Transition metal-catalyzed alkane dehydrogenation

A computational study of ethane dehydrogenation by the 14-electron complex Ir(PH3)(2)H (1) is presented. The first step is C-H oxidative addition of ethane to 1. The intrinsic reaction coordinate (IRCs) for ethane C-H oxidative addition are consistent with an experimental trajectory derived from analysis of the crystal structures of agostic complexes. Ethane binds to 1 as strongly as, if not stronger than, methane. However, calculation of the IRC suggests that for ethane, unlike methane, an alkane adduct of 1 does not lie along the path to C-H oxidative addition. The product of oxidative addition is a non-agostic Ir-III-ethyl complex. Oxidative addition is followed by beta-H transfer to yield an Ir-III-ethylene complex. Following the IRC for beta-H transfer from the transition state towards reactants shows the reactant to be an agostic Ir-III-ethyl isomer, approximate to 4 kcal mol(-1) lower in energy than its non-agostic isomer (i.e., the product of oxidative addition). Thus, calculations support experimental suggestions about the importance of agostic interactions in p-hydride transfer (and the microscopic reverse, olefin insertion into M-H bonds). After olefin dissociation, complex 1 is regenerated by H-2 reductive elimination to complete the catalytic cycle. The product of H-2 reductive elimination from the catalyst is an eta(2)-dihydrogen complex. The present calculations support the experimental inference that the H-H distance remains constant along the IRC (at approximate to 0.82 Angstrom) until very close to the transition state for oxidative addition, after which it undergoes rapid lengthening and scission.