ZINC(II) N2S2 SCHIFF-BASE COMPLEXES INCORPORATING PYRAZOLE - SYNTHESES, CHARACTERIZATION, TAUTOMERIC EQUILIBRIA AND RACEMIZATION KINETICS

Chiral four-co-ordinate zinc(II) N2S2 complexes with bi- or tetra-dent ate Schiff-base ligands have been prepared and their properties invest igated by spectroscopic methods. The single-crystal structures have be en determined for two different crystalline modifications of -1-methyl -3-phenylpyrazole-5-thiolato-N,S]zinc(II) 1, a triclinic modification (1a) and an orthorhombic one (1b), and for -3-methyl-1-phenylpyrazole- 5-thiolato-N,S)zinc(II) 2. The co-ordination geometry of 1 is similar to that found for the active site of horse liver alcohol dehydrogenase . The asymmetric units in the structures of 1 contain two pseudo-tetra hedral complexes slightly distorted towards trans-planar co-ordination geometries; the angles theta between the N-Zn-S and S'-Zn-N' planes a re 97.0 and 96.8 (modification 1a), 92.5 and 98.6 degrees (1b) for the two molecules, respectively. The structure of 2 also reveals a pseudo -tetrahedral geometry and a slight distortion towards a cis-planar co- ordination (theta = 82.63 degrees) typical of M(II)N(2)S(2) complexes. The racemization kinetics for the process Delta reversible arrow Delt a have been investigated for four complexes by temperature-dependent H -1 NMR spectroscopy and the activation parameters derived for complexe s 1 [Delta H double dagger = 80.7 kJ mol(-1), Delta S double dagger = -2.9 J K-1 mol(-1), Delta G double dagger (25 degrees C) = 81.6 kJ mol (-1)] and 2 [Delta H double dagger = 67.2 kJ mol(-1), Delta S double d agger = -57.8 J K-1 mol(-1), Delta G double dagger (25 degrees C) = 84 .4 kJ mol(-1)]. The complexes carrying tetradentate ligands do not rac emize below 90 degrees C. The tautomeric equilibria for the protonated pro-ligands and the preferred mesomer of the complexes have been inve stigated by NMR and UV spectroscopy. The pro-ligands are mainly in the thione form, while the ligands are thiolate-like when co-ordinated to zinc(II). The evolution of the electronic spectra with time, however, reveals the development of thiol- or thiolate-like forms for both pro -ligands and complexes.