DFT Study on the Mechanism of Formic Acid Decomposition by a Well-Defined Bifunctional Cyclometalated Iridium(III) Catalyst: Self-Assisted Concerted Dehydrogenation via Long-Range Intermolecular Hydrogen Migration

Abstract

DFT calculations have been performed to gain insight into the mechanism of formic acid (HCOOH) decomposition into H2 and CO2, catalyzed by a well-defined bifunctional cyclometalated iridium(III) complex (a Ir–H hydride) based on a 2-aryl imidazoline ligand with a remote NH functionality. It is shown that the reaction features the direct protonation of the Ir–H hydride by HCOOH with the hydrogen shuttling between the NH group and the carbonyl group of HCOOH. Importantly, the simultaneous presence of two HCOOH molecules is proposed to be important for the dehydrogenation, where one works as a hydrogen source and the other acts as a hydrogen shuttle to assist the long-range intermolecular hydrogen migration. The dehydrogenation mechanism is referred to as HCOOH self-assisted concerted hydrogen migration. With such a mechanism, the energetic span, i.e. the apparent activation energy of the catalytic cycle, is calculated to be 17.3 kcal/mol, which is consistent with the observed rapid dehydrogenation of HCOOH ...