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.