Racemization kinetics and spin states of four-co-ordinate nickel(II) N2X2 Schiff-base and aza complexes with bi- or tetra-dentate ligands incorporating pyrazole (X = NH or S)

Abstract

Bis(bidentate ligand) and tetradentate ligand nickel(II) N ₂ X ₂ Schiff-base and aza complexes (X = NH or S) have been prepared and their properties investigated by spectroscopic methods. In the bis(bidentate ligand) complexes the aza function stabilizes the low-spin S = 0 state compared with the imine function. The aza complexes are low spin both in the solid state and in solution; the Schiff-base complexes are either low or high spin (S = 1) in the solid state, and are either in spin equilibrium (S = 0 ⇌ S = 1) or high spin in solution. The crystal structure has been solved for the high-spin complex bis(4-isopropyliminomethyl-1,3-diphenylpyrazol-5-ylaminato)nickel( II). The co-ordination of Ni is pseudo-tetrahedral, the angle between the N–Ni–NH and NH′–Ni–N′ planes being 93.8(1)°. The Ni–N (imine) bond lengths are 1.999(2) and 2.003(3) Å, significantly longer than the Ni–N (amine) bond lengths of 1.919(2) Å. In the tetradentate ligand complexes the two identical halves of the ligands are linked by aliphatic four-carbon chains. When the linkage is CMe ₂ (CH ₂ ) ₂ CMe ₂ the complexes are fully paramagnetic in the solid state and in solution, while complexes bridged by unsubstituted (CH ₂ ) ₄ are low spin in the solid state and in spin equilibrium in solution. The crystal structure of [N,N′-bis(1,3-dimethyl-5-sulfanylpyrazol-4- ylmethylene)butane-1,4-diaminato]nickel(II) reveals an almost planar co-ordination geometry, the angle between the N–Ni–S and S′–Ni–N′ planes being 7.9(3)°. The Schiff-base complexes are chiral and all show evidence of racemization in solution. Thermodynamic parameters for the spin-equilibrium process in CD ₂ Cl ₂ [ΔG(25 °C) from -4.32 to 0.71 kJ mol ^-1 for the bis(bidentate ligand) systems, from 3.72 to 11.3 kJ mol ^-1 for the tetradentate ligand systems] and kinetic parameters for the racemization process in CD ₂ Cl ₂ or CDCl ₂ CDCl ₂ [ΔG ^‡ (25 °C) from 40.5 to 53.3 kJ mol ^-1 for the bis(bidentate ligand) complexes, 46.5 to 63.1 kJ mol ^-1 for the tetradentate ligand complexes] have been derived using variable-temperature ¹ H NMR spectroscopy. Sulfur donor atoms and aryl substituents favour the low-spin state. Ligand-field parameters for the Schiff-base complexes have been derived from the electronic spectra.