Proton-Coupled Electron Transfer in Ruthenium(II)−Pterin Complexes: Formation of Ruthenium-Coordinated Pterin Radicals and Their Electronic Structures
- Soushi Miyazaki ,
- Takahiko Kojima ,
- Taisuke Sakamoto ,
- Tetsuya Matsumoto ,
- Kei Ohkubo , and
- Shunichi Fukuzumi
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Department of Material and Life Science, Graduate School of Engineering, Osaka University and SORST (JST), Suita, Osaka 565-0871 and Department of Chemistry, Faculty of Sciences, Kyushu University, 6-10-1 Hakozaki, Higashi-Ku, Fukuoka 812-8581, Japan
Abstract
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.
†
Osaka University and SORST (JST).
*
To whom correspondence should be addressed. E-mail: [email protected] chem.eng.osaka-u.ac.jp (T.K.) and [email protected] (S.F.).
‡
Kyushu University.
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