Enantioselective extraction mediated by a chiral cavitand-salen covalently assembled on a porous silicon surface

Article abstract of DOI:10.1039/c4cc00034j

A chiral organic-inorganic hybrid material, based on a porous silicon surface functionalized with a chiral cavitand, was designed and synthesized. The affinity of this device in water toward a bromine-marked alkyl-ammonium salt has been evaluated using XPS detection. UV and CD measurements highlight the enantioselective extraction from a racemic mixture in water of the S-enantiomer of the selected guest (ee ≥ 80%).

Full text of DOI:10.1039/c4cc00034j

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Enantioselective extraction mediated by a chiral
cavitand–salen covalently assembled on a
porous silicon surface†
Alessandro D’Urso,‡a Cristina Tudisco,‡ab Francesco P. Ballistreri,a
Guglielmo G. Condorelli,ab Rosalba Randazzo,a Gaetano A. Tomaselli,a
Rosa M. Toscano,a Giuseppe Trusso Sfrazzetto*a and Andrea Pappalardo*a
Cite this: Chem. Commun., 2014,
50, 4993
Received 2nd January 2014,
Accepted 24th January 2014
DOI: 10.1039/c4cc00034j
www.rsc.org/chemcomm
A chiral organic–inorganic hybrid material, based on a porous
Cavitand receptors have been recently bonded to flat Si (100) to
silicon surface functionalized with a chiral cavitand, was designed perform molecular recognition tasks at the silicon–liquid13 and at the
and synthesized. The affinity of this device in water toward a silicon–air14 interfaces. Among these systems, porous silicon (PSi) is a
bromine-marked alkyl-ammonium salt has been evaluated using material with unique properties, which includes, besides the men-
XPS detection. UV and CD measurements highlight the enantio- tioned high surface area, convenient surface chemistry and easy
selective extraction from
S-enantiomer of the selected guest (ee Z 80%).
a
racemic mixture in water of the integration in silicon-based devices.15
Recently, we reported on the synthesis of diastereomeric triqui-
noxaline cavitands, containing a salen chiral framework, and their
Chirality is a property of matter that plays a key role in several uranyl complexes which were proved to achieve excellent selective
branches of science, from optics to medicine.1 Many biological molecular recognition of chiral ammonium ion pairs.16 In particular,
processes involve chiral molecules in order to achieve highly specific we verified the extraction properties of these salen cavitands in a
and selective interactions. The separation of chiral isomers is an biphasic system (chloroform–water) towards the chiral guest N,N,N-
appealing burgeoning field that captures the interest of many trimethyl-a-methylbenzyl ammonium iodide.16a The anchoring of
researchers.2 Different methods have been employed to resolve a these supramolecular hosts, whose synthesis is expensive and time-
racemic mixture in solution, including crystallization,3 chromato- consuming, onto a surface is highly desirable since it allows the
graphy,4 and magnetic stirring.5 Usually, these techniques exploit multiple use of the solid device and reduces the purification steps of
the enantioselective recognition using chiral metal–ligand com- the hosts. Taking into account these considerations, we designed and
plexes,6 chiral polymers,7 micelles,8 gels9 and enzymes.10
synthesized a chiral organic–inorganic hybrid material (PSi–SalCav,
An interesting approach for harnessing the full potential of Fig. 1)17 based on a porous silicon surface functionalized with the
molecular receptors consists of their arrangement in mono- chiral SalCav, and studied the enantioselection properties in water of
layers hosted on an inorganic surface.11 Moreover, the use of a this device towards (R/S)-N,N,N-trimethyl-a-methyl-p-bromo-benzyl-
heterogeneous solid device for enantioselective recognition ammonium iodide (Br-MBA) by X-ray photoelectron spectroscopy
would allow reutilization of the solid device, thus increasing (XPS) and circular dichroism (CD) measurements.
the enantiomeric separation under eco-friendly conditions. To
The cavitand monolayer was prepared from a solution of SalCav
date, although some examples of enantiomeric recognition bearing an undecylenic foot, via thermal hydrosilylation of the double
assisted by chiral surfaces have been reported in the litera-
ture,12 to our knowledge, no examples of chiral host molecules
anchored on a solid support, able to enantiodiscriminate, have
been hitherto described.
a
`
Dipartimento di Scienze Chimiche, Universita di Catania, Viale Andrea Doria 6,
95125, Catania, Italy. E-mail: andrea.pappalardo@unict.it,
giuseppe.trusso@unict.it
b INSTM Udr of Catania, Viale Andrea Doria 6, 95125, Catania, Italy
† Electronic supplementary information (ESI) available: Synthesis of guest,
porous silicon preparations, procedure for the extraction, XPS and CD data using
an unfunctionalized PSi surface, CD calibration, UV spectra of Br-MBA solution
after extraction and reversibility experiments. See DOI: 10.1039/c4cc00034j
‡ These authors contributed equally.
Fig. 1 Molecular structures of the chiral cavitand-salen receptor (SalCav)
and the selected guest (R/S)-N,N,N-trimethyl-a-methyl-p-bromo-benzyl-
ammonium iodide (R/S-Br-MBA), and the schematic representation of
SalCav grafted onto the PSi surface (PSi–SalCav).
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bonds on H-terminated PSi, prepared by metal-assisted chemical The XPS of the Br 3d spectral region of the treated Psi surface did not
etching.†17 The affinity of the PSi–SalCav surface towards alkyl- show any evidence of the guest, thus indicating that interaction did
ammonium salts has been evaluated using the bromine-marked not occur without SalCav functionalization.†
guest (Æ)N,N,N-trimethyl-a-methyl-p-bromobenzylammonium iodide
To evaluate the enantioselectivity properties of the functionalized
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(Br-MBA), to allow easy XPS detection.
surface and to estimate the enantiomeric excess towards Br-MBA, CD
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Complexation was accomplished through the immersion of measurements were carried out. The device was dipped in a racemic
PSi–SalCav into a racemic R/S-Br-MBA solution in water. The solution of Br-MBA (1 mM, Fig. 2A). After the removal of the
complexation of the Br-MBA guest on PSi–SalCav has been functionalized surface, a CD spectrum of the resulting Br-MBA
assessed by XPS measurements.
solution, diluted to 30 mM, was obtained.18 As shown in Fig. 2D, a
In Fig. 2B the XPS of the Br 3d spectral region of PSi–SalCav before small positive CD signal at 223 nm is observed, indicating an excess
complexation shows no signal for any Br. After dipping the PSi– of the R-Br-MBA enantiomer in the resulting solution. Thus,
SalCav into the Br-MBA solution, the presence of the Br 3d bands in these data suggest that PSi–SalCav preferentially recognizes the
the XPS pattern of PSi–SalCav confirms the complexation ability of S-enantiomer of Br-MBA.
the cavitand-functionalized surface (PSi–SalCavÁBr-MBA, Fig. 2C).
Considering that enantiomer molecules show mirror image CD
Note that complete decomplexation of Br-MBA was obtained by a spectra, the selective extraction of the S-enantiomer of Br-MBA by the
simple immersion of PSi–SalCavÁBr-MBA in water for 15 min, as chiral surface has to be confirmed by recording the CD spectrum of
shown by the disappearance of the Br 3d signal (Fig. 2E). These the extracted guest. As expected, the immersion of the PSi–SalCavÁBr-
experiments give rise to the reversible recognition of Br-MBA on the MBA in water for 15 min led to a desorbed solution, whose CD
PSi–SalCav surface, due to the inclusion of the guest inside the spectrum is the mirror image of the R-Br-MBA enantiomer, establish-
cavities of the cavitands. Furthermore, to rule out the possibility that ing the selective extraction of the S-enantiomer (Fig. 3).
the recognition on the PSi surface occurs even without SalCav
To quantify the enantiomeric excess CD and UV data were
functionalization, control experiments were performed. Unfunction- combined. The CD spectrum of the solution of the Br-MBA salt, after
alized PSi surfaces were immersed for 2 h into a solution of Br-MBA. the immersion of PSi–SalCav, showed an excess of 3.28 mM of the
Fig. 2 XPS and CD experiments of the PSi–SalCav surface: (A) the CD spectrum of the racemic mixture R/S-Br-MBA in water; (B) the XPS pattern of the
Br region of PSi–SalCav; (C) the XPS pattern of the Br region of PSi–SalCav complexed with Br-MBA (PSi–SalCavÁBr-MBA); (D) the CD spectrum of the
resulting mixture of R/S-Br-MBA after the immersion of PSi–SalCav; (E) the XPS pattern of the Br region of PSi–SalCav after decomplexation with water;
(F) the CD spectrum of the resulting water solution used for the decomplexation of PSi-SalCavÁBr-MBA.
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In conclusion, a promising chiral organic–inorganic hybrid
device, based on a chiral cavitand–salen covalently assembled
on a porous silicon surface, having enantioselection properties
in water solution, and exploiting non-covalent interaction, was
disclosed. This chiral surface paves the way for the develop-
ment of synthetic heterogeneous enantioselective catalysts.
Furthermore, the facile handling of the chiral surface and its
successive reusability, make this device an ideal tool for the
enantioselective recognition of chiral guests.
The authors thank University of Catania and CIRCC, for
financial support through (FIRB 2012 RBFR12WB3W project),
FIRB ‘‘RINAME Rete Integrata per la NAnoMEdicina’’ (RBA-
P114AMK), MIUR PRIN 2010–2011 – 2010N3T9M4_004, and
Prof. Roberto Purrello for useful discussions.
Fig. 3 CD spectra of the resulting solution of Br-MBA diluted to 30 mM
after the immersion of PSi–SalCav (black curve), desorbed solution
(red curve) and 3 mM solution of R-Br-MBA (dashed blue curve).
Notes and references
1 G. H. Wagniere, On Chirality and the Universal Asymmetry, Wiley-
VCH, Zurich, Weinheim, 2007.
2 (a) G. A. Hembury, V. V. Borovkov and Y. Inoue, Chem. Rev., 2008,
108, 1; (b) Y. Liu, W. Xuan and Y. Cui, Adv. Mater., 2010, 22, 4112.
3 C. Viedma and P. Cintas, Chem. Commun., 2011, 47, 12786.
4 B. Schuur, B. J. V. Verkuijl, A. J. Minnaard, J. G. de Vries, H. J. Heeres
and B. L. Feringa, Org. Biomol. Chem., 2011, 9, 36.
5 A. D’Urso, R. Randazzo, L. Lo Faro and R. Purrello, Angew. Chem.,
Int. Ed., 2010, 49, 108.
6 (a) G. Bianchini, A. Cavarzan, A. Scarso and G. Strukul, Green Chem.,
2009, 11, 1517; (b) C. Jahier, M. Coustou, M. Cantuel,
N. D. McClenaghan, T. Buffeteau, D. Cavagnat, M. Carraro and
S. Nlate, Eur. J. Inorg. Chem., 2011, 727.
7 (a) E. Yashima, K. Maeda and Y. Okamoto, Nature, 1999, 399, 449;
(b) V. Berl, I. Huc, R. G. Khoury, M. J. Krische and J.-M. Lehn, Nature,
2000, 407, 720; (c) T. Miyabe, H. Iida, M. Banno, T. Yamaguchi and
E. Yashima, Macromolecules, 2011, 44, 8687; (d) F. Song, G. Wei,
X. Jiang, F. Li, C. Zhu and Y. Cheng, Chem. Commun., 2013, 49, 5772.
8 C. Bombelli, C. Bernardini, G. Elemento, G. Mancini, A. Sorrenti and
C. Villan, J. Am. Chem. Soc., 2008, 130, 2732.
Fig. 4 Concentration of: (A) the racemate (Br-MBA salt racemic solution,
30 mM); (B) resulting solution of the racemate after immersion for 2 h of
Psi–SalCav; (C) and (D) concentrations of the pure S enantiomer (green)
and the racemic mixture of R- and S-Br-MBA (blue) extracted by the
surface from the initial racemic mixture; (E) concentration of desorbed
solution after the immersion of the device in 2 mL of water.
9 (a) X. Chen, Z. Huang, S.-Y. Chen, K. Li, X.-Q. Yu and L. Pu, J. Am. Chem.
¨
Soc., 2010, 132, 7297; (b) T. Tu, W. Fang, X. Bao, X. Li and K. H. Dotz,
R-Br-MBA enantiomer.† UV data of this solution indicated a
concentration decrease of 4.1 mM (from 30 mM to 25.9 mM,
Fig. 4B), suggesting that 8.2 Â 10À2 mmol of R/S-Br-MBA were
extracted from the initial racemic solution.19 Bearing in mind
that 3.28 mM is the concentration of the pure S extracted
enantiomer (i.e. 6.56 Â 10À2 mmol, calculated from CD data,
Fig. 4D, green area), the remaining 0.82 mM consists in an
equimolar mixture of S and R enantiomers (1.64 Â 10À2 mmol,
Fig. 4D, blue area), thus obtaining an enantiomeric excess
Z80%.20 Notably, the concentration value of the solution
obtained by dipping the device into 2 mL of water is B4.1 mM
(Fig. 4E), in good agreement with the total amount of R/S-Br-MBA
extracted by PSi–SalCav (Fig. 4D), thus confirming the whole
decomplexation as previously reported by XPS experiments.
Hence, the complexation–decomplexation reversibility of the PSi–
SalCav surface with a racemic mixture of the Br-MBA guest was
tested. Interestingly, upon three successive experiments in which the
PSi–SalCav surface was first rinsed in different freshly prepared
racemic solutions of Br-MBA for two hours, and then immersed in
water for 15 min, the recorded CD spectra exhibited a positive curve
centered at 223 nm, indicating the enantiomeric excess of R-Br-
MBA,† demonstrating the robustness and the reusability of this
functionalized surface.
Angew. Chem., Int. Ed., 2011, 50, 6601; (c) H. Jintoku, M. Takafuji,
R. Odac and H. Ihara, Chem. Commun., 2012, 48, 4881; (d) Q. Jin,
L. Zhang, X. Zhu, P. Duan and M. Liu, Chem.–Eur. J., 2012, 18, 4916.
10 A. Ghanem, M. N. Aboul-Enein, A. El-Azzouny, M. F. El-Behairy,
E. Al-Humaidi, A. A. Alaidan, K. Amin and M. N. Al-Ahdal, Chirality,
2008, 20, 871.
`
11 (a) G. G. Condorelli, A. Motta, M. Favazza, I. L. Fragala, M. Busi,
E. Menozzi, E. Dalcanale and L. Cristofolini, Langmuir, 2006, 22, 11126;
(b) Y. L. Bunimovich, Y. S. Shin, W.-S. Yeo, M. Amori, G. Kwong and
J. R. Heath, J. Am. Chem. Soc., 2006, 128, 16323; (c) A. Motta, C. Tudisco
and G. G. Condorelli, Sci. Adv. Mater., 2011, 3, 362.
12 (a) M. O. Lorenzo, C. J. Baddeley, C. Muryn and R. Raval, Nature, 2000,
404, 376; (b) V. Humblot, S. Haq, C. Muryn, W. A. Hofer and R. Raval,
J. Am. Chem. Soc., 2002, 124, 503; (c) A. Kuhnle, T. R. Linderoth,
B. Hammer and F. Besenbacher, Nature, 2002, 415, 891; (d) R. Ouyang,
J. Lei, H. Ju and Y. Xue, Adv. Funct. Mater., 2007, 17, 3223; (e) Y. Fu,
L. Wang, Q. Chen and J. Zhou, Sens. Actuators, B, 2011, 155, 140;
( f ) Q. Han, Q. Chen, Y. Wang, J. Zhou and Y. Fu, Electroanalysis, 2012,
24, 332; (g) Y. Yun and A. J. Gellman, Angew. Chem., Int. Ed., 2013,
52, 3394; (h) R. Kaminker, X. de Hatten, M. Lahav, F. Lupo, A. Gulino,
G. Evmenenko, P. Dutta, C. Browne, J. R. Nitschke and M. E. van der
Boom, J. Am. Chem. Soc., 2013, 135, 17052.
`
13 (a) E. Biavardi, M. Favazza, A. Motta, I. L. Fragala, C. Massera,
L. Prodi, M. Montalti, M. Melegari, G. G. Condorelli and
E. Dalcanale, J. Am. Chem. Soc., 2009, 131, 7447; (b) F. Lupo,
C. Capici, G. Gattuso, A. Notti, M. F. Parisi, A. Pappalardo,
S. Pappalardo and A. Gulino, Chem. Mater., 2010, 22, 2829;
(c) D. A. Cristaldi, I. Fragala, A. Pappalardo, R. M. Toscano,
F. P. Ballistreri, G. A. Tomaselli and A. Gulino, J. Mater. Chem.,
2012, 22, 675; (d) E. Biavardi, C. Tudisco, F. Maffei, A. Motta,
`
This journal is ©The Royal Society of Chemistry 2014
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C. Massera, G. G. Condorelli and E. Dalcanale, Proc. Natl. Acad. Sci.
U. S. A., 2012, 109, 2263.
A. Pappalardo, G. A. Tomaselli, R. M. Toscano and G. Trusso
Sfrazzetto, J. Org. Chem., 2012, 77, 7684.
14 (a) K. D. Schierbaum, T. Weiss, E. U. Thoden van Veizen, 17 C. Tudisco, G. Trusso Sfrazzetto, A. Pappalardo, A. Motta,
¨
`
J. F. J. Engbersen, D. N. Reinhoudt and W. Gopel, Science, 1994,
G. A. Tomaselli, I. L. Fragala, F. P. Ballistreri and G. G. Condorelli,
265, 1413; (b) G. G. Condorelli, A. Motta, M. Favazza, E. Gurrieri,
Eur. J. Inorg. Chem., 2011, 2124.
P. Betti and E. Dalcanale, Chem. Commun., 2010, 46, 288; 18 Due to the high extinction coefficient of Br-MBA (33 253 cmÀ1MÀ1),
(c) C. Tudisco, P. Betti, A. Motta, R. Pinalli, L. Bombaci,
E. Dalcanale and G. G. Condorelli, Langmuir, 2012, 28, 1782.
15 B. Chen, S. Xiang and G. Qian, Acc. Chem. Res., 2010, 43, 1115.
16 (a) M. E. Amato, F. P. Ballistreri, S. D’Agata, A. Pappalardo,
in order to record the CD spectra, the initial racemic solution was
diluted from 1 mM to 30 mM (Fig. 4A).
19 Considering a total volume of 2 mL of the solution, the decreasing
of 4.1 mM corresponded to 8.2 Â 10À2 mmol of R/S-Br-MBA.
G. A. Tomaselli, R. M. Toscano and G. Trusso Sfrazzetto, Eur. 20 Enantiomeric excess: ee = ([S] À [R])/([S] + [R]), where [S] =
J. Org. Chem., 2011, 5674; (b) M. E. Amato, F. P. Ballistreri,
(3.28 + 0.41) mM and [R] = 0.41 mM.
4996 | Chem. Commun., 2014, 50, 4993--4996
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