Amino acid silica hybrid materials with mesoporous structure and enantiopure surfaces

Article abstract of DOI:10.1002/anie.200803405

(Figure Presented) Pure and porous: Several hundreds of square meters of enantiopure surface per gram of material were generated by introducing amino acids into periodically ordered mesoporous organosilica materials. The stereochemical nature of the surfa

Full text of DOI:10.1002/anie.200803405

Angewandte
Chemie
DOI: 10.1002/anie.200803405
Porous Materials
Amino Acid Silica Hybrid Materials with Mesoporous Structure and
Enantiopure Surfaces**
Andreas Kuschel, Heiko Sievers, and Sebastian Polarz*
Stereochemistry and in particular chirality is of crucial
importance both for basic scientific research and application
purposes. Proteins, representing the most important operating
entity in organisms, are composed of stereochemically pure
and specified amino acids. That is why the applications of
chemical products in biological context are often associated
directly with the ability to allocate them in an enantiomer-
ically pure form. In an extreme case the same compound can
either be a drug or a toxin depending on the enantiomeric
form. Enantioselective synthesis has reached a profound stage
of development.[1] However, as long as the enantiomeric
excess of catalytic reactions is below 100% further improve-
ment of the reactions and additional purification of the
products is necessary.[2]
porous materials contain the chiral entity only in minor
amounts, typically in the range 5–15 mol%.[7] There is an
alternative approach to equip mesoporous silica materials
with a higher content of organic functionality. Periodically
ordered mesoporous organosilica materials (PMOs) are
prepared by pure and undiluted silsesquioxane precursors
[10]
ꢀ ꢀ
possessing a bridging organic group (R’O)3Si R Si(OR’)3.
Because the composition of the resulting mesoporous net-
work can be described as RSi2O3, the interfacial accessibility
of the organic group is maximized.[11] The PMO field has been
described recently in excellent review articles.[7,12] Examples
for real PMOs (ca. 100% organic modification) in which
chiral building blocks have been used are rare. Our group
applied enantioselective catalytic hydroboration to the pre-
ꢀ
=
ꢀ
The idea to equip a nanoporous material with chiral
interfaces is tempting.[3] The interaction of confined guests
with the pore walls can be triggered by the adjustment of pore
size and the target-oriented variation of surface functional
groups. Such materials are valuable in chiral separation
technology. Furthermore, they could eventually act as a
stereo-directing reaction field.[4] It has been documented that
higher values of enantiomeric excess can be achieved when
the reaction is conducted in a chiral matrix through indirect
stereochemical induction.[5] Chiral groups can be attached to
the walls of mesoporous silica materials such as MCM-41 or
SBA-15 by grafting,[6] or through the condensation of terminal
organosilanes R*Si(OR’)3 (R* = chiral, organic group) with
cursor (EtO)3Si CH CH Si(EtO)3 to give a chiral PMO
ꢀ
ꢀ
ꢀ
with “Si C*H(OH) CH2 Si” entities embedded in the
walls.[13] Thomas and co-workers used a chiral hydroboration
agent directly instead of a stereoselective catalyst.[14] Froeba
and co-workers could prepare a PMO with chiral benzylic
ether bridges in 88% ee also by enantioselective catalysis.[15]
The synthesis of a stable, nanoporous matrix that is
exclusively made from a chiral building block is difficult. This
high goal was first reached in the field of metal–organic
framework solids (MOFS) containing chiral linkers.[16] Even
for MOFS there are only limited reports describing amino
acids as linker groups.[17] Furthermore, similar to other
crystalline, microporous materials, it is difficult to extend
the pore size beyond 2 nm. As a consequence, it is challenging
to use an enantiomerically pure compound from the natural
pool as a bridging entity in a respective silsesquioxane
precursor for PMO synthesis. Herein, we present a novel
solid material constructed by the controlled assembly of
amino acid derivatives into a well-defined mesoporous
structure possessing above 600 m2 gꢀ1 of internal chiral sur-
face.
ꢀ
surface-bound silanol groups (Si OH) resulting in a stable
[7]
ꢀ ꢀ
Si O Si linkage. Examples exist for amino acids and short
peptide sequences.[8] Aida and co-workers reported a differ-
ent, highly interesting method.[7,9] A triblock surfactant, the
so-called lizard template, containing an alkoxysilane head
group covalently bound to an amino acid group attached to a
long alkyl chain together with approximately 90% of a source
for unmodified SiO2 was employed for the one-pot synthesis
of a mesoporous material with ordered porosity.[9] However,
the inherent disadvantage of the described methods is that the
We previously reported a PMO material (UKON2a)
containing a bridging benzoic acid function along the pore
walls (Figure 1a).[18] The benzoic acid groups in the pore walls
are accessible for chemical modifications. The mesoporous
[*] A. Kuschel, Prof. Dr. S. Polarz
Department of Chemistry, Universitꢀt Konstanz
78457 Konstanz (Germany)
Fax: (+49)7531-884-406
E-mail: sebastian.polarz@uni-konstanz.de
Homepage: http://www.uni-konstanz.de/polarz
ꢀ
ꢀ
organosilica UKON2a was treated with H2N Ala OMe
(Ala = alanine) to give the material denoted UKON3a or
ꢀ
ꢀ
ꢀ
with the “dipeptide” H2N Ala Asp (OMe)2 (Asp = aspara-
gine) to give UKON3b (see the Experimental Section in the
Supporting Information). The modification of the benzoic
acid groups can be monitored by NMR spectroscopy (spectra
shown in the Supporting Information) and small-angle X-ray
scattering (SAXS; Figure 1b). The 13C-MAS NMR spectra of
UKON3a and UKON3b are markedly different from that of
UKON2a, which is characterized by the signals for the
COOH group (d = 173 ppm) and the aromatic carbon atoms
Dr. H. Sievers
Quantachrome GmbH & Co. KG
Rudolf-Diesel Strasse 12, 85235 Odelzhausen (Germany)
[**] The project is funded by the Deutsche Forschungsgemeinschaft
(DFG; project 780/6-1).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200803405.
Angew. Chem. Int. Ed. 2008, 47, 9513 –9517
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9513

Communications
is successful, it can not be excluded that not all the benzoic
acid groups present in UKON2a react. For example, in the
13C NMR spectrum of UKON3a two signals seem to be
superposed in the spectral region typical for the -COX
function (d1 = 167 ppm; d2 = 173 ppm) indicating that the
composition of the materials has to be described as
[SiC6H3COOAlaOMeSiO3]n[SiC6H3COOHSiO3]m. The same
argument accounts for UKON3b.
Consequently, it would be better to ensure that the entire
material contains exclusively the desired amino acid entities.
This is only possible by the bottom-up construction from a
new bridged silsesquioxane precursor shown in Scheme 1.
Activation of the -COOH group in 1 can be achieved by
conversion of 1 into the benzoyl chloride derivative 2. The
latter reacts with an allyl ester protected alanine, and after
cleavage of the protecting group the desired compound 3 can
be isolated. The compounds 2 and 3 were unambiguously
characterized by NMR, IR, UV/Vis, and circular dichroism
(CD) spectroscopy, as well as electron impact mass spec-
trometry (EI-MS) (see the Supporting Information). Precur-
sor 3 was used for the preparation of a PMO material using a
liquid crystalline phase of an amphiphilic block copolymer as
a template. Template removal was achieved by liquid–liquid
extraction. The resulting material UKON3c was character-
ized by transmission electron microscopy (TEM), SAXS, and
nitrogen physisorption measurements, as well as solid-state
NMR, FTIR, UV/Vis, and CD spectroscopy.
Four reflexes (q100 = 0.62 nmꢀ1, q110 = 1.03 nmꢀ1, q200
=
1.23 nmꢀ1, and q210 = 1.66 nmꢀ1) can be observed in SAXS,
and in TEM a cylindrical, hexagonally aligned channel
structure can be seen (Figure 2a and Supporting Informa-
tion). The prepared PMO is well-ordered and possesses an
average pore size of 5.2 nm and a BET surface area of
633 m2 gꢀ1, as determined from N2 physisorption measure-
ments (see the Supporting Information). From these results
Figure 1. a) Synthesis of the PMO materials UKON3a,b by post-
modification of the PMO UKON2a. b) SAXS patterns recorded for
UKON2a (g), UKON3a (a), and UKON3b (c).
(d = 141, 135 ppm).[18] The former spectra
are in good agreement with known data
for classical amino acid compounds. For
example, the three signals at d = 19.5,
35.0, and 53.5 ppm in the spectrum of
Scheme 1. Synthesis of a PMO material with l-(+)-alanine embedded in the pore wall.
UKON3b correspond to alanine CH3,
ꢀ
aspargine CH2, and amino acid CH/O
CH3 groups, respectively. The main reflec-
tion observed in SAXS shifts from 0.715 nmꢀ1 for UKON2a
to 0.801 nmꢀ1 for UKON3a or 0.832 nmꢀ1 for UKON3b.
Because the latter two materials originate from the “parent”
material UKON2a, it is unlikely that the grafting of amino
acids changes the periodicities of the pore system to such
extent (DdUKON2a!UKON3a = ꢀ0.943 nm and DdUKON3a!
UKON3b = ꢀ0.293 nm). The shift of the diffraction maximum
in SAXS can be rationalized if one considers that, because of
the amino acid groups at the surfaces (Figure 1a), UKON3a
and UKON3b can no longer be regarded as strict two-phase
systems (solid pore wall and empty pores). The latter
assumption is also supported by the Porod behavior of the
scattering patterns at high angles. The curves for UKON3a/
3b decay significantly slower than that of UKON2a (/
qꢀ4).[19] Although the described grafting method (Figure 1a)
and those from the SAXS and TEM analysis it can be
concluded that the pore walls are rather thick (ca. 4.5 nm).
The molecular composition of UKON3c was analyzed by 13C-
MAS NMR spectroscopy (Figure 2b). In the 13C-MAS NMR
spectrum, only the signals related to the phenyl-Ala entity can
be observed. Comparison with the spectrum of the precursor
reveals that the alkoxysilane groups (d = 64.8, 24.3 ppm) have
been cleaved in the course of the sol–gel process. The
carbonyl–amide bond (d = 170.4 ppm) as well as the amino
acid function were fully retained. 29Si-MAS NMR spectros-
copy proves that the carbon–silicon bond is also stable under
the chosen conditions (see the Supporting Information). Only
the three signals typical for monosubstituted organosilica
materials RSi(OH)2(OSi) (d = ꢀ69.5 ppm), RSi(OH)(OSi)2
(d = ꢀ78.1 ppm), RSi(OSi)3 (d = ꢀ87.3 ppm) are found.
9514
www.angewandte.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 9513 –9517

Angewandte
Chemie
amino acid through the -COX
function of the latter requires a
new starting compound, 3,5-
bis(triisopropoxysilyl)aniline
(4) as indicated in Scheme 2.
Compound 4, which is
reported herein for the first
time, could be prepared by
Pd-catalyzed coupling of 3,5-
bis(triisopropoxysilyl)bromo-
benzene and zinc bis(trimethyl-
silyl)amide (see the Supporting
Information). It should be
noted that 4 itself can be con-
verted into an interesting PMO
material. However, the latter
aspect is beyond the scope of
the current manuscript and will
be reported elsewhere. Herein,
we concentrate on the possibil-
ity of attaching N-protected
amino acids to 4 to obtain
new, amino-terminated precur-
sors 5 (see the Experimental
Section in the Supporting
Information).
Figure 2. Selected analytical results for UKON3c: a) TEM micrograph and SAXS pattern (inset).
b) 13C NMR spectra of the mesoporous solid (black; solid state) and the precursor 3 (gray; recorded in
CDCl3) as a reference. c) CD spectra of a thin mesoporous film (black) and the precursor 3 (gray;
measured in solution) as a reference.
Then, the synthesis of the
corresponding PMO UKON3d
proceeds in analogy to that of
UKON3c using Pluronic P123
Therefore, the composition of UKON3c can be described as
[SiC6H4COAlaSiO3]n.
(see Scheme 1) as structuring agent in an acidic medium. The
resulting mesoporous organosilica was analyzed by the same
analytical techniques (see the Supporting Information). The
Furthermore, UKON3c is optically active. A transparent
film thin enough to allow CD measurements was prepared by
spin coating on a quartz plate. UKON3c gives a complicated
CD spectrum. A solution of precursor 3 was measured as a
reference and both spectra are compared in Figure 2c. It can
be concluded that the formal composition and stereochem-
istry of UKON3c is l-(+)-AlaC6H4Si2O3. The essential
question remaining is if the internal surfaces of UKON3c
possess chiral properties. Recording physisorption data using
chiral gases is potentially able to answer this question. To the
best of our knowledge our results presented below are the
first proof of the chirality of surfaces using physisorption
measurements.
Propylene oxide was selected as a chiral gas because of its
low boiling point, sufficient stability, and ready availability of
both enantiomers in pure form. Adsorption isotherms
recorded at 313.15 K are shown in Figure 3. UKON3c adsorbs
significant amounts of propylene oxide over the entire
pressure range. (R)-Propylene oxide is markedly less adsor-
bed than the S enantiomer. The differences are more
pronounced at low pressures. This result is reasonable, since
the strongest effects are expected for low coverage, which
occurs at low pressure.
The surfaces of UKON3c are covered with (chiral)
carboxylic groups. It could be interesting also to prepare an
analogous material with surfaces characterized by chiral,
alkaline groups. The linkage of the precursor backbone to an
Figure 3. Top: Adsorption of the two enantiomers of propylene oxide
on the chiral surfaces of the UKON3c material. Bottom: Adsorption
data recorded at T=313.15 K. R enantiomer: red; S enantiomer: blue.
Angew. Chem. Int. Ed. 2008, 47, 9513 –9517
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
9515

Communications
by the signals typical for T-linked silicon centers (see the
Supporting Information). One further advantage of 3,5-
bis(triisopropoxysilyl)aniline (4) is that alternative, more
functional natural amino acids can be attached. Therefore, a
new sol–gel precursor containing histidine (6) and the
corresponding organosilicate (UKON3e) was prepared as a
proof of concept (see Scheme 2). A comparison of the
13C NMR spectrum of the precursor in CDCl3 with that of
the resulting (C6H4NH-His)Si2O3 product is shown in Fig-
ure 4b. The corresponding 29Si-NMR spectrum is given in the
Supporting Information. SAXS (see the Supporting Informa-
tion) reveals one reflection at q = 0.62 nmꢀ1, indicating the
expected meso order.
Scheme 2. Preparation PMO materials characterized by surfaces con-
taining amino-terminated alanine or histidine.
Summarizing, two methods have been discussed for the
preparation of mesoporous materials containing enantiomer-
ically pure amino acids located at the internal surfaces. The
precursor approach allows the preparation of a PMO material
composed exclusively of the amino acid building block. Pore
walls terminated with carboxylic acid or amino groups could
be generated. The chirality of the surfaces was probed by
applying a unique experiment, namely, the measurement of
adsorption of a chiral gas on the mesoporous solids.
TEM and SAXS results (main reflection at q = 0.61 nmꢀ1) are
consistent with a wormhole pore system. The 13C-MAS NMR
spectrum of UKON3d is given in Figure 4a. The signals at d =
20 ppm and 52 ppm are typical for the CH3 and CH group in
alanine. The peak at d = 172 ppm indicates that the CON
bond is still intact. Similarly to the spectrum of UKON3c, the
29Si-MAS NMR spectrum of UKON3d is also characterized
Experimental Section
A detailed description of the preparation and characterization of all
compounds and materials discussed herein is given in the Supporting
Information.
Received: July 14, 2008
Published online: October 31, 2008
Keywords: chiral materials · mesoporous materials ·
.
organic–inorganic hybrid composites · surface chemistry
[1] a) H. C. Kolb, M. S. Vannieuwenhze, K. B. Sharpless, Chem. Rev.
1994, 94, 2483; b) R. H. Grubbs, S. Chang, Tetrahedron 1998, 54,
4413; c) Advanced Asymmetric Synthesis (Ed.: G. R. Stephen-
son), Chapman and Hall, London, 1996; d) G. Procter, Stereo-
selectivity in Organic Synthesis, Oxford University Press, Oxford,
1998; e) Preparative Enantioselective Chromatography (Ed.:
G. B. Cox), Blackwell Publishing, Oxford, 2005; f) P. I. Dalko,
L. Moisan, Angew. Chem. 2001, 113, 3840; Angew. Chem. Int.
Ed. 2001, 40, 3726; g) P. I. Dalko, L. Moisan, Angew. Chem. 2004,
116, 5248; Angew. Chem. Int. Ed. 2004, 43, 5138.
[2] a) Y. Hayashi, M. Matsuzawa, J. Yamaguchi, S. Yonehara, Y.
Matsumoto, M. Shoji, D. Hashizume, H. Koshino, Angew. Chem.
2006, 118, 4709; Angew. Chem. Int. Ed. 2006, 45, 4593; b) V. A.
Soloshonok, Angew. Chem. 2006, 118, 780; Angew. Chem. Int.
Ed. 2006, 45, 766; c) M. Klussmann, T. Izumi, A. J. P. White, A.
Armstrong, D. G. Blackmond, J. Am. Chem. Soc. 2007, 129,
7657; d) T. Satyanarayana, H. B. Kagan, Tetrahedron 2007, 63,
6415; e) V. A. Soloshonok, H. Ueki, M. Yasumoto, S. Mekala,
J. S. Hirschi, D. A. Singleton, J. Am. Chem. Soc. 2007, 129, 12112.
[3] a) S. Polarz, B. Smarsly, J. Nanosci. Nanotechnol. 2002, 2, 581;
b) S. Che, J. Nanosci. Nanotechnol. 2006, 6, 1557; c) C. Li, Catal.
Rev. Sci. Eng. 2004, 46, 419; d) C. Li, H. D. Zhang, D. M. Jiang,
Q. H. Yang, Chem. Commun. 2007, 547.
Figure 4. a) 13C-MAS NMR spectrum of UKON3d (black) compared
with the 13C NMR spectrum of the precursor 5 (gray); b) 13C-MAS
NMR spectrum of UKON3e (black) compared with the 13C NMR
spectrum of precursor 6 (gray).
[4] a) S. Polarz, A. Kuschel, Chem. Eur. J. 2008, DOI: 10.1002/
chem.200800674; b) J. M. Thomas, R. Raja, Acc. Chem. Res.
2008, 41, 708.
[5] J. Ding, W. Armstrong, Chirality 2005, 17, 281.
9516
www.angewandte.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 9513 –9517

Angewandte
Chemie
[6] a) C. T. Kresge, M. Leonowicz, W. J. Roth, J. C. Vartuli, J. S.
Beck, Nature 1992, 359, 710; b) P. Yang, D. Zhao, D. I.
Margolese, B. F. Chmelka, G. D. Stucky, Nature 1998, 396, 152.
[7] F. Hoffmann, M. Cornelius, J. Morell, M. Frꢀba, Angew. Chem.
2006, 118, 3290; Angew. Chem. Int. Ed. 2006, 45, 3216.
[8] M. Luechinger, A. Kienhofer, G. D. Pirngruber, Chem. Mater.
2006, 18, 1330.
[15] J. Morell, S. Chatterjee, P. J. Klar, D. Mauder, I. Shenderovich, F.
Hoffmann, M. Frꢀba, Chem. Eur. J. 2008, 14, 5935.
[16] a) B. Kesanli, W. B. Lin, Coord. Chem. Rev. 2003, 246, 305; b) D.
Bradshaw, J. B. Claridge, E. J. Cussen, T. J. Prior, M. J. Rossein-
sky, Acc. Chem. Res. 2005, 38, 273; c) W. B. Lin, J. Solid State
Chem. 2005, 178, 2486.
[17] a) R. Murugavel, G. Anantharaman, D. Krishnamurthy, M.
Sathiyendiran, M. G. Walawalkar, Proc. Indian Acad. Sci. Chem.
Sci. 2000, 112, 273; b) R. Vaidhyanathan, D. Bradshaw, J. N.
Rebilly, J. P. Barrio, J. A. Gould, N. G. Berry, M. J. Rosseinsky,
Angew. Chem. 2006, 118, 6645; Angew. Chem. Int. Ed. 2006, 45,
6495; c) Y. Xie, Y. Yan, H. H. Wu, G. P. Yong, Y. Cui, Z. Y.
Wang, L. Pan, J. Li, R. Fan, R. P. Li, Y. C. Tian, G. Q. Pan, L. S.
Sheng, X. Li, Inorg. Chim. Acta 2007, 360, 1669; d) Y. Xie, Z. P.
Yu, X. Y. Huang, Z. Y. Wang, L. W. Niu, M. Teng, J. Li, Chem.
Eur. J. 2007, 13, 9399.
[9] Q. M. Zhang, K. Ariga, A. Okabe, T. Aida, J. Am. Chem. Soc.
2004, 126, 988.
[10] a) T. Asefa, M. J. MacLachan, N. Coombs, G. A. Ozin, Nature
1999, 402, 867; b) S. Inagaki, S. Guan, Y. Fukushima, T. Ohsuna,
O. Terasaki, J. Am. Chem. Soc. 1999, 121, 9611; c) B. J. Melde,
B. T. Holland, C. F. Blanford, A. Stein, Chem. Mater. 1999, 11,
3302.
[11] M. J. MacLachlan, T. Asefa, G. A. Ozin, Chem. Eur. J. 2000, 6,
2507.
[12] B. Hatton, K. Landskron, W. Whitnall, D. Perovic, G. A. Ozin,
Acc. Chem. Res. 2005, 38, 305.
[13] S. Polarz, A. Kuschel, Adv. Mater. 2006, 18, 1206.
[14] A. Ide, R. Voss, G. Scholz, G. A. Ozin, A. Antonietti, A. Thomas,
Chem. Mater. 2007, 19, 2649.
[18] A. Kuschel, S. Polarz, Adv. Funct. Mater. 2008, 18, 1272.
[19] a) G. Porod, Kolloid Z. Z. Polym. 1951, 124, 83; b) G. Porod,
Kolloid Z. Z. Polym. 1952, 125, 51.
Angew. Chem. Int. Ed. 2008, 47, 9513 –9517
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
9517