From Solution to the Solid State: Control of Niobium Oxide–Fluoride [NbOxFy]n− Species

In this study, we describe the crystallization of specific niobium oxide–fluoride anions (either [NbOF4]− or [NbOF5]2–) by increasing the fluoride concentration with the appropriate use of organic bases with varied corresponding pKa values to create suitable equilibria for the formation of each anion. HpyNbOF4 (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 [NbO2/2F4]∞ chains in I was controlled with the use of weak base pyridine (pKa = 5.23), while isolated [NbOF5]2– crystallized in II with strong base 4,4′-bipyridyl (pKa = 10.5). This approach can be broadly applied to target-specific basic building units for fundamentally new and potentially functional solid-state materials.