Bimetallic, Spirocyclic, Methylene-Bridged Carbene Complexes of

Feb 20, 2009 - Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada, T6G 2G2. Organometallics , 2009, 28 (6), pp 1652–1665...
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Organometallics 2009, 28, 1652–1665

Bimetallic, Spirocyclic, Methylene-Bridged Carbene Complexes of Rhodium and Palladium Derived by Stepwise Metalations of Lithiated Bis(diphenylphosphoranotrimethylsilylimido)methandiide Min Fang, Nathan D. Jones, Kristina Friesen, Guanyang Lin, Michael J. Ferguson,# Robert McDonald,# Robert Lukowski, and Ronald G. Cavell* Department of Chemistry, UniVersity of Alberta, Edmonton, Alberta, Canada, T6G 2G2 ReceiVed June 18, 2008

The Li-C-Rh, bimetallic, phosphinimine spirocyclic, bridged carbene complex 1 was readily prepared from dilithiobis(diphenylphosphoranosilylimido)methandiide, [Li2L]2, and an equimolar (Rh/Li, 1:1) amount of [Rh(cod)Cl]2. Subsequent reaction with a different metal precursor, in the present cases, [Pd(allyl)Cl]2, yielded the spirocyclic Rh-Pd heterobimetallic 2 or, if a second equivalent of [Rh(cod)Cl]2 was used, the spirocyclic Rh-Rh homobimetallic 3 was obtained; thus the system presents an effective synthetic route for the preparation of homo- or heterobimetallic complexes. All of the complexes have constrained symmetry, which creates in some cases isomeric and enantiomeric structures. The Li-Rh complex 1 thermally rearranged to the chelated, orthometalated, Rh complex 6, wherein the Rh(I) center is bound to the ipso carbon of a phenyl ring and a methine carbon formed from protonation of the spiro carbon. Remarkably, controlled hydrolysis of 6 reversed the orthometalation and provided an iminophosphorane-methine chelate complex of Rh(I), 7. Reactions of the cod-substituted complexes 2, 3, and 7 with CO resulted in the replacement of cod substituents to form parallel Rh carbonyl complexes (4, 5, and 8). The allyl substituents in the system were not affected. The carbonyl complexes 5 and 8 could be made independently from [Rh(CO)2Cl]2 and either the dilithium methandiide or the methine form of the ligand as appropriate. Complexes 2-7 have been structurally characterized herein. Complexes 3 and 5 are chiral (because they lack rotation-reflection axes), but the products here are unresolved racemic mixtures according their NMR spectra. The complex 2 and its derivative 4 exist as unequal diastereomeric pairs according to the NMR. Fluxional NMR behavior is observed for 1, wherein the cod appears to partly dissociate and invert, for 2 and 3 where the cod exchange appears to involve a different mechanism being assisted by dissociation of one imine substituent, for 2 and 4 wherein allyl syn-syn, anti-anti exchange occurs between the diastereomers, and for 7 wherein the coordinated and uncoordinated imine groups exchange. Introduction Bimetallic complexes containing two directly bonded or proximal nonbonded metal atoms constitute an important class of organometallic compounds. A special subset of such complexes, defined by the presence of a methylene carbon bridge between the metals (which, as a result, are often called bridging carbene complexes), has received considerable attention. Overall the studies are encouraged by the possibility that suitable combinations of metals linked by appropriate ligands would show enhanced cooperative catalytic behavior and new transformation chemistry perhaps rivaling the efficacy of natural enzymic systems.1,2 Heterobimetallic complexes are also of particular interest as starting materials for the preparation of cluster compounds.3 Bimetallic complexes have been utilized * Corresponding author. E-mail: [email protected]. # Crystallography Laboratory. (1) (a) Moss, J. R.; Scott, L. G. Coord. Chem. ReV. 1984, 60, 171–190. (b) Herrmann, W. A. AdV. Organomet. Chem. 1982, 20, 159–263. (c) Hahn, J. E. Prog. Inorg. Chem. 1984, 31, 205–264. (d) Casey, C. P.; Audett, J. D. Chem. ReV. 1986, 86, 339–352. (e) Herrmann, W. A. J. Organomet. Chem. 1983, 250, 319–343. (f) Gavrilova, A. L.; Bosnich, B. Chem. ReVs 2004, 104, 349–383. (g) Singh, S.; Roesky, H. W. Dalton Trans. 2007, 1360– 1370. (2) Zhou, M.; Xu, Y.; Tan, A.-M.; Leung, P.-H.; Mok, K. F.; Koh, L.L.; Hor, T. S. A. Inorg. Chem. 1995, 34, 6425–6429.

to stabilize an unusual planar-tetracoordinated carbon,4 and several bimetallic complexes exhibit a rich variety of photochemical reactions.5,6 Systems of this type have been designed to provide simplified models of heterogeneous catalytic processes occurring on metal surfaces (e.g., the industrially important Fischer-Tropsch process).1b-d,7 Overall, however, there is a lack of simple, efficient, and general synthetic routes to bimetallic species. The existing preparations are often tedious with poor yields. Much work has been done on ligand design, and very complex ligand structures are often involved.1f Here we develop from the unique (3) Braunstein, P.; Rose, J. In ComprehensiVe Organometallic Chemistry (II); Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon: Oxford, U. K., 1995; Vol. 10. (4) Ro¨ttger, D.; Erker, G.; Fro¨hlich, R.; Grehl, M.; Silverio, S. J.; HylaKryspin, I.; Gleiter, R. J. Am. Chem. Soc. 1995, 117, 10503–10512. (5) Geoffroy, G. L.; Wrighton, M. S. Organometallic Photochemistry; Academic Press: New York, 1979. (6) Meyer, T. J.; Caspar, J. V. Chem. ReV. 1985, 85, 187–218. (7) (a) Dell’Amico, M. M.; Trepanier, S. J.; McDonald, R.; Cowie, M. Organometallics 2001, 20, 88–99. (b) Rowsell, B. D.; Trepanier, S. J.; Lam, R.; McDonald, R.; Cowie, M. Organometallics 2002, 21, 3228–3237. (c) Ristic-Petrovic, D.; Anderson, D. J.; Torkelson, J. R.; McDonald, R.; Cowie, M. Organometallics 2003, 22, 4647–4657. (d) Trepanier, S. J.; Dennett, J. N. L.; Sterenberg, B. T.; McDonald, R.; Cowie, M. J. Am. Chem. Soc. 2004, 126, 8046–8058. (e) Rowsell, B. D.; McDonald, R.; Cowie, M. Organometallics 2004, 23, 3873–3883.

10.1021/om8005678 CCC: $40.75  2009 American Chemical Society Publication on Web 02/20/2009

Methylene-Bridged Carbene Complexes of Rh and Pd Scheme 1. Bimetallic Spirocycles

Organometallics, Vol. 28, No. 6, 2009 1653 Scheme 2. Formation (via Thermal Conversion of 1) and Hydrolysis Reaction of 6

Chart 1. Previously Reported Homobimetallic Complexes 9,15 10,16 and 1118c,d

Li-C-Rh, bimetallic, phosphinimine-bridged carbene complex 18 (readily prepared from dilithiobis(diphenylphosphoranotrimethylsilylimido)methandiide, [Li2L]2,9-11 which, in turn, is easily derived from bis(diphenylphosphoranotrimethylsilylimido)methane, [H2L]9), effective systematic routes to both homo- and heterobimetallic bridged spirocyclic carbene complexes. The preparations and reactions are illustrated in Schemes 1 and 2. Much previous methandiide chemistry has been concerned with “pincer” and metal-chelate carbenes,12-14 which contain tri- or bidentate linkages to one metal with concomitant formation of at least one M-C bond. This MdC bond is supported by coordination or binding of one or more of the N (imine) parts of the ligand. Here we develop a bridged bimetallic carbene system, again supported by coordination of the imine substituent, which is easily synthesized. We have previously reported a few homobimetallic15,16 complexes such as the methylene-bridged, bimetallic complexes of Al, 9,15 and Cr, 1016 (Chart 1). Complex 9 proved to be an excellent catalyst for polymerization of ethylene.17 Leung and co-workers18a-d have also used the same or functionally related (8) Fang, M.; Jones, N. D.; Lukowski, R.; Tjathas, J.; Ferguson, M. D.; Cavell, R. G. Angew. Chem., Int. Ed. 2006, 45, 3097–3101. (9) The parent methylene-bridged ligand with the formula H2C(Ph2PdNSiMe3)2 is represented as [H2L], the dimeric dilithiated derivative Li2C(Ph2PdNSiMe3)2, as [Li2L]2, and the monolithiated derivative Li(H)C(R2PdNSiMe3)2, as [Li(H)L]. (10) Kasani, A.; Babu, R. P. K.; McDonald, R.; Cavell, R. G. Angew. Chem., Int. Ed. 1999, 38, 1483–1484. (11) Ong, C. M.; Stephan, D. W. J. Am. Chem. Soc. 1999, 121, 2939– 2940. (12) Cavell, R. G.; Babu, R. P. K.; Aparna, K. J. Organomet. Chem. 2001, 617, 158–169. (13) Jones, N. D.; Cavell, R. G. J. Organomet. Chem. 2005, 690, 5485– 5496. (14) Cavell, R. G. Chapter 14 In The Chemistry of Pincer Compounds; Morales-Morales, D., Jensen, C. M., Eds.; Elsevier: Amsterdam, 2007. (15) Aparna, K.; McDonald, R.; Ferguson, M.; Cavell, R. G. Organometallics 1999, 18, 4241–4243. (16) Kasani, A.; McDonald, R.; Cavell, R. G. Chem. Commun. 1999, 1993–1994.

methylene-bridged phosphorus-based nitrogen ligands to prepare homobimetallic bridged carbene systems (Pb-C-Pb and Sn-C-Sn) of tin and lead (e.g., complexes 11 (Chart 1)),18c,d and recently analogous complexes of Ca19a and Ba19b have been reported by Harder and co-workers. A substantial body of earlier work on iminophosphorane metal complexes also exists.20-25 Most closely related to the present work are the monometallic complexes such as 12 and 13 (Chart 2 and Table S1 in Supporting Information).24,25 The present system evolves from the dilithiated ligand [Li2L]29-11 which has proven to be a versatile reagent. Lithium reagents are widely used and are extremely important organic and organometallic synthetic agents.26 The nature and structure of these reagents has been the subject of extensive investigation (17) Cavell, R. G.; Aparna, K.; Babu, R. P. K.; Wang, Q. J. Mol. Catal., A 2002, 189, 137–143. (18) (a) Leung, W.-P.; Wong, K.-W.; Wang, Z.-X.; Mak, T. C. W. Organometallics 2006, 25, 2037–2044. (b) Leung, W.-P.; So, C.-W.; Kan, K.-W.; Chan, H.-S.; Mak, T. C. W. Organometallics 2005, 24, 5033–5037. (c) Leung, W.-P.; Ip, Q. W.-Y.; Wong, S.-Y.; Mak, T. C. W. Organometallics 2003, 22, 4604–4609. (d) Leung, W.; So, C.; Wang, J.; Mak, T. C. W. Organometallics 2003, 22, 4305–4311. (19) (a) Orzechowski, L.; Jansen, G.; Harder, S. J. Am. Chem. Soc. 2006, 128, 14676–14684. (b) Orzechowski, L.; Harder, S. Organometallics 2007, 26, 5501–5506. (20) Dehnicke, K.; Weller, F. Coord. Chem. ReV. 1997, 158, 103–169. (21) Dehnicke, K.; Krieger, M.; Massa, W. Coord. Chem. ReV. 1999, 182, 19–65. (22) Johnson, A. W.; Kaska, W. C.; Starzewski, K. A. O.; Dixon, D. A. Ylides and Imines of Phosphorus; John Wiley & Sons, Inc.: New York, 1993. (23) Stephan, D. W. Organometallics 2005, 24, 2548–2560. (24) Imhoff, P.; Nefkens, C. A.; Elsevier, C. J.; Goubitz, K.; Stam, C. H. Organometallics 1991, 10, 1421–1431. (25) Imhoff, P.; Elsevier, C. J. J. Organomet. Chem. 1989, 361, C61C65.

1654 Organometallics, Vol. 28, No. 6, 2009 Chart 2. Relevant Related Monometallic Iminophosphorane Complexes 1224 and 1325

for more than two decades.27,28 They continue to excite interest from synthetic, structural, and theoretical perspectives.28,29 The number of carbon-bridged Li-C compounds is small, and to date, only 10 carbon-bridged Li-C-precious metal compounds have been structurally characterized. Only three of these have the Li-C-Rh structural motif,30,31 and in no case is the bridging carbon of such Li-C-Rh complexes substituted with phosphorus. The Li complexes that contain phosphorus in the backbone and so have structural relevance to this work are complexes 14 to 19 (Chart 3).32-37 There is also some interesting Li chemistry in this system. In addition to that described herein, we have shown elsewhere that the bimetallic Li-Rh spirocyclic carbene 1 reacts readily with CO to form a unique, dimeric Li-Rh ketene complex8 (A, Scheme 3), which is destroyed by water, liberating CO, to form 8. The chemistry of A has not yet been developed. It is also worth noting that several structures encountered in this study are unique. Beyond 2 and 4, only one structurally characterized carbon-bridged Rh-C-Pd complex (complex 20 (Chart 4) with a CO bridge)38 has been reported. Although there (26) (a) Huryn, D. M. In ComprehensiVe Organic Synthesis; Schreiber, S. L., Ed.; Pergamon: Oxford, 1991; Vol. 1; pp 49-75. (b) Beswick, M. A.; Wright, D. S. In ComprehensiVe Organometallic Chemistry (II); Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon: Oxford, 1995; Vol. 1; pp 1-34. (c) Boche, G. Angew. Chem., Int. Ed. 1989, 28, 277–297. (d) Seebach, D. Angew. Chem., Int. Ed. 1988, 27, 1624–1654. (27) Streitwieser, A.; Bachrach, S. M.; Dorigo, A.; Schleyer, P. v. R. In Lithium Compounds: Principles and Applications; Sapse A.-M., Schleyer, P. v. R., Eds.; Wiley: New York, 1995. (28) Sorger, K.; Schleyer, P. v. R.; Seeger, R.; Pople, J. A. J. Mol. Struct. (THEOCHEM) 1995, 338, 317–346. (29) (a) Jemmis, E. D.; Subramanian, G.; Kos, A. J.; Schleyer, P. v. R. J. Am. Chem. Soc. 1997, 119, 9504–9512. (b) Davies, R. P.; Raithby, P. R.; Snaith, R. Angew. Chem., Int. Ed. 1997, 36, 1215–1217. (c) Sorger, K.; Schleyer, P. v. R.; Fleischer, R.; Stalke, D. J. Am. Chem. Soc. 1996, 118, 6924–6933. (d) Sygula, A.; Fronczek, F. R.; Rabideau, P. W. J. Organomet. Chem. 1996, 526, 389–391. (e) Harder, S.; Lutz, M.; Streitwieser, A. J. Am. Chem. Soc. 1995, 117, 2361–2362. (f) Ashe, A. J.; Kampf, J. W.; Savla, P. M. Organometallics 1993, 12, 3350–3353. (g) Ludvig, M.; Lagow, R. J. J. Org. Chem. 1990, 55, 4880–4883. (h) Kawa, H.; Manley, B. C.; Lagow, R. J. Polyhedron 1988, 7, 2023–2025. (i) Harder, S.; Boersma, J.; Brandsma, L.; Kanters, J. A. J. Organomet. Chem. 1988, 339, 7–15. (30) Kulzick, M. A.; Andersen, R. A.; Muetterties, E. L.; Day, V. W. J. Organomet. Chem. 1987, 336, 221–236. (31) Cipot, J.; Wechsler, D.; Stradiotto, M. Organometallics 2003, 22, 5185–5192. (32) Lo´pez-Ortiz, F.; Pela´ez-Arango, E.; Tejerina, B.; Pe´rez-Carren˜o, E.; Garcı´a-Granda, S. J. Am. Chem. Soc. 1995, 117, 9972–9981. (33) Said, M.; Thornton-Pett, M.; Bochmann, M. Organometallics 2001, 20, 5629–5635. (34) Hitchcock, P. B.; Lappert, M. F.; Uiterweerd, P. G. H.; Wang, Z. J. Chem. Soc., Dalton Trans. 1999, 3413–3417. (35) Hitchcock, P. B.; Lappert, M. F.; Wang, Z. Chem. Commun. 1997, 1113–1114. (36) Steiner, A.; Stalke, D. Angew. Chem., Int. Ed. 1995, 34, 1752– 1755. (37) Babu, R. P. K.; Aparna, K.; McDonald, R.; Cavell, R. G. Organometallics 2001, 20, 1451–1455. (38) County, G. R.; Dickson, R. S.; Fallon, G. D. J. Organomet. Chem. 1998, 565, 11–18.

Fang et al. Chart 3. Related Lithium-Containing Complexes 14,32 15,33 16,34 17,34 18,36 and 1937

Scheme 3. Dimeric Rh-Li Ketene Complex Formed from 1 with CO and Degraded to 8 with Water8

are many bimetallic Rh-Pd complexes, the examples demonstrate either directly bound metals or metals bridged by more than a single carbon.39-44 Numerous carbon-bridged Rh-C-Rh complexes have been previously reported, but none of these possesses phosphorus substitutents on the carbon, so 3 and 5 offer new structural environments. A unique complex containing a RhCCPC ring, 6, was identified in this study as the result of an orthometalation process, which, remarkably, could be reversed (to 7) by controlled hydrolysis. Our previous Pt carbene derivatives 21, 22, and 2345,46 and related Pd and Rh cyclic imine and amine systems 2447 and 2548 (Chart 4) provide structural comparisons, and the NHC-Pd-allyl 2649 (Chart 4) illustrates a noncyclic carbene. Therefore, the system that evolves from 1 with its unique reactive lithium center offers further substitution opportunities and provides a potentially useful reagent for systematic and efficient synthesis of bimetallic, phosphinimine, bridged carbene complexes. (39) Balch, A. L.; Fossett, L. A.; Olmstead, M. M.; Oram, D. E., Jr. J. Am. Chem. Soc. 1985, 107, 5272–5274. (40) Tejel, C.; Shi, Y.-M.; Ciriano, M. A.; Edwards, A. J.; Lahoz, F. J.; Modrego, J.; Oro, L. A. J. Am. Chem. Soc. 1997, 119, 6678–6679. (41) Oro, L. A.; Ciriano, M. A.; Tejel, C.; Bordonaba, M.; Graiff, C.; Tiripicchio, A. Chem.-Eur. J. 2004, 10, 708–715. (42) Farr, J. P.; Olmstead, M. M.; Balch, A. L. J. Am. Chem. Soc. 1980, 21, 6654–6656. (43) Arena, C. G.; Rotondo, E.; Faraone, F. Organometallics 1991, 10, 3877–3885. (44) Schiavo, S. L.; Rotondo, E.; Bruno, G.; Faraone, F. Organometallics 1991, 10, 1613–1620.

Methylene-Bridged Carbene Complexes of Rh and Pd Chart 4. The Only Previously Structurally Characterized Rh-C-Pd Complex, 20,38 Our Related Pt Imine-Derived Complexes 21-23,45,46 and Reported Structurally Related Imine and Carbene Complexes 24,47 25,48 and 2649

Results and Discussions Syntheses and Reactivity. The syntheses of bimetallic complexes 1, 2, and 3 and the derivatives thereof (4 and 5) are illustrated in Scheme 1. Complex 1 is readily obtained from the reaction of [Li2L]2 with a stoichiometric quantity of [(cod)RhCl]2.8 The other noble metal bimetallics described herein were accessed by reaction of 1 with appropriate chloro-bridged bimetallic transition metal precursors (either with [(allyl)PdCl]2 to give the Pd-C-Rh spirocyclic bimetallic 2 or with more [(cod)RhCl]2 to form, quantitatively, the Rh-C-Rh bimetallic phosphinimine spirocyclic bridged carbene complex 3). Although there are many examples wherein an organometallic complex is prepared by metathetical replacement of Li in a lithiated precursor,12,16,35,45,50 to the best of our knowledge, a synthetic route of this type has not been systematically applied to prepare bimetallic complexes. The successful syntheses of complexes 2 and 3 from 1 suggest that complex 1 has the potential to become a general useful precursor to bridged bimetallic Rh-C-M2 carbene complexes by treatment of 1 with suitable M2 precursors. As mentioned elsewhere,8 complex 1 is unstable but can be isolated and stored at -20 °C. The slow thermal conversion of (45) Jones, N. D.; Lin, G.; Gossage, R. A.; McDonald, R.; Cavell, R. G. Organometallics 2003, 22, 2832–2841; Erratum: Organometallics 2003, 22, 5378. . (46) Lin, G.; Jones, N. D.; Gossage, R. A.; McDonald, R.; Cavell, R. G. Angew. Chem., Int. Ed. 2003, 42, 4054–4057. (47) Masuda, J. D.; Wei, P.; Stephan, D. W. J. Chem. Soc., Dalton Trans. 2003, 3500–3505. (48) van der Zeijden, A. A. H.; van Koten, G.; Nordemann, R. A.; KojicProdic, B.; Spek, A. L. Organometallics 1988, 7, 1957–1966. (49) Viciu, M. S.; Zinn, F. K.; Stevens, E. D.; Nolan, S. P. Organometallics 2003, 22, 3175–3177. (50) Leung, W.; Wang, Z.; Li, H.; Mak, T. C. W. Angew. Chem., Int. Ed. 2001, 40, 2501–2503.

Organometallics, Vol. 28, No. 6, 2009 1655 Scheme 4. Proposed Reaction Pathways for the Conversion of 1 to 6

1 leads to the orthometalated lithiated methine complex 6 (Scheme 2), a reaction that is reminiscent of that previously observed for the orthometalation of Pt complex 21 to 22 (Chart 4). It was proposed in that case45 that the process involved a dissociation of the Pt-N bond. However the solution NMR behavior of 1 (see below) suggested that the Rh-N bond does not dissociate, but, rather, the NMR exchange process here involves the dissociation of one of the Rh-olefinic (cod) bonds. It is also known that coordination unsaturation in square-planar complexes is an important prerequisite for orthometalation and that C-H activations at Rh(I) typically proceed by oxidative addition routes.51,52 Therefore we propose (Scheme 4) that the orthometalation proceeds via the partial dissociation of the cod ligand followed by oxidative addition of one ortho hydrogen of one of the phenyl rings of the LiNPPh2 fragment to Rh(I) to form the Rh(III) hydride. The formation of the Rh-C bond weakens the Rh-N dative bonds, leading to the formation of a new Li-N bond, which results in an increase in electron density on the C bound to Li (via an inductive effect because Li-C bonds are quite polar). In addition, the high oxidation state of Rh(III) confers protonic properties on the Rh-H unit. Thus a reductive elimination via a proton shift to the negatively charged C with the recoordination of the cod occurs, resulting in 6. Rather surprisingly, complex 6 undergoes hydrolysis (under controlled conditions) to form complex 7 (Scheme 2), which can be regarded as a reverse orthometalation process. This is rather unexpected because, overall, a short planar Rh-C(sp2) (sum of angles at C(12) ) 359.5°) bond (2.043(3) Å) to the phenyl ring is broken in this process instead of the longer, and presumably weaker, Rh-C(sp3) bond with C(1) (2.164(3) Å). Reverse orthometalation reactions are relatively rare processes but have been previously observed, for example, in a case wherein silyl ligands are bound to iridium (eq 1).53

The driving force in the present case is probably the formation of solid LiOH with its high lattice energy. A reaction pathway (51) Ryabov, A. D. Chem. ReV. 1990, 90, 403–424. (52) Foley, P.; DiCosimo, R.; Whitesides, G. M. J. Am. Chem. Soc. 1980, 102, 6713–6725. (53) Aizenberg, M.; Milstein, D. Organometallics 1996, 15, 3317–3322.

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Fang et al.

Scheme 5. Proposed Reaction Pathway for the Formation of 7 from 6, a “Reverse Orthometalation”

for the process of transforming 6 to 7 is proposed in Scheme 5. The reaction probably proceeds by initial hydrolytic attack at Li with formation of a LiOH precipitate, generating a proton, which attacks the sp2 (phenyl) C bonded to Rh instead of the sp3 C due to the higher electronegativity of the former, regenerating the phenyl group (of course still attached to phosphorus). This attack selection obviates the concern expressed above. We do not think that activation of H2O by Rh(I) to form an intermediate hydroxide salt of a five-coordinated Rh(III) hydride is operative because the Rh(III)-H would probably attack the cod. Similar activation of H2O species by Rh(I) complexes following this route has, however, been reported.54 Although this reductive elimination is unexpected from the bond strength perspective discussed above, Milstein et al.55 reported that aryl-H reductive eliminations are more facile than aliphatic C-H reductive eliminations because (a) the aryl C-H bond is stronger and (b) aryl agostic intermediates (having metal η2-C-H coordination) favor a lower energy transition state. The aryl-H bond reductive elimination also recovers the aromaticity of the phenyl ring. We note that the two vicinal C-C bond distances were significantly lengthened when the ring was bonded to the metal (Supporting Information (SI) Figure S11). Complex 7 can also be synthesized directly by the reaction of the monolithiated ligand, Li(H)L, with [(cod)RhCl]2. It should also be noted that hydrolysis with unlimited quantities of water destroys the complex system completely. Reactions of CO with 2, 3, and 7 (Schemes 1 and 2) were undertaken to evaluate whether insertion into M-C bonds or, especially, into N-Si bonds might occur, given the precedents established by the insertion of CO into 1 to form the ketene dimer8 and with 21 (Chart 4) to give the unusual tris(carbene) carbonyl platinum complex 23 (Chart 4).46 In the present system, however, no insertion reactions were observed; in all cases the CO simply replaced cod on the Rh centers to form 4, 5, and 8. Notably, the allyl substituents on Pd were not affected. The reaction of 3 was clean, but as complex 2 was being transformed to 4, it was noted that a black precipitate gradually formed, which could be Pd(0) produced by the reduction of Pd(II) in either 2 or 4 by CO. Synthesis of 4 in good yield was achieved by recovering the carbonylated product from the solution as quickly as possible after complete reaction had been achieved. Complex 8, derived here from 7 (and formed by hydrolysis of (54) Yoshida, T.; Okano, T.; Ueda, Y.; Otsuka, S. J. Am. Chem. Soc. 1981, 103, 3411–3422. (55) Rybtchinski, B.; Cohen, R.; Ben-David, Y.; Martin, J. M. L.; Milstein, D. J. Am. Chem. Soc. 2003, 125, 11041–11050.

Figure 1. Perspective view of complex 2 showing the atom-labeling scheme. Non-hydrogen atoms are represented by Gaussian ellipsoids at the 20% probability level. Only the ipso carbon atoms of the phenyl rings are shown. Hydrogen atoms are not shown. Selected bond distances (Å) for 2: C(1)-Pd, 2.149(3); Rh-C(1), 2.137(3); P(1)-C(1), 1.739(3); C(1)-P(2), 1.733(3); N(1)-P(1), 1.603(3); P(2)-N(2), 1.612(2); Pd-N(1), 2.147(2); N(2)-Rh, 2.181(3). Selected angles (deg): P(1)-N(1)-Pd, 93.1(1); N(1)-P(1)-C(1), 101.6(1); P(1)-C(1)-Pd, 89.3(1); C(1)-Pd-N(1), 74.2(1); Pd-C(1)-Rh, 87.7(1); P(1)-C(1)-P(2), 130.9(2); C(1)-P(2)-N(2), 101.6(1); P(2)-N(1)-Rh, 90.6(1); C(1)-Rh-N(2), 73.8(1); Rh-C(1)-P(2), 88.9(1); Pd-C(1)-P(2), 124.2(2); P(1)-C(1)-Rh, 131.2(2); plane Pd-P(1)-C(1)/plane Rh-P(2)-C(1), 66.59(6); plane P(1)-N(1)-Pd/plane Pd-P(1)-C(1), 14.26(7); plane RhP(2)-C(1)/plane P(2)-N(2)-Rh-Si(2), 24.2(1). Nonbonded separations: Pd-Rh, 2.9683(4); Pd-P(1), 2.748(1); Rh-P(2), 2.726(1). Additional details are given in the SI.

A, Scheme 3),8 can also be accessed by the reaction of Li(H)L9 with [ClRh(CO)2]2. Molecular Structures of the Bimetallic Spirocycles 1-5. All five complexes have a spirocyclic carbon center and so are axially chiral because of the geometry imposed at this center. Complexes 3 and 5 are bis-Rh complexes, whereas 1, 2, and 4 have a mixed metal composition. All look like propellors with different edges, because either the two blades have different composition (1, 2, and 4) or the edges have different orientations (3 and 5). The molecular structures and the atom-numbering schemes of complexes 2-5 are shown in Figures 1-4. Additional information is provided as SI (Figures S1-S7). Complex 1 crystallizes from Et2O as a monomer with one Et2O coordinated to the lithium atom. As complex 1 was prepared from optically inactive reactants, a racemic mixture of both 1-R and 1-S crystals was obtained. Interestingly the crystal used for structure solution revealed only the 1-S enantiomer (Flack parameter 0.01). No racemic twinning was observed. The crystal size and visual morphology were such that physical resolution of enantiomers was not attempted. Reported in some detail elsewhere8 we give the molecular structure and atomic numbering scheme (of the 1-S enantiomer) in the SI (Figure S0) along with fuller structural details for convenient comparisons. Some selected bond distances and angles of 1 are compared with related structures 2-5 and literature examples, 12 and 13, in Table 1. Although complexes 2-5 have chiral carbon centers, the products are racemic because the precursors are racemic; in these cases, no crystallization resolution occurred and R- and S-type enantiomers are present in the unit cells. Complex 6 is also a bimetallic system, but it is not a spirocycle. Its structure is shown in Figure 5 and will be discussed below. Complex 7 is a monometallic methine. It was structurally characterized and is illustrated in Figure 6. The related carbonylated methine, complex 8, was not structurally characterized.

Methylene-Bridged Carbene Complexes of Rh and Pd

Figure 2. Perspective view of complex 3 showing the atom-labeling scheme. Non-hydrogen atoms are represented by Gaussian ellipsoids at the 20% probability level. Only the ipso carbon atoms of the phenyl rings are shown. Hydrogen atoms are not shown. Selected bond distances (Å) for 3: C(1)-Rh(1), 2.148(2); Rh(2)-C(1), 2.168(2); P(1)-C(1), 1.732(2); C(1)-P(2), 1.738(2); Rh(1)-P(1), 2.723(1); Rh(2)-P2, 2.723(1); Rh(1)-N(1), 2.197(2); N(2)-Rh(2), 2.181(2); N(1)-P(1), 1.605(2); P(2)-N(2), 1.611(2). Selected angles (degree) are: P(1)-N(1)-Rh(1), 90.08(9); N(1)-P(1)-C(1), 100.3(1); P(1)-C(1)-Rh(1), 88.5(1); C(1)-Rh(1)-N(1), 72.26(8); Rh(1)-C(1)-Rh(2), 89.2(2); P(1)-C(1)-P(2), 129.7(1); C(1)-P(2)N(2), 100.4(1); P(2)-N(2)-Rh(2), 90.51(9); N(2)-Rh(2)-C(1), 72.59(8); Rh(2)-C(1)-P(2), 87.7(1); Rh(1)-C(1)-P(2), 131.3(1); P(1)-C(1)-Rh(2), 128.0(1); N(2)-Rh(2)-C(1), 72.59(8); plane P(1)C(1)Rh(1)/plane P(2)C(1)Rh(2), 65.45(4). The nonbonded Rh(1)-Rh(2) separation is 3.0310(3) Å. Additional information is given in the SI.

Figure 3. Perspective view of complex 4 showing the atom-labeling scheme. Non-hydrogen atoms are represented by Gaussian ellipsoids at the 20% probability level. Only the ipso carbon atoms of the phenyl rings are shown. Hydrogen atoms are not shown. Selected bond distances (Å): Rh-C(10), 2.135(3); Pd-C(10), 2.152(3); P(1)-C(10), 1.736(3); P(2)-C(10), 1.732(3); N(1)-P(1), 1.593(3); P(2)-N(2), 1.609(3); N(2)-Rh, 2.119(3); Pd-N(1), 2.149(3); Rh-C(4), 1.881(4); Rh-C(5), 1.839(4); C(4)-O(4), 1.115(4); C(5)-O(5), 1.146(4). Selected angles (deg): P(1)-N(1)-Pd, 92.2(1); N(1)-P(1)-C(10), 101.1(2); P(1)-C(10)-Pd, 88.2(1); C(10)-Pd-N(1), 73.5(1); Pd-C(10)-Rh, 90.1(1); P(1)-C(10)-P(2), 132.4(2); P(2)-C(10)-Rh, 90.4(1); C(10)-Rh-N(2), 74.2(1); Rh-N(2)-P(2), 94.5(1); N(2)-P(2)-C(10), 100.2(1); C(5)-C(4)N(2)-C(10), -4.8(2). Additional details are given in the SI.

Three complexes, 1, 2, and 3, have one or more Rh-cod units and so possess many structural similarities. Rh-C and Rh-N bond distances of 1-5 are within the normal range and are similar to those in the simple chelates 1224 and 1325 (Chart 2), the only two RhNPC ring containing structures that, according to the Cambridge Structure Database (CSD), have been previously described. The Rh-C(1) (spirocyclic carbon) bond of 1 (2.105(6) Å) has the shortest length in this set of Rh spirocycles (1-5) (where values range from 2.105(6) to 2.183(1) Å (Table 1)), implying that this case represents the greatest Rh-C bond valence contribution.56 Correspondingly, the Rh-N bond of 1 is the longest of the set

Organometallics, Vol. 28, No. 6, 2009 1657

Figure 4. Perspective view of the complex 5 showing the atomlabeling scheme. Non-hydrogen atoms are represented by Gaussian ellipsoids at the 50% probability level. Only the ipso carbon atoms of the phenyl rings are shown. Hydrogen atoms are not shown. Selected bond distances (Å) for 5: C(10)-Rh(1), 2.144(2); C(10)-Rh(2), 2.145(2); C(10)-P(1), 1.743(2); P(2)-C(10), 1.740(2); P(1)-N(1), 1.605(2); N(2)-P(2), 1.607(2); N(1)-Rh(1), 2.115(2); Rh(2)-N(2), 2.117(2); C(1)-O(1) 1.136(3); C(2)-O(2) 1.138(3); C(3)-O(3) 1.133(3); C(4)-O(4) 1.137(3). Selected angles (deg): P(2)-N(2)-Rh(2), 92.45(7); N(2)-P(2)-C(10), 99.37(8); P(2)C(10)-Rh(2), 87.89(8); P(2)-C(10)-P(1), 131.3(1); P(1)-C(10)Rh(1), 90.01(8); C(10)-P(1)-N(1), 100.51(8); P(1)-N(1)-Rh(1), 94.97(7); N(1)-Rh(1)-C(10), 74.45(6), Rh(2)-C(10)-P(1), 124.81(9); P(2)-C(10)-Rh(1), 127.35(9); C(10)-Rh(1)-N(1)-P(1), -1.73(7); C(10)-Rh(2)-N(2)-P(2), -18.05(7); C(2)-C(1)-Rh(1)C(10), 170.0(8). The nonbond Rh(1)-Rh(2) separation is 3.0922(2) Å. Additional information is given in the SI.

(range 2.081(8)-2.197(4) Å), implying the smallest bond valence contribution. This agrees with the bond valence conservation rule, which requires that the sum of bond valences around the central atom (Rh(I)) in each complex should be constant and equal to the valence of Rh(I).57 The short Rh-C(1) and P-C(1) bonds produce larger P-C-Rh and N-P-C angles (compared, for example, to those which appear in 12 and 13 (Chart 2)). The square-planar coordination mode of each Rh, defined by the two olefinic centers of the cod (taking (for example) for 3) m1 and m2 as the midpoints of C(51)-C(52) and C(55)-C(56), respectively), the N(2) and C(1) atoms, is slightly distorted. In 1, the Rh lies 0.073 Å below the plane defined by m1, m2, C(1), and N(1) (torsion angle of m1-m2-N(1)-C(1) ) 2.2(3)°). Very similar separations and distortions prevail throughout the set 1, 2, and 3 (and 12 and 13) (Table 1). In 1 the Rh-m1 separation distance (2.001(6) Å, trans to N) is shorter than Rh-m2 (2.036(7) Å, trans to C(1)), a difference that has also been observed in 12 and 13. This difference can be ascribed to the higher trans influence of the covalent carbon atom compared to the dative nitrogen atom. These similar separation parameters (which also prevail in 6 and 7) suggest that similar total bond valence56,57 contributions of the olefinic Rh bonds prevail in each complex. The bond angles and distances within the M-N-P-C rings found in 1-5 are similar with some slight shortening in some cases (Table 1). These M-N-P-C rings are essentially planar (Table 1) with small deviations. The ring constraints within the M-N-P-C rings are responsible for the small angles at N(1), P(1), C(1), and M. For example in 1: Rh-N(1)-P(1) ) (56) In the purely empirical bond valence model,57 bonds between atom pairs are assigned bond valences, s, which are functions of only the bond distances, r, and the atom types involved. The rule of bond valence conservation requires that at any atom type the valence sum is constant in all bonding situations. Different expressions for s have been proposed, but most commonly used is s ) exp((r - r0)/b), where r0 is the single-bond distance and b is a constant. Typically b is around 0.37 Å. (57) Brown, I. D. In Structure and Bonding in Crystals; O’Keefe M., Navrotsky, A., Ed.; Academic Press: London, 1981.

1658 Organometallics, Vol. 28, No. 6, 2009

Fang et al.

Table 1. Selected Bond Distances (Å) and Bond and Torsion Angles (deg) for Spirocyclic Complexes 2, 4, 3, 5, and 1 and Corresponding Values for the Monomeric Complex 7 and for Literature Four-Membered Ring Monomers 12 and 13 2 structure portion

a

Rh ring

4 Pd ring

a

Rh ring

a

3 a

Pd ring

M-Na M-C(spiro)a P-N (within M ring)a P-C (within M ring)a N-Sib m1-Mc m2-Mc

2.181(3) 2.137(3) 1.612(2) 1.733(3) 1.720(3) 1.990(4) 2.0027(4)

2.147(2) 2.149(3) 1.603(3) 1.739(3) 1.710(3)

P -N -Md N-P-C(spiro)d P-C(spiro)-Md C(spiro)-M-Nd C(spiro)-M-m1cd N-M-m2cd m1-M-m2cd M-C(spiro)-P-Nd

90.6(1) 101.6(1) 88.9(1) 73.8(1) 100.9(1) 99.8(1) 86.8(2) 19.4(1)

93.1(1) 101.6(1) 89.3(1) 74.2(1)

94.5(1) 100.2(1) 90.4(1) 74.2(1)

92.2(1) 101.1(2) 88.2(1) 73.5(1)

11.3(1)

-7.2(1)

-19.1(1)

-179.6(4)

139.5(3)

-154.0(4)

-166.8(4)

d

P-N-Si-M e

plane a

2.119(3) 2.135(3) 1.609(3) 1.732(3) 1.717(3)

66.59(6)

70.47(11)

2.149(3) 2.183(4) 1.593(3) 1.736(3) 1.717(3)

5 f

Rh ring

Distances 2.197(2) 2.148(2) 1.605(2) 1.732(2) 1.722(2) 2.007(3) 2.028(3) Angles 90.08(9) 100.3(1) 88.5(1) 72.26(8) 102.2(1) 99.0(1) 86.3(1) 25.5(1) 162.2(3) (178.4(3)) 65.45(4)

b

1 f

Rh ring 2.115(2) 2.144(2) 1.605(2) 1.743(2) 1.728(2)

Li ring

a

2.02(1) 2.20(1) 1.579(5) 1.695(6) 1.692(4)

12

13

7

Rh ring

Rh ring

Rh ring

Rh ring

2.160(4) 2.105(6) 1.611(4) 1.706(6) 1.706(4) 2.001(6) 2.036(7)

2.132(3) 2.128(3) 1.624(2) 1.750(3)

2.081(8) 2.22(1) 1.613(8) 1.77(1)

1.974(4) 2.053(3)

1.999(1) 2.026(1)

2.184(3) 2.182(4) 1.599(3) 1.776(4) 1.733(3) 1.992(4) 2.058(2)

91.5(2) 99.1(2) 88.3(2) 74.2(2) 94.7(1) 103.3(1) 87.6(1)

94.9(5) 98.3(7) 85.9(6) 73.1(1) 102.3(6) 97.7(6) 86.45(5) -22.6(6)

87.7(1) 103.5(2) 83.5(2) 74.8(1) 97.20(9) 100.8(2) 96.95(6) 27.6(2)

a

94.97(7) 100.51(8) 90.01(8) 74.45(6)

90.4(4) 108.1(3) 81.5(4) 77.7(4)

-2.05(8) (-21.49(8)) -150.2(2) (-173.7(2)) 71.31(6)

12.5(4)

92.6(2) 101.5(3) 91.8(2) 74.1(2) 100.5(2) 99.15(2) 86.3(3) 1.1(3)

-163.9(8)

170.5(6)

171.8(5)

67.0(3)

c

M is the metal center defining the ring. The silicon bound to the ring N indicated. Midpoint (cod ring first double bond) to metal (e.g., for 2 C(51)-C(52)), midpoint (cod ring second double bond) to metal (e.g., for 2 C(55)-C(56)). d Atoms listed are ring atoms centered on M as specified. Coordination plane: M1-C(spiro)-P(within first metal ring)/plane M2-C(spiro)-P(within second metal ring). f There are two chemically identical rings surrounding the metal atom. Data in parentheses are for the second of the two rings in homogeneous metal complexes.

e

Figure 5. Perspective view of complex 6 showing the atom-labeling scheme. Non-hydrogen atoms are represented by Gaussian ellipsoids at the 20% probability level. The hydrogen atom attached to carbon C(1) is shown with an arbitrarily small thermal parameter; the remaining hydrogen atoms are not shown. Only the ipso carbon atoms of the phenyl rings are shown. Selected bond distances (Å): C(1)-Rh, 2.164(2); Rh-C(12), 2.043(2); C(1)-P(1), 1.794(2); C(1)-P(2),1.786(2); C(11)-P(1), 1.799(2); N(1)-P(1), 1.591(2); N(2)-P(2), 1.580(2); N(1)-Li 2.027(5); N(2)-Li, 2.034(5); Li-O, 1.927(5); Si(1)-N(1), 1.705(2); N(2)-Si(2), 1.708(2); Rh-m1 (midpoint of C(2) and C(3)), 2.11(2); Rh-m2 (midpoint of C(6) and C(7)), 2.02(1). Selected angles (deg): P(1)-C(1)-P(2), 109.7(1); N(1)-P(1)-C(1),118.5(1);C(1)-P(2)-N(2),114.8(1);P(1)-C(1)-Rh, 100.4(1); C(1)-Rh-C(12), 86.16(8); Rh-C(12)-C(11), 114.4(2); N(2)-Li-N(1), 108.7(2); O-Li-N(1), 123.9(2); O-Li-N(2), 127.3(3); Li-N(2)-Si(2), 119.8(2); C(1)-Rh-C(12), 86.15(9); m2-Rh-m1, 86.7(4); m1-Rh-C(1), 94.3(2). Nonbonded separations (Å): Li-P(1), 2.856(5); Li-P(2), 2.947(5); Li-C(1), 3.536(5). Additional details are in the SI.

92.6(2)°, N(1)-P(1)-C(1) ) 101.5°, P(1)-C(1)-Rh ) 91.8(2)°, and C(1)-Rh-N(1) ) 74.1(2)°. Analysis of Li · · · H and Rh · · · H distances in 1 (SI) found few under 3.00 Å; hence nonbonded interactions are not expected.52 Structurally related LiNPC ring containing complexes32-35,58 with Li-C σ bonds (bond distance around 2.27 Å33) are few in (58) Mu¨ller, A.; Neumu¨ller, B.; Dehnicke, K. Chem. Ber. 1996, 129, 253–257.

Figure 6. Perspective view of complex 7 showing the atom-labeling scheme. Non-hydrogen atoms are represented by Gaussian ellipsoids at the 20% probability level. Hydrogen atoms on cod and C(1) are shown with arbitrarily small thermal parameters. The phenyl and trimethylsilyl hydrogens are not shown. Only the ipso carbon atoms of the phenyl rings are shown. Selected bond distances (Å) and angles (deg): C(1)-Rh, 2.182(4); N(1)-Rh, 2.184(3); P(1)-C(1), 1.776(4); C(1)-P(2), 1.788(4); N(1)-P(1), 1.599(3); P(2)-N(2), 1.544(3); Rh-m1 (midpoint of C(2) and C(3)), 2.058(2); Rh-m2 (midpoint of C(6) and C(7)), 1.992(4). Selected angles (deg): P(1)-C(1)-Rh, 83.5(2); C(1)-Rh-N(1), 74.8(1); Rh-N(1)-P(1), 87.7(1); N(1)-P(1)-C(1), 103.5(2); C(1)-P(2)-N(2), 115.7(2); C(1)-Rh-N(1), 74.8(1); N(1)-Rh-C(1)-P(1), -20.0(1). Additional details are given in the SI.

number. Relevant complexes, 14-17, are shown in Chart 3. The bond angles and distances of 1 in the Li ring structure are generally in good agreement with those of related LiNPC fourmembered-ring compounds (shown in Table S2) especially those of 14-16 (Chart 3) (with values ranging from 2.00(1) to 2.039(3) Å). One Li atom in 17 (Chart 3) also has a Li-N distance in this range (2.000(9) Å), but the other Li-N bond is much longer than those observed in 1 and 14-16 (2.166(9) Å); this seems to be a weakly bound dicoordinate Li. The angle between the LiCP and RhCP planes of 1 is 67.0(3)°. The angles between the M1CP and M2CP ring planes of complexes 2-5

Methylene-Bridged Carbene Complexes of Rh and Pd

are similar (Table 1). The small angles within the Rh or Li fourmembered rings (Table 1) reveal the presence of ring constraints. Notably the Li center in 1 is three coordinate rather than the usual coordination number of 4, the most common coordination number found in organolithium chemistry.59 The Li-P(2) distance of 2.57(1) Å might suggest a bonding interaction60 since the sum of covalent radii of Li and P is (1.34 + 1.10 )) 2.44 Å and the sum of van der Waals radii of Li and P is (1.80 + 1.85 )) 2.65 Å, but the rigidity of the molecule would also force these atoms to be close. A bonding interaction between Li and P would likely appear as a lengthening of the Li-OEt2 bond, but it lies in the expected range (1.89(1) Å) and is about the same as the bond distance of the tricoordinated Li-OEt2 in 18 (Chart 3) (1.897(5) Å).36 The mixed Rh-Pd complexes 2 and 4 are unusual examples of this combination of metals. Only one structurally characterized, carbon-bridged Rh-C-Pd complex (complex 20 (Chart 3)) has been reported,38 and, although there are many structurally characterized bimetallic Rh-Pd complexes (the Cambridge Database lists 25),39-44 only one has a Pd-N-P-C ring (24, Chart 4)47 analogous to the present structures. The P-C bond distance (1.739(3) Å) within the Pd-N-P-C ring of 2 (and 4) is slightly ( Li-C-Rh, which is in agreement with the fact that Pd has greater formal charge (two) than do Rh and Li (one) and that Li is more covalent than Rh. Although quite a few PCM1M2P (where M1 and M2 are metal elements)-containing compounds have been synthesized,16,17,50,73 few 13C chemical shifts of the quaternary carbons in these types of compounds have been reported. The 13C chemical shift of the PC(H)(H)P of CH2(PPh2NSiMe3)2 occurs at 37.9 ppm (t, JPC ) 68).11 The two phosphorus centers of 3 or 5 are not magnetically equivalent; therefore the 31P spectrum shows a second-order pattern. The 1H chemical shift of 3 is unusual in that two CH atoms (H8 as shown in Figure 7) of the cod have a very high chemical shift value (6.97 ppm). The X-ray crystal structure of 3 showed that CH hydrogen atoms H(62) and H(55) (assigned to H5 shown in Figure 7) lie in close proximity to the phenyl ring H(12)-C(12)-C(13) and H(41)-C(42)-C(43) (angles: H(62)-H(12)-C(12)-C(13) ) 110.2(2)°; H(55)-H(41)-C(42)-C(43) ) 109.3°) (shown in Figure S5(b)), allowing shielding of their environment by phenyl ring magnetic fields, which perturbs the environment so that these particular atoms show shift values closer to those of the phenyl hydrogen atoms themselves. As was the case for 22 (Chart 4),45 the P-Rh coupling in 1, 2, and 4 is mediated via the P-N-M rather than the P-C-M linkage. The values lie in the range 17-19 Hz. This was confirmed in the NMR characterizations of complex 7. The 13C NMR of the CH3 carbon of one Si(CH3)3 group is coupled to Rh (3JCRh ) 0.5 Hz), so this Si(CH3)3 group has to be the one connected to the NRh fragment. It is also coupled to the phosphorus having a chemical shift of 49.3 ppm (3JCP ) 2.8 Hz), indicating that this phosphorus is connected to the NRh fragment, and only this phosphorus couples to Rh in the 31P spectrum. A P-Rh coupling mediated through the P-C-Rh linkage of 6 was also observed, but is much smaller (4.0 Hz). The P-P coupling through the quaternary C is much bigger (57-80 Hz) in 1, 2, and 4 than it is through the tertiary C (8-15 Hz in 6 and 7). Similar patterns appear in 21 (Chart 4) (67 Hz) and 22 (Chart 4) (5.9 Hz).45 Solution Dynamic Behavior of 3. The cod CH and CH2 proton NMR peaks were correlated with the X-ray crystal structural data of 3 as shown in Figure 7 based on the connectivity information revealed from the 1H-1H 2D COSY (SI, Figure S12) and NOE NMR spectra (SI, Figure S14). Detailed results and assignments are given in the Supporting Information. The correspondence of the solution NMR results with X-ray crystal structure of 3 indicates that solid and solution structures of 3 are very similar. The TRoESY spectrum (Figure S13) of 3 showed exchanging cross-peaks between olefinic hydrogens of different bonds (H5 (73) Aparna, K.; McDonald, R.; Cavell, R. G. J. Am. Chem. Soc. 2000, 122, 9314–9315.

Organometallics, Vol. 28, No. 6, 2009 1661

Figure 7. Proton assignments of cod ligands of complex 3 drawn at the 20% probability level. H1-H10 are the proton peaks shown in Figure S12. Selected distances (Å): H3(H57B)-H4(H58A), 2.161(0); H4 (H57B)-H10(H57A), 1.597(0), H4(H57B)-H8(H56), 2.298(1); H8(H56)-H10(H57A), 2.512(0); H1(H54A)-H2(H53B), 2.161(0); H5(H55)-H1(H54A), 2.239(0); H5(H55)-H8(H56), 2.191(1); H6(H51)-H7(H52), 2.212(1); H8(H61)-H6 (H51), 2.822(1). Scheme 7. Exchange Pattern of the cod Hydrogens in Complex 3 with a Proposed Mechanism

Scheme 8. Exchange Pattern of Allyl Groups of Complexes 2 and 4

T H6 and H7 T H8), as shown in Scheme 7 and CH2 hydrogens (H1 T H3, H2 T H4) of each cod ligand (proton labeling is shown in Figure 7), which indicates that cod ligands suffer 180° oscillation. A proposed pathway for the dynamic processes in complex 3 based on rupture of Rh-N bonds and rotations around Rh-C bonds is illustrated in Scheme 8. Similar mechanistic proposals have been advanced elsewhere to account for the dynamic behavior of Rh(I) complexes.48 Solution Dynamic Behavior of 2 and 4. Two sets of diasteromers (each set consists of a racemic pair) are shown as indicated by the 1H and 31P spectra of complexes 2 and 4. The two types of diastereomers observed in complexes 2 (3.6:1 ratio) and 4 (1.6:1) probably arise from the different relative position of the central allyl C-H vector (up or down) with respect to the PdNPC ring. In the crystal structures of 2 and 4, only one type of diastereomer was observed. The 1H-1H 2D TRoESY spectra of 2 and 4 (SI, Figure S16 and S19) in C6D6 reveal that the allyl groups of the two sets of diasteromers are under syn-syn, anti-anti type of exchange, as shown in Scheme 9. To study the exchange patterns of allyl groups of isomers of complex 2, THF-d8 solvent was used instead of C6D6, which provides a greater dispersion of the 1H

1662 Organometallics, Vol. 28, No. 6, 2009 Scheme 9.

1

H Assignments and Exchange Processes of cod Ligands of Complex 2

chemical shifts of the allyl groups, thereby enabling a more advantageous separation of the proton allyl hydrogens: Hb2 from Ha1; Hb3 from Ha3 and Ha4 (labeling shown in Scheme 9). In the crystal structure of 2, only one type of diastereomer was observed. For the syn-syn, anti-anti exchange (apparent rotation of the allyl group) of Pd-allyl complexes, several mechanisms have been suggested by others: (i) a pseudorotation within a pentacoordinate palladium intermediate;74-76 (ii) a process whereby the bond to one nitrogen is broken followed by rearrangement and recombination.77-79 Either mechanism is consistent with the evidence available for this system. NOE cross-peaks (SI, Figure S14) between Ha4 and Hb1; Ha2 and Hb2 were observed in complex 4 (SI, Figure S17). When a more concentrated (2X) sample (32 mg/mL) was used, additional exchanges between Ha3 and Hb3; Ha1 and Hb4 were observed (Scheme 9). Similarly NOE cross-peaks were observed for 2 (Ha3 and Hb1, Ha1 and Hb4, Ha4 and Hb3, Ha2 and Hb2 as shown in the SI, Figures S20 and S23). The solution exchange process (Scheme 9) affecting the cod hydrogen atoms of 2 is the same as that proposed for complex 3: rupture of the Rh-N bond followed by rotation of the cod ligands around the Rh-olefinic bond axis (see Scheme 7). The 1 H-1H 2D TRoESY study of complex 2 in C6D6 (SI, Figure S22) also revealed an exchange cross-peak between Ha4 and Hb3, indicating that the two sets of cod ligands also exchange. However the exact exchange pattern is not clear because of some signal overlaps. In summary, 2 shows complicated exchange processes in solution wherein allyl and cod ligand exchange between diasteromers and intramolecular cod hydrogen exchange occur simultaneously. Solution Dynamic Behavior of Complex 1. The NMR proton peaks due to the cod ligand were successfully assigned to the cod hydrogen atoms in the crystal structure of 1 (SI, Figure S24) based on 1H-1H COSY (SI, Figure S25) and NOE spectral data (SI, Figure S27). The exchange pattern of the cod hydrogens, revealed by the TRoESY spectrum (SI, Figure S26), and a proposed mechanism is illustrated in Scheme 10. Possible steps are (i) dissociation of a rhodium-olefinic bond; (ii) rotation of the η2-cod ligand around the remaining rhodium-olefin bond axis; (iii) inversion of the flexible η2-cod ligand config(74) Hansson, S.; Norrby, P.; Sjo¨gren, M. P. T.; Åkermark, B.; Cucciolito, M. E.; Giordano, F.; Vitagliano, A. Organometallics 1993, 12, 4940–4948. (75) van Haaren, R. J.; Goubitz, K.; Fraanje, J.; van Strijdonck, G. P. F.; Oevering, H.; Coussens, B.; Reek, J. N. H.; Kamer, P. C. J.; van Leeuwen, P. W. N. M. Inorg. Chem. 2001, 40, 3363–3372. (76) Jalo´n, F. A.; Manzano, B. R.; Moreno-Lara, B. Eur. J. Inorg. Chem. 2005, 100–109. ¨ rnebro, J.; Grennberg, H.; Ba¨ckvall, J. J. Am. Chem. (77) Gogoll, A.; O Soc. 1994, 116, 3631–3632. (78) Torre, F. G. d. l.; Hoz, A. d. l.; Jalo´n, F. A.; Manzano, B. R.; Otero, A. O.; Rodrı´guez, A. M.; Rodrı´guez-Pe´rez, M. C.; Elguero, J. Inorg. Chem. 1998, 37, 6606–6614. (79) Torre, F. G. d. l.; Hoz, A. d. l.; Jalo´n, F. A.; Manzano, B. R.; Rodrı´guez, A. M. Inorg. Chem. 2000, 39, 1152–1162.

Fang et al. Scheme 10. Proposed Motions Describing the Dynamic NMR Exchange Process for cod Occurring in Complex 1

uration; and (iv) reassociation to form the η4-cod structure. This type of cod exchange pattern is rare, but it has been suggested elsewhere.80 It is strange that the cod exchange pattern of complex 1 seems to be entirely different from that of complexes 2 and 3 (described above), in which it appears that a Rh-N bond is broken. However, the X-ray crystal structures of complex 1 and complex 3 do show that the Rh-N bond of complex 3 (Rh-N 2.197(2) Å) is longer than that of complex 1 (2.160(4) Å) and one of the Rh-olefinic bonds of complex 1 (2.036(7) Å) is longer than those of complex 3 (Rh-m1 2.001(6) Å; Rh-m2 2.036(7) Å), so it should not be too surprising that dirhodium, Rh-Pd, and Rh-Li complexes have different dynamic NMR exchange properties.

Experimental Section General Considerations. All manipulations were performed either in an Ar-filled glovebox or under an Ar or N2 atmosphere using standard Schlenk techniques. Et2O and THF were dried over Na/benzophenone and distilled under an Ar atmosphere before use. The dilithiated methylenebis(diphenylphosphoranimine) salt ([Li2L]2) was prepared as previously described.10,11 All other reagents were used as received from commercial sources. Unless otherwise indicated, reactions were carried out in a Pyrex glass bulb with one valve or a 5 mm NMR tube. All spectroscopy was performed within the Department of Chemistry at the University of Alberta. NMR spectra were recorded at ambient probe temperature using C6D6 or [d8]THF as solvent on a Varian i400 spectrometer (161.9 MHz for 31P, 100.6 MHz for 13C, 155.4 MHz for 7Li) and referenced to residual solvent proton (1H), solvent carbon (13C), external 85% aqueous H3PO4 (31P), or external 0.5 M LiCl in D2O (7Li). For NMR data, J values are given in Hz; abbreviations used: s ) singlet, d ) doublet, t ) triplet, m ) multiplet, and p ) pseudo. Microscope-IR spectra were recorded on a Bomem MB100 instrument and are reported in cm-1; abbreviations used are s ) strong, m ) medium, and w ) weak. Elemental analyses (C, H, N) were performed using a Carlo Erba EA1108 elemental analyzer in the Department of Chemistry, University of Alberta. The synthesis of 1 is described elsewhere.8 Structural information is provided in the SI. Synthesis of 2. To a benzene solution (∼5 mL) of 1 (0.157, 0.185 mmol) was added [PdCl(allyl)]2 (0.038 g, 0.104 mmol) in a single portion at room temperature. The mixture was stirred for 15 min, over which time the color of the solution changed from yellow to green-brown. Some black particles had formed in the solution, which were removed by filtration, and the solution was then stirred (80) Crociani, B.; Antonaroli, S.; Di Vona, M. L.; Licoccia, S. J. Organomet. Chem. 2001, 631, 117–124.

Methylene-Bridged Carbene Complexes of Rh and Pd for another 40 min. The solvent was removed under dynamic vacuum, and the resultant solid was extracted with ether (5 mL). The insoluble material (LiCl) was removed by centrifugation. The organic portion was reduced in vacuo to approximately 1 mL. This mixture was kept at -20 °C overnight in the drybox, whereupon the crystalline solid, 2, precipitated (0.108 g, yield: 63.7%). 2 is moisture sensitive; however, in the solid state, 2 is stable to air for short periods of time. Anal. Calcd for C42H55N2P2PdRhSi2: C, 55.1; H, 6.1; N, 3.1. Found: C, 55.2; H, 6.0; N, 2.9. Crystals suitable for X-ray diffraction were grown at -20 °C from an ether solution of 2. A mixture of two isomers was obtained with a ratio of 3.6:1. For the major diastereomers (a mixture of 1:1 ratio enantiomers): H NMR (C6D6): δ 0.112 (s, 9H, Si(CH3)3) and 0.133 (s, 9H, Si(CH3)3), 1.47 (m, 1H, cod CH2), 1.54 (m, 1H, cod CH2), 1.69 (m, 1H, cod CH2), 1.85 (m, 1H, cod CH2), 1.93 (m, 1H, cod CH2), 2.11 (m, 1H, cod CH2), 2.29 (m, 1H, cod CH2), 2.48 (m, 1H, cod CH2), 3.14 (d, 1H, allyl CH2, JHH ) 12), 3.18 (m, 1H, cod CH), 3.60 (m, 1H, allyl CH2), 4.17 (dd, 1H, allyl CH2, JHH ) 3 and 7), 4.21 (dd, 1H, allyl CH2, JHH ) 2 and 12), 4.42 (m, 1H, cod CH), 4.84 (m, 1H, cod CH), 5.23 (m, 1H, cod CH), 5.37 (m (7 peaks), 1H, allyl CH), 6.69 (dt, 2H, Ph H, J ) 2.2 and 7.8), 6.82 (dt, 1H, Ph H, J ) 1.6, 7.5), 6.91-7.22 (m, 11H, Ph H), 7.42 (ddd, 2H, Ph H, J ) 1.3, 8.3, 11.2), 7.78 (ddd, 2H, Ph H, J ) 1.6, 8.0, 11.3), 7.98 (ddd, 2H, Ph H, J ) 1.3, 8.3, 11.0). 31P{1H} NMR (C6D6): δ 30.9 (dd, PNRh, JPP ) 71, JPRh ) 17), 41.8 (d, PNPd, JPP ) 71, JPRh ) 0). 13C{1H} NMR (C6D6): δ 4.2 and 4.7 (d, Si(CH3)3),3JCP ) 3.3 and 3.7), 29.6 (s, cod CH2), 31.5 (s, cod CH2), 31.8 (s, cod CH2), 32.6 (s, cod CH2), 49.2 (s, allyl CH2), 61.1 (s, allyl CH2), 68.1 (d, cod CH, 1JCRh ) 12), 72.9 (d, cod CH, 1JCRh ) 15), 75.6 (d, cod CH, 1JCRh ) 11), 79.6 (d, cod CH, 1JCRh ) 11), 105.2 (s, allyl CH), 127-134 (Ph CH), 137-140 (Ph C), PCP carbon not observed. 1

For the minor diastereomers (a mixture of 1:1 ratio enantiomers): H NMR (C6D6): δ 0.097 and 0.115 (s, 18H, two Si(CH3)3), 1.40-2.42 (m, 8H, cod CH2), 2.20 (d, 1H, allyl CH2, JHH ) 11), 3.15 (d, 1H, allyl CH2, JHH ) 13), 3.60 (m, 1H, cod CH), 4.19 (d, 1H, allyl CH2, J obscured), 4.45 (m, 1H, cod CH), 4.71 (m, 1H, cod CH), 4.84 (m, 1H, cod CH), 5.03 (d, 1H, allyl CH2, JHH ) 7.0), 5.42 (m (7 peaks), 1H, allyl CH), 6.75 (dt, 2H, Ph H, J ) 2.1, 7.9), 6.88 (dt, 1H, Ph H, J ) 1.4, 7.4), 6.91-7.28 (m, 11H, Ph H), 7.50 (ddd, 2H, Ph H, J ) 1.2, 9.7, 11.2), 7.84 (ddd, 2H, Ph H, J ) 1.6, 7.5, 10.8), 8.01 (close to dd, 2H, Ph H, J obscured). 31P{1H} NMR (C6D6): δ 34.8 (dd, JPP ) 71, JPRh ) 17), 40.6 (d, JPP ) 71, JPRh ) 0). 13C{1H} NMR (C6D6): δ 4.3 and 4.7 (d, Si(CH3)3), 30.5 (s, cod CH2), 30.8 (s, cod CH2), 32.7 (s, cod CH2), 46.4 (s, allyl CH2), 61.3 (s, allyl CH2), 67.3 (d, cod CH, 1JCRh ) 12), 75.6 (d, cod CH, 1JCRh ) 11), 78.1 (d, cod CH, 1JCRh ) 12), 108.5 (s, allyl CH), 127-134 (Ph CH), 137-140 (Ph C), PCP carbon not observed. 1

The compound remains unchanged after 1-day exposure to air (Microscope IR). Microscope IR: 663 (m), 694 (m), 713 (m), 734 (m), 744 (m), 767 (s), 832 (vs), 849 (vs), 907 (w), 940 (vw), 999 (vw), 1030 (vw), 1075 (vs), 1096 (s), 1118 (m), 1145 (s), 1210 (vw), 1240 (s), 1244 (s), 1254 (s), 1327 (vw), 1432 (s), 1479 (m), 2822 (m), 2870 (m), 2908 (m), 2950 (m), 3056 (m), 3073 (m). Syntheses of 3. (a) From [Li2L]2 and [Rh(cod)Cl]2. Under an argon atmosphere, THF (∼3 mL) was added to the solid mixture of [Li2L]2 (0.106 mg, 0.093 mmol) and [Rh(cod)Cl]2 (0.110 g, 0.223 mmol, 20% in excess) to form an orange-red solution. The solution turned to red-green while being stirred at room temperature for 4 days. No precipitate was observed. The in situ 31P NMR spectrum showed that 79 mol % of [Li2L]2 had been converted to 3. The solvent was removed under a dynamic vacuum, and the resultant solid was extracted with ether (∼10 mL). The insoluble material (LiCl) was removed by centrifugation. Ether was then removed under dynamic vacuum, and the resultant solid was washed with ether (∼2 mL) and dried under vacuum to yield 0.117 g of a yellow

Organometallics, Vol. 28, No. 6, 2009 1663 solid. The ether solution (∼2 mL) that was recovered from washing was kept at -20 °C for 1 h, which yielded another 0.010 g of yellow solid. The total yield of complex 3 · Et2O was 0.127 mg (0.121 mmol, 65%). Anal. Calcd for C51H72N2OP2Rh2Si2: C, 58.2; H, 6.9; N, 2.7. Found: C, 57.7; H, 6.7; N, 2.5. 1H NMR (C6D6): δ -0.03 (s, 18H, Si(CH3)3), 1.87 (m, 4H, cod CH2), 1.99 (m, 4H, cod CH2), 2.43 (m, 1H, cod CH2), 2.56 (m, 1H, cod CH2), 2.74 (m, 1H, cod CH2), 2.95 (m, 1H, cod CH2), 4.01 (m, 2H, cod CH), 4.72 (m, 2H, cod CH), 5.04 (m, 2H, cod CH), 6.87 (t, 4H, Ph H, no change in 1 H{31P}), 6.97 (t, 4H, two cod CH and two Ph H, tt in 1H {31P}), 7.06 (m, 6H, Ph H), 7.67 (q, 4H, Ph H, doublet in 1H {31P}), 8.08 (br s, 4H, Ph H). 31P{1H} NMR (C6D6): δ 32.6(close to dd, JPRh ) 7.6 and 9.2). 31P{1H} NMR (THF-d8): δ 32.9(close to dd, JPRh ) 7.6 and 9.2). 13C{1H} NMR (C6D6): δ 5.1 (d, Si(CH3)3), 3JCP ) 1.6), 11.9 (observed in 13C{1H, 31P}, t, RhCRh, 1JCRh )17), 30.4 (s, cod CH2), 30.5 (s, cod CH2), 32.1 (s, cod CH2), 32.7 (s, cod CH2), 49.2 (s, allyl CH2), 70.4 (d, cod CH, 1JCRh ) 12), 74.5 (m, cod CH), 76.0 (m, cod CH), 79.9 (m, cod CH), 127-134 (Ph CH), 137.7 (observed in 13C{1H, 31P}, t, Ph C, 3JCRh ) 4.6), 141.5 (observed in 13C{1H, 31P}, t, Ph C, 3JCRh ) 6.5). The sample is unchanged after exposure to air for 1 day according to Microscope IR. Microscope IR: 664 (m), 692 (m), 706 (m), 719 (m), 739 (s), 766 (s), 832 (vs), 847 (vs), 949 (w), 975 (w), 999 (w), 1028 (w), 1066 (vs), 1099 (s), 1106 (s, sh), 1114 (s, sh), 1175 (w), 1211 (w), 1243 (s), 1254 (s), 1327 (w), 1433 (s), 1479 (m), 2826 (s), 2871 (s), 2911 (s), 2945 (s), 3056 (m). (b) From Complex 1 and [RhCl(cod)]2. Under an argon atmosphere in a 5 mm NMR tube, THF (∼0.8 mL) was added to the solid mixture of complex 1 (58 mg, 0.068 mmol) and [RhCl(cod)]2 (0.024 g, 0.049 mmol, 43% in excess) to form an orange-red solution. The solution was kept at room temperature for 4 days. A very small amount of yellow precipitate was formed over this period, and the solution remained orange-red. The in situ 31 P NMR spectrum showed that 95 mol % of complex 1 was converted to 3. The solvent was removed under dynamic vacuum, and the resultant solid was extracted with ether (∼3 mL) and filtered. Ether was then removed under dynamic vacuum, and the yellow solid was washed with ether (∼0.5 mL) and dried under vacuum to provide 0.032 g of a yellow solid. The wash ether solution (∼0.5 mL) was kept at -20 °C for 1 h, to yield a further 0.008 g. The total yield of complex 3 · Et2O is 0.040 mg (0.038 mmol, 56%). Crystals of 3 · Et2O suitable for X-ray diffraction were grown at -20 °C from an ether solution. Synthesis of 4. A degassed orange ether solution (10 mL) of compound 2 (0.248 g, 0.271 mmol) (degassed beforehand) was reacted with CO at 1 atm pressure in a 50 mL bulb tube with valve at room temperature. A yellow solution with small amounts of black precipitate was obtained after the mixture was stirred for 1.5 h. In situ 31P NMR showed 2 was converted to 4 quantitatively. The solvent was removed by dynamic vacuum for 1 h. The product was a yellow powder. The product was extracted with 10 mL of THF and filtered. A small amount of black, insoluble material was observed. The solution was reduced to 0.5 mL and put into a freezer, to yield 0.210 g (0.265 mmol) of 4 (yield 90.0%) as a yellow powder. Compound 4 is very soluble in THF (>100 mg/mL) and benzene (>112 mg/mL), but not as soluble in ether and the least soluble in hexane. Anal. Calcd for C36H43N2O2P2Rh1Pd1Si2: C, 50.09; H, 5.02; N, 3.25. Found: C, 50.23; H, 4.88; N, 3.06. In the starting compound 2, the molar ratio of the major diasteromers to the minor diasteromers is 100:26, close to 4:1. In the product, the ratio of major diasteromers/minor diasteromers is 1.4:1 (based on 1 H recorded in C6D6). In THF-d8, the ratio of major diasteromers/ minor diasteromers is 1.6:1. For the major disateromers: 1H NMR (C6D6): δ 0.12 (s, 9H, Si(CH3)3) and 0.14 (s, 9H, Si(CH3)3), 2.82 (d, 1H, 3JHH ) 13, allyl CH2 (Ha1), 2.97 (td, 1H, JHH ) 6.6, 1.7, allyl CH2 (Ha2)), 3.41 (d, 1H, 3JHH ) 12, allyl CH2 (Ha3)), 3.93 (dd, 1H, JHH ) 7.0, 2.2, allyl

1664 Organometallics, Vol. 28, No. 6, 2009 CH2 (Ha4)), 4.72 (m (7 peaks), 1H, allyl CH (Ha5)), 6.8-7.4 (m, 16H, Ph H), 7.51 (ddd, 2H, Ph H), 7.85 (ddd, 2H, Ph H). 1H NMR (THF-d8) (12May05): δ -0.113 (s, 9H, Si(CH3)3) and -0.174 (s, 9H, Si(CH3)3), 2.79 (d, 1H, 3JHH ) 13, allyl CH2 (Ha1)), 2.84 (d, 1H, 3JHH ) 6.4, allyl CH2 (Ha2)), 3.25 (d, 1H, 3JHH ) 12, allyl CH2 (Ha3)), 3.95 (dd, 1H, JHH ) 6.8, 2.4, allyl CH2 (Ha4)), 4.91 (m (7 peaks), 1H, allyl CH (Ha5)), 6.9-7.5 (m, 18H, Ph H), 7.57 (ddd, 2H, Ph H). 31P{1H} NMR (C6D6): δ 49.3 (dd, PNRh, 2JPP ) 57, 2 JPRh ) 18), 39.5 (d, 2JPP ) 57, 2JPRh ) 0). 31P{1H} NMR (THFd8) (12May05): δ 54.6 (dd, PNRh, 2JPP ) 57, 2JPRh ) 18), 45.9 (d, 2 JPP ) 57, 2JPRh ) 0). 13C{1H} NMR (C6D6): δ 0.9 (d in 13C{1H, 13 P}, PdCRh, 1JCRh ) 14.0), 3.87 (d, Si(CH3)3, 3JCP ) 3.1), 4.1 (dd, Si(CH3)3, 3JCRh ) 0.57, 3JCP with P at 49.3 ppm ) 3.0), 49.7 (s, allyl CH2), 61.1 (s, allyl CH2), 105.4 (s, allyl CH), 127.2-133.1 (Ph CH), 137.5 (d, Ph C, 1JCP ) 79), 138.6 (dd, Ph C, 1JCP ) 77, 3 JCPh ) 1.4), 138.7 (dd, Ph C, 1JCP ) 82, 3JCPh ) 1.3), 139.2 (dd, Ph C, 1JCP ) 79, 3JCPh ) 0.9), 188.4 (d, CO, 1JCRh ) 64), 189.9 (d, CO, 1JCRh ) 70). For the minor diasteromers: 1H NMR (C6D6): δ 0.07 (s, 9H, Si(CH3)3) and 0.17 (s, 9H, Si(CH3)3), 1.34 (d, 1H, 3JHH ) 12, allyl CH2 (Hb1)), 2.59 (d, 1H, 3JHH ) 13, allyl CH2 (Hb2)), 3.98 (dd, 1H, JHH ) 7.1, 2.3, allyl CH2 (Hb3)), 4.03 (dd, 1H, JHH ) 6.4, 1.9, allyl CH2 (Hb4)), 4.89 (m (7 peaks (Hb5)), 1H, allyl CH), 6.8-7.4 (m, 16H, Ph H), 7.66 (ddd, 2H, Ph H), 8.25 (ddd, 2H, Ph H). 1H NMR (THF-d8): δ -0.107 (s, 9H, Si(CH3)3) and -0.221 (s, 9H, Si(CH3)3), 1.19 (d, 1H, 3JHH ) 12, allyl CH2 (Hb1)), 2.68 (d, 1H, 3JHH ) 13, allyl CH2 (Hb2)), 3.81 (d, 1H, JHH ) 6.4, 1.8, allyl CH2 (Hb4)), 3.97 (dd, 1H, JHH ) 7.2, 2.4, allyl CH2 (Hb3)), 5.05 (m (7 peaks (Hb5)), 1H, allyl CH), 6.9-7.5 (m, 18H, Ph 8.02 (ddd, 2H, Ph H). 31 P{1H} NMR (C6D6): δ 56.5 (dd, PNRh, 2JPP ) 57, 2JPRh ) 18), 36.5 (d, 2JPP ) 58, 2JPRh ) 0). 31P{1H} NMR (THF-d8): δ 61.8 (dd, PNRh, 2JPP ) 57, 2JPRh ) 18), 42.5 (d, 2JPP ) 57, 2JPRh ) 0). 13 C{1H} NMR (C6D6): δ 0.2 (d in 13C{1H, 13P}, PdCRh, 1JCRh ) 14.2), 3.89 (d, Si(CH3)3, 3JCP obsured), 4.0 (dd, 3JCRh ) 0.57, 3JCP with P at 56.5 ppm ) 2.8), 48.0 (s, allyl CH2), 67.8 (s, allyl CH2), 109.7 (s, allyl CH), 127.2-133.1 (Ph CH), 138.0 (d, Ph C, 1JCP ) 74), 138.1 (dd, Ph C, 1JCP ) 73, 3JCPh ) 1.2), 138.9 (dd, Ph C, 1JCP ) 77, 3JCPh ) 1.4), 139.7 (dd, Ph C, 1JCP ) 76, 3JCPh ) 0.7), 189.0 (d, CO, 1JCRh ) 64), 189.5 (d, CO, 1JCRh ) 70). The compound is unchanged after 1-day exposure to air (Microscope IR). Microscope IR: 664 (m), 695 (s), 717 (s), 740 (s), 770(s), 834 (br s), 922 (m), 999 (w), 1012 (w), 1028 (w), 1095 (br s), 1138 (s), 1161 (s), 1250 (s), 1311 (w), 1399 (w), 1436 (s), 1482 (m), 1572 (w), 1777 (w), 1823 (w), 1925 (m, sh), 1954 (vs, br, υ(CO)), 2009 (m, sh, υ(CO)), 2027 (vs, br, υ(CO)), 2894 (m), 2964 (m), 3055 (m). Synthesis of 5. A degassed orange THF/ether (1:1 ratio) solution (6 mL) of compound 3 (0.058 g, 0.059 mmol) was reacted with CO at 1 atm pressure in a 50 mL bulb tube with valve at room temperature. The mixture was stirred for 24 h. No obvious color change was observed. In situ 31P NMR showed 3 was converted to 5 quantitatively. The solvent was removed by dynamic vacuum for 1 h, leaving an oily product. The product was then extracted with ether (3 mL) and filtered, and then solvent was removed in dynamic vacuum for 12 h to yield 37 mg (0.042 mmol) of 5 (yield: 71%) as a yellow powder. Anal. Calcd for C35H38N2O4P2Rh2Si2: C, 48.1; H, 4.4; N, 3.2. Found: C, 47.6; H, 4.3; N, 2.8. 1H NMR (C6D6): δ 0.10 (s, 18H, Si(CH3)3), 6.83 (m, 4H, Ph H), 6.97 (m, 6H, Ph H), 7.05 (m, 2H, Ph H), 7.47 (m, 4H, Ph H), 7.62 (m, 4H, Ph H). 31P{1H} NMR (C6D6): δ 52.1 (close to dd, J ) 7.2, 9.4). 31 P{1H} NMR (THF-d8): δ 52.9 (close to dd, JPRh ) 6.9 and 9.2). 13 C{1H} NMR (C6D6): δ 3.7 (s, Si(CH3)3)), 127.7-133.4 (Ph CH), 135.4 (d, 1JCP ) 83, Ph C), 136.2 (d, 1JCP ) 78, Ph C), 187.4 (d, CO, 1JCRh ) 67), 188.4 (dt, CO, 1JCRh ) 68, 3JCP ) 2.0), RhCRh not observed. Microscope IR: 667 (m), 692 (s), 722 (m), 740 (m), 773 (s), 834 (vs), 998 (w), 1028 (w), 1067 (s), 1091 (s), 1106 (s), 1122 (m), 1184 (w), 1248 (s), 1312 (w), 1435 (s), 1481 (w), 1589

Fang et al. (w), 1640 (br, m), 1832 (br, w), 1971 (vs, ν(CO)), 2029 (vs, ν(CO)), 2047 (vs, ν(CO)), 2895 (w), 2952 (m), 3057 (w), 3410 (br, m). Synthesis of 6. A yellow THF solution (7 mL) of compound 1 (0.224 g, 0.264 mmol) was heated for 16 h at 60 °C, resulting in formation of 6 (100% yield by in situ 31P). The THF was removed under dynamic vacuum, and the solid was dissolved in ether (5 mL). The solution was reduced in volume to approximately 1 mL by vacuum and was kept at -20 °C overnight in the drybox to produce 0.131 g of an orange, crystalline solid (6). The supernatant liquid was decanted, further reduced in volume, and cooled again to obtain a second crop (0.030 g) of 6. The overall yield of 6 was 0.161 g (71.8%). At room temperature, 30% of 1 (16 mg in 0.5 mL dry C6D6, 31P) was converted to 6 over 12 days. The conversion of 1 (25 mg in 0.5 mL dry C6D6) to 6 was completed (100% yield by 31P) after 36 h heating at 54 °C. Complex 6 is very soluble in benzene (>176 mg/ml) and ether (>300 mg/ml). It is sensitive to moisture; however it remains stable in dry benzene at 60 °C for 5 days. Anal. Calcd for C43H60LiN2OP2RhSi2: C, 60.8; H, 7.1; N, 3.3. Found: C, 60.7; H, 7.1; N, 3.3. A quantity of 6 (0.088 g) was dissolved in 0.5 mL of C6H6 under argon in a 5 mm NMR tube equipped with a Teflon cap. Red crystals of 6 · 1.5C6H6 were formed after the sample was kept at room temperature for about 1 week, and its crystal structure was determined. 1H NMR (C6D6): δ -0.245 (s, 9H, Si(CH3)3), -0.098 (s, 9H, Si(CH3)3), 1.08 (t, 6H, (CH3CH2)2O, 3JHH ) 7.1), 1.15 (m, 1H, cod CH2), 1.47 (m, 1H, cod CH2), 1.57 (m, 2H, cod CH2), 1.80 (m, 1H, cod CH2), 1.94 (m, 2H, cod CH2), 2.41 (m, 1H, cod CH2), 2.90 (ddd, PC(H)P, 2 JHP ) 8.0 with 31P peak at 29 ppm, 2JHP ) 13.0 with 31P peak at 19 ppm, 2JHRh ) 2.8), 3.31 (q, 4H, (CH3CH2)2O, 3JHH ) 7.1), 3.75 (m, 2H, cod CH), 3.93 (m, 1H, cod CH), 4.35 (m, 1H, cod CH), 7.03 (m, 3H, Ph H), 7.08 (m, 3H, Ph H), 7.21 (m, 5H, Ph H), 7.57 (m, 3H, Ph H), 7.68 (t, 1H, Ph H, 3JHP ) 7.3 Hz with P at 18 ppm, 3 JHH ) 7.3), 7.87 (dd, 2H, Ph H, 3JHP ) 12.8 Hz coupled with P at 28 ppm), 8.35 (m, 2H, Ph H, 3JHP ) 10.7 Hz with P at 28 ppm, 3 JHH ) 7.8). 31P{1H} NMR (C6D6): δ 28.9 (dd, 2JPP ) 14.8, 2JPRh ) 4.0, PPh2), 18.5 (d, 2JPP ) 14.8, 2JPRh ) 0, PPh). 13C{1H} NMR (C6D6): δ 3.64 (d, Si(CH3)3), 3JCP ) 4.1), 4.14 (d, Si(CH3)3), 3JCP ) 3.6), 14.6 (s, Et2O), 30.1, 30.2, 30.7, 31.8 (s, cod CH2), 37.9 (m, PC(H)P, 1JCRh ) 25.8 (obtained in 13C{1H, 31P}), 1JCP obscured), 65.5 (s, Et2O CH2), 76.5 (d, cod CH, 1JCRh ) 11.9), 77.1 (d, cod CH, 1JCRh ) 10.9), 82.3 (d, cod CH, 1JCRh ) 6.7), 82.3 (d, cod CH, 1 JCRh ) 7.8), 122.5-135.1 (m, Ph CH), 137.1 (d, 2 Ph C, 1JCP ) 96.8, 3JCP ) 5.6), 140.4 (d, Ph C, 1JCP ) 89.6, 3JCP ) 3), 142.5 (d, Ph C, 1JCP ) 92.5, 3JCP ) 10.0), 145.4 (d, Ph C, 1JCP ) 134.7, 3JCP not observed), 177.8 (t, CRh, 1JCRh ) 36.0, 2JCP ) 35.8). 7Li{1H} NMR (C6D6): δ 0.99 (s). Syntheses of 7. (a) Directly from LiC(H)(Ph2PdNSiMe3)2 and [RhCl(cod)]2. To a benzene solution (10 mL) of [RhCl(cod)]2 (0.132 g, 0.268 mmol) was added 4.7 mL of a benzene/pentane solution of LiC(H)(Ph2PdNSiMe3)2 (0.1137 M, total moles: 0.534 mmol) dropwise. The resultant yellow solution was stirred for 3 days at room temperature. The solvent was removed under vacuum, and the yellow solids obtained were extracted with 20 mL of pentane followed by 10 mL of hexane. The insoluble solid (LiCl) was removed by centrifugation. The orange solution thus obtained was reduced in vacuo to approximately 2 mL and kept at -15 °C overnight. Orange crystals of 7 (0.160 g) were formed, from which crystals suitable for X-ray diffraction were obtained. The supernatant liquid was decanted, further reduced in volume, and cooled again to obtain a second crop (0.208 g) of 7. The overall yield of 7 was 0.368 g (0.479 mmol, 89.6%). Anal. Calcd for C39H51N2P2RhSi2: C, 60.9; H, 6.7; N, 3.6. Found: C, 61.3; H, 6.5; N, 3.5. 1H NMR (C6D6): δ 0.198 (s, 18H, Si(CH3)3), 1.47 (m, 4H, cod CH2), 1.86 (t, 1H, PC(H)P, 2JHP ) 10.6), 2.17 (br s, 4H, cod CH2), 4.22 (br s, 4H, cod CH), 6.88 (m, 6H, Ph H), 7.15 (m, 2H, Ph H), 7.22 (m, 4H, Ph H), 7.51 (m, 4H, Ph H), 8.28 (m, 4H, Ph H). 1H NMR (THF-d8): δ -0.09 (s, 18H, Si(CH3)3), 1.54 (m, 4H, cod CH2), 1.72

Methylene-Bridged Carbene Complexes of Rh and Pd (dt, 1H, PC(H)P, 2JHP ) 9.6, 2JHRh ) 1.6), 2.14 (m, 4H, cod CH2), 4.02 (br s, 4H, cod CH), 7.04 (dt, 4H, Ph H), 7.17 (dt, 2H, Ph H), 7.38 (ddd, 4H, Ph H), 7.42-7.52 (m, 6H, Ph H), 8.09 (m, 4H, Ph H). 31P{1H} NMR (C6D6): δ 21.49 at 5 °C (br s), 21.43 at 25 °C (br s), 20.76 (br d, 2Jpp ) 8.3) at 75 °C. 31P{1H} NMR (THF-d8): δ 20.2 at 25 °C (br s). 13C{1H} NMR (C6D6): δ 4.4 (d, Si(CH3)3), 3 JCP ) 3.2), 8.1 (dt, PCP, 1JCP ) 67, 1JCRh ) 18), 30.7 (s, cod CH2), 77.1 (s, cod CH), 127.3-132.1 (m, Ph CH), 138.3 (dd, two Ph C, 1JCP ) 83, 3JCRh ) 3.6), 138.4 (dd, two Ph C, 1JCP ) 92, 3 JCRh ) 4.6). (b) Synthesis of 7 via Hydrolysis of Complex 6. To the dry THF solution (0.43 mL) of 6 (16 mg, 0.019 mmol) in a 5 mm NMR tube equipped with a Teflon cap was added 0.3 mL of a THF solution of H2O (concentration: 0.065 mmol/mL H2O, total moles of H2O: 0.020 mmol). The solution color changed immediately from yellow to yellow-green, and a white precipitate (LiOH) formed. The 31P NMR spectrum taken at this time showed only peaks due to compound 7. The THF was removed by dynamic vacuum and C6D6 (about 0.5 mL) added to the 5 mm NMR tube; the 1H and 31P (21.07) of the resultant solution showed it was compound 7. Synthesis of 8. A degassed orange solution (6 mL) of compound 7 (0.073 g, 0.095 mmol) in ether was reacted with CO at 1 atm pressure in a 50 mL bulb at room temperature equipped with a vacuum valve. A yellow solution with small amounts of black precipitate was obtained after the mixture was stirred for 1 h. In situ 31P showed that 7 was quantitatively converted to 8. The product was extracted by hexane (6 mL) and filtered. Some brown insoluble material was observed. Removing the solvent from the product by overnight pumping in a dynamic vacuum yielded 53 mg (0.074 mmol) of 8 (yield: 78%) as a light yellow powder. Compound 8 is soluble in benzene and hexane and can be purified by recrystallization in hexane. Anal. Calcd for C33H39N2O2P2Rh1Si2: C, 55.3; H, 5.5; N, 3.9. Found: C, 55.2; H, 5.3; N, 3.7. 1H NMR (C6D6): δ 0.22 (s, 18H, Si(CH3)3), 2.16 (dt, 1H, PC(H)P, 2JHP ) 11, 2JHRh ) 2.0), 6.85 (dt, 4H, Ph H, JHH ) 7.5, 2.3), 6.92 (m, 2H, Ph H, JHH ) 7.5, 1.5), 7.10 (m, 6H, Ph H), 7.45 (ddd, 4H, Ph H, JHH ) 7.5, 1.2, 3JHP ) 12), 8.07 (ddd, 4H, Ph H, JHH ) 8.3, 1.4, 3JHP ) 12). 1 H NMR (THF-d8): δ 0.003 (s, 18H, Si(CH3)3), 2.17 (dt, 1H, PC(H)P, 2JHP ) 9.4, 2JHRh ) 1.7), 7.09 (dt, 4H, Ph H, JHH ) 7.6, 2.8), 7.24 (dt, 2H, Ph H, JHH ) 7.2, 1.6), 7.36-7.54 (m, 10H, Ph H), 7.96 (ddd, 4H, Ph H, JHH ) 7.5, 1.2, 3JHP ) 12). 31P{1H} NMR (C6D6): δ 30.5 (br s). 31P{1H} NMR (THF-d8): δ 28.3 (br s). 13 C{1H} NMR (C6D6): δ 4.3 (d, Si(CH3)3),3JCP ) 2.9), 5.5 (dt, PC(H)P, 1JCRh )14, 1JCP ) 66), 127.6-132.2 (Ph CH), 136.1 (dd, Ph C, 1JCP ) 91, 3JCRh ) 3.6), 187.0 (d, CO, 1JCRh ) 68). 13C{1H} NMR (THF-d8): δ 4.3 (d, Si(CH3)3), 3JCP ) 2.7), 5.4 (dt, PC(H)P, 1 JCRh )14, 1JCP ) 66), 128.2 (d, JCP ) 12, Ph CH), 128.9 (d, JCP ) 12, Ph CH), 131.3 (s, Ph CH), 131.7 (s, Ph CH), 132.5 (d, JCP ) 10, Ph CH), 137.1 (dd, Ph C, 1JCP ) 90, 3JCRh ) 4.2), 187.7 (d, CO, 1JCRh ) 67). Microscope IR: 670 (m), 700 (s), 731 (s), 742 (s), 773 (s), 850 (br, s), 906 (m), 1000 (mw), 1028 (mw), 1109 (s), 1131 (s), 1185 (w), 1239 (s), 1251 (s), 1264 (m, sh), 1306 (w, sh), 1361 (s, br), 1436 (s), 1482 (mw), 1590 (w), 1946 (w, sh, ν(CO)), 1975 (vs, ν(CO)), 2047 (vs, ν(CO)), 2894 (m), 2949 (s), 3057 (mw), 3076 (mw).

Summary and Conclusions The sequential reaction of the dilithiated bis(diphenylphosphoranotrimethylsilylimido)methandiide, [Li2L]2 with selected

Organometallics, Vol. 28, No. 6, 2009 1665

Pd and Rh precursors, has provided a controlled stepwise route to spirocyclic carbon-bridged, bimetallic complexes with the combinations Li-Rh (1), Pd-Rh (2), and Rh-Rh (3) in good yields. The cod ligands on Rh are readily replaced with CO (suggesting that interesting and useful derivative chemistry could be developed), but no easy replacement of the allyl substituents occurred on Pd. This system would appear to provide a systematic route to simple, spirocyclic bridged, bimetallic complexes, which deserves further exploration. The principal Li-Rh complex, 1, achieved by replacement of one Li with Rh, gave, upon reaction with CO, a dimeric ketene complex, which has been discussed elsewhere.8 This complex 1 thermally converts to an orthometalated Li-Rh methine species (6) with a cyclic ring structure linking the Rh and Li. Interestingly controlled hydrolysis with Very limited water reversed the orthometalation (and removed the Li) to form a simple Rh(cod) bis(diphenylphosphoranotrimethylsilylimido)methine complex (7), which reacted with CO to replace cod (8). This CO-methine complex (8) was previously encountered as the result of the hydrolysis of the ketene complex.8 Notably throughout the system, the ligand constraints remove the molecular symmetry so that the bimetallic complexes possess enantiomeric structures. This aspect has not yet been explored, and further studies directed toward the isolation of enantiomeric bimetallic complexes should be pursued. The allyl and cod NMR spectra indicated that these substituents were fluxional. Interestingly the cod exchange pattern for 1 is different from that operative in 2 and 3. Overall the system offers many new avenues for study, as it is likely that general metal sequential substitutions can be devised, given the breadth of metal chemistry that has been demonstrated by the bis(diphenylphosphoranotrimethylsilylimido)methandiide system. A notable advantage is offered by the fact that these bimetallic spirocycles are not unduly sensitive to moisture (whereas the MdC diide complexes often readily add E-H fragments across the MdC bond to create methines),12-16 and thus these spirocycles may offer enhanced capability for building bimetallic catalytic systems.

Acknowledgment. We thank the Natural Sciences and Engineering Council (NSERC) of Canada, the Killam Foundation of the University of Alberta (for a Postdoctoral Fellowship to N.D.J.), the Petroleum Research Foundation of the American Chemical Society (ACS-PRF), our industrial sponsor, NOVA Chemicals, and the University of Alberta (UA) for support. We thank the University of Alberta for the provision and maintenance of excellent NMR and crystallography facilities in the department, which have proven to be essential for this work. Supporting Information Available: The cif files, detailed crystal structure information including unit cells and packings for complexes 1-7, and 2D-NMR COSY and TRoESY spectra of 1-4. This material is available free of charge via the Internet at http://pubs.acs.org. OM8005678