Copper(II) complexes of sterically hindered phenolate ligands as structural models for the active site in galactose oxidase and glyoxal oxidase: x-ray crystal structure and spectral and redox properties

Abstract

Copper(II) complexes of a series of tripodal ligands containing a phenolate moiety, viz. 2-(bis(1-methylbenzimidazol-2-yl)aminomethyl)-4-nitrophenol [H(L¹)] and N,N-dimethyl-N'-(pyrid-2-ylmethyl)-N'-(2-hydroxy-4-nitrobenzyl)-ethylenediamine [HL²] have been isolated and characterised by electronic absorption and EPR spectroscopy and cyclic and differential pulse voltammetry. The X-ray crystal structure of the binuclear perchlorate complex [Cu(L²)]₂(ClO₄)₂ reveals a square based pyramidal distorted trigonal bipyramidal (SBPDTB) coordination geometry around Cu(II). The CuN₂O trigonal coordination plane is comprised of N,N-dimethyl substituted amine nitrogen, one pyridyl nitrogen and a phenolate oxygen of the ligand. The axial positions are occupied by the tertiary amine nitrogen and the phenolate ion from the second coordination sphere resulting in the dimerisation. The ligand field and EPR spectra of all the complexes are consistent with a square-based geometry in solution. An intense band observed around 390 nm may originate from Cu(II) → O^- (phenolate, axial) and/or equatorial (phenolate) O^- → Cu(II) CT transitions. The g|| values lie in the range 2.24-2.28 indicating the presence of CuN₂O₂ or CuN₃O chromophores. The incorporation of sterically hindered N,N-dimethyl substituted nitrogen and bulky 1-methylbenzimidazole groups tends to enhance the trigonal distortion and raise the Cu(II)/Cu(I) redox potential.