Mechanistic basis for tuning iridium hydride photochemistry from H2 evolution to hydride transfer hydrodechlorination

Abstract

The photochemistry of metal hydride complexes is dominated by H₂ evolution, limiting access to reductive transformations based on photochemical hydride transfer. In this article, the innate H₂ evolution photochemistry of the iridium hydride complexes [Cp*Ir(bpy-OMe)H]⁺ (1, bpy-OMe = 4,4ʹ-dimethoxy-2,2′-bipyridine) and [Cp*Ir(bpy)H]⁺ (2, bpy = 2,2ʹ-bipyridine) is diverted towards photochemical hydrodechlorination. Net hydride transfer from 1 and 2 to dichloromethane produces chloromethane with high selectivity and exceptional photochemical quantum yield (Φ ≤ 1.3). Thermodynamic and kinetic mechanistic studies are consistent with a non-radical-chain reaction sequence initiated by “self-quenching” electron transfer between excited state and ground state hydride complexes, followed by proton-coupled electron transfer (PCET) hydrodechlorination that outcompetes H–H coupling. This unique photochemical mechanism provides a new hope for the development of light-driven hydride transfer reactions.