Proton-Coupled Electron Transfer in Ruthenium(II)−Pterin Complexes: Formation of Ruthenium-Coordinated Pterin Radicals and Their Electronic Structures

Ruthenium(II)−pterin complexes were prepared using tetradentate and tripodal tris(2-pyridylmethyl)amine (TPA)
and tris(5-methyl-2-pyridylmethyl)amine (5-Me3-TPA) as auxiliary ligands together with 2-(N,N-dimethyl)-6,7-dimethylpterin (Hdmdmp) and 6,7-dimethylpterin (Hdmp) as pterin derivatives for ligands. Characterization was
made by spectroscopic methods, X-ray crystallography, and electrochemical measurements. The pterin ligands
coordinated to the ruthenium centers as monoanionic bidentate ligands via the 4-oxygen of the pyrimidinone moiety
and the 5-nitrogen of the pyrazine parts. The striking feature is that the coordinated dmp- ligand exhibits a quinonoid
structure rather than a deprotonated biopterin structure, showing a short C−N bond length for the 2-amino group.
Those complexes exhibit reversible two-step protonation for both pterin derivatives coordinated to the ruthenium
centers to give a drastic spectral change in the UV−vis spectroscopy. Doubly protonated Ru(II)−pterin complexes
were stabilized by π-back-bonding interaction and exhibited clear and reversible proton-coupled electron transfer
(PCET) to give ruthenium-coordinated neutral monohydropterin radicals as intermediates of PCET processes. Those
ESR spectra indicate that the unpaired electron delocalizes onto the PCET region (N5−C6−C7−N8) of the pyrazine
moiety.