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

Abstract

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 Rb(2)Ga(9)(PO(4))(8)(HPO(4))(OH)F(6).2N(2)C(6)H(18).7H(2)O. The structure is a network of hexameric units of Ga(3)(PO(4))(3)F(2) and Ga(3)(PO(4))(2)(HPO(4))F(3) 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 (31)P 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 Cmc2(1) (n degrees 36), a = 32.1510(2), b = 17.2290(3), c = 10.2120(1) A. 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.

Is Water Templating Nanoporous Materials?

“…Referring back to Figure 4, we now readily anticipate that the naive pentagonal picture (bottom left) cannot be reached by the H-bond network owing to the presence of the crystalline lattice. A further demonstration of this conclusion is provided by the consideration of the hydrogen-relaxed (H 2 O) 17 water cluster. This new structure was obtained by exactly the same procedure as before, but after removal of all network atoms and after complete removal of translation symmetries (direct space finite summation instead of Madelung summation).…”

“…In the present case, the crystal has deposited its characteristic structural signature in the hydrogen network of the water cluster. Upon removal of the inorganic framework, a very stable (H 2 O) 17 cluster forms by complete reorganization of its H-bond pattern. Owing to its very large bond energy (E HB~À 36 kJ mol À1 ), if such a cluster could form spontaneously it would probably not be quickly destroyed by heating or by dilution with other water molecules so easily.…”

“…A simplex optimization procedure has also been applied to the (H 2 O) 17 water cluster isolated from its crystalline environment. For the 16 outer O atoms, two H atoms have been added, constraining the two OÀH bond lengths at 97 pm and the H-O-H bond angle to 1058.…”

“…As explained in the Method section, Figure 2 displays the distribution of H-bond angles and H-bond energies for two clusters sharing the same O 17 backbone. The first (H 2 O) 17 geometry was obtained when considering the cluster placed inside a crystalline lattice displaying the P4 ≈ 3n space group symmetry. As far as H-bond angles are concerned, it is characterized by a striking bimodal distribution, with a first set characterized by q(HÀO¥¥¥O) < 58 and a second set displaying much wider values q(HÀO¥¥¥O) % 508.…”

Mu‐35: A Fluorogallophosphate Obtained by In Situ Generation of the Template

“…The structure could also be described as an arrangement of hexameric units (Ga 3 P 3 ). These hexameric units are common in fluorinated gallium phosphates, as it has been observed in many 2D and 3D structures (ULM-8, [25] MIL-30, [11] MIL-50 [26] ). These units are joined together by two opposite orientations, which then generates the pseudomembered rings described below.…”