Dianionic Tris[oxalato(2—)]silicate and Tris[oxalato(2—)]germanate Complexes: Synthesis, Properties, and Structural Characterization in the Solid State and in Solution

Abstract

Reaction of tetramethoxysilane with three molar equivalents of oxalic acid and two molar equivalents of 1-(2-hydroxyethyl)-pyrrolidine or 1-(2-hydroxyethyl)piperidine in tetrahydrofuran yielded the λ 6 Si-silicates 1-(2-hydroxyethyl)pyrrolidinium tris[oxalato(2Ϫ)]silicate (4) and 1-(2-hydroxyethyl)piperidinium tris[oxalato(2Ϫ)]silicate (5). The related germanium compounds 1-(2-hydroxyethyl)piperidinium tris[oxalato(2Ϫ)]germanate (6) and triethylammonium tris[oxalato(2Ϫ)]germanate (7) were synthesized analogously, starting from tetramethoxygermane and using three molar equivalents of oxalic acid and two molar equivalents of 1-(2-hydroxyethyl)piperidine or triethylamine. Compounds 4Ϫ7 Dianionische Tris[oxalato(2؊)]silicat-und Tris[oxalato(2؊)]germanat-Komplexe: Synthese, Eigenschaften und strukturelle Charakterisierung im Festkörper und in LösungInhaltsübersicht. Die Umsetzung von Tetramethoxysilan mit drei Mol-Äquivalenten Oxalsäure und zwei Mol-Äquivalenten 1-(2-Hydroxyethyl)pyrrolidin oder 1-(2-Hydroxyethyl)piperidin in Tetrahydrofuran führte zu den λ 6 Si-Silicaten 1-(2-Hydroxyethyl)pyrrolidinium-tris[oxalato(2Ϫ)]silicat (4) und 1-(2-Hydroxyethyl)piperidinium-tris[oxalato(2Ϫ)]silicat (5). Die verwandten GermaniumVerbindungen 1-(2-Hydroxyethyl)piperidinium-tris[oxalato(2Ϫ)]-germanat (6) und Triethylammonium-tris[oxalato(2Ϫ)]germanat (7) wurden auf analoge Weise durch Umsetzung von Tetramethoxygerman mit drei Mol-Äquivalenten Oxalsäure und zwei Mol-Äqui-

On the interactions of alkyl 2-hydroxycarboxylic acids with alkoxysilanes 2. Complexation and esterification of di- and tricarboxylic acids

“…Figures in parentheses indicate yields in absence of Si(OMe) 4 .More recently Tacke et al prepared the triethylammonium salt of 1 from OA, tetraethoxysilane, Si(OEt) 4 , and triethylamine in toluene under reflux, confirmed the structure by X-ray crystallography, and showed 1 decomposes in H 2 O. 16,17 Sugahara et al studied the reaction of OA with Si(OEt) 4 in ethanol (EtOH) and observed oligomerisation of siloxanes and esterification of OA, without identifying any OA-silicon complex, 18 but crucially they did not consider ester formation in the absence of Si(OEt) 4 , so it is unclear whether the reaction is accelerated by Si(OEt) 4 or not. OA has also been used in silica production from Si(OEt) 4 in aqueous conditions: the mixture OA-Si(OEt) 4 -H 2 O gave silica with higher surface area than when formic or acetic acids were used,19 which might suggest OA-silicon complexes play a role in silica precipitation.Complexes of silicon with 2HOAs were first produced from Si(OMe)4 and Si(OEt) 4 by Mehrotra et al in the 1960s, 5 but structures were assigned based only on the quantities of alcohol eliminated and elemental analyses of the residues.…”

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

“…To explain this phenomenon, it has been postulated that silicon may be stored as silicon(IV) complexes with organic ligands. − However, the chemical nature of these hypothetical silicon species in the soluble silicon pool is still completely unknown, and the biological relevance of such silicon complexes (especially those with penta- or hexacoordinate silicon atoms) is discussed very controversially. Despite our own (up to now unsuccessful) efforts to verify this hypothesis experimentally, − we would like to suggest again an alternative (or additional) mechanism for the storage of “dissolved” silicon in biological systems: inspired by the results of the present study and the data reported in refs and , silicon storage based on sols is proposed that are built up by aggregates of polysilicic acid, which are stabilized by organic components (in this context, see also ref ). However, to verify this speculative suggestion, further extensive studies are necessary.…”

“…In context with our studies on higher-coordinate silicon(IV) complexes and their potential role in silicon biochemistry, − we have reported on the synthesis and structural characterization of racemic tri( n -butyl)ammonium bis[(citrato(3−)- O 1 , O 3 , O 6 ]silicate ( 1 ) . Most recently, we have succeeded in synthesizing the related compounds 2 ·2MeOH ( meso -configuration) and 3 ·1.73MeOH (racemic), which contain a bis[citrato(3−)- O 1 , O 3 , O 6 ]silicate dianion and a bis[citrato(4−)- O 1 , O 3 , O 6 ]silicate tetraanion, respectively (for a recent review dealing with higher-coordinate silicon compounds with Si O 5 and Si O 6 skeletons, see ref ).…”

“…To get more information about the above-mentioned sol−gel process, small-angle neutron scattering (SANS) studies of aqueous solutions of 1 were performed. For reasons of comparison, the germanium analogue 5 (which was expected to be hydrolytically stable; in this context, see refs , , and 6) was included in these studies. We report here on the synthesis and structural characterization of 5 and SANS studies of aqueous solutions of 1 and 5 .…”

Heterodimetallic Germanium(IV) Complex Structures with Transition Metals

“…The chemical moiety common to structures 1-4 is the divalent tris(oxalate-O,O′)germanate anion, [Ge(C 2 O 4 ) 3 ] 2- (Figure 2 and Scheme 1). A search in the literature and in the CSD 39,40 reveals only a handful of crystallographic reports [41][42][43][44] describing this anion which, invariably, shows identical coordination geometry for all compounds, including those reported herein. The Ge 4+ center appears coordinated to three oxalate anions (coordinated via a typical anti,anti-chelate bidentate fashion), describing a slightly distorted {GeO 6 } octahedral coordination fashion as depicted in Figure 2.…”