👤 Serhiy Demeshko

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. Show less

Iron complexes of tetradentate macrocyclic ligands containing N-heterocyclic carbene (NHC) donors have been referred to as organometallic heme analogues, but they usually lack the redox noninnocence under oxidizing conditions that is characteristic of porphyrins. Here we report a novel NHC/N-donor hybrid macrocyclic ligand containing two trans NHC moieties, a pyridine and a redox active carbazolide fragment. Its Fe^II, Fe^III and formal Fe^IV complexes have been isolated and comprehensively characterized, where UV/vis and ⁵⁷Fe Mössbauer spectroscopies, SQUID magnetometry and density functional theory (DFT) calculations reveal that the latter are best described as Fe^III systems antiferromagnetically coupled to a carbazolide-based organic π-radical. Two different redox series are obtained depending on the axial ligands: nitriles such as MeCN give low-spin (LS) configurations of the metal ion, while in case of weakly coordinating solvents and triflate anions the iron adopts an intermediate-spin (IS) configuration; MeCN binding constants have been determined. As in other heme analogues with NHC-based macrocycles, the strong equatorial σ-donor character raises the energy of the Fe(d_x²-y²) orbital, making high-spin (HS) iron species inaccessible. The combined features of equatorial ligand redox noninnocence, restriction to LS/IS surfaces and tunability via the axial coligands makes this a promising platform for bioinspired reactivity such as the generation of reactive Fe/O_x intermediates. Show less