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 H 2 and CO 2 , catalyzed by a well-defined bifunctional cyclometallated iridium(III) complex (a Ir-H hydride) based on 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 the 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 under mild conditions (40 °C). On one hand, the effectiveness of the self-assisted catalytic system is attributed to the d-pπ conjugation between the Ir center and the proximal nitrogen, which increases the electron density at the Ir center and hence promotes the Ir-H bond cleavage. On the other hand, the effectiveness is also closely related to the hydrogen-shared three-center-four-electron (3c-4e) bond between formate and formic acid, which stabilizes the transition states and hence reduces the free energy barriers of the reaction. In addition, calculated results also emphasize the importance of the concerted catalysis of the bifunctional catalyst: when γ-NH functional group does not participate in the reaction or is replaced by O atom, the reaction becomes remarkably less favorable. The present work rationalizes the experimental findings and provides important insights for understanding the catalysis of the bifunctional cyclometallated iridium(III) complexes.

Hydrogen production from formic acid catalyzed by a phosphine free manganese complex: investigation and mechanistic insights

“…43 Last but not least, previous investigations on FA dehydrogenation using noble metal complexes pointed out several subtle aspects that should be mentioned here. The computational studies performed by Zhang et al 44 showed the capacity of formic acid to form stable dimers. As a result, the transition state leading to the hydride protonation would be assisted by the N-H moiety.…”

Mechanistic Considerations on Homogeneously Catalyzed Formic Acid Dehydrogenation

“…However, theoretical calculations performed by Zhang et al discard the pathway proposed by Xiao's group and suggest an alternative mechanism by which two molecules of FA are required – one acts as hydrogen source and the other as H‐shuttle (Figure ) . This mechanism is based on the capacity of FA molecules to form stable dimers in solution by means of hydrogen bonding interactions, since a single FA molecule is incapable of linking the two sites of the bifunctional catalysts (namely, Ir and NH) .…”

An Update on Formic Acid Dehydrogenation by Homogeneous Catalysis

“…Mechanistic studies suggest the long‐range metal‐ligand cooperation between FA and the distal NH functional group from the imidazole moiety. The subsequent computational study by Zhang's group corroborated the reaction mechanism involving two FA molecules, where one acts as a H 2 source, and the other participates as a proton shuttle to favor the long‐range activation ( 8‐TS , Figure ).…”