New hybrid organic-inorganic solids with helical morphology via H-bond mediated sol-gel hydrolysis of silyl derivatives of chiral (R,R)- or (S,S)-diureidocyclohexane [6]

Full text of DOI:10.1021/ja003843z

J. Am. Chem. Soc. 2001, 123, 1509-1510
1509
creation of porous hybrids with well-ordered mesostructures.10
The control of the pore structure is certainly of great importance
in the preparation of catalytic materials. However, for potential
applications such as electronic, optic, advanced catalysis the
control of the structure at macroscopic and nanoscopic level is
also important. Efforts have been made to control the macroscopic
morphology of silicas using surfactant-mediated fabrication;
interesting helical silica structures were produced using chiral
gelators as templates.11 Regarding porous hybrids, a surfactant-
mediated approach was only recently used to generate a cubic
crystal with decaoctahedral shape.12
New Hybrid Organic-Inorganic Solids with Helical
Morphology via H-Bond Mediated Sol-Gel
Hydrolysis of Silyl Derivatives of Chiral (R,R)- or
(S,S)-Diureidocyclohexane
Joe¨l J. E. Moreau,* Luc Vellutini, Michel Wong Chi Man, and
Catherine Bied
Laboratoire de Chimie Organome´tallique, UMR CNRS 5076
Ecole Nationale Supe´rieure de Chimie de Montpellier
34296 Montpellier Cedex 05, France
In all of the above cases, the structure of the material was
controlled by the addition of an external organic template which
directs the organization of the inorganic phase. Silsesquioxane
hybrids offer much larger potentialities and seem very interesting
because, owing to the covalent bond between the organic and
the inorganic fragments, the auto-assembly of the organic
substructure can control the formation of the inorganic silicate
components and eventually direct the structure at the meso- or
macroscopic level. A judicious choice of the organic substructure
in the precursor may lead to auto-organized solids upon simple
hydrolysis. We wanted to test the use of intermolecular H-bond
interactions between precursor molecules during the solid forma-
tion, as a mean to control the three-dimensional structure of the
hybrid network.
Our current interest in the design of chiral hybrids containing
(R,R)- or (S,S)-diamino-cyclohexane units and of new enantiose-
lective catalytic materials13 has led us to study the use of chiral
organic substructures capable of auto-association. Here we report
the creation of a hybrid with helical morphology via H-bond-
mediated hydrolysis of a single precursor. A left- or right-handed
helix is auto-generated, according to the configuration of the
organic substructure.
Diureido derivatives, which are low-molecular weight gelators,
were chosen because of their ability to auto-associate through
H-bonds.14 Compound (R,R)-2, [R]D +4.3 (c ) 3, CHCl3), was
obtained from trans-(1R,2R)-diaminocyclohexane (R,R)-1 upon
reaction with γ-isocyanatopropyl-triethoxysilane as shown in
Scheme 1. Despite the absence of a long hydrocarbon chain
substituent14 in compound 2, it easily auto-associates by H-bonds
in solution and was found to form organogel upon dissolution in
organic solvents such as cyclohexane or mesitylene, at ∼3-5
ReceiVed NoVember 1, 2000
Hybrid materials1 have attracted the attention of chemists and
material scientists due to commercial interest in their applications
as well as scientific interest in the challenges posed by their
synthesis. Even though major strides have been made in the past
decade, the goal of the synthesis of materials and the tailoring of
their properties by design from molecules remains largely elusive.
In this context, silsesquioxane hybrids,2-4 with well-defined
composition and resulting from the sol-gel hydrolysis condensa-
tion of functionalized organic molecules, constitute an interesting
class of materials for study. The combination of organic and
inorganic substructures offers unique possibilities for designing
new synthetic routes. However, the tailoring of these materials
is complicated by the fact that they are metastable products made
under kinetic rather than thermodynamic control, and many factors
influence the structure of the synthetic materials. The morphology
of the solid varies according to the nature of the organic molecules
and according to the experimental conditions of the synthesis.3a,4a-6
Also, weak intermolecular interactions have been shown to
influence the kinetic parameters, to modify the texture and to
generate anisotropy in the resulting hybrid materials.5b,7 Clear
examples of templating and structure-directing have also been
demonstrated. The use of the silicas templating agents: molecular
assemblies,8 organogelators,9 has been recently extended to the
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10.1021/ja003843z CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/26/2001

1510 J. Am. Chem. Soc., Vol. 123, No. 7, 2001
Communications to the Editor
Scheme 1
Figure 1. SEM images of hybrid silicas made from (a, b) precursor (R,R)-
2, and from (c, d) (S,S)-2.
mg‚mL-1. This indicates that quite facile and stable auto-
association of molecules of 2 by H-bonds occurs. Evidence for
H-bond association was obtained from the FTIR spectrum: at
low concentration (6 mg‚mL-1 CHCl3), (R,R)-2 shows single
absorption in the NH stretch region (3368 cm-1) and in the amide
I and II regions (1656 and 1550 cm-1) characteristic for the
presence of non-hydrogen-bonded urea groups and at higher
concentration or in the organogel, shifts are observed toward
shorter wavenumbers for the NH and amide I (respectively 3330
and 1630 cm-1) and toward higher wavenumbers for the amide
II (1572 cm-1) in agreement with strong association of the urea
groups.14
domain, ranging from 5 to 50 nm. The voids of the fibrous
structure may account for the observed porosity.
Supramolecular polymeric architectures with helical morphol-
ogies have been obtained by the auto-association of chiral
structures on the basis of H-bonding interactions.15 H-bonds also
lead to the auto-association of organic molecules to form
organogels,16 and recently chiral low-molecular weight gelators
were used as templates in the sol-gel hydrolysis of TEOS and
interestingly led to the creation of helical silica structures.11
The present results show the first example of self-organized
chiral hybrid material. The nature of the intermolecular interaction
between molecules of precursor 2 and the structure of the organic
assembly before hydrolysis are probably similar to those of related
nonsilylated molecular gelators.14 However, further studies are
necessary to clarify the mechanistic pathway leading from 2 to
H and how the large-scale nanostucture relates to the structure
of organogels. It is interesting to note that the formation of strong
siloxane bonds during the polycondensation of compounds 2, to
some extent, can be controlled by weak intermolecular H-bonds
between chiral monomeric units or growing oligomers, creating
chirality in the entire hybrid material. However, the formation of
siloxane bridges seemed to influence the arrangement of the
organic substructures and lead to some variability in the structure
of the material. Despite an overall rope-like morphology, varying
pitches appeared, and the spirals range from about 0.5 to 1 µm.
The self-assembly properties which can be assigned to the
organic units, offer possibilities for achieving a structure control
at different scales. Material with chiral morphology and properties
are of great interest for heterogeneous catalysis. Whereas in
solution, very efficient and highly enantioselective molecular
catalytic species have been obtained by designing the coordination
sphere of transition metal complexes,17 the design of heterogenised
catalyst with activity and enantioselectivity similar to those of
homogeneous species remains a challenge.18 We already showed
that enhanced enantioselectivity can result from supramolecular
effect in an amorphous chiral hybrid host matrix.13 Controlled
formation of the three-dimensional chiral network in hybrid silicas
may permit a tailoring of the environment around the immobilized
species analogous to that of molecular species. This may lead in
the future to a rational design of chiral hybrid catalyst.
The hybrid gel (R,R)-H was then synthesized under acidic
hydrolysis in a purely aqueous medium. In a typical synthesis, a
mixture containing molar ratio of (R,R)-2:1, H2O:600, HCl:0.2
was vigorously stirred in an oil-bath at 80 °C for 2 h and then
allowed to stand for 2 days at the same temperature. The resulting
white slurry solution was filtered and washed with water until
neutral pH was attained and again washed with ethanol. Careful
drying at 110 °C of the gel led to a white solid (R,R)-H. The
precursor (S,S)-2,[R]D -4.8 (c ) 3.1, CHCl3), and hybrid gel
(S,S)-H were similarly obtained from trans-(1S,2S)-diaminocy-
clohexane (S,S)-1. For both hybrids, the 29Si CP-MAS NMR
spectra exhibit two signals at -58 and -67 ppm assigned to SiC-
(OH)(OSi)2 (T2) and SiC(OSi)3 (T3). Consistent with the existence
of covalent Si-C bonding between the organic diureido structure
and the silicate units, no signal corresponding to SiO4 (Q)
substructures was detected. The 13C solid-state spectra show a
peak at 160 ppm attributed to the CdO signal in addition to the
different sp3 carbon atoms at 55, 43, 34, 25, and 10 ppm. The
FTIR spectrum showed vibrations at 3344, 1636, and 1576 cm-1
;
these values, close to those observed in the organogel gave
evidence for H-bonded urea groups within the solid.
Scanning electron microscopy (SEM) images reveal that the
whole materials have a fiber-like structure and (R,R)-H (Figure
1a,b) and (S,S)-H (Figure 1c,d) are essentially composed of helical
fiber-like structures assembled in bundles within the range of
0.3-1 µm width and up to 15 µm length. The helices were always
right-handed for the materials obtained from the (R,R)-2 and left-
handed in the case of those arising from the (S,S)-2. TEM images
confirm the fiber-like structure of the materials, the average width
of a fiber being approximately 2-5 nm.
It represents the first example of the creation of a chiral
morphology in hybrids. It was not observed in the case of the
sol-gel condensation of related chiral diaminocyclohexane
precursors not possessing auto-association properties,13 therefore
providing here evidence for the role of self-association by H-bonds
during the solid synthesis.
Acknowledgment. Support from the “Ministe`re de la Recherche” and
CNRS is gratefully acknowledged.
JA003843Z
(15) Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1990, 29, 1304-1319 and
references therein.
(16) Terech, P.; Weiss, R. G. Chem. ReV. 1997, 97, 3133-3159 and
references therein.
Nitrogen sorption experiments of (R,R)-H revealed a type II
adsorption-desorption isotherm and a BET surface area of 120
m2‚g-1. It showed a broad distribution of pore size in the mesopore
(17) Noyori, R. Asymmetric Catalysis in Organic Synthesis; Wiley-
Interscience: New York, 1994 and references therein.
(18) Blaser, H. U. Chem. ReV. 1992, 92, 935-952.