Targeting Oxide Concentration in Tantalum Oxide-Fluoride Anions

The design of both molecular and non-molecular solid materials with specific properties fundamentally relies on the controlled synthesis of crystals with desired functional groups, bonding motifs, polarity, chirality, and more. To this end, fluoride and oxide-fluoride anions have been utilized as basic building units (BBUs) in the synthesis of noncentrosymmetric racemic materials for their ability to create polar axes that facilitate the breaking of an inversion center as demonstrated in a series of compounds with [MF₆]^2- anions (M = Ti, Zr, Hf). Targeting an analog with a [TaOF₅]^2- anion, the phase space of (CuO, Ta₂O₅)/bpy/HF_(aq)/H₂O (bpy = 2,2′-bipyridine) was investigated and three new compounds with Cu-bpy cations and Ta-fluoride or Ta-oxyfluoride anions were synthesized: [Cu(bpy)₂][TaF₆], [Cu(bpy)₂][Ta₂OF₁₀], and [Cu(bpy)F(H₂O)₂]₂[TaF₇]∙H₂O with the anions [TaF₆]^-, [Ta₂OF₁₀]^2-, and [TaF₇]^2-, respectively. The formation of these anions was found to be a product of both the concentration of hydrofluoric acid in solution and the ratio of metal-oxide starting materials to ligand. This work contributes to the understanding of mixed anion formation in the solid state.