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 RbGa(3)(PO(4))(2)(HPO(4))F(4)·C(5)N(2)H(16)·2H(2)O (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 GaO(3)F(3) and GaO(4)F(2) 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. (1)H, (13)C, (15)N and (87)Rb 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. (71)Ga 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 (19)F and (31)P and (19)F-(31)P two-dimensional NMR experiments have been recorded, which are in full agreement with the proposed structural model. Finally, possible assignments of the (19)F and (31)P resonances to the crystallographic sites in RbGa(3)(PO(4))(2)(HPO(4))F(4)·C(5)N(2)H(16)·2H(2)O have been determined by comparing adjacency matrices build-up from 2D NMR correlation spectra and from the structural data.

High-Resolution Structural Characterization of Two Layered Aluminophosphates by Synchrotron Powder Diffraction and NMR Crystallographies

“…By sorting the cations according to their increasing isotropic shifts, the adjacency matrix of 2 has been constructed (Table b) from the correlation peaks extracted of the 27 Al– 31 P 2D MQ- D -R-INEPT NMR spectrum. Taking these two matrices as input data, a homemade program returns all secondary submatrices resulting from permutations of all rows and columns of the Al–P submatrix, that match the two matrices. Exhaustive details about the calculation procedure and hypotheses are given in SI.…”

Spies Within Metal-Organic Frameworks: Investigating Metal Centers Using Solid-State NMR

“…14 WURST pulses afford broadband excitation of powder patterns and may be followed by an echo (WURST-echo) for powder spectra or accompanied by a CPMG echo train (WURST-CPMG) to increase the S/N at the cost of spectral resolution when transverse magnetization relaxation (T 2 ) values are sufficiently long. 15−18 Because of the difficulties involved, SSNMR studies of metal centers in MOFs have been largely limited to select nuclei that display favorable NMR properties, including 27 Al, 19 71 Ga, 20 and 45 Sc. 21 Significantly more challenging nuclei, such as 25 Mg and 67 Zn, have recently been studied with the help of a 21.1 T ultrahigh magnetic field.…”

“…Several tools are available to NMR spectroscopists to overcome these obstacles, , such as ultrahigh magnetic fields of 18–23.5 T to enhance S/N and narrow the width of second-order quadrupolar interaction (SOQI)-dominated powder patterns and pulse sequences based on the wideband uniform-rate smooth-truncation (WURST) and quadrupolar Carr–Purcell–Meiboom–Gill (QCPMG) sequences . WURST pulses afford broadband excitation of powder patterns and may be followed by an echo (WURST-echo) for powder spectra or accompanied by a CPMG echo train (WURST-CPMG) to increase the S/N at the cost of spectral resolution when transverse magnetization relaxation ( T 2 ) values are sufficiently long. − Because of the difficulties involved, SSNMR studies of metal centers in MOFs have been largely limited to select nuclei that display favorable NMR properties, including 27 Al, 71 Ga, and 45 Sc . Significantly more challenging nuclei, such as 25 Mg and 67 Zn, have recently been studied with the help of a 21.1 T ultrahigh magnetic field. ,− Many other challenging NMR-active metals, such as 47/49 Ti, 91 Zr, 115 In, and 139 La, appear prominently in several interesting MOF structural motifs and could potentially provide a wealth of structural information.…”