Synthesis, crystal structure, and characterization of the first Nb₃ triangular cluster compound bonded to fluorine ligands: Association of Nb₃IⁱFⁱ₃ F^a₈L^a units and Nb^IVL₆ octahedra with L = O and F

The synthesis and the crystal structure of the first compound containing Nb₃ triangular clusters bonded to fluorine ligands are presented in this work. The structure of Nb₃IF₇L(NbL₂)_0.25 with L = O and F, determined by single-crystal X-ray diffraction, is based on a Nb₃IⁱFⁱ₃F^a ₈L^a unit and a NbL₆ octahedron (tetragonal, space group 14/m, a = 13.8638(3) Å, c = 8.9183(2) Å, V = 1714.14(7) ų, Z = 8). Two crystallographic positions (noted L5 and L6) are randomly occupied by fluorine and oxygen with two different F:O occupancies. These L ligands build an octahedral site for a single niobium atom, located between the units. The four L5 ligands of the NbL₆ octahedron are shared with four Nb₃ cluster units, while the two other L6 ligands are terminal. The Nb₃ cluster is face-capped by one iodine and edge-bridged by three fluorine ligands. Two of the three niobium atoms constituting the cluster are bonded to three additional apical fluorine ligands, while the third one is bonded to two fluorines and one L5 ligand. The Nb₃ cluster is linked to six adjacent ones via all the apical fluorine ligands. The developed formula of the unit is therefore Nb₃IⁱFⁱ₃ F^a-a_8/2L^a according to the Schäfer and Schnering notation. The oxidation state of the single niobium and the random distribution of fluorine and oxygen on the two L sites will be discussed on the basis of structural analysis, the bond valence method, and IR and EPR measurements. The structural results will be compared to those of previously reported niobium compounds containing NbF₆ or Nb(F,O)₆ octahedra.