Theoretical studies of inorganic and organometallic reaction mechanisms. 18. Catalytic transfer dehydrogenation of alkanes by an iridium(III) pincer complex

Density functional theory has been used to investigate the mechanism of tra nsfer dehydrogenation of ethane catalyzed by (PCP ' )Ir(H)(2) [PCP ' = eta (8)-C6H3(CH2PH2)(2)-1,3] With ethylene as the hydrogen acceptor. Our calcul ations show that the transfer dehydrogenation of ethane by (PCP ' )Ir(H)(2) involves two stages: first, (PCP ' )Ir(H)(2) is dehydrogenated by the hydr ogen acceptor to produce the key intermediate (PCP ' )Ir; second, ethane is dehydrogenated by (PCP ' )Ir to produce the product ethylene and regenerat e the catalyst (PCP ' )Ir(H)(2). The three critical steps in this reaction are hydride transfer to ethylene, ethane oxidative addition, and dissociati on of the coordinated ethylene, with barriers of 14.0, 11.6, and 23.4 kcal/ mol, respectively. In contrast to acceptorless dehydrogenation catalyzed by the same pincer complex, where Ir(V) species are energetically accessible, here, the alternative path for transfer dehydrogenation involving Ir(V) in termediates is shown to be too endoergic.