Efficient synthesis and characterization of novel bibenzimidazole oligomers and polymers as potential conjugated chelating ligands

Article abstract of DOI:10.1021/jo051515h

A simple and mild condensation route for the synthesis of novel bibenzimidazole oligomers and polymers is reported here using methyl 2,2,2-trichloroacetimidate as a key starting material. The dimer, trimer, tetramer, and polymers of bibenzimidazole were synthesized as a new series of potential conjugated chelating ligands for metallopolymer studies. The polymers show a maximum absorption at around 400 nm. The optical band gap of the polymer was estimated to be 2.68 eV.

Full text of DOI:10.1021/jo051515h

Efficient Synthesis and Characterization of Novel
Bibenzimidazole Oligomers and Polymers as Potential Conjugated
Chelating Ligands
Jun Yin and Ronald L. Elsenbaumer*
Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, Texas 76019
elsenbaumer@uta.edu
Received July 20, 2005
A simple and mild condensation route for the synthesis of novel bibenzimidazole oligomers and
polymers is reported here using methyl 2,2,2-trichloroacetimidate as a key starting material. The
dimer, trimer, tetramer, and polymers of bibenzimidazole were synthesized as a new series of
potential conjugated chelating ligands for metallopolymer studies. The polymers show a maximum
absorption at around 400 nm. The optical band gap of the polymer was estimated to be 2.68 eV.
Introduction
achieved by better matching of orbital energies between
the metal-based dπ orbitals and the polymer π or π*
orbitals.
Metal-complexed conjugated polymers have attracted
significant attention in recent years.1-4 A variety of
π-conjugated heteroaromatic polymers which serve as
electrically conducting, rigid chelating ligands have been
prepared, and their metal complexes have been inves-
tigated.5-7 They offer much promise in the development
of nanoelectronics, electrocatalysts, sensors, and optical
devices.
It has been demonstrated by Pickup and co-workers
that the existence of superexchange interactions between
metal centers coordinated to conjugated polymer back-
bones enhanced the rate of electron transport through
the polymer in a series of Ru- and Os-complexed poly-
benzimidazoles based on benzimidazole-pyridine or benz-
imidazole-pyrazine as the repeat units.7-9 It was pre-
dicted that higher rates of electron transport could be
To probe the orbital interactions between metals and
conjugated polymers, poly(bibenzimidazoles) with 2,2′-
bibenzimidazole (1) as the repeat unit were chosen in our
research as the π-conjugated backbones. It appears that
polybenzimidazoles are attractive choices for a number
of reasons.4 They tend to be very robust and remain
stable under considerable thermal and chemical stress.10
The studies of binuclear benzimidazole complexes have
shown that they possess significant electronic coupling
between two metals.11-13 In addition, removal of the
imidazole proton allows pH control of the electron density
on the conjugated ligands.14-21
(10) Buckley, A.; Stuetz, D. E.; Serad, G. A. In Encyclopedia of
Polymer Science and Engineering; Kroschwitz, J. I., Ed.; Wiley: New
York, 1988; Vol. 11, pp 572-601.
(11) Haga, M.; Bond, A. M. Inorg. Chem. 1991, 30, 475-480.
(12) Haga, M.; Matsumura-Inoue, T.; Yamabe, S. Inorg. Chem. 1987,
26, 4148-4154.
(13) Haga, M.-A. Inorg. Chim. Acta 1980, 45, L183-L184.
(14) Bond, A. M.; Haga, M. Inorg. Chem. 1986, 25, 4507.
(15) Haga, M. Compr. Coord. Chem. II 2004, 1, 125.
(16) Haga, M.; Ano, T.; Kano, K.; Yamabe, S. Inorg. Chem. 1991,
30, 3843-3849.
(17) Haga, M. A. Inorg. Chim. Acta 1983, 75, 29-35.
(18) Haga, M.-A.; Ali, M. M.; Arakawa, R. Angew. Chem., Int. Ed.
Engl. 1996, 35, 76-78.
(19) Haga, M.-A.; Ali, M. M.; Koseki, S.; Fujimoto, K.; Yoshimura,
A.; Nozaki, K.; Ohno, T.; Nakajima, K.; Stufkens, D. J. Inorg. Chem.
1996, 35, 3335-3347.
(20) Haga, M.-A.; Ali, M. M.; Maegawa, H.; Nozaki, K.; Yoshimura,
A.; Ohno, T. Coord. Chem. Rev. 1994, 132, 99-104.
(21) Haga, M.-A.; Takasugi, T.; Tomie, A.; Ishizuya, M.; Yamada,
T.; Hossain, M. D.; Inoue, M. Dalton Trans. 2003, 10, 2069-2079.
* To whom correspondence should be addressed. Phone: 1-817-272-
1021. Fax: 1-817-272-2625.
(1) Holliday, B. J.; Swager, T. M. Chem. Commun. 2005, 1, 23-36.
(2) Kingsborough, R. P.; Swager, T. M. Prog. Inorg. Chem. 1999,
48, 123-231.
(3) Liu, Y.; Li, Y.; Schanze, K. S. Photochem. Photobiol., C 2002, 3,
1-23.
(4) Pickup, P. G. J. Mater. Chem. 1999, 9, 1641-1653.
(5) Hayashida, N.; Yamamoto, T. Bull. Chem. Soc. Jpn. 1999, 72,
1153-1162.
(6) MacLean, B. J.; Pickup, P. G. J. Mater. Chem. 2001, 11, 1357-
1363.
(7) MacLean, B. J.; Pickup, P. G. J. Phys. Chem. B 2002, 106, 4658-
4662.
(8) Cameron, C. G.; MacLean, B. J.; Pickup, P. G. Macromol. Symp.
2003, 196, 165-171.
(9) Cameron, C. G.; Pittman, T. J.; Pickup, P. G. J. Phys. Chem. B
2001, 105, 8838-8844.
10.1021/jo051515h CCC: $30.25 © 2005 American Chemical Society
Published on Web 10/14/2005
9436
J. Org. Chem. 2005, 70, 9436-9446

Synthesis of Bibenzimidazole Oligomers and Polymers
SCHEME 1. Formation of 2,2′-Bibenzimidazole and Byproduct
SCHEME 2. Holan’s Synthesis of
Polybenzimidazoles are generally synthesized via melt
polymerization of aromatic bis(o-diamine)s with aromatic
dicarboxylic acid derivatives.22-24 Another common method
is solution polymerization performed in poly(phosphoric
acid).25 However, the drawbacks with these protocols
often include high temperatures, long reaction times, and
strongly acidic conditions. In addition, the structures and
the molecular weights of the obtained polymers are
difficult to determine. Our initial attempts using these
methods for the synthesis of poly(bibenzimidazole) met
with limited success when using them for further metal
complexation.
In this regard, the synthesis and investigation of a
discrete set of oligomers with precise lengths has been
an exciting approach to provide specific information for
interpreting structural and conformational properties of
the corresponding polymers.26 This is because oligomers
serve as excellent models for the solution, electronic,
photonic, thermal, and morphological properties of their
corresponding polymers.26 In addition, the metal com-
plexes of the oligomers would also provide further
information about the metallopolymers. However, the
synthesis of well-defined bibenzimidazole oligomers has
not been reported.
In this paper, we report a concise and mild route for
the first synthesis of bibenzimidazole oligomers using
methyl 2,2,2-trichloroacetimidate (2) as the key starting
material. From a retrosynthetic standpoint, a bibenzim-
idazole unit is constructed via a ring-closure reaction of
o-phenylenediamine (3) with 2-trichloromethylbenzim-
idazole (4) or its derivative. This methodology does not
involve tedious precursor synthesis and metal-catalyzed
aryl-aryl bond formation, which is commonly used for
heterocyclic aromatic oligomer and polymer synthesis.27
2,2′-Bibenzimidazole
(ca. 10%).29 Interestingly, the formation of 6 in this
reaction was never reported in the literature. Due to the
similar solubility as that of 2,2′-bibenzimidazole, the
byproduct 6 was very difficult to separate from the
desired monomer. Furthermore, this method cannot be
extended to the synthesis of either bibenzimidazole
oligomers or polymers due to the nature of the reaction
and the formation of the mixed products.
Therefore, an alternative and more efficient approach
was required. In an older report, in 1967, Holan and co-
workers published a series of papers describing their
excellent studies on the synthesis and reactions of
2-trichloromethylbenzimidazole (4).30-33 A mild and ef-
ficient synthesis of 2,2′-bibenzimidazole (1) was also
reported by simply reacting o-phenylenediamine (3) with
methyl 2,2,2-trichloroacetimidate (2) in methanol at room
temperature (Scheme 2). The pure product 1 was ob-
tained in an excellent 90% yield after a simple filtration.
The advantages of this method are obvious: the reaction
occurred at room temperature; the desired product
precipitated out from the reaction mixture, which allowed
easy separation and purification; no byproduct formed;
and recrystallization was not necessary. Surprisingly,
despite the practical convenience, high yields, simple
product purification, and mild reaction conditions, this
approach for the synthesis of bibenzimidazole has been
overlooked in the literature.
Intrigued by these advantages, we explored the syn-
thesis of bibenzimidazole oligomers by utilizing similar
methodology for the construction of the bibenzimidazole
unit. The successful extension of this efficient method for
the synthesis of bibenzimidazole oligomers (n ) 2-4, n
is the number of the repeat units in the oligomers) and
the applications for the synthesis of bibenzimidazole
polymers are described in this paper.
Synthesis of Bibenzimidazole Dimer. Following
Holan’s procedures,30 the key intermediate 2-trichloro-
methylbenzimidazole (4) for the synthesis of the oligo-
mers was prepared by reacting o-phenylenediamine (3)
with methyl 2,2,2-trichloroacetimidate (2) in acetic acid.
The use of a weak acid, e.g., acetic acid, was the key to
Results and Discussion
Synthesis of Bibenzimidazole Monomer. Initial
attempts for the synthesis of 2,2′-bibenzimidazole mono-
mer (1) followed the most commonly used method in the
literature via a condensation reaction of o-phenylenedi-
amine (3) and oxamide (5) in ethylene glycol at reflux,
which was reported by Fieselmann et al. in 1978 (Scheme
1).28 However, the drawbacks of this approach are the
high reaction temperature (around 196 °C), low product
yield, and difficulties in the purification of the desired
product due to the formation of an isomeric byproduct 6
(22) Dudgeon, C. D.; Vogl, O. J. Polym. Sci., Polym. Chem. Ed. 1978,
16, 1815-1830.
(23) Dudgeon, C. D.; Vogl, O. J. Polym. Sci., Polym. Chem. Ed. 1978,
16, 1831-1852.
(29) Kaupp, G.; Naimi-Jamal, M. R. Eur. J. Org. Chem. 2002, 8,
1368-1373.
(24) Vogel, H.; Marvel, C. S. J. Polym. Sci. 1961, 50, 511-539.
(25) Osaheni, J. A.; Jenekhe, S. A. Macromolecules 1995, 28, 1172-
1179.
(30) Holan, G.; Samuel, E. L.; Ennis, B. C.; Hinde, R. W. J. Chem.
Soc., Perkin Trans. 1 1967, 1, 20-25.
(31) Holan, G.; Samuel, E. L. J. Chem. Soc., Perkin Trans. 1 1967,
1, 25-29.
(26) Tour, J. M. Chem. Rev. 1996, 96, 537-553.
(27) Babudri, F.; Farinola, G. M.; Naso, F. J. Mater. Chem. 2004,
14, 11-34.
(28) Fieselmann, B. F.; Hendrickson, D. N.; Stucky, G. D. Inorg.
Chem. 1978, 17, 2078-2083.
(32) Ennis, B. C.; Holan, G.; Samuel, E. L. J. Chem. Soc., Perkin
Trans. 1 1967, 1, 30-33.
(33) Ennis, B. C.; Holan, G.; Samuel, E. L. J. Chem. Soc., Perkin
Trans. 1 1967, 1, 33-39.
J. Org. Chem, Vol. 70, No. 23, 2005 9437

Yin and Elsenbaumer
SCHEME 3. Holan’s Synthesis and Reactions of Trichloride 4
SCHEME 4. Synthesis of Bibenzimidazole Dimer 10
SCHEME 5. Synthesis of 4-Bibenzimidazole-Substituted o-Phenylenediamine 11
TABLE 1. Comparison of the Reactions that Occurred
at Different Temperatures in Scheme 5
the successful suppression of the formation of bibenz-
imidazole (1) and isolation of the relatively stable trichlo-
ride 4. Holan and co-workers have demonstrated that 4
can react with a variety of o-bifunctional nucleophiles 7
to generate heterocyclic ring systems 8 (Scheme 3).33
We have found that when 3,3′-diaminobenzidine (9)
was employed as a bis-o-bifunctional nucleophile and
treated with two equiv of trichloride 4 under similar
reaction conditions at reflux, the desired bibenzimidazole
dimer 10 was obtained in 70% yield (Scheme 4).
The X-ray structure determination reveals that it is a
rigid coplanar structure, and this will be discussed in
detail below (Figure 2).
diamine dimer byproduct
11
(%)
10
(%)
12
(%)
entry
reaction conditions
1
2
RT 5 h, then 130 °C 44 h
RT 5 h, then 75 °C 7-10 h
51
63
6
10
30
0
Table 1), for which 1H NMR spectroscopy showed a
singlet proton at 9.25 ppm. In accordance with the
spectroscopy data, the structure of the byproduct was
assigned as 12, presumably produced by the cyclization
of the diamine 11 with DMF.
Synthesis of Diamino Compound 11. The key
nonsymmetric building block for the synthesis of bibenz-
imidazole trimer and the tetramer is the 4-bibenzimid-
azole-substituted o-phenylenediamine derivative 11
(Scheme 5). It possesses one bibenzimidazole unit with
an o-phenylenediamino functionality, which could further
participate in a similar cyclization process with either
imidate 2 or trichloride 4 as o-phenylenediamine (3) does
(Schemes 2 and 3) to generate more bibenzimidazole
units.
To synthesize compound 11, the cyclization of 3,3′-
diaminobenzidine 9 with trichloride 4 has to occur on one
side of 3,3′-diaminobenzidine, which could be achieved
by controlling the stoichiometry. Therefore, when trichlo-
ride 4 was treated with an excess of 3,3′-diaminobenzi-
dine in DMF at room temperature for several hours and
then heated at 130 °C for 44 h, the desired amine 11 was
obtained in 51% yield, along with 6% of the dimer 10. A
third byproduct was also isolated in 30% yield (entry 1,
Subsequent control experiments provided supportive
evidence for the formation of byproduct 12. For example,
when diamine 11 was heated in DMF in the presence or
absence of Et3N, 12 was formed exclusively in ca. 76%
yield. It is interesting to note that the formation of
benzimidazole derivative by the cyclization of aromatic
diamine with DMF without an appropriate activating
agent has not been reported in the literature.34
To prevent the further cyclization of diamine 11 with
DMF, the reaction of trichloride 4 with 3,3′-diaminoben-
zidine (9) was performed at a relatively low tempera-
ture: room temperature for several hours, then at 75-
80 °C for 7-10 h. Under these conditions, the formation
of byproduct 12 was completely suppressed and the
desired diamine 11 was obtained in a satisfactory 63%
yield, along with a small amount of the dimer 10 (ca.
(34) Desaubry, L.; Wermuth, C. G.; Bourguignon, J.-J. Tetrahedron
Lett. 1995, 36, 4249-4252.
9438 J. Org. Chem., Vol. 70, No. 23, 2005

Synthesis of Bibenzimidazole Oligomers and Polymers
SCHEME 6. Synthesis of Bibenzimidazole Trimer 13 and Tetramer 15
10%) (entry 2, Table 1), which was readily separated from
11 by fractional precipitation.
imidazoles have Tg’s greater than 430 °C.10 The thermal
properties are discussed in the following section.
Synthesis of Bibenzimidazole Trimer and Tet-
ramer. With 11 in hand, the synthesis of the trimer 13
was readily achieved by condensation of two equiv of 11
with imidate 2 at room temperature in 72% yield,
following the same strategy used for the synthesis of 2,2′-
bibenzimidazole (1) from o-phenylenediamine (Scheme 6).
Another important symmetric difunctional building
block for the synthesis of the tetramer, bis(trichloride)
(14), was synthesized using the same strategy as that
used for 2-trichloromethylbenzimidazole (4). 3,3′-Diami-
nobenzidine (9) was treated with 2.2 equiv of imidate 2
in acetic acid to provide the desired bis(trichloride) 14,
which precipitated out from the reaction medium and was
isolated in an excellent 85% yield after a simple filtration.
With both diamine 11 and bis(trichloride) 14 accessible,
the tetramer 15 was synthesized by condensation of two
equiv of 11 with one equiv of 14 using the previous
diamine trichloride cyclization conditions (Scheme 6). The
product 15 precipitated from the reaction medium and
was isolated in 60% yield. A small amount of DMF
cyclization byproduct 12 was also formed under the
reaction conditions and was removed by trituration of the
crude product in DMF under heating (around 130 °C).
The melting point of the monomer bibenzimidazole 1
was greater than 300 °C.29 The melting points of the
oligomers (10, 13, 15) were not measured due to the high
temperature. The thermal properties of polybenzimid-
azoles have been widely investigated.10 The polybenz-
The dimer, trimer, and tetramer are all brownish
yellow powders and are soluble in concentrated sulfuric
acid and methanesulfonic acid. The dimer is soluble in
DMAC, DMF, DMSO, CF3COOH, and HCOOH. The
trimer and tetramer are partially soluble in DMAC and
HCOOH and sparingly soluble in DMF, DMSO, NMP,
and CF3COOH.
Synthesis of Bibenzimidazole Polymer. Poly-
(bibenzimidazoles) were synthesized by two methods
(Scheme 7). One method is the polycondensation reaction
between diaminobenzidine 9 and acetimidate 2 in dif-
ferent solvents. For polymer 16a, the interfacial poly-
condensation of equal amounts of 3,3′-diaminobenzidine
(9) and methyl 2,2,2-trichloroacetimidate (2) was carried
out in a nonsolvent ethanol. The reaction was initiated
at room temperature and then heated to reflux for 82 h
to ensure that the cyclization went to completion. The
brown polymer 16a was obtained by simple filtration in
70% yield. The number average molecular weight (Mn)
was around 19 000 based on the viscosity determination
(discussed in the viscosity section).
The synthesis of polymer 16b was carried out in a
homogeneous solution using DMF as the solvent in the
presence of ethanol. The brown polymer 16b precipitated
out from the solution in 60% yield with a number average
molecular weight (Mn) around 15 000.
The comparison of these two reaction conditions indi-
cates that the interfacial polymerization gave a higher
J. Org. Chem, Vol. 70, No. 23, 2005 9439

Yin and Elsenbaumer
SCHEME 7. Synthesis of Poly(bibenzimidazole) 16a-da
a 16a: In ethanol, RT, then reflux. 16b: In DMF in the presence of ethanol, RT, then 75 °C. 16c: RT, then 75-80 °C. 16d: RT, then
75 °C, then 120-130 °C, and then 150 °C.
molecular weight polymer (16a) than the solution po-
lymerization, which produces polymer 16b. On the basis
of Holan’s study of the reaction between the diamino
compound and methyl 2,2,2-trichloroacetimidate (2),30 the
reactions need to be initiated at room temperature for
at least several hours and then heated to around 70 °C
to ensure that the cyclization goes to completion. If the
reaction is heated directly to 70 °C without being stirred
at room temperature, it would not provide the polymer
with the expected structure.
The other method of synthesizing poly(bibenzimida-
zole) (16c-d) is the polycondensation reaction between
diaminobenzidine 9 and bis(trichloride) 14 in DMF under
different conditions. The direct polycondensation of equal
amounts of 3,3′-diaminobenzidine (9) and bis(trichloride)
14 was carried out in a homogeneous DMF solution in
the presence of Et3N and ethanol. For polymer 16c, the
reaction was initiated at room temperature for 5 h, then
heated to 75 °C, and kept at 75-80 °C for 72 h. The
orange-colored polymer was obtained by addition of water
to the reaction mixture and precipitated out in 83% yield.
For polymer 16d, the reaction was initiated at room
temperature for 5 h, then heated to 75 °C, then to 120-
130 °C, and eventually to a maximum of 150 °C. The
greenish yellow polymer 16d precipitated out from the
reaction mixture and was isolated in 65% yield.
effect,35 CF3COOD was used as the solvent, as well as
forming the trifluoroacetate salts of the oligomers. The
crystal structure of dimer 10 shows that the rings are
coplanar and the molecule is centrosymmetric. In the
aromatic region of the 1H NMR spectrum, the dimer
shows five groups of peaks (H1, H2, H3, H4/4′, and H5/5′).
The singlet (H1) is at 8.43 ppm; there are two double
doublets at 8.28 and 8.26 ppm, which represent the
protons H2 and H3 in the repeat unit; there are two
double doublets at 8.12 and 7.93 ppm, which represent
the protons H4/4′ and H5/5′ on both sides of the end groups.
The NMR spectrum of trimer 13 and dimer 10 are very
similar, which indicates that the trimer might have the
coplanar structure. But the spectrum of the tetramer 15
is a little different from those of the dimer and the trimer.
There are three singlets representing H1, H1′, and H1′′,
respectively, instead of one singlet in the dimer and
trimer; H2/2′/2′′ and H3/3′/3′′ show multiplets instead of the
two double doublets in the dimer and the trimer. This
indicates that H1, H1′, H1′′ are different, as well as H2,
H2′, H2′′ and H3, H3′, H3′′. The tetramer may not have the
coplanar structure that the dimer does. This noncen-
trosymmetric nature may lead to the nonequivalence of
the protons in the repeat units.
X-ray Structure of the Dimer. Yellow block single
crystals of the CF3COOH salt of dimer 10 were grown
by slow evaporation of a CF3COOH solution at room
temperature [monoclinic space group P21/C (Figure 2)].
The X-ray crystal structure analysis reveals the almost
coplanar structure of the dimer. One dimer of bibenz-
imidazole was protonated with four CF3COOH molecules,
and the rest of the CF3COOH molecules are in the lattice
to support the crystals by intermolecular H-bonding
interactions. The protonated bis(bibenzimidazole) formed
a zigzag pattern with alternating stacking columns. The
structure indicates an efficient π-stacking in the solid
state with an intermolecular spacing of 7.90 Å.
Polymer 16c has a higher molecular weight (Mn
)
9600) than does polymer 16d (Mn ) 6400). Keeping the
reaction temperature at 75 °C leads to the formation of
the higher molecular weight polymer 16c. For polymer
16d, the temperature was eventually increased to 150
°C, which might lead to the cyclization of the diamino
groups with DMF and therefore terminate the chain
growth.
Polymers 16a,b are partially soluble in concentrated
sulfuric acid, methanesulfonic acid, and HCOOH but are
not soluble in CF3COOH, DMAC, DMF, DMSO, and
NMP. Polymer 16c is soluble in all the above solvents.
Polymer 16d is soluble in concentrated sulfuric acid and
methanesulfonic acid; partially soluble in HCOOH and
CF3COOH; not soluble in DMAC, DMF, DMSO, and
NMP. The solubility of polymer 16c is different from
those of polymers 16a,b, and d, probably because the
reaction temperature of 75 °C is not high enough for the
complete cyclization in the polymer chains.
UV-vis Properties. The UV-vis properties of the
oligomers and the polymers were investigated in the
protonated forms in methanesulfonic acid (Figures 3-6,
Table 2). The bibenzimidazole monomer 1 has a maxi-
mum absorption at 340 nm, which represents the π-π*
transition. The X-ray crystal analysis of dimer 10 shows
the coplanar structure. Its λmax (π-π* transition) is at
373 nm, which indicates the extension of the conjugation.
NMR Studies of the Oligomers. 1H NMR spectra of
the oligomers (1, 10, 13, and 15) are shown in Figure 1.
To clarify the NMR spectra by removing the tautomer
(35) Zucco, C.; Dall’Oglio, L.; Salmoria, G. V.; Gallardo, H.; Neves,
A.; Rezende, M. C. J. Phys. Org. Chem. 1998, 11, 411-418.
9440 J. Org. Chem., Vol. 70, No. 23, 2005

Synthesis of Bibenzimidazole Oligomers and Polymers
FIGURE 1. NMR spectra of the oligomers in CF3COOD; the CF3COO- counterions were omitted from the structures for clarity;
“a” indicates that the NMR of the tetramer was in CF3COOD/D2O to improve the solubility.
FIGURE 2. Crystal structure and packing pattern of [bis(bibenzimidazole)]2(CF3COOH)12.
The λmax of the trimer 13 is at 382 nm, which indicates
further extension of conjugation, but not as much as from
the monomer to the dimer. The tetramer 15 only red-
shifted 7 nm to 389 nm compared to that of the trimer,
which indicates the large chain length extension does not
produce the expected further decrease of the HOMO-
LUMO gap. This phenomenon indicates that the struc-
ture of the tetramer might not be coplanar.
Figure 4 shows the linear relationship between the
extinction coefficient and the number of the repeat unit
J. Org. Chem, Vol. 70, No. 23, 2005 9441

Yin and Elsenbaumer
FIGURE 6. UV-vis spectra of the polymers 16c,d in meth-
anesulfonic acid (10-6 M).
FIGURE 3. UV-vis spectra of the oligomers in methane-
sulfonic acid (10-6 M).
TABLE 2. Photophysical Properties of Bibenzimidazole
Oligomers and Polymers
ꢀ
Stokes
shift
(nm)
abs
em
λmax
(104,
Eg λmax
compound
MW
(nm) M-1‚cm-1
)
(eV) (nm)
monomer 1
dimer 10
trimer 13
tetramer 15
polymer 16a 19000
polymer 16b 15000
polymer 16c 9600
polymer 16d 6400
234.28 340
466.54 373
698.74 382
930.98 389
4.22
9.21
14.82
20.72
414
2.89 495
2.81 495
2.78 490
2.68 498
2.65 495
2.70 505
2.75 495
74
122
113
101
98
400
400
388
397
95
117
98
extinction coefficients of the trimer and the tetramer are
larger than 140 000 M-1‚cm-1, which is unusually large
for the short chain oligomers.
FIGURE 4. Extinction coefficient vs the number of repeat unit
in the oligomers. (a, solid line): Experimental value; there are
interactions between the repeat units due to the π orbital
overlap. (b, dotted line): Theoretical values, assuming the
repeat units are isolated with no interactions.
The λmax values of the polymers 16a-d were at 388-
400 nm, which were very similar to that of the tetramer
(Figures 5 and 6). From the onset of the absorptions, the
band gap of polymer 16a in methanesulfonic acid was
estimated at 2.68 eV. An optical saturation or near
saturation occurred at around 400 nm. 16a shows a
narrow absorption band centering at 400 nm, while 16b
shows a broad absorption band centering at around 350
nm with a shoulder at 400 nm. This indicates that the
interfacial polycondensation reaction in ethanol gives
polymer 16a with a uniform molecular weight distribu-
tion, while the solution polymerization in DMF gives
polymer 16b with a broad molecular weight distribution.
Photoluminescence Properties. The photolumines-
cence spectra of the oligomers and the polymers in the
diluted solution of methanesulfonic acid are shown in
Figures 7 and 8. All the oligomers and the polymers have
similar emission bands with the maxima at 490-505 nm.
These results indicate that there is little effect of the
chain length on the solution luminescence of the proto-
nated oligomers and the polymers. Large excitation and
emission energy differences were observed for the oligo-
mers and polymers in solution. From the dimer to the
polymer, the Stokes shifts between the excitation and
emission decreased from 122 to 95 nm (Table 2).
Viscosity and Molecular Weight of the Polymers.
The viscosities of the poly(bibenzimidazole) (16a-d) were
determined in methanesulfonic acid at 30.00 °C (Figure
9, Table 3). (The plots of 16b-d are similar to the plot
of 16a shown in Figure 9.) The inherent viscosity ηinh is
0.705 and 0.553 dL/g at a concentration of 0.400 g/dL for
FIGURE 5. UV-vis spectra of the polymers 16a,b in meth-
anesulfonic acid (10-6 M).
in the oligomers. The solid line, a, is the experimental
data; the dotted line, b, is the theoretical data assuming
that there is no interaction between the repeat units. The
slope of the solid line a (5.51 × 104) is larger than that of
the dotted line b (4.22 × 104), which indicates that there
are some interactions between the repeat units due to
the π orbital overlap. The extinction coefficients ꢀ
(M-1‚cm-1) per repeat unit were found to be 4.22 × 104
in the monomer, 4.61 × 104 in the dimer, 4.94 × 104 in
the trimer, and 5.18 × 104 in the tetramer. This suggests
that in the tetramer, one repeat unit absorbs 1.2 times
more light than one repeat unit in the monomer. The
9442 J. Org. Chem., Vol. 70, No. 23, 2005

Synthesis of Bibenzimidazole Oligomers and Polymers
TABLE 3. Viscosities of the Polymers 16a-d in
Methanesulfonic Acid
ηinh (dL/g)
at 0.400 g/dL
entry
[η] (dL/g)
Mn
polymer 16a
polymer 16b
polymer 16c
polymer 16d
0.705
0.553
0.425
0.287
0.720
0.521
0.387
0.344
19000
15000
9600
6400
TABLE 4. Thermostability of the Oligomers and
Polymers
onset of
decomposition temp
(Td, °C)
residue at
800 °C
(%)
entry
FIGURE 7. Photoluminescence spectra of the oligomers in
methanesulfonic acid based on the same molar concentration
(5.7 × 10-8 mol/L). (The excitation wavelength was 340 nm
for the monomer, 373 nm for the dimer, 382 nm for the trimer,
and 389 nm for the tetramer.)
dimer 10
trimer 13
tetramer 15
polymer 16a
polymer 16b
polymer 16c
polymer 16d
516
561
572
656
646
639
662
17.44
52.78
59.30
47.39
50.80
54.90
68.98
estimated to be around 19 000, and the Mn of 16b is
around 15 000 based on the molecular weight-inherent
viscosity relationship established for poly[2,2′-(m-phen-
ylene)-5,5′-bibenzimidazole].10 The inherent viscosity ηinh
is 0.425 and 0.287 dL/g at a concentration of 0.400 g/dL
for 16c and 16d, respectively. The intrinsic viscosity [η]
of 16c is 0.387 dL/g, and [η] of 16d is 0.344 dL/g obtained
by extrapolating to zero concentration. The number
average molecular weight (Mn) of 16c can be roughly
estimated to be around 9600, and the Mn of 16d is around
6400.
Infrared Analysis of the Polymers. The infrared
spectra of a variety of polybenzimidazoles have been
discussed previously.25,37,38 The characteristic infrared
peaks of the benzimidazoles were observed in this oligo-
mers and polymer series. In the spectral region between
3500 and 2500 cm-1, a very broad peak was observed. A
relatively sharp peak at 3403 cm-1 was attributed to the
stretching absorption of isolated, non-hydrogen-bonded
N-H groups. The very broad, asymmetric absorption,
approximately centered at 3145 cm-1, was assigned to
hydrogen-bonded N-H groups. The low intensity peak
at 3053 cm-1 was attributed to the stretching modes of
the aromatic C-H groups.
FIGURE 8. Photoluminescence spectra of the polymers
16a-d in methanesulfonic acid. (The excitation wavelength
was 400 nm for polymers 16a,b, 388 nm for polymer 16c, and
397 nm for polymer 16d.)
The region at 1660-1480 cm-1 is very characteristic
of benzimidazoles. The CdC/CdN stretching vibrations
(1622 cm-1) were observed in this region, as well as ring
modes which are characteristic of the conjugation be-
tween the benzene and the imidazole rings (1585 cm-1).
Strong absorptions due to in-plane ring modes are found
at 1422 and 1397 cm-1. An imidazole ring-breathing
mode gives rise to a peak at 1280 cm-1. The in-plane C-H
bending bands, characteristic of substituted benzimid-
azoles, are found at 1230-1090 cm-1. For the benzene
C-H out-of-plane bending modes, typically the trisub-
stituted benzene ring modes can be observed in the range
FIGURE 9. Plots of inherent viscosity ηinh and reduced
specific viscosity ηsp/c vs concentration of poly(bibenzimidazole)
(16a) at 30.00 °C in methanesulfonic acid. Solid line: ηred
ηsp/c. Dotted line: ηinh ) (lnηr)/c.
)
16a and 16b, respectively. The intrinsic viscosity [η] of
16a is 0.720 dL/g, and [η] of 16b is 0.521 dL/g obtained
by extrapolating to zero concentration.36 The number
average molecular weight (Mn) of 16a can be roughly
of 950-675 cm-1, especially at 947 and 802 cm-1
.
(37) Chen, C. C.; Wang, L. F.; Wang, J. J.; Hsu, T. C.; Chen, C. F.
J. Mater. Sci. 2002, 37, 4109-4115.
(38) Guerra, G.; Choe, S.; Williams, D. J.; Karasz, F. E.; MacKnight,
W. J. Macromolecules 1988, 21, 231-234.
(36) Huggins, M. L. J. Am. Chem. Soc. 1942, 64, 2716-2718.
J. Org. Chem, Vol. 70, No. 23, 2005 9443

Yin and Elsenbaumer
6.6, 3.3 Hz, 4H). 13C NMR (125 MHz, CF3COOD): δ ) 144.0,
135.3, 134.8, 134.3, 134.1, 133.7, 132.5, 132.3, 118.2, 117.0,
116.1. 13C NMR (125 MHz, D2SO4) (for comparison with the
spectra of trimer, tetramer, and polymers): δ ) 143.1, 133.0,
132.6, 132.5, 132.3, 132.2, 130.5, 117.5, 116.4, 116.3, 115.3,
115.2. ESI-MS (m/z): 467.2 [M + H]+. IR (KBr pellet, cm-1):
3500-2500 broad (3350, 3203, 3052), 1650, 1619, 1586, 1560,
1520, 1447, 1315, 1276, 809, 740. Anal. Calcd for C28H18N8‚
1.5H2O: C, 68.14; H, 4.29; N, 22.71. Found: C, 67.60; H, 3.79;
N, 22.27.
Polymers 16c,d made by the reaction of 3,3′-diami-
nobenzidine (9) and bis-trichloride 14 show the charac-
teristic IR peaks described above. In addition, they show
weak absorption at around 1670 cm-1, which might be
due to the formation of the amide functionality in the
polymer chain or at the end groups by hydrolysis in the
presence of base and ethanol.30
Thermostability. The oligomers and the polymers are
thermally stable (Table 4). The onset decomposition
temperatures (Td) are all higher than 500 °C. With the
chain length increase, the Td increases.
[(4-(2-(1H-Benzo[d]imidazol-2-yl)-1H-benzo[d]imidazol-
5-yl)benzene-1,2-di amine)] (Compound 11). 3,3′-Diami-
nobenzidine (9) (4.00 g, 19 mmol) was dissolved in 200 mL of
DMF. 2-Trichloromethylbenzimidazole (4) (2.00 g, 8.6 mmol)
was dissolved in 45 mL of DMF and added dropwise to the
above solution, followed by addition of absolute ethanol (2.5
mL). Triethylamine (6 mL) was added dropwise to the above
solution. The solution was stirred at room temperature for 5
h and heated at 75-80 °C under nitrogen for 7-10 h. The
reaction mixture was cooled and filtered to remove the small
amount of dark brown impurity. To the filtrate, 150 mL of
water was added to precipitate the solid, which was bis(2,2′-
bibenzimidazole) ([bis(BiBzImH2)]) (10) (0.40 g, 0.857 mmol,
yield 20%). Then to the filtrate of the above solution after 10
was removed by filtration, around 600 mL of water was added
to precipitate compound 11 (1.86 g, 5.46 mmol, yield 63%). In
total, 7.17 mmol of 2-trichloromethylbenzimidazole (4) was
consumed to form compounds 10 and 11. The overall yield is
83% based on the consumed 2-trichloromethylbenzimidazole.
Product 11 can be further purified as follows: dissolve 100
mg of crude sample in 15 mL of DMF, then add 30 mL of water
dropwise to precipitate the desired product. The collected solid
was washed with 300 mL of water (3×) to remove DMF and
dried in a vacuum oven for 20 h at 60 °C. The final product 11
was a yellow powder. 1H NMR (500 MHz, CF3COOD): δ )
8.33 (s, 1H), 8.26 (d, J ) 1.65 Hz, 1H), 8.24 (d, J ) 8.8 Hz,
1H), 8.12 (dd, J ) 8.8, 1.7 Hz, 1H), 8.09 (dd, J ) 6.6, 3.3 Hz,
2H), 8.09 (d, J ) 8.3 Hz, 1H), 8.02 (d, J ) 8.3 Hz, 1H), 7.92
(dd, J ) 6.6, 3.3 Hz, 2H). 13C NMR (125 MHz, CF3COOD):
δ ) 145.6, 142.0, 135.6, 134.9, 134.6, 134.0, 133.7, 132.9, 132.5,
131.4, 129.6, 127.9, 126.8, 126.0, 118.3, 117.0, 115.7. ESI-MS
(m/z): 341 [M + H]+. IR (KBr pellet, cm-1): 3500-2500 broad
(3345, 3177, 3058, 2970), 1621, 1587, 1524, 1424,1399, 1337,
1274, 805, 766, 748, 738. Anal. Calcd for C20H16N6‚H2O: C,
67.02; H, 5.06; N, 23.45. Found: C, 67.65; H, 4.75; N, 23.89.
Byproduct (12). The following is the procedure for the
formation of compound 12 as a byproduct. 3,3′-Diamino-
benzidine (9) (4.00 g, 19 mmol) was dissolved in 150 mL of
DMF. 2-Trichloromethylbenzimidazole (4) (1.76 g, 7.50 mmol)
was dissolved in 20 mL of DMF and added dropwise to the
above solution, followed by the addition of absolute ethanol (3
mL, around 6.8 equiv). Triethylamine (7 mL, around 6.7 equiv)
was added dropwise to the above solution. The solution was
stirred at room temperature for 6 h and heated at 130 °C under
nitrogen for 44 h. The reaction mixture was cooled and filtered
to remove the small amount of dark brown impurity. To the
filtrate, 200 mL of water was added to precipitate the solid,
which was bis(2,2′-bibenzimidazole) (10) (0.300 g, 0.643 mmol).
Then, to the filtrate of the above solution after 10 was removed
by filtration, around 200 mL of water was added dropwise to
precipitate compound 12 (1.17 g, 3.34 mmol). Then, to the
filtrate of the above solution after 12 was removed by filtration,
around 200 mL of water was added dropwise to precipitate
compound 11 (1.85 g, 5.43 mmol). In total, 6.73 mmol of
2-trichloromethylbenzimidazole (4) was consumed to form
compounds 10 and 11. The overall yield is 89% based on the
consumed 2-trichloromethylbenzimidazole (4). Product 12 can
be further purified by dissolving 100 mg of the crude sample
in 15 mL of DMF. Then 20 mL of water is added dropwise to
precipitate the desired product. The product was washed with
300 mL of water (3×) to remove DMF and dried in a vacuum
oven at 70 °C for 20 h. The final product 12 was a beige-colored
Conclusions
The oligomers of bibenzimidazole (dimer, trimer, and
tetramer) were first synthesized using a concise and
efficient synthetic strategy. This methodology is very
practical overall, requires few synthetic steps, and is
suitable for the synthesis of the bibenzimidazole poly-
mers. The X-ray structure of the dimer shows that it is
coplanar and centrosymmetric. This series of oligomers
provided useful information for the analysis of the
polymers. The UV-vis spectra show that the chain
extension results in a decrease of the HOMO-LUMO
gap, and the maximum absorption saturates at around
400 nm. The optical band gap of the polymer was
estimated to be 2.68 eV.
The synthesis and properties of the metal complexes
of the oligomers and the polymers are under investiga-
tion.
Experimental Section
2-Trichloromethylbenzimidazole (4). This is a known
compound,30 but NMR data was not reported. To a cooled
suspension of the o-phenylenediamine (3) (5.41 g, 0.05 mol)
and glacial acetic acid (100 mL), methyl 2,2,2-trichloroace-
timidate (2) (8.82 g, 6.19 mL, 0.05 mol) was added slowly.
When the resultant exothermic reaction subsided, the reaction
mixture was kept at the room temperature for 10 h. The
precipitated white powdered 2-trichloromethylbenzimidazole
(4) was collected by filtration, washed with a small amount of
water, and vacuum-dried at 50 °C for 15 h. Yield: 85%. 1H
NMR (500 MHz, CF3COOD): 7.94 (dd, J ) 6.6, 3.3 Hz, 2H),
7.85 (dd, J ) 6.6, 3.3 Hz, 2H). 13C NMR (125 MHz, CF3-
COOD): δ ) 149.6, 132.4, 131.8, 116.7, 83.6. ESI-MS (m/z):
234 [M - H]+. IR (KBr pellet, cm-1): 3500-2500 broad (3061,
2983, 2908, 2829, 2724, 2658, 2603, 2478), 1620, 1590, 1448,
1428, 1309, 1278, 1228, 1043, 892, 816, 741, 683, 501.
Bis(2,2′-bibenzimidazole)‚1.5H2O (10). In a suspension
of 3,3′-diaminobenzidine (9) (1.24 g, 5.8 mmol), 2-trichloro-
methylbenzimidazole (4) (3.00 g, 12.7 mmol), and absolute
ethanol (50 mL), triethylamine (5.86 g, 58 mmol) was added
dropwise. The suspension was stirred at room temperature for
12 h and then refluxed under nitrogen for 36 h. The precipi-
tated bis(2,2′-bibenzimidazole) ([bis(BiBzImH2)]) was collected
by filtration and washed with hot glacial acetic acid. Then it
was stirred in diluted ammonium hydroxide solution to
neutralize the acid residue. The final product was a yellow
powder. Yield: 70%. (DMF can also be the solvent to replace
ethanol. If DMF is used as the solvent, then 10 equiv of
absolute ethanol is needed to facilitate the reaction. The
reaction in DMF was stirred at room temperature under
nitrogen for 5-10 h and then heated at 75-80 °C for 24 h.
The produced dimer is soluble in DMF and can be precipitated
by adding water dropwise to the DMF solution. The final
1
product was collected by filtration.) H NMR (500 MHz, CF3-
COOD): δ ) 8.43 (s, 2H), 8.29 (d, J ) 8.8 Hz, 2H), 8.26 (d,
J ) 8.8 Hz, 2H), 8.12 (dd, J ) 6.6, 3.3 Hz, 4H), 7.94 (dd, J )
9444 J. Org. Chem., Vol. 70, No. 23, 2005

Synthesis of Bibenzimidazole Oligomers and Polymers
powder. 1H NMR (500 MHz, CF3COOD): δ ) 9.25 (s, 1H), 8.35
(s, 1H), 8.25 (d, J ) 8.6 Hz, 1H), 8.24 (s, 1H), 8.20 (d, J ) 8.6
Hz, 1H), 8.11 (dd, J ) 6.1, 3.1 Hz, 2H), 8.10 (d, J not
determined due to the overlap, 2H), 7.93 (dd, J ) 6.1, 3.1 Hz,
2H). 13C NMR (125 MHz, CF3COOD): δ ) 144.7, 142.0, 141.4,
141.3, 135.0, 134.7, 134.0, 133.9, 133.7, 132.7, 132.5, 132.4,
132.3, 132.2, 130.1, 117.3, 117.0, 115.8. ESI-MS (m/z): 351
[M + H]+. IR (KBr pellet, cm-1): 3500-2500 broad (3048, 2968,
2887, 2794), 1398, 1377, 1344, 1282, 948, 812, 738. Anal. Calcd
for C21H14N6: C, 71.99; H, 4.03; N, 23.99. Found: C, 71.38; H,
4.08; N, 23.60.
134.9, 134.4, 134.3, 133.9, 133.7, 132.1, 131.8, 116.8. 13C NMR
(125 MHz, D2SO4; the tetramer dissolves well in D2SO4 but
does not dissolve well in CF3COOD, so the 13C NMR spectrum
was taken in D2SO4 for comparison): δ ) 143.3, 143.2, 133.2,
133.1, 132.7, 132.6, 132.5, 132.4, 132.0, 130.6, 117.6, 116.5,
116.4, 115.5, 115.3. ESI-MS (m/z): 931 [M + H]+ (CF3COOH
as solvent for ESI). IR (KBr pellet, cm-1): 3500-2500 broad
(3411, 3058, 2970, 2863, 2779), 1622, 1585, 1422, 1396, 1372,
1335, 1280, 947, 814, 783, 749. Anal. Calcd for C56H34N16‚
2.5H2O: C, 68.91; H, 4.03; N, 22.96. Found: C, 69.09; H, 3.69;
N, 22.81.
Tris(2,2′-bibenzimidazole) (13). Compound 11 (0.20 g,
0.588 mmol) was suspended in 20 mL of absolute ethanol and
stirred at room temperature for 1 h. Methyl 2,2,2-tricholoace-
timidate (2) (0.060 g, 0.294 mmol, 0.04 mL) was added by
syringe dropwise. The above suspension was stirred at room
temperature for 23 h and filtered to collect the solid. The crude
product was purified by heating in 50 mL of ethylene glycol
at 145 °C for 24 h to remove the soluble impurities. After the
suspension was cooled to room temperature, the brownish
yellow trimer was collected by filtration, washed with water,
and dried under vacuum at 60 °C for 40 h. Yield: 72%. 1H
NMR (500 MHz, CF3COOD): δ ) 8.43 (s, 4H), 8.29 (dd, J )
8.8 Hz, 3.3 Hz, 4H), 8.26 (dd, J ) 8.8 Hz, 3.3 Hz, 4H), 8.11
(dd, J ) 6.6, 3.3 Hz, 4H), 7.93 (dd, J ) 6.6, 3.3 Hz, 4H). 13C
NMR (125 MHz, D2SO4; 13 dissolves well in D2SO4 but does
not dissolve well in CF3COOD, so the 13C NMR spectrum was
taken in D2SO4): δ ) 143.2, 143.0, 133.1, 133.0, 132.5, 132.4,
132.3, 132.2, 131.8, 130.6, 117.4, 116.3, 115.3. ESI-MS (m/z):
699 [M + H]+ (CF3COOH as solvent for ESI). IR (KBr pellet,
cm-1): 3500-2500 broad (3402, 3057, 2965, 2858, 2774), 1622,
1585, 1422, 1396, 1372, 1335, 1280, 947, 810, 783, 745. Anal.
Calcd for C42H26N12‚2H2O: C, 68.65; H, 4.12; N, 22.88.
Found: C, 68.85; H, 3.88; N, 21.91.
Bis(2-trichloromethylbenzimidazole) (14). To a solution
of the 3,3′-diaminobenzidine (9) (2.14 g, 0.01 mol) and glacial
acetic acid (50 mL), methyl 2,2,2-trichloroacetimidate (2) (4.41
g, 3.10 mL, 0.025 mol) was added dropwise. The reaction
mixture was kept at room temperature for around 15 h. The
precipitated pale yellow product bis(trichloride) 14 was col-
lected by filtration and washed with a small amount of water.
Yield: 85%. 1H NMR (500 MHz, DMSO-d6): 7.93 (s, 2H), 7.78
(d, J ) 8.7 Hz, 2H), 7.71 (d, J ) 8.7 Hz, 2H). 13C NMR (125
MHz, DMSO-d6): δ ) 151.2, 138.8, 137.8, 136.9, 123.7, 116.9,
114.2, 88.7. 13C NMR (125 MHz, CF3COOD): δ ) 150.8, 143.5,
133.3, 132.5, 131.6, 117.8, 116.1, 83.5. ESI-MS (m/z): 431
[M - Cl + H]+. IR (KBr pellet, cm-1): 3500-2500 broad (3427,
3075, 2916, 2823), 1626, 1584, 1419, 1293, 1206, 1028, 876,
815, 776, 679, 514. Anal. Calcd for C16H8Cl6N4: C, 40.98; H,
1.72; N, 11.95. Found: C, 40.62; H, 1.62; N, 11.82.
Poly(2,2′-bibenzimidazole) (16a). 3,3′-Diaminobenzidine
(9) was purified following the known procedures by making
the tetrahydrochloride salt of it and regenerating it into the
pure 3,3′-diaminobenzidine using 5% NaOH solution.24 The
slightly pink 3,3′-diaminobenzidine (9) (500 mg, 2.33 mmol)
was suspended in 50 mL of absolute ethanol, and the solution
was degassed under nitrogen for 10 min. Methyl 2,2,2-
trichloroacetimidate (2) (411 mg, 0.288 mL, 2.33 mmol) was
added dropwise. The reaction suspension was stirred at room
temperature for 26 h and then refluxed for 82 h. After the
suspension was cooled to room temperature, the precipitated
orange brown polymer was collected by filtration. Then it was
stirred in 300 mL of deionized water (3×) for around 15 h each
time to remove solvent ethanol. The final orange brown
polymer was collected by filtration and dried under vacuum
oven at 70 °C for 80 h. Yield: 70%. It is partially soluble in
concentrated sulfuric acid, methanesulfonic acid, and HCOOH
and not soluble in CF3COOH, DMAC, DMF, DMSO, and NMP.
The NMR spectra were taken in D2SO4, due to the low
solubility in CF3COOD. 1H NMR (500 MHz, D2SO4): δ ) 8.48
(s), 7.92 (s, br), 7.85-7.70 (multiplet), 7.58 (weak multiplet).
13C NMR (125 MHz, D2SO4, signals were relatively weak
compared to those of the 13C NMR of polymer 16d, due to the
lower solubility of 16a): δ ) 143.3 (br), 133.1, 132.5 (br), 131.8,
131.4, 131.0, 117.5 (br), 115.3 (br). FTIR (KBr pellet, cm-1):
3500-2500 (broad), 1624, 1585, 1420, 1394, 1275, 947, 804.
Anal. Calcd for (C14H8N4‚H2O)n: C, 67.19; H, 4.03; N, 22.39.
Found: C, 66.53; H, 3.73; N, 21.95.
Poly(2,2′-bibenzimidazole) (16b). The slightly pink, puri-
fied 3,3′-diaminobenzidine (9) (700 mg, 3.27 mmol) was dis-
solved in 50 mL of DMF, and the solution was degassed under
nitrogen for 15 min. Methyl 2,2,2-trichloroacetimidate (2) (576
mg, 0.404 mL, 3.27 mmol) was added dropwise, followed by
the addition of absolute ethanol (1.9 mL, 33 mmol). The deep
orange brown reaction solution was stirred at room temper-
ature for 54 h and then heated at 60-70 °C for 40 h. After the
mixture was cooled to room temperature, the precipitated
orange brown polymer was collected by filtration. Then it was
stirred in 300 mL of deionized water (3×) for around 1 day
each time to remove solvent DMF. The final orange brown
polymer was collected by filtration and dried under vacuum
at 70 °C for 80 h. Yield: 60%. It is partially soluble in
concentrated sulfuric acid, methanesulfonic acid, and HCOOH
and not soluble in CF3COOH, DMAC, DMF, DMSO, and NMP.
The NMR spectra were taken in D2SO4, due to the low
solubility in CF3COOD. 1H NMR (500 MHz, D2SO4): δ ) 8.46
(s), 7.92-7.57 (multiplet, br).13C NMR (125 MHz, D2SO4): not
obtained due to the low solubility. FTIR (KBr pellet, cm-1):
3500-2500 (broad), 1624, 1585, 1420, 1394, 1275, 947, 804.
Anal. Calcd for (C14H8N4‚1.5H2O)n: C, 64.86; H, 4.28; N, 21.61.
Found: C, 64.48; H, 3.60; N, 21.11.
Tetra(2,2′-bibenzimidazole)‚2.5H2O (15). To a suspen-
sion of compound 11 (0.700 g, 2.05 mmol) in 40 mL of DMF,
bis(trichloride) 14 (0.385 g, 0.82 mmol) and absolute ethanol
(0.50 mL) were added, followed by the dropwise addition of
triethylamine (0.83 g, 8.20 mmol, 1.15 mL). The suspension
was stirred at room temperature for 10 h and then heated at
75 °C under nitrogen for 34 h. The precipitated yellow tetra-
(2,2′-bibenzimidazole) ([tetra(BiBzImH2)]) (15) was collected
by filtration and washed with water. Then it was heated in
90 mL of ethylene glycol or DMF at around 130 °C for 24 h to
remove the soluble impurities. After the suspension was cooled
to room temperature, the product was collected by filtration
and washed with 300 mL of water (3×). The final product was
a brownish yellow powder after vacuum-drying at 70 °C for
40 h. Yield: 60%. The NMR solvent was a combination of CF3-
COOD/D2O, since [tetra(BiBzImH2)] does not dissolve well in
Poly(2,2′-bibenzimidazole) (16c). The slightly pink, puri-
fied 3,3′-diaminobenzidine (9) (365.5 mg, 1.706 mmol) was
dissolved in 50 mL of DMF, and the solution was degassed
under nitrogen for 15 min, followed by the addition of absolute
ethanol (2 mL, 34 mmol). Bis(trichloride) (14) (800.0 mg, 1.706
mmol) was added in one portion. Then triethylamine (4.75 mL,
34 mmol) was added dropwise to the above solution over 10
min. (After around 4 equiv of triethylamine was added, the
reaction mixture became gel-like and difficult to stir, with the
formation of precipitate.) The color of the mixture was brown-
1
CF3COOD. H NMR (500 MHz, CF3COOD/D2O): δ ) 8.47 (s,
2H), 8.45 (s, 2H), 8.44 (s, 2H), 8.25 (multiplet, 12H), 8.11 (dd,
J ) 6.6, 3.3 Hz, 4H), 7.91 (dd, J ) 6.6, 3.3 Hz, 4H). 13C NMR
(125 MHz, CF3COOD/D2O) (some peaks are overlapped and
show multiplets, and some peaks are overlapped with CF3-
COOD between δ 120-113 ppm): δ ) 143.6, 135.2, 135.0,
J. Org. Chem, Vol. 70, No. 23, 2005 9445

Yin and Elsenbaumer
ish orange. The reaction mixture was stirred at room temper-
ature for 8 h, then heated at 70-75 °C. The precipitate was
almost completely dissolved after 8 h at 75 °C. The mixture
was kept at 70-75 °C for a total of 72 h and then cooled. The
orange polymer was precipitated out from the reaction mixture
by the addition of deionized water and collected by filtration.
Then it was stirred in 300 mL of deionized water (3×) for
around 1 day each time to remove DMF. The final orange
powdered polymer was collected by filtration and dried under
vacuum at 70 °C for 80 h. Yield: 83%. It is soluble in
concentrated sulfuric acid, methanesulfonic acid, HCOOH,
CF3COOH, DMAC, DMF, DMSO, and NMP. 1H NMR (500
MHz, D2SO4): δ ) 8.49 (s), 7.93-7.50 (multiplet, br).13C NMR
(125 MHz, D2SO4): δ ) 143.2, 140.2 (br, weak), 133.1, 132.6
(br), 132.4, 131.9, 131.4, 127.8, 117.6, 116.6, 115.3. FTIR (KBr
pellet, cm-1): 3500-2500 broad (3060), 1624, 1585, 1420, 1379,
1334, 1283, 946, 803. Anal. Calcd for (C14H8N4‚H2O)n: C, 67.19;
H, 4.03; N, 22.39. Found: C, 67.13; H, 3.62; N, 21.61.
Poly(2,2′-bibenzimidazole) (16d). The slightly pink, puri-
fied 3,3′-diaminobenzidine (9) (365.5 mg, 1.706 mmol) was
dissolved in 50 mL of DMF, and the solution was degassed
under nitrogen for 15 min, followed by the addition of absolute
ethanol (2 mL, 34 mmol). Bis(trichloride) (14) (800.0 mg, 1.706
mmol) was added in one portion. Then triethylamine (4.75 mL,
34 mmol) was added dropwise to the above solution over 10
min. (After around 4 equiv of triethylamine was added, the
reaction mixture became gel-like and difficult to stir, with the
formation of precipitate. The color of the mixture was brownish
orange.) The reaction mixture was stirred at room temperature
for 7 h and then heat at 75-80 °C; the precipitate was almost
completely dissolved after 8 h at 78 °C. The reaction was
maintained at 75-80 °C for 20 h, then heated at 120-130 °C.
After around 20 h, a gray yellow precipitate formed. The
mixture was heated at 120-130 °C for an additional 20 h, then
heated at 145-150 °C for 46 h. After the mixture was cooled
to room temperature, the precipitated greenish yellow polymer
was obtained by filtration. Then it was stirred in 300 mL of
deionized water (3×) for around 1 day each time to remove
DMF. The final yellow powdered polymer was collected by
filtration and dried under vacuum at 70 °C for 80 h in 65%
yield. It is soluble in concentrated sulfuric acid and methane-
sulfonic acid; partially soluble in CF3COOH and HCOOH; not
soluble in DMAC, DMF, DMSO, and NMP. The NMR solvent
1
was D2SO4, due to the low solubility in CF3COOD. H NMR
(500 MHz, D2SO4): δ ) 8.48 (s), 7.93 (s, br), 7.85-7.70
(multiplet), 7.58 (weak multiplet).13C NMR (125 MHz, D2-
SO4): δ ) 143.2 (br), 140.3 (br, weak), 133.1, 132.6 (br), 132.4,
131.9, 131.4, 131.0, 129.3, 117.6, 116.7, 115.3, 114.8. FTIR
(KBr pellet, cm-1): 3500-2500 broad (3060), 1624, 1585, 1420,
1379, 1334, 1283, 946, 803. Anal. Calcd for (C14H8N4‚H2O)n:
C, 67.19; H, 4.03; N, 22.39. Found: C, 67.87; H, 3.54; N, 22.70.
Acknowledgment. We thank the Robert A. Welch
Foundation for financial support under Grant No.
Y-1291. X-ray diffraction was performed at the Robert
A. Welch Foundation funded Texas Center for Crystal-
lography at Rice University. We thank Dr. Douglas
Ogrin at the Department of Chemistry at Rice Univer-
sity and Dr. Simon G. Bott at the Department of
Chemistry at University of Houston for solving the
X-ray crystal structure of the dimer (10). We also
thank Professor Martin Pomerantz, Zoltan Schelly, and
Frederick M. MacDonnell at the University of Texas at
Arlington for helpful discussions.
Supporting Information Available: 1H, COSY, and 13
C
NMR spectra of compounds 10, 11, and 12; FTIR spectra of
compounds 1, 4, 10-16; X-ray crystallography data of dimer
10; CIF files for the X-ray analysis of dimer 10. This material
is available free of charge via the Internet at http://pubs.acs.org.
JO051515H
9446 J. Org. Chem., Vol. 70, No. 23, 2005