Single-Electron Oxidation of Monomeric Copper(I) Alkyl Complexes: Evidence for Reductive Elimination through Bimolecular Formation of Alkanes Laurel A. Goj, ² Elizabeth D. Blue, ² Samuel A. Delp, ² T. Brent Gunnoe,* ,² 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 R 2 . Experimental studies suggest that the reductive elimination of R 2 from Cu(II) occurs through a nonradical bimolecular mechanism. Computational studies of the Cu-C methyl yield bond dissociation enthalpies of [(SIPr)Cu-CH 3 ] n+ (80 kcal/mol for n ) 0 {Cu(I)} and 38 kcal/mol for n ) 1 {Cu(II)}). Introduction 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 reduction of the metal), C-C elimination from two metal centers (formal single-electron reduction of each metal), or initial metal-carbon bond homolysis (formal single-electron reduction of the metal; Scheme 1). 5,8-24 Copper complexes have played a prominent role in homo- geneous catalysis and metal-mediated synthesis of organic molecules. 25-28 Despite the substantial utility of copper com- * To whom correspondence should be addressed. E-mail: brent_gunnoe@ ncsu.edu. ² North Carolina State University. ‡ University of North Texas. § West Virginia University. (1) Simo˜es, J. A. M. Energetics of Organometallic Species; Kluwer Academic Publishers: Boston, 1992; Vol. 367. (2) Marks, T. J. Bonding Energetics in Organometallic Compounds; American Chemical Society: Washington, DC, 1990; Vol. 428. (3) Simo˜es, J. A. M.; Beauchamp, J. L. Chem. ReV. 1990, 90, 629-688. (4) Halpern, J. Inorg. Chim. Acta 1985, 100, 41-48. (5) Tilset, M.; Fjeldahl, I.; Hamon, J.-R.; Hamon, P.; Toupet, L.; Saillard, J.-Y.; Costuas, K.; Haynes, A. J. Am. Chem. Soc. 2001, 123, 9984-10000. (6) Skagestad, V.; Tilset, M. J. Am. Chem. Soc. 1993, 115, 5077-5083. (7) Ingleson, M. J.; Pink, M.; Huffman, J. C.; Fan, H.; Caulton, K. G. Organometallics 2006, 25, 1112-1119. (8) Halpern, J. Inorg. Chim. Acta 1982, 62, 31-37. (9) Halpern, J. Acc. Chem. Res. 1982, 15, 332-338. (10) Vollhardt, K. P. C.; Cammack, J. K.; Matzger, A. J.; Bauer, A.; Capps, K. B.; Hoff, C. D. Inorg. Chem. 1999, 38, 2624-2631. (11) Heyduk, A. F.; Nocera, D. G. J. Am. Chem. Soc. 2000, 122, 9415- 9426. (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- 41. (14) Wegman, R. W.; Brown, T. L. J. Am. Chem. Soc. 1980, 102, 2494- 2495. (15) Whitesides, G. M.; Casey, C. P.; Krieger, J. K. J. Am. Chem. Soc. 1971, 93, 1379-1389. (16) Tam, W.; Wong, W.-K.; Gladysz, J. A. J. Am. Chem. Soc. 1979, 101, 1589-1591. (17) Jones, W. D.; Huggins, J. M.; Bergman, R. G. J. Am. Chem. Soc. 1981, 103, 4415-4423. (18) Jones, W. D.; Bergman, R. G. J. Am. Chem. Soc. 1979, 101, 5447- 5449. (19) Warner, K. E.; Norton, J. R. Organometallics 1985, 4, 2150-2160. (20) Norton, J. R. Acc. Chem. Res. 1979, 12, 139-145. (21) Riordan, C. G.; Halpern, J. Inorg. Chim. Acta 1996, 243, 19-24. (22) Ng, F. T. T.; Rempel, G. L.; Mancuso, C.; Halpern, J. Organome- tallics 1990, 9, 2762-2772. (23) Collman, J. P.; McElwee-White, L.; Brothers, P. J.; Rose, E. J. Am. Chem. Soc. 1986, 108, 1332-1333. (24) Seyler, J. W.; Safford, L. K.; Fanwick, P. E.; Leidner, C. R. Inorg. Chem. 1992, 31, 1545-1547. (25) Taylor, R. J. K. Organocopper Reagents: A Practical Approach; Harwood, L. M., Moody, C. J., Ed.; Oxford University Press: Oxford, 1994. Scheme 1. Possible Pathways for Elimination of Metal-Carbon Bonds 4097 Organometallics 2006, 25, 4097-4104 10.1021/om060409i CCC: $33.50 © 2006 American Chemical Society Publication on Web 07/21/2006