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Alkyl Complexes: Evidence for Reductive Elimination Through Bimolecular Formation of Alkanes

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Alkyl Complexes: Evidence for Reductive Elimination Through Bimolecular Formation of Alkanes Organometallics 2006, 25, 4097-4104 4097 Single-Electron Oxidation of Monomeric Copper(I) Alkyl Complexes: Evidence for Reductive Elimination through Bimolecular Formation of Alkanes Laurel A. Goj,† Elizabeth D. Blue,† Samuel A. Delp,† T. Brent Gunnoe,*,† Thomas R. Cundari,‡ and Jeffrey L. Petersen§ Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695-8204, Center for AdVanced Scientific Computing and Modeling (CASCaM), Department of Chemistry, UniVersity of North Texas, Box 305070, Denton, Texas 76203-5070, and C. Eugene Bennett Department of Chemistry, West Virginia UniVersity, Morgantown, West Virginia 26506-6045 ReceiVed May 12, 2006 Monomeric Cu(I) alkyl complexes (NHC)Cu(R) (NHC ) N-heterocyclic carbene; R ) Me or Et) and (dtbpe)Cu(Me) (dtbpe ) 1,2-bis(di-tert-butylphosphino)ethane) have been prepared, isolated, and characterized. Single-electron oxidation of the Cu(I) alkyl complexes upon reaction with AgOTf to form putative Cu(II) intermediates of the type [(L)Cu(R)]+ (L ) NHC or dtbpe, R ) Me or Et) results in the rapid production of (L)Cu(X) (X ) OTf) and R2. Experimental studies suggest that the reductive elimination of R2 from Cu(II) occurs through a nonradical bimolecular mechanism. Computational studies of the n+ Cu-Cmethyl yield bond dissociation enthalpies of [(SIPr)Cu-CH3] (80 kcal/mol for n ) 0 {Cu(I)} and 38 kcal/mol for n ) 1 {Cu(II)}). Introduction Scheme 1. Possible Pathways for Elimination of Metal-Carbon Bonds In the area of homogeneous catalysis, the development of well-defined and selective catalytic cycles often requires access to even-electron transformations in preference to odd-electron radical processes. Therefore, relatively strong M-C bonds can be desirable for organometallic systems, and understanding the factors that control M-C bond dissociation enthalpies (BDEs) as well as other metal-ligand BDEs is of fundamental importance.1-7 In addition to the propensity of a system to initiate M-C bond homolysis and radical chemistry, the mechanism of C-C reductive elimination processes depends on M-C bond energies. For example, the net reductive elimination of M-C bonds can proceed by direct C-C bond formation from a single metal center (formal two-electron Copper complexes have played a prominent role in homo- reduction of the metal), C-C elimination from two metal centers geneous catalysis and metal-mediated synthesis of organic 25-28 (formal single-electron reduction of each metal), or initial molecules. Despite the substantial utility of copper com- metal-carbon bond homolysis (formal single-electron reduction of the metal; Scheme 1).5,8-24 (12) Nappa, M. J.; Santi, R.; Diefenbach, S. P.; Halpern, J. J. Am. Chem. Soc. 1982, 104, 619-621. (13) Nappa, M. J.; Santi, R.; Halpern, J. Organometallics 1985, 4,34- * To whom correspondence should be addressed. E-mail: brent_gunnoe@ 41. ncsu.edu. (14) Wegman, R. W.; Brown, T. L. J. Am. Chem. Soc. 1980, 102, 2494- † North Carolina State University. 2495. ‡ University of North Texas. (15) Whitesides, G. M.; Casey, C. P.; Krieger, J. K. J. Am. Chem. Soc. § West Virginia University. 1971, 93, 1379-1389. (1) Simo˜es, J. A. M. Energetics of Organometallic Species; Kluwer (16) Tam, W.; Wong, W.-K.; Gladysz, J. A. J. Am. Chem. Soc. 1979, Academic Publishers: Boston, 1992; Vol. 367. 101, 1589-1591. (2) Marks, T. J. Bonding Energetics in Organometallic Compounds; (17) Jones, W. D.; Huggins, J. M.; Bergman, R. G. J. Am. Chem. Soc. American Chemical Society: Washington, DC, 1990; Vol. 428. 1981, 103, 4415-4423. (3) Simo˜es, J. A. M.; Beauchamp, J. L. Chem. ReV. 1990, 90, 629-688. (18) Jones, W. D.; Bergman, R. G. J. Am. Chem. Soc. 1979, 101, 5447- (4) Halpern, J. Inorg. Chim. Acta 1985, 100,41-48. 5449. (5) Tilset, M.; Fjeldahl, I.; Hamon, J.-R.; Hamon, P.; Toupet, L.; Saillard, (19) Warner, K. E.; Norton, J. R. Organometallics 1985, 4, 2150-2160. J.-Y.; Costuas, K.; Haynes, A. J. Am. Chem. Soc. 2001, 123, 9984-10000. (20) Norton, J. R. Acc. Chem. Res. 1979, 12, 139-145. (6) Skagestad, V.; Tilset, M. J. Am. Chem. Soc. 1993, 115, 5077-5083. (21) Riordan, C. G.; Halpern, J. Inorg. Chim. Acta 1996, 243,19-24. (7) Ingleson, M. J.; Pink, M.; Huffman, J. C.; Fan, H.; Caulton, K. G. (22) Ng, F. T. T.; Rempel, G. L.; Mancuso, C.; Halpern, J. Organome- Organometallics 2006, 25, 1112-1119. tallics 1990, 9, 2762-2772. (8) Halpern, J. Inorg. Chim. Acta 1982, 62,31-37. (23) Collman, J. P.; McElwee-White, L.; Brothers, P. J.; Rose, E. J. Am. (9) Halpern, J. Acc. Chem. Res. 1982, 15, 332-338. Chem. Soc. 1986, 108, 1332-1333. (10) Vollhardt, K. P. C.; Cammack, J. K.; Matzger, A. J.; Bauer, A.; (24) Seyler, J. W.; Safford, L. K.; Fanwick, P. E.; Leidner, C. R. Inorg. Capps, K. B.; Hoff, C. D. Inorg. Chem. 1999, 38, 2624-2631. Chem. 1992, 31, 1545-1547. (11) Heyduk, A. F.; Nocera, D. G. J. Am. Chem. Soc. 2000, 122, 9415- (25) Taylor, R. J. K. Organocopper Reagents: A Practical Approach; 9426. Harwood, L. M., Moody, C. J., Ed.; Oxford University Press: Oxford, 1994. 10.1021/om060409i CCC: $33.50 © 2006 American Chemical Society Publication on Web 07/21/2006 4098 Organometallics, Vol. 25, No. 17, 2006 Goj et al. plexes and the integral role of transition metal complexes that possess metal-alkyl or metal-aryl ligands in the field of homogeneous catalysis, the number of copper hydride, alkyl, or aryl complexes that are isolable and well-defined are few, and this is especially true of monomeric systems.29-32 Copper systems that possess Cu-R(R) alkyl or aryl) or Cu-H bonds are often highly reactive substrates. The reduction of Cu(II) halides with tetraalkyllead has been studied, which may involve Cu(II) alkyl systems.33 Whitesides et al. have studied the mechanism of decomposition of Cu(I) alkyl complexes in the presence of phosphine ligands,34,35 and, most germane here, the elimination of alkanes upon reaction of dialkylcuprates with molecular oxygen has been explored.36 Recently, Sadighi et al. have reported the synthesis, isolation, and full characterization of the two-coordinate Cu(I) methyl ) complex (IPr)Cu(Me) (IPr 1,3-bis(2,6-diisopropylphenyl)- Figure 1. Partially labeled ORTEP of (SIPr)Cu(OAc) (1)at30% 30 imidazol-2-ylidene). Given the diversity of reactivity available probability (hydrogen atoms omitted). Selected bond distances for the Lewis acidic Cu(II) oxidation state, we became interested (Å): Cu1-O1 1.838(2), Cu1-C1 1.886(3), O1-C28 1.276(4), in exploring the possible preparation of Cu(II) alkyl complexes. O2-C28 1.222(6), C1-N1 1.322(6), C1-N2 1.324(6), C2-C3 To our knowledge, no isolable Cu(II) alkyl complexes have been 1.512(8). Selected bond angles (deg): C1-Cu1-O1 172.5(2), reported. Herein, we report that single-electron oxidation of a Cu1-O1-C28 125.3(3), O1-C28-O2 125.2(4), O1-C28-C29 series of two-coordinate Cu(I) complexes of the type (NHC)- 120.2(4), Cu1-C1-N2 126.5(3), Cu1-C1-N1 124.4(4), N1-C1- Cu(R) (NHC ) N-heterocyclic carbene, R ) methyl or ethyl) N2 109.1(3). and the three-coordinate complex (dtbpe)Cu(Me) {dtbpe ) 1,2- bis(di-tert-butylphosphino)ethane} results in the formation of unstable systems that rapidly undergo reductive elimination of the alkyl ligand to return to the Cu(I) oxidation state. Results and Discussion Synthesis of Copper(I) Complexes. The complexes (SIPr)- Cu(OAc) (1) and (IMes)Cu(OAc) (2) (SIPr ) 1,3-bis(2,6- diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene; IMes ) 1,3- bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) can be prepared in reactions analogous to the synthesis of (IPr)Cu(OAc).30 Complexes 1 and 2 have been characterized by 1H and 13C NMR spectroscopy and elemental analysis. In addition, single-crystal X-ray diffraction studies of 1 and 2 have confirmed their identity (Figures 1 and 2 and Table 1). Other examples of structural characterization of Cu-carboxylate complexes, including both Cu(I) and Cu(II) oxidation states, have been reported.30,37-44 Figure 2. ORTEP of (IMes)Cu(OAc) (2) at 30% probability - (26) Ley, S. V.; Thomas, A. W. Angew. Chem., Int. Ed. 2003, 42, 5400 (hydrogen atoms omitted). Selected bond distances (Å): Cu1-O1 5449. - - - (27) Krause, N. Modern Organocopper Chemistry; Wiley-VCH: New 1.867(3), Cu1 C1 1.859(3), O1 C22 1.233(5), O2 C22 1.223- York, 2002. (6), C1-N1 1.354(3), C1-N2 1.356(3), C2-C3 1.349(4). Selected (28) Johnson, J. S.; Evans, D. A. Acc. Chem. Res. 2000, 33, 325-335. bond angles (deg): C1-Cu1-O1 171.1(1), Cu1-O1-C22 116.3- (29) Dempsey, D. F.; Girolami, G. S. Organometallics 1988, 7, 1208- (3), O1-C22-O2 124.4(4), O1-C22-C23 117.0(4), Cu1-C1- 1213. N2 127.6(2), Cu1-C1-N1 128.8(2), N1-C1-N2 103.5(2). (30) Mankad, N. P.; Gray, T. G.; Laitar, D. S.; Sadighi, J. P. Organo- metallics 2004, 23, 1191-1193. (31) Coan, P. S.; Folting, K.; Huffman, J. C.; Caulton, K. G. Organo- - - - The structures of 1 and 2 reveal nearly linear C1 Cu1 O1 metallics 1989, 8, 2724 2728. ° ° (32) Mankad, N. P.; Laitar, D. S.; Sadighi, J. P. Organometallics 2004, linkages with bond angles of 172.5(2) and 171.1(1) , respec- 23, 3369-3371. tively. The Cu-O bond distance of 1 is 1.838(2) Å, while that (33) Clinton, N.
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