Variation of nuclearity in NiII complexes of a Schiff base ligand: crystal structures and magnetic studies

Abstract

Four new NiII complexes, [Ni(L1)](NO3)·1.5H2O (1), [Ni(L1)][Ni(HL2)](NO3)2·H2O (2), [Ni4(L2)2(μ3-OCH3)2(NO3)](NO3)·CH3OH (3), and [Ni6(L2)2(o-van)2(μ3-OH)4](NO3)2·H2O (4), where HL1 = 2-{[2-(2-aminoethylamino)ethylimino]methyl}-6-methoxyphenol and H2L2 = N,N′-bis(3-methoxysalicylidene)diethylenetriamine, have been synthesized by changing the reaction conditions and stoichiometry of the reactants. Single-crystal X-ray diffraction study reveals that complexes 1 and 2 are mononuclear with a square planar geometry around the NiII centre, whereas complexes 3 and 4 possess tetranuclear and hexanuclear structures, respectively, with octahedral NiII centres. The differences in nuclearity, coordination numbers and geometry of the NiII centres in the complexes have been explained considering the flexible coordination modes and denticity of the Schiff base ligands. Variable temperature molar magnetic susceptibility measurements show that the tetranuclear NiII4 complex (3) is ferromagnetically coupled with two exchange couplings, J1 = 0.64 cm−1 and J2 = 8.41 cm−1, whereas the hexanuclear NiII6 complex (4) is antiferromagnetically coupled with four exchange couplings, J1 = −5.60 cm−1, J2 = −9.39 cm−1, J3 = −5.18 cm−1 and J4 = 3.72 cm−1. The sign and magnitude of these exchange couplings in complexes 3 and 4 are correlated with similar types of reported cubane or defective cubane-like NiII complexes. Electrochemical measurements of these complexes reveal that the NiII centres are irreversibly reduced with one reduction potential for 1, 3 and 4, whereas for 2 the NiII centres are irreversibly reduced with two reduction potentials.