Intramolecular mobility and phase transitions in ammonium oxofluoroniobates (NH4)2NbOF5 and (NR4)3NbOF6, a NMR and DFT study

Molecular structure, ionic mobility and phase transitions in six- and seven-coordinated ammonium oxofluoroniobates (NH4)(2)NbOF5 and (NH4)(3)NbOF6 were studied by F-19, H-1 NMR and DFT calculations. Equatorial fluorine atoms (F-cq) in [NbOF5](2-) and [NbOF6](3-) are characterized by high F-19 NMR chemical shifts while axial fluorine atoms (F-ax) have those essentially lower. The high-temperature ionic mobility in (NH4)(2)NbOF5 does not stimulate the ligand exchange F-eq <-> F-ax, whereas it is observed in (NH4)(3)NbOF6 as pseudorotation typical for seven-coordinated polyhedra. The transformation of pentagonal bipyramidal structure (BP) of [NbOF6](3-) into capped trigonal prismatic (CTP) one takes place during the phase transition (PT) at 260 K. The PT of order-disorder type in (NH4)(2)NbOF5 is accompanied by transition of anionic sublattice to a rigid state. The F-19 and H-1 NMR data corroborate the independent motions of NH4 groups and anionic polyhedra in (NH4)(2)NbOF5 while they are coordinated in (NH4)(3)NbOF6. (C) 2011 Elsevier B.V. All rights reserved.