Inter-digitation approach to encapsulation of C60: [C60 ? (p-phenylcalix[5]arene)2]

Article abstract of DOI:10.1039/b203688f

p-Phenylcalix[5]arene shows a strong and selective binding of C₆₀ in toluene resulting in crystallisation of a hemispherically inter-digitated 2:1 complex; the receptor itself crystallises from toluene with self assembly through phenyl group inclusion.

Full text of DOI:10.1039/b203688f

Inter-digitation approach to encapsulation of C60: [C60 7
(p-phenylcalix[5]arene)2]†
Mohamed Makha, Michaele J. Hardie and Colin L. Raston*
Department of Chemistry, University of Leeds, Leeds, UK LS2 9JT.
E-mail: c.l.raston@chemistry.leeds.ac.uk; Fax: +44 0113 2336401; Tel: +44 0113 233655
Received (in Cambridge, UK) 16th April 2002, Accepted 28th May 2002
First published as an Advance Article on the web 14th June 2002
p-Phenylcalix[5]arene shows a strong and selective binding
of C60 in toluene resulting in crystallisation of a hemi-
spherically inter-digitated 2+1 complex; the receptor itself
crystallises from toluene with self assembly through phenyl
group inclusion.
outer rim phenyl group is disordered over two orientations and
all are twisted away from the plane of their adjoining inner
phenyl ring at torsion angles ranging from 1.4 to 40.2°. The
calixarenes show close associations with one another through
two modes; a host–guest interaction and phenolic back-to-back
stacking. The host–guest association has one arm of a calixarene
protruding into the bowl of an adjacent symmetry equivalent
host molecule, Fig. 1.
The last decade has seen developments in the host–guest
chemistry of fullerenes involving cavitands, especially calixar-
enes.1 Initially this was concerned with practical separation
techniques of fullerene C60, then crystal engineering and
supramolecular chemistry including materials applications such
as high temperature superconductivity.2 Progress in separation
of fullerenes deals with retrieval of C60 via complexation with
tBu-calix[8]arene3 and the lower cyclic phenolic oligomers
calix[5,6]arenes.4,5 Calix[5]arenes usually adopt the cone
Two inner phenyl rings within this association are coplanar at
centroid…centroid separation 4.35 Å. The outer-rim phenyl
group sits slightly displaced from the center of the calixarene
bowl, the distance between the phenyl centre and centre of the
host O5 plane at 4.07 Å. Calixarene molecules within the
extended structure stack back-to-back with intermolecular
phenolic O…O separations 2.88–3.30 Å. These distances are
consistent with the formation of hydrogen bonds, however the
hydrogen atoms were not located and are more likely to be
involved in intramolecular hydrogen bonding where the O…O
separations are 2.75–2.78 Å. This back-to-back arrangement is
similar to that found in the complex of unsubstituted calix[5]ar-
ene, (o-carborane)(calix[5]arene)·(CH2Cl2).13 Despite the large
number of rigid phenyl groups within the structure, there is only
one intermolecular association that could be regarded as a p
interaction, notably an edge-to-face interaction from an outer
rim phenyl to an inner calixarene phenyl at C–H…p distance
2.76 Å. The solvent toluene molecules are significantly
disordered, and hence cannot be commented on with any degree
of veracity.
The structure of the complex [C60 7 (1)2] was similarly
determined.‡ The asymmetric unit comprises one super-
molecule and 1.5 ordered toluene molecules, Fig. 2. The
fullerene is entirely disordered, and was treated as a sphere of
electron density.‡ The calixarene molecules are ordered and
both have a more symmetrical cone conformation than in
1·(toluene)1.75, with centroid…centroid…centroid angles be-
tween adjacent inner phenyl groups ranging from 107.0 to
109.0° and 105.1 to 112.7°. As for 1·(toluene)1.75, the outer rim
phenyl groups are twisted away from the plane of their
adjoining inner phenyl rings.
conformation which has dimensional and curvature com-
6–8
plementarity for binding a single molecule of C60
thereby
optimising host–guest p…p interactions,7 and this has been
observed in the solid state for a number of complexes.6,7,9 We
have previously established C5 symmetry alignment of p-
benzylcalix[5]arene with a C5 symmetry axis of C60 in the solid
state which manifests itself in high selectivity for binding C60
over other fullerenes.7 In contrast calix[6,8]arenes adopt a
double cone conformation with a fullerene in each shallow
cavity.5,10
As part of our ongoing endeavours in host–guest calixarene–
fullerenes chemistry, we recently reported the synthesis of the
deep cavity p-phenylcalix[5]arene, 1, accessible by a direct ‘one
pot’ procedure.11 Herein we report the improved synthesis of 1
along with its solid state structure and the synthesis and
structure of inter-digitated [C60 7 (1)2], and C60 binding and
separation studies.
The improved synthesis of the receptor, 1, minimises the use
of noxious organic solvents such as tetralin and phenylether,
and dispenses with the use of a multiple step process to prepare
such compounds.12 The procedure consists of heating a slurry of
p-phenylphenol, KOH and formaldehyde solution for 2 hours at
120 °C, and the resulting solid is transferred to a Kugelrohr and
rapidly heated at 290 °C in vacuo for 1 hour. Work up of the
brown solid then followed the published procedure, affording 1
in 10% yield (cf. 5%),11 Scheme 1.
The two calixarenes form a capsule with C60 and are in a
staggered arrangement, however, the capsule is considerably
skewed, with an angle of 157.5° between the centre of the
fullerene and the centres of the phenolic O5 planes of the
calixarenes. Hence the equatorial region of the capsule is
unsymmetrical, with a closed arrangement of outer rim phenyl
Crystals of 1 were obtained as a toluene solvate, 1·(tolu-
ene)1.75, and the structure determined by X-ray crystallog-
raphy.‡ The calixarene is in an unsymmetrical cone conforma-
tion with centroid…centroid…centroid angles between
adjacent inner phenyl groups ranging from 97.5 to 115.3°. One
Scheme 1
† Dedicated to Professor Jerry L. Atwood on the occasion of his 60th
birthday.
Fig. 1 Structure of p-phenylcalix[5]arene, 1·(toluene)1.75
.
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groups at one end, Fig. 2(a), and open at the other, Fig. 2(b). The
closed end features inter-digitation of outer rim phenyl groups
from the two calixarene molecules with closest C…C separation
3.55 Å. In contrast, the opposite end of the capsule has C…C
separations typically > 6.5 Å. The fullerene is slightly closer to
one of the two calixarene molecules, with distances from the
fullerene centroid to the centres of the phenolic O5 planes of the
calixarenes at 6.87 and 7.01 Å.
The extended structure of [C60 7 (1)2] features layers of
host–guest capsules projected in the bc plane and stacking in the
a direction. Within the layers of capsules there are numerous
inter-digitation interactions between adjacent capsules, with
C…C separations typically ~ 3.6 Å. As for the structure of the
toluene solvate of the host calixarene, there are few obvious p
interactions between the phenyl groups. The solvent toluene
molecules are located between the layers of capsules.
UV–vis studies involving 1 and C60 in toluene were
undertaken. Stoichiometry considerations from the Job plot
indicate more than one species present in solution, and the
interplay between the two species can be modelled as a two
component system, with the formation of 1+1 then 2+1
supermolecules, eqn. (1).
In conclusion, we have demonstrated a more facile access to
p-phenylcalix[5]arene and its high selectivity towards binding
C60. The skewed bicapped C60 supermolecule with an equato-
rial gap suggests complexation of mono-derivatised C60 is
possible. The lack of binding of C70 is surprising given that the
larger fullerene can in principle be accommodated by two
molecules of 1.
Support of this work by the EPSRC is gratefully acknowl-
edged.
Notes and references
† Dedicated to Professor Jerry L. Atwood on the occasion of his 60th
birthday.
‡
Crystal data: All data were collected at 150(1) K on an Enraf–Nonius
Kappa CCD diffractometer with Mo-Ka radiation. The structures were
solved by direct methods (SHELXS-97) and refined with a block matrix
least-squares refinement on F2 (SHELXL-97).
(p-phenylcalix[5]arene)(toluene)1.75: There was considerable difficulty
in locating a suitable crystal with the majority having a mosaicity > 2°. Data
was collected on a colorless needle of dimensions 0.20 3 0.12 3 0.10 mm
and mosaicity 0.983(2)°. Residual electron density was modelled as 1.75
toluene molecules disordered over 5 positions with rigid body refinement.
The methyl group of one such toluene could not be located. C77.25H54O5: Mr
= 1062.20, monoclinic, P21/c, a = 13.7426(4), b = 22.0634(7), c =
21.7546(7) Å, b = 99.941(2)°, U = 6497.1(3) Å3, Z = 4, rcalc = 1.086 g
mol21, m = 0.067 mm21 (no correction), qmax = 25.0°, 42001 data
(1)
This is analogous to the p-benzylcalix[5]arene system, although
there the 2+1 supermolecules have the p-benzyl groups directed
away from the centroid of the 2+1 supermolecule,7 at least in the
solid state structure, rather than partial inter-digitation in the
present study. The binding constants were determined by the
Benesi–Hildebrand method and by the procedure outlined by
Connors,14 with K1 = 3000 ± 200, K2 = 250 ± 50 M21 (cf. K1
of other calix[5]arene and C60 systems in toluene, 30 ± 2, 2800
± 200, 2120 ± 110, 1673 ± 70 and 588 ± 70 M21 respectively for
calix[5]arene, p-benzylcalix[5]arene, p-(I)2(Me)3-calix[5]ar-
ene, p-Me-calix[5]arene and p-(H)2(Me)3-calix[5]arene).5,7
Complex [C60 7 (1)2] selectively crystallises from a toluene
solution of fullerite and the calixarene. However, the separation
of C60 from the complex is rather difficult. Addition of
methylene chloride results in slow dissolution of the complex
and, more interestingly, the spontaneous formation of a new
crystalline material, possibly the 1+1 complex prior to complete
dissociation achieved only on evaporation of the solvent.
collected, 11382 unique (Rint = 0.122), 731 parameters, no restraints, R1
0.1063 (5935 data I > 2s(I)), wR2 = 0.3556 (all data), S = 1.056.
=
[C60 7 (1)2](toluene)1.5: Data was collected on a dark red needle of
dimensions 0.15 3 0.10 3 0.08 mm. Crystals were small and weakly
diffracting with subsequent problems with the data:parameter ratio. The
disordered fullerene was modelled with 60 C atoms over 120 sites each at
50% occupancy and with all Uij values restrained to be approximately equal
to those of neighbouring atoms. C200.5H112O10: Mr = 2680.90, monoclinic,
C2/c, a = 30.4539(2), b = 17.3036(1), c = 50.3034(5) Å, b = 96.676(1)°,
U = 26328.2(3) Å3, Z = 8, rcalc = 1.353 g mol21, m = 0.082 mm21 (no
correction), qmax = 25.0°, 71483 data collected, 22692 unique (Rint
=
0.085), 2469 parameters, 2230 restraints, R1 = 0.0642 (12507 data I >
2s(I)), wR2 = 0.1961 (all data), GoF = 1.037, restrained S = 1.013. CCDC
184114 and 184115. See http://www.rsc.org/suppdata/cc/b2/b203688f/ for
crystallographic files in .cif or other electronic format.
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Fig. 2 Space filling projections of the molecular capsules in [C60 7 (1)2].
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