Donor-substituted peralkynylated "radiaannulenes": Novel all-carbon macrocycles with an intense intramolecular charge-transfer

Article abstract of DOI:10.1039/b304130a

A novel class of planar, highly conjugated all-carbon macrocycles, which we christened "radiaannulenes", have been prepared based on acetylenic scaffolding using tetraethynylethene (TEE) building blocks; these structures are powerful electron acceptors an

Full text of DOI:10.1039/b304130a

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Donor-substituted peralkynylated “radiaannulenes”: novel all-carbon
macrocycles with an intense intramolecular charge-transfer†
Frieder Mitzel,a Corinne Boudon,b Jean-Paul Gisselbrecht,b Paul Seiler,a Maurice Grossb and François
Diederich*a
a Laboratorium für Organische Chemie, ETH-Hönggerberg, CH-8093 Zürich, Switzerland.
E-mail: diederich@org.chem.ethz.ch
b Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide, UMR 7512, C.N.R.S., Université
Louis Pasteur, 4, rue Blaise Pascal, 67000 Strasbourg, France
Received (in Cambridge, UK) 14th April 2003, Accepted 16th May 2003
First published as an Advance Article on the web 9th June 2003
A novel class of planar, highly conjugated all-carbon
macrocycles, which we christened “radiaannulenes”, have
been prepared based on acetylenic scaffolding using tetra-
ethynylethene (TEE) building blocks; these structures are
powerful electron acceptors and, upon peripheral substitu-
tion with electron-donating N,N-dialkylanilino groups, dis-
play intense intramolecular charge-transfer.
peralkynylated dehydro[18]annulene (11, see ESI†) (21.36 V
in THF).2 The replacement of two anilino groups in 2 by
nitrophenyl groups (3) shifts the first reduction potential
anodically to 21.07 V. The bicyclic cores 4 and 5 display
extremely low first reduction potentials at 20.81 and 20.98 V,
respectively. In fact, the potential of 4 is significantly lower than
the first reduction potential of buckminsterfullerene C60 (21.02
V under comparable conditions),7 which is touted as a very
good electron acceptor.
The electron-accepting power of the acetylenic cores in
combination with the peripheral electron donor groups gives
rise to intense intramolecular charge-transfer (CT) absorptions.
The longest-wavelength absorption maximum of the hexa-
anilino-substituted monocycle 2 appears at lmax = 615 nm
(2.02 eV, e = 99800 M21 cm21) (Fig. 2). Upon acidification of
the solution with p-toluenesulfonic acid (PTSA) and protona-
tion of the donor moieties, this band disappears; neutralisation
with triethylamine regenerates the original spectrum (ESI†),
which proves the CT-character of this absorption. Radiaannu-
lene 1 also undergoes intramolecular CT, but the CT-band is
Recently, we reported perethynylated expanded radialenes1 and
perethynylated dehydroannulenes2 bearing peripheral electron-
donating N,N-dialkylanilino groups and showed that these
compounds exhibit strong intramolecular charge-transfer ab-
sorptions. Here we present a novel class of mono- (1–3) and bi-
cyclic (4, 5) expanded acetylenic chromophores which, from a
structural viewpoint, are hybrids between perethynylated dehy-
droannulenes and expanded radialenes and which we therefore
call perethynylated radiaannulenes.
The synthesis of 1–3 proceeded via the acyclic precursors
6–8 by intramolecular oxidative acetylene coupling (Scheme
1).‡ Compounds 6–8 in turn were assembled from the
appropriate known mono- and cis-bis-deprotected tetraethyny-
lethenes (TEEs),1–5 also by acetylene coupling (ESI).
Single crystals of 1, suitable for X-ray crystallography, were
grown by slow diffusion of hexane into a chloroform sol-
ution.§The cyclic framework is virtually planar, with a mean
out-of-plane deviation of 0.040 Å and a maximum deviation of
0.091 Å (C(6)) (Fig. 1). The bond angles around the C(1)–C(16)
double bond are all close to the ideal angle of 120°
(117.0–122.0°). Strain in the 16-membered ring is expressed
mainly at the macrocyclic C·C–C(sp2) angles (C(4)–C(5)–
C(6), C(6)–C(7)–C(8), C(9)–C(10)–C(11) and C(11)–C(12)–
C(13)) with a bending from ideally 180° to approximately
163°.
The synthesis of the bicyclic scaffolds 4 and 5 was achieved
by double intramolecular oxidative coupling of the novel
acyclic TEE-pentamers 9 and 10 (Scheme 2), after removal of
the silyl-protecting groups. The acetylenic pentamers in turn
were obtained from fully deprotected TEE (C10H4) and
appropriate mono-deprotected TEEs by acetylenic coupling
(ESI†).
The new macrocyclic compounds presented here show
several reversible, exceedingly low reduction potentials in
cyclic voltammetry experiments (CH2Cl2 + 0.1 M Bu4NPF6;
potentials vs. Fc/Fc+), which demonstrates their strong electron-
accepting power (ESI†). For instance, the first reduction
potential of 1 occurs at 21.19 V, compared to 21.96 V for
tetrakis(trimethylsilyl)-protected tetraethynylethene.6 The in-
troduction of four more electron-donating anilino-groups in 2
results in a more negative first reduction potential (21.34 V)
which is virtually identical to that of the recently reported,
structurally related hexakis(N,N-dimethylanilino)-substituted
† Electronic supplementary information (ESI) available: schemes describ-
ing the full synthesis of 1–5, preparation and full spectral characterisation of
4, complete electronic absorption spectra, complete electrochemical data.
See http://www.rsc.org/suppdata/cc/b3/b304130a/
Scheme 1 Synthesis of monocycles 1–3. Reagents and conditions: i,
Bu4NF, THF, 0 °C, 10 min; ii, CuCl, N,N,NA,NA-tetramethylethylenediamine
(TMEDA), O2, acetone, r.t., 2 h, 48% (1), 32% (2), 14% (3).
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CHEM. COMMUN., 2003, 1634–1635
This journal is © The Royal Society of Chemistry 2003

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weaker and only observed as a shoulder at 588 nm (2.11 eV) as
a result of the smaller number of electron donor moieties. The
introduction of two p-nitrophenyl moieties into 3 shifts the end-
absorption bathochromically by approximately 100 nm (0.22
eV) compared to that of 2.
Fig. 2 Electronic absorption spectra of 2 (blue), 3 (green) and 5 (red) in pure
CHCl3 and after addition of PTSA (2: yellow, 5: black).
The bicyclic radiaannulene 5 displays an unusually strong
CT-absorption with an end-absorption at approximately 850 nm
(1.46 eV), the lowest-energy end-absorption known for any
TEE-oligomers. Again, the CT-band of 5 can be reversibly
removed and regenerated by acidification/neutralisation (Fig.
S2, ESI†).
In summary, two-dimensional acetylenic scaffolding based
on TEE-building blocks has been advanced to the preparation of
radiaannulenes, an unprecedented class of perethynylated all-
carbon macrocycles that are hybrids between expanded radia-
lenes and dehydroannulenes. The new carbon sheets are
powerful electron acceptors and, upon peripheral donor-
substitution, exhibit strong intramolecular charge-transfer ab-
sorptions.
Fig. 1 ORTEP plot of 1 with arbitrary numbering. H-atoms are omitted for
clarity. Atomic displacement parameters at 183 K are drawn at the 30%
probability level. Selected bond angles (°): C(4)–C(5)–C(6) 163.18(18),
C(5)–C(6)–C(7) 111.76(14), C(6)–C(7)–C(8) 164.86(18), C(9)–C(10)–
C(11) 163.88(19), C(10)–C(11)–C(12) 111.18(15), C(11)–C(12)–C(13)
162.78(17).
Support by the ETH Research Council and the German Fonds
der Chemischen Industrie is gratefully acknowledged. We thank
Prof. H. Hopf (Braunschweig) for useful discussions.
Notes and references
‡ All new compounds were fully characterised by IR, UV/Vis, 1H and 13
C
NMR, mass spectrometry and microanalysis or HR-MS. All mono- and bi-
cyclic radiaannulenes are stable at room temperature in the air for
months.
§ Crystal data for 1 at 183 K for (C70H40N2, Mr = 909.04): triclinic, space
group P1 (no. 2), Dc = 1.167 g cm23, Z = 2, a = 13.6643(5), b =
¯
14.0449(6), c = 15.9105(8) Å, a = 66.12(1), b = 68.03(1), g = 83.06(1)°,
V = 2587.6(2) Å3. Bruker-Nonius Kappa-CCD diffractometer, Mo-Ka
radiation, l = 0.7107 Å. A black single crystal with linear dimensions of ca.
0.3 3 0.07 3 0.05 mm was obtained by slow diffusion of hexane into a
CHCl3 solution. Final R(F) = 0.060, wR(F2) = 0.129 for 690 parameters
and 7437 reflections with I > 2s(I) and 2.65 < q < 27.49° (corresponding
R-values based on all 11698 reflections are 0.108 and 0.151 respectively).
CCDC reference number 208124. See http://www.rsc.org/suppdata/cc/b3/
b304130a/ for crystallographic data in CIF or other electronic format.
1 M. B. Nielsen, M. Schreiber, Y. G. Baek, P. Seiler, S. Lecomte, C.
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2 F. Mitzel, C. Boudon, J.-P. Gisselbrecht, M. Gross and F. Diederich,
Chem. Commun., 2002, 2318.
3 For a review on carbon-rich acetylenic scaffolding, see: F. Diederich,
Chem. Commun., 2001, 219.
4 J. Anthony, A. M. Boldi, Y. Rubin, M. Hobi, V. Gramlich, C. B. Knobler,
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7 (a) D. Dubois, G. Moninot, W. Kutner, M. T. Jones and K. M. Kadish, J.
Phys. Chem., 1992, 96, 7137; (b) L. Echegoyen, F. Diederich and L.
Echegoyen, in Fullerenes: Chemistry, Physics and Technology, eds. K.
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Scheme 2 Synthesis of bicycles 4 and 5. Reagents and conditions: i, Bu4NF,
THF, 0 °C, 10 min; ii, CuCl, TMEDA, O2, acetone/PhH, THF, r.t., 2 h, 88%
(4), 15% (5).
CHEM. COMMUN., 2003, 1634–1635
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