Theoretical Studies of Inorganic and Organometallic Reaction Mechanisms. 14. β-Hydrogen Transfer and Alkene/Alkyne Insertion at a Cationic Iridium Center

Recent experimental work shows that alkanes can be activated by Cp*Ir(PMe 3 )(CH 3 ) + 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 β-H transfer from the bound alkyl. Ab initio calculations and density functional theory (DFT) studies of ethane activation by CpIr(PH 3 )(CH 3 ) + ( 1 ) to generate CpIr(PH 3 )(η 2 -C 2 H 4 )(H) + ( 7 ) show that the β-H transfer from CpIr(PH 3 )(C 2 H 5 ) + ( 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(PMe 3 )(CH 3 ) + 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(PH 3 )(CH 3 ) + 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(PH 3 )(CH 3 ) + complex is more favorable. Thus, the dimerization of terminal alkynes catalyzed by cationic iridium complexes is plausible.