Transition metal-catalyzed alkane dehydrogenation

A computational study of ethane dehydrogenation by the 14-electron complex Ir(PH₃)₂H (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 β-H transfer to yield an Ir^III-ethylene complex. Following the IRC for β-H transfer from the transition state towards reactants shows the reactant to be an agostic Ir^III-ethyl isomer, ≈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 β-hydride transfer (and the microscopic reverse, olefin insertion into M-H bonds). After olefin dissociation, complex 1 is regenerated by H₂ reductive elimination to complete the catalytic cycle. The product of H₂ reductive elimination from the catalyst is an η²-dihydrogen complex. The present calculations support the experimental inference that the H-H distance remains constant along the IRC (at ≈0.82 Å) until very close to the transition state for oxidative addition, after which it undergoes rapid lengthening and scission.