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

Molecular structure, ionic mobility and phase transitions in six- and seven-coordinated ammonium oxofluoroniobates (NH4)2NbOF5 and (NH4)3NbOF6 were studied by 19F, 1H NMR and DFT calculations. Equatorial fluorine atoms (Feq) in [NbOF5]2− and [NbOF6]3− are characterized by high 19F NMR chemical shifts while axial fluorine atoms (Fax) have those essentially lower. The high-temperature ionic mobility in (NH4)2NbOF5 does not stimulate the ligand exchange Feq ↔ Fax, whereas it is observed in (NH4)3NbOF6 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)2NbOF5 is accompanied by transition of anionic sublattice to a rigid state. The 19F and 1H NMR data corroborate the independent motions of NH4 groups and anionic polyhedra in (NH4)2NbOF5 while they are coordinated in (NH4)3NbOF6.