Five-Coordinate Hydrido−Ruthenium(II) Complexes Featuring N

Aug 24, 2010 - Research School of Chemistry, Institute of Advanced Studies, The Australian National University, Canberra, ACT 0200, Australia. ‡ Ins...
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Organometallics 2010, 29, 4012–4017 DOI: 10.1021/om100146m

Five-Coordinate Hydrido-Ruthenium(II) Complexes Featuring N-Heterocyclic Silylene and Carbene Ligands Ian A. Cade,† Anthony F. Hill,*,† Alexander K€ampfe,†,‡ and J€ org Wagler‡ †

Research School of Chemistry, Institute of Advanced Studies, The Australian National University, Canberra, ACT 0200, Australia, and ‡Institut f€ ur Anorganische Chemie, Technische Universit€ at Bergakademie Freiberg, D-09596 Freiberg, Germany Received February 24, 2010

The reactions of [RuCl2(dCHPh)(PCy3)2] or [RuHCl(η-H 2 )(PCy3)2] with the N-heterocyclic silylene :Si(NtBuCH)2 (ISitBu) affords the coordinatively unsaturated silylene complex [RuHCl(ISitBu)(PCy3)2]. For comparative purposes, the corresponding complex [RuHCl(IiPr)(PCy3)2] (IiPr= :C(NiPrCH)2) was obtained from [RuHCl(η-H2)(PCy3)2] and IiPr.

Introduction The rapidity with which N-heterocyclic carbenes (NHCs) have been embraced as spectator ligands in catalyst design is truly remarkable.1 Though NHC complexes were previously *To whom correspondence should be addressed. E-mail: a.hill@anu. edu.au. (1) Recent reviews include: (a) Marion, N.; Nolan, S. P. Acc. Chem. Res. 2008, 41, 1440. (b) Marion, N.; Nolan, S. P. Chem. Soc. Rev. 2008, 37, 1776. (c) W€ urtz, S.; Glorius, F. Acc. Chem. Res. 2008, 41, 1523. (d) Marion, N.; Diez-Gonzalez, S.; Nolan, S. P. Angew. Chem., Int. Ed. 2007, 46, 2988. (e) Kantchev, E. A. B.; O'Brien, C. J.; Organ, M. G. Angew. Chem., Int. Ed. 2007, 46, 2768. (f) Kuhl, O. Chem. Soc. Rev. 2007, 36, 592. (2) (a) Lappert, M. F. J. Organomet. Chem. 2005, 690, 5467. (b) Lappert, M. F. J. Organomet. Chem. 1988, 358, 185. (c) Lappert, M. F. J. Organomet. Chem. 1975, 100, 139. (3) Arduengo, A. J., III; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1991, 113, 361. (b) Arduengo, A. J., III. Acc. Chem. Res. 1999, 32, 913. (4) (a) Denk, M.; Lennon, R.; Hayashi, R.; West, R.; Belyakov, A. V.; Verne, H. P.; Haaland, A.; Wagner, M.; Metzler, N. J. Am. Chem. Soc. 1994, 116, 2691. (b) Hill, N. J.; West, R. J. Organomet. Chem. 2004, 689, 4165. (c) Tomasik, A. C.; Mitra, A.; West, R. Organometallics 2009, 28, 378. (d) Li, W.; Hill, N. J.; Tomasik, A. C.; Bikzhanova, G.; West, R. Organometallics 2006, 25, 3802. (e) Evans, W. J.; Perotti, J. M.; Ziller, J. W.; Moser, D. F.; West, R. Organometallics 2003, 22, 1160. (f) Schmedake, T. A.; Haaf, M.; Paradise, B. J.; Millevolte, A. J.; Powell, D. R.; West, R. J. Organomet. Chem. 2001, 636, 17. (g) Schmedake, T. A.; Haaf, M.; Paradise, B. J.; Powell, D.; West, R. Organometallics 2000, 19, 3263. (h) Haaf, M.; Schmedake, T. A.; West, R. Acc. Chem. Res. 2000, 33, 704. (5) (a) Gehrhus, B.; Lappert, M. F.; Heinicke, J.; Boese, R.; Blaser, D. J. Chem. Soc., Chem. Commun. 1995, 1931. (b) Gehrhus, B.; Lappert, M. J. Organomet. Chem. 2001, 617-618, 209. (c) Gehrhus, B.; Hitchcock, P. B.; Pongtavornpinyo, R.; Zhang, L. Dalton Trans. 2006, 1847. (d) Gehrhus, B.; Hitchcock, P. B.; Jansen, H. J. Organomet. Chem. 2006, 691, 811. (e) Delawar, M.; Gehrhus, B.; Hitchcock, P. B. Dalton Trans. 2005, 2945. (f) Avent, A. G.; Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F.; Maciejewski, H. J. Organomet. Chem. 2003, 686, 321. (g) Cai, X.; Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F. Can. J. Chem. 2000, 78, 1484. (h) Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F.; Maciejewski, H. Organometallics 1998, 17, 5599. (6) (a) Driess, M.; Yao, S.; Brym, M.; Van Wuellen, C.; Lentz, D. J. Am. Chem. Soc. 2006, 128, 9628. (b) Driess, M.; Yao, S.; Brym, M.; Van Wuellen, C. Angew. Chem., Int. Ed. 2006, 45, 6730. (c) Xiong, Y.; Yao, S.; Driess, M. Chem.;Eur. J. 2009, 15, 8542. (d) Xiong, Y.; Yao, S.; Bill, E.; Driess, M. Inorg. Chem. 2009, 48, 7522. (e) Xiong, Y.; Yao, S.; Driess, M. Chem.;Eur. J. 2009, 15, 5545. (f) Xiong, Y.; Yao, S.; Driess, M. J. Am. Chem. Soc. 2009, 131, 7562. (g) Meltzer, A.; Prasang, C.; Driess, M. J. Am. Chem. Soc. 2009, 131, 7232. (h) Meltzer, A.; Praesang, C.; Milsmann, C.; Driess, M. Angew. Chem., Int. Ed. 2009, 48, 3170. (i) Xiong, Y.; Yao, S.; Driess, M. Organometallics 2009, 28, 1927. pubs.acs.org/Organometallics

Published on Web 08/24/2010

prevalent, primarily from the work of Lappert,2 the discovery that free NHCs were isolable reagents revolutionized their use.3 In contrast, isolable N-heterocyclic silylenes (NHSi’s, Chart 1, 1-4) discovered by West,4 Gehrhus and Lappert,5 and Driess6 have not yet enjoyed the same degree of application to catalysis, though F€ urstner7a and more 7b recently Roesky have demonstrated their use as co-ligands in palladium-mediated cross-coupling reactions. The bonding in NHSi’s has been computationally interrogated at various levels of theory8 primarily to address the question of their comparative σ-basicity and π-acidity relative to the more familiar NHC ligands. In general, these studies concluded that both processes are electronically more favorable for NHSi than NHC ligands, steric factors notwithstanding (longer M-Si bond, contracted NSiN angle). Although π-retrodonation from the metal is considered to be marginally enhanced for NHSis relative to NHCs, this component of the synergic bonding remains at best modest. The relevant frontier orbitals of the model compounds A(NMeCH)2 (A = C, IMe 5a, Si ISiMe 5b); Chart 2) for binding to a metal display a similar topology, though for A = Si these orbitals appear more localized on silicon and are better matched energetically for combination with metal d-orbitals. Perhaps the most celebrated example of the use of NHCs as phosphine substitutes in catalysis involves the conversion of Grubbs’ first-generation catalyst [RuCl2(dCHPh)(PCy3)2] (6)9 into the imidazolylidene derivative [RuCl2(dCHPh)(PCy3)(IMes)] (7, IMes = 2,5-dimesitylimidazolylidene, Chart 3)10 (and related variants). This results in dramatic differences in (7) (a) F€ urstner, A.; Krause, H.; Lehmann, C. W. Chem. Commun. 2001, 2372. (b) Zhang, M.; Liu, X.; Shi, C.; Ren, C.; Ding, Y.; Roesky, H. W. Z. Anorg. Allg. Chem. 2008, 634, 1755. (8) (a) Dhiman, A.; M€ uller, T.; West, R. W.; Becker, J. Y. Organometallics 2004, 23, 5689. (b) Zeller, A.; Bielert, F.; Haerter, P.; Herrmann, W. A.; Strassner, T. J. Organomet. Chem. 2005, 690, 3292. (c) Boehme, C.; Frenking, G. Organometallics 1998, 17, 5801. (d) McGuinness, D. S.; Yates, B. F.; Cavell, K. J. Organometallics 2002, 21, 5408. (e) Tuononen, H. M.; Roesler, R.; Dutton, J. L.; Ragogna, P. J. Inorg. Chem. 2007, 46, 10693. (f) Denk, M.; Green, J. C.; Metzler, N.; Wagner, M. J. Chem. Soc., Dalton Trans. 1994, 2045. (g) Besora, M.; Maseras, F.; Lledo, A.; Eisenstein, O. Inorg. Chem. 2002, 41, 7105. (9) Schwab, P.; France, M. B.; Ziller, J. W.; Grubbs, R. H. Angew. Chem., Int. Ed. Engl. 1995, 34, 2039. r 2010 American Chemical Society

Article

Organometallics, Vol. 29, No. 18, 2010 Chart 1. Isolable N-Heterocyclic Silylenes4-6

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Chart 3. Selected NHC Ligands and Their Steric Profiles: (a) ItBu; (b) IiPr; (c) IMes

Chart 2. Comparative Topology of N-Heterocyclic Carbene (IMe) and Silylene (ISiMe) Frontier Orbitals (B3LYP, 6-311þþG**)

Scheme 1

performance in the various permutations of alkene metathesis reactions. We have begun an investigation of the organotransition metal chemistry of West’s NHSi, ISitBu (1), with a view to exploring its potential as a co-ligand in catalysis. We report herein our attempt to prepare an NHSi analogue of 7. The mode of operation of 7 is presumed to involve preferential phosphine dissociation, with the IMes ligand remaining strongly bound. Accordingly, the increased σ-donation and π-acceptance by an NHSi ligand in place of IMes should, in principle, result in stronger NHSi binding to the ruthenium center. Furthermore, the stronger σ-donation should help labilize the trans PCy3 ligand. It however transpires that the reaction of 6 with ISitBu does not proceed as anticipated, but rather affords inter alia a novel five-coordinate ruthenium hydrido NHSi complex, devoid of an alkylidene ligand.

Results and Discussion Treating a solution of 6 in tetrahydrofuran with 1 results in a complex mixture of products as indicated by 31P, 1H, and 29 Si NMR spectroscopy. No single product predominates in sufficient quantity to be isolated in pure form; however slow concentration of the crude mixture did afford a few crystals of one compound suitable for crystallographic analysis. The (10) (a) Scholl, M.; Trnka, T. M.; Morgan, J. P.; Grubbs, R. H. Tetrahedron Lett. 1999, 40, 2247. (b) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18. (c) Weskamp, T.; Schattenmann, W. C.; Spiegler, M.; Herrmann, W. A. Angew. Chem., Int. Ed. 1998, 37, 2490.

substandard crystallographic data set thus obtained yielded a structural model of poor precision, which was however sufficiently definitive to identify the product as [RuHCl(PCy3)2(ISitBu)] (8, Scheme 1). The complexity of the reaction mixture precluded mechanistic conjecture as to how the complex forms and the ultimate fate of the benzylidene ligand. However, the identity of the product could be established via an unequivocal synthesis. Treating a solution of [RuHCl(η2-H2)(PCy3)2] (9)11 in THF under argon with 1 resulted in the visible evolution of gas and the formation (31P NMR) of a deep orange solution from which could be isolated the complex [RuHCl(PCy3)2(ISitBu)] (8) in moderate yields (compromised by its solubility, Scheme 2). A 31P NMR spectrum of the crude reaction mixture after 1.5 h (Supporting Information, Figure S2) indicated the presence of three compounds, viz., unreacted starting complex 9 (14%), the desired product 8 (78%), and an as yet unidentified compound (7%, vide infra). Similar ratios of hydride resonances were observed in the 1H NMR spectrum (Supporting Information, Figure S3, vide infra). (11) Chaudret, B.; Chung, G.; Eisenstein, O.; Jackson, S. A.; Lahoz, F. J.; Lopez, J. A. J. Am. Chem. Soc. 1991, 113, 2314.

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Cade et al. Table 1. Selected Structural and Spectroscopic Data for Complexes of the Form [RuHCl(L)(PCy3)2]11,12 8

11

12 a

13

14

L

NHSi

NHC

CO

N2

BMesb

δH δP 2 JPH P-Ru-P0 H-Ru-Cl L-Ru-Cl H-Ru-L geometrye

-27.8 43.3 20.8 162° 129° 152 79 Y

-29.0 42.5 23.9 159° 129° 149 82 Y

-24.7 46.6 18.0 180° c 97° 178 83 T

-27.3 47.3 18.3 180° c 100° 176 83 T

-14.9 50.8 18.1 165° 89° d 114 75d Y

a

Figure 1. Molecular structure of 8 in the crystal structure of 8 3 (C6H6)2.5 (cyclohexyl groups, solvent, and most hydrogen atoms omitted for clarity). Selected bond lengths [A˚] and angles [deg]: Ru1-Si1 2.2123(7), Ru1-P1 2.3782(6), Ru1-P2 2.3628(7), Ru1-Cl1 2.4318(6), Ru1-H1 1.46(4), Ru1 3 3 3 H5032 2.77, Si1Ru1-P1 95.90(2), Si1-Ru1-P2 94.90(2), P1-Ru1-P2 161.71(2), Si1-Ru1-Cl1 151.94(3), P1-Ru1-Cl1 89.82(2), P2-Ru1-Cl1 87.54(2), Si1-Ru1-H1 79(2), P1-Ru1-H1 86(2), P2-Ru1-H1 82(2), Cl1-Ru1-H1 129(2), Ru1-Si1-N2 141.58(8), Ru1Si1-N5 127.87(8), N2-Si1-N5 90.5(1). Scheme 2. Synthesis of NHSi and NHC Complexes of Ruthenium

Given that this compound begins to form before all of the starting material 9 is consumed, 1.5 h represents the optimum reaction duration for maximizing the accumulation of 8. The isolated yield of pure 8 was however compromised by its solubility and losses during fractional crystallization. The possibility that the formation of 8 from the reaction of 6 with ISitBu might have actually arisen due to contamination of the commercial sample by adventitious 9 was excluded by 31 P{1H} NMR spectroscopic assay (Supporting Information Figure S1). Our suggested formulation of 8 follows from a consideration of spectroscopic data (vide infra); however it was also possible to obtain crystallographic grade crystals of the

Cl and CO disordered. b Mes = C6H2Me3-2,4,6. c Crystallographically imposed. d H not located; position surmised from DFT calculations. e In the context of ref 32a.

solvate 8 3 (C6H6)2.5 from benzene upon standing. The results of the crystallographic study are summarized in Figure 1 and discussed below. Spectroscopic data of note include a triplet resonance in the 29Si{1H} NMR spectrum, at δSi = 94.8 (2JPSi = 27.6 Hz) in a region characteristic of the threecoordinate silicon nuclei of NHSi ligands.4 The conventional hydride resonance in the 1H NMR spectrum appears at δH = -27.81, also as a sharp triplet (2JHP = 20.7 Hz, Table 1). The nature of the side product observed in both the 1H and 31 P NMR spectra (Supporting Information) remains unknown; however the multiplicities and coupling constant (δH = -24.72 triplet, δP = 45.70 doublet, 2JPH = 18.15 Hz) are consistent with a mer-trans “RuH(PCy3)2” moiety. Thus the most likely candidates would appear to be either the complex [RuHCl(PCy3)2(ISitBu)2] or the salt [RuH(PCy3)2(ISitBu)2]Cl; however this material was not isolated. There is a growing range of 16-electron complexes of the form [RuHCl(PCy3)2(L)] with various two-electron ligands “L” (L = CO12a N2,12b dBC6H2Me3-2,4,6,12c dCdCH2, dCHMe,12d H2CdCH2,12e PF312f). In the present context, the most relevant examples that are however missing are the L = NHC adducts [RuHCl(PCy3)2(NHC)], which might have provided a benchmark against which to interrogate the structural and spectroscopic features of 8. The closest analogue would be Caulton’s complex [RuHCl(PiPr3)2(IMe)],13 involving the less sterically demanding and less basic phosphine PiPr3, while Fogg has recently shown that the reaction of 9 with the bulky NHC “IMes” (Chart 3) results in phosphine substitution, affording [RuHCl(H2)(PCy3)(IMes)] with retention of the dihydrogen ligand.14 In contrast, Whittlesey has recently shown that [RuHCl(PPh3)3] reacts with IMe in CH2Cl2 to afford three isomers of the complex [RuHCl(PPh3)2(IMe)], while the same NHC with [RuHCl(PiPr3)2]2 affords a single isomer of [RuHCl(PiPr3)2(IMe)]15 akin to Caulton’s earlier example.13 (12) (a) Louie, J.; Grubbs, R. H. Organometallics 2002, 21, 2153. (b) Olivan, M.; Caulton, K. G. Inorg. Chem. 1999, 38, 566. (c) Alcarez, G.; Helmstadt, U.; Clot, E.; Vendier, L.; Sabo-Etienne, S. J. Am. Chem. Soc. 2008, 130, 12878. (d) Stuer, W.; Weberndoerfer, B.; Wolf, J.; Werner, H. Dalton Trans. 2005, 1796. (e) Lachaize, S.; Sabo-Etienne, S.; Donnadieu, B.; Chaudret, B. Chem. Commun. 2003, 214. (f) Werner, H.; Stuer, W.; Jung, S.; Weberndorfer, B.; Wolf, J. Eur. J. Inorg. Chem. 2002, 1076. (13) Ho, V. M.; Watson, L. A.; Huffman, J. C.; Caulton, K. G. New J. Chem. 2003, 27, 1446. (14) Beach, N. J.; Blacquiere, J. M.; Drouin, S. D.; Fogg, D. E. Organometallics 2009, 28, 441. (15) (a) Burling, S.; Mas-Marza, E.; Valpuesta, J. V.; Mahon, M. F.; Whittlesey, M. K. Organometallics 2009, 28, 6676. (b) Burling, S.; KociokK€ohn, G.; Mahon, M. F.; Whittlesey, M. K.; Williams, J. M. J. Organometallics 2005, 24, 5868.

Article

To evaluate the parallel between NHSi and NHC coordination in this system and because few structural data are available for directly comparable pairs of NHC/NHSi complexes,16-21 we investigated the reaction of 9 with the more sterically modest N,N0 -diisopropylimidazolylidene (IiPr),22 given Fogg’s observation that more sterically demanding NHC ligands result in phosphine displacement.14 For phosphines, if one assumes free rotation about the metal-phosphorus bond, the steric profile of phosphines may be parametrized in terms of the Tolman cone angle.23 For NHC ligands, representing the steric profile is less straightforward, and two approaches may be employed. Calculations of the buried volume of the ligand24 provide a single parameter. However, the nonconic and enormously variable shape of NHC ligands dictate that a single parameter is unlikely to suffice in many cases. The standard platforms for benchmarking the steric and electronic features of NHC ligands are either Tolman’s original pseudoconic “Ni(CO)3” fragment25 or the “MCl(CO)2” (M = Rh, Ir) unit,26 for which coordination of an NHC completes a square-planar coordination sphere. However, both of these potentially underestimate potential interligand steric conflict in that either the co-ligands bend away from the NHC (Ni) or the metal center has two vacant coordination sites (Ir) that may help accommodate steric bulk associated with the N-substituents. Nolan has also defined two angular parameters, AL and AH (Chart 4), to represent the length and height of the fence around the metal provided by the NHC, and in cases where metals adopt higher coordination numbers, these become especially useful. On the basis of a consideration of these parameters, the IiPr ligand was chosen as most likely to afford a stable analogue of 8 given that (i) interactions between the tBu substituents of ISitBu and the ruthenium center in 8 had been observed; (ii) the ItBu is especially predisposed to agostic C-H 3 3 3 M interactions, which on occasion proceed to C-H activation; (iii) Fogg’s demonstration that the IMes ligand (16) Structural data are available for NHC analogues of the structurally characterized NHSi complexes [M(L)2(CO)4] (M = Cr, Mo, W; L = 1, 2, 3),17 [Fe(1)(CO)4],18 [Ru(1)2(CO)3],19 [Ni(L)3] (L = 1, 2),20 and [Ni(1)2(CO)2].21 (17) (a) Ackermann, K.; Hofmann, P.; Kohler, F. H.; Kratzer, H.; € Krist, H.; Ofele, K.; Schmidt, H. R. Z. Naturforsch. 1983, 38B, 1313. (b) Scheidsteger, O.; Huttner, G.; Bejenke, V.; Gartzke, W. Z. Naturforsch. 1983, 38B, 1598. (c) Lappert, M. F.; Pye, P. L.; Rogers, A. J.; McLaughlin, G. M. J. Chem. Soc., Dalton Trans. 1981, 701. (d) Lappert, M. F.; Pye, P. L.; McLaughlin, G. M. J. Chem. Soc., Dalton Trans. 1977, 1272. (18) Huttner, G.; Gartzke, W. Chem. Ber. 1972, 105, 2714. (19) (a) Ellul, C. E.; Saker, O.; Mahon, M. F.; Apperley, D. C.; Whittlesey, M. K. Organometallics 2008, 27, 100. (b) Chantler, V. L.; Chatwin, S. L.; Jazzar, R. F. R.; Mahon, M. F.; Saker, O.; Whittlesey, M. K. Dalton Trans. 2008, 2603. (20) Schaub, T.; Backes, M.; Radius, U. Organometallics 2006, 25, 4196. (21) (a) Schaub, T.; Radius, U. Chem.;Eur. J. 2005, 11, 5024. (b) Scott, N. M.; Clavier, H.; Mahjoor, P.; Stevens, E. D.; Nolan, S. P. Organometallics 2008, 27, 3181. (22) Burling, S.; Paine, B. M.; Nama, D.; Brown, V. S.; Mahon, M. F.; Prior, T. J.; Pregosin, P. S.; Whittlesey, M. K.; Williams, J. M. J. J. Am. Chem. Soc. 2007, 129, 1987. (23) Tolman, C. A. Chem. Rev. 1977, 77, 313. (24) Kelly, R. A.; Clavier, H.; Giudice, S.; Scott, N. M.; Stevens, E. D.; Bordner, J.; Samardjiev, I.; Hoff, C. D.; Cavallo, L.; Nolan, S. P. Organometallics 2008, 27, 202. (25) (a) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. Rev. 2000, 100, 39. (b) Díez-Gonzalez, S.; Nolan, S. P. Coord. Chem. Rev. 2007, 251, 874. (c) Dorta, R.; Stevens, E. D.; Scott, N. M.; Costabile, C.; Cavallo, L.; Hoff, C. D.; Nolan, S. P. J. Am. Chem. Soc. 2005, 127, 2485. (26) (a) Chianese, A. R.; Li, X.; Janzen, M. R. H. Organometallics 2003, 22, 1663. (b) Chianese, A. R.; Kovacevic, A.; Zeglis, B. M.; Faller, J. W.; Crabtree, R. H. Organometallics 2004, 23, 2461.

Organometallics, Vol. 29, No. 18, 2010

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Chart 4. Steric Profiles of NHC Ligands in Terms of Nolan’s AH and AL Parametersa

a

Values taken from ref 27b.

resulted in phosphine displacement, presumably due to the incompatibility of a PCy3 ligand coordinating cis to an NHC with a large AH value (70°); and (iv) Whittlesey’s isolation of [RuHCl(PiPr3)2(Me2IiPr)],15 indicating that N-iPr substituents could be accommodated between two bulky cis-phosphines. Athough AH and AL parameters have not appeared for ItBu or IiPr NHC ligands, it may be assumed that these are close to those for IAd and ICy (Chart 4), respectively. A smooth reaction ensues with gas evolution under ambient conditions to cleanly afford the red complex [RuHCl(PCy3)2(IiPr)] (11). The identity of 11 followed from spectroscopic data and was confirmed by a crystallographic study of the solvate 11 3 (CH2Cl2)1.5, the results of which are summarized in Figure 2. These may be directly compared to those of 8 and analogues with other two-electron ligands (Table 1).12 The Ru-Si bond (2.2123(7) A˚) in 8 is shorter than that observed in Sabo-Etienne’s five-coordinate σ-silyl complex [Ru(SiMeCl2)Cl(η2-H2)(PCy3)2] (Ru-Si = 2.2727(5) A˚),29 being the shortest on record between ruthenium and trigonal silicon (2.220-2.336 A˚).4,30 The most immediately conspicuous feature of the molecular structures of both 8 and 11 is that the ruthenium centers would be five-coordinate (coordinatively unsaturated) were it not for the presence in each case of an anagostic interaction31 between the ruthenium center and one nitrogen substituent (C-H 3 3 3 Ru = 2.77 for 8, 2.72 A˚ for 11). In the case of 8, this results in a (27) (a) Jafarpour, L.; Nolan, S. P. Adv. Organomet. Chem. 2000, 46, 191. (b) Huang, J.; Schanz, H.-J.; Stevens, E. D.; Nolan, S. P. Organometallics 1999, 18, 2370. (28) Dorta, R.; Stevens, E. D.; Nolan, S. P. J. Am. Chem. Soc. 2004, 106, 5054. (29) Lachaize, S.; Cabalierpo, A.; Vendier, L.; Sabo-Etienne, S. Organometallics 2007, 26, 3713. (30) (a) Amoroso, D.; Haaf, M.; Yap, G. P. A.; West, R.; Fogg, D. E. Organometallics 2002, 21, 534. (b) Yoo, H.; Carroll, P. J.; Berry, D. H. J. Am. Chem. Soc. 2006, 128, 6038. (c) Dysard, J. M.; Tilley, T. D. Organometallics 2000, 19, 4726. (d) Ochiai, M.; Hashimoto, H.; Tobita, H. Angew. Chem., Int. Ed. 2007, 46, 8192. (e) Grumbine, S. K.; Tilley, T. D.; Arnold, F. P.; Rheingold, A. L. J. Am. Chem. Soc. 1994, 116, 5495. (f) Grumbine, S. K.; Mitchell, G. P.; Straus, D. A.; Tilley, T. D.; Rheingold, A. L. Organometallics 1998, 17, 5607. (g) Takaoka, A.; Mendiratta, A.; Peters, J. C. Organometallics 2009, 28, 3744. (h) Neumann, E.; Pfaltz, A. Organometallics 2005, 24, 2008. (31) Braga, D.; Grepioni, F.; Tedesco, E.; Biradha, K.; Desiraju, G. R. Organometallics 1997, 16, 1846.

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Figure 2. Molecular structure of 11 in the crystal structure of 11 3 (CH2Cl2)1.5 (cyclohexyl groups, solvent, and most hydrogen atoms omitted for clarity). Selected bond lengths [A˚] and angles [deg]: Ru1-Cl1 2.466(1), Ru1-P1 2.368(1), Ru1-P2 2.358(1), Ru1-C1 1.987(5), Ru1-H1 1.46(5), Ru1 3 3 3 H501 2.72, Cl1Ru1-P1 90.40(4), Cl1-Ru1-P2 87.43(4), P1-Ru1-P2 159.32(4), Cl1-Ru1-C1 148.7(1), P1-Ru1-C1 96.0(1), P2-Ru1-C1 96.5(1), Cl1-Ru1-H1 129 (2), P1-Ru1-H1 85(2), P2-Ru1-H1 81(2), C1-Ru1-H1 82(2), N5-C1-N2 102.5(4), N5-C1Ru1-120.6(3), N2-C1-Ru1 136.9(3).

tilting of the silylene (Ru1-Si1-N2 141.58(8)°, Ru1-Si1N5 127.87(8)°), and a similar though even more pronounced distortion (C-H 3 3 3 Ru = 2.33 A˚, Ru-Si-N = 116, 153°) has been noted for the complex [RuCl{κ3-C,P,P0 -CH(CH2PCy2)C2H4PCy2}(ISitBu)].30a In the case of 11, the interaction is with a methyne rather than methyl hydrogen atom of the iPr substituent, resulting in a comparable C-H 3 3 3 Ru separation (2.72 A˚), which also requires swivelling of the NHC ligand (N5-C1-Ru1-120.6(3)°, N2-C1-Ru1 136.9(3)°) due to the shorter Ru-C1 bond length (1.987(5) A˚). While neither of the C-H 3 3 3 Ru separations could be described as short (i.e., strong), the requisite distortion of the NHC or NHSi ligand presumably comes at an energetic cost balanced by the attractive anagostic C-H 3 3 3 Ru interactions. Perusal of structural data for NHSi complexes in general (Table S1, Supporting Information) reveals that in most cases the silylene is symmetrically bound with M-Si-N angles differing by less than 6°. Exceptions to this are the complexes [RuCl{CH(CH2PCy2)C2H4PCy2}(ISitBu)] (mean difference: 36.9°),30a [Ru(N2){py(CMedNMes)2-2,6}{Si(NCH2Ph)2C6H4}] (md: 8.2°),30b [Rh(ISitBu)4]þ (md: 7.2°),30h and (8) (md: 13.7°). Berry has also observed a modest pyramidalization of the silicon center in a series of ruthenium NHSi complexes (angle sum at silicon ca. 355°).30b The coordination at ruthenium for both 8 and 11 approaches neither idealized trigonal-bipyramidal nor squarebased-pyramidal geometries, though they are perhaps closer to the latter with the hydride occupying the pseudoaxial site. In both cases, the NHC or NHSi ligands in addition to the phosphines bend toward the sterically unassuming hydride ligand. Structural data are available for the complexes [RuHCl(L)(PCy3)2] (L = CO (12),12a N2(13),12b BMes (14)12c), and a comparison of their geometries with those of 8 and 11 (Table 1) leads to the conclusion that the irregular geometry is not steric in origin, but rather a response to electronic factors. Eisenstein has (32) (a) Riehl, J. F.; Jean, Y.; Eisenstein, O.; Pelissier, M. Organometallics 1992, 11, 729. (b) Jean, Y.; Eisenstein, O. Polyhedron 1988, 7, 405.

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computationally investigated the geometric preferences for d6-trans-ML3(PR3)2 complexes,32 concluding that when a single π-donor is present (Cl for 8 and 11), the angle between the remaining two pseudoequatorial ligands will contract to afford a Y-shaped distorted trigonal-bipyramidal geometry. In contrast, for a single π-acidic ligand, a conventional square-based pyramidal T-geometry is to be expected. Thus the Y-geometries of 8 and 11 may be taken as suggestive of neglible π-acidity when ISitBu or IiPr function as ligands. Concluding Remarks. Although a simple NHSi analogue of Grubbs’ second-generation catalysts does not appear to result from the reaction of 6 with ISitBu under the conditions employed here, the reaction does provide a rare example of a hydrido complex ligated by an NHSi. Though 8 is only the second30b hydrido-NHSi complex to have been isolated, Eisenstein has computationally evaluated the hypothetical complex [PtH{dSi(NMe2)2}(H2PC2H4PH2)]þ, concluding that it is more stable than the migratory-insertion σ-silyl tautomer [Pt{SiH(NMe2)2}(H2PC2H4PH2)]þ.8g In contrast Yates has concluded that the NHSi complex [Pd(CH3)(ISiMe)(PH3)2]þ would be considerably more prone to migratory insertion than the corresponding NHC analogue [Pd(CH3)(IMe)(PH3)2]þ.8d Jutzi has demonstrated the formal insertion of ISitBu into the W-H bond of [WH2(η-C5H5)2];33 however, given that the tungstenocene precursor is electron precise, coordination of 1 prior to insertion seems less likely than a proton-transfer/ion-pair collapse mechanism. The compatibility of hydride and silylene ligands in 8 and also toward coordinated dihydrogen in the presumed but transient intermediate [RuHCl(η-H2)(1)(PCy3)2] would appear to bode well for the use of NHSi ligands in catalyst design. The hydrido complex 8, being coordinatively unsaturated, includes the requisite design features typical of a reduction catalyst, given the notional parallels with, for example, [RuHCl(PPh3)3].34

Experimental Section General Considerations. Syntheses were carried out under an inert atmosphere (dry, oxygen-free argon) using standard Schlenk, vacuum line, and drybox techniques and dried, distilled solvents. 1H, 13C, 29Si, and 31P solution NMR spectra were recorded on a Varian INOVA 300 spectrometer using TMS as internal standard for 1H, 13C, and 29Si and 85% D3PO4 as external standard for 31P. Elemental analysis was performed by the microanalytical services of the Research School of Chemistry, ANU. The complex [RuHCl(H2)(PCy3)2] (9)11 and the ligands ISitBu7a and IiPr22 were prepared according to the indicated published procedures. All other reagents, including [RuCl2(dCHPh)(PCy3)2] (6, Supporting Information Figure S1), were obtained from commercial sources. Synthesis of [RuHCl(PCy3)2(ISitBu)] (8). To a solution of [RuHCl(H2)(PCy3)2] (9; 0.548 g, 0.783 mmol) in THF (30 mL) was added a solution of ISitBu (1; 0.155 g, 0.89 mmol) in THF (5 mL). The resulting orange-brown solution was then stirred at room temperature for 1.5 h and then freed of volatiles under high vacuum. The residue was extracted with benzene (see Supporting Information for 1H and 31P NMR spectra), filtered, and concentrated to afford orange-brown crystals of 8, which were isolated by decantation and dried under high vacuum. Yield: 0.138 g (0.150 mmol, 19.2%). Anal. Found: C, 62.12; H, (33) Petri, S. H. A.; Eikenberg, D.; Neumann, B.; Stammler, H.-G.; Jutzi, P. Organometallics 1999, 18, 2615. (34) Hallman, P. S.; Evans, D.; Osborn, J. A.; Wilkinson, G. Chem. Commun. 1967, 305.

Article 9.70; N, 2.80. Calcd for C46H87ClN2P2RuSi: C, 61.75; H, 9.80; N, 3.13. NMR (25 °C). 1H (CD2Cl2): δH -27.81 (t, 1 H, 2JPH = 20.7, RuH), 1.54, 1.56 (s  2, 9 H  2, CH3), 1.07-2.25 (m x 6, 66 H, PC6H11), 6.33, 6.43 (d  2, 1 H  2, NCHCHN, 3JHH = 4.3 Hz). 29Si{1H} (C6D6): δSi 94.8 (t, 2JPSi = 27.6 Hz). 29Si (C6D6): δSi 95.9 (d, 2JHSi = 27.3 Hz). 31P (CD2Cl2): δP 43.25 (d, 2 JPH = 30.0 Hz). 13C{1H}: δC 118.6 (NCHCHN), 55.10, 53.80 (NCMe3), 36.06, 35.26 (NCCH3), 26-32 (PCy). IR (THF): ν 1903 (RuH) cm-1. Crystal data for 8 3 (C6H6)2.5: C61H102ClN2P2RuSi, Mr = 1090.05, T = 200(2) K, triclinic, space group P1 (No. 2), a = 10.5987(1) A˚ , b = 14.3344(1) A˚, c = 20.7699(3) A˚, R = 94.7153(7)°, β = 100.6340(7)°, γ = 101.9604(7)°, V = 3010.07(6) A˚3, Z = 2, Fcalcd = 1.203 Mg m-3, μ(Mo KR) = 0.415 mm-1, F(000) = 1174, 2θmax = 60.0°, 69 700 collected reflections, 17 588 unique reflections, 617 parameters, R1 = 0.0364 (I > 2σ(I)), wR2 = 0.1258, max./min. residual electron density þ0.82/-1.06 e A˚-3 (CCDC 747688). Synthesis of [RuHCl(PCy3)2(IiPr)] (11). To a solution of [RuHCl(H2)(PCy3)2] (9; 0.092 g, 0.132 mmol) in THF (2 mL) was added a solution of IiPr (0.020 g, 0.132 mmol) in THF (2 mL). The resulting red-brown solution was stored at room temperature for 15 h, and then the resulting red precipitate was filtered off, washed with a minimum of THF, and freed of volatiles. The residue was dissolved in DCM (see Supporting Information for 31P NMR spectrum) and cooled (-20 °C) to afford bright red crystals of 11, which were isolated by decantation and dried in vacuo. Yield: 0.058 g (0.068 mmol, 52%). Anal. Found: C, 61.83; H, 9.07; N, 3.08. The crystals, shown by X-ray crystallography to be 11 3 (CH2Cl2)1.5, rapidly but incompletely desolvate in the absence of solvent. Grinding of the sample under argon and subsequent drying under high vacuum failed to completely remove residual CH2Cl2. Calcd for C45H83ClN2P2Ru.(CH2Cl2)0.4: C, 61.64; H, 9.55; N, 3.17. HR ESI-MS (þve ion): m/z 847.5015 (calcd [M - Cl þ O2]þ 847.4973). LR: m/z 847.5 [M - Cl þ O2], 607.5, 311.3, 281.2. NMR (C6D6, 25 °C). 1H: δH -28.99 (t, 1 H, 2JPH = 23.5, RuH), 1.22, 1.37 (d  2, 6 H  2, 3 JHH = 6.7, CH3), 0.8-2.7 (m, 66 H, PCy), 5.28, 5.35 (h  2, 1 H  2, 3JHH = 6.7 CHMe2), 6.28, 6.43 (d  2, 1 H  2, 3JHH = 2.4 Hz NCHCHN). 31P{1H}: δP 69.7. 13C{1H}: δC (NHC resonance not unequivocally identified), 115.4, 114.7 (NCHCHN), 50.2, 48.6 (Me2CHN), 28.54 [m, C2,6(C6H11)], 27.47 [tv, C1(C6H11), JPC = 69.5)], 27.15 [C4(C6H11)], 26.80 [m, C3,5(C6H11)], 24.53, 24.04 (CHCH3). IR (CH2Cl2): νRuH 2134 cm-1. IR (Nujol): ν 2136 (νRuH), 1261, 1090, 1017, 800 cm-1 (NHC fingerprint). Crystal data for 11 3 (CH2Cl2)1.5: C45H83ClN2P2Ru(CH2Cl2)1.5, Mr = 978.04, T = 200(2) K, triclinic, space group P1 (no. 2), a = 10.1582(3) A˚, b = 13.6688(4) A˚, c = 18.9367(4) A˚, R = 96.311(2)°, β = 90.107(2)°, γ = 105.557(1)°,

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V = 2516.3(1) A˚3, Z = 2, Fcalcd = 1.291 Mg m-3, μ(Mo KR) = 0.620 mm-1, F(000) = 1042, 2θmax = 50.0°, 32 239 collected reflections, 8949 unique reflections, 519 parameters, R1 = 0.0526 (I > 2σ(I)), wR2 = 0.1371, max./min. residual electron density þ2.03, -0.98 e A˚-3. Reaction of 1 with [RuCl2(dCHPh)(PCy3)2] (6); Formation inter alia of [RuHCl(PCy3)2(ISitBu)] (8). A solution of ISitBu (1: 0.051 g, 0.26 mmol) in THF (5 mL) was cooled (dry ice/acetone), and to this was added a solution of [RuCl2(dCHPh)(PCy3)2] (6; 0.107 g, 0.13 mmol) in THF (3 mL). The resulting intensely pink colored mixture was stirred at -78 °C for 30 min. The mixture was subsequently allowed to warm to ambient temperature slowly over a period of 2 h, during which time the color of the mixture changed to reddish-brown. The mixture was stirred at room temperature for 14 h and then freed of volatiles under reduced pressure. The entire residue was dissolved in C6D6 and a 31 P{1H} NMR spectrum measured, comprising free PCy3 (δP = 10.4, 66%), 8 (δP = 44.5, 3.5%), 6 (δP = 36.8, 1%), and OdPCy3 (δP = 51.7, 4%) in addition to unidentified resonances at δP = 25.4 (1%), 27.5 (2%), 41.4 (br, 5%), 52.1(br, 8%), and 59.4 (br, 8%). The broadness of resonances designated as “br” might be indicative of incomplete proton decoupling for hyrido complexes given that a multiplet is also observed at δH -27.3 ppm in the 1H NMR spectrum. Poor signal/noise compromised the diagnostic utility of the 29Si{1H} NMR spectrum, which however included inter alia major peaks at δSi 117-119 (br), 8.69, -6.03, -17.3 -21.5, -25.7, -26.4, -27.3, -28.8, -29.7, -30.0, and -31.3. All volatiles were removed under reduced pressure and the residue extracted with pentane (5 mL), filtered through diatomaceous earth, and freed of volatiles. The residue was then dissolved in toluene (0.5 mL) and cooled (-18 °C) for 2 days to afford a small number of crystals, which were identified crystallographically as nonsolvated 8 (vide supra). The data set thus obtained, while unambiguously confirming the identity of 8, was of inadequate quality to afford a satisfactory structural model for detailed analysis (R = 0.111).

Acknowledgment. This work was supported by the Studienstiftung des Deutschen Volkes and by the Australian Research Council (DP0771497). Supporting Information Available: Full details of the crystal structure determinations of 8 3 (C6H6)2.5 (CCDC 747688) and 11 3 (CH2Cl2)1.5 (CCDC 747689), respectively, in CIF format; table of selected structural data for N-heterocyclic silylene complexes; selected 1H and 31P NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.