Isolation of a Zinc Phosphate Primary Building Unit, [C6N2H18]2+[Zn(HPO4)(H2PO4)2]2−, and Its Transformation to an Open-Framework Phosphate, [C6N2H18]2+[Zn3(H2O)4(HPO4)4]2−

A 4-membered ring zinc phosphate monomer, I, [C6N2H18]2+[Zn(HPO4)(H2PO4)2]2−, has been isolated by the reaction of the Zn2+ with the amine, N,N,N′,N′-tetramethylethylenediamine (TMED) and H3PO4 at room temperature over an extended period of time. It could also be prepared by the reaction of the amine phosphate, [C6N2H18][HPO4]2H2O, with Zn2+ in the temperature range 30–80°C. The phosphate monomer consists of a network of ZnO4, PO2(OH)2, and PO3(OH) tetrahedral units linked by their vertices, forming isolated 4-membered rings that are held by hydrogen bond interactions. Interestingly, I, on mild heating, transform into a layered structure, II, [C6N2H18]2+[Zn3(H2O)4(HPO4)4]2−. The structure of II consists of a network of ZnO4, ZnO2(H2O)4, and PO3(OH) moieties sharing the vertices, forming a layered architecture. The formation of a layered structure from the 4-membered ring monomer suggests that the 4-membered ring is likely to be the primary building unit of the open-framework phosphates.