Herein, we report the synthesis of a novel heterohexanuclear complex (1) of a heteroaromatic cofactor, pyrroloquinolinequinone (PQQ). The crystal structure of 1 was determined to reveal that two PQQ-bridged Ru(II)Ag(I) units were linked by two [Ag(I)(OTf)2](-) units (OTf = CF3SO3(-)). A solvent-bound Ru(II)Ag(I) heterodinuclear complex (2) was formed from 1 in a coordinating solvent such as acetone to show an intense metal-to-ligand charge-transfer band at 709 nm.

Ruthenium(ii) complexes of PQQTME, a trimethyl ester derivative of redox-active PQQ (pyrroloquinolinequinone), were prepared using a tridentate ligand, 2,2':6',2''-terpyridine (terpy) as an auxiliary ligand. The characterization of the complexes was performed by spectroscopic methods, X-ray crystallography, and electrochemical measurements. In one complex, the pyridine site of PQQTME binds to the [Ru(II)(terpy)] unit as a tridentate ligand, and a silver(i) ion is coordinated by the quinone moiety in a bidentate fashion. In contrast, another complex includes the [Ru(II)(terpy)] unit at the bidentate quinone moiety of the PQQTME ligand. The difference in the coordination modes of the complexes exhibits a characteristic difference in the stability of metal coordination and also in the reversibility of the reduction processes of the PQQTME ligand. It should be noted that an additional metal-ion-binding to the PQQTME ligand largely raises the 1e(-)-reduction potential of the ligand. In addition, we succeeded in the characterization of the 1e(-)-reduced species of the complexes, where the unpaired electron was delocalized in the π-conjugated system of the PQQTME˙(-) ligand, using UV-Vis absorption and ESR spectroscopies.

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 (1OX) exhibits higher electron-acceptability than that of 1e(-)-oxidized 2 (2OX). Bond dissociation enthalpies (BDE) of the two Ru(II) complexes having Hdmdmp as a ligand in proton-coupled electron transfer (PCET) to generate 1OX and 2OX were calculated to be 85 kcal mol(-1) for 1OX and 78 kcal mol(-1) for 2OX. 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 1OX and 2OX. 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 1OX and -0.44 for 2OX. The difference between 1OX and 2OX 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 1OX and 2OX. The more basic 2OX attracts a proton from a C-H bond via a more polarized transition state than that of 1OX; on the contrary, the more electron-deficient 1OX forms less polarized transition states in PCET oxidation reactions of C-H bonds.