Zinc(II) N2S2 Schiff-base complexes incorporating pyrazole: syntheses, characterization, tautomeric equilibria and racemization kinetics

Abstract

Chiral four-co-ordinate zinc(II) N ₂ S ₂ complexes with bi- or tetra-dentate Schiff-base ligands have been prepared and their properties investigated by spectroscopic methods. The single-crystal structures have been determined for two different crystalline modifications of bis[4-(2,6-dimethylphenyliminomethyl)-1-methyl-3-phenylpyrazole-5- thiolato-N,S]zinc(II) 1, a triclinic modification (1a) and an orthorhombic one (1b), and for bis(4-isopropyliminomethyl-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-tetrahedral complexes slightly distorted towards trans-planar co-ordination geometries; the angles θ between the N–Zn–S and S′–Zn–N′ planes are 97.0 and 96.8 (modification 1a), 92.5 and 98.6 ° (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 (θ = 82.63°) typical of M ^II N ₂ S ₂ complexes. The racemization kinetics for the process Δ Λ have been investigated for four complexes by temperature-dependent ¹ H NMR spectroscopy and the activation parameters derived for complexes 1 [ΔH ^‡ = 80.7 kJ mol ^-1 , ΔS ^‡ = -2.9 J K ^-1 mol ^-1 , ΔG ^‡ (25 °C) = 81.6 kJ mol ^-1 ] and 2 [ΔH ^‡ = 67.2 kJ mol ^-1 , ΔS ^‡ = -57.8 J K ^-1 mol ^-1 , ΔG ^‡ (25 °C) = 84.4 kJ mol ^-1 ]. The complexes carrying tetradentate ligands do not racemize below 90 °C. The tautomeric equilibria for the protonated pro-ligands and the preferred mesomer of the complexes have been investigated 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.