Transformations of the low-dimensional zinc phosphates to complex open-framework structures. Part 2: one-dimensional ladder to two-and three-dimensional structures

Abstract

Open-framework zinc phosphates with one-dimensional ladder structures are shown to transform, under simple reaction conditions, to two- and three-dimensional structures. Thus, the one-dimensional ladder, [C₆N₄H₂₂]_0.5[Zn(HPO₄)₂ ], I, on heating with piperazine in aqueous solution gives a layer phosphate, [C₄N₂H₁₂][Zn₂(PO₄)₂], III, and the three-dimensional phosphates [C₂N₂H₁₀]_0.5[Zn(PO₄)], IV, [C₆N₄H₂₂]_0.5[Zn₂(PO₄)₂], V and [C₆N₄H₂₁]₄ [Zn₂₁(PO₄)₁₈], VI. On heating in water in the absence of any amine, I transforms to a three-dimensional solid, [C₆N₄H₂₂]_0.5[Zn₃(PO₄)₂(HPO₄)], VII, with 16-membered channels. Of these, III and IV are the only new compounds. The phosphates formed by the transformations of I exhibit unique structural features. Thus, in III, the layers are formed only with 3- and 4-membered rings and have step-like features due to the presence of infinite Zn-O-Zn linkages. Compound IV has a structure similar to that of the naturally occurring aluminosilicate, gismondine, and VI possesses unusual Zn₇O₆ clusters. The ladder zinc phosphate, [C₃N₂H₁₂] [Zn(HPO₄)₂], II, transforms to two layered compounds, [C₃N₂H₁₂][Zn₄(PO₄)₂ (HPO₄)₂], VIII, and [C₃N₂H₁₂] [Zn₂(HPO₄)₃], IX, on heating with zinc acetate and water, respectively. II, on heating in water in the presence of other amines, forms a ladder, [C₃N₂H₅][Zn(HPO₄)], X, and a three-dimensional phosphate, [C₃N₂H₁₂]₂[Zn₅(H₂O)(PO₄)₄(HPO₄)], XI. The syntheses and structures of VIII-XI have already been reported. What is interesting is that the majority of the transformations seem to occur through the process of deprotonation of the phosphoryl group and elimination of the -HPO₄ unit. The transformations of the ladder phosphates to higher dimensional structures reported in the present study not only demonstrate the seminal role of the one-dimensional structures as basic building units, but also the likely occurrence of self-assembly of these one-dimensional units in the building-up process.