title: Targeted oxidase reactivity with a new redox-active ligand incorporating N2O2 donor atoms. Complexes of Cu(II), Ni(II), Pd(II), Fe(III), and V(V).
abstract: The coordination chemistry of the tetradentate ligand N,N'-bis(2-hydroxy-3,5-di-tert-butylphenyl)-2,2'-diaminobiphenyl H(4)L has been studied with the copper(II), nickel(II), palladium(II), iron(III), and vanadium(V) ions. The ligand is non-innocent in the sense that it is readily oxidized in the presence of air to its o-iminobenzosemiquinonato (L(**))(2-) radical form. The crystal structures of the diradical compounds, [Cu(II)(L(**))] 1, [Ni(II)(L(**))] 2, [Pd(II)(L(**))] 3, the monoradical high-spin compound [Fe(III)(HL(*))Cl] 4, and the di(mu-methoxo)divanadium(V) compound [L(2)V(2)(mu-OCH(3))(2)] 5 without a radical have been determined by X-ray crystallography at 100 K. The biphenyl backbone of the ligand induces a tetrahedral distortion of the metal(II) geometry in 1, 2, and 3 having a N(2)O(2) coordination environment. The dihedral angles between the metal planes are 35.5 degrees for 1, 30.8 degrees for 2, and 22.2 degrees for 3. Variable-temperature (2-290 K) magnetic susceptibility measurements together with Mossbauer and electron paramagnetic resonance (EPR) spectroscopy establish the electronic structures of the complexes. Electrochemical cyclic voltammetric measurements indicate four one-electron reversible redox processes of the ligand for 1, 2, and 3. Complex 1 is found to catalyze the aerial oxidation of benzylalcohol to benzaldehyde, thus modeling the catalytic function of the copper-containing enzyme Galactose Oxidase (GO). Kinetic measurements in conjunction with EPR and UV-vis spectroscopic studies have been used to decipher the catalytic oxidation process. A ligand-derived redox activity has been proposed as a mechanism in which complex 1 disproportionates in a basic medium to generate the catalytically active species. An "on-off" mechanism of the radicals without apparent participation of the metal center is invoked for the catalytic process, whose intimate mechanism thus differs from that of the enzyme Galactose Oxidase.
authors.affiliation: Max-Planck-Institute for Bioinorganic Chemistry, Stiftstrasse 34-36, D-45470 Mülheim an der Ruhr, Germany.
authors.forename: Chandan
authors.initials: C
authors.lastname: Mukherjee
authors.forename: Thomas
authors.initials: T
authors.lastname: Weyhermüller
authors.forename: Eberhard
authors.initials: E
authors.lastname: Bothe
authors.forename: Phalguni
authors.initials: P
authors.lastname: Chaudhuri
mesh_terms: D001894:Borohydrides / Q000737:chemistry; D003300:Copper / Q000737:chemistry*; D018360:Crystallography, X-Ray / Q000379:methods; D005692:Galactose Oxidase / Q000737:chemistry; D007501:Iron / Q000737:chemistry*; D008024:Ligands; D008958:Models, Molecular; D008968:Molecular Conformation; D009532:Nickel / Q000737:chemistry*; D009574:Nitro Compounds / Q000737:chemistry; D009589:Nitrogen Oxides / Q000737:chemistry*; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000378:metabolism*; D010165:Palladium / Q000737:chemistry*; D014639:Vanadium / Q000737:chemistry*
chemical_list: D001894:Borohydrides; D008024:Ligands; D009574:Nitro Compounds; D009589:Nitrogen Oxides; D014639:Vanadium; D010165:Palladium; D003300:Copper; D009532:Nickel; D007501:Iron; D010088:Oxidoreductases; D005692:Galactose Oxidase