Rigidity of donor atoms' topology: synthesis and spectroscopic characterization of copper(II) and copper(II)-doped zinc(II) complexes having the chromophore MN₄

Abstract

A new acyclic ligand has been synthesized by Schiff base condensation of 2-dimethylaminoethyl amine with diisobutyraldehyde disulphide, which can coordinate to a metal ion through four nitrogens. Introduction of a large bite angle for two nitrogens forces the coordination geometry around a metal ion to distort from planarity towards tetrahedral. The presence of a disulphide linkage along with two imine linkages make the ligand somewhat neoplastic so that the four nitrogens can impose a pseudo-tetrahedral geometry upon a metal ion overcoming its own preference for a particular geometry. The electronic spectral results when compared with the square planar complex Cu(L)₂(ClO₄)₂ (L = 2-dimethylaminoethyl amine) are consistent with a pseudo-tetrahedral coordination geometry for copper(II). We have tested the rigidity of the donor atoms' topology through synthesis and characterization of neat copper(II) and copper(II)-doped zinc(II) complexes. ESR spectral results show that the neat copper(II) as well as the copper(II)-doped zinc(II) complexes give axial spectra and almost identical g_| and A_| values, which are attributable to similar pseudo-tetrahedral coordination. THus, the ligand-field driven tendencies of zinc(II) to form tetrahedral complexes of copper(II) to form square planar complexes with four nitrogen donors are overcome through ligand design. The neat copper(II) complex shows a quasi-reversile copper(II)/copper(I) couple at E _½ = + 0.18 V (vs SCE), which is shifted by more than +450 mV compared to the square-planar analogue Cu(L)₂(ClO₄)₂.