Theoretical studies of inorganic and-organometallic reaction mechanisms. 15. Catalytic alkane dehydrogenation by iridium(III) complexes

Alkane dehydrogenation catalyzed by the Ir(III) complexes (PCP')IT(H)(2) (1 ) [PCP' = eta(3)-C6H3(CN2-PH2)(2)-1,3] and CpIr(PH3)(H)(+) (10) [Cp = eta(5 )-C5H5] is investigated with density functional theory (DFT). For both syst ems the theoretical results show that catalytic alkane dehydrogenation to a lkene proceeds through (i) alkane oxidative addition, (ii) dihydride reduct ive elimination, (iii) beta-H transfer from alkyl ligand to metal, and fina lly (iv) elimination of the olefin. Barriers for steps (i), (ii), and(iv) a re critical for the catalytic cycle. The (PCP')Ir(H)(2) system is special b ecause these three barriers are balanced (16, 15, and 22 kcal/mol, respecti vely), whereas in the CpIr(PH3)(H)(+) system these three barriers are unbal anced (9, 24, and 41 kcal/mol,respectively). Thus, in the catalytic cycle f or alkane dehydrogenation by (PCP')Ir(H)(2) the reaction endothermicity is achieved gradually. The higher stability of the formally Ir(V) complexes an d the eta(2)-alkene complex, which has some Ir(V)-like character, in the Cp Ir(PH3)(H)(+) system is responsible for the larger barriers in these critic al steps. In the key role played by the ligand systems, PCP'(H) vs Cp(PH3), the former increases the energy-of the metal-ligand fragment's triplet sta te relative to that of the singlet and thus destabilizes all the Ir(V)-like species.