Where Fluoride Is Present, Hexafluorosilicate Might Be Encountered: Supramolecular Binding of the SiF6 2- Anion by Nanojars.

Abstract

While the fact that HF etches glass is rather common chemical knowledge, the reaction of fluoride ions in an organic solvent with glassware under alkaline conditions has not yet been documented. It becomes apparent that, in general, whenever any fluorine-containing material is handled in silicon-based glassware, the possible presence of SiF6 2- should be considered regardless of the pH of the reaction medium. Subsequent to the initial, inadvertent synthesis by the reaction of CuF2 with laboratory glassware, hexafluorosilicate-entrapping supramolecular assemblies (nanojars) of the formula [SiF6⊂{cis-CuII(μ-OH)(μ-pz)} n ]2- (Cu n SiF 6 ; n = 28, 30, 32, 34) were rationally prepared by the reaction of Cu2+ with pyrazole (Hpz) in the presence of a base and SiF6 2- ions or by combining Cu n CO 3 with H2SiF6 or HF. This work demonstrates that nanojars are excellent anion-binding agents not only for trigonal planar/pyramidal or tetrahedral anions but also for larger, octahedral anions, which are bound exclusively by charge-assisted hydrogen bonds. The SiF6 2- anion guest is preferentially bound by a larger Cu32 host, whereas CO3 2- favors a smaller Cu27 nanojar. The Cu n SiF 6 mixture was characterized by electrospray ionization mass spectrometry as well as variable-temperature, paramagnetic 1H and 19F NMR spectroscopy in solution, complemented by single-crystal X-ray diffraction studies in the solid state, which reveal the unprecedented structure of a Cu8+14+10 nanojar (where Cu8, Cu10, and Cu14 are the component Cu x metallamacrocycles of the Cu32 nanojar).