METAL(II) N2S2 SCHIFF-BASE COMPLEXES INCORPORATING PYRAZOLE OR ISOXAZOLE (M=NI, CU OR ZN) - SPIN STATES, RACEMIZATION KINETICS AND ELECTROCHEMISTRY

Schiff-base complexes incorporating a five-membered aromatic heterocyc le (isoxazole or pyrazole) and biologically important 3d metal(II) ion s (M = Ni, Cu or Zn) have been synthesized. The crystal structures hav e been determined for azol-4-ylmethylene)butane-1,4-diaminato]copper(I I) 1b and for azol-4-ylmethylene)butane-1,4-diaminato]nickel(II) 2a. I n both structures the metal atom is co-ordinated to two nitrogen and t o two sulfur atoms which form a flattened tetrahedron with the dihedra l angles between the two N-M-S planes being 48.6(1) and 12.7(1)degrees respectively. Their physicochemical properties have been studied in s olution. The spin-equilibrium process S = 0 reversible arrow S = 1 and the racemization process Delta reversible arrow Lambda for the nickel (II) complexes have been investigated by temperature-dependent H-1 NMR spectroscopy. Complex 2a (Delta G = 7.8 kJ mol(-1) at 25 degrees C) i s more stabilized in the low-spin state than is 1a (Delta G = 3.2 kJ m ol(-1)). The racemization rates are 2.3 x 10(4) and 1.7 x 10(4) s(-1) at 25 degrees C for 1a and 2a, respectively. The stereochemistry of th e zinc(II) and copper(II) complexes in solution seems to be independen t of the ligand. For the former the configurations were evaluated from the chemical shift differences between diastereotopic protons. For th e latter structural similarities are seen in the electronic and ESR sp ectra. The electrochemical properties have been investigated by cyclic voltammetry. The isoxazole ligand stabilizes a high potential for the M(II)-M(I) reduction ( E(1/2) approximate to 0 for Cu) compared with the pyrazole ligand (E(1/2) = -407 mV for Cu).