A molecular chalice with hydrophobic walls and a hydrophilic rim: Self-assembly and complexation properties

Article abstract of DOI:10.1039/c1cc15125h

We describe the synthesis of a diphenylglycoluril/dibenzo-crown-6 molecular chalice, the self-assembly at the air/water interface and its complexation properties in solution and at the water/chloroform interface.

Full text of DOI:10.1039/c1cc15125h

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COMMUNICATION
A molecular chalice with hydrophobic walls and a hydrophilic rim:
self-assembly and complexation propertieswz
Marc Vidal,ya Vanessa Kairouz,ya Christine DeWolfb and Andreea R. Schmitzer*a
Received 17th August 2011, Accepted 14th October 2011
DOI: 10.1039/c1cc15125h
We describe the synthesis of a diphenylglycoluril/dibenzo-
crown-6 molecular chalice, the self-assembly at the air/water
interface and its complexation properties in solution and at the
water/chloroform interface.
Molecular recognition at interfaces has relevance to biological
systems but is also important for modern applications such as
high sensitivity sensors. Selective binding of guest molecules in
solution to host molecules located at a solid surface has been
described during the past two decades using Langmuir mono-
layers, self-assembled monolayers and lipid assemblies as
recognition media.1 Diphenylglycoluril2 1 is a rigid, concave
molecule which can be easily functionalized in order to obtain
excellent building blocks for synthetic molecular clips and
tweezers with interesting molecular recognition properties,
such as self-assembling and molecular recognition of small
electron-deficient guest molecules.3 On the other hand, crown
ethers as macrocyclic polydentate ligands are used most often
as a catalyst in phase transfer processes4 by forming complexes
with inorganic salts. Their polyfunctionality makes them
particularly useful for the construction of various supramolecular
systems,5 to optically control the extraction of ions, as elements
of photo-switchable molecular devices, or as novel materials
for recording, storing, and processing optical information.6
The combination of diphenylglycoluril with crown ethers has
been previously described for the synthesis of molecular
baskets, where crown ethers were used as handles.7
Scheme 1 Synthesis of molecular chalice 5.
polar rim and the hydrophobic walls, as well as the particular
rigidity, we were interested in the extraction properties and
ability to form monomolecular films at the water/organic
solvent interface of this molecule. The first synthetic steps
outlined in Scheme 1 were previously reported in the literature
for preparing 4 from diphenylglycoluril.8 The final step in the
synthesis of 5 is the addition of the aromatic side walls and the
crown ether moiety by a quadruple Friedel–Crafts reaction of
4 and the dibenzo[18]crown-6 (DB18C6). As side products of
this reaction, mono- and double-attached DB18C6 were
formed, as previously reported for Friedel–Crafts reactions
with 4.8 Due to its particular shape, one of the most promising
applications of this molecular chalice would be cation recognition
and application in separation processes. Crystals of molecular
chalice 5 suitable for X-ray crystal structure determination
were obtained by diffusion of ether to a chloroform solution of
5 (CCDC 831478).
Here we report the design, synthesis and properties of a
chalice-shaped molecular receptor, with a diphenylglycoluril
bottom and a crown ether rim. Because of the duality of the
a
´
Department of Chemistry, Universite de Montreal, 2900 Edouard
Montpetit CP 6128 Succursalle Centre-ville, Montreal, Quebec,
´
The aromatic walls define a squared cavity, with the centers
of the benzene rings at 5.32 A distance (Fig. 1). The two fused
five-member rings of glycoluril form an electron rich shallow
floor, with two hydrogen bond acceptor sites (two carbonyl
oxygen atoms) separated by 5.56 A. The most unusual structural
feature of 5 lies in the asymmetric conformation of the crown
ether moiety, which may bind only small alkali metals, compared
to the free DB18C6.
´
´
Canada H3C3J7. E-mail: ar.schmitzer@umontreal.ca
b Department of Chemistry and Biochemistry, Concordia University,
´
´
7141 Sherbrooke Street West, Montreal, Quebec, Canada H4B 1R6
w Electronic supplementary information (ESI) available: Experimental
details, synthetic procedure, NMR, BAM data. CCDC 831478. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c1cc15125h
z We are grateful to the Natural Sciences and Engineering Research
Council of Canada, the Centre of Green Chemistry and Catalysis and
Universite
(U. Montre
des Materiaux, and Dr Rolf Schmidt for isotherm interpretation and
´
de Montre
´
´
al for financial support. We thank Prof. A. Badia
Alkali metal complexation in solution was followed by
NMR. If for Na+ and K+ cations complexation can be
observed by NMR, no significant NMR chemical shifts were
observed for Rb+ or Cs+. While an upfield shift is generally
al) for the use of instruments, Laboratoire de Caracte
´
risation
´
Dr Michel Simard for X-ray acquisition.
y Equal contribution.
c
12834 Chem. Commun., 2011, 47, 12834–12836
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Table 1 Alkali metal picrates extraction data
Percentage of picrate extracted
Na+
K+
Rb+
Cs+
Hosta
DB18C6
0.25 mM
5.1%
2.03 mM
40.6%
1.20 mM
23.9%
0.49 mM
9.8%
5
0.75 mM
15%
0.07 mM
1.5%
0.04 mM
0.8%
0 mM
0%
a
Extraction conditions: 5 mM of 5 or DB18C6 in water-saturated
CHCl3 solvent. The aqueous phase was 5 mM in alkali metal picrate.
Values given are the average of 3 independent extraction experiments.
Extraction samples were shaken for 0.5 h at r.t. and allowed to
equilibrate for 12 h prior to undertaking UV spectrophotometric
analysis of the aqueous phase at 374 nm.
It is not surprising that the alkali metal selectivity is completely
different when comparing the conformation of free DB18C6
and the crown ether moiety of 5.12 But the most interesting
observation of this extraction process is that the molecular
chalice 5 is absolutely not soluble in water, and the extraction
takes place at the interface. Only in the presence of sodium or
potassium picrates, precipitates were formed at the water–
chloroform interface, showing that the molecular recognition
took place at this interface (see ESIw Fig. S3: different surface
tension properties can be observed for the different alkali
metal picrate solutions). The discrimination ability of 5 towards
Na+ compared to K+ is also reflected by the amount of
precipitate formed at the interface during the extraction process.
In order to understand the 2D organisation of 5 at an
interface, a solution of the molecular chalice was spread on
a pure water surface from a 1 mM solution in chloroform. The
film structure was investigated by means of p–A isotherms and
microscopic observation of the textures of Langmuir mono-
layers was possible by Brewster Angle Microscopy (BAM) and
ellipsometric microscopy techniques.13 The surface pressure/
area compression isotherms of 5 are shown in Fig. 2, where it
can be seen that chalice molecules form a coherent monolayer
film. The complete isotherm shown in Fig. 2 was obtained by
combining and correcting the different isotherms obtained
when spreading different volumes of the same 1 mM solution
of 5. Since chalice 5 is not a classical amphiphile, this strategy was
necessary to fully understand its behaviour at the air–water
Fig. 1 ORTEP view of 5 (ellipsoids are drawn at 30% probability level).
obtained when a metal is complexed by a crown ether, due to
the electron withdrawing effect of the metal, during the titration
of 5 with NaPF6 or KPF6, an upfield shift of the multiplet at
3.64–3.55 ppm and a downfield shift of the multiplet at
4.36–4.28 ppm can be observed. This suggests that the cation
complexation induces a conformation change of the crown
ether moiety, and the chemical shifts are a combination of
the electron withdrawing effect and geometry change. Indeed,
as shown in the crystal structure of 5, the crown ether’s
conformation is not symmetrical, with one oxygen atom
pointing towards the exterior of the cavity and the other
oxygen atom pointing towards the interior of the cavity. The
complexation may induce a conformation where both oxygen
atoms point outside the cavity to chelate the cation. From the
variation of the chemical shifts of H1 and H2 protons in the
1H NMR spectra, Job’s plots were drawn and the maximum
observed at 50% of the total concentration and the shape of
the curve revealed a 1 : 1 stoichiometry (see ESIw). To determine
the association constants, NMR titrations were done with
solutions having a constant concentration of 5 and varying
concentrations of NaPF6 or KPF6. By the non-linear curve-
fitting methods,9 the association constant for a 5:Na+ complex
was estimated to be about 1350 ꢀ 80 M–1 and 500 ꢀ 28 Mꢁ1
for 5:K+. If for Na+ the association constant is 20 times
lower than the one for DB18C6 with Na+, for K+ the
association constant of 5 is 200 times lower than that for the
corresponding crown ether. The crown ether moiety of 5
clearly distinguishes between the Na+ and K+ cation, with
a higher selectivity for Na+.
interface. The film remains in a gaseous phase (0 mN mꢁ1
)
The amount of picrate ions extracted by 5 from an aqueous
solution was analyzed by the difference in UV spectrophoto-
metric absorbance of the aqueous solutions before and after
extraction with a chloroform solution containing the macro-
cycle.10 DB18C6 ether was used as a control macrocycle,
which extracts only sizeable amounts of the potassium picrate,
in agreement with previous reports.11 Inspection of the data
presented in Table 1 reveals that 5 displays low selectivity
toward extraction of K+, Rb+ and Cs+ picrates from aqueous
solution, but a high selectivity for Na+, compared to DB18C6.
Fig. 2 Surface pressure (p)/molecular area isotherms of 5 on (a) pure
water; (b) NaCl 10 mM solution and (c) KCl 10 mM solution.
c
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until a molecular area of B50 A2 per molecule is reached, when
an increase in pressure was observed. The slope suggests that
the film reached a condensed phase at that point. At B40 A2
per molecule (corresponding to the cross-sectional area of
5 B42 A2 in the crystal structure), the change of the slope
represents the reorganization of the film structure, indicating
controlled collapse of the monolayer towards multilayers
formation. As molecular chalice 5 showed a high selectivity
for sodium cations in solution, a solution of 5 was spread on a
10 mM NaCl subphase from a 1 mM solution in chloroform
(Fig. 2). The isotherm shows a significant shift to higher
molecular areas, however, when higher volumes of 5 were
spread on the 10 mM NaCl subphase, i.e. isotherms starting at
40 A2/molecule, the isotherms are identical in both the
presence and absence of NaCl. This suggests that the molecular
chalice may complex sodium in the monolayer state i.e. before
the first reorganization transition but upon spreading into a
multilayer, either the sodium cation is not incorporated in the
layers or does not affect their organization. Having said this,
the same isotherms were obtained when spread on a 10 mM
KCl subphase, which indicates that the behaviour of 5 at the
air–water interface is not affected by the nature of the cation.
Thus complexation may not induce a conformation change of
the monolayer at the interface and counterions may additionally
interact non-specifically with the chalice to expand the mono-
layer. BAM was utilized to characterize the morphology of
films of 5 during the course of compression in the absence of
ions (see ESIw, Fig. S4). High molecular areas showed large
regions of material which was already aggregated. Once the
pressure began to increase, the film appears to comprise a
uniform distribution of aggregates of varying heights and/or
optical properties. Only once the plateau is reached does it
appear that the gaseous phase is completely removed and the
aggregates have coalesced.
film deposited at 5 mN mꢁ1 on silicon. Seven ellipsometric
thicknesses at different locations of the LB film were measured,
giving an average of 12.3 ꢀ 0.2 A. Thus, this result suggests
that the molecular chalices form a monolayer, knowing that
the distance from the crown ether moiety and the phenyl
groups in the crystal structure is 12.86 A (Fig. 1). AFM was
used to investigate the structure of the Langmuir–Blodgett film
when deposited on a mica surface. When transferred during the
first regime compression at p o 10 mN mꢁ1 at 25 1C, a film is
formed with a height of 12.1 ꢀ 0.4 A (measured at seven
different pinhole locations), corresponding to the formation
of a monolayer (Fig. 3). When the films are transferred at
p 4 20 mN mꢁ1 at 25 1C the underlying monolayer is more
continuous and uniform (with the absence of pinholes and less
height variation). Furthermore the observed aggregates are
26.1 ꢀ 0.4 A above the background monolayer, indicating the
formation of a double layer.
In conclusion we have synthesized and characterized a
molecular chalice able to selectively bind Na+ cations. This
unique molecular recognition property should be useful for
future technology, for example in creating new chemical
sensors and in fabricating two-dimensional molecular lattices
comprising multiple functional units for molecular separations
and water depollution.
Notes and references
1 (a) F. Hof, S. L. Craig, C. Nuckolls and J. Rebek, Jr., Angew.
Chem., Int. Ed., 2002, 41, 1488–1508; (b) A. E. Rowan, J. A. A.
W. Elemans and R. J. M. Nolte, Acc. Chem. Res., 1999, 32, 995;
(c) P. E. J. Sanderson, J. D. Kilburn and W. C. Still, J. Am. Chem.
Soc., 1989, 111, 8314; (d) W. Lee, S. Samal, N. Selvapalam,
H. J. Kim and K. Kim, Acc. Chem. Res., 2003, 36, 621.
2 (a) Z.-G. Wang, B.-H. Zhou, Y.-F. Chen, G.-D. Yin, Y.-T. Li,
A.-X. Wu and L. Isaacs, J. Org. Chem., 2006, 71, 4502; (b) K. Kim,
N. Selvapalam, H. K. Young, M. P. Kyeng, D. Kim and J. Kim,
Chem. Soc. Rev., 2007, 36, 267; (c) J. Lagona, P. Mukhopadhyay,
S. Chakrabarti and L. Isaacs, Angew. Chem., Int. Ed., 2005,
44, 4844.
3 (a) J. W. H. Smeets, R. P. Sijbesma, F. G. M. Niele, A. L. Spek, W.
J. J. Smeets and R. J. M. Nolte, J. Am. Chem. Soc., 1987, 109, 928;
(b) L. Isaacs, D. Witt and J. C. Fettinger, Chem. Commun., 1999,
2549; (c) A. Sygula, F. R. Fronczek, R. Sygula, P. W. Rabideau
and M. M. Olmstead, J. Am. Chem. Soc., 2007, 129, 3842.
4 P. E. Stott, J. S. Bradshaw and W. W. Parish, J. Am. Chem. Soc.,
1980, 102, 4810.
In order to confirm this organization, spectroscopic ellipsometry
was used to determine the thickness of a Langmuir–Blodgett
5 H. Kuhn and D. Mobius, Monolayer assemblies, in Investigations
of Surfaces and Interfaces, ed. B. W. Rossiter and R. C. Baetzold,
Wiley, New York, 2nd edn, 1993, vol. IXB.
6 S. P. Gromov and M. V. Alfimov, Russ. Chem. Bull., 1997, 46, 611.
7 (a) J. W. H. Smeets, L. van Dalen, V. E. M. Kaats-Richter and
R. J. M. Nolte, J. Org. Chem., 1990, 55, 454; (b) R. P. Sijbesma and
R. J. M. Nolte, J. Org. Chem., 1991, 56, 3122; (c) H. K. A.
C. Coolen, H. Engelkamp, J. N. H. Reek, A. H. Priem, P. N. W.
M. van Leeuwen and R. J. M. Nolte, Recl. Trav. Chim. Pays-Bas,
2010, 114, 65.
8 R. P. Sijbesma and R. J. M. Nolte, Recl. Trav. Chim. Pays-Bas,
2010, 112, 643.
9 K. Connors, Binding Constants. The Measurement of Molecular
Complex Stability, Wiley, New York, 1987.
10 A. H. Bond, M. L. Dietz and R. Chiarizia, Ind. Eng. Chem. Res.,
2000, 39, 3442.
11 B. G. Cox and H. Schneider, Coordination and Transport Properties of
Macrocyclic Compounds in Solution, Elsevier, 1992.
12 P. Dapporto, P. Paoli, I. Matija and L. Tusek-Bosic, Inorg. Chim.
Acta, 1996, 252, 383–389.
13 (a) D. Vollhardt, Adv. Colloid Interface Sci., 1996, 64, 143;
(b) D. Hoenig and D. Moebius, J. Phys. Chem., 1991, 95, 4590;
(c) S. Henon and J. Meunier, Rev. Sci. Instrum., 1991, 62, 936.
Fig. 3 AFM images of the film transferred onto mica.
12836 Chem. Commun., 2011, 47, 12834–12836
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