Theoretical Studies of Inorganic and Organometallic Reaction Mechanisms 13: Methane, Ethylene, and Acetylene Activation at a Cationic Iridium Center
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The oxidative-addition/reductive-elimination (OA/RE) reactions of methane, ethylene and acetylene with the CpIr(PH3)(CH3)+ complex are investigated by ab initio methods and density functional theory (DFT). The calculated results shows that the OA reaction from CpIr(PH3)(CH3)(agostic-alkane)+ to CpIr(PH3)(CH3)(H)(alkyl)+ is endothermic by 4.4 and 0.8 kcal/mol with a low barrier of 11.5 and 10.0 kcal/mol at the DFT-B3LYP and coupled cluster with singles and doubles (CCSD) levels of theory, respectively. The RE reaction from CpIr(PH3)(CH3)(H)(alkyl)+ to a -agostic complex, CpIr(PH3)(alkyl)+, is exothermic with a low barrier of 7.1 and 9.2 kcal/mol. A strong stabilizing interaction between either ethylene or acetylene and CpIr(PH3)(CH3)+ leads to a high activation barrier (24-36 kcal/mol) for the OA processes of either one. Compared to ethylene, the OA/RE reaction of acetylene with CpIr(PH3)(CH3)+ complex is more favorable. Thus, the dimerization of terminal alkynes catalyzed by cationic iridium complexes is plausible.
