THEORETICAL-STUDIES OF INORGANIC AND ORGANOMETALLIC REACTION-MECHANISMS - 14 - BETA-HYDROGEN TRANSFER AND ALKENE ALKYNE INSERTION AT A CATIONIC IRIDIUM CENTER/

Recent experimental work shows that alkanes can be activated by CpIr( PMe3)(CH3)(+) at room temperature to generate olefin complexes. The re action begins with alkane activation by oxidative addition (OA) follow ed by reductive elimination (RE) of methane and then olefin formation by the beta-H transfer from the bound alkyl. Ab initio calculations an d density functional theory (DFT) studies of ethane activation by CpIr (PH3)(CH3)(+) (1) to generate CpIr(PH3)(eta(2)-C2H4)(H)(+) (7) show th at 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 s tep in alkane dehydrogenation to olefin complexes by CpIr(PMe3)(CH3)( +) is the alkane OA step. These results are in very good agreement wit h the experimental work of Bergman and co-workers. A strong stabilizin g interaction between either ethylene or acetylene and CpIr(PH3)(CH3)( +) leads to high activation barriers (25-36 kcal/mol) for the insertio n processes of ethylene or acetylene. In comparison to ethylene, the i nsertion reaction of acetylene with the CpIr(PH3)(CH3)(+) complex is m ore favorable. Thus, the dimerization of terminal alkynes catalyzed by cationic iridium complexes is plausible.