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

Copper(II) complexes of a series of tripodal ligands containing a phenolate moiety, viz. 2-(bis(1-methylbenzimidazol-2-yl)aminomethyl)-4-nitrophenol [ H(L-1)] and N,N-dimethyl-N'-(pyrid-2-ylmethyl)-N'-(2-hydroxy-4-nitrobenzyl) -ethylenediamine [HL2] have been isolated and characterised by electronic a bsorption and EPR spectroscopy and cyclic and differential pulse voltammetr y. The X-ray crystal structure of the binuclear perchlorate complex [Cu(L-2 )](2)(ClO4)(2) reveals a square based pyramidal distorted trigonal bipyrami dal (SBPDTB) coordination geometry around Cu(II). The CuN2O trigonal coordi nation plane is comprised of NN-dimethyl substituted amine nitrogen, one py ridyl nitrogen and a phenolate oxygen of the ligand. The axial positions ar e occupied by the tertiary amine nitrogen and the phenolate ion from the se cond coordination sphere resulting in the dimerisation. The ligand field an d EPR spectra of all the complexes are consistent with a square-based geome try 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(parallel to) values lie in the range 2.24-2.28 indi cating the presence of CuN2O2 or CuN3O chromophores. The incorporation of s terically hindered NN-dimethyl substituted nitrogen and bulky 1-methylbenzi midazole groups tends to enhance the trigonal distortion and raise the Cu(I I)/Cu(I) redox potential. (C) 2001 Elsevier Science BN. All rights reserved .