Highly effective pincer-ligated iridium catalysts for alkane dehydrogenation. DFT calculations of relevant thermodynamic, kinetic, and spectroscopic properties

The p-methoxy-substituted pincer-ligated iridium complexes, (MeO-(PCP)-P-tBu)IrH4 ((PCP)-P-R = kappa(3)-C6H3-2,6-(CH2PR2)(2)) and (MeO-(PCP)-P-iPr)IrH4, are found to be highly effective catalysts for the dehydrogenation 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" ((POCOP)-P-R = kappa(3)-C6H3-2,6(OPR2)(2)) pincer-ligated catalysts, which also show catalytic activity higher than unsubstituted PCP analogues (Gottker-Schnetmann, I.; White, P.; Brookhart, M. J. Am. Chem. Soc. 2004, 126, 1804). On the basis of v(CO) 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 --> C(aryl) pi-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 a-withdrawal from the phosphorus atoms. The high v(CO) value of (POCOP)Ir(CO) is attributable to electrostatic effects, rather than decreased ir-CO pi-donation or increased OC-Ir sigma-donation.