(C 5 H 14 N 2 )[Zn 3 (HPO 4 ) 4 ], a New 3D Open‐framework Zincophosphate Synthesized by Hydrothermal Reversible Transformation of (C 5 H 14 N 2 )[Zn 2 (HPO 4 ) 3 ]·H 2 O

In the system ZnO/H 3 PO 4 /H 2 O/1,4‐diazacycloheptane (C 5 H 12 N 2 ), a new zincophosphate (ZnPO), (C 5 H 14 N 2 )[Zn 3 (HPO 4 ) 4 ] ( I ), was prepared by hydrothermal transformation (180 °C) of the known ZnPO hydrate (C 5 H 14 N 2 )[Zn 2 (HPO 4 ) 3 ]·H 2 O ( II ). The thermally‐induced transformation is reversible; upon keeping the heterogeneous mixture of I and mother liquor at 80 °C recrystallization of II was observed. Single‐crystal X‐ray crystallography revealed that I possesses a unique three‐dimensional (3D) open‐framework structure built from corner‐linked ZnO 4 and HPO 4 tetrahedra. The (3,4)‐connected framework of I differs considerably from the 3D open‐framework ZnPO structure of II . Crystal data for I : Monoclinic system, space group Cc (No. 9) , Z = 4, a = 9.1389(6), b = 23.627(2), c = 9.3073(6) Å, β = 109.463(7)°, T = 298 K.