Five- and sixfold coordinated silicon in silicodiphosphonates: short range order investigation by solid-state NMR spectroscopy

The structure of amorphous silicodiphosphonates made from diphosphonic acids and tetraethoxysilane was investigated by 1H, 13C, 29Si, and 31P single pulse and cross polarization (CP) MAS NMR spectroscopy as well as 1H-31P{29Si} REDOR, 1H-31P-29Si and 1H-29Si-13C double CP MAS experiments. The combination of these solid-state NMR techniques leads to a structural understanding of the short range order. The structure is dominated by [Si(OP)6] units. Besides few [SiO4] groups, we noticed that the HEDP based silicodiphosphonate possess further five- and sixfold coordinated silicon moieties. Another synthesis route to obtain Si-O-P compounds with higher coordinated silicon species is the reaction of trimethylsilyl-substituted diphosphates and diphosphonates with tetramethoxysilane. The 29Si CP NMR spectra demonstrate polymeric structures with also different [SiO4] units in the solids. Both silyl-substituted phosphorus precursors show a suprising triplet signal in the 29Si INEPT NMR spectra which we interpret by higher order coupling pattern.