Chiral four-co-ordinate zinc() 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-5thiolato-N,S]zinc() 1, a triclinic modification (1a) and an orthorhombic one (1b), and for bis(4-isopropyliminomethyl-3-methyl-1-phenylpyrazole-5-thiolato-N,S)zinc() 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 SЈ᎐Zn᎐NЈ planes are 97.0 and 96.8 (modification 1a), 92.5 and 98.6 Њ (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 = 82.63Њ) typical of M II N 2 S 2 complexes. The racemization kinetics for the process ∆ Λ have been investigated for four complexes by temperature-dependent 1 H NMR spectroscopy and the activation parameters derived for complexes 1 [∆H ‡ = 80.7 kJ mol Ϫ1 , ∆S ‡ = Ϫ2.9 J K Ϫ1 mol Ϫ1 , ∆G ‡ (25 ЊC) = 81.6 kJ mol Ϫ1 ] and 2 [∆H ‡ = 67.2 kJ mol Ϫ1 , ∆S ‡ = Ϫ57.8 J K Ϫ1 mol Ϫ1 , ∆G ‡ (25 ЊC) = 84.4 kJ mol Ϫ1 ]. The complexes carrying tetradentate ligands do not racemize below 90 ЊC. 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(). The evolution of the electronic spectra with time, however, reveals the development of thiol-or thiolate-like forms for both pro-ligands and complexes.
Structural characterization of an iron(iii) complex of a hexadentate tetrakis(picoly1)diamine ligand reveals a novel tetranuclear structure in which two p-0x0-bis(p-acetate)-bridged { Fe2(0)(MeC02)2L6}2+ cores are linked b y aliphatic chains; the electronic structures of this complex and related complexes are similar t o those of the m e t forms of hemeryt h ri n.
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