Targeted Oxidase Reactivity with a New Redox-Active Ligand Incorporating N₂O₂Donor 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.