Composition Space of the (CuO, ¹/₂Nb₂O₅)/(HF)_x·pyridine/H₂O System. Structure and Synthesis of [pyH⁺]₂[CuNb₂(py)₄O₂F₁₀]^2- and CuNb(py)₄OF₅

Single crystals of [pyH⁺]₂[CuNb₂(py)₄O₂F₁₀]^2- and CuNb(py)₄OF₅ were synthesized in a (HF)_x·pyridine/pyridine/water solution (150 °C, 24 h, autogeneous pressure) using CuO and Nb₂O₅ as reagents. The compound [pyH⁺]₂[CuNb₂(py)₄O₂F₁₀]^2- contains clusters of [CuNb₂(py)₄O₂F₁₀]^2- anions linked through N−H⁺···F hydrogen bonds to the [pyH⁺] cations. In contrast CuNb(py)₄OF₅ is a unidimensional compound consisting only of chains, perpendicular to the c axis, of alternating [Cu(py)₄(O/F)_2/2]^0.5+ and [NbF₄(O/F)_2/2]^0.5- octahedra. The chains change direction between the [110] and [11̄0] every c/2. Crystal data for [pyH⁺]₂[CuNb₂(py)₄O₂F₁₀]^2-:  tetragonal, space group I4₁22 (No. 98),with a = 11.408(3) Å, c = 30.36(1) Å, and Z = 4. Crystal data for CuNb(py)₄OF₅:  monoclinic, space group C2/c (No. 15), with a = 10.561(3) Å, b = 13.546(6) Å, c = 16.103(4) Å, β = 97.77(2)°, and Z = 4.