Trigonal bipyramid Ni₃Cl₃O₂ cluster showing the high-spin S = 3 ground state with uniaxial magnetic anisotropy

We have prepared a novel nickel(II) trinuclear compound [Ni₃(tmen)₃(μ₂-Cl)₃(μ₃-OMe)(μ₃-OH)](BPh₄)·0.5MeOH·0.5CH₂Cl₂ (1; tmen: N,N,N′,N′-tetramethylethylenediamine) with a trigonal bipyramid molecular structure, where the basal plane consists of Ni₃(μ₂-Cl)₃ atoms. Two known compounds [Ni₃(tmen)₃X₄(OH)]X (X = Cl (2), Br (3)) were also investigated. Magnetic study clarified the ground high-spin (S = 3) state, and the intramolecular exchange coupling parameters were determined as 2J/k_B = 18.0 ± 0.2, 27.2 ± 0.2, and 19.6 ± 0.2K for 1, 2, and 3, respectively. Crystallographic analysis clarified the unique geometry of 1. The molecular axes are aligned almost parallel in the crystallographic b axis. Relatively short intermolecular Cl⋯Cl distances (3.725(1) and 3.847(2)Å) are found among linearly arrayed complex ions. A single-crystal magnetic study gave a zero-field-splitting parameter DS=3/k_B = -2.2 ± 0.1 K. An intermolecular exchange coupling parameter was determined as 2j/k_B = -0.15 ± 0.01K from simulation. A pulsed-field magnetization study at 0.5K for 1 showed a jump around 1.2 T, which is explained in terms of intermolecular interaction. Alternating current magnetic susceptibility measurements revealed the Arrhenius behavior with E_a = 10.1 ± 0.8K at zero bias field. Therefore, the magnetic properties of 1 can be understood as an antiferromagnetic chain made of potential single-molecule magnets.