Theoretical Studies of Inorganic and Organometallic Reaction Mechanisms. 18. Catalytic Transfer Dehydrogenation of Alkanes by an Iridium(III) Pincer Complex

Abstract

Density functional theory has been used to investigate the mechanism of transfer dehydrogenation of ethane catalyzed by (PCP‘)Ir(H)2 [PCP‘ = η3-C6H3(CH2PH2)2-1,3] with ethylene as the hydrogen acceptor. Our calculations show that the transfer dehydrogenation of ethane by (PCP‘)Ir(H)2 involves two stages: first, (PCP‘)Ir(H)2 is dehydrogenated by the hydrogen acceptor to produce the key intermediate (PCP‘)Ir; second, ethane is dehydrogenated by (PCP‘)Ir to produce the product ethylene and regenerate the catalyst (PCP‘)Ir(H)2. The three critical steps in this reaction are hydride transfer to ethylene, ethane oxidative addition, and dissociation 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) intermediates is shown to be too endoergic.