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 groun Show more
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
Herein, we investigate the proton-coupled electron transfer (PCET) reactivity of a cobalt(I) complex with a proton-responsive pyridin-4-ol PNP pincer-type ligand (HLPNP = 2,6-bis((bis-te Show more
Herein, we investigate the proton-coupled electron transfer (PCET) reactivity of a cobalt(I) complex with a proton-responsive pyridin-4-ol PNP pincer-type ligand (HLPNP = 2,6-bis((bis-tert-butylphosphaneyl)-methyl)pyridin-4-ol). The cobalt(II) complexes [(LPNP)CoIICl], 1, and [(LPNP)CoII(MeCN)]+, 2+, with the deprotonated ligand and [(HLPNP)CoII(MeCN)2]2+, 2H2+, with the protonated ligand, were synthesized and characterized. 2H2+ has a pKa of 18 ± 1, and the reduction of 2H2+ appears at -1.08 V vs. FeCp2+|0 in MeCN. This leads to a bond dissociation free energy (BDFE) of the OH bond in [(HLPNP)CoI(MeCN)]+, 2H+, of 52 kcal mol-1, which is supported by DFT calculations. The solution BDFE of 2H+ equals the BDFE of 1/2 H2, and indeed, 2H+ slowly loses dihydrogen. Kinetic analysis revealed a first-order rate law in 2H+ with a reaction rate constant k of 3.2 × 10-4 s-1 at 25 °C and a positive activation entropy ΔS‡ of 9.4 ± 0.6 cal (ΔH‡ = 24.3 ± 0.2 kcal mol-1) for H2 loss. Based on these kinetic results, H/D labeling studies, and DFT calculations, a unimolecular mechanism is proposed. However, H atom transfer from 2H+ to acceptors such as (2,2,6,6-tetramethylpiperidin-1-yl)oxyl or 2,4,6-tert-butylphenoxide is very fast (k2 of 104 s-1 M-1 for the reaction of 2H+ with TEMPO•) and H2 loss can be easily outcompeted. Show less