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, Ba 3 [(VO) 2 (C 2 O 4 ) 5 (H 2 O) 6 ]·(H 2 O) 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 Ba 3 [(VO) 2 (C 2 O 4 ) 5 (H 2 O) 4 ]·(H 2 O) 2, 2, and Ba 3 [(VO) 2 (C 2 O 4 ) 5 (H 2 O) 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 C 2/ c and P 2 1 / 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.