Ferromagnetically coupled tetranuclear Ni(ii)-2-oxy-aceto- or benzo-phenonate complexes

Abstract

Reaction of 2-hydroxy-acetophenone (HL) or 2-hydroxy-benzophenone (HL′) with nickel(II) acetate provides the tetrakis-[(μ₃-methanolato-κ³O:O:O)(methanol-κO)(2-oxyacetophenone-κ²O,O′)nickel(II)], [Ni(L)(μ₃-CH₃O)(CH₃OH)]₄ (1) or tetrakis-[(μ₃-methanolato-κ³O:O:O)(aqua-κO)(2-oxybenzophenone-κ²O,O′)nickel(II)] monohydrate, [Ni(L′)(μ₃-CH₃O)(H₂O)]₄·H₂O (2). Molecular structure determination demonstrates each nickel(II) ion is six-coordinate with a distorted octahedral geometry defined by three oxygen atoms from three methoxide fragments, a methanol (1) or water (2) molecule, and two oxygen atoms from the acetophenonate (L⁻) or benzophenonate (L′⁻) ligand, such that the four nickel atoms and four methoxide groups represent a cubane-type structural topology with each methoxide fragment bridging three of the metal centers. Stabilization of the cubane core occurs via intramolecular O–H⋯O hydrogen bonds. Solid-state magnetic measurements along with computational modeling confirm dominant ferromagnetic interactions for the compounds, attributed to their cubane topology and the Ni–O–Ni angles adopting values lower than 100°. Thermogravimetric analysis (TGA) suggests thermal decomposition of the complexes with successive release of the lattice water, coordinated solvents (MeOH or H₂O), OCH₃ groups and fragmented ligand species, supported by differential scanning calorimetry (DSC) studies. Cyclic voltammetry reveals a quasi-reversible two electrons charge transfer process in N,N-dimethylformamide.