Selective anion binding and solid-state host-guest chemistry of an extended cavity calix[6]pyrrole

Article abstract of DOI:10.1039/b007788g

Easily prepared, cone-like, extended cavity calix[6]pyrrole is shown to form strong complexes with iodine and other halide ions as well as with trihaloalkanes and electron deficient aromatic systems.

Full text of DOI:10.1039/b007788g

Selective anion binding and solid-state host–guest chemistry of an extended
cavity calix[6]pyrrole
Boaz Turner,a Alexander Shterenberg,a Moshe Kapon,a Kinga Suwinskab and Yoav Eichen*ac
a Department of Chemistry, Technion–Israel Institute of Technology, Technion City, 32000 Haifa, Israel.
E-mail: chryoav@tx.technion.ac.il
b Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, PL-01 224 Warszawa, Poland
c
Solid-State Institute, Technion–Israel Institute of Technology, Technion City, 3200 Haifa, Israel
Received (in Cambridge, UK) 26th September 2000, Accepted 7th November 2000
First published as an Advance Article on the web
Easily prepared, cone-like, extended cavity calix[6]pyrrole is
shown to form strong complexes with iodine and other
halide ions as well as with trihaloalkanes and electron
deficient aromatic systems.
determination of binding constants between different guests and
calix[6]pyrrole, 2.6 Quantitative assessments of anion binding
constants were made by following the induced shifts in the 1H
NMR spectra of the host as a function of the concentration of the
guest in an acetonitrile–chloroform (1+9) solution at room
temperature (298 K). Table 1 lists the association constants
between 2 and the different guests. For comparison, the
complexation of octamethylcalix[4]pyrrole with the same guest
species was studied under the same conditions (Table 1).
In accordance with previous studies performed by Sessler and
coworkers octamethylcalix[4]pyrrole displays a clear prefer-
ence towards fluoride ions over larger anions, the binding order
being F2 > > Cl2 > Br2 > I2. In contrast, probably due to its
extended cavity, 2 exhibits a clear preference to iodide over
smaller halides. Here the binding order switches to I2 > F2 > >
Cl2 > Br2. We interpret this binding order and the high affinity
towards iodide in terms of the geometrical fit between the
extended cavity of 2 and the iodide ion, allowing full binding of
the anion by up to six pyrrole rings. Being the smallest halide,
the fluoride ion may fit and bind efficiently to only part of the
pyrrolemethane ring. Similar effects have previously been
reported for the binding of cations to the cavities of crown
ethers.7
The host–guest chemistry of calixpyrroles has been the subject
of intensive research aimed at gaining both understanding and
control on guest recognition and binding.1 Recently, Sessler and
coworkers2 reported that calix[4]pyrroles 1 are effective and
selective receptors for anions and neutral guest species.
Calixpyrroles that have been tailored to the size, shape and
binding modes of target guests were shown to exhibit improved
binding ability.3 Here we report the selective binding of anions,
haloalkanes and aromatic guests by recently reported, cone-like,
Calix[6]pyrrole 2 is found to bind also to trihalogenated
species such as trichloroethanol, trifluoroethanol, tetrafluoro-
borate and trifluoroacetate and forms significantly stronger
complexes with such guests than with their nonhalogenated
ananolgs (Table 1). The reason for this is revealed from the
crystal structure† of such a complex between 2,2,2-tri-
chloroethanol and 2 (Fig. 1). Unlike simple calixpyrroles that
extended-cavity
1,1,3,3,5,5-meso-hexaphenyl-2,2,4,4,6,6-
meso-hexamethylcalix[6]pyrrole.2,4 Calix[6]pyrroles are a new
class of molecules characterized by a six-pyrrolemethane rim.
The cross section of the hexapyrrolemethane rim is significantly
larger than that of calix[4]pyrroles, ca. 110 cf. 24 Å2.
Additionally, in the stable conformation of calix[6]pyrroles,
such as 2, three of the meso phenyl substituents form a trigonal
cavity with a volume of ca. 500 Å3. Such extended-cavity
receptors may allow efficient and selective binding of electron
defficient aromatic guests as well as large anions such as I2.
These substrates can not be efficiently recognized nor com-
plexed by the smaller calix[4]pyrrole systems.3b,5
bind their guests predominantly through X2…H–N bonds,2,3b
2
anchors the 2,2,2-trichloroethanol guest through one (dis-
ordered) H–O…H–N hydrogen bond with the hydroxy group,
d(O84b…H–N18)
d(O84a…H–N70)
=
=
2.411 Å; a(O–H–N)
2.412 Å; a(O–H–N)
=
=
169.11°;
175.95°;
d(O84a…H–N44) = 2.725 Å; a(O–H–N) = 156.97°, and three
phenyl…Cl–C bonds between the Cl atoms of the guest and the
p electron clouds of the three axial meso-phenyl groups forming
p
Calix[6]pyrrole 2 was prepared using a modification of a
previously reported method.4a Benzophenone (5 g, 27.4 mmol),
pyrrole (5 mL, 72.3 mmol) and BF3·OEt (5 mL, 39.5 mmol)
were dissolved in dry ethanol (250 mL) and stirred at room
temperature for 5 days under an inert atmosphere. The
precipitate was then filtered off and washed with cold ethanol,
offering di(phenyl)di(pyrrol-2-yl)methane 3 as a colorless solid
(40% yield). An additional crop of 3 could be isolated from the
mother-liquor using column chromatography [silica, dichloro-
methane–hexane (1+10), 50% total yield for 3]. Calix[6]pyrrole
2 was obtained by stirring a solution of 3 (300 mg, 1 mmol) in
60 mL of dry acetone–ethanol (1+1) containing CF3CO2H for 5
days under inert atmosphere. The product was isolated by
filtration (43% yield). An additional crop of 2 could be isolated
from the mother-liquor using column chromatography [silica,
dichloromethane–hexane (3+7), 52% total yield for 2].
Table
1 Binding constants of calix[6]pyrrole 2 and 1,2,3,4-meso-
octamethylcalix[4]pyrrole 1, with different guest molecules
Guest
Calix[4]pyrrole
Calix[6]pyrrole
F2
23 800
6800
270
< 10
< 10
< 10
< 10
400
1080
650
150
6600
< 10
150
2350
< 10
1150
< 10
80
Cl2
Br2
I2
SCN2
2
p-MeC6H4SO3
BF4
MeCO2
CF3CO2
EtOH
CF3CH2OH
CCl3CH2OH
2
2
2
70
< 10
< 10
< 10
Similarly to earlier studies by Sessler and coworkers on the
complexation of calix[4]pyrroles with different guest species,
proton NMR spectroscopy was found to be a useful tool for the
60
DOI: 10.1039/b007788g
Chem. Commun., 2001, 13–14
This journal is © The Royal Society of Chemistry 2001
13

trihalogenated compounds. The application of calix[6]pyrroles
to the separation and identification of such compounds is under
investigation.
This work was supported by the Israel Science Foundation,
Administrated by the Israel Academy of Sciences and Human-
ities and by the Fund for the Promotion of Sponsored Research
at the Technion.
Notes and referenecs
† Crystal data: for 2·2.5CCl3CH2OH·1.5CHCl3·3CH3CH2OH·5.5H2O:
grown in the dark from 2,2,2-trichloroethanol–chloroform–ethanol. A
single crystal was mounted on the Nonius Kappa CCD diffractometer,8 and
cooled to 170 K under a nitrogen stream. Data were collected with graphite-
monochromated Mo-Ka radiation (l = 0.71070 Å) by applying o and w
rotations. Data reduction was performed using DENZO-SMN software.9
The structure was solved using direct methods (SHELXS-9710) and refined
by SHELXL-97.11 All non-H atoms of the macrocycle and the tri-
chloroethanol inside the cavity, excluding the disordered hydroxy oxygen,
were refined anisotropically. Hydrogen atoms of these moieties were placed
at calculated positions and refined as riding on their carbon and nitrogen
atoms. Difference Fourier maps based on the macrocycle and the guest
inside, revealed another moderately disordered trichloroethanol bound to
the macrocycle outside the cavity, and another four sites of severely
disordered molecules such as trichloroethanol, chloroform, ethanol and
water. All the disordered positions of the solvent molecules were refined
isotropically. 38 hydrogen atoms belonging to some of the disordered
solvent molecules were not allocated. Mr = 1804.83, monoclinic, space
group P21/n, a = 17.773(10), b = 20.2090(10), c = 26.2590(10) Å, b =
Fig. 1 The molecular structure of the complex between 2,2,2-tri-
chloroethanol and calix[6]pyrrole 2. Solvent and other molecules not
situated in the cavity of the host have been omitted for clarity.
the pseudo-threefolded cavity of 2: d(Cl79–p(C34–C39)) =
3.38 Å; d(Cl80–p(C60–C65)) = 3.00 Å; d(Cl81–p(C8–C13))
= 3.45 Å.
The stable conformation of 2 brings two electron-rich pyrrole
rings, situated in a 1,4 position to one another, into a parallel and
cofacial orientation. These two rings are spaced ca. 7.1 Å apart.
Being an electron rich ring system, the hexapyrrolemethane ring
is suitable for hosting electron poor conjugated species in
between a pair of cofacial pyrrole rings. The additional four
pyrrole rings are capable of forming multiple hydrogen bonds
with appropriate guests, making the system an interesting host
for different nitro- and carboxy-aromatic compounds. Fig. 2
shows the crystallographic structure† of a complex between p-
nitrotoluene/nitrobenzene and 2. Interestingly, though crystal-
lized from a solution containing nitrobenzene and p-nitro-
toluene in a 10+1 ratio, the crystal structure clearly indicates the
1+1 inclusion of nitrobenzene and p-nitrotoluene within the
cavity of 2. As can be seen in Fig. 2, the nitroaromatic guest is
fixed to the cavity of the host through short range p–p
interactions between the nitro group of the guest and the two
sandwiching pyrrole rings of the host, d(nitro(plane)…pyrrole-
(plane)) = 3.55 Å. Three of the other four pyrrole rings are
involved in hydrogen bonding with the nitro group of the
encapsulated guest, d(NH1…O80) = 2.23 Å, d(N1…O80) =
3.06 Å, a(O–H–N) = 172.94°, d(NH31…O80) = 2.38 Å,
107.730(3)°, V = 8983.6(8) Å3, T = 170.0(1) K, Z = 4, m = 0.076 mm21
,
14 385 relections measured, 14 385 unique which were used in all
calculations. The final R(F2) was 0.1168 [I > 2s(I)].
For 2·2.5C6H5NO2·0.5C7H9NO2: grown in the dark by slow evaporation
of a chloroform solution. A single crystal was mounted on the Nonius
Kappa CCD diffractometer, at 293 K. Data collection and reduction as
above. The structure was also solved and refined as above. All non-H atoms
of the macrocycle and the guests were refined anisotropically. Hydrogen
atoms were placed at calculated positions and refined as riding on their
carbon and nitrogen atoms except for the N–H hydrogen atoms of the
pyrrole rings which were localized on a Fourier difference map and refined
isotropically. Mr
= 1392.66, monoclinic, space group P21/n, a =
19.455(1), b = 19.762(1), c = 22.027(1) Å. b = 115.405(2)°, V =
7649.8(5) Å3, T = 293 K, Z = 4, m = 0.076 mm21, 16 018 relections
measured, 15 694 unique which were used in all calculations. The final
R(F2) was 0.0740 [I > 2s(I)].
CCDC 182/1864. See http://www.rsc.org/org/suppdata/cc/b0/b007788g/
for crystallographic files in .cif format.
d(N31…O80)
=
3.18 Å, a(O–H–N)
=
143.67°,
1 J. M. Lehn, Supramolecular Chemistry, Concepts and Prespectives,
VCH, Weinheim, 1995.
2 P. A. Gale, J. L. Sessler, V. Kral and V. Lynch, J. Am. Chem. Soc., 1996,
118, 5140.
d(NH19…O81) = 2.45 Å, d(N19…O81) = 3.23 Å, a(O–H–N)
= 165.11°.
3 (a) P. Anzenbacher, Jr., K. Jursikova, V. M. Lynch, P. A. Gale and J. L.
Sessler, J. Am. Chem. Soc., 1999, 121, 11 020; (b) G. Cafeo, F. H.
Kohnke, G. L. La Torre, A. J. P. White and D. J. Williams, Chem.
Commun., 2000, 1207; (c) S. Camiolo and P. A. Gale, Chem. Commun.,
2000, 1129.
4 (a) B. Turner, M. Botoshansky and Y. Eichen, Angew. Chem., Int. Ed.,
1998, 37, 2475; B. Turner, M. Botoshansky and Y. Eichen, Angew.
Chem., 1998, 110, 2633; (b) for an alternative approach to
calix[6]pyrroles see: G. Cafeo, F. H. Kohnke, G. L. La Torre, A. J. P.
White and D. J. Williams, Angew. Chem., Int. Ed., 2000, 39, 1496.
5 P. A. Gale, L. J. Twyman, C. I. Handlin and J. L. Sessler, Chem.
Commun., 1999, 1851.
6 W. E. Allen, P. A. Gale, C. T. Brown, V. M. Lynch and J. L. Sessler,
J. Am. Chem. Soc., 1996, 118, 12 471; H. Miyaji, P. Anzenbacher, J. L.
Sessler, E. R. Bleasdale and P. A. Gale, Chem. Commun., 1999, 1723;
P. A. Gale, J. L. Sessler and V. Kral, Chem. Commun., 1998, 1; P. A.
Gale, J. L. Sessler, W. E. Allen, N. A. Tvermoes and V. Lynch, Chem.
Commun., 1997, 665.
7 A. Alvanipour, J. L. Atwood, S. G. Bott, P. C. Junk, U. H. Kynast and
H. Prinz, J. Chem. Soc., Dalton Trans., 1998, 1223; J. L. Atwood, S. G.
Bott, S. Harvey and P. C. Junk, Organometallics, 1994, 13, 4151.
8 Nonius, KappaCCD Server software. Nonius BV, Delft, The Nether-
lands.
Fig. 2 The molecular structure of the complex between calix[6]pyrrole and
nitrotoluene/nitrobenzene. Molecules that are not situated in the cavity of
the host have been omitted for clarity.
In conclusion, calix[6]pyrrole 2 shows a wealth of binding
modes to different substrates, ranging from simple anions to
aromatic derivatives. The axial meso phenyl groups form a
genuine preorganized cavity and actively participate in binding
9 Z. Otwinowski and W. Minor, Methods Enzymol., 1997, 276, 307.
10 G. M. Sheldrick, Acta Crystaltogr. Sect. A, 1990, 46, 467.
11 G. M. Sheldrick, SHELXL97, University of Göttingen, Germany.
14
Chem. Commun., 2001, 13–14