Fullerene-templated synthesis of a cyclic porphyrin trimer using olefin metathesis

Article abstract of DOI:10.1039/c0cc04474a

An olefination approach to the construction of covalently linked cyclic metalloporphyrin trimers is presented using fullerenes such as C₆₀ or C₇₀ as a template. Yields of the trimer approach 60%. In the absence of a template, the maj

Full text of DOI:10.1039/c0cc04474a

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COMMUNICATION
Fullerene-templated synthesis of a cyclic porphyrin trimer using olefin
metathesisw
Amy R. Mulholland, Clint P. Woodward and Steven J. Langford*
Received 18th October 2010, Accepted 29th November 2010
DOI: 10.1039/c0cc04474a
An olefination approach to the construction of covalently linked
cyclic metalloporphyrin trimers is presented using fullerenes
such as C60 or C70 as a template. Yields of the trimer approach
The synthesis of porphyrin 1 bearing antipodal 1-butenyl
side chains was achieved using a [2+2] MacDonald-type
condensation with subsequent oxidation of the porphyrinogen
12
6
0%. In the absence of a template, the major product is the
cyclic dimer (50% yield) with only a small amount of trimer
o10%) formed, indicating this is a template-directed approach.
intermediate (Scheme 1). Under optimized reaction conditions,
the key precursors were combined with a catalytic amount of
trifluoroacetic acid in dry, degassed DCM and stirred at room
temperature for 6 h, followed by oxidation with DDQ.
Silica-gel chromatography was used to separate the target
compound from isomeric and polymeric by-products, giving
the target porphyrin 1 in 22% yield. The 5,10-disubstituted
and 5,10,15-tributenyl substituted isomers were also isolated in
5 and 3% yield, respectively.
(
Supramolecular chemistry has evolved into an efficient and
versatile paradigm for the preparation of functional molecular
1
architectures. Through careful control of self-assembly and
self-organisation, it is possible to direct the formation of
complex molecular systems with application in such fields as
2
3
light harvesting, biomimicry and molecular recognition. In
order to avoid issues associated with the instability of supra-
molecular architectures, covalent capture can be conducted by
linking key points around the supramolecular ensemble with
4
,5
covalent bonds, enhancing structural integrity. The olefin
metathesis reaction (OM) is well suited for the purpose
of covalent capture, as it can be performed under mild
6
conditions, is reversible (enabling structural error checking),z
and most importantly to this work the activity of the catalyst is
7
not adversely affected by either porphyrin or fullerene.y
Seeking to expand upon our previously reported work in the
8
OM chemistry of porphyrins, and in particular where templation
was used as a synthetic strategy for the formation of a
9
tetrameric macrocycle, we considered the use of fullerenes
as an organizing template capable of complementary p–p
1
0
interactions with the 18-p electron porphyrin macrocycle.
Using a fullerene as a template for formation of the
macrocycle also implies that the resulting construct will
be
a useful receptor for that fullerene, with further
potential for use in new materials involving the organization
1
of fullerene and/or the modulation of fullerene properties.
1
Here, we present a new method for the construction
of a covalently-linked cyclic metalloporphyrin trimer using
either C60 or C70 as a template and discuss the product
distribution based on the geometric differences of the
template used.
School of Chemistry, Monash University, Clayton, Victoria, Australia.
E-mail: steven.langford@monash.edu; Fax: +613 9905 4597;
Tel: +613 9905 4597
Scheme 1 Synthetic process in forming multiporphyrin assemblies
w Electronic supplementary information (ESI) available. Synthesis of
1–4, and characterisation details. See DOI: 10.1039/c0cc04474a
including template-directed formation of triporphyrin 2 using C60
.
1
494 Chem. Commun., 2011, 47, 1494–1496
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+
Templation studies were conducted using a 3 : 1 stoichio-
metric ratio of the porphyrin 1 to C60 (or C70) and a 20 mol%
loading (10 mol% per alkene) of 1st generation Grubbs’
catalyst in CH Cl over a 48 h period (Scheme 1) resulting
which corresponded well to the calculated value for [M + H]
(see Fig. 1). Extended exposure to the cinnamic acid matrix
used in MALDI-TOF analysis resulted in partial demetallation
of the trans–trans–cis isomer of the cyclic metalloporphyrin
trimer, with additional peaks observed at m/z 2558, 2495 and
2
2
in the formation of a purple solid later identified as 1 : 1
1
+
+
trimeric porphyrin/C60 complex, 2*C . The H-NMR
2432 corresponding to [M ꢀ Zn + 3H] , [M ꢀ 2Zn + 5H]
60
+
spectrum of the crude material displayed considerable
complexity in the distribution of the proton resonances. Closer
examination revealed a loose correlation between the ratio of
cis:trans olefinic bonds and the other signals arising from the
b-pyrrolenic, b-pyrrolic, methyl ester and phenyl protons.
Small amounts of a cyclic porphyrin dimer 4 (4% yield) were
also detected as a by-product of this reaction. The fullerene
could be removed from the system by loading the crude
material onto a short column of silica and eluting with toluene
until the eluent became colourless.
and [M ꢀ 3Zn + 7H] , respectively.
In the absence of a template, the predominant product is a
cyclic porphyrin dimer 4 (50% yield, Scheme 1), with the cyclic
trimer 3 forming as a minor product (9% yield). This result
suggests that the fullerene template allows for the tuning
of the cavity size in favour of a larger (trimeric) porphyrin
macrocycle. The trimeric result complements the solid state
1
0c
findings of Boyd et al., who reported a prismatic porphyrin/
fullerene co-crystallite, which displays a tight encasement of a
C60 by three (pentafluorophenyl)porphyrin molecules and the
result presented here demonstrates an appropriate translation
of the solid-state finding to solution. The use of C70 as a
template resulted in a product distribution similar to that of
the C60 templation (B50% of 2); unfortunately there was no
evidence that the use of a slightly larger fullerene (along the
endohedral axis) altered the product distribution in favour of
multimeric porphyrin macrocycles with a greater cavity size.
The use of even larger fullerenes may have greater bearing on
the products obtained using this method.
After removing the template from the system the isomeric
mixture was zinc metallated by treatment with Zn(OAc)
2
in
refluxing CHCl /MeOH. The product mixture was then
3
separated using silica gel chromatography and as a result of
this, it was possible to isolate single isomers of the form
trans–trans–trans; cis–trans–trans and cis–cis–trans with the
cis–trans–trans being the dominant isomer. The trimeric
porphyrin macrocycle 2 was isolated in 59% yield from the
previous step. The fact that trimer 2 forms in such good yield
with three new carbon–carbon double bonds being formed
so efficiently indicates a strong cooperativity between the
spherical template and each porphyrin unit.
The association between the metalloporphyrin trimer 3
(as the cis–trans–trans isomer) and C60 was assessed using a
set of UV-vis and fluorescence quenching experiments.
Titration of a solution of C60 in toluene into a solution of
the trimer 3 in toluene did not induce the spectral changes
normally associated with the formation of an inclusion
complex. By substituting toluene with chloroform as the
porphyrin solvent and conducting the titration as previously
described, a slight bathochromic shift in the Soret band with a
concomitant reduction in intensity was observed (see ESIw).
Clear isosbestic points were also detected at 415 and 436 nm.
Qualitatively, this finding is consistent with previous research
1
As expected the H-NMR spectrum of the all-trans isomer
showed considerably less complexity than the two isomers of
lower symmetry, the predominant change (cf. 1) being in the
position and splitting pattern of the alkene protons. In the
1
H-NMR spectra of the cis–trans–trans and cis–cis–trans
isomers, it was interesting to find that the olefinic bonds had
quite a dramatic effect not just on the adjacent methylene
protons, but also on the position of the signals attributed to
methyl ester, phenyl and b-pyrrolic protons, suggesting
complex conformational interactions. The Soret bands of the
cis–trans–trans and cis–cis–trans isomers were red-shifted
3
,11a
in porphyrin–fullerene interactions,
however the affinity of
metalloporphyrin trimer 3 for C is considerably lower
0
6
3
ꢀ1
(lmax 429 and 430 nm, respectively) and slightly less symmetrical
(2.7 ꢁ 10 M ) than reported values for other dimeric and
trimeric porphyrin systems, no doubt as a result of the lower
level of preorganisation of 3. The ease with which the C60
template was removed from the system post-synthesis is
reflected in the result obtained here.z
than the all-trans isomer, which displayed a peak maximum
at 427 nm. MALDI-TOF mass spectrometry provided
further evidence in support of trimer formation, yielding a
predominant ion peak at m/z 2621 (for the all-trans isomer)
In summary, olefin CM was employed, creating a discrete
and novel trimeric porphyrin macrocycle in high yield in which
three new covalent bonds were made in a single pot. The use of
C60 leads to excellent product selectivity. To the best of our
knowledge these results represent the first examples of
covalent templation around fullerene guest molecules via
olefin metathesis. Further investigations to thoroughly assess
supramolecular properties of 3 and its reduced analogue are
currently underway. These porphyrinic trimers possess
functionality conducive to the growth of columnar arrays
and other related supramolecular assemblies, further details
of which will be reported in due course.
We wish to thank Ms Shane Reeve for her assistance with
mass spectral analysis, Dr Peter Nichols for assistance
with NMR and the Sir James McNeill foundation for a
Fig. 1 MALDI-TOF spectrum of a single isomer of the cyclic
metalloporphyrin trimer 3.
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 1494–1496 1495

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postgraduate research scholarship (to AM). This project is
supported by Australian Research Council Discovery Projects
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stoichiometric amount of C60 verified that fullerenes have no effect on
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quite different in the absence of toluene, which has a marked effect on
binding. The same titration in 1 : 5 v/v mixes of toluene and CHCl
shows very weak binding. The insolubility of C60 in dichloromethane
and chloroform precluded determination of the binding constant in
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496 Chem. Commun., 2011, 47, 1494–1496
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