MIL-50, an Open-Framework GaPO with a Periodic Pattern of Small Water Ponds and Dry Rubidium Atoms: a Combined XRD, NMR, and Computational Study

A new fluorinated gallium phosphate, MIL-50, has been synthesized under mild hydrothermal
conditions using 1,6-diaminohexane. The chemical formula of MIL-50 is Rb2Ga9(PO4)8(HPO4)(OH)F6·2N2C6H18·7H2O. The structure is a network of hexameric units of Ga3(PO4)3F2 and Ga3(PO4)2(HPO4)F3 via
corner sharing. It creates a three-dimensional open-framework delimiting 6- and 18-ring channels running
along the c axis. The diprotonated 1,6-diaminohexane and water molecules are trapped within the 18-ring
pores, whereas the rubidium cations reside in the 6-ring ones. A double quantum 31P NMR experiment
and partial charge calculations indicate that water molecules are present under the form of periodic small
clusters, lowering the multiplicity of one phosphorus site, P3. Though water hops within the clusters, the
motion leaves the water pattern periodic. Rubidium is so tightly embedded into the framework that water
moving in the large 18-ring channels does not reach it, leaving it therefore dry. The crystal framework may
be ascribed to the orthorhombic space group Cmc21 (n° 36), a = 32.1510(2), b = 17.2290(3), c = 10.2120(1)
Å. The periodic water pattern has a different symmetry than that of the framework. A method has been
devised to superpose the two sublattices that coexist in the same unit cell in order to have full occupancy
of each site and to perform Madelung summations. This original method is of general interest for most
zeolitic materials exhibiting a different symmetry for the framework and the template sublattices.