A porphyrin nanobarrel that encapsulates C60

Article abstract of DOI:10.1021/ja107814s

A porphyrin nanobarrel, 1, that can encapsulate C₆₀ effectively was prepared via a concise coupling route. The structures of both 1 and C ₆₀@1 were confirmed by single-crystal X-ray diffraction analysis.

Full text of DOI:10.1021/ja107814s

Published on Web 10/29/2010
A Porphyrin Nanobarrel That Encapsulates C60
Jianxin Song,† Naoki Aratani,*,†,‡ Hiroshi Shinokubo,*,§ and Atsuhiro Osuka*,†
Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Sakyo-ku, Kyoto 606-8502, Japan,
PRESTO, Japan Science and Technology Agency, and Department of Applied Chemistry, Graduate School of
Engineering, Nagoya UniVersity, Chikusa-ku, Nagoya 463-8603, Japan
Received August 30, 2010; E-mail: aratani@kuchem.kyoto-u.ac.jp; hshino@apchem.nagoya-u.ac.jp;
osuka@kuchem.kyoto-u.ac.jp
Scheme 1. Synthetic Route to 1a
Abstract: A porphyrin nanobarrel, 1, that can encapsulate C60
effectively was prepared via a concise coupling route. The
structures of both 1 and C60@1 were confirmed by single-crystal
X-ray diffraction analysis.
The bottom-up “designed organic synthesis” of benzene-based
nanocarbon materials such as fullerenes1 and single-walled carbon
nanotubes (SWNTs)2 has been extensively attempted, as such a
a Conditions: (a) 2,6-dibromopyridine, Pd2(dba)3, PPh3, Cs2CO3, CsF,
synthetic approach would allow for tailored fine-tuning of the
structures, properties, and functions of these materials. These
attempts are also important in view of bent aromatic molecules.3
Such distorted aromatic systems are expected to display novel
electronic properties that are not shared by normal planar aromatic
molecules. Porphyrin is a representative planar and aromatic
macrocycle that serves as a key chromophore or catalyst in both
the natural and chemical worlds. In recent years, it has been
increasingly realized that porphyrins are structurally and electroni-
cally rather flexible, depending upon perturbations such as periph-
eral substitution and steric congestion, as exemplified by extensively
conjugated cases and extremely distorted or bent examples.4
The finding that bacterial photosynthetic light-harvesting antenna
LH2 are supramolecular wheel-shaped assemblies has stimulated
synthetic efforts directed toward large porphyrin wheels for use in
the study of excitation energy transfer and electronic coupling within
the wheel.5 As a consequence, many porphyrin wheels have been
explored. However, with a few exceptions,6 the vast majority lack
a whole π conjugation, and none of the reported examples involve
a doubly linked wheel bearing a void space that is comparable to
fullerenes and SWNTs. When these structural requirements are met,
the porphyrin wheel may be regarded as a porphyrin nanotube. Quite
recently, we reported the synthesis of ꢀ,ꢀ′-doubly 2,6-pyridylene-
bridged Ni(II)-porphyrin belts,7 which exhibit remarkably bent
conformations due to constraints arising from the ꢀ,ꢀ′-double 2,6-
pyridylene bridges. These structures have strongly encouraged the
synthetic extension to cyclic porphyrin tubes.8
toluene, DMF, reflux, 22 h; (b) 2, Pd2(dba)3, PPh3, Cs2CO3, CsF, toluene,
DMF, reflux, 48 h.
Figure 1. X-ray crystal structure of 1: (a) top view; (b) perspective view.
Thermal ellipsoids represent 50% probability. Hydrogen atoms and solvent
molecules in both views and the tert-butyl groups in the perspective view
have been omitted for clarity.
1). The matrix-assisted laser desorption ionization time-of-flight
(MALDI-TOF) mass spectrum of 1 displays the parent ion peaks
at m/z 3574.501 (calcd for C232H216N24Ni4, m/z 3574.512 [M]+).
1
The H NMR spectrum of 1 in CDCl3 at -25 °C reveals only a
single set of signals that consists of two singlet peaks at 10.42 and
8.64 ppm due to the meso and ꢀ protons, respectively, and signals
due to the other aromatic protons in the range 8.23-6.97 ppm.
These data indicate that the porphyrin barrel 1 takes a D4h-symmetric
structure in solution.
Here we report the synthesis of porphyrin nanobarrel 1 via
consecutive cross-coupling reactions at multiple sites starting from
a simple monomer. Our synthetic strategy involves the use of
Suzuki-Miyaura cross-coupling reactions for construction of the
tubular framework. Specifically, tetraborylporphyrin 29 and an
excess amount of 2,6-dibromopyridine were treated with 6 mol %
Pd catalyst in DMF/toluene at reflux under an inert atmosphere to
give tetra(6-bromopyridyl)porphyrin 3 in 50% yield; 3 was then
coupled with 2 to furnish porphyrin barrel 1 in 10% yield (Scheme
The final structural proof was obtained from single-crystal X-ray
diffraction analysis (Figure 1). The tetramer 1 displays a barrel
structure with a diameter of ∼14 Å (Ni-Ni distance). The
constitutional porphyrins take a ruffled structure, forming a concave
wall. The pyridyl bridges are held tilted to the neighboring pyrroles
with an average dihedral angle of 43.5° and pyridine-pyrrole bond
lengths of ∼1.47 Å. The tilted pyridyl bridges seemingly serve as
a conjugative mediator to cause overall moderate π conjugation.
The inside of the barrel was filled with many solvent molecules of
THF and chloroform (Figure S8 in the Supporting Information).
† Kyoto University.
‡ PRESTO.
§ Nagoya University.
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10.1021/ja107814s  2010 American Chemical Society

C O M M U N I C A T I O N S
Figure 3. X-ray crystal structure of C60@1. For clarity, only one isomer
of encapsulated C60 molecules is shown. Thermal ellipsoids represent 50%
probability. The C60 molecule is depicted as a sphere model. Solvent
molecules, two extracapsular C60 molecules, and hydrogen atoms have been
omitted for clarity.
Figure 2. UV-vis absorption spectra of 1 (2.0 µM) in toluene in the
presence of various amounts of C60 (0 < [C60] < 36 µM) at 25 °C. Arrows
indicate the changes in absorption with increasing [C60]. Inset: plot of
∆A431nm vs [C60].
C60. We are currently exploring the synthesis of free-base and Zn(II)
counterparts of C60@1 and their electron-transfer chemistry.
The UV-vis absorption spectrum of 1 is spread over a wide range
of the visible region relative to that of the monomer, indicating the
effective electronic interaction between the porphyrin units though
the pyridyl bridges (Figure 2).
Supporting Information Available: Preparation and analytical data
for samples and crystallographic data (CIF) for 1 and C60@1. This
material is available free of charge via the Internet at http://pubs.acs.org.
In the next step, the encapsulation of C60 into 1 was examined,
as the diameter of the interior cavity of 1 is ∼14 Å, which is nicely
fit to the diameter of C60.10 Actually, the addition of C60 into a
toluene solution of 1 changed the absorption spectrum as a result
of electronic interactions between the two components (Figure 2).
The encapsulation was also confirmed by 13C NMR spectroscopy:
a 1:1 mixture solution of 1 and C60 in CDCl3 showed a signal at
139.6 ppm that was distinctly different from the signal of free C60
observed at 143.1 ppm and hence assignable to C60@1. The
complexation stoichiometry was determined to be 1:1 on the basis
of the Job’s plot, and the association constant of C60@1 was
estimated from the UV-vis absorption changes to be (5.3 ( 0.1)
× 105 M-1, which is certainly large but comparable to or slightly
smaller than those reported for cyclic porphyrin dimers11a,b and a
cyclic porphyrin timer.11c
Fortunately, the complex structure was unambiguously confirmed
by single-crystal X-ray diffraction analysis (Figure 3). In the solid
state, the porphyrin units of C60@1 have a structure similar to that
of 1 with respect to the pyridine-pyrrole distance (1.45-1.50 Å),
the dihedral angles of the pyridines with respect to the porphyrins
(50-53°), and the void space (14 Å diameter). As shown in Figure
3, a C60 molecule is nicely captured within the void space with an
average distance of ∼3.6 Å. Closer inspection of the crystal structure
revealed that the constitutional ruffled porphyrins protrude their
convex faces toward the interior void space, which interacts with
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C60 in a cooperative manner. Interestingly, the porphyrin barrels in
the crystal are interconnected through extracapsular C60 molecules
that interact with their concave faces, forming an infinite three-
dimensional grid structure (Figure S9).
In summary, the porphyrin barrel 1 was synthesized via a concise
synthetic route. This barrel exhibits an effective electronic interac-
tion over the molecule as well as an encapsulating ability toward
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J. AM. CHEM. SOC. VOL. 132, NO. 46, 2010 16357