Nickel(II) N2X2 Schiff-base complexes incorporating pyrazole (X = NH, O or S): syntheses and characterization†

Abstract

A series of four-co-ordinate nickel(II) N₂X₂ complexes (X = NH, O or S) with tetradentate Schiff-base ligands incorporating pyrazole has been prepared in order to compare the influence on the nickel(II) centre of the donor atoms in N₂(NH)₂ and N₂S₂ complexes versus N₂O₂ complexes. In each case, two identical parts of the ligand are linked by a two- or three-carbon aliphatic chain (n = 2 or 3). Crystal structures have been determined for a n = 3 N₂(NH)₂ complex, for two n = 3 N₂S₂ complexes and for a n = 2 N₂O₂ complex. All of the complexes have been investigated in solution by spectroscopic methods (UV/VIS/NIR, ¹H NMR). The n = 2 and 3 N₂(NH)₂ complexes and the n = 2 N₂S₂ complexes are fully diamagnetic. An extension of the chain linking the two halves of the ligand from two to three carbon atoms induces a spin-equilibrium process (S = 0 ⇄ S = 1) for the resulting N₂S₂ complex (ΔG = 19–26 kJ mol^–1 at 50 °C). Ligand-field parameters have been derived from the electronic spectra and the electrochemical properties of the N₂S₂ complexes have been investigated by cyclic voltammetry. From these results it is clear that low-spin character is emphasized in N₂(NH)₂ and N₂S₂ complexes of nickel(II) when compared with the corresponding N₂O₂ complexes.