Single-crystal XRD and solid-state NMR structural resolution of a layered fluorinated gallium phosphate: RbGa3(PO4)2(HPO4)F4·C5N2H16·2H2O (MIL-145)

Abstract

A new two-dimensional fluorinated gallium phosphate RbGa₃(PO₄)₂(HPO₄)F₄·C₅N₂H₁₆·2H₂O (MIL-145) has been hydrothermally synthesized (180 °C for 36 h) in the presence of 1,5-diaminopentane and rubidium fluoride. Its structural model has been determined by means of single-crystal X-ray diffraction analysis. The structure contains corrugated infinite ribbons of GaO₃F₃ and GaO₄F₂ octahedra linked through edge- and corner-sharing mode via fluoride anions. These chains are then connected to each other via phosphate groups to create a layered network delimiting 6-ring channels trapping rubidium cations. The inorganic sheets are intercalated by diprotonated 1,5-diaminopentane and water molecules, ensuring the three-dimensional cohesion via hydrogen bond scheme. ¹H, ¹³C, ¹⁵N and ⁸⁷Rb solid-state NMR spectra show the presence of two inequivalent amines as well as two Rb cations, confirming the choice of the space group, which was ambiguous from the diffraction data. ⁷¹Ga NMR spectra, acquired at several magnetic fields, contain two different sets of Ga signals, corresponding to the two types of gallium environments in the structure. One-dimensional ¹⁹F and ³¹P and ¹⁹F–³¹P two-dimensional NMR experiments have been recorded, which are in full agreement with the proposed structural model. Finally, possible assignments of the ¹⁹F and ³¹P resonances to the crystallographic sites in RbGa₃(PO₄)₂(HPO₄)F₄·C₅N₂H₁₆·2H₂O have been determined by comparing adjacency matrices build-up from 2D NMR correlation spectra and from the structural data.