Behavior of tri(n-butyl)ammonium bis[citrato(3-)-O1,O3,O6]silicate in aqueous solution: analysis of a sol-gel process by small-angle neutron scattering.

The racemic hexacoordinate silicon(IV) complex tri(n-butyl)ammonium bis[citrato(3-)-O1,O3,O6]silicate (1) was synthesized by treatment of Si(OMe)4 with 2 molar equiv of citric acid and 2 molar equiv of N(n-Bu)3. The corresponding germanium analogue, tri(n-butyl)ammonium bis[citrato(3-)-O1,O3,O6]germanate (5; structurally characterized by single-crystal X-ray diffraction), was obtained analogously, starting from Ge(OMe)4. Upon dissolution in water, the lambda6Si-silicate dianion of 1 hydrolyzes spontaneously (formation of Si(OH)4 and citric acid), whereas the lambda6Ge-germanate dianion of 5 was found to be stable in water. Aqueous "solutions" of 1, with concentrations that are significantly higher than the saturation concentration of Si(OH)4, look absolutely clear over a period of several weeks; however, in reality, these solutions are sols with very small particles that slowly grow with time and finally form a gel that precipitates. This sol-gel process was monitored by small-angle neutron scattering (SANS). For reasons of comparison, an aqueous solution of the hydrolytically stable germanium compound 5 was also studied by the SANS technique.