-I(2) 89.2(1); C(6)-Pt(1)-I(1) 87.5(2).

1-7 (Table 1). They generally give three products, a (R isomer), b (β(Z)), and c (β(E)), together with a dehydrogenative silylation product d. The major products are generally a and c, whereas b and d are the minor products. Some trace amounts of reductive

side products of styrene and ethylbenzene are also detected by GC-MS, although they are largely insignificant compared to a-d. A notable exception is found in 4, which gives b (34%) and d (40%) as the major isomeric products with the usual β(E) (c) form reduced to 20% yield. This is accomplished within 1 h, giving 97% conversion. The reported Pt(0) NHC catalysts almost invariably give the β(E) isomer as the major product.4 Complex 4 thus represents an unusual example of a Pt catalyst that selects the β(Z) isomer and enhances the formation of the usually suppressed silylation product.18,19 In all cases, no double hydrosilylation products (i.e., further hydrosilylation of R,β(E) and β(Z) vinylsilanes) are observed. The hydrosilylation mechanism and its selectivities have been vigorously examined.4b Formation of vinylsilanes from the hydrosilylation of phenylacetylene catalyzed by Pt(II) has also been observed.8a A redox cycle involving Pt(II/IV), similar to that reported for alkene hydrosilylation,14d is probably operative. Complex 2, which also has the bulky benzyl substituents, gives the highest selectivity for β(E) (c) isomer (Table 1, No. 5). This is achieved with good conversion (94%) in a relatively short duration (4 h). The β(E)/R selectivity ratio of ∼3 is considerably higher than those in other related Pt(II) imidazolium-based chelating hybrid monocarbene (C-N) systems.8a On the other hand, we recorded high R/β(E) selectivities in 1, 5, 6, and 7 which tend to produce at least two-fold more of the R (a) product than the most thermodynamically stable product (c). Similar reports of Marko` et al.4a suggested that the unusual regioselectivity trend observed for phenylacetylene with an electron-withdrawing group could be reversed in a substrate such (15) (a) Trost, B. M.; Ball, Z. T. J. Am. Chem. Soc. 2001, 123, 12726. (b) Kawanami, Y.; Sonoda, Y.; Mori, T.; Yamamoto, K. Org. Lett. 2002, 4, 2825. (16) (a) Doyle, M. P.; High, K. G.; Nelsoney, C. L.; Clayton, J. W.; Lin, J. Organometallics 1991, 10, 1225. (b) Mori, A.; Takahisa, E.; Yamamura, Y.; Kato, T.; Mudalige, A. P.; Kajiro, H.; Hirabayahi, K.; Nishihara, Y.; Hiyama, T. Organometallics 2004, 23, 1755. (c) Victoria Jimenez, M.; Perez-Torrente, J. J.; Bartolome, M. I.; Gierz, V.; Lahoz, F. J.; Oro, L. A. Organometallics 2008, 27, 224. (17) (a) Tanke, R. S.; Crabtree, R. H. J. Am. Chem. Soc. 1990, 112, 7984. (b) Tanke, R. S.; Crabtree, R. H. Chem. Commun. 1990, 1056. (c) Jun, C. H.; Crabtree, R. H. J. Organomet. Chem. 1993, 447, 177. (d) Esteruelas, M. A.; Olivan, M.; Oro, L. A.; Tolosa, J. I. J. Organomet. Chem. 1995, 487, 143. (e) Sridevi, V. S.; Fan, W. Y.; Leong, W. K. Organometallics 2007, 26, 1157. (18) Vicent, C.; Viciano, M.; Mas-Marza´, E.; Sanau´, M.; Peris, E. Organometallics 2006, 25, 3713.

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Figure 5. (a) Crystal structure of complex 5; hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Pt(1)-C(1) 1.973(8); Pt(1)-C(5) 1.971(8); Pt(1)-Br(1) 2.489(1); Pt(1)-Br(2) 2.495(1); C(1)-Pt(1)-C(5) 85.0(3); Br(1)-Pt(1)-Br(2) 88.63(3); C(1)-Pt(1)-Br(1) 92.5(2); C(5)-Pt(1)-Br(2) 93.5(2). (b) Crystal packing diagram of complex 5 showing the C8-H8A · · · Br1 ) 2.720(1) Å and C15-H15A · · · Br2 ) 2.815(1) Å intramolecular H-bonding interactions as well as the Pt1-Br1 · · · H3A ) 2.757(1) Å intermolcular hydrogen bonds which link individual molecules together to form a “supramolecular polymer”.

Figure 6. Crystal structure of complex 6; hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Pt(1)-C(1) 1.977(9); Pt(1)-C(9) 1.977(9); Pt(1)-I(1) 2.665(1); Pt(1)-I(2) 2.649(1); C(1)-Pt(1)-C(5) 84.4(4); I(1)-Pt(1)-I(2) 92.03(2); C(1)-Pt(1)-I(1) 91.2(3); C(5)-Pt(1)-I(2) 91.3(3).

Figure 7. Crystal structure of complex 7 showing the hydrogen bonds between [PtBr6]2- and [Bzim]+ (where [Bzim]+ ) N,Ndibenzylbenzimidazolium). Selected bond lengths (Å): Pt(1)-Br(2) 2.465(1); Pt(1)-Br(2A) 2.465(1), Pt(1)-Br(1)2.471(2); Pt(1)-Br(1A) 2.471(2); Pt(1)-Br(3) 2.475(1); Pt(1)-Br(3A) 2.475(1); C1-H1 · · · Br2 2.867(1) Å); C1A-H1A · · · Br2A (2.867(1) Å).

as 1-octyne that carries a more electron-donating group, thus pointing to the influence of the electronic nature of the alkyne on the regioselectivity.

The chelating carbenes, represented by 5 and 6, show similar selectivities compared to the monocarbene analogue (1) and the non-carbene Pt(IV) system (7), except that their reactions are generally more sluggish. The bromo form tends to show higher rate compared to its iodo counterpart. The hydrosilylation of phenylacetylene with the more inert bis(trimethylsiloxy)methylsilane4,8a has also been carried out at 100 °C in toluene using 1 mol % of catalyst 1-7 (Table 2). Hydrosilylation of alkynes by other dialkoxysilanes similar to bis(trimethylsiloxy)methylsilane is known to show modest selectivity.14a The mixed ligand monocarbene catalysts generally show good activities, with significant selectivities toward the R and β(E) isomers. Complex 2 and the non-carbene Pt(IV) complex (7) are most selective toward the R-isomer which is unusual since the majority of the Pt-catalyzed hydrosilylation using bis(trimethylsiloxy)methylsilane would selectively yield the β(E) isomer.4,8a,14 The biscarbene 4 is unique in this system in selecting the β(Z) isomer in 41% yield, albeit with significantly lower conversion yield and rate. The chelating complexes 5 and 6 behave similarly to the monocarbene counterparts (1-3) but are consistently less active. To demonstrate further the electronic effect of the alkyne, hydrosilylation of trimethylsilylacetylene with triethylsilane has been performed at a temperature just below the boiling point of trimethylsilylacetylene (52-53 °C) using 1 mol % of catalysts 1, 2, 4, 5, and 7 (Table 3). All complexes tested are active except the non-carbene 7. The catalyst activity trend in near-reversal of what is observed for phenylacetylene, with the chelating and biscarbene complexes 4 and 5 (96-98% conversions after 4 h), performs better than the monocarbenes 1 and 2. These catalysts generally favor the β(E) over the R isomer. Trace amounts of dehydrogenative silylation product are also detected when 4 is used. No β(Z) isomer is observed or detected. The hydrosilylation of trimethylsilylacetylene with the relatively inert bis(trimethylsiloxy)methylsilane has been similarly conducted at 50 °C in toluene using 1 mol % of catalysts of 1, 2, 4, 5, and 7. All of them show little activity after 19 h, except 5, which gives ∼13% conversion. This study demonstrates that Pt(II) NHCs show a significant and diverse range of inter- and intramolecular anagostic and H-bonding interactions in their solid states. Although the R/β regioselectivities and E/Z stereoselectivities toward hydrosilylation may not be synthetically significant, they nevertheless

NoVel Pt(II) Mono- and Biscarbene Complexes

Organometallics, Vol. 28, No. 4, 2009 1217

Scheme 5. Distribution of Products in Hydrosilylation and Dehydrogenative Silylation Reactions

Table 1. Hydrosilylation of Phenylacetylene with Triethylsilane Catalyzed by 1-7a

a Reaction conditions: alkyne/silane ratio is 1.2, solvent is toluene, T ) 100 °C; yield and selectivity of products were measured by 1H NMR and GC-MS. b Alkyne/silane ratio is 10.0.

Table 2. Hydrosilylation of Phenylacetylene with Bis(trimethylsiloxy)methylsilane Catalyzed by 1-7a

a Reaction conditions: alkyne/silane ratio is 1.2, solvent is toluene, T ) 100 °C; yield and selectivity of products were measured by 1H NMR and GC-MS.

provide a common platform for comparisons of a monocarbene versus biscarbene and a monodentate versus chelating carbene ligand. These complexes can also be electronically tuned by adjusting the exocyclic substituent on the carbene or even the heterocyclic atoms. They collectively provide an incentive for the development of biscarbene and mixed ligand Pt(II) complexes with a single strongly σ-donating NHC ligand balanced by a weaker and more labile ligand such as DMSO. It is also possible to compare with other hydrosilylations catalyzed by non-Pt metals. The nonchelating biscarbene system shows different selectivities with the chelating biscarbene. Its selectivity

for the dehydrogenative silylation product and β(Z) vinylsilane is surprising. Work is in progress to understand these anomalies and expand the silylation to other unsaturated systems.

Experimental Section General Considerations. Unless otherwise stated, all manipulations were performed without taking precautions to exclude air and moisture. All solvents were used as received unless otherwise noted. PtBr2 and PtI2 were purchased from Strem. All imidazolium and benzimidazolium salts were synthesized according to the literature

1218 Organometallics, Vol. 28, No. 4, 2009

Hu et al.

Table 3. Hydrosilylation of Trimethylsilylacetylene with Triethylsilane Catalyzed by 1, 2, 4, 5, and 7a

a Reaction conditions: alkyne/silane ratio is 1:2, solvent is toluene, duration ) 4 h; temp ) 50 °C; product yield and selectivity of products were measured by 1H NMR and GC-MS.

procedures20 or with slight modifications. NMR spectra were recorded on Bruker ACF300 300 MHz and AMX500 500 MHz FT NMR spectrometers. Mass spectra were measured using a Finnigan MAT LCQ(ESI) spectrometer. Elemental analyses were performed by the microanalytical laboratory within the department. cis-Dibromo(1,3-dibenzylbenzimidazolin-2-ylidene)(dimethylsulfoxide)platinum(II) (1). PtBr2 (50 mg, 0.14 mmol), anhydrous NaOAc (24 mg, 0.30 mmol), and 1,3-dibenzylbenzimidazolium bromide (108 mg, 0.28 mmol) were dissolved in Me2SO (5 mL) and heated to 80 °C for 2 h. The resultant yellow solution was further heated to 120 °C for 2 h. The pale yellow mixture thus obtained was cooled to room temperature and filtered over Celite. The filtrate was stripped of the solvent under vacuum and the residue washed successively with deionized H2O (2 × 2 mL), MeOH (2 × 2 mL), and Et2O (2 × 10 mL) and dried under vacuum to give 1 (2 mg, 70%) in the form of a white powder. Single crystals suitable for X-ray diffraction were grown by slow evaporation of a sample solution in CH2Cl2. 1H NMR (300.13 MHz, CD2Cl2-d2): δ ) 7.45-7.21 (m, 14H, phenyl H), 6.06 (dd, 2J(H, H) ) 15.7 Hz, 4H, CH2), 3.16 (t, 3J(Pt, H) ) 25.2 Hz, 6H, (CH3)2SO). 13C{1H} NMR (75.48 MHz, CD2Cl2-d2): δ ) 157.87 (s, NCN), 134.8, 133.9, 128.84, 128.15, 127.39, 123.99, 111.85, (s, Ar-C), 52.24 (s, CH2), 45.9 (s, (CH3)2SO). Anal. Calcd: C, 37.8; H, 3.3; N, 3.9. Found: C, 38.5; H, 3.3; N, 3.9. MS (ESI): m/z ) 1029.5 [M + C21H19N2]+. Crystallographic data, C23H24Br2N2OPtS (1): orthorhombic space group P212121; a ) 9.091(3), b ) 11.053(3), c ) 22.995(6) Å; R ) 90.00, β ) 90.00, γ ) 90.00°; V ) 2310.6(11) Å3; Z ) 4; GOF ) 1.014; independent reflections: 5300; final R indices: R1 ) 0.0697; wR2 ) 0.1376; R indices (all data): R1 ) 0.1046; wR2 ) 0.1499; largest diff. peak: 2.851 (e Å-3). cis-Dibromo(1,3-dibenzylimidazolin-2-ylidene)(dimethylsulfoxide)platinum(II) (2). PtBr2 (46 mg, 0.13 mmol), anhydrous NaOAc (24 mg, 0.30 mmol), and 1,3-dibenzylimidazolium bromide (90 mg, 0.27 mmol) were dissolved in Me2SO (5 mL) and heated to 80 °C for 2 h. The resultant yellow solution was further heated to 120 °C for another 2 h. The pale yellow mixture thus obtained was cooled to room temperature and filtered over Celite. The filtrate was stripped of solvent under vacuum and the residue washed successively with deionized water (2 mL) and Et2O (10 mL) and dried under vacuum in the form of a light gray powder. Single crystals suitable for X-ray diffraction were grown by slow evaporation of a sample solution in CH2Cl2. 1H NMR (300.13 MHz, CD2Cl2-d2): δ ) 7.40 (s, 10H, phenyl H), 6.85 (s, 2H, NCH) 5.65 (s, 4H, CH2), 3.21 (t, 3J(Pt, H) ) 25.2 Hz, 6H, (CH3)2SO). 13C{1H} (19) Sprengers, J. W.; Mars, M. J.; Duin, M. A.; Cavell, K. J.; Elsevier, C. J. J. Organomet. Chem. 2003, 679, 149. (20) (a) Cheng, Y.; Liu, M.-F.; Fang, D.-C.; Lei, X.-M. Chem.sEur. J. 2007, 13, 4282. (b) Zhang, Y.; Ngeow, K. C.; Ying, J. Y. Org. Lett. 2007, 18, 3495. (c) Dzyuba; Sergei, V.; Bartsch, J.; Richard, A. J. Heterocycl. Chem. 2001, 38, 265. (d) Olofson, R. A.; Thompson, W. R.; Michelman, J. S. J. Am. Chem. Soc. 1964, 86, 1865. (e) Lee, H. M.; Lu, C. Y.; Chen, C. Y.; Chen, W. L.; Lin, H. C.; Chiu, P. L.; Cheng, P. Y. Tetrahedron 2004, 60, 5807.

NMR (75.48 MHz, CD2Cl2-d2): δ ) 146.28 (s, NCN), 135.52, 129.29, 128.79, 128.67, 121.66, (s, Ar-C), 54.61 (s, CH2), 46.40 (s, (CH3)2SO). Anal. Calcd: C, 33.5; H, 3.3; N, 4.1. Found: C, 34.5; H, 3.0; N, 4.6. MS (ESI): m/z ) 929.62 [M + C17H17N2]+. Crystallographic data, C19H22Br2N2OPtS (2): orthorhombic space group Pna21; a ) 10.9117(9), b ) 8.815(2), c ) 19.431(5) Å; R ) 90.00, β ) 91.427(6), γ ) 90.00°; V ) 1740.8(8) Å3; Z ) 4; GOF ) 1.032; independent reflections: 4899; final R indices: R1 ) 0.0298; wR2 ) 0.0610; R indices (all data): R1 ) 0.0686; wR2 ) 0.1527; largest diff. peak: 1.512 (e Å-3). cis-Dibromo(1-methyl-3-(3-methylbutyl)imidazolin-2-ylidene)(dimethylsulfoxide)platinum(II) (3). PtBr2 (75 mg, 0.21 mmol), anhydrous NaOAc (80 mg, 0.97 mmol), and 1-methyl-3-(3methylbutyl)imidazolium bromide (210 mg, 0.90 mmol) were dissolved in Me2SO (5 mL) and heated to 80 °C for 2 h. The resultant pale yellow solution was further heated to 120 °C for another 2 h. The mixture obtained was cooled to room temperature and filtered over Celite. The filtrate was stripped of solvent by vacuum distillation and the residue washed successively with deionized H2O (2 mL) and Et2O (10 mL) and dried under vacuum in the form of a white powder. Single crystals suitable for X-ray diffraction were grown by slow diffusion of Et2O into a sample solution in CH2Cl2. 1H NMR (300.13 MHz, CD2Cl2-d2): δ ) 6.95 (t, 3J(H, H) ) 4.8 Hz, 2H, NCH), 4.35 (t, 3J(H, H) ) 8.1 Hz, 2H, NCH2), 3.93 (s, 3H, NCH3), 3.53 (t, 3J(Pt, H) ) 25.9 Hz, 3H, CH3SO), 3.52 (t, 3J(Pt, H) ) 25.3 Hz, 6H, CH3SO), 1.86-1.67 (m, 3H, CCH2CH), 1.02-0.99 (dd, 6H, C(CH3)2). 13C{1H} NMR (75.48 MHz, 24 °C, CD2Cl2-d2): δ ) 122.13 (s, NCH), 120.25 (s, NCH), 48.98 (s, NCH2), 46.78 (s, SCH3), 46.37 (s, SCH3), 38.62 (s, NCH3), 37.59 (s, CH2), 25.62 (s, CH), 22.14 (s, CH3), 22.10 (s, CH3); carbene carbon signal was not detected. Anal. Calcd: C, 22.6; H, 3.8; N, 4.8. Found: C, 22.7; H, 3.5; N, 4.9. MS (ESI): m/z ) 738.61 [M + C9H17N2]+. Crystallographic data, C11H22Br2N2OPtS (3): monoclinic space group P21/n; a ) 10.166(3), b ) 11.053(3), c ) 22.995(6) Å; R ) 90.00, β ) 90.00, γ ) 90.00°; V ) 2310.6(11) Å3; Z ) 4; GOF ) 1.014; independent reflections: 3063; final R indices: R1 ) 0.0568; wR2 ) 0.1446; R indices (all data): R1 ) 0.1046; wR2 ) 0.1499; largest diff. peak: 4.920 (e Å-3). cis-Diiodobis(1,3-dimethylimidazolin-2-ylidene)platinum(II) (4). PtI2 (50 mg, 0.11 mmol), anhydrous NaOAc (20 mg, 0.25 mmol), and 1,3-dimethylimidazolium iodide (57 mg, 0.25 mmol) were dissolved in Me2SO (5 mL) and heated to 80 °C for 2 h. The resultant red solution was further heated to 120 °C for another 2 h. The mixture obtained was cooled to room temperature and filtered over Celite. The filtrate was stripped of solvent by vacuum distillation and the residue washed successively with deionized water (2 mL) and Et2O (10 mL) and dried under vacuum in the form of a pale yellow powder. Single crystals suitable for X-ray diffraction were grown by slow evaporation of a sample solution in CH2Cl2. 1H NMR (500.13 MHz, DMSO-d6): δ ) 7.43-7.27 (d, 4H, NCH), 3.85-3.69 (d, 12H, NCH3). 13C{1H} NMR (125.7 MHz, CD2Cl2-d2): δ ) 149.4 (NCN), 123.80, 123.52 (s, NCH), 37.95,

NoVel Pt(II) Mono- and Biscarbene Complexes 37.30 (s, NCH3). MS (ESI): m/z ) 737.6 [M + C5H9N2]+. Crystallographic data, C10H16I2N4Pt (4): monoclinic space group P21/n; a ) 8.9311(11), b ) 16.229(2), c ) 11.239(2) Å; R ) 90.00, β ) 94.783(2), γ ) 90.00°; V ) 1623.3(3) Å3; Z ) 4; GOF ) 1.041; independent reflections: 3713; final R indices: R1 ) 0.0288; wR2 ) 0.0685; R indices (all data): R1 ) 0.0332; wR2 ) 0.0702; largest diff. peak: 1.962 (e Å-3). cis-Dibromo(1,1′-Dibenzyl-3,3′-methylene-4-diimidazolin-2,2′diylidene)platinum(II) (5). PtBr2 (50 mg, 0.14 mmol), anhydrous NaOAc (23 mg, 0.28 mmol), and 1,1′-dibenzyl-3,3′-methylene diimidazolium dibromide (69 mg, 0.14 mmol) were added to Me2SO (5 mL) and heated to 80 °C for 2 h. The temperature was then increased to 100 °C for another 1 h. The resultant mixture was cooled to room temperature and filtered over Celite. The filtrate was stripped of solvent and the residue washed successively with deionized water (5 mL), CH2Cl2 (2 mL), and Et2O (10 mL) and dried under vacuum to give a light gray product. Single crystals suitable for X-ray diffraction were grown by slow diffusion of Et2O into a sample solution in CH3CN. 1H NMR (500.13 MHz, DMSOd6): δ ) 7.53-7.22 (m, 14H phenyl and aryl H), 6.15 (d, 1H, NCH2N, 2J(H, H) ) 13.2 Hz), 6.06 (d, 1H, NCH2N, 2J(H, H) ) 13.2 Hz), 6.08 (d, 2H, NCH2Ph, 2J(H, H) ) 14.5 Hz), 5.27 (d, 2H, NCH2Ph, 2J(H, H) ) 14.5 Hz). 13C{1H} NMR (125.7 MHz, DMSOd6): δ ) 145.1 (s, NCN), 136.5, 128.6, 128.0, 127.9, 121.2 (s, aryl C), 62.1 (s, NCH2N), 52.8 (s, NCH2Ph). Anal. Calcd: C, 36.9; H, 3.0; N, 8.2. Found: C, 36.6; H, 3.4; N, 7.7. MS (ESI): m/z ) 602.9 [M - Br]+. Crystallographic data, C21H20Br2N4Pt.CH3CN (5): monoclinic space group P2(1)/n; a ) 11.7655(7), b ) 8.6377(5), c ) 25.2325(15) Å; R ) 90.00, β ) 90.656(2), γ ) 90.00°; V ) 2564.1(3) Å3; Z ) 4; GOF ) 1.013; independent reflections: 5848; final R indices: R1 ) 0.0512; wR2 ) 0.1176; R indices (all data): R1 ) 0.0823; wR2 ) 0.1297; largest diff. peak: 2.649 (e Å-3). cis-Diiodo(1,1′-dibenzyl-3,3′-methylene-4-diimidazolin-2,2′diylidene)platinum(II) (6). PtI2 (50 mg, 0.11 mmol), anhydrous NaOAc (18 mg, 0.23 mmol), and 1,1′-dibenzyl-3,3′-methylene diimidazolium diiodide (65 mg, 0.11 mmol) were added to Me2SO (5 mL) and heated to 75 °C for 3 h. The resultant mixture was cooled to room temperature and filtered over Celite. The filtrate was stripped of solvent, and the residue obtained was washed successively with deionized water (5 mL), CH2Cl2 (2 mL), and Et2O (10 mL) and dried under vacuum in the form of a pale yellow powder. Single crystals suitable for X-ray diffraction were grown by slow diffusion of Et2O into a sample solution in CH3CN. 1H NMR (500.13 MHz, DMSO-d6): δ ) 7.71-7.19 (m, 14H phenyl and aryl H), 6.16 (d, 1H, NCH2N, 2J(H, H) ) 13.2 Hz), 6.09 (d, 1H, NCH2N, 2J(H, H) ) 13.2 Hz), 5.93 (d, 2H, NCH2Ph, 2J(H, H) ) 15.1 Hz), 5.29 (d, 2H, NCH2Ph, 2J(H, H) ) 15.1 Hz). 13C{1H} NMR (125.7 MHz, DMSO-d6): δ ) 149.6 (s, NCN), 136.0, 128.5, 127.9, 121.2, 121.1 (s, aryl C), 62.1 (s, NCH2N), 54.0 (s, NCH2Ph). MS (ESI): m/z ) 650.0 [M - I]+. Crystallographic data, C21H20I2N4Pt.CH3CN (6): monoclinic space group C2/c; a ) 28.406(2), b ) 8.8110(6), c ) 22.1674(16) Å; R ) 90.00, β ) 115.553(2), γ ) 90.00°; V ) 5005.5(6) Å3; Z ) 8; GOF ) 1.167; independent reflections: 5744; final R indices: R1 ) 0.0586; wR2 ) 0.1304; R indices (all data): R1 ) 0.0678; wR2 ) 0.1346; largest diff. peak: 3.602 (e Å-3). Bis(1,3-dibenzylbenzimidazolium)hexabromoplatinate(IV) (7). PtBr2 (50 mg, 0.14 mmol), anhydrous NaOAc (26 mg, 0.32 mmol), and 1,3-dibenzylbenzimidazolium bromide (108 mg, 0.29 mmol) were added to CH3CN (10 mL). The suspension was refluxed for 3 h. The resultant orange suspension was filtered over Celite, and the red residue collected was washed successively with CH3CN (2 mL) and Et2O (10 mL) and dried under vacuum in the form of a red powder. Single crystals suitable for X-ray diffraction were grown by slow diffusion of Et2O into a sample solution in CH3CN. 1 H NMR (300.13 MHz, DMSO-d6): δ ) 10.08 (s, 1H, NCHH), 7.99-7.41 (m, 14H, phenyl H), 5.80 (s, 4H, NCH2). 13C{1H}NMR (75.47 MHz, DMSO-d6): δ ) 142.7 (s, NCHN), 133.9, 131.1, 129.0,

Organometallics, Vol. 28, No. 4, 2009 1219 128.7, 128.3, 126.8, 114.0 (s, aryl C), 50.0 (s, NCH2). Anal. Calcd: C, 39.6; H, 3.0; N, 4.4. Found: C, 41.3; H, 3.1; N, 4.5. MS (ESI): m/z ) 974.3 [C21H19N2]+[PtBr6]2-. Crystallographic data, C42H38Br6N4Pt (7): triclinic space group P1j; a ) 8.5907(9), b ) 11.9725(13), c ) 12.6266(13) Å; R ) 63.442(2), β ) 84.174(2), γ ) 71.003(2)°; V ) 1096.9(2) Å3; Z ) 1; GOF ) 0.982; independent reflections: 7505; final R indices: R1 ) 0.0598; wR2 ) 0.1104; R indices (all data): R1 ) 0.0843; wR2 ) 0.1196; largest diff. peak: 2.991 (e Å-3). General Procedure for the Hydrosilylation of Terminal Alkynes with Triethylsilane and Bis(trimethylsiloxy)methylsilane. In a Schlenk tube, a solution of 1 (0.005 mmol) in toluene (5.0 mL) was prepared. Phenylacetylene (0.6 mmol) or trimethylsilylacetylene (0.6 mmol), together with triethylsilane (0.5 mmol) or bis(trimethylsiloxy)methylsilane (0.5 mmol), was added in quick successions via syringe. The yellow solution was stirred at 100 °C for 24 h. Samples were taken periodically in intervals for GC-MS analysis. After 24 h, the reaction was stopped and the crude mixture was filtered through a capillary tube filled with silica gel, stripped of solvent, and analyzed by 1H NMR spectroscopy. A similar procedure was applied to the use of 2-7. Identification of Alkenylsilanes. The molecular weights of the alkenylsilane were determined from the molecular ion peaks in the GC-MS analysis. The regiochemistry and stereochemistry of the alkenylsilane isomers were determined from 1H NMR based on the olefinic coupling constants.8a,17c For the β(Z) and β(E) isomers, the two CHdCH′ vinyl resonances from 5 to 7.5 ppm show characteristic coupling. When these protons are mutually trans, 3J(H,H′) coupling is in the typical range of 18-20 Hz. When the protons are mutually cis, 3J(H,H′) coupling is typically between 12 and 16 Hz. For the R-isomer, the vinylic protons are geminal and the 2J(H,H′) coupling found in this case is much smaller than the 3J(H,H′) coupling. The hydrosilylation of phenylacetylene with triethylsilane yields three isomers: R-(triethylsilyl)styrene, cis-β(triethylsilyl)styrene, and trans-β-(triethylsilyl)styrene. These three products were unambiguously determined on the basis of the olefinic coupling constants in the 1H NMR spectra: β(Z)-isomer 1H NMR (CD2Cl2) δ 7.58 (d, J ) 15.1 Hz), 5.89 (d, J ) 15.1 Hz); β(E)isomer 1H NMR (CD2Cl2) δ 7.04 (d, J ) 19.2 Hz), 6.56 (d, J ) 19.2 Hz); R-isomer 1H NMR (CD2Cl2) δ 5.98 (d, J ) 3.0 Hz), 5.70 (d, J ) 3.0 Hz). GC-MS: m/z ) 218. The hydrosilylation of phenylacetylene with bis(trimethylsiloxy)methylsilane also yields three isomers: β(Z)-isomer 1H NMR (CDCl3) δ 7.45 (d, J ) 15.5 Hz), 5.70 (d, J ) 15.5 Hz); β(E)-isomer 1H NMR (CDCl3) δ 7.01 (d, J ) 19.2 Hz), 6.31 (d, J ) 19.2 Hz); R-isomer 1H NMR (CDCl3) δ 5.96 (d, J ) 3.1 Hz), 5.75 (d, J ) 3.1 Hz). GC-MS: m/z ) 324. The products from the hydrosilylation of phenylacetylene with triethylsilane and bis(trimethylsilyloxy)methylsilane have been previously reported in the literature.8a,17c The hydrosilylation of trimethylsilylacetylene with triethylsilane yields two isomers: β(E)isomer 1H NMR (CDCl3) δ 6.56 (d, J ) 22.7 Hz), 6.45 (d, J ) 22.7 Hz); R-isomer 1H NMR (CDCl3) δ 6.33 (d, J ) 5.0 Hz), 6.22 (d, J ) 5.0 Hz). GC-MS: m/z ) 214. The hydrosilylation of trimethylsilylacetylene with bis(trimethylsiloxy)methylsilane also yields two isomers: β(E)-isomer 1H NMR (CDCl3) δ 6.71 (d, J ) 22.7 Hz), 6.43 (d, J ) 22.7 Hz); R-isomer 1H NMR (CDCl3) δ 6.38 (d, J ) 5.0 Hz), 6.29 (d, J ) 5.0 Hz). GC-MS: m/z ) 320. Identification of Dehydrogenative Silylation Products. The products of dehydrogenative silylation reaction were identified based on their m/z data and characteristic MS fragmentation patterns, determined by GC-MS, as given below. Triethyl(phenylethynyl)silane. GC-MS: m/z ) 216. MS fragmentation pattern: 216, 187, 159, 131, and 105. Bis(trimethylsilyloxyl)methyl(phenylethynyl)silane. GC-MS: m/z ) 322. MS fragmentation pattern: 322, 307, 219, 189, 159, and 73.

1220 Organometallics, Vol. 28, No. 4, 2009 Ethynyltrimethylsilane. GC-MS: m/z ) 212. MS fragmentation pattern: 212, 183, 155, 127, and 73. X-ray Diffraction Studies. The crystals were mounted on quartz fibers, and the diffraction data were collected on a Bruker AXS APEX diffractometer equipped with a rotation anode at 223 K for complexes 1, 2, 5, and 6 and at 295 K for complexes 3, 4, and 7 using graphite-monochromated Mo KR radiation (λ ) 0.71073 Å). For 6, one of the phenyl groups was disordered into two parts with an occupancy ratio of 50:50. The data were corrected for Lorentz and polarization effects with the SMART suite of programs and for absorption effects with SADABS. Structure solution and refinement were carried out with the SHELXTL suite of programs.21 The structures were solved by direct methods to locate the heavy atoms, followed by difference maps for the light non-hydrogen (21) Sheldrick, G. M. SHELXL-97, Program for crystal structure refinement; University of Go¨ttingen: Go¨ttingen, Germany, 1997.

Hu et al. atoms. The data collection and processing parameters are given in the Experimental Section.

Acknowledgment. This work was supported by the Agency for Science, Technology & Research (Singapore) (R143-000-277-305 and R143-000-364-305), Ministry of Education (R143-000-361-112), and the National University of Singapore. Helpful discussions with Dr. S. Bai and Dr. W. Zhang are acknowledged. Technical support from staff at the department is appreciated. Supporting Information Available: Crystallographic data for complexes 1-7 as CIF files. This material is available free of charge via the Internet at http://pubs.acs.org. OM800978J