{Co(HBpe)2}(V4O12): pedal motion induced order–disorder P → C transition and disrupted C → C2/m displacive transition due to thermal instability

Abstract

The one dimensional {Co(HBpe)₂}(V₄O₁₂) inorganic–organic compound, where Bpe is 1,2-di(4-pyridyl)ethene (C₁₂H₁₀N₂), has been synthesized using mild hydrothermal conditions under autogenous pressure at 120 °C for three days, obtaining single-crystals suitable for X-ray structure determination. The compound crystallizes in the triclinic system, space group P, with a = 15.4705(3) Å, b = 11.8919(3) Å, c = 7.8490(2) Å, α = 88.252(2)°, β = 95.429(2)° and γ = 92.534(2)° at 100 K. The crystal structure possesses two crystallographically independent units, strong pseudo-symmetry elements, and a unit cell with a possible monoclinic metric at higher temperatures. At low temperature (100–293 K) the crystallographically independent units present perfectly ordered conformers of the HBpe ligand. The thermal evolution of the cell parameters was studied by single X-ray and powder X-ray diffraction, showing a clear anomaly in the 90–165 °C temperature range. The crystal structures determined at different temperatures (100, 293, 438, 503 and 533 K) show that a complete disorder of the HBpe ligand through pedal motion occurs between 90 °C and 165 °C. This dynamical process is completely reversible and gives rise to the P to C order–disorder structural transition. At higher temperatures the crystal structure tends to the monoclinic C2/m symmetric crystal structure, however, the thermal instability disrupts this hypothetical displacive structural transition. To the best of our knowledge this is the second polymeric compound in which the pedal motion has been reported. The spectroscopic and magnetic properties were also determined. The IR-spectrum shows the characteristic bands related to the organic ligand and V₄O₁₂ cycles. The thermal evolution of the magnetic susceptibility is related to magnetically isolated Co(II) centers exhibiting the spin–orbit coupling.