Insight into systematic formation of hexafluorosilicate during crystallization via self-assembly in a glass vessel

Formation of the unexpected hexafluorosilicate (SiF₆²⁻) anion during crystallization via self-assembly in glassware is scrutinized. Self-assembly of M(BF₄)₂ (M²⁺ = Cu²⁺ and Zn²⁺) with tridentate N-donors (L) in a mixture solvent including methanol in a glass vessel gives rise to an SiF₆²⁻-encapsulated Cu₃L₄ double-decker cage and a Zn₂L₄ cage, respectively. Induced reaction of CuX₂ (X⁻ = PF₆⁻ and SbF₆⁻) instead of Cu(BF₄)₂, with the tridentate ligands, produces the same species. The formation time of SiF₆²⁻ is in the order of anions BF₄⁻ < PF₆⁻ < SbF₆⁻ under the given reaction conditions. The SiF₆²⁻ anion, acting as a cage template or cage-to-cage bridge, seems to be formed from the reaction of polyatomic anions containing fluoride with the SiO₂ of the surface of the glass vessel.