A C84 selective porphyrin macrocycle with an adaptable cavity constructed through alkyne metathesis

Article abstract of DOI:10.1039/c2cc32571c

A bisporphyrin macrocycle was constructed from a porphyrin-based diyne monomer in one step through alkyne metathesis. The fullerene binding studies (C₆₀, C₇₀ and C₈₄) showed the highest binding affinity of the macrocycle for C₈₄, which is in great contrast to its bisporphyrin four-armed cage analogue that showed the strongest binding with C₇₀.

Full text of DOI:10.1039/c2cc32571c

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Cite this: Chem. Commun., 2012, 48, 6172–6174
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COMMUNICATION
A C84 selective porphyrin macrocycle with an adaptable cavity
constructed through alkyne metathesiswz
Chenxi Zhang,a Hai Longb and Wei Zhang*a
Received 11th April 2012, Accepted 27th April 2012
DOI: 10.1039/c2cc32571c
A bisporphyrin macrocycle was constructed from a porphyrin-based
diyne monomer in one step through alkyne metathesis. The fullerene
binding studies (C60, C70 and C84) showed the highest binding
affinity of the macrocycle for C84, which is in great contrast to its
bisporphyrin four-armed cage analogue that showed the strongest
binding with C70.
from a C60-enriched fullerene mixture (C60/C70, 10/1 mol/mol)
through the ‘‘Selective Complexation–Decomplexation’’ strategy.
Fullerenes have attracted enormous research interest due to their
unique physical properties and important applications in photo-
voltaics, electronics and the biopharmaceutical industry, etc.1
Despite the fascinating progress in fullerene chemistry, their
practical applications have been hindered by the cost and
energy intensive purification process as well as the poor
solution processibility.2 Synthetic host molecules capable of
noncovalently interacting with fullerenes have been extensively
used as the selective complexation agents to enhance the
solubility of fullerenes or assist the purification of a particular
fullerene from an organic solution of fullerene mixtures.3 As a
consequence, the search for fullerene receptors with high
binding affinity and/or high selectivity has been an intensive
research area.
Recently, we have synthesized a novel 3-D molecular cage
(covalent organic polyhedron, COP-5) as a host molecule
for fullerenes through alkyne metathesis, a highly efficient
dynamic covalent chemistry (DCC) approach, in one step
from a readily accessible porphyrin-based precursor.4 The
shape-persistent COP-5 showed strong binding interactions
with fullerenes. It forms 1 : 1 complexes with C60 and C70
with the association constants of 1.4 ꢀ 105 Mꢁ1 (C60) and
1.5 ꢀ 108 Mꢁ1 (C70) in toluene, strongly favoring binding with
C70 over C60 (KC70/KC60 > 1000). Moreover, the binding
between COP-5 and fullerenes was fully reversible under the
acid–base stimuli, thus allowing successful separation of C70
ð1Þ
In this communication, we report a bisporphyrin macrocycle (1)
as a host for fullerenes. Its binding interactions with various
fullerenes (C60, C70 and C84) were studied and compared to those
of the bisporphyrin cage COP-5. It turned out that COP-5 binds
most strongly with C70, while macrocycle 1 exhibits the highest
binding affinity for C84, thus showing the feasibility of tailoring
the fullerene binding selectivity of aromatic host molecules by fine
tuning their chemical structures.
The bisporphyrin macrocycle 1 was prepared from porphyrin-
based diyne monomer 2 through one-step alkyne metathesis
(eqn (1)), catalyzed by our recently developed multidentate
Mo(VI) alkylidyne catalyst.5 Porphyrin diyne 2 was synthesized
from N-hexadecyl-3-formyl-6-iodocarbazole and 5-(4-tert-butyl-
phenyl)-dipyrromethane through ring cyclization under the
standard Lindsey conditions,6 followed by Sonogashira coupling
reaction to install the end groups for precipitation-driven alkyne
metathesis.7 The metathesis reaction was conducted at 45 1C for
a Department of Chemistry and Biochemistry, University of Colorado,
Boulder, CO 80309, USA. E-mail: wei.zhang@colorado.edu;
Fax: +1 303-492-5894; Tel: +1 303-492-0652
b National Renewable Energy Laboratory, Golden, CO 80401, USA
w This article is part of the ChemComm ‘Aromaticity’ web themed
issue.
z Electronic supplementary information (ESI) available: Experimental
procedures, GPC trace of macrocycle 1, NMR, MALDI-MS, and
UV-Vis spectra of selected compounds and host–guest complexes. See
DOI: 10.1039/c2cc32571c
c
6172 Chem. Commun., 2012, 48, 6172–6174
This journal is The Royal Society of Chemistry 2012

Fig. 1 UV-Vis titration of 1 with C60 (a), C70 (b), C84 (c), and COP-5
with C84 (d). The titration was conducted in toluene, the concentration of 1
was 10ꢁ6 mol Lꢁ1, and the concentration of COP-5 was 2 ꢀ 10ꢁ6 mol Lꢁ1
.
Inset: plot of normalized DA425nm vs. equivalent of fullerene added. The
absorbance at 425 nm was used for the titration of 1. The absorbance at
428 nm was used for the titration of COP-5.
Fig. 2 Energy-minimized models of 1 ((a) top view; (b) side view),
COP-5 ((c) top view; (d) side view), C84@1 (e), and C84@COP-5 (f).
Methyl groups were used in the calculation instead of hexadecyl chains
for simplification. The height of 1, and COP-5 was defined as
the distance between the top and bottom porphyrin panels, and the
diameters of the inside cavity of 1, and COP-5 were defined as the
distance between the two ethynylene groups in the diagonal edges.
16 hours to give the macrocycle 1 in 60% isolated yield.y
The gel permeation chromatography (GPC) trace of the crude
reaction mixture showed the transformation of monomer 2
into the target macrocycle 1 without initial formation of a
large amount of oligomers or polymers along the reaction
process (Fig. S1, ESIz). Macrocycle 1 was purified by column
chromatography, and characterized by 1H, 13C NMR, MALDI-
MS and GPC.
that bisporphyrin macrocycle 1 forms the most stable complex
with C84, while the four-armed cage analog COP-5 binds with
C70 most strongly. Both macrocycle 1 and COP-5 have two
porphyrin moieties connected by the same type of rigid spacers.
In order to gain some insight into this different binding preference
of COP-5 and 1, we took a close look at the structural features of
these two host molecules. The energy-minimized model reveals
that macrocycle 1 is conformationally rather flexible although
it consists of highly rigid aromatic building blocks. The free
macrocycle 1 adopts a collapsed conformation with a minimal
internal cavity. The 1H NMR spectrum of 1 at 20 1C shows four
sets of aromatic proton signals for the 4-tert-butylphenyl rings,
which supports the conformation shown in Fig. 2a and b.9
However, upon addition of the fullerene guest, macrocycle 1
can adapt in size to accommodate the guest molecules, under-
going a remarkable geometry change, with an increase in the
internal macrocyclic cavity size from 4.8 A to 19.3 A. However,
cage COP-5 is relatively rigid, and the size and geometry of the
cage do not change much upon the fullerene encapsulation
(Fig. 2c and d). Due to the high rigidity (shape-persistency) of
COP-5, presumably C70 fits perfectly inside of the cage cavity,
while C60 fits more loosely, and C84 fits a little bit too tight.
Compared to the closed cage COP-5, macrocycle 1 has only
two side arms, which allows the more bulky C84 to fit best in its
more open cavity.
Given the successful bisporphyrin macrocycle synthesis, we
next explored its fullerene binding behavior. Macrocycle 1
showed a strong binding interaction with fullerenes. The binding
of 1 with C60, C70, and C84 was characterized by UV-Vis titration
experiments in toluene (Fig. 1a–c). In all cases, we observed the
decrease of the absorption peak at lmax E 425 nm, and the
appearance of a new peak at lmax E 430 nm. Macrocycle 1 forms
a 1 : 1 host–guest complex with C60, C70, and C84, respectively,
based on the Job plot. We also observed the peaks corresponding
to the complex C70@1, and C84@1 on the MALDI-TOF mass
spectra of the cage–fullerene mixtures (Fig. S3, ESIz). The
association constants of C60, C70 and C84 with macrocycle 1
were calculated based on the 1 : 1 binding mode and fitting of
the UV-Vis adsorption data at 425 nm at different fullerene
concentrations. The results are summarized in Table 1. Macro-
cycle 1 forms the most stable complex with C84 with a binding
constant of 2.2 ꢀ 107 Mꢁ1, which is three orders of magnitude
(KC84/KC60 > 1500) higher than that of complex C60@1.8
Previously, we reported that the four-armed bisporphyrin cage
COP-5 shows a much higher binding affinity for C70 over C60.4
For comparison purpose, we also measured the binding
constant of COP-5 with C84 (Fig. 1d), which is smaller than that
of COP-5 with C70 (KC70/KC84 B 10). It is interesting to note
TFA
Ð
C84@1
C84 þ ½1 þ 2Hþꢂ
ð2Þ
Et3N
Table 1 Binding constants of 1 and COP-5 with fullerenes
Given the high binding selectivity of macrocycle 1 toward C84,
next we examined the feasibility of the pH-controlled release
of C84 and regeneration of host macrocycle 1. Previously, we
reported the pH-driven reversible association and dissociation
KC60 (Mꢁ1
)
KC70 (Mꢁ1
)
KC84 (Mꢁ1
)
1
COP-5
1.3 ꢀ 104
2.0 ꢀ 106
2.2 ꢀ 107
1.4 ꢀ 105
1.5 ꢀ 108
2.4 ꢀ 107
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 6172–6174 6173

of COP-5 and fullerene complexes, and successfully demonstrated
the easy and efficient separation of C70 from a fullerene mixture.
Using a similar approach, we show here that C84@1 is also
capable of undergoing the reversible association and dissociation
process under acid–base stimuli (eqn (2)). Addition of excess TFA
(100 equiv.) to the solution of C84@1 in toluene protonates the
porphyrin ring and weakens the porphyrin–fullerene interaction,
thus leading to the dissociation of C84 and 1. As a consequence,
we observed the broadening and red shift of the adsorption band
of C84@1 with the appearance of a new absorption band at
675 nm (Fig. S7, ESIz). The absorbance of the acidified C84@1
complex is in good agreement with the acidified 1 itself, which
indicates that C84 released from the cage. Subsequent addition of
triethylamine (100 equiv.) to the abovementioned mixture
neutralized the porphyrin ring and restored the binding inter-
action between 1 and C84. Remarkably, the acid–base-mediated
association–dissociation of the host–guest complex could be
repeated many times without obvious change in the absorbance.
The over 1500 times stronger binding interaction of macrocycle 1
with C84 over C60, and the reversible nature of this host–guest
binding triggered by pH open the possibility of using such
a ‘‘Selective Complexation–Decomplexation’’ approach for
purification of higher fullerenes (e.g. C84).
Notes and references
y Procedure for the synthesis of macrocycle 1: the tris(arylmethyl)amine
ligand (1.5 mg, 0.0032 mmol) and the Mo(VI) carbyne precursor
(2.0 mg, 0.0031 mmol) were premixed in dry carbon tetrachloride
(3 mL) for 5 minutes to generate the catalyst in situ. Subsequently, the
monomer 2 (77 mg, 0.040 mmol) was added and the reaction mixture
was stirred at 45 1C for 16 hours. The reaction mixture was
then filtered to remove the byproduct. The filtrate was concentrated
and subjected to column chromatography over alumina (CH2Cl2/
hexane, 1/2, v/v). The pure product was obtained as a purple solid
(33 mg, 60%).
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Technology, Wiley-Interscience, New York, 2000; (b) A. Pivrikas,
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2007, 15, 677; (c) S. S. Babu, H. Mohwald and T. Nakanishi, Chem.
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G. A. Olah, Proc. Natl. Acad. Sci. U. S. A., 1994, 91, 9019;
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In conclusion, bisporphyrin macrocycle 1 with an adaptable
cavity was synthesized via alkyne metathesis with high efficiency.
The macrocycle showed a high binding selectivity for C84 over
lower fullerenes, C70 and C60, which is in great contrast to its
four-armed more rigid cage analogue COP-5 that shows the
highest binding affinity for C70. The significant difference in
fullerene binding affinities and selectivity due to the relatively
small structure variation of host molecules highlights the subtleness
of such molecular recognition events involving fullerene guests. Our
findings will contribute to the future design of novel 2-D or 3-D
host–guest systems and composite materials, which would facilitate
broad applications of fullerenes in various fields.
8 The competitive binding of C84 and C60 with macrocycle 1 was
characterized by 13C NMR, see Fig. S4, ESIz.
9 At elevated temperature, the four sets of aromatic proton signals
coalesced into two sets due to the rapid conformational interconversion
of macrocycle 1. The signal coalesce was observed in both
toluene and chloroform as evidenced by VT NMR characterization,
see Fig. S5 and S6, ESIz.
We acknowledge National Science Foundation (DMR-1055705)
for the funding support, and Dr Yinghua (Alice) Jin for the help
with the manuscript preparation.
c
6174 Chem. Commun., 2012, 48, 6172–6174
This journal is The Royal Society of Chemistry 2012