Theoretical studies of inorganic and organometallic reaction mechanisms. 14. beta-hydrogen transfer and alkene/alkyne insertion at a cationic iridium center

Abstract

Recent experimental work shows that alkanes can be activated by Cp*Ir(PMe3)(CH3)(+) at room temperature to generate olefin complexes. The reaction begins with alkane activation by oxidative addition (OA) followed by reductive elimination (RE) of methane and then olefin formation by the beta-H transfer from the bound alkyl. Ab initio calculations and density functional theory (DFT) studies of ethane activation by CpIr(PH3)(CH3)(+) (1) to generate CpIr(PH3)(eta(2)-C2H4)(H)(+) (7) show that the beta-H transfer from CpIr(PH3)(C2H5)(+) (5) to 7 is exothermic by 12 and 16 kcal/mol with a very low barrier of 0.7 and 0.4 kcal/mol at the DFT and CCSD levels, respectively. Thus, the rate-determining step in alkane dehydrogenation to olefin complexes by Cp*Ir(PMe3)(CH3)(+) is the alkane OA step. These results are in very good agreement with the experimental work of Bergman and co-workers. A strong stabilizing interaction between either ethylene or acetylene and CpIr(PH3)(CH3)(+) leads to high activation barriers (25-36 kcal/mol) for the insertion processes of ethylene or acetylene. In comparison to ethylene, the insertion reaction of acetylene with the CpIr(PH3)(CH3)(+) complex is more favorable. Thus, the dimerization of terminal alkynes catalyzed by cationic iridium complexes is plausible.