Solvent-induced structural diversity in tetranuclear Ni(ii) Schiff-base complexes: the first Ni4 single-molecule magnet with a defective dicubane-like topology

Abstract

Two tetranuclear Ni^II complexes, namely [Ni₄(L)₄(CH₃OH)₃(H₂O)]·CH₃OH (1) and (Pr₃NH)₂[Ni₄(L)₄(CH₃COO)₂] (2, Pr₃N = tripropylamine), were synthesized from a tridentate Schiff base ligand H₂L (2-[(E)-(2-hydroxybenzylidene)amino]phenol) and Ni(CH₃COO)₂·4H₂O, using different solvents and their ratios (CH₃OH and/or CH₂Cl₂). The prepared Ni₄ complexes are of different structural types, involving an Ni₄O₄ cubane-like core (1) and Ni₄O₆ defective dicubane-like core (2), with all the Ni atoms hexacoordinated. The complexes were characterized by elemental analysis, FT-IR spectroscopy, variable temperature and field magnetic measurements, and single crystal X-ray analysis. The DFT and CASSCF/NEVPT2 theoretical calculations were utilized to reveal information about the isotropic exchange parameters (J_ij) and single-ion zero-field splitting parameters (D_i, E_i). The variable temperature magnetic data suggested the competition of the antiferromagnetic and ferromagnetic intracluster interactions in compound 1, which is in contrast to compound 2, where all intracluster interactions are ferromagnetic resulting in the ground spin state S = 4 with an easy-axis type of anisotropy quantified by the axial zero-field splitting parameter D = −0.81 cm⁻¹. This resulted in the observation of a field-induced slow-relaxation of magnetization (U = 3.3–6.7 K), which means that the complex 2 represents the first Ni₄ single-molecule magnet with the defective dicubane-like topology.