title: Aerial Oxidation of a V(IV)-Iminopyridine Hydroquinonate Complex: A Trap for the V(IV)-Semiquinonate Radical Intermediate.
abstract: The reaction of 2,5-bis[N,N'-bis(2-pyridyl-aminomethyl)aminomethyl]-p-hydroquinone (H2bpymah) with VO(2+) salts in acetonitrile or water at a low pH (2.2-3.5) results in the isolation of [{V(IV)(O)(Cl)}2(μ-bpymah)], the p-semiquinonate complex [{V(IV)(O)(Cl)}2(μ-bpymas)](OH), the cyclic mixed-valent hexanuclear compound [{V(V)(O)(μ-O)V(IV)(O)}(μ-bpymah)]3, and [(V(V)O2)2(μ-bpymah)]. [{V(IV)(O)(Cl)}2(μ-bpymas)](OH) is an intermediate of the radical-mediated oxidation of [{V(IV)(O)(Cl)}2(μ-bpymah)] from O2. At lower pH values (2.2), a reversible intramolecular electron transfer from the metal to the ligand of [{V(IV)(O)(Cl)}2(μ-bpymas)](OH) is induced with the concurrent substitution of chlorine atoms by the oxygen-bridging atoms, resulting in the formation of [{V(V)(O)(μ-O)V(IV)(O)}(μ-bpymah)]3. The metal complexes were fully characterized by X-ray crystallography, infrared (IR) spectroscopy, and magnetic measurements in the solid state, as well as by conductivity measurements, UV-vis spectroscopy, and electrochemical measurements in solution. The oxidation states of the metal ions and ligands were determined by the crystallographic data. The [{V(IV)(O)(Cl)}2(μ-bpymah)]-[{V(IV)(O)(Cl)}2(μ-bpymas)](OH) redox process is electrochemically reversible. The V(IV) ion in the semiquinonate compound exhibits a surprisingly low oxophilicity, resulting in the stabilization of OH(-) counterions at acidic pH values. An investigation of the mechanism of this reaction reveals that these complexes induce the reduction of O2 to H2O2, mimicking the activity of enzymes incorporating two redox-active centers (metal-organic) in the active site.
authors.affiliation: †Department of Chemistry, University of Cyprus, 1678 Nicosia, Cyprus.
authors.forename: Marios
authors.initials: M
authors.lastname: Stylianou
authors.affiliation: ‡ Department of Agriculture Production, Biotechnology and Food Science, Cyprus University of Technology, 3036 Limasol, Cyprus.
authors.forename: Chryssoula
authors.initials: C
authors.lastname: Drouza
authors.affiliation: §Department of Mechanical and Manufacturing Engineering, University of Cyprus, 1678 Nicosia, Cyprus.
authors.forename: John
authors.initials: J
authors.lastname: Giapintzakis
authors.affiliation: §Department of Mechanical and Manufacturing Engineering, University of Cyprus, 1678 Nicosia, Cyprus.
authors.forename: Georgios I
authors.initials: GI
authors.lastname: Athanasopoulos
authors.affiliation: †Department of Chemistry, University of Cyprus, 1678 Nicosia, Cyprus.
authors.forename: Anastasios D
authors.initials: AD
authors.lastname: Keramidas
mesh_terms: D056831:Coordination Complexes / Q000737:chemistry*; D004563:Electrochemistry; D005609:Free Radicals / Q000737:chemistry; D006873:Hydroquinones / Q000737:chemistry*; D008024:Ligands; D008958:Models, Molecular; D008968:Molecular Conformation; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry; D011725:Pyridines / Q000737:chemistry*; D014639:Vanadium / Q000737:chemistry*; D014867:Water / Q000737:chemistry
chemical_list: D056831:Coordination Complexes; D005609:Free Radicals; D006873:Hydroquinones; D008024:Ligands; D011725:Pyridines; D014639:Vanadium; D014867:Water; C023666:pyridine; D010100:Oxygen; C031927:hydroquinone