Synthesis, characterization and magnetic properties of halogenated tetranuclear cubane-like nickel(ii) complexes

Abstract

Four Ni₄O₄ cubanes with formula [Ni₄(Cl₂-L)₄(μ₃-OMe)₄(MeOH)₄] (1), [Ni₄(Br₂-L)₄(μ₃-OMe)₄(MeOH)₄] (2), [Ni₄(I₂-L)₄(μ₃-OMe)₄(MeOH)₄] (3) and [Ni₄(BrCl-L)₄(μ₃-OMe)₄(MeOH)₄] (4) (Cl₂-LH = 3,5-dichlorosalicylaldehyde, Br₂-LH = 3,5-dibromosalicylaldehyde, I₂-LH = 3,5-diiodosalicylaldehyde, BrCl-LH = 3-bromo-5-chlorosalicylaldehyde) were obtained by self-assembly from commercially available 3,5-halogen-substituted salicylaldehydes and NiCl₂·6H₂O in high yields at room temperature. The X-ray crystal structure analysis of 4 showed a cubane-type structural topology as each Ni(II) ion is six-coordinated in a distorted octahedral geometry defined by four oxygen atoms from the MeOH molecule (three as bridging methoxy-oxygens and one as terminal group) and two oxygen atoms from the 3-bromo-5-chlorosalicylaldehyde ligands. The cubane core is stabilized via an intramolecular O–H⋯O hydrogen bond, while in the crystal, complex molecules are linked into the zigzag chain to the a-axis via strong Br⋯Br interaction. Both SQUID and μSQUID measurements indicate the coexistence of ferromagnetic and antiferromagnetic interactions in two complexes. The fitting of the magnetic data yields different magnetic parameters for each complex, clearly highlighting the differences in the magnetic properties among these systems. DFT calculations have also been performed on compounds 1–4 to explain the exchange interaction mechanisms between the Ni(II) ions, supporting both the magnitude and sign of the coupling constants.