Chirality transcription of molecular precursors to hybrid silicas

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 intermolecular 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 (Delta v = 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 C-13 NMR spectra exhibit in all cases a signal at 160 ppm attributed to the <(C)under bar>=O, and a series of chemical shifts at 55, 43, 34, 25 and 10 ppm corresponding to the different sp(3) carbon atoms. The preservation of the C-Si bonds was confirmed by the solid state Si-29 CP-MAS spectra in which only T-2 ( 58 ppm) and T 3 ( 67 ppm) structures, respectively assigned to C-(S) under bari(OH)(OSi)(2) and C-(S) under bari(OSi)(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.