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

(C6N2H16)(0.5)[(VO)(HAsO4)F] 1 has been synthesized using mild hydrothermal conditions under autogenous pressure. Above 70 degrees 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) angstrom b = 7.2717(4) angstrom c = 10.396(1) angstrom beta = 104.290(8)degrees for compound 1 and a = 23.025(1) angstrom, b = 7.322(1) angstrom, c = 10.344(1) angstrom and beta = 109.250(6)degrees 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 degrees C, whereas the limit of thermal stability for 2 is approximately 150 degrees C. Both phases coexist in the temperature range from 80 to -15 degrees 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 degrees C. The diffuse reflectance spectrum of 1 confirms the presence of vanadyl ions, in which the vanadium(IV) cations have a d(1) 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.