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

Soushi Miyazaki, Takahiko Kojima, James M. Mayer, Shunichi Fukuzumi

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Abstract

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 pKa and bond dissociation energy (BDE) of multistep PCET processes in acetonitrile (MeCN) were determined for ruthenium-pterin complexes, [RuII(Hdmp)(TPA)] (ClO4)2 (1), [RuII(Hdmdmp)(TPA)](ClO 4)2 (2), [RuII(dmp-)(TPA)]ClO 4 (3), and [RuII(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 [RuIII(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.

Original languageEnglish
Pages (from-to)11615-11624
Number of pages10
JournalJournal of the American Chemical Society
Volume131
Issue number32
DOIs
StatePublished - 19 Aug 2009

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