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⁻

Abstract

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.