Synthesis, crystal structures and properties of ternary copper(II) complexes having 2,2'-bipyridine and a-amino acid salicylaldiminates as models for the type-2 sites in copper oxidases

Abstract

Ternary copper(II) complexes [Cu(salgly)(bpy)]·4H₂O (1·4H₂O), [Cu(salala)(bpy)]·4H₂O·(2·4H₂O) and [Cu(salphe)(bpy)] (3), where salgly, salala and salphe are tridentate Schiff base ligands derived from the condensation of salicylaldehyde with glycine, ^L-alanine and ^L-phenylalanine, respectively, and bpy is 2,2'-bipyridine, are prepared and structurally characterized. Crystal structures of the complexes display a distorted square-pyramidal (4+1) coordination geometry in which the ONO-donor Schiff base is bonded to the metal in the basal plane. The chelating bpy ligand displays axial-equatorial mode of bonding. Complexes 1 and 2 show chemically significant hydrogen bonding interactions. In addition, complex 2 exhibits ∏-∏ stacking, while complex 3 shows attractive CH(imine)-∏ (phenyl) non-covalent interactions. The one-electron paramagnetic complexes display axial EPR spectra in DMF-glass at 77 K indicating a {d_x²-y²}¹ ground state. The complexes show a d-d band near 670 nm and a charge transfer band near 440 nm in methanol. The complexes are redox active and exhibit a quasi-reversible Cu(II)-Cu(I) couple in DMF-0.1 M TBAP near -0.8 V versus SCE. The complexes are catalytically active in the oxidation of ascorbic acid by dioxygen. The process which is effective with a mole ratio of ascorbic acid to complex as approximately 30, models the ascorbate oxidation property of the type 2 sites in ascorbate oxidase and dopamine β-hydroxylase. The complexes with a N₃O₂ coordination environment model the active site structure in amine oxidase and mediate deamination reactions in which benzylamine gets oxidized to benzaldehyde in the presence of hydrogen peroxide.