In general, Si-O-P compounds are structurally interesting materials because they possess many different structural motives including higher coordinated silicon atoms. Well-known examples are crystalline silicophosphates like SiP2O7. These and other Si-O-P compounds can be synthesized by different reaction routes at ambient pressure. We established a so-called “water-free” sol-gel route,[2,3] which can be adapted as a general synthetic procedure to obtain various Si-O-P compounds by using diphosphoric or diphosphonic acids instead of crystalline phosphoric acid.
Here, the reaction of tetraethoxysilane with either diphosphoric acid (SiOP1) or 1-hydroxyethane 1,1-diphosphonic acid (SiOP2) yielded amorphous products. The local structure of these Si-O-P materials could be elucidated primarily by solid-state NMR spectroscopy. The evidence for higher coordinated silicon atoms was given by 29Si MAS NMR spectroscopy due to chemical shift data. Additionally, 13C and 31P MAS NMR spectra could verify the existence of diphosphate and diphosphonate structures. Furthermore, FSLG-HETCOR and 31P-29Si CP-REDOR experiments were able to deliver, inter alia, detailed information about 1H-29Si and 1H-31P heteronuclear correlations and the first coordination sphere of the phosphate and phosphonate groups. In the case of SiOP1, the phosphorous atoms of the main structure are connected via oxygen to three silicon atoms.
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