Unusual Structural Types in Nickel Cluster Chemistry from the Use of Pyridyl Oximes: Ni₅, Ni₁₂Na₂, and Ni₁₄ Clusters

Syntheses, crystal structures, and magnetochemical characterization are reported for the three new nickel(II) clusters [Ni₁₄(OH)₄(N₃)₈(pao)₁₄(paoH)₂(H₂O)₂](ClO₄)₂ (1), [Ni₁₂Na₂(OH)₄(N₃)₈(pao)₁₂(H₂O)₁₀](OH)₂ (2), and [Ni₅(ppko)₅(H₂O)₇](NO₃)₅ (3) (paoH = pyridine-2-carbaldehyde oxime, ppkoH = di-2-pyridyl ketone oxime). The reaction of Ni(ClO₄)₂·6H₂O with paoH and NBuⁿ₄N₃ in H₂O/MeCN in the presence of NEt₃, gave 1 in 65% yield. Complex 2 was obtained in 60% yield from the reaction of NiCl₂·6H₂O with paoH and NaN₃ in H₂O/MeCN in the presence of NaOH. The reaction of Ni(NO₃)₂·6H₂O with ppkoH in EtOH in the presence of LiOH afforded complex 3 in 75% yield. The complexes all contain novel core topologies. The core of 1 comprises a central Ni₄ rhombus between two Ni₅. Complex 1 is the largest metal/oxime cluster discovered to date, as well as the first Ni^II₁₄ coordination complex and the largest Ni^II/N₃⁻ cluster. Complex 2 has a Ni₁₂Na₂ topology that is very similar to that of 1, but with two central Ni^II atoms of 1 replaced with Na^I atoms. The core of 3 consists of four Ni^II atoms forming a highly distorted tetrahedron, with the fifth Ni^II atom lying almost on one of the edges. Variable-temperature, solid-state dc susceptibility and magnetization studies were carried out on complexes 1−3, and these were complemented with ac susceptibility data for 1 and 2. Fitting of the obtained M/(Nμ_B) vs H/T data by matrix diagonalization and including axial zero-field splitting (D) gave ground-state spin (S) and D values of S = 6, D = −0.12(3) cm⁻¹ for 1 and S = 3, D = −0.20(5) cm⁻¹ for each of the two essentially noninteracting S = 3 Ni₆ subunits of 2. The data for 3 indicate antiferromagnetic exchange interactions and an S = 1 ground state. A simple 2-J model was found to be adequate to describe the variable-temperature dc susceptibility data. The combined work demonstrates the ligating flexibility of pao⁻ and ppko⁻, and their usefulness in the synthesis of polynuclear Ni_x clusters with or without the presence of ancillary ligands.