Mechanistic Insights into C-H Oxidations by Ruthenium(III)-Pterin Complexes: Impact of Basicity of the Pterin Ligand and Electron Acceptability of the Metal Center on the Transition States

A ruthenium(II) complex, [Ru(dmdmp)Cl(MeBPA)] (2) (Hdmdmp = N,N-dimethyl-6,7-dimethylpterin, MeBPA = N-methyl-N,N-bis(pyridylmethyl)amine), having a pterin derivative as a proton-accepting ligand, was synthesized and characterized. Complex 2 shows higher basicity than that of a previously reported Ru^II-pterin complex, [Ru(dmdmp) (TPA)]⁺ (1) (TPA = tris(2-pyridylmethyl)amine). On the other hand, 1e^--oxidized species of 1 (1_OX) exhibits higher electron-acceptability than that of 1e^--oxidized 2 (2_OX). Bond dissociation enthalpies (BDE) of the two Ru^II complexes having Hdmdmp as a ligand in proton-coupled electron transfer (PCET) to generate 1_OX and 2_OX were calculated to be 85 kcal mol^-1 for 1_OX and 78 kcal mol^-1 for 2_OX. The BDE values are large enough to perform H atom transfer from C-H bonds of organic molecules to the 1e^--oxidized complexes through PCET. The second-order rate constants (k) of PCET oxidation reactions were determined for 1_OX and 2_OX. The logarithms of normalized k values were proportional to the BDE values of C-H bonds of the substrates with slopes of -0.27 for 1_OX and -0.44 for 2_OX. The difference between 1_OX and 2_OX in the slopes suggests that the transition states in PCET oxidations of substrates by the two complexes bear different polarization, as reflection of difference in the electron acceptability and basicity of 1_OX and 2_OX. The more basic 2_OX attracts a proton from a C-H bond via a more polarized transition state than that of 1_OX; on the contrary, the more electron-deficient 1_OX forms less polarized transition states in PCET oxidation reactions of C-H bonds.