Synthesis, spectra and redox properties of mononuclear five-co-ordinate copper(II) complexes with non-communicable pyrazole/pyridyl containing ligands: X-ray structure of [2,6-bis(3,5-dimethyl-pyrazol-1-ylmethyl)pyridine][2-(3,5-dimethylpyrazol-1-ylmethyl)pyridine]copper(II) diperchlorate

Abstract

Synthesis and characterization of a series of five-co-ordinate mononuclear copper(II) compounds (1-8) have been achieved using 1 tridentate / bidentate heterocyclic nitrogen donor ligands with mixed hard-soft donor sets [L¹ = 2,6-bis(pyrazol-1-ylmethyl)pyridine; L² = 2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine; L³ = 2-(pyrazol-1-ylmethyl)pyridine; L⁴ = 2-(3,5-dimethylpyrazol-1-ylmethyl)- pyridine]. Two types of complexes have been synthesized: (i) the anion bound complexes [Cu(L¹/L² )X₂] (X=Cl^- 1and 2; N^-₃ 3 and 4; SCN^- 5 and 6) and (ii) mixed-ligand complexes [Cu(L¹/L²)(L³/L⁴)](ClO₄)₂ 7 and 8. Complexes 1-8 belong to only a handful of copper(II) complexes with this class of non-planar ligands where pyrazole and pyridine rings are separated by methylene spacer(s). A representative complex [Cu(L²)(L⁴)](ClO₄)₂ 8, having CuN₅ co-ordination sphere, has been structurally characterized, to reveal square pyramidal stereochemistry at the copper(II) centre. Conductivity measurements revealed that one of the co-ordinated anions in 3, 5 and 6 is dissociated in solution, with the vacant site occupied by solvent molecule. Absorption and EPR spectral features of 1-8 represent that these copper(II) complexes adopt essentially a square pyramidal geometry. However, for 1 both in the solid state as well as in solution and for 2 in solution we propose that the copper(II) centre could be six-co-ordinate. Cyclic voltammetric measurements reveal the following trends in their E_½ values (Cu^II -Cu^I redox process): (i) relative stabilization of Cu^II state by anionic ligands (N^-₃, Cl^- or NCS^- ) and a strongly co-ordinating solvent like DMF and (ii) predominance of steric over electronic effect (L¹ vs. L²).