Combining Azide, Carboxylate, and 2-Pyridyloximate Ligands in Transition-Metal Chemistry: Ferromagnetic Ni^II₅ Clusters with a Bowtie Skeleton

The combined use of the anion of phenyl(2-pyridyl)ketone oxime (ppko−) and azides (N3−) in nickel(II) carboxylate chemistry has afforded two new NiII5 clusters, [Ni5(O2CR′)2(N3)4(ppko)4(MeOH)4] [R′ = H (1), Me (2)]. The structurally unprecedented {Ni5(μ-N3)2(μ3-N3)2}6+ cores of the two clusters are almost identical and contain the five NiII atoms in a bowtie topology. Two N3− ions are end-on doubly bridging and the other two ions end-on triply bridging. The end-on μ3-N3− groups link the central NiII atoms with the two peripheral metal ions on either side of the molecule, while the Ni···Ni bases of the triangles are each bridged by one end-on μ-N3− group. Variable-temperature, solid-state direct- (dc) and alternating-current (ac) magnetic susceptibility, and magnetization studies at 2.0 K were carried out on both complexes. The data indicate an overall ferromagnetic behavior and an S = 5 ground state for both compounds. The ac susceptibility studies on 1 reveal nonzero, frequency-dependent out-of-phase (χM′′) signals at temperatures below ∼3.5 K; complex 2 reveals no χM′′ signals. However, single-crystal magnetization versus dc field scans at variable temperatures and variable sweep rates down to 0.04 K on 1 reveal no noticeable hysteresis loops, except very minor ones at 0.04 K assignable to weak intermolecular interactions propagated by nonclassical hydrogen bonds.