A SYNCHROTRON SINGLE-CRYSTAL X-RAY STRUCTURE DETERMINATION OF (NH4)3MO4P3O16 - A MICROPOROUS MOLYBDENUM PHOSPHATE WITH MO4O4(6+) CUBES

Reaction of MoO3, Mo, (NH4)2HPO4, H3PO4, and H2O in a mole ratio of 1.4:1:3.6:6:120 at 360°C for 16 hr gives a nearly quantitative yield of black cubes of (NH4)3Mo4P3O16 (1). The structure of (1) was solved from data collected on a 30 × 30 × 30 μm3 crystal at the National Synchrotron Light Source at Brookhaven National Laboratory. The compound is cubic, space group P43m, with a = 7.736(2) Å, and was refined to residuals of R(Rw) = 0.035(0.049). Phosphate (1) is isotypic with Cs3Mo4P3O16 and is related to the iron arsenate mineral pharmacosiderite. Unlike the Cs+ compound, (1) can be rendered microporous by thermal removal of the NH+4 cations to give ammonia with the charge compensating proton remaining behind in the lattice. Water absorption isotherms show the reversible uptake of 5.6 wt% water, which corresponds to over 15 vol% void space in (1) after the NH3 removal. The framework consists of Mo4O6+4 cubes, with six MoMo contacts of 2.570(4) Å, joined together together by (PO4) 6 2 along 〈100〉 to form a 3-D network composed of tetramers of triply edge-sharing Mo-centered octahedra and phosphate groups alternating along all 〈100〉 directions. The windows and cavities in (1) are large enough that the NH+4 cations occupy several different positions in the unit cell. © 1991.