From solution to the solid state: Control of niobium oxide-fluoride [NbO_xF_y]^n- species

In this study, we describe the crystallization of specific niobium oxide-fluoride anions (either [NbOF₄]^- or [NbOF ₅]^2-) by increasing the fluoride concentration with the appropriate use of organic bases with varied corresponding pK_a values to create suitable equilibria for the formation of each anion. HpyNbOF ₄ (I; py = pyridine) contains the [NbOF₄]^- anion, while [H₂(4,4′-bpy)]NbOF₅] (II; 4,4′-bpy = 4,4′-bipyridyl) contains the [NbOF₅] ^2- anion; their identity is correlated with reagent ratios. The increase of basic species (proton acceptors) results in an increase in the fluoride concentration and high fluoride-containing anions. The crystallization of [NbOF₄]^- in [NbO_2/2F₄] _∞ chains in I was controlled with the use of weak base pyridine (pK_a = 5.23), while isolated [NbOF₅]^2- crystallized in II with strong base 4,4′-bipyridyl (pK_a = 10.5). This approach can be broadly applied to target-specific basic building units for fundamentally new and potentially functional solid-state materials.