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 18crown-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 center dot H3O][NbF6] (1), [18C6 center dot H3O][TaF6] (2), [15C5 center dot H5O2][TaF6] (3), [(B15C5)(2)center dot H5O2][(B15C5)(2)center dot H3O][TaF6](2) (4) and [(B12C4)(2)center dot 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 center dot center dot center dot O hydrogen bonds. Complexes 3-5 represent sandwich-like adducts with the [H3O](+) or [H5O2](+) cations encapsulated. A plethora of weak C-H center dot center dot center dot F interactions with NbF6- and TaF6- anions is responsible for the extended supramolecular architectures. (C) 2007 Elsevier Ltd. All rights reserved.