TETRAHEDRALLY DISTORTED TETRAGONAL COPPER-COMPLEXES OF A TRIDENTATE LIGAND

Copper(II) complexes with a tridentate chelating ligand within the general 2 N, O donor class, containing a bis(benzimidazole) ether donor set, have been synthesized and characterized. Complexes of 1:1 stoichiometry form on mixing Cu(ClO4)2.6H2O with the ligand in acetonitrile. X-band EPR spectra of the above complexes examined as a frozen acetonitrile solution indicate g parallel-to > g perpendicular-to. The g parallel-to and A parallel-to values are consistent with the binding of less than three nitrogen atoms to the complex and border on Peisach and Blumberg plots of A parallel-to vs g parallel-to for two nitrogen and two oxygen atoms in the equatorial plane. Copper complexes of 1:2 stoichiometry form in the presence of an added exogenous ligand, viz. azide, acetate, and benzimidazole. X-Band EPR spectra of the above complexes indicate a severe distortion of the tetragonal symmetry. All of these show less than four g parallel-to components and a broadening of the g perpendicular-to component. Superimposed on the broad g perpendicular-to component are eleven lines due to super hyperfine interaction (shf), with an average coupling constant of B(N) = 14 +/- 1 G. Only in one case does the m(I) = -3/2 component in the g parallel-to region show well-resolved shf, with nine lines and A(N) = 11 +/- 1 G, and the intensity pattern of the shf lines is in fair agreement with that observed for splitting due to four nitrogen atoms, i.e. 1:4:10:16:19:16:10:4:1. The intensity pattern of the shf lines on the g perpendicular-to component matches quite well with the ratios 1:4:11:20:29:32:29:20:11:4:1. Such a pattern is known to emerge if two sets of nine lines overlap to give an eleven-line pattern. In the present case the two sets of nine lines originate from overlapping of g parallel-to and g perpendicular-to components. The fractions of s and p character (f(s) and f(sigma)) have been evaluated for nitrogen bonding orbitals. We find that the ratio of f(s)/f(sigma) is slightly lower than that expected for sp2 hybridization, implying higher p character in the bonding orbitals of nitrogen atom. Molecular orbital coefficients beta(1), beta(2), and beta'(1) have been evaluated after Maki and McGarvey. The trends of beta(1) and beta'(1) indicate increasing electron delocalization as we change the exogenous ligand from benzimidazole to acetate to azide, while the trend of g parallel-to and A parallel-to indicates marked tetrahedral distortion of the tetragonal site. The above complexes may have potential relevance to mononuclear copper sites in the turacin superoxide dismutase and to coordination geometries adopted by type 3 copper sites in rhus laccase and ascorbate oxidase.