Giant Voids in the Hydrothermally Synthesized Microporous Square Pyramidal−Tetrahedral Framework Vanadium Phosphates [HN(CH2CH2)3NH]K1.35[V5O9(PO4)2]·xH2O and Cs3[V5O9(PO4)2]·xH2O
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Abstract
The use of a novel mixed-valence pentavanadate phosphate cluster as a building block has made possible the low-temperature hydrothermal self-assembly of the new three-dimensional square pyramidal−tetrahedral framework vanadium phosphates [HN(CH2CH2)3NH]K1.35[V5O9(PO4)2]·xH2O (1) and Cs3[V5O9(PO4)2]·xH2O (2), from structurally simple starting materials. These materials possess some of the lowest framework densities and the largest cavities thus far observed in open-framework solid-state materials. The degree of curvature, size, shape, and charge of the mixed valence {V5O9(PO4)2} V(4+/5+) pentameric building block, which resembles a portion of an arc of a circle, favors the formation of very large cavities. Phosphate 1 crystallizes in space group I4̄3m with a = 26.247(3) Å and has very large cubic-shaped cavities that display 4̄3m point symmetry, each of which enclose nearly 50 positive charges. These charges are distributed among 12 HN(CH2CH2)3NH2+, 32 K+ cations, and several waters of crystallization. Each cavity, which could contain a sphere of approximately 13.0 Å diameter and which displays an enormous 32-ring at its maximum diameter, communicates via six 16-ring windows to other similar supercages via intervening smaller cavities. Phosphate 2 is built up from the same pentamers as those in 1 but arranged in a different manner in space group Fd3̄m with a = 32.306(4) Å such that one pentamer lies on each of the six faces of a cube. The three-dimensional structure of 2 consists of two types of cavity, larger ones with all six pentamers curved in an outward fashion, with a free diameter along the diagonal of the cubic-shaped cavity of approximately 20 Å and exhibiting a 24-ring diameter, and smaller ones with all six curved to the inside with cavity diagonals of 7.5 Å. The large cavities are interconnected to one another in a manner that is topologically identical to the arrangement of the carbon atoms in diamond. The ion exchange and sorption properties of these two phosphates are also to be presented.
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