Mechanistic insights into the light-driven hydrogen evolution reaction from formic acid mediated by an iridium photocatalyst

Abstract

A novel light-triggered hydrogen evolution reaction from formic acid mediated by an Ir(III) photocatalyst has been experimentally reported recently. However, its reaction mechanism remains elusive. Herein, we have employed the density functional theory (DFT) method to explore this photocatalytic reaction in detail. On the basis of the results, we have proposed a possible photocatalytic reaction mechanism. In the formation of the metal hydride [Cp*Ir(bpy)(H)]⁺ (5), formic acid acts as a bridge assisting proton shuttling. Upon irradiation, two nonadiabatic excited-state decay pathways quickly populate the lowest triplet T₁ state of the metal hydride from its initially populated excited singlet S₁ state. In the T₁ state, water and formic acid facilitate excited-state hydride/proton transfers from the Ir center to Cp* and bpy ligands producing several energetically lower triplet-state isomers demonstrating that the triplet-state metal hydride 5* could not be the only precursor for the photocatalysis. Adiabatic H₂ evolution in the T₁ state is energetically unfavorable. These T₁ isomers hop, through radiationless T₁ → S₀ intersystem crossings via T₁/S₀ crossing points, to the S₀ state in which H₂ evolution takes place. In these reactions, solvents acting as assistants and catalysts reduce reaction barriers, thereby accelerating H₂ release and enhancing the overall photocatalytic performance. Our current work provides significant mechanistic insights into light-induced hydrogen-evolution reactions of iridium-containing photocatalysts.