Two new “onium” fluorosilicates, the products of interaction of fluorosilicic acid with 12-membered macrocycles: structures and spectroscopic properties

Abstract

Two novel compounds, (L¹H)₂[SiF₆]·2H₂O (1) and (L²H)₂[SiF₅(H₂O)]₂·3H₂O (2), resulting from the reactions of H₂SiF₆ with 4′-aminobenzo-12-crown-4 (L¹) and monoaza-12-crown-4 (L²), respectively, were studied by X-ray diffraction and characterised by IR and ¹⁹F NMR spectroscopic methods. Both complexes have ionic structures due to the proton transfer from the fluorosilicic acid to the primary amine group in L¹ and secondary amine group incorporated into the macrocycle L². The structure of 1 is composed of [SiF₆]²⁻ centrosymmetric anions, N-protonated cations (L¹H)⁺, and two water molecules, all components being bound in the layer through a system of NH⋯F, NH⋯O and OH⋯F hydrogen bonds. The [SiF₆]²⁻ anions and water molecules are assembled into inorganic negatively-charged layers via OH⋯F hydrogen bonds. The structure of 2 is a rare example of stabilisation of the complex anion [SiF₅(H₂O)]⁻, the labile product of hydrolytic transformations of the [SiF₆]²⁻ anion in an aqueous solution. The components of 2, i.e., [SiF₅(H₂O)]⁻, (L²H)⁺, and water molecules, are linked by a system of NH⋯F, NH⋯O, OH⋯F, OH⋯O hydrogen bonds. In a way similar to 1, the [SiF₅(H₂O)]⁻ anions and water molecules in 2 are combined into an inorganic negatively-charged layer through OH⋯F and OH⋯O interactions.