Catalytic performance of the high and low temperature polymorphs of (C6N2H16)0.5[(VO)(HAsO4)F]: structural, thermal, spectroscopic and magnetic studies

Abstract

(C₆N₂H₁₆)_0.5[(VO)(HAsO₄)F] 1 has been synthesized using mild hydrothermal conditions under autogenous pressure. Above 70 °C, this phase has a polymorph with the same chemical composition 2 in which the organic 1,4-diamincyclohexane molecule adopts a different conformation. The crystal structures have been solved from single-crystal X-ray diffraction data. The phases crystallize in the C2/c monoclinic space group with the unit-cell parameters a = 21.065(2) Åb = 7.2717(4) Åc = 10.396(1) Åβ = 104.290(8)° for compound 1 and a = 23.025(1) Å, b = 7.322(1) Å, c = 10.344(1) Å and β = 109.250(6)° for compound 2. These phases exhibit a layered inorganic framework, with the template molecule linking the layers via electrostatic interaction and hydrogen bonds. In both phases, the structure is built from secondary building units SBU-4, which are constructed from two [V₂O₈F₂] edge-shared dimeric vanadyl octahedra, connected by the vertices of two hydrogenarsenate (HAsO₄) tetrahedra. The repetition of this SBU unit gives sheets along the [010] direction. Polymorph 1 exists below 70 °C, whereas the limit of thermal stability for 2 is approximately 150 °C. Both phases coexist in the temperature range from 80 to −15 °C. By means of the DSC technique it has been possible to verify that the temperature of the structural transition is between 70 and 100 °C. The diffuse reflectance spectrum of 1 confirms the presence of vanadyl ions, in which the vanadium(IV) cations have a d¹ electronic configuration in a slightly distorted octahedral environment. ESR spectra of both phases are isotropic with mean g values of 1.96 and 1.99 for 1 and 2, respectively. Magnetic measurements for 1 indicate the existence of antiferromagnetic exchange couplings. Both phases are effective and selective catalysts in the oxidation of organic sulfides to sulfoxides and 3,7-dimethylocta-1,6-dien-3-ol.