Structure of Dibenzocrown Ethers and their H-bonded Adducts. 3. Isolation of Oxonium Ions by Biphenyl-20-crown-6 and [1.5]Dibenzo-18-Crown-6 in Complexes with [NbF6]− and [TaF6

Abstract

The reaction of Nb2O5 and Ta2O5 with an aqueous solution of hydrofluoric acid, HF in the presence of biphenyl-20-crown-6 (BP20C6, L1) or [1.5]dibenzo-18-crown-6 ([1.5]DB18C6, L2) results in the complexes [L1·(H3O)][NbF6] (1), [L1 (H3O)][TaF6] (2), [2L2·(H7O3)][NbF6] (3) and [2L2·(H7O3)][TaF6] (4). Complexes 1–4 were identified by the elemental and X-ray structural analysis and IR spectroscopy. Complexes 1 and 2 are isostructural, with the H3O+ oxonium ion embedded in one crown molecule via OH···O hydrogen bonds. Complexes 3 and 4 represent the supramolecular isomers distinctive by the crown conformations and crystal packing, with the (H7O3)+ cation enclosed in the cage of two crown molecules. Being poor H-bond acceptors, NbF 6 − and TaF 6 − anions do not compete with the crown oxygen atoms for the oxonium hydrogen atoms, but are involved in the numerous C–H···F short contacts responsible for the extended supramolecular architectures in all cases. A change of crown ethers’ conformation in complexes 1–4 and a correlation between the degree of proton hydration and an accessibility of the crown ether oxygen atoms is observed.