Synthesis and properties of [Cu(L⁵)₂][ClO₄]₂·H₂O having square planar and pseudo-octahedral geometries in the same unit cell, and anion-bound complexes [Cu(L⁵)₂X][ClO₄] (X=Cl^-, NCS^-, N₃^-) [L⁵=2-(3,5-dimethylpyrazol-1-ylmethyl)pyridine]

Abstract

Using a bidentate heterocyclic ligand with mixed hard-soft nitrogen donor set [L⁵=2-(3,5-dimethylpyrazol-1-ylmethyl)pyridine] two types of copper(II) complexes have been synthesized and characterized: (i) [Cu(L⁵)₂][ClO₄]₂·H₂O (1) and (ii) [Cu(L⁵)₂X][ClO₄] (X=Cl^- (2), NCS^- (3) and N₃^- (4)). Structural analysis of 1 reveals that its unit cell consists of a 1:1 mixture of square planar [Cu(L⁵)₂]²⁺ (coordination by two pyrazoles and two pyridines) and tetragonal [Cu(L⁵)₂(H₂O)₂]²⁺, having additional weak interaction with the water molecules in the axial positions. Preliminary X-ray structural analysis of 3 with a poor data set, which is not reported here, revealed that the thiocyanate group is S-coordinated. For each complex the stereochemistry of the copper(II) centre has been investigated in the solid state as well as in MeCN solution, by absorption and EPR spectroscopy. In the polycrystalline state as well as in MeCN solution complex 1 displays axial EPR spectra having a d_x2_-y2 ground state. However, solid samples of 2-4 exhibit rhombic EPR spectral features, characteristic of a compressed d_z² ground state. In the solution phase (80 K) 2 exhibits an isotropic signal; 3 and 4 exhibit axial spectra, characteristic of a d_x2_-y2 ground state. The observed E_½ values for the Cu^II-Cu^I redox couple [cyclic voltammetry: MeCN solution; V vs. saturated calomel electrode (SCE)] are fairly positive: 0.41 (1), 0.29 (2), 0.30 (3) and 0.19 (4).