DFT/ECP study of C-H activation by (PCP)Ir and (PCP)Ir(H)(2) (PCP = eta(3)-1,3-C6H3(CH2PR2)(2)). Enthalpies and free energies of associative and dissociative pathways

(PCP)Ir(H)(2) (PCP = eta (3)-1,3-C6H3(CH2PR2)(2)) complexes are highly effe ctive catalysts for the dehydrogenation of alkanes; in particular, they are the first efficient molecular catalysts for alkane dehydrogenation that do not require a sacrificial hydrogen acceptor. Using density functional theo ry/effective core potential methods, we have examined C-H bond cleavage in alkanes and arenes by both (PCP)Ir and (PCP)Ir(H)(2). C-H addition to the d ihydride is accompanied by loss of H-2; both associative and dissociative p athways for this exchange reaction have been examined. The energetic barrie r (DeltaE double dagger) for associative displacement of H-2 by benzene is much lower than the barrier for a dissociative pathway involving initial lo ss of H-2; however, the pathways have very comparable free energy barriers (DeltaG double dagger). Extrapolation to the higher temperatures, bulkier p hosphine ligands, and the alkane substrates used experimentally leads to th e conclusion that the pathway for the "acceptorless" dehydrogenation of alk anes is dissociative. For hydrocarbon/hydrocarbon exchanges, which are requ ired for transfer-dehydrogenation, dissociative pathways are calculated to be much more favorable than associative pathways. We emphasize that it is t he free energy, not just the internal energy or enthalpy, that must be cons idered for elementary steps that show changes in molecularity.