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

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 pi-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.

Proton-Coupled Electron Transfer of Ruthenium(III)−Pterin Complexes: A Mechanistic Insight

“…Reversible one-electron redox couples assigned to Ru(II)/Ru(III) were observed at 0.55 V (vs Fc/Fc + , Δ E = 60 mV) for 1 and 0.52 V (Δ E = 64 mV) for 2 . Those values are 0.28 and 0.26 V higher than those observed for 3 (0.27 V) and 4 (0.26 V), respectively, however, only 0.2 V lower than those of corresponding doubly protonated species (see Scheme 2 below) 22a. In contrast to the reversible redox behavior of deprotonated and monoanionic pterin ligands in 3 and 4 and that of corresponding doubly protonated and monocationic species, monoprotonated and neutral pterin ligands in 1 and 2 exhibited irreversible redox behavior due to “proton shift” upon one-electron reduction 21.…”

“…The reversible two-step protonation/deprotonation behavior was observed in absorption spectra of 3 and 4 in aqueous buffer solutions and in MeCN by adding HClO 4 or base as reported previously 22,23. The spectroscopic titrations were carried out to determine p K a of 3 and 4 in MeCN by using Hdcba (p K a = 17.6) for the first protonation steps.…”

“…Phenols were further purified by the standard methods 24. 2,4,6-Tri- tert -buthylphenol- d ,25 [Ru(Hdmp)(TPA)](ClO 4 ) 2 ( 1 ),21 [Ru(Hdmdmp)(TPA)](ClO 4 ) 2 ( 2 ),21 [Ru(dmp − )(TPA)]ClO 4 ( 3 ),22 and [Ru(dmdmp − )(TPA)]ClO 4 ( 4 ),22,23 were prepared according to literature methods. NMR measurements were performed on JEOL AL-300 and Bruker AV-300 spectrometers.…”

“…Among those, ruthenium complexes of pterins have been reported to exhibit PCET with clear reversibility for both the metal center and the pterin ligands 20. We have reported on synthesis and characterization of the ruthenium-pterin complexes as shown in Chart 1, [Ru II (Hdmp)(TPA)](ClO 4 ) 2 ( 1 ),21 [Ru II (Hdmdmp)(TPA)](ClO 4 ) 2 ( 2 ),21 [Ru II (dmp − )(TPA)]ClO 4 ( 3 ),22 and [Ru II (dmdmp − )(TPA)]ClO 4 ( 4 ),22,23 in light of their structures and redox properties including PCET. No report has appeared, however, to demonstrate any reactivity of transition metal complexes of pterins toward external entities, including PCET reactions with substrates.…”

Pterin chemistry and its relationship to the molybdenum cofactor

“…Ru(+2) coordination makes the pterin ligand more basic so that it can be protonated twice [107,108]. The doubly protonated species can be 1e − reduced to a H 2 pterin radical, observable by EPR.…”