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

Seth M Barrett; Bethany M Stratakes; Matthew B Chambers; Daniel A Kurtz; Catherine L Pitman; Jillian L Dempsey; Alexander J M Miller

doi:10.1039/d0sc00422g

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

Chem Sci. 2020 Mar 6;11(25):6442-6449. doi: 10.1039/d0sc00422g.

Authors

Seth M Barrett^{1

2}, Bethany M Stratakes¹, Matthew B Chambers^{1

3}, Daniel A Kurtz¹, Catherine L Pitman¹, Jillian L Dempsey¹, Alexander J M Miller¹

Affiliations

¹ Department of Chemistry, University of North Carolina at Chapel Hill Chapel Hill North Carolina 27599-3290 USA ajmm@email.unc.edu.
² Department of Chemistry, Muskingum University New Concord OH 43762-1118 USA.
³ Department of Chemistry, Louisiana State University Baton Rouge LA 70803-1804 USA.

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 = 4,4'-dimethoxy-2,2'-bipyridine) and [Cp*Ir(bpy)H]⁺ (2, bpy = 2,2'-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.