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

## TL;DR

This paper studies how spin–orbit coupling affects the thermochemistry of a rhenium-based redox catalyst during a proton-coupled electron transfer reaction.

## Contribution

The study reveals significant spin–orbit coupling effects in a rhenium complex, challenging assumptions about their negligible role in organometallic species.

## Key findings

- A rhenium(III) complex shows a 6 kcal·mol–1 deviation in PCET thermochemistry from coupled-cluster computations.
- The deviation is attributed to sizable spin–orbit coupling in the amine precursor, which is reduced in the rhenium(IV) product.
- The study highlights limitations in using coupled-cluster methods for benchmarking heavy d-block catalysts.

## Abstract

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.