Table 1 ITC results for Zn-2 with DABCO and Bpy in CHCl3
(0.4 mM ligand titrated into ca. 0.04 mM porphyrin dimer). ITC data
for the titration of M-2 with C60 were not obtained due to the low
solubility of the C60 complex in CHCl3
KITC
/
DHu/
TDSu/
DGu/
Dimer
1066M21
kJ mol21
kJ mol21
kJ mol21
DABCO
BPy
7.75
0.50
280.8
268.8
241.4
236.3
239.3
232.5
(ITC) indicate a smaller favourable enthalpy of binding for BPy, as
expected for an aromatic amine compared to an aliphatic one
(Table 1). The same order of magnitude was found for TDS in
both BPy and DABCO adducts, suggesting that the entropic
contributions are dominated by the restriction in conformational
freedom induced by complexation.
Fig. 1 Molecular structure of the inclusion complex between receptor
2H-2 and C60 (hydrogen atoms omitted for clarity).
1H NMR (293 K, CDCl3, 500 MHz) showed that the meso
signals of the free porphyrin host can be found between 8.5 and
9.5 ppm as broad signals, due to an off-set p–p stacking and the
presence of a variety of conformations.16–18 Binding to DABCO or
BPy led to the ‘freezing’ of the host into a single conformation, and
consequently a clearer region in 1H NMR: meso-hydrogens
resonate at 10 ppm (BPy complex) or 9.5 ppm (DABCO complex)
as sharp singlets.
co-workers’ complexes,20 a 5 : 6 ring-juncture of C60 was located
above the central metal ion. In our case, the best-fit plane of the
˚
porphyrin–C60 outer shell distance was found to be 2.87 A,
21
˚
significantly less than the sum of the van der Waals radii (3.09 A).
This observation suggests the presence of a p interaction. The
ethyloxydisulfide linkers of the dimer are folded to adjust the
porphyrin–porphyrin distance in an asymmetric fashion. This
results in a tilt angle between the porphyrin planes of 11u, much
smaller that the angle reported by Reed et al. in their tweezer
host (42u).11
In conclusion, we have developed a new porphyrin receptor for
C60 through dynamic reversible chemistry. Use of an ‘extendable’
linker allowed the construction of the receptor with templates of
differing steric requirements.
1H NMR spectroscopy (293 K, 500 MHz, CS2 : CDCl3 9 : 1)
was used to monitor titration of C60 into solutions of dimers.
Changes in the aromatic region were observed. For 2H-2, a
single sharp meso-H was seen at 9.88 ppm, and the aromatic region
was simplified with respect to the spectrum of the free host. This
indicates an increase in the rigidity of the system upon C60
complexation. It can only be assumed that the inner ortho-H
vicinal to the alkyl chain (7.03 ppm) points into the cavity as it has
been shown to do in the case of BPy (7.36 ppm) or DABCO
(7.49 ppm) binding. This upfield shift indicates that the proton falls
into a strongly shielding region of the guest, in contrast to the
binding of DABCO or BPy, where deshielding by the porphyrin
ring current was observed. A similar upfield shift is seen for the
–OCH2CH2S– linker protons, which again points towards C60
We thank Prof Paul Raithby, Drs David Watkin, Andrew
Cowley and Simon Teat for assistance with crystallography, and
EPSRC for financial support.
Amy L. Kieran, Sofia I. Pascu, Thibaut Jarrosson and
Jeremy K. M. Sanders*
Department of Chemistry, University of Cambridge, Lensfield Rd,
Cambridge, UK CB2 1EW. E-mail: jkms@cam.ac.uk;
Fax: +44 1223 336017; Tel: +44 1223 336411
1
encapsulation within the dimer. Whilst the H spectrum suggests
that the complex adopts one conformation, it gives no indication
about the rate of exchange of the host and guest. However,
attempts to monitor the binding by 13C NMR (293 K, CDCl3,
125 MHz) suggested that it was in fast exchange on the NMR
timescale. Signal averaging resulted in just one signal for C60 being
observed at 142.3 ppm due to the rapid exchange of complexed
and free fullerene on the NMR timescale (N.B. d for free C60 in
CDCl3 143.1 ppm).
Notes and references
{ Use of extended tripyridyl templates leads to efficient templated synthesis
of the trimer.
§ Layering of a concentrated solution of the 2H-2?C60 complex (CDCl3 :
CS2 1 : 9) with hexane yielded small deep red triangular-shaped crystals.
Crystallographic data were collected using the synchrotron radiation source
at Station 9.8, Daresbury SRS, UK, on a Bruker SMART CCD
diffractometer. The structures were solved by direct methods using the
program programs SIR9222 The refinement (on F) and graphical
calculations were performed using the CRYSTALS23 program suite.
Crystal data: C128H168N8O4S4 C60, M 5 2731.72, Z 5 4, monoclinic, space
The X-ray diffraction structure shows formation of the inclusion
complex (Fig. 1).§ The porphyrin macrocycles are saddled in
order to accommodate the convex C60. A difference Fourier map
suggested that C60 is rotationally disordered within the cavity
(consistent with the 13C NMR spectrum) and more than two
atoms became necessary to model each of the C60 sites. Therefore,
a ‘smeared-out’ electron density over the surface of the sphere
provided an alternative model for C60 with a refined radius of
˚
˚
˚
˚
group P21/n, a 5 20.3893(19) A, b 5 29.840(3) A, c 5 23.181(2) A,
21
,
3
b 5 90.746(2)u, U 5 14103(2) A , T 5 150 (2) K, m 5 0.133 mm
˚
synchrotron radiation l 5 0.68920 A. Of 26067 reflections measured, 11009
were independent (Rint 5 0.03). Final R 5 0.1651 (3780 reflections with
suppdata/cc/b4/b417951j/ for crystallographic data in .cif or other electronic
format.
3.531(4) A.19 As a result, it could not be determined with precision
˚
which edge or face of the C60 has the closest approach to the
porphyrin core. The hexyl side chains of the porphyrin dimer
are folded around the C60, creating a cage-like species. In Aida and
1 H. W. Kroto, J. R. Heath, S. C. O’Brien, R. F. Curl and R. F. Smalley,
Nature, 1985, 318, 162.
2 F. Diederich and M. Go´mez-Lo´pez, Chem. Soc. Rev., 1999, 28, 263.
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 1276–1278 | 1277
Kieran, Amy L.
