An oxacalix[2]arene[2]pyrimidine-bis(Zn-porphyrin) tweezer as a selective receptor towards fullerene C70

Article abstract of DOI:10.1016/j.tetlet.2010.02.137

An oxacalix[2]arene[2]pyrimidine-bis(Zn^II-porphyrin) conjugate was readily prepared via nucleophilic aromatic substitution of a phenolic AB₃-Zn-porphyrin on the upper rim of a (1,3-alternate) 5,17-bis(methylsulfonyl)oxacalix[4]arene precursor. Efficient 1:1 complex formation between the 'jaws' bisporphyrin tweezer and fullerene C₇₀ was evidenced by ¹H NMR titrations (K = 3.0 × 10⁴ M^-1), while no detectable complexation could be observed with C₆₀. On the other hand, an analogous oxacalix[4]arene-bis(Cu-corrole) conjugate did not show any measurable (C₆₀ or C₇₀) fullerene binding.

Full text of DOI:10.1016/j.tetlet.2010.02.137

Tetrahedron Letters 51 (2010) 2423–2426
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Tetrahedron Letters
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An oxacalix[2]arene[2]pyrimidine-bis(Zn-porphyrin) tweezer as a selective
receptor towards fullerene C70
a,
a,c
Wim Van Rossom a, Ondrej Kundrát b, Thien Huynh Ngo a, Pavel Lhoták b, , Wim Dehaen , Wouter Maes
*
*
a Molecular Design and Synthesis, Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium
b Department of Organic Chemistry, Prague Institute of Chemical Technology, Technická 5, 166 28 Prague 6, Czech Republic
c Institute for Materials Research (IMO), Research Group Organic and (Bio)Polymeric Chemistry, Hasselt University, Universitaire Campus, Agoralaan Building D,
B-3590 Diepenbeek, Belgium
a r t i c l e i n f o
a b s t r a c t
An oxacalix[2]arene[2]pyrimidine-bis(ZnII-porphyrin) conjugate was readily prepared via nucleophilic
aromatic substitution of a phenolic AB3-Zn-porphyrin on the upper rim of a (1,3-alternate) 5,17-bis(meth-
ylsulfonyl)oxacalix[4]arene precursor. Efficient 1:1 complex formation between the ‘jaws’ bisporphyrin
tweezer and fullerene C70 was evidenced by 1H NMR titrations (K = 3.0 ꢀ 104 Mꢁ1), while no detectable
complexation could be observed with C60. On the other hand, an analogous oxacalix[4]arene-bis(Cu-cor-
role) conjugate did not show any measurable (C60 or C70) fullerene binding.
Article history:
Received 30 January 2010
Revised 18 February 2010
Accepted 22 February 2010
Available online 1 March 2010
Keywords:
Ó 2010 Elsevier Ltd. All rights reserved.
Oxacalixarenes
Porphyrinoids
Fullerenes
Supramolecular receptors
Traditional methylene-bridged calixarenes and thiacalixarenes
are well-known, generally applied building blocks and scaffolds
within supramolecular chemistry.1,2 These macrocycles are rela-
tively easy to prepare and functionalize, and show particular confor-
mational preferences. The N- and O-bridged counterparts (aza- and
oxacalixarenes, respectively) are less familiar members of the hete-
racalixarene subclass.3–6 Oxacalixarene chemistry has been rejuve-
nated in recent years,4–6 but supramolecular applications have
only scarcely been explored. A particular advantage of oxacalixa-
renes in respect to most other members of the calixarene family is
the 1,3-alternate conformation of the smallest and most rigid oxaca-
lix[4]arene, as generally observed in solid-state X-ray structures.4–7
Thisconformationappearstobe (most)promisingtogeneratepreor-
ganized supramolecular receptors, for example, for cations, anions
or fullerenes.
the assumed preorganization in a 1,3-alternate conformation,7 this
prompted us to investigate supramolecular host–guest complexa-
tion by introducing receptor motifs on the oxacalixarene
framework.
Designing effective host molecules capable of (size-) selective
complexation of fullerenes is highly important to reduce costs due
to laborious fullerene purification and isolation procedures, and
facilitate their application in (bio)chemistry and (photoactive)
material sciences.11 The geometrical complementarity of concave
calixarene cavities and spherical fullerenes has evidently aroused
the interest of calixarene chemists. As indicated in seminal work
by the Atwood and Shinkai groups, particular calixarene derivatives
indeed show high affinity for fullerenes.12 The (classical) p-tert-
butylcalix[8]arene binds to the fullerene surface through van der
Waals concave–convex p–p interactions and is selective for [60]ful-
Recently, the Leuven group has developed versatile and selec-
tive synthetic protocols towards both oxacalix[2]arene[2]pyrimi-
dines,8 applying a convenient one-pot procedure, and enlarged
calixarene macrocycles (oxacalix[6]- and oxacalix[8]arenes),9
using efficient fragment-coupling approaches. The most appealing
aspect of oxacalix[m]arene[m]pyrimidines is the ease and wide
scope of derivatization of the outer perimeter, for example, via
nucleophilic aromatic substitution (SNAr) reactions.10 Alongside
lerene (C60). Inspired by this early observation, many other research
groups have studied fullerene complexation properties with various
other calixarenes and derivatives thereof. Noteworthy for the pre-
sented results are the fullerene affinities reported for hetero-atom
bridged calix(hetero)aromatics. Azacalix([m]arene)[n]pyridines
and N,O-bridged calix[1]arene[4]pyridines exhibit rather high
association constants (up to K = 1.4 ꢀ 105 Mꢁ1) for C60 and/or C70
,
as revealed by UV–vis and fluorescence quenching titration experi-
ments,13 while very modest binding constants were observed for
homooxacalix[3]arenes.14
The spontaneous attraction of fullerenes to the centre of a
(metallo)porphyrin is an intriguing supramolecular recognition
* Corresponding authors. Tel.: +420 220 445 055; fax: +420 220 444 288 (P.L.);
tel.: +32 16 327390; fax: +32 16 327990 (W.D.).
E-mail addresses: pavel.lhotak@vscht.cz (P. Lhoták), wim.dehaen@chem.kuleu
ven.be (W. Dehaen).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.02.137

2424
W. Van Rossom et al. / Tetrahedron Letters 51 (2010) 2423–2426
element that can also be applied to produce discrete host-guest
systems. To obtain strong interaction of the curved -surface of
the fullerene sphere with the electropositive centre of the planar
porphyrin -surface (in addition to the regular attraction
and differential solvation effects), a suitable arrangement of the
porphyrin units with respect to one another appears to be cru-
cial.15 The use of (thia)calixarene scaffolds (among others11) has
shown to position porphyrins at an appropriate mutual distance
of 12 Å (diameter C60/C70 ꢂ10 Å), affording calixarene-porphyrin
tweezers, often referred to as ‘jaws’ porphyrins, with strong fuller-
ene binding properties.16 Besides the (centre of the) flat porphyrin
depicted in Scheme 1.19 Inspired by the post-macrocyclization pro-
cedures previously optimized for bis(methylsulfonyl)oxacalix[2]
arene[2]pyrimidine 1,10 more sophisticated functional units such
as porphyrins and corroles could be introduced to obtain preorga-
nized tweezer-type host molecules. Since tert-butyl substituents
were previously shown to enhance the fullerene binding properties
p
p
p–p
(by p
–CH3 interactions),16c,20 5,10,15-tris(3,5-di-tert-butylphenyl)-
20-(4-hydroxyphenyl)porphyrinato zinc (4) was selected as the
nucleophilic porphyrin. The porphyrin moiety, synthesized accord-
ing to a reported method,21 was coupled to the upper rim of oxaca-
lix[4]arene 1 via a SNAr reaction employing K2CO3 as a base in DMF
at 70 °C to afford oxacalix[2]arene[2]pyrimidine-bis(ZnII-porphy-
rin) receptor 2 in 73% yield (Scheme 1).22 Previous activities of the
Leuven group in the corrole field ignited our interest to study analo-
gous corrole derivatives.23 To our knowledge, supramolecular com-
plexation of fullerenes with corrole derivatives has not been
demonstrated so far. Since Cu-metallated corroles usually show an
improved (oxidative) stability over their free-base counterparts,
nucleophilic Cu-corrole precursor 5 was selected, available to us
from ongoing corrole research,24 and a similar SNAr approach was
then used to obtain the first ‘jaws’ biscorrole receptor. Treatment
of oxacalix[4]arene substrate 1 with a small excess (2.2 equiv) of
Cu-corrole 5 in the presence of NaH base in acetonitrile at 70 °C re-
sulted in the formation of oxacalix[2]arene[2]pyrimidine-bis
(Cu-corrole) receptor 3, which could be isolated in 80% yield
(Scheme 1).25
The complexation abilities of the novel conjugates 2 and 3 to-
wards fullerenes C60/C70 were studied by 1H NMR titrations in ben-
zene-d6. Upon addition of an excess of C70 to a solution of receptor
2, the signals corresponding to the b-pyrrolic protons of the por-
phyrin moieties shifted upfield (complexation-induced chemical
shifts (CIS) >50 Hz after the addition of 5 equiv C70), while the sig-
nals of the oxacalixarene skeleton remained almost unchanged.
These changes in chemical shifts clearly point to direct interactions
p-system, the cavity of the calixarene itself can possibly also con-
tribute to the inclusion properties.
To determine the fullerene binding strength of the oxacalixarene
framework, several oxacalix[n]arenes were selected from the small
library available to us from previous work. 5,17-Diphenyloxaca-
lix[2]arene[2]pyrimidine and the analogous oxacalix[8]arene,
enlarging the cavity’s
p
-surface and approaching the shape of a
-surfacecould
moleculartweezer, wereinitiallyselected.17 Alarger
p
also be achieved via oxacalix[2]arene[2]quinazolines.18 Both a syn-
and anti-oxacalix[2]arene[2]quinazoline as well as expanded syn-
and anti-oxacalix[2]naphthalene[2]quinazolines were studied.17
Unfortunately, 1HNMR titrations indicated that none of these oxaca-
lixarenes shows any detectable binding with either fullerene C60 or
C70. This was not completely unexpected considering the (too) small
calix[4]arene cavity, the supposed 1,3-alternate conformation and
the more electron-deficient structure compared to azacalix[n]are-
nes. Although the contribution to actual fullerene binding might
be negligible, the oxacalixarene framework can still serve as a versa-
tilescaffoldenablingappropriatearrangementof porphyrinoidunits
with recognition potential for fullerenes.
Hence, a strategy for the construction of tweezer-type bis-
porphyrinoid receptors has been explored. The synthetic protocol
towards oxacalix[4]arene-porphyrinoid conjugates 2 and 3 is
Scheme 1. Reagents and conditions: (i) K2CO3, 18-crown-6, DMF, 70 °C, 24 h (73%); (ii) NaH, MeCN, 70 °C, 15 min (80%).

W. V. Rossom et al. / Tetrahedron Letters 51 (2010) 2423–2426
2425
between the porphyrin and fullerene entities. The complexation-
induced chemical shifts were used for the construction of a titra-
tion curve (Fig. 1), which was analyzed by a nonlinear least-squares
method assuming 1:1 stoichiometry.26 The rather high complexa-
tion constant K = 30000 4600 Mꢁ1 indicates that cooperation of
both porphyrin units likely occurs during the binding process.27
The 1:1 stoichiometry of the C70-bisporphyrin 2 complex was also
established by an independent Job plot analysis (Fig. 2). By con-
trast, upon the addition of C60, the porphyrin aromatic resonances
for compound 2 showed only negligible CIS (<5 Hz), which did not
allow the construction of the corresponding titration curve.28
Higher Kass for C70 compared to C60 have previously been attributed
to a maximization of ovoid-shaped C70–porphyrin interactions.29
Unfortunately, in the case of biscorrole receptor 3 no salient 1H
NMR CIS were observed in the presence of either C60 or C70, which
might be attributed to the additional conformational flexibility
introduced by the methylene linker or to sterical effects imposed
by the meso-mesitylmoieties.30 As both systems (porphyrin2 vs cor-
role 3) are not fully structurally comparable, one of the fundamental
questions—whether corroles can be used instead of porphyrins for
the construction of efficient fullerene receptors—remains partly
unanswered at this stage. To achieve efficient binding with ‘jaws’
corroles, more work should be devoted to optimize the subtle bal-
ance between optimal receptor properties and corrole stability, for
which electron-deficient and sterically bulky ortho,ortho0-substi-
tuted meso-aryl units are often favourable.
contribution of the oxacalix[4]arene part to fullerene binding
seems small, the oxacalixarene skeleton is a versatile building
block towards efficient tweezer-type supramolecular receptors.
Further studies will be directed towards exploration of the binding
capacity of ‘jaws’ corrole hosts and fullerene receptors based on
larger oxacalix[n]arene scaffolds (n = 6, 8) with additional com-
plexing porphyrinoid units.
Acknowledgements
We thank the FWO (Fund for Scientific Research—Flanders) for
financial support and a postdoctoral fellowship to W. Maes, and
the K. U. Leuven and the Ministerie voor Wetenschapsbeleid for
continuing financial support. T. H. Ngo is grateful to the IWT (Insti-
tute for the Promotion of Innovation through Science and Technol-
ogy in Flanders) for a doctoral fellowship. P.L. and O.K. thank the
Czech Science Foundation (Grant 203/09/0691).
Supplementary data
Supplementary data (experimental procedures and character-
ization data for all porphyrin/corrole precursors, and 1H NMR
spectra of receptors 2 and 3) associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.2010.02.137.
References and notes
In conclusion, novel tweezer-type bisporphyrinoid receptors
were easily obtained via straightforward and high-yielding syn-
thetic SNAr procedures based on a preorganized 1,3-alternate
oxacalix[2]arene[2]pyrimidine platform and a nucleophilic por-
phyrin or corrole reaction partner. Solution-phase binding studies
with fullerenes C60 and C70 were conducted by 1H NMR titration
experiments, which revealed pronounced selectivity for the egg-
shaped C70 employing oxacalixarene-linked bisporphyrin 2, with
a significant binding constant (K = 3.0 ꢀ 104 Mꢁ1). Although the
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resembles the solid-state 1,3-alternate conformation (Ref. 5).
Figure 1. 1H NMR titration of receptor 2 with C70 (C6D6, 300 MHz, 298 K).
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Figure 2. Job plot for the 2 + C70 system (1H NMR, C6D6, 300 MHz, 298 K;
[2] + [C70] = 5 ꢀ 10ꢁ3 M).

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W. Van Rossom et al. / Tetrahedron Letters 51 (2010) 2423–2426
15. For a review on fullerene-porphyrin constructs, see: Boyd, P. D. W.; Reed, C. A.
Acc. Chem. Res. 2005, 38, 235–242.
24. The synthetic procedure towards this metallocorrole can be found in the
Supplementary data file.
16. (a) Arimura, T.; Nishioka, T.; Suga, Y.; Murata, S.; Tachiya, M. Mol. Cryst. Liq.
25. Preparation of oxacalix[2]arene[2]pyrimidine-bis(Cu-corrole) conjugate 3: 5,17-
ˇ
Cryst. Sci. Technol., Sect. A 2002, 379, 413–418; (b) Dudic, M.; Lhoták, P.; Stibor,
bis(methylsulfonyl)oxacalix[4]arene 1 (30 mg, 0.05 mmol), Cu-corrole 5
ˇ ˇ
I.; Petrícková, H.; Lang, K. New J. Chem. 2004, 28, 85–90; (c) Hosseini, A.; Taylor,
(77 mg, 0.11 mmol, 2.2 equiv) and NaH (60% dispersion, 7 mg, 0.15 mmol)
were dissolved in CH3CN (2 mL) and the resulting mixture was heated at 70 °C
for 15 min (under Ar). Subsequently, ethyl acetate (50 mL) was added and the
mixture was washed with water (3 ꢀ 25 mL), dried over MgSO4, filtered and
the solvents were removed under vacuum. After column chromatographic
purification (silica, eluent heptane/ethyl acetate 7:3), oxacalix[4]arene-
biscorrole conjugate 3 (78 mg, 80%) was isolated in pure form as a red-
S.; Accorsi, G.; Armaroli, N.; Reed, C. A.; Boyd, P. D. W. J. Am. Chem. Soc. 2006,
128, 15903–15913; (d) Káš, M.; Lang, K.; Stibor, I.; Lhoták, P. Tetrahedron Lett.
2007, 48, 477–481; (e) Kundrát, O.; Káš, M.; Tkadlecová, M.; Lang, K.; Cvacka, J.;
ˇ
Stibor, I.; Lhoták, P. Tetrahedron Lett. 2007, 48, 6620–6623.
17. Structural representations of these oxacalix[n]arenes can be found in the
Supplementary data file.
18. Van Rossom, W.; Kishore, L.; Robeyns, K.; Van Meervelt, L.; Dehaen, W.; Maes,
W., submitted for publication.
19. A few related oxacalixarene-porphyrin conjugates have recently been prepared
by other research groups (Refs. 4c,e,i).
20. Georghiou, P. E.; Tran, A. H.; Stroud, S. S.; Thompson, D. W. Tetrahedron 2006,
62, 2036–2044.
brown solid. mp >350 °C; MS (ESI+) m/z 1798.6 [M+H]+, 899.9 [M+2H]2+ 1H
;
NMR (400 MHz, CDCl3) d 7.98 (sbr, 4H; Hb-pyr), 7.64–7.59 (m, 8H; Hb-pyr), 7.35
(sbr, 4H; Hb-pyr), 7.20–7.15 (m, 8H), 7.02 (s, 8H; Mes), 6.90 (s, 4H; 4,6-orc), 6.67
(s, 2H; 2-orc), 5.59 (s, 4H; CH2), 5.06 (s, 2H; 5-pyr), 2.39 (s, 12H; CH3-Mes), 2.37
(s, 6H; CH3-orc), 2.06 (s, 24H; CH3-Mes); 13C NMR (100 MHz, CDCl3) d 174.1 (C;
4,6-pyr), 166.1 (C; 2-pyr), 152.9 (C; 1,3-orc), 148.9 (C), 144.3 (C), 143.8 (C; 5-
orc), 138.5 (br; CHb), 137.7 (C; Mes), 136.4 (C), 132.6 (br, CHb), 131.4 (CH),
128.2 (CH; Mes), 127.8 (CH), 121.0 (CH), 120.6 (CH; 4,6-orc), 111.9 (CH; 2-orc),
81.5 (CH; 5-pyr), 69.6 (CH2), 21.6 (CH3; orc), 21.4 (CH3; Mes), 19.9 (CH3; Mes);
21. Maes, W.; Vanderhaeghen, J.; Smeets, S.; Asokan, C. V.; Van Renterghem, L. M.;
Du Prez, F. E.; Smet, M.; Dehaen, W. J. Org. Chem. 2006, 71, 2987–2994.
22. Preparation of oxacalix[2]arene[2]pyrimidine-bis(Zn-porphyrin) conjugate 2: 5,17-
bis(methylsulfonyl)oxacalix[4]arene 1 (30 mg, 0.05 mmol), AB3-porphyrin 4
(117 mg, 0.11 mmol, 2.2 equiv), K2CO3 (22 mg, 0.15 mmol) and 18-crown-6
(5 mg, 0.02 mmol) were dissolved in DMF (2 mL) and the mixture was heated at
70 °C for 24 h (under Ar). DMF was removed under vacuum, and the residue was
redissolved in ethyl acetate (50 mL) and washed with water (3 ꢀ 25 mL). The
organic fraction was dried over MgSO4, filtered and the solvent was removed
under vacuum. After column chromatographic purification (silica, eluent
heptane/ethyl acetate 9:1), oxacalix[4]arene-bisporphyrin conjugate 2 (96 mg,
73%) was isolated in pure form as a pink-purple solid. mp >350 °C; MS (ESI+) m/z
2457.0 [M+H]+; 1H NMR (300 MHz, CDCl3) d 9.07–9.01 (m, 16H; Hb-pyr), 8.31 (d,
3J = 8.2 Hz, 4H; Ph-O), 8.10–8.08 (m, 12H; t-Bu-Ph), 7.79 (s, 6H; t-Bu-Ph), 6.67 (d,
3J = 8.2 Hz, 4H; Ph-O), 7.02 (s, 4H; 4,6-orc), 6.84 (s, 2H; 2-orc), 5.27 (s, 2H; 5-pyr),
2.45 (s, 6H; CH3-orc), 1.53 (s, 108H; t-Bu); 13C NMR (75 MHz, CDCl3) d 174.6 (C;
4,6-pyr), 166.3 (C; 2-pyr), 159.9, 153.0 (C; 1,3-orc), 152.3 (C), 150.6 (C), 150.3 (C),
148.7 (C; Ci-t-Bu), 144.0 (C; 5-orc), 142.0 (C), 140.6 (C), 135.6 (CH), 132.6/132.5/
132.4/132.0 (CHb), 129.9 (CH), 129.8 (CH), 122.9 (C), 122.7 (C), 120.9 (CH), 119.8
(CH; 4,6-orc), 112.1 (CH; 2-orc), 82.6 (CH; 5-pyr), 35.2 (C; t-Bu), 32.0 (CH3; t-Bu),
UV–vis (CH2Cl2) kmax (log e) 256 (4.856), 403 (5.123), 538 (4.175).
26. The binding constants were calculated using the computer program OPIUM
freely available at http://www.natur.cuni.cz/~kyvala/opium.html.
27. This binding constant is comparable to those observed for a thiacalixarene-
based tweezer employing the same porphyrin entity (Ref. 16e) and a related
traditional calixarene-based bisporphyrin (Ref. 16b).
28. Since the CIS were too small (in the range of the accuracy of the NMR
measurements) to allow quantitative determination of the binding constants,
one can state that there is virtually no measurable binding under the applied
conditions and method (although in principle, strong binding can occur even
with very small CIS).
29. Zheng, J. Y.; Tashiro, K.; Hirabayashi, Y.; Kinbara, K.; Saigo, K.; Aida, T.;
Sakamoto, S.; Yamaguchi, K. Angew. Chem., Int. Ed. 2001, 40, 1858–1861.
30. Another factor to consider is the role of corrole metallation (somewhat variable
trends are observed for metalloporphyrins). For Cu-corroles, inherent saddling
of the corrolato ligand and the partial radical character might have an influence
as well. Electronic effects—corroles are in general more electron rich than
porphyrins—cannot be excluded either.
21.8 (CH3; orc); UV–vis (CH2Cl2) kmax (log e) 402sh (4.992), 422 (6.073), 549
(4.679), 588 (4.134).
23. (a) Maes, W.; Ngo, T. H.; Vanderhaeghen, J.; Dehaen, W. Org. Lett. 2007, 9,
3165–3168; (b) Ngo, T. H.; Van Rossom, W.; Dehaen, W.; Maes, W. Org. Biomol.
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