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

Abstract

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 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 ²⁹Si 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 ²⁹Si INEPT NMR spectra that we interpret using higher order coupling patterns.