In different crystalline silicophosphates like SiP2O7 or Si5P6O25 the silicon is coordinated to six oxygen atoms. Of particular interest is the role of silicon pyrophosphate phases. Materials with such [SiO6] moieties can be obtained by various reaction routes. We established a “water-free” synthetic route based on the reaction of phosphorous precursors like crystalline phosphoric acid with tetra(alkoxy)silanes in organic solvents.[2,3]
During the reaction of pyrophosphoric acid with different tetra(alkoxy)silanes solids were precipitated. According to 29Si and 31P MAS NMR chemical shift data hydrogen phosphate structures are formed. The products were characterized via 2D HETCOR and other solid state NMR experiments. Additionally, a DFT calculated structure model was developed, which was verified by solid state NMR spectroscopy. To monitor the reaction progress and to investigate the mechanism 31P NMR spectroscopy measurements at different temperatures were used. The studies showed redistribution of the P-O-P linkage during the reaction. Furthermore, the stability of the given silicophosphates in water and other aqueous solutions were analysed by in situ mixing of the solvent and the solid in a Wilmad-LabGlass Reaction Monitoring System. 31P solution NMR spectroscopy was used to investigate the formation of molecular intermediates.
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