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).
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.
Mukherjee
Chandan
C
Max-Planck-Institute for Bioinorganic Chemistry, Stiftstrasse 34-36, D-45470 Mülheim an der Ruhr, Germany.
Weyhermüller
Thomas
T
Bothe
Eberhard
E
Chaudhuri
Phalguni
P
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*
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