Specific chemistry of the anions: [TaOF₅]^2-, [TaF₆]^-, and [TaF₇]^2-

The controlled crystallization of specific tantalum oxide-fluoride and tantalum fluoride anions ([TaOF₅]^2-, [TaF ₆]^-, and [TaF₇]^2-) is demonstrated using organic reagents with varied corresponding pK_a values in the presence of aqueous hydrofluoric acid. The identity of tantalum oxide-fluoride or fluoride anions of [TaOF₅]^2-, [TaF₆] ^-, and [TaF₇]^2- are shown to crystallize successively from solution to solid state by increasing the corresponding pK_a of organic reagents, which lead to the subsequent increase of fluoride concentration in the hydrofluoric acid solution. With the use of this methodology, three new hybrid crystal structures were targeted: [H ₂(2,2′-bpy)]TaOF₅ (2,2′-bpy = 2,2′-bipyridyl) 1, [Hdpa]TaF₆ (dpa = 2,2′-dipyridylamine) 2, and [H₂En]TaF₇ (En = ethylenediamine) 3, respectively. The applicability and comparison of this methodology for tantalum and previously prepared niobium compounds show that it can be broadly used to design new materials with specific functionalities for other transition metal oxide-fluorides.