Sodium thiosulfonate salts: Molecular and supramolecular structural features and solution radiolytic properties

Abstract

Three sodium thiosulfonate salts, NaMeS₂O₂·H₂O, NaPhS₂O₂ and NaMeC₆H₄S₂O₂ have been prepared by the direct reaction of the sodium sulfinate salts with elemental sulfur, a clean, benign route that produces no by-products. The structures of the phenyl (which crystallised as a hydrate, NaPhS₂O₂·1.5H₂O) and p-tolyl compounds were determined by X-ray crystallography. For the p-tolyl derivative, NaMeC₆H₄S₂O₂, the unexpected coordination of the pendant sulfur atom was found, a feature not reported previously for thiosulfonate salts, and observed only in two of the more common thiosulfate salts. Intermolecular CH/π interactions are postulated to contribute to the driving force of sulfur coordination, otherwise a different orientation of the aromatic rings would be expected. For NaPhS₂O₂·1.5H₂O, the water ligands and thiosulfonate anions each contribute three oxygen atoms to form a NaO₆ coordination sphere. The thiosulfonate and water oxygens bridge to other sodium atoms forming a three-dimensional layer structure consisting of sheets of NaPhS₂O₂·1.5H₂O with a hydrophilic interior layer, comprising the sodium ions, water ligands and -S₂O₂⁻groups, and a hydrophobic exterior formed by the phenyl substituent. The structure is further stabilised by an extensive H-bonding network between the ligated water and the non-coordinating thiosulfonate sulfur atom forming part of the hydrophilic layer and by weak intermolecular edge-to-face CH/π interactions between the sheets. Investigation of the radical chemistry of the three salts using pulse radiolysis indicated that oxidation of NaMeS₂O₂·H₂O involves formation of a sulfur-centred radical rather than hydrogen abstraction from the methyl substituent, whereas oxidation of the aromatic ring is the preferred pathway for the phenyl and p-tolyl derivatives.