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

Abstract

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 λ 6 Si-silicate dianion of 1 hydrolyzes spontaneously (formation of Si(OH) 4 and citric acid), whereas the λ 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.

Crystal structure of bis[cis-diaquabis(phenanthroline)cobalt(II)] bis(citrato)germanate(IV) dinitrate

“…A search of the Cambridge Structural Database (CSD, Version 5.42, update November 2020; Groom et al, 2016) for the Ge(HCit) 2 2À anion yielded 15 structures containing this anion (Seiler et al, 2005;Seifullina et al, 2006Seifullina et al, , 2007Seifullina et al, , 2015Seifullina et al, , 2016Seifullina et al, , 2017aMartsinko et al, 2011Martsinko et al, , 2013Martsinko et al, , 2018a. In these structures, the Ge-O bond lengths for the hydroxyl, -carboxylate and -carboxylate oxygen atoms are in the ranges 1.793-1.840, 1.881-1.914 and 1.904-1.955 Å , respectively.…”