Phase transition from disordered high-temperature to twinned low-temperature structure of tris(ethylenediamine)zinc(II) selenate

The tris(ethylenediamine)zinc(II) selenate, [Zn(C2H8N2)3](SeO4), has been successfully synthesized and crystallographically characterized. It crystallizes at 293 K in the non-centrosymmetric space group P31c of the trigonal system with the following unit cell parameters: a = 9.1137(1) A, c = 9.8369(2) A, V = 707.58(2) A3, and Z = 2. Its crystal structure consists of zinc(II) atom coordinated by three ethylenediamine molecules in N-octahedral geometry and dynamically disordered selenate anion linked together by hydrogen bonds. The title complex undergoes a reversible phase transition at about 241 K detected by DSC. Below this temperature, the structure adopts the non-centrosymmetric space group P3 with the following unit cell parameters: a = 9.0260(2) A, c = 9.9010(2) A, V = 698.55(3) A, and Z = 2. The resulting main change undergoing below the transition temperature is a full ordering of selenate anion due to strengthening of hydrogen bonds.