Four-electron oxidative dehydrogenation induced by proton-coupled electron transfer in ruthenium(III) complex with 2-(1,4,5,6-tetrahydropyrimidin-2-yl) phenolate

New ruthenium(II or III) complexes with general formula [Ru(O-N)(bpy) ₂]ⁿ⁺ (O-N = unsymmetrical bidentate phenolate ligand; bpy = 2,2'-bipyridine) were synthesized, and their crystal structures and electrochemical properties were characterized. Ru^II complexes with 2-(2-imidazolinyl)phenolate (Himn^-) or 2-(1,4,5,6- tetrahydropyrimidin-2-yl)phenolate (Hthp^-) could be deprotonated by addition of excess KO^tBu, although the deprotonated species were easily reprotonated by exposure to air. Unlike these Ru^II complexes, their Ru^III analogs showed interesting ligand oxidation reactions upon addition of bases. With [Ru^III(Himn)(bpy)₂] ²⁺, two-electron oxidation of Himn^- and reduction of the Ru^III center resulted in conversion of the 2-imidazolinyl group to a 2-imidazolyl group. On the other hand, the corresponding Hthp^- complex exhibited four-electron oxidation of the ligand to form 2-(2-pyrimidyl)phenolate (pym^-). These aromatization reactions of imidazolinyl and 1,4,5,6-tetrahydropyrimidyl groups were also achieved by the electrochemically generated Ru^III complexes.