Behavior Of Tri(N-Butyl)Ammonium Bis[Citrato(3-)-O-1,O-3,O-6]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-)-O-1,O-3,O-6]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-)-O-1,O-3,O-6]germanate (5; structurally characterized by single-crystal X-ray diffraction), was obtained analogously, starting from Ge(OMe)(4). Upon dissolution in water, the lambda Si-6-Silicate dianion of 1 hydrolyzes spontaneously (formation of Si(OH)(4) and citric acid), whereas the lambda(6) Ge-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.