A reaction mechanism for the hydrogenation of CO2 to formate catalyzed by an Ir(III)pincer trihydride complex has been investigated with density functional theory calculations. Two routes for the formation of formate were considered: (I) the insertion of CO2 into the IrH bond with the assistance of the metal center, and (II) the direct addition of hydride to CO2 via nucleophilic attack. Route II is energetically more favorable than route I. Molecular orbital and natural bond orbital analyses showed that this trihydride complex consists of two kinds of hydrides with distinct IrH bond properties, and the hydride in the plane vertical to the pyridine moiety is highly active. The whole catalytic cycle for CO2 hydrogenation to formate is exothermic by 30.3 kcal mol1, and the rate-limiting step is the regeneration of the active complex, which involves a barrier of 15.6 kcalmol1. The theoretical results are in good agreement with and give a reasonable explanation to the experimental observations. Moreover, the results imply that the type of a metalhydride bond might determine which route the CO2 insertion takes, route I or route II.
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