Octanuclear nickel phosphonate core forming extended and molecular structures

Abstract

Three new nickel phosphonate complexes {[Na₂Ni₈(L)₆]·nSolv}_m (L = SAA³⁻ (1), BSAA³⁻ (2), NAA³⁻ (3); Solv = H₂O, MeOH; m = ∞ (1, 2), 1 (3)) were synthesized. All three complexes possess a novel octanuclear {Ni₈} phosphonate core, which consists of four dinuclear doubly oxygen-bridged units, further interconnected to each other by phosphonate oxygen bridges. The steric features of the ligands influence the aggregation degree. Molecules of 1 and 2 are interconnected by sodium cations into 2D layered and 1D chain extended structures, respectively, while the molecules of 3 with the bulkiest ligand are not bonded with each other. Magnetic properties of the obtained {Ni₈} core unit were studied for complex 1 as a representative of this family of compounds and are reported in detail. Magnetic susceptibility at low temperature is indicative of a singlet ground state. The absence of saturation and the magnetization behavior points to zero-field splitting (ZFS). Simulation of the magnetization data revealed an easy-plane magnetic anisotropy with an axial ZFS parameter D = 7.4 cm⁻¹. The magnetic properties of 1 were also studied by broken-symmetry DFT calculations (BS-DFT), which revealed the presence of ferromagnetic exchange interactions within the dinuclear units of the {Ni₈} core with a dominant antiferromagnetic interaction between these dinuclear entities. These results are in good agreement with coupling constants derived from the experimental susceptibility and magnetization data.