Transformations of low-dimensional zinc phosphates to complex open-framework structures. Part 1: zero-dimensional to one-, two-and three-dimensional structures

Abstract

Zero-dimensional 4-membered zinc phosphate monomers, [C₆N₂H₁₈][Zn(HPO₄)(H₂PO₄ )₂], I, and [C₆N₄H₂₁][Zn(HPO₄)₂(H₂PO₄)], II, transform under simple reaction conditions to one-, two- and threedimensional structures. Monomer II, on heating with zinc acetate dihydrate (Zn(OAc)₂) in aqueous solution, gives a layered phosphate [C₆N₄H₂₁][NH₄][Zn₆(PO₄ )₄(HPO₄)₂]H₂O, III. A novel three-dimensional structure [C₆N₄H₂₁]₄[Zn₇(PO₄)₆]₃, IV, with channels comprising Zn₇O₆ clusters is obtained on heating II in water under hydrothermal conditions. The monomer I transforms to a one-dimensional ladder, [C₃N₂H₅][Zn(HPO₄)], V, on heating with imidazole and to a three-dimensional structure, [C₄N₂H₁₂][Zn₂(H₂O)(PO₄ )(HPO₄)]₂, VI, on heating with piperazine under ordinary conditions. I also transforms to a layered zinc phosphate, [C₆N₂H₁₈][Zn₃(H₂O)₄(HPO₄)₄], VII, on heating in water. In addition to the monomer, II, compounds III-VI have been obtained for the first time. The structures formed by the transformations of the monomers also exhibit unique structural features. Thus, in the ladder structure, V, the imidazole molecule is linked to the Zn center similar to the phosphate unit in a typical ladder structure, while in the layered phosphate, III, onedimensional tubules are linked via ZnO₄ tetrahedra and the three-dimensional structure, IV, possesses Zn₇O₆ clusters. Isolation of several related solids encompassing a variety of architectures through the transformations of zero-dimensional monomeric phosphates demonstrates not only that the 4-membered ring is a basic structural building unit in these open-framework materials, but also sheds light on the building-up process involved in their formation.