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 H-1, C-13, Si-29, and P-31 single pulse and cross polarization (CP) MAS NMR spectroscopy as well as H-1-P-31{Si-29} REDOR, H-1-P-31-Si-29 and H-1-Si-29-C-13 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 [SiO4] 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 Si-29 CP NMR spectra also demonstrate polymeric structures with different [SiO4] units in the solids. Both silyl-substituted phosphorus precursors show a surprising triplet signal in the Si-29 INEPT NMR spectra that we interpret using higher order coupling patterns.