Chirality transcription of molecular precursors to hybrid silicas

Abstract

Silylated racemic, chiral (R,R)- and (S,S)-diureidocyclohexane derivatives have been synthesized in quantitative yields by the reaction of respectively trans racemic, (R,R)- or (S,S)-1,2-diaminocyclohexanes with 3-isocyanatopropyltriethoxysilane. The enantiomeric compounds have the ability to auto-associate through inter-molecular hydrogen bonding owing to the presence of the urea groups. Consequently they formed organic gels in non-polar solvents such as cyclohexane and mesitylene at very low concentration (ca < 10 mg per ml). Such gelating properties were not observed in the case of the racemic mixture. Under acidic aqueous media the controlled hydrolysis-condensation of the (R,R)- and (S,S)-enantiomers led to the hybrid silicates with fibrous structures. Interestingly right- or left-handed helices were respectively observed by Scanning Electronic Microscopy (SEM) images. Conversely, a featureless structure was obtained from the racemic mixture under the same conditions. A significantly reduced wavelength distance (Δ ν = 60 cm−1) between the C=O stretching mode and the NH bending mode was observed by FTIR spectroscopy, respectively at 1636 and 1576 cm−1. These results confirm the presence of the H-bonded urea groups in the solid materials. The solid state 13C NMR spectra exhibit in all cases a signal at 160 ppm attributed to the C=O, and a series of chemical shifts at 55, 43, 34, 25 and 10 ppm corresponding to the different sp3 carbon atoms. The preservation of the C–Si bonds was confirmed by the solid state 29Si CP-MAS spectra in which only T2 (−58 ppm) and T3 (−67 ppm) structures, respectively assigned to C—Si(OH)(OSi)2 and C—Si(Si)3, were observed. The formation of the right- and left-handed helical morphologies demonstrates the transcription of chirality from the enantiomers to the hybrid solids.