Theoretical Studies on the Mechanism of Iridium-Catalyzed Alkene Hydrogenation by the Cationic Complex [IrH2(NCMe)3(PiPr3)]+

Abstract

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 σ-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.