Four-Electron Oxidative Dehydrogenation Induced by Proton-Coupled Electron Transfer in Ruthenium(III) Complex with 2‑(1,4,5,6-Tetrahydropyrimidin-2-yl)phenolate

Abstract

New ruthenium­(II or III) complexes with general formula [Ru­(O-N)­(bpy)₂]^<i>n</i>+ (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^<i>t</i>Bu, 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