Structure of oxonium hexafluoroniobate and hexafluorotantalate complexes with crown ethers of different dimensionality

Two synthetic pathways, the interaction of Nb2O5 or Ta2O5 with an aqueous solution of hydrofluoric acid, HF, in the presence of 18-crown-6 (18C6), 15-crown-5 (15C5), benzo-15-crown-5 (B15C5) or benzo-12-crown-4 (B12C4), and the direct interaction of the corresponding pentafluorides, MF5 (M = Ta, Nb), in the presence of HF with the same crown ethers result in crystalline complexes of the same composition, [18C6 · H3O][NbF6] (1), [18C6 · H3O][TaF6] (2), [15C5 · H5O2][TaF6] (3), [(B15C5)2 · H5O2][(B15C5)2 · H3O][TaF6]2 (4) and [(B12C4)2 · H3O][TaF6] (5). Complexes 1–5 were identified by elemental and X-ray structural analysis. Compounds 1 and 2 are isostructural, with the [H3O]+ oxonium ion embedded in one crown molecule via OH⋯O hydrogen bonds. Complexes 3–5 represent sandwich-like adducts with the [H3O]+ or [H5O2]+ cations encapsulated. A plethora of weak C–H⋯F interactions with NbF6- and TaF6- anions is responsible for the extended supramolecular architectures.

Graphical abstractFive novel complexes, [18C6 · H3O][NbF6], [18C6 · H3O][TaF6], [15C5 · H5O2][TaF6], [(B15C5)2 · H5O2][(B15C5)2 · H3O][TaF6]2 and [(B12C4)2 · H3O][TaF6] were synthesized and identified by elemental and X-ray structural analysis.Figure optionsDownload full-size imageDownload as PowerPoint slide