Highly Effective Pincer-Ligated Iridium Catalysts for Alkane Dehydrogenation. DFT Calculations of Relevant Thermodynamic, Kinetic, and Spectroscopic Properties Keming Zhu, Patrick D. Achord, Xiawei Zhang, Karsten Krogh-Jespersen,* and Alan S. Goldman* Contribution from the Department of Chemistry and Chemical Biology, Rutgers, The State UniVersity of New Jersey, New Brunswick, New Jersey 08903 Received May 5, 2004; E-mail: agoldman@rutchem.rutgers.edu; krogh@rutchem.rutgers.edu Abstract: The p-methoxy-substituted pincer-ligated iridium complexes, (MeO- tBu PCP)IrH4 ( R PCP ) κ 3 - C6H3-2,6-(CH2PR2)2) and (MeO- iPr PCP)IrH4, are found to be highly effective catalysts for the dehydroge- nation of alkanes (both with and without the use of sacrificial hydrogen acceptors). These complexes offer an interesting comparison with the recently reported bis-phosphinite “POCOP” ( R POCOP ) κ 3 -C6H3-2,6- (OPR2)2) pincer-ligated catalysts, which also show catalytic activity higher than unsubstituted PCP analogues (Go ¨ ttker-Schnetmann, I.; White, P.; Brookhart, M. J. Am. Chem. Soc. 2004, 126, 1804). On the basis of νCO values of the respective CO adducts, the MeO-PCP complexes appear to be more electron-rich than the parent PCP complexes, whereas the POCOP complexes appear to be more electron-poor. However, the MeO-PCP and POCOP ligands are calculated (DFT) to show effects in the same directions, relative to the parent PCP ligand, for the kinetics and thermodynamics of a broad range of reactions including the addition of C-H and H-H bonds and CO. In general, both ligands favor (relative to unsubstituted PCP) addition to the 14e (pincer)Ir fragments but disfavor addition to the 16e complexes (pincer)IrH2 or (pincer)- Ir(CO). These kinetic and thermodynamic effects are all largely attributable to the same electronic feature: O f C(aryl) π-donation, from the methoxy or phosphinito groups of the respective ligands. DFT calculations also indicate that the kinetics (but not the thermodynamics) of C-H addition to (pincer)Ir are favored by σ-withdrawal from the phosphorus atoms. The high ν CO value of (POCOP)Ir(CO) is attributable to electrostatic effects, rather than decreased Ir-CO π-donation or increased OC-Ir σ-donation. Introduction The development of systems for the selective catalytic func- tionalization of alkanes, and “unactivated” C-H bonds more generally, is one of the most significant challenges in modern catalysis. Organometallic systems have demonstrated great pro- mise in this context. Particular progress has been made over the past two decades in the development of catalysts for the dehydrogenation of alkanes, either with or without the use of sacrificial olefins as hydrogen acceptors. 1,2 Presently, the most promising systems seem to be (κ 3 -P,C,P-pincer)-ligated iridium catalysts (e.g., complexes of the form (X- R PCP)Ir ( R PCP ) κ 3 -C 6 H 3 -2,6-(CH 2 PR 2 ) 2 )), first introduced as catalysts for transfer dehydrogenation 3 and later found to be highly effective for acceptorless dehydrogenation as well. 4 We recently reported the synthesis of iridium complexes bearing a p-methoxy-substituted PCP ligand (MeO- tBu PCP) and experimental and computational studies of the effect of p- methoxy and other p-substituents on the thermodynamics of addition reactions. 5-7 DFT calculations predicted that additions of C-H bonds and H 2 to the 14-electron fragments (X-PCP)- Ir are favored by π-donating X groups such as methoxy. The mechanism of dehydrogenation by “( tBu PCP)Ir” precursors 8 has been shown to operate via C-H addition to this 14e fragment (which is rate-determining under certain conditions) and to proceed via formation of ( tBu PCP)IrH 2 . 9 Thus, factors that favor addition of either C-H bonds or H 2 are of obvious relevance in this context. Herein we report that p-methoxy-PCP deriva- tives, in particular the previously unreported complex (MeO- iPr PCP)Ir, 10 afford unprecedented levels of catalytic dehydro- genation activity under suitable conditions. (1) (a) Burk, M. J.; Crabtree, R. H.; Parnell, C. P.; Uriarte, R. J. Organometallics 1984, 3, 816-817 (and references therein for stoichiometric dehydroge- nations). (b) Burk, M. J.; Crabtree, R. H. J. Am. Chem. Soc. 1987, 109, 8025-8032. (c) Felkin, H.; Fillebeen-Khan, T.; Holmes-Smith, R.; Lin, Y. Tetrahedron Lett. 1985, 26, 1999-2000. (2) (a) Maguire, J. A.; Goldman, A. S. J. Am. Chem. Soc. 1991, 113, 6706- 6708. (b) Maguire, J. A.; Petrillo, A.; Goldman, A. S. J. Am. Chem. Soc. 1992, 114, 9492-9498. (c) Wang, K.; Goldman, M. E.; Emge, T. J.; Goldman, A. S. J. Organomet. Chem. 1996, 518, 55-68. (3) (a) Gupta, M.; Hagen, C.; Flesher, R. J.; Kaska, W. C.; Jensen, C. M. Chem. Commun. 1996, 2083-2084. (b) Liu, F.; Pak, E. B.; Singh, B.; Jensen, C. M.; Goldman, A. S. J. Am. Chem. Soc. 1999, 121, 4086-4087. (4) Xu, W.; Rosini, G. P.; Gupta, M.; Jensen, C. M.; Kaska, W. C.; Krogh- Jespersen, K.; Goldman, A. S. Chem. Commun. 1997, 2273-2274. (5) Krogh-Jespersen, K.; Czerw, M.; Zhu, K.; Singh, B.; Kanzelberger, M.; Darji, N.; Achord, P. D.; Renkema, K. B.; Goldman, A. S. J. Am. Chem. Soc. 2002, 124, 10797-10809. (6) Kanzelberger, M.; Singh, B.; Zhu, K.; Goldman, A. S. Abstracts of Papers; 219th National Meeting of the American Chemical Society, San Francisco, CA; American Chemical Society: Washington, DC, 2000; INOR-366 AN 2000:331138. Published on Web 09/17/2004 13044 9 J. AM. CHEM. SOC. 2004, 126, 13044-13053 10.1021/ja047356l CCC: $27.50 © 2004 American Chemical Society