Annularly fused hexapyrrolohexaazacoronenes: An extended π system with multiple interior nitrogen atoms displays stable oxidation states

Article abstract of DOI:10.1002/anie.200701452

(Figure Presented) The heteroatom-stabilized annularly fused hexapyrrolohexaazacoronene shown (N blue, F green) was synthesized and isolated as a representation of a new class of nitrogen-containing molecules with extended π systems. The reversible redox

Full text of DOI:10.1002/anie.200701452

Communications
DOI: 10.1002/anie.200701452
Extended p Systems
Annularly Fused Hexapyrrolohexaazacoronenes: An Extended
p System with Multiple Interior Nitrogen Atoms Displays Stable
Oxidation States**
Masayoshi Takase, Volker Enkelmann, Daniel Sebastiani, Martin Baumgarten, and
Klaus Müllen*
There is a considerable demand for the synthesis of large two-
dimensional molecules with extended p systems such as
polycyclic aromatic hydrocarbons (PAHs), in part because of
their technological applications.[1] Nitrogen-containing PAHs
are of particular interest since such heteroatoms influence the
electronic nature without modifying the structure. For
instance, all-carbon triphenylenes are p-type and hexaazatri-
phenylenes n-type semiconductors.[2,3] In contrast, nitrogen
atoms in pyrrole derivatives make the p systems electron rich,
and oligo- and polypyrroles are conductive in their oxidized
forms.[4] Their polycyclic analogues—fused oligopyrroles—
have remained elusive, and only the fusion of two pyrrole
rings has been reported.[5,6]
While the products resulting from the oxidation of
hexapyrrolylbenzene 1 have been analyzed by MALDI-
TOF mass spectrometry,[7] the isolation and spectroscopic
properties of ring-fused analogues have not yet been
reported. Herein, we describe the synthesis, X-ray crystal
structure, and spectroscopic properties of annularly fused
hexapyrrolohexaazacoronenes 2 (HPHAC)[8] and their cor-
responding dications 22+. In HPHAC 2, the nitrogen atoms
fully contribute to the stabilization of higher oxidation states
of the HPHAC core, and this is the first example with multiple
interior nitrogen atoms within the extended disk-type p
system.
mixture in which the parent molecular ion of HPHAC 2a was
the main component. To isolate 2 in the neutral form, more
electron-deficient pyrroles, namely, b-dibromopyrrole and b-
di(4-trifluoromethylphenyl)pyrrole were chosen as building
blocks leading to 1b and 1c, respectively. Whereas cyclo-
dehydrogenation of 1c successfully afforded HPHAC 2c as an
orange solid in 53% yield, 2b was obtained from 1c as a
mixture with mono- and didebrominated compounds in 81%
yield (for full characterization see the Supporting Informa-
tion). HPHAC 2b showed remarkable thermal stability up to
5358C in thermogravimetric analysis (TGA) under N2-flow
conditions.
The 1H NMR spectrum of HPHAC 2c in CD2Cl2 indicates
a completely fused structure.[11] Signals for the a protons of 1c
(d = 6.72 ppm) are absent, and doublets due to the peripheral
4-trifluoromethylphenyl groups appear at d = 6.81and
6.57 ppm with upfield shifts of Dd = 0.71and 0.64 ppm,
respectively, relative to the signals of 1c (see the Supporting
Information). This reflects the ring-current effect of the
nearby phenyl groups associated with the ring fusion of
pyrroles. In the UV/Vis absorption spectrum of 2c the
absorption maxima at 313, 373, 485, and 520 nm (Figure 1)
are hypsochromically shifted compared to those of corre-
sponding all-benzenoid hydrocarbons such as hexa-peri-
hexabenzocoronene (HBC).[12] Surprisingly enough, 2c exhib-
its a bathochromically shifted red fluorescence at 570, 617,
and 673 nm (Figure 1), whereas HBC fluoresces at much
shorter wavelengths (lmax = 465, 484, 492, 517, and 528 nm).[12]
Such a bathochromically shifted fluorescence could be useful
for scintillators, solar collectors, and biological fluorescence
probes.[13]
The molecular structure of HPHAC 2c was confirmed by
single-crystal X-ray diffraction (Figure 2a).[14a] In contrast to
the expected bowl-shaped structure arising from the annually
fused five-membered rings, which was predicted by DFT
calculations of parent HPHAC 2a (see the Supporting
Information), a nearly coplanar conformation was detected.
The distance between two phenyl groups of neighboring
pyrroles is less than that between the two phenyl groups
attached to the same pyrrole. Furthermore, the centers of
gravity of the twelve peripheral phenyl groups are not in the
same plane but above and below the plane forming a wavelike
pattern; this possibly plays a crucial role in the planarity of the
HPHAC core. In the packing structure, 2c is arranged as
slipped parallel sheets, and the p planes are approximately
4.9 Š apart (see the Supporting Information).
Hexapyrrolylbenzene 1 was synthesized by nucleophilic
aromatic substitution of hexafluorobenzene with pyrrolyl
sodium salts (Scheme 1).[7,9] In a first attempt to synthesize
HPHAC 2, hexapyrrolylbenzene 1a was oxidized with iron-
(III) trichloride and subsequently neutralized with hydrazine.
MALDI-TOF mass spectrometry (solid-state analyte pre-
pared in a 7,7,8,8-tetracyano-p-quinodimethane (TCNQ)
matrix[10]) of the resulting black powder showed a complex
[*] Dr. M. Takase, Dr. V. Enkelmann, Dr. D. Sebastiani,
Dr. M. Baumgarten, Prof. Dr. K. Müllen
Max-Planck-Institut für Polymerforschung
Ackermannweg 10, 55128 Mainz (Germany)
Fax: (+49)6131-379-350
E-mail: muellen@mpip-mainz.mpg.de
[**] This work was financially supported by the European Commission
Project NAIMO (NMP4-CT-2004-500355). The authors thank Dr. P.
Cameron for the help with the CV and DPV measurements, A.
Mavrinskiy for the help with the ESR measurements, and Dr.
Christopher G. Clark for useful discussions. M.T. is grateful to the
Alexander von HumboldtFoundation for a research fellowship.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Chemie
the spin density is localized
within the HPHAC core,
treatment of 2c with an
excess amount of NOSbCl6
(10 equiv) in CD2Cl2 at
room temperature gave
the ESR-silent dication
2c2+ (see the Supporting
Information). The 1H
NMR
resonances
of
peripheral
4-trifluoro-
methylphenyl groups are
shifted downfield (Dd =
1.45 and 0.80 ppm), while
the 13C NMR resonances of
the three sp2 carbons of the
HPHAC core appear with
Scheme 1. Synthesis of annularly fused hexapyrrolohexaazacoronenes 2 (HPHAC). Reagents and conditions:
a) NaH or NaOMe, DMF; then hexafluorobenzene; b) FeCl3, CH3NO2/CH2Cl2; then hydrazine.
considerable
downfield
shifts into the region of
common polycyclic aro-
matic nitrogen heterocycles such as quinoline, acridine, and
phenazine (d = 141.01, 124.95, and 122.78 ppm).[15] This is due
to the decrease in electron density of the HPHAC core.[15,16a]
Slow evaporation of the CD2Cl2 solution gave single
crystals of dication 2c2+ (2c2+[SbCl6 ]2). No decomposition of
À
the dication crystal was observed when it was stored at room
temperature under air for several days.[16] The molecular
structure of 2c2+ was determined by X-ray diffraction for the
first time as a dication of fused pyrroles (Figure 2b).[14b] The
two hexachloroantimonate counter anions are located above
and below opposite nitrogen atoms, and no intermolecular p-
stacking between the HPHAC cores was observed as in the
case of the neutral form.
The bond-length alternation in molecules with extended p
systems is important for discussing the nature of the p system.
Table 1shows selected bond lengths for pyrrole units of
neutral 2c and dication 2c2+. The neutral 2c exhibits bond-
length alternations in the pyrrole units[17] with bond lengths
comparable to those in the nonfused hexapyrrolylbenzene
1a,[9] and the “new” bonds, Ca–Ca of neighboring pyrroles in
2c are rather long. On the other hand, in the case of dication
2c2+, the averaged Ca–Cb bonds are elongated and the
averaged Cb–Cb and Ca–Ca bonds are shortened compared
to those of 2c. The tentative conclusion that the dication of
parent HPHAC 2a2+ possesses overall aromaticity is sup-
ported by a calculation of three-dimensional nucleus-inde-
pendent chemical shifts (see the Supporting Informa-
tion).[18,19]
In brief, we have reported the synthesis and isolation of
annularly fused hexapyrrolohexaazacoronenes by cyclodehy-
drogenation of hexapyrrolylbenzene and subsequent neutral-
ization. Electrochemical oxidation of HPHAC 2c revealed
that interior nitrogen atoms play an important role in
stabilizing higher oxidation states of HPHAC. Thus it was
possible to record spectroscopic properties of 2c and isolate
its dication under ambient conditions. This reversible redox
behavior makes the present p system of potential interest as
an electrochromic molecule and a charge-carrier material in
organic electronics. In line with this, studies on the synthesis
Figure 1. UV/Vis absorption and fluorescence (lex =313 nm) spectra of
2c in CH2Cl2. The insert shows an enlargement of the absorption
spectrum between 450 and 550 nm.
The electrochemical properties of HPHAC 2c were
studied by cyclic voltammetry (CV) and differential pulse
voltammetry (DPV) (CH3CN, RT, Ag/Ag+), revealing four
reversible one-electron oxidation steps with the following
half-wave potentials: Eox1 = 0.355 V, Eox2 = 0.520 V, Eox3
1.52 V, Eox4 = ca. 1.88 V (see the Supporting Information).
=
Since the interior nitrogen atoms of 2 can thus be
considered to stabilize higher oxidation states, 2c was
oxidized with antimony pentachloride (SbCl5) in CH2Cl2 at
room temperature. Incremental addition of SbCl5 under
anaerobic conditions afforded the green-colored monocation
2c+C (absorption maxima at 845 and 965 nm together with a
broad band centered at 1450 nm) and then the dark green-
colored dication 2c2+ (absorption maxima at 676 and 739 nm
together with a broad band centered at 1150 nm; see the
Supporting Information). The oxidation could be reversed by
addition of tetrabutylammonium iodide (Bu4NI), which
reduced the dication 2c2+ back to the starting 2c quantita-
tively. While the ESR spectrum of the monocation 2c+C
exhibits a sharp single-line signal (g = 2.0023), indicating that
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Communications
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Figure 2. Molecular structures (top and side views) of a) HPHAC 2c and b) 2c2+[SbCl6À]2.
[8] Hexapyrrolo[2,1,5-bc:2’,1’,5’-ef:2’’,1’’,5’’-
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form 2c and b) the dication salt, 2c2+[SbCl6À]2.
À
Bond
2c
2c2+[SbCl6
]
2
N–Ca
Ca–Cb
Cb–Cb
Ca–Ca
1.40Æ0.006
1.39Æ0.007
1.44Æ0.01
1.48Æ0.009
1.39Æ0.03
1.42Æ0.03
1.40Æ0.05
1.45Æ0.04
[a] Averaged values are shown with standard deviations calculated by the
following equation: {ꢀ(xiÀ<x>)2/(nÀ1)}1/2
.
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Received: April 4, 2007
Published online: June 20, 2007
Keywords: annulation · aromaticity · fused-ring systems ·
.
nitrogen heterocycles · redox chemistry
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3028.43, orthorhombic, space group Pbc21, a = 15.8638(5), b =
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Chemie
21526 unique reflections, Rmerge = 0.0732, 16127 observed reflec-
tions (I > 2.0s(I)); structure solution by direct methods (Shelxs),
refinement by full matrix least squares on F with anisotropic
temperature factors for the non-H atoms. Final R = 0.0535, Rw =
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