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‘)Ir(H) 2 ( 1 ) [PCP‘ = η 3 -C 6 H 3 (CH 2 PH 2 ) 2 -1,3] and CpIr(PH 3 )(H) + ( 10 ) [Cp = η 5 -C 5 H 5 ] is investigated with density functional theory (DFT). For both systems the theoretical results show that catalytic alkane dehydrogenation to alkene proceeds through (i) alkane oxidative addition, (ii) dihydride reductive elimination, (iii) β-H transfer from alkyl ligand to metal, and finally (iv) elimination of the olefin. Barriers for steps (i), (ii), and (iv) are critical for the catalytic cycle. The (PCP‘)Ir(H) 2 system is special because these three barriers are balanced (16, 15, and 22 kcal/mol, respectively), whereas in the CpIr(PH 3 )(H) + system these three barriers are unbalanced (9, 24, and 41 kcal/mol, respectively). Thus, in the catalytic cycle for alkane dehydrogenation by (PCP‘)Ir(H) 2 the reaction endothermicity is achieved gradually. The higher stability of the formally Ir(V) complexes and the η 2 -alkene complex, which has some Ir(V)-like character, in the CpIr(PH 3 )(H) + system is responsible for the larger barriers in these critical steps. In the key role played by the ligand systems, PCP‘(H) vs Cp(PH 3 ), the former increases the energy of the metal−ligand fragment's triplet state relative to that of the singlet and thus destabilizes all the Ir(V)-like species.