Theoretical studies on the mechanism of iridium-catalyzed alkene hydrogenation by the cationic complex IrH2(NCMe)3(PiPr3)+

摘要

A mechanistic DFT study has been carried out on the ethene hydrogenation catalyzed by the [IrH2(NCMe)3(PiPr3)]+ complex (1). First, the reaction of (1) with ethene has been theoretically characterized, and three mechanistic proposals (A-C) have been made for an identification of the preferred pathways for the alkene hydrogenation catalytic cycle considering Ir(I)/Ir(III) and Ir(III)/Ir(V) intermediate species. Theoretical calculations reveal that the reaction path with the lowest energy starts at an initial ethene migratory insertion into the metal-hydride bond, followed by dihydrogen coordination into the vacancy. Ethane is formed via ?-bond metathesis between the bound H2 and the Ir-ethyl moiety, being the rate-determining step, in agreement with the experimental data available. The calculated energetic span associated with the catalytic cycle is 21.4 kcal mol-1. Although no Ir(V) intermediate has been found along the reaction path, the Ir(V) nature of the transition state for the proposed key σ-bond metathesis step has been determined by electron localization function and geometrical analysis.