Topological description of a 3D self-catenated nickel hybrid vanadate Ni(bpe)(VO3)2. Thermal stability, spectroscopic and magnetic properties

Abstract

The three-dimensional Ni(bpe)(VO₃)₂ hybrid compound, where bpe is 1,2-di(4-pyridyl)ethene, (C₁₂H₁₀N₂), has been synthesized using mild hydrothermal conditions under autogeneous pressure at 140 °C during five days, obtaining green emerald prismatic single-crystals suitable for X-ray structure determination. The compound crystallizes in the orthorhombic system, space groupPbcn, with a = 14.9066(5), b = 7.6269(2) and c = 26.9624(10) Å, Z = 8, and R1 = 0.0373 for 4149 observed reflections. Single-crystal X-ray diffraction reveals that the crystal structure is composed of a 3D self-catenated 10-connected uninodal net constructed from 3⁶-hxl like {NiV₂O₆} inorganic sheets linked through bpe ligands. The thermal evolution of the crystal parameters shows three different tendencies during the heating process: (i) initial contraction, (ii) thermal expansion, (iii) structural collapse due to the thermal instability of the organic ligand. The IR spectrum shows the vibrational modes of the bpe organic molecules and those of the (VO₄)³⁻ tetrahedral oxoanions. Diffuse reflectance electronic absorption spectroscopy shows the characteristic bands of the Ni(II) d⁸-high spin cation in slightly distorted octahedral geometry. From the positions of the bands in the electronic spectrum the Dq (940 cm⁻¹) and Racah, B (930 cm⁻¹) and C (3350 cm⁻¹), parameters have been calculated. Magnetic measurements indicate the existence of antiferromagnetic interactions between the Ni(II) centres of the dinuclear units, with a value of the J/k = −59.4, with g = 2.076.