Chiral open-framework uranyl molybdates. 3. Synthesis, structure and the C2221 → P212121 low-temperature phase transition of [C6H16N]2[(UO2)6(MoO4)7(H2O)2](H2O)2

A new chiral 6:7 open-framework uranyl molybdate, [C6H16N]2[(UO2)6(MoO4)7(H2O)2](H2O)2, has been synthesized by hydrothermal methods. The structure has been refined using single-crystal X-ray diffraction data collected at 20 and −127 °C. The 20 °C structure [orthorhombic, C2221, a = 11.3045(14), b = 19.962(6), c = 24.416(5) Å, V = 5510(2) Å3] has been refined to R1 = 0.046 on the basis of 6093 unique observed reflections. The −127 °C structure [orthorhombic, P212121, a = 11.211(4), b = 19.880(10), c = 24.421(8) Å, V = 5443(4) Å3] has been refined to R1 = 0.047 on the basis of 6951 unique observed reflections. The structures are based upon topologically identical frameworks of corner-sharing UO7 pentagonal bipyramids and MoO4 tetrahedra. The extra-framework H2O groups and protonated triethylamine molecules reside in the framework cavities. In the C2221 structure at 20 °C, H2O and [C6H16N]+ molecules filling the chiral channels along [0 0 1] are disordered, whereas, in the P212121 structure at −127 °C, they are perfectly ordered. The symmetry difference between structures at 20 °C and −127 °C is the result of a C2221 → P212121 second order phase transition that involves ordering of extra-framework protonated amine molecules and H2O groups, and distortion of the flexible [(UO2)6(MoO4)7(H2O)2]2− uranyl molybdate framework. On the basis of measurements of intensities of reflections that violate absence conditions of C-centering cell, the temperature of the phase transition is estimated as −11 °C.