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.