Synthesis and Characterization of a New Family of Thermally Stable Open-Framework Zincophosphate/Arsenate Phases: M3Zn4O(XO4)3·nH2O (M = Na, K, Rb, Li, ...; X = P, As; n = ∼3.5−6). Crystal Structures of Rb3Zn4O(PO4)3·3.5H2O, K3Zn4O(AsO4)3·4H2O, and Na3Zn4O(PO4)3·6H2O

Abstract

The syntheses, representative crystal structures, and some properties of a new family of microporous zincophosphate/arsenate materials, denoted M3Zn4O(XO4)3·nH2O (M = Li, Na, K, Rb, Cs, ...; X = P, As; n = ∼3.5−6), are reported. All these materials are based on a flexible, anionic Zn4O(XO4)3 network built up from vertex-linked tetrahedral ZnO4 and XO4 units. Four ZnO4 units share a vertex, resulting in novel OZn4 centers. The use of mixed four-coordinate anion types (XO43-, O2-) results in a number of interesting structural and stability properties. The M3Zn4O(XO4)3·nH2O network contains only pairs of 3-rings (a spiro-5 unit) and 8-rings, and no 4-rings or 6-rings: it encloses roughly spherical cavities connected by a three-dimensional network of 8-ring channels propagating in the orthogonal [100], [010], and [001] directions (cubic unit-cell axes). Extraframework cations and water molecules occupy these cavities and channels. These phases display typical “zeolitic” dehydration/rehydration and ion-exchange reactions with thermal stabilities up to 600 °C, the highest known for this type of open-framework Zn(P/As)O material. The included guest cation (Na, Rb, K, Li, ...) has a crucial effect on crystal structure, by ordering in and modifying the configuration of the intercavity 8-ring channels. The crystal structures of Rb3Zn4O(PO4)3·3.5H2O, K3Zn4O(AsO4)3·4H2O, and Na3Zn4O(PO4)3·6H2O are presented. Crystal data: Rb3Zn4O(PO4)3·3.5H2O, Mr = 881.40, cubic, space group F4̄3c (No. 219), a = 15.3423(9) Å, V = 3611.4(6) Å3, Z = 8, Rp = 5.12%, Rwp = 6.78%, 27 parameters, 1874 data points (X-ray Rietveld refinement). K3Zn4O(AsO4)3·4H2O, Mr = 883.63, cubic, space group F4̄3c (No. 219), a = 15.4638(2) Å, V = 3697.9(2) Å3, Z = 8, Rp = 8.69%, Rwp = 11.42%, 24 parameters, 2124 data points (X-ray Rietveld refinement). Na3Zn4O(PO4)3·6H2O, Mr = 739.49, trigonal, space group R3c (No. 161), a = 10.7489(3) Å, α = 60.114(2)°, V = 880.4(4) Å3, Z = 2, R = 2.49%, Rw = 2.61%, 180 parameters, 846 observed reflections with I > 3σ(I) (single-crystal data).