Observation of Multiple Crystal-to-Crystal Transitions in a New Reduced Vanadium Oxalate Hybrid Material, Ba3[(VO)2(C2O4)5(H2O)6]·(H2O)3, Prepared via a Mild, Two-Step Hydrothermal Method

A new reduced vanadium oxalate, Ba3[(VO)2(C2O4)5­(H2O)6]­·(H2O)3, 1, was synthesized via a two-step, mild hydrothermal method using oxalic acid as the reducing agent. This compound undergoes multiple single-crystal to single-crystal phase transitions as a function of water content to form Ba3[(VO)2(C2O4)5­(H2O)4]·(H2O)2, 2, and Ba3[(VO)2(C2O4)5­(H2O)2], 3, after heating 1 to 50 and 100 °C, respectively. In addition, exposing 1 to a vacuum for 12 h also transforms it into 2, but not 3. All three compositional variants were structurally characterized by single crystal X-ray diffraction and are found to crystallize in the monoclinic space groups of C2/c and P21/c, for 1 and 2, and 3, respectively. Vanadium in these structures retains the +4 oxidation state, indicating exceptional crystal stability. The magnetic susceptibility of these compounds was measured, and all three materials were found to be paramagnetic down to 2 K. Compound 1 was further characterized by IR, UV-vis spectroscopy, and thermal analysis.