Molecularly defined shape-persistent 2D oligomers: The covalent-template approach to molecular spoked wheels

Article abstract of DOI:10.1002/anie.200701614

(Figure Presented) A template-directed sixfold dimerization of alkyne units is the final step in the synthesis of the title compound leading to nanoscale 2D rigid wheel-shaped oligo-(phenylene ethynylene)s. Solubilizing side groups allow full characteriza

Full text of DOI:10.1002/anie.200701614

Communications
DOI: 10.1002/anie.200701614
Two-Dimensional Oligomers
Molecularly Defined Shape-Persistent 2D Oligomers:
The Covalent-Template Approach to Molecular Spoked
Wheels**
Dennis Mössinger, Jens Hornung, Shengbin Lei, Steven De Feyter,* and
Sigurd Höger*
Angewandte
Chemie
6802
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Two-dimensional rigid structures have received increasing
attention in materials science, both in basic research and
industrial application. For example, polymer–clay nanocom-
posites have been investigated thoroughly with regard to their
stiffness, strength, thermal stability, and gas and liquid
permeability.[1] Uniformly disk-shaped synthetic clay platelets
(e.g. Laponite RD, d ꢀ 25 nm) have also been analyzed
recently in order to understand fundamental physical and
chemical aspects of polymer–clay composite materials.[2,3]
Alternatively, rigid two-dimensional (2D) organic oligom-
ers and polymers with similar lateral expansions could be
employed as model compounds or even used for the
formulation of new composites. While linear shape-persistent
organic oligomers are well investigated as models for the
corresponding polymers and also have their own field of
applications (e.g. in photooptics), rigid two-dimensional
oligomers are relatively unexplored.[4–8] However, this
approach has numerous potential benefits as these structures
are molecularly defined and well characterizable, allow
introduction of functionality at the rim as well as above and
below the molecular plane, and can be obtained in different
sizes (Figure 1).[9] Although some defined rigid 2D structures
have already been described, an approach towards objects in
a size range of the inorganic counterparts can be rarely
found.[10] Here we describe the first example of new molecular
objects that fulfill the aforementioned criteria. Scanning
tunneling microscopic (STM) investigations can be used to
visualize the molecules and confirm their proposed size and
shape.
Our synthetic approach to large two-dimensional oli-
go(arylene ethynylene)s is outlined in Scheme 1. Target
structure C consists of hub, spoke, and rim subunits based
[*] Dr. S. Lei, Prof. Dr. S. De Feyter
Department of Chemistry
Laboratory of Photochemistry and Spectroscopy, and
INPAC—Institute for Nanoscale Physics and Chemistry
Katholieke Universiteit Leuven
Celestijnenlaan 200-F, 3001 Leuven (Belgium)
Fax: (+32)16-327-990
E-mail: steven.defeyter@chem.kuleuven.be
Homepage: http://www.chem.kuleuven.be/research/mds/
mds_en.html
Dipl.-Chem. D. Mössinger, Prof. Dr. S. Höger
KekulØ-Institut für Organische Chemie
Biochemie der Universität Bonn
Gerhard-Domagk-Strasse 1, 53121 Bonn (Germany)
Fax: (+49)228-73-5662
E-mail: hoeger@uni-bonn.de
Homepage: http://www.chemie.uni-bonn.de/oc/ak_ho/
Dipl.-Chem. J. Hornung[+]
Institut für Technische Chemie und Polymerchemie
Universität Karlsruhe
Figure 1. Schematic illustration of rigid 2D oligomers with side groups
above and below the molecular plane and at the periphery (top).
Structure of the first-generation 2D oligomer 1 (bottom).
Engesserstrasse 18, 76131 Karlsruhe (Germany)
[+] Present address: ETH Zürich
Laboratorium für Organische Chemie
HCI G 306
Wolfgang-Pauli-Strasse 10, 8093 Zurich (Switzerland)
[**] This work was supported by the Deutsche Forschungsgemeinschaft,
the SFB 624, the Belgian Federal Science Policy Office through IAP-
6/27, and the Fund for Scientific Research—Flanders (FWO). The
authors thank K. U. Leuven (GOA 2006/2) and Marie-Curie RTN
“Chextan” for financial support. The help of Carsten Siering in
creating graphical representations is gratefully acknowledged.
Scheme 1. Two different rigid building blocks (A) undergo sixfold
coupling to form star-shaped molecule B, which is cyclized to give the
rigid 2D oligomer C. A repetitive synthesis using C as a hubshould
enable further radial growth to form higher generations, for example,
D as a representative of the second generation.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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on rigid building blocks. The sixfold rotational symmetry
products have the same Rf values on TLC, the reaction
progress can be determined only by 1H NMR spectroscopy or
mass spectrometry. Column chromatographic purification at
this point of the synthesis was not possible. However, after
deprotection the corresponding desilylated five- (and four-
fold) coupling products have greater Rf values so that 16 could
be purified by radial chromatography.
The intramolecular dimerization of acetylene units was
performed under pseudo-high-dilution conditions by adding a
dilute solution of 16 in pyridine over four days to a slurry of
CuCl/CuCl2 in the same solvent. The reaction mixture was
stirred for an additional 16 h before the crude reaction
product was analyzed by gel permeation chromatography
(GPC). When the reaction was performed at room temper-
ature, the reaction mixture contained about 65% of 1 along
permits a feasible synthesis requiring only two different
modules for the formation of B and an intramolecular
coupling to give C. Our concept allows functionalization
above and below the plane (hub and spokes) as well as at the
periphery (corners and edges of the rim). Radial expansion
with similar modules should make “higher generations”
accessible (D). If we apply dendrimer terminology to describe
our concept, the spoked wheel C represents a first-generation
rigid 2D oligomer. Spiderweb D, yet not realized, would
represent a second-generation structure.[11]
This approach is not limited to specific bond-forming
reactions, and alternatives including bond formation under
thermodynamic control can also be envisaged. In our first
À
approach we focused on Pd-catalyzed C C coupling reactions
for the formation of B and oxidative acetylene coupling for
the cyclization giving C.[12,13] We have chosen butadiyne
formation as the cyclization reaction because
previously our group demonstrated that tem-
plate-supported (intramolecular) acetylene
dimerization gives nearly quantitative yields of
the corresponding macrocyclic coupling prod-
ucts.[14–16]
with 20% of
a
low-molecular-weight impurity (Mw
ꢀ 2000 gmolÀ1) of unknown origin and structure and 15%
Scheme 2 displays the synthesis of the novel
2D oligomer 1. Sixfold Suzuki reaction of
hexa(4-bromophenyl)benzene (2)[18] with the
aryl boron pinacolate 3[19] and subsequent iodo-
desilylation using iodine monochloride gave the
“hub” component 5. Although 5 is barely soluble
in chloroform, the twelve ethyl side groups made
it possible to purify 5, a prerequisite for the
success of the following reaction sequence.
Component 6, accessible from the corresponding
pyrylium salt,[20] was coupled with two equiva-
lents of the boronate 7 to yield 8.[21] Prior to the
subsequent Hagihara–Sonogashira coupling with
(3-cyanopropyl)dimethylysilyl(CPDMS)-acetyl-
ene, 8 was transformed into the more reactive
iodo compound 9 by halogen–metal exchange
and treating the intermediate with diiodoethane.
The CPDMS group is a polar analogue of the
trimethylsilyl group and simplifies the chromato-
graphic separation of coupling products, starting
material, and dehalogenated side products.[14h,22]
Base-catalyzed deprotection of 10, coupling with
12, and removal of the CPDMS groups gave 14.
Here again, thanks to the CPDMS group,
purification of 13 by column chromatography
was straightforward even on a hundred-milli-
gram scale. Furthermore, the consecutive polar-
ity change of the products in the reaction
Scheme 2. a) PEPPSI catalyst,[17] KOH, THF, 5 h reflux, 66%; b) ICl, CHCl3, 0–208C,
24 h, 91%; c) [Pd(PPh3)4], Cs2CO3, H2O, THF, 3 d reflux, 26%; d) nBuLi, C2H4I2, THF,
À788C, 2 h, 208C, 94%; e) [(3-cyanopropyl)dimethylsilyl]acetylene (CPDMS-acetylene),
[PdCl2(PPh3)2], CuI, PPh3, THF, piperidine, 208C, 24 h, 62%; f) K2CO3, THF/MeOH
(2:1), 3 h, 208C, 98%; g) [PdCl2(PPh3)2], CuI, PPh3, THF, piperidine, 208C, 4 h, 65%;
h) K2CO3, THF/MeOH (2:1), 16 h, 208C, 92%; i) [PdCl2(PPh3)2], CuI, PPh3, THF,
piperidine, 708C, 3 d; j) excess TBAF, 1m in THF, 5% H2O, 208C, 3 d, 46% (2 steps);
k) CuCl, CuCl2, pyridine, 508C, 4 d, 59%. TBAF=[NBu4]F, TIPS=triisopropylsilyl,
sequence guaranteed that 14 was not contami-
nated even with traces of side product, an
absolute requirement for the next crucial cou-
pling steps. The coupling of 5 with nine equiv-
alents of 14 at 708C for 3 d gave high yields of the
sixfold coupling product 15 along with less than
5% of the fivefold coupling product.[23] Since
intermediate 15 and the incomplete coupling TMS=trimethylsilyl.
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of impurities of slightly higher molecular weight, presumably
dimers of 1 (Figure 2). However, as we have shown previously
on other systems, using the same coupling conditions at
slightly elevated temperatures (508C) has a strong influence
on the product distribution and, in this case, suppressed the
Figure 3. MALDI-TOF mass spectra of precursor molecule 16 (left) and
cyclic 2D oligomer 1 (right); matrix material: 2-[2E-3-(4-tert-butyl-
phenyl)-2-methylprop-2-enylidene]malononitrile (DCTB)
(250.34 gmolÀ1).
conformation, and self-assembling properties of organic
molecules can be characterized on atomically flat conductive
substrates with submolecular resolution, even under ambient
conditions.[29,30] 2D oligomer 1 was adsorbed on the surface of
highly oriented pyrolytic graphite (HOPG) after application
of one microliter of a 0.1 mgmLÀ1 solution of the compound
in toluene. STM characterization was carried out under
ambient conditions at room temperature. The images reveal
some isolated bright disks or clusters on top of a featureless
background (Figure 4). By comparison of the diameter and
Figure 2. Molecular-weight distributions obtained by GPC analysis
(THF, polystyrene calibration). From top to bottom: precursor mole-
cule 16, crude products of the oxidative cyclizations at 208C and 508C,
and 2D oligomer 1 after GPC purification. Note that the hydrodynamic
radius of 16 is larger than that of 1. w(logM)=weight fraction.
formation of higher-molecular-weight side products nearly
completely.[24,25] As also observed for other template-directed
acetylene dimerizations, the hydrodynamic volume of 1 is
smaller than that of 16.[14]
Purification by preparative GPC gave pure 1 in 59%
yield. Oligomer 1 is a slightly yellow solid which does not
melt below 2508C (decomp.). It is moderately soluble in
THF ( ꢀ 1.5 mgmLÀ1
) and well soluble in chloroform
(ꢀ7.5 mgmLÀ1)
and
tetrachlororethane
(TCE)
(>15 mgmLÀ1), and exhibits a violet-blue fluorescence in
solution.[26]
The cyclization is accompanied by an increase of rigidity
and, therefore, the 1H NMR spectrum of 2D oligomer 1
exhibits broadened signals at room temperature even in good
solvents, most probably due to hindered rotation of the p-
phenylene units.[27] The resolution in high-temperature meas-
urements in TCE at 908C is comparable to that of precursor
16 at room temperature. The precursorꢀs characteristic signal
at d = 3.2 ppm for the terminal alkyne units is absent in the
spectrum of 1, which shows, together with the high symmetry
of the spectrum, that the cyclization is complete (see the
Supporting Information).
Figure 4. Large-scale (a) and high-resolution (b,c) STM images of 2D
oligomer 1 immobilized on graphite. An optimized molecular model is
superimposed on the left molecule of the STM image in (c). Tunneling
conditions are It =16 pA, Vt =1.04 V for (a); It =15 pA, Vt =À1.04 V for
(b,c).
This assumption is further supported by MALDI-TOF
mass spectra of both 16 and 1 (Figure 3). Beside matrix adduct
peaks there are distinct molecule peaks at m/z 8779.8 (16,
calcd.: 8779.7) and m/z 8767.2 (1, calcd.: 8767.6), respectively.
The difference of 12.6(Æ 0.5) m/z units fits perfectly with the
loss of twelve acetylene protons during the cyclization. Minor
peaks in the mass spectrum of 2D oligomer 1 originate from
doubly charged molecules (m/z 4383.9), as well as dimers and
trimers (see the Supporting Information).[28]
the internal structure of the disks (see below) with a
molecular model, it can be safely concluded that the disks
correspond to individual molecules. Sometimes only the
upper part of such a disk is visible. This can be attributed to
the lateral manipulation of individual molecules induced by
the STM tip. The 2D oligomers could often be revealed with
submolecular resolution. The hub, spoke, and rim subunits of
the molecule can be clearly distinguished and appear bright
due to the high tunneling probability of the phenylene
ethynylene moieties. The sixfold symmetry and the 5.7-nm
Oligomer 1 is indeed a molecular spoked wheel as
confirmed by STM. With this technique the structure,
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[12]P. Siemsen, R. C. Livingston, F. Diederich, Angew. Chem. 2000,
112, 2740 – 2767; Angew. Chem. Int. Ed. 2000, 39, 2632 – 2657.
[13]For recent reviews on cyclic phenylene ethynylene structures
see, e.g.: a) J. S. Moore, Acc. Chem. Res. 1997, 30, 402 – 413; b) S.
Höger, J. Polym. Sci. Part A 1999, 37, 2685 – 2698; c) M. M.
Haley, J. J. Pak, S. C. Brand, Top. Curr. Chem. 1999, 201, 81 – 130;
d) C. Grave, A. D. Schlüter, Eur. J. Org. Chem. 2002, 3075 –
3098; e) D. Zhao, J. S. Moore, Chem. Commun. 2003, 807 –
818; f) Y. Yamaguchi, Z. Yoshida, Chem. Eur. J. 2003, 9, 5430 –
5440; g) S. Höger, Chem. Eur. J. 2004, 10, 1320 – 1329; h) S.
Höger, Angew. Chem. 2005, 117, 3872 – 3875; Angew. Chem. Int.
Ed. 2005, 44, 3806 – 3808; i) W. Zhang, J. S. Moore, Angew.
Chem. 2006, 118, 4524 – 4548; Angew. Chem. Int. Ed. 2006, 45,
4416 – 4439.
[14]a) S. Höger, A.-D. Meckenstock, H. Pellen, J. Org. Chem. 1997,
62, 4556 – 4557; b) S. Höger, J. Polym. Sci. Part A 1999, 37, 2685 –
2698; c) S. Höger, A.-D. Meckenstock, Tetrahedron Lett. 1998,
39, 1735 – 1736; d) S. Höger, A.-D. Meckenstock, Chem. Eur. J.
1999, 5, 1686 – 1691; e) M. Fischer, S. Höger, Eur. J. Org. Chem.
2003, 441 – 446; f) M. Fischer, S. Höger, Tetrahedron 2003, 59,
9441 – 9446; g) A. Ziegler, W. Mamdouh, A. Ver Heyen, M.
Surin, H. Uji-i, M. M. S. Abdel-Mottaleb, F. C. De Schryver, S.
De Feyter, R. Lazzaroni, S. Höger, Chem. Mater. 2005, 17, 5670 –
5683; h) K. Becker, P. G. Lagoudakis, G. Gaefke, S. Höger, J. M.
Lupton, Angew. Chem. 2007, 119, 3520 – 3525; Angew. Chem.
Int. Ed. 2007, 46, 3450 – 3455.
measured distance between two opposite apexes are consis-
tent with an optimized molecular model. The STM observa-
tion provides clear evidence of the rigidity and shape-
persistence of these giant wheel-like molecules.
We have described the first representative of new shape-
persistent two-dimensional oligomers. In a rational modular
synthesis we obtained a molecularly defined, stable, highly
symmetrical, and rigid disk-shaped compound measuring
seven nanometers in diameter (opposite t-butyl groups).
Peripheral t-butyl groups and alkyl chains attached to the
planes of the molecule provide sufficient solubility such that
the compound can be fully characterized. The immobilized
oligomer was also visualized on HOPG by means of STM.
Our synthetic concept permits functionalization at various
positions in the plane of the molecules, so that the rim can be
reserved for the construction of the next generation in a
repetitive approach. Ongoing studies focus on this topic as
well as detailed investigations of functionalized and meso-
morphic derivatives of 1.
Received: April 12, 2007
Published online: June 26, 2007
[15]For a recent macrocyclization by Yamamoto coupling with the
assistance of a covalently bound template see: S.-H. Jung, W.
Pisula, A. Rouhanipour, H. J. Rꢁder, J. Jacob, K. Müllen, Angew.
Chem. 2006, 118, 4801 – 4806; Angew. Chem. Int. Ed. 2006, 45,
4685 – 4690.
[16]For a supramolecular-template approach towards rigid arylene
ethynylene macrocycles see, e.g.: D. W. J. McCallien, J. K. M.
Sanders, J. Am. Chem. Soc. 1995, 117, 6611 – 6612.
Keywords: macrocycles · molecular wheels ·
scanning probe microscopy · template synthesis
.
[1]E. P. Giannelis, Adv. Mater. 1996, 8, 29 – 35.
[2]D. J. Voorn, W. Ming, A. M. van Herk, Macromolecules 2006, 39,
4654 – 4656.
[3]A. Loiseau, J.-F. Tassin, Macromolecules 2006, 39, 9185 – 9191.
[4]U. H. F. Bunz, Chem. Rev. 2000, 100, 1905 – 1944.
[5]a) G. Wegner, Macromol. Symp. 2003, 201, 1 – 9; b) G. Wegner,
Macromol. Chem. Phys. 2003, 204, 347 – 357.
[6]Most covalent 2D structures have limited expansions, e.g.: J. A.
Marsden, M. J. OꢀConnor, M. M. Haley, Org. Lett. 2004, 6, 2385 –
2388.
[7]For multinanometer-sized substructures of graphdiyne, although
not wheel-shaped, see, e.g.: J. A. Marsden, M. M. Haley, J. Org.
Chem. 2005, 70, 10213 – 10226.
[8]For supramolecularly reinforced large shape-persistent macro-
cycles with “spokes” from the central “hub” to the “rim” see,
e.g.: S. Rucareanu, A. Schuwey, A. Gossauer, J. Am. Chem. Soc.
2006, 128, 3396 – 3413. However, supramolecular approaches do
not guarantee a binding in diluted solution.
[9]For some 2D organic molecules see, e.g.: a) J. Wu, W. Pisula, K.
Müllen, Chem. Rev. 2007, 107, 718 – 747; b) M. D. Watson, A.
Fechtenkötter, K. Müllen, Chem. Rev. 2001, 101, 1267 – 1300;
c) Z. I. Niazimbetova, H. Y. Christian, Y. J. Bhandari, F. L.
Beyer, M. E. Galvin, J. Phys. Chem. B 2004, 108, 8673 – 8681;
d) J. van Herrikhuyzen, P. Jonkheijm, A. P. H. J. Schenning,
E. W. Meijer, Org. Biomol. Chem. 2006, 4, 1539 – 1545.
[10]Lately, large arylene ethynylene macrocycles have received
increasing interest, e.g.: a) M. Mayor, C. Didschies, Angew.
Chem. 2003, 115, 3284 – 3287; Angew. Chem. Int. Ed. 2003, 42,
3176 – 3179; b) K. Nakao, M. Nishimura, T. Tamachi, Y. Kuwa-
tani, H. Miyasaka, T. Nishinaga, M. Iyoda, J. Am. Chem. Soc.
2006, 128, 16740 – 16747; Although these macrocycles are based
on rigid building blocks they are likely to become flexible with
increasing ring size: A. Godt, M. Schulte, H. Zimmermann, G.
Jeschke, Angew. Chem. 2006, 118, 7722 – 7726; Angew. Chem.
Int. Ed. 2006, 45, 7560 – 7564.
[17]PEPPSI = pyridine-enhanced precatalyst preparation, stabiliza-
tion, and initiation; C. J. OꢀBrien, E. A. B. Kantchev, C. Valente,
N. Hadei, G. A. Chass, A. Lough, A. C. Hopkinson, M. G.
Organ, Chem. Eur. J. 2006, 12, 4743 – 4748.
[18]J. Wu, M. D. Watson, L. Zhang, Z. Wang, K. Müllen, J. Am.
Chem. Soc. 2004, 126, 177 – 186.
[19]V. Hensel, A.-D. Schlüter, Liebigs Ann./Recl. 1997, 303 – 309.
[20]a) S. Klyatskaya, N. Dingenouts, C. Rosenauer, B. Müller, S.
Höger, J. Am. Chem. Soc. 2006, 128, 3150 – 3151; b) S. Höger, S.
Rosselli, A.-D. Ramminger, V. Enkelmann Org. Lett. 2002, 4,
4269 – 4272; c) T. Zimmermann, G. W. Fischer, J. Prakt. Chem.
1987, 329, 975 – 984.
[21]A. Godt, ꢂ. ꢃnsal, M. Roos, J. Org. Chem. 2000, 65, 2837 – 2842.
[22]S. Höger, K. Bonrad, J. Org. Chem. 2000, 65, 2243 – 2245.
[23]Shorter reaction times (2 d) at 20–50 8C lead to a product
distribution of four-, five-, and sixfold coupling products of
approximately 0.7:1:1; all side products were identified by
1H NMR spectroscopy and MALDI-TOF mass spectrometry.
[24]S. Höger, K. Bonrad, L. Karcher, A.-D. Meckenstock, J. Org.
Chem. 2000, 65, 1588 – 1589.
[25]The cyclization can be performed at elevated temperatures
because the covalent-template approach is independent of the
association equilibrium that is in effect for supramolecular
templates.
[26]Emission maximum at 417 nm, shoulder at 436 nm (in CH 2Cl2),
see the Supporting Information.
[27]T. C. Bedard, J. S. Moore, J. Am. Chem. Soc. 1995, 117, 10662 –
10671.
[28]S. Höger, J. Spickermann, D. L. Morrison, P. Dziezok, H. J.
Rꢁder, Macromolecules 1997, 30, 3110 – 3111.
[29]J. P. Rabe, S. Buchholz, Science 1991, 253, 424 – 427.
[30]S. De Feyter, F. C. De Schryver, J. Phys. Chem. B 2005, 109,
4290 – 4302.
[11]G. R. Newkome, C. Moorefield, F. Vögtle, Dendritic Molecules.
Concepts, Syntheses, Perspectives, Wiley-VCH, Weinheim, 1999.
6806 www.angewandte.org
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