Proton-coupled electron transfer of ruthenium(III)-Pterin complexes: A mechanistic insight

Ruthenium(II) complexes having pterins of redox-active heteroaromatic coenzymes as ligands were demonstrated to perform multistep proton transfer (PT), electron transfer (ET), and proton-coupled electron transfer (PCET) processes. Thermodynamic parameters including pK_a and bond dissociation energy (BDE) of multistep PCET processes in acetonitrile (MeCN) were determined for ruthenium-pterin complexes, [Ru^II(Hdmp)(TPA)] (ClO₄)₂ (1), [Ru^II(Hdmdmp)(TPA)](ClO ₄)₂ (2), [Ru^II(dmp^-)(TPA)]ClO ₄ (3), and [Ru^II(dmdmp^-)-(TPA)]ClO₄ (4) (Hdmp = 6,7-dimethylpterin, Hdmdmp = N,N-dimethyl-6,7-dimethylpterin, TPA ) tris(2-pyridylmethyl)amine), all of which had been isolated and characterized before. The BDE difference between 1 and one-electron oxidized species, [Ru ^III(dmp^-)(TPA)]²⁺, was determined to be 89 kcal mol-1, which was large enough to achieve hydrogen atom transfer (HAT) from phenol derivatives. In the HAT reactions from phenol derivatives to [Ru ^III(dmp^-)(TPA)]²⁺, the second-order rate constants (k) were determined to exhibit a linear relationship with BDE values of phenol derivatives with a slope (-0.4), suggesting that this HAT is simultaneous proton and electron transfer. As for HAT reaction from 2,4,6-tri-tert-buthylphenol (TBP; BDE = 79.15 kcal mol^-1) to [Ru ^III(dmp^-)(TPA)]²⁺, the activation parameters were determined to be ΔH^‡ = 1.6 ± 0.2 kcal mol^-1 and ΔS^‡)-36 ( 2 cal K-1 mol-1. This small activation enthalpy suggests a hydrogenbonded adduct formation prior to HAT. Actually, in the reaction of 4-nitrophenol with [Ru^III(dmp ^-)(TPA)]²⁺, the second-order rate constants exhibited saturation behavior at higher concentrations of the substrate, and low-temperature ESI-MS allowed us to detect the hydrogen-bonding adduct. This also lends credence to an associative mechanism of the HAT involving intermolecular hydrogen bonding between the deprotonated dmp ligand and the phenolic O-H to facilitate the reaction. In particular, a two-point hydrogen bonding between the complex and the substrate involving the 2-amino group of the deprotonated pterin ligand effectively facilitates the HAT reaction from the substrate to the Ru(III)-pterin complex.