Five- and six-fold 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 1 H, 13 C, 29 Si, and 31 P single pulse and cross polarization (CP) MAS NMR spectroscopy as well as 1 H- 31 P{ 29 Si} REDOR, 1 H- 31 P- 29 Si and 1 H- 29 Si- 13 C 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 a few [SiO 4 ] groups, we noticed that HEDP based silicodiphosphonate possesses 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 29 Si CP NMR spectra also demonstrate polymeric structures with different [SiO 4 ] units in the solids. Both silyl-substituted phosphorus precursors show a surprising triplet signal in the 29 Si INEPT NMR spectra that we interpret using higher order coupling patterns. Silicodiphosphonates synthesized by two different pathways show interesting chemical shifts of five- and sixfold coordinated silicon.