To cite this article: J. Černák, N. Farkašová, M. Tomás, V. Kavečanský, E. Čižmár & M. Orendáč (2015): [Ni(bpy)(ox)]: a candidate in the class of Haldane gap systems (bpy = 2,2ʹ-bipyridine, ox = oxalate), Journal of Coordination Chemistry,[Ni(bpy)(ox)] (bpy = 2,2ʹ-bipyridine; ox = oxalate) was solvothermally prepared in the microcrystalline form and identified by chemical analyses and IR spectroscopy. X-ray powder diffractometry indicates its isostructural character with analogous complexes [M(bpy)(ox)], and thus, its chain-like crystal structure formed of {Ni(bpy)} building units linked by bridging oxalate anions coordinated in a bis(chelate) fashion. The temperature dependence of the magnetic susceptibility and the field dependence of the magnetization reveal that the studied compound belongs to the class of systems with a Haldane gap. The estimated magnitude of the gap E g ≈ 13.4 cm −1 is comparable with those found in archetypal Haldane gap systems.

The reaction of NiCl2, K2C2O4·H2O and 2,2'-bipyridine (bpy) in water-ethanol solution at 281 K yields light-purple needles of the new pentahydrate of bis(2,2'-bipyridine)oxalatonickel(II), [Ni(C2O4)(C10H8N2)2]·5H2O or [Ni(ox)(bpy)2]·5H2O, while at room temperature, deep-pink prisms of the previously reported tetrahydrate [Ni(ox)(bpy)2]·4H2O [Román, Luque, Guzmán-Miralles & Beitia (1995), Polyhedron, 14, 2863-2869] were gathered. The asymmetric unit in the crystal structure of the new pentahydrate incorporates the discrete molecular complex [Ni(ox)(bpy)2] and five solvent water molecules. Within the complex molecule, all three ligands are bonded as chelates. The complex molecules are involved in an extended system of hydrogen bonds with the solvent water molecules. Additionally, π-π interactions also contribute to the stabilization of the extended structure. The dehydration of the pentahydrate starts at 323 K and proceeds in at least two steps as determined by thermal analysis.

The title hydrated ionic complex, [Ni(CH3COO)(C12H12N2)2]ClO4·H2O or [Ni(ac)(5,5'-dmbpy)2]ClO4·H2O (where 5,5'-dmbpy is 5,5'-dimethyl-2,2'-bipyridine and ac is acetate), (1), was isolated as violet crystals from the aqueous ethanolic nickel acetate-5,5'-dmbpy-KClO4 system. Within the complex cation, the Ni(II) atom is hexacoordinated by two chelating 5,5'-dmbpy ligands and one chelating ac ligand. The mean Ni-N and Ni-O bond lengths are 2.0628 (17) and 2.1341 (15) Å, respectively. The water solvent molecule is disordered over two partially occupied positions and links two complex cations and two perchlorate anions into hydrogen-bonded centrosymmetric dimers, which are further connected by π-π interactions. The magnetic properties of (1) at low temperatures are governed by the action of single-ion anisotropy, D, which arises from the reduced local symmetry of the cis-NiO2N4 chromophore. The fitting of the variable-temperature magnetic data (2-300 K) gives g(iso) = 2.134 and D/hc = 3.13 cm(-1).