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

Abstract

Four new Ni^II 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 Ni^II centre, whereas complexes 3 and 4 possess tetranuclear and hexanuclear structures, respectively, with octahedral Ni^II centres. The differences in nuclearity, coordination numbers and geometry of the Ni^II 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 Ni^II₄ complex (3) is ferromagnetically coupled with two exchange couplings, J₁ = 0.64 cm⁻¹ and J₂ = 8.41 cm⁻¹, whereas the hexanuclear Ni^II₆ 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 Ni^II complexes. Electrochemical measurements of these complexes reveal that the Ni^II centres are irreversibly reduced with one reduction potential for 1, 3 and 4, whereas for 2 the Ni^II centres are irreversibly reduced with two reduction potentials.