The First Zwitterionic, Optically Active Disilicate with Pentacoordinate Silicon

Synthesis, structures and photophysical properties of hexacoordinated organosilicon compounds with 2-(2-pyridyl)phenyl groups

We synthesised novel hexacoordinated organosilicon compounds with two 2-(2-pyridyl)phenyl groups. Single-crystal X-ray structure analyses indicated that Lewis acid-base interactions exist between the silicon atom and two nitrogen atoms of the pyridine rings, and that hexacoordinated organosilicon compounds have slightly distorted octahedral structures in the solid state. The hexacoordinated organosilicon compounds are stable in air, water, heat, acids, and bases. The fluorescent quantum yield increased dramatically and a significant red-shift of the maximum fluorescence wavelength was observed with the introduction of amino groups on the 2-(2-pyridyl)phenyl aromatic rings. The fluorescence colours of a hexacoordinated organosilicon compound with two amino groups can be switched by protonation and deprotonation (neutralisation) of the amino groups.

Optically active zwitterionic lambda(5)Si,lambda(5)Si'-disilicates: syntheses, crystal structures, and behavior in aqueous solution

The zwitterionic lambda(5)Si,lambda(5)Si'-disilicates 1-8 were synthesized and characterized by solid-state and solution NMR spectroscopy. In addition, compounds 26 H(2)O, 32 CH(3)CN, 45/2 CH(3)CN, 6CH(3)OH, 7, and 8CH(3)OHCH(3)CN were studied by single-crystal X-ray diffraction. The optically active (Delta,Delta,R,R,R,R)-configured compounds 1-8 contain two pentacoordinate (formally negatively charged) silicon atoms and two tetracoordinate (formally positively charged) nitrogen atoms. One (ammonio)alkyl group is bound to each of the two silicon centers, and two tetradentate (R,R)-tartrato(4-) ligands bridge the silicon atoms. Although these lambda(5)Si,lambda(5)Si'-disilicates contain SiO(4)C skeletons, some of them display a remarkable stability in aqueous solution as shown by NMR spectroscopy and ESI mass spectrometry.

On the interactions of alkyl 2-hydroxycarboxylic acids with alkoxysilanes: selective esterification of simple 2-hydroxycarboxylic acids

The interactions of a range of monocarboxylic acids with tetramethoxysilane Si(OMe)(4) (TMOS), in methanol (MeOH), have been investigated by using (1)H, (13)C and (29)Si solution-phase NMR spectroscopy and electrospray mass spectrometry (ESMS). Si(OMe)(4) acts as a catalyst/reagent in the selective methylation of 2-hydroxycarboxylic acids (2HOAs) in MeOH at room temperature: glycolic acid, lactic acid and 2-hydroxybutyric acid are esterified more than a hundred times faster in MeOH and Si(OMe)(4) than in MeOH alone. No acceleration of methylation is observed for carboxylic acids lacking the 2-hydroxy group. Methylation of the 2HOAs is associated with the condensation of individual siloxane units to form oligomers. A mechanism is proposed in which 2HOAs attach to silicon via the alkoxy group, then subsequently via the carboxyl group in an intramolecular rearrangement to form an unstable and reactive cyclic intermediate. This intermediate may lead to accelerated methylation of the carboxylic acid via nucleophilic attack of MeOH at the carbonyl group, while a separate reaction pathway leads to condensation of silanols and/or alkoxysilanes leading to oligosiloxanes. The mechanism has implications for the use of 2HOAs as templates in sol-gel silica preparation.