Metal(II) N2S2 Schiff-base complexes incorporating pyrazole or isoxazole (M = Ni, Cu or Zn). Spin states, racemization kinetics and electrochemistry

Abstract

Schiff-base complexes incorporating a five-membered aromatic heterocycle (isoxazole or pyrazole) and biologically important 3d metal(II) ions (M = Ni, Cu or Zn) have been synthesized. The crystal structures have been determined for [N,N′-bis(3-phenyl-5-sulfanylisoxazol-4- ylmethylene)butane-1,4-diaminato]copper(II) 1b and for [N,N′-bis(1-methyl-3-phenyl-5-sulfanylpyrazol-4- ylmethylene)butane-1,4-diaminato]nickel(II) 2a. In both structures the metal atom is co-ordinated to two nitrogen and to two sulfur atoms which form a flattened tetrahedron with the dihedral angles between the two N–M–S planes being 48.6(1) and 12.7(1)° respectively. Their physicochemical properties have been studied in solution. The spin-equilibrium process S = 0 S = 1 and the racemization process Δ Λ for the nickel(II) complexes have been investigated by temperature-dependent ¹ H NMR spectroscopy. Complex 2a (ΔG = 7.8 kJ mol ^-1 at 25 °C) is more stabilized in the low-spin state than is 1a (ΔG = 3.2 kJ mol ^-1 ). The racemization rates are 2.3 × 10 ⁴ and 1.7 × 10 ⁴ s ^-1 at 25 °C for 1a and 2a, respectively. The stereochemistry of the zinc(II) and copper(II) complexes in solution seems to be independent of the ligand. For the former the configurations were evaluated from the chemical shift differences between diastereotopic protons. For the latter structural similarities are seen in the electronic and ESR spectra. The electrochemical properties have been investigated by cyclic voltammetry. The isoxazole ligand stabilizes a high potential for the M ^II –M ^I reduction (E _½ ≈ 0 for Cu) compared with the pyrazole ligand (E _½ = -407 mV for Cu).