Nickel(II) N2O2 Schiff-base complexes incorporating pyrazole: syntheses, characterization and acidity of the metal centre towards co-ordinating solvents

Abstract

A series of four-co-ordinate nickel(II) complexes have been prepared containing a tetradentate N₂O₂ aldimine or ketoimine Schiff-base ligand formed by condensation of 4-formyl-5-hydroxy- or 4-benzoyl-5-hydroxypyrazoles with aliphatic diamines containing two or three carbon atoms (n= 2 or 3). In non-donor solvents the low-spin (S= 0) state is favoured. The complexes with n= 2 are fully diamagnetic; the spin-equilibrium process (S= 0)⇌(S= 1) is, however, induced when n= 3. The crystal structure has been determined of {N,N′-bis[(5-hydroxy-1,3-diphenylpyrazolyl)phenylmethylene]propane-1,3-diaminato}nickel(II)1a which has a flattened tetrahedral geometry, and the spin-equilibrium process has been investigated for selected complexes by spectroscopic methods [ΔH 16–21 kJ mol^–1, ΔS 2–22 J K^–1 mol^–1, and ΔG(50 °C) 14–19 kJ mol^–1]. The four-co-ordinate complexes reversibly increase their co-ordination numbers in donor solvents (all) or by molecular association in non-donor solvents (the aldimine complexes only). Solid six-co-ordinate products from the reactions with dimethylsulfoxide (dmso), pyridine and water have been isolated, and the crystal structures determined of the product (1b) from the reaction of 1a with dmso and of that (3b) from the reaction of {N,N′-bis[(5-hydroxy-1,3-diphenylpyrazolyl)(p-tolyl)methylene]propane-1,3-diaminato}nickel(II) with dmso. Complexes 1b and 3b have trans distorted-octahedral geometries with O-bound dmso molecules in axial positions. The adduct formation with dmso [–ΔH 10–27 kJ mol^–1, –ΔS 57–107 J K^–1 mol^–1, ΔG(25 °C) 0.9–7.0 kJ mol^–1] and with pyridine [ΔG(25 °C)– 19 kJ mol^–1] has been investigated for selected complexes by spectroscopic methods. Angular overlap model ligand-field parameters have been derived from the electronic absorption spectra.