Evaluating Spin-Orbit Effects on the Thermochemistry of Proton-Coupled Electron Transfer

Many heavy transition metal compounds are active redox catalysts. Their redox potentials can be offset by differential spin-orbit coupling (SOC) effects in the case of strong perturbation of the ground-state energy of the oxidized or the reduced state. However, SOC effects are often considered negligible in the case of organometallic species, anticipating energetically well-separated, nondegenerate spin ground states for metal ions in strong ligand fields with low symmetry. We here report a rhenium(III) aminodiphosphine complex that undergoes proton-coupled electron transfer with a phenoxyl radical as a hydrogen abstractor. Experimental derivation of the PCET thermochemistry shows a deviation from coupled-cluster computations in the range of 6 kcal·mol^-1. The deviation can be attributed to a sizable SOC contribution by the amine precursor, which is largely quenched in the rhenium(IV) amido product. Our case study emphasizes potential pitfalls for coupled-cluster benchmarking of the reaction energetics of heavy d-block catalysts.