Examining the Out-of-Center Distortion in the [NbOF₅]^2- Anion

Out-of-center “primary” electronic distortions are inherent to the oxide fluoride anions of the early d0 transition metals. In the [NbOF5]2- anion, the Nb5+ moves from the center of the octahedron toward the oxide ligand to form a short NbO bond and long trans Nb−F bond. The combined results of single-crystal X-ray diffraction and electronic structure calculations indicate that the primary distortion of the [NbOF5]2- anion is affected by the coordination environment that is created by the three-dimensional extended structure. The formation of bonds between an M(L)42+ (M = Cd2+, Cu2+; L = 3-aminopyridine, 4-aminopyridine) cation and the oxide and/or trans-fluoride ligands of the [NbOF5]2- anion weakens the π component of the NbO bond. At the same time, hydrogen bond interactions between the equatorial fluorides and the aminopyridine groups both lengthen the equatorial Nb−F bonds and can further reduce the symmetry of the [NbOF5]2- anion. These combined three-dimensional bond network interactions that serve to lengthen the NbO bond and thereby decrease the primary distortion of the [NbOF5]2- anion are illustrated in the structures of three new niobium oxide fluoride phases, [4-apyH]2[Cu(4-apy)4(NbOF5)2] (4-apy = 4-aminopyridine), Cd(3-apy)4NbOF5 (3-apy = 3-aminopyridine), and Cu(3-apy)4NbOF5, that were synthesized and characterized using X-ray diffraction. Crystal data for [4-apyH]2[Cu(4-apy)4(NbOF5)2]: tetragonal, space group I41/acd (No. 142), with a = 20.8745(8) Å, c = 17.2929(9) Å, and Z = 8. Cd(3-apy)4NbOF5: tetragonal, space group P43 (No. 78), with a = 8.4034(4) Å, c = 34.933(3) Å, and Z = 4. Cu(3-apy)4NbOF5: monoclinic, space group P21/n (No. 14), with a = 8.822(1) Å, b = 16.385(3) Å, c = 8.902(1) Å, β = 109.270(3)°, and Z = 2.