Intramolecular mobility and phase transitions in ammonium oxofluoroniobates (NH 4) 2NbOF 5 and (NH 4) 3NbOF 6, a NMR and DFT study

Abstract

Molecular structure, ionic mobility and phase transitions in six- and seven-coordinated ammonium oxofluoroniobates (NH 4) 2NbOF 5 and (NH 4) 3NbOF 6 were studied by 19F, 1H NMR and DFT calculations. Equatorial fluorine atoms (F eq) in [NbOF 5] 2− and [NbOF 6] 3− are characterized by high 19F NMR chemical shifts while axial fluorine atoms (F ax) have those essentially lower. The high-temperature ionic mobility in (NH 4) 2NbOF 5 does not stimulate the ligand exchange F eq ↔ F ax, whereas it is observed in (NH 4) 3NbOF 6 as pseudorotation typical for seven-coordinated polyhedra. The transformation of pentagonal bipyramidal structure (BP) of [NbOF 6] 3− into capped trigonal prismatic (CTP) one takes place during the phase transition (PT) at 260 K. The PT of order-disorder type in (NH 4) 2NbOF 5 is accompanied by transition of anionic sublattice to a rigid state. The 19F and 1H NMR data corroborate the independent motions of NH 4 groups and anionic polyhedra in (NH 4) 2NbOF 5 while they are coordinated in (NH 4) 3NbOF 6.