Regulation of Redox behavior and Proton-coupled Electron Transfer of Transition Metal Complexes with Heteroaromatic Coenzymes as Ligands

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Synthesis and characterization were made on ruthenium complexes of heteroaromatic coenzymes with tris(2-pyridylmethyl)amine and its derivatives. Those novel complexes were characterized by spectroscopic and electrochemical measurements, in addition to X-ray crystallography.1) Ruthenium-Pterin ComplexesIn order to clarify the role of the 2-amino group of the pterin framework, we synthesized a ruthenium(II)-TPA complex with 6,7-dimethylpteridine (H_2dmp) in place of 2-N, N-dimethy1-6,7-dimethylpteridine (H_2dmdmp). The new complex, [Ru(Hdmp)(TPA)]CIO_4, was characterized by X-ray crystallography, spectroscopic methods, and electrochemical measurements.The crystal structure was revealed to exhibit intermolecular hydrogen bonding between the 2-amino group and the 8-position of the pterin skeleton to form a dimeric motif in the crystal. This indicates the high basicity of the 8-nitrogen, which accepts proton first in the two-step reversible protonation-deprotonation process of the pterin li … More gands in the Ru-TPA environments.The results obtained in the spectroscopic and electrochemical approach to evaluate the proton-coupled electron transfer indicated following points:1. Hydrogen bonding toward the 2-amino group enhances the proton accessibility of the pterin and improves the reversibility of the redox processes of pterins..2. The 2-amino group plays important roles in promoting smooth proton-coupled electron transfer of pterins.2) Ruthenium-Alloxazine Complexes.We synthesized a novel ruthenium(II)-TPA complex with alloxazine which is an analogue of flavins and succeeded in the crystal structure determination to reveal an unprecedented coordination mode of the alloxazine ligand. The unusual coordination mode allowed us to observe the hydrogen bonding adduct with a complementary hydrogen bonding receptor of 2,6-bis(acetoamide)pyridine. The crystal structure of the adduct was determined to demonstrate unambiguously the three-point recognition of the receptor by the alloxazine complex. This hydrogen bonding not only altered the redox potential of the alloxazine ligand, but perturbed the electronic structure of the one-electron reduced π-anion radical of the coordinated alloxazine. These results imply that the hydrogen bonding toward the flavin cofactors in the enzymes play crucial roles to regulate the reactivity of the coenzymes, which result in the control of the reactivity of the enzymes. A paper about this will appear in Angew. Chem., Int. Ed. as a Very Important Paper (VIP). Less