Cyclodehydrogenation of di- and tetra(benzimidazol-2-yl)benzenes to give model heteroaromatic discotic systems

Article abstract of DOI:10.1039/b301512m

Di-and tetra(benzimidazol-2-yl)benzenes upon oxidation undergo cyclodehydrogenation with formation of N-N bonds to form planarized polycyclic compounds which are models for the cores of heteroatom-containing discotic materials, and which can be readily reduced back to the original compounds, thus demonstrating a molecular redox switch.

Full text of DOI:10.1039/b301512m

Cyclodehydrogenation of di- and tetra(benzimidazol-2-yl)benzenes to
give model heteroaromatic discotic systems
Weicheng Wu, Andrew. C. Grimsdale and Klaus Müllen*
Max-Planck-Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany.
E-mail: muellen@mpip-mainz.mpg.de; Fax: +49 6131 379350; Tel: +49 6131 379150
Received (in Cambridge, UK) 6th February 2003, Accepted 28th February 2003
First published as an Advance Article on the web 7th April 2003
Di-and tetra(benzimidazol-2-yl)benzenes upon oxidation
undergo cyclodehydrogenation with formation of N–N
bonds to form planarized polycyclic compounds which are
models for the cores of heteroatom-containing discotic
materials, and which can be readily reduced back to the
original compounds, thus demonstrating a molecular redox
switch.
For the conversion of 1 to 6 by means of cyclodehydrogen-
ation to create a N–N bond (Scheme 2), a number of different
oxidizing agents were tested, including FeCl3, I2, PbO2,
Pb(OAc)4, NaIO4, AgClO4 and SbCl5. It was found that only
AgClO4, PbO2–K2CO3, SbCl5 and NaIO4 gave an oxidation
product (30–65% yield) but because of the difficulty in handling
AgClO4 in which traces of Ag can reduce the yield, PbO2–
K2CO3, NaIO4 and SbCl5 were preferable reagents. The best
yield (65%) was obtained by stirring 6 with a large excess of
lead dioxide (17 eq.) and K2CO3 (5.7 eq.) in a mixture of
DMSO–nitrobenzene (1+5) at 120 °C under argon for 24 h.
The mass spectrum of the product showed the loss of 2 mass
units from the structure 1 corresponding to loss of 2 hydrogens
which is consistent with either 6 or 7. While 6 has an
intramolecular –N–N– linkage, 7 is an alternative possible
oxidation product as oxidation of a 2, 2A-bisbenzimidazole 3 is
known to produce a quinoid structure.12
The UV absorption spectra (Fig. 1) of the precursor 6 has a
lmax = 324 nm and the product a maximum at 356 nm, but the
quinoid 7 should have a larger red shift than the one obtained
here (32 nm). The 1H NMR spectrum contains signals between
7.2 and 8.2 ppm which proved that the oxidation product was 6
not 7 whose protons should appear only in the range of 5–7
ppm.
Cyclodehydrogenation is a well-known method for preparing
polycyclic aromatic hydrocarbons from oligophenylene pre-
cursors, e.g. oxidation of o-diphenyl- and hexaphenylbenzene
produces triphenylene1 and peri-hexabenzocoronene (HBC),2
respectively. Reductive cyclodehydrogenation of hexaphe-
nylbenzene with lithium affords the dianion of diphenylte-
trabenz[a,c,h,j]anthracene.3 However, to the best of our knowl-
edge, there is no report of similar cyclodehydrogenations being
used to make bonds other than C–C bonds. The ability to make
other bonds, e.g. C–N or N–N bonds, would enable the
synthesis of discotic molecules containing heteroatoms which
would have higher electron densities and perhaps higher charge
carrier mobilities in columnar mesophases.4 The change in the
molecular conformation from the twisted to the flat two-
dimensional structure will also improve the molecules’ ability
to self-assemble and thus potentially their charge carrying
properties. In addition the well-known ability of N–N bonds to
be reductively cleaved5 offers the possibility to use materials
containing them in a molecular redox switch. Imidazoles are
promising building blocks for electron rich materials as due to
their having six p-electrons delocalised over only five atoms,
they have higher electron densities than six-membered rings.
Further, benzimidazole derivatives can easily pick up or release
protons and form networks via hydrogen bond bridges.6
Therefore they are promising building units for the development
of proton conducting materials. †
As shown in Scheme 2, the compound 6 can be quantitatively
reduced back to 1 by stirring with hydrazine in methanol at
room temperature overnight. Thus 1 and 6 act as a redox
molecular switch. With the linkage or cleavage of –N–N–
bonds, a reversible change between three-dimensional and two-
dimensional molecular conformations is obtained. This change
should lead to major changes in the phase-forming and
electronic properties of materials containing these cores as is
seen for triphenylene and HBC.
The reversible redox behaviour of the dibenzimidazolyl
benzene 1 extends to tetrabenzimidazolyl benzene 2 (Scheme
2). The precursor 2 and N–N– bridged product 8 were
We report the synthesis of di- and tetra(benzimidazol-2-yl)-
benzenes 1 and 2 which are the precursors to models for
heteroatom-containing discotic mesogens. Such compounds are
of interest as active materials in electronic devices.7
The compounds 1 (colorless powder) and 2 (yellow powder)
were obtained in yields of 84% and 77% respectively by
condensation of phthalic acid and 1,2,4,5-benzenetetracar-
boxylic acid respectively with o-diaminobenzene in a mixture
of phosphorus pentoxide and methanesulfonic acid (PPMA) at
120 °C for 24 h (Scheme 1). PPMA was prepared by adding
phosphorus pentoxide (30 g) to methane sulfonic acid (210 ml)
in portions and stirring the mixture at 80 °C until the phosphorus
pentoxide had completely dissolved. The main advantage of this
method over using polyphosphoric acid (PPA) which is
commonly employed for preparation of benzimidazole deriva-
tives8 is that PPMA is less viscous than PPA and so the reactions
can proceed at lower temperatures (110–125 °C, cf. 220 °C).
Bis(benzimidazole)s linked vis C–C bonds e.g. 3, or C,N–
bonds, e.g. 4, are readily obtained by simple procedures.9,10
Dimers linked via N, NA– bonds e.g. 5 can only be prepared by
multistep routes involving sequential closing of the imidazole
rings.11 Despite many attempts, there is to date no report of the
coupling of two benzimidazole units via formation of a N–N
bond.
Scheme 1 Synthesis of di (1) and tetra(benzimidazol-2-yl)benzene (2).
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CHEM. COMMUN., 2003, 1044–1045
This journal is © The Royal Society of Chemistry 2003

Scheme 2 Reversible ring-closure of 1 and 2.
Research Program (project SISITOMAS) and the Bundesminis-
terium für Bildung und Forschung (Zentrum für Multi-
funktionelle Werkstoffe und Miniaturisierte Funktionseinhei-
ten).
Notes and references
† Physical data for 1,2,6 and 8. 1: 1H NMR d(500 MHz, DMSO-d6, 393 K):
8.19–8.17 (m, 2H); 7.69–7.67 (m, 2H); 7.61 (br s, 4H); 7.22–7.20 (m, 4H);
13C NMR d(125 MHz, DMSO-d6, 293 K): 140.92; 128.66; 124.73; 118.55;
116.36; 115.41; 110.84; UV/Vis: l (max) = 324 nm; MS (FD, 8kv): m/z
(%) = 310.0 (100) [M+] (calcd for C20H14N4 = 310.36) 6: 1H NMR d(250
MHz, DMSO-d6, 293 K): 8.17–8.13 (m, 2H); 7.94–7.70 (m, 2H); 7.65–7.62
(m, 4H); 7.42–7.38 (m, 4H); 13C NMR d(125 MHz, DMSO-d6, 293 K):
160.43; 143.23; 141.40; 133.31; 130.40; 128.75; 119.57; 117.93; 116.32;
UV/Vis: l (max) = 356 nm; MS (FD, 8kv): m/z (%) = 308.1 (100) [M+]
(calcd for C20H12N4 = 308.35) 2: 1H NMR d(250 MHz, DMSO-d6, 293 K):
8.96 (s, 2H); 7.71–7.68 (m, 8H); 7.39–7.35 (m, 8H); 13C NMR d(125 MHz,
D2SO4+ CDCl3, 293 K): 140.71; 137.13; 131.12; 129.98; 127.27; 114.72;
UV/Vis: l (max) = 319 nm; MS (FD, 8kv): m/z (%) = 542.7 (100) [M+]
(calcd for C34H22N8 = 542.61) 8: 1H NMR d(250 MHz, DMSO-d6, 293 K):
9.75 (s, 2H); 7.75–7.71 (m, 8H); 7.19–7.15 (m, 8H); 13C NMR d(125 MHz,
DMSO-d6, 393 K): 153.89; 140.87; 134.47; 129.32; 120.00; 114.90; UV/
Vis: l (max) = 334 nm; MS (FD, 8kv): m/z (%) = 538.3 (100) [M+] (calcd
for C34H18N8 = 538.61)
Fig. 1 UV absorption spectra of 1,2,6 and 8 in DMSO: solid line = 1, short
dashed line = 6 dashed line = 2 dash-dotted line = 8.
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characterized by 1H NMR spectra. The two protons on the
central benzene of 8 undergo a large shift to lower magnetic
field (8.96 to 9.75 ppm) while the protons on the benzimidazole
substituents show only minor changes. Because of poor
solubility, compound 2 was dissolved in a different solvent
(D2SO4+ CDCl3) from 8 (DMSO-d6) for 13C NMR measure-
ment.
The UV absorption spectra displayed a red shift from lmax
=
319 nm for molecule 7 to 334 nm for molecule 8 because of the
more planar conformation of 2. Compound 8 was readily and
quantitatively reduced by hydrazine back to 2 under the same
conditions as for reduction of 6 to 2.
To conclude, di- and tetra(benzimidazol-2-yl)benzenes have
been prepared in high yields. These were oxidised to produce
N–N bonded polycyclic molecules, which could be readily
reduced back to the starting compounds, thus demonstrating a
reversible redox system. The planarized products are models for
the cores of novel heteroatom-containing discotic mesogens.
Derivatives of these materials functionalised with suitable
substituents should show mesomorphic properties.
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Research into discotic systems at Mainz is supported by the
European Commission (project DISCEL) the TMR European
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