Easy synthesis of phenyl oligomers using a Ni complex

Article abstract of DOI:10.1039/b400236a

In this work are described the syntheses of p-sexiphenyl and p-octiphenyl starting from 4-bromo-p-terphenyl and 4-bromo-p-quaterphenyl, respectively, by using a nickel complex in the presence of bipyridine with DMF as solvent. This type of synthesis was shown to give an improved yield as well as easy preparation and purification of these phenylene oligomers.

Full text of DOI:10.1039/b400236a

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Easy synthesis of phenyl oligomers using a Ni complex
Laura O. Péres, Françoise Guillet and Gérard Froyer *
LPC, IMN, 2 Rue de la Houssinière, Nantes, France. E-mail: Gerard.Froyer@cnrs-imn.fr;
Fax: ϩ33(2) 4037 3991; Tel: ϩ33(2) 4037 3988
Received 7th January 2004, Accepted 7th January 2004
First published as an Advance Article on the web 21st January 2004
In this work are described the syntheses of p-sexiphenyl
and p-octiphenyl starting from 4-bromo-p-terphenyl and
4-bromo-p-quaterphenyl, respectively, by using a nickel
complex in the presence of bipyridine with DMF as
solvent. This type of synthesis was shown to give an
improved yield as well as easy preparation and purification
of these phenylene oligomers.
sation of dihaloaromatic compounds to give π-conjugated
polymers in the presence of Ni(COD)2. To avoid the above-
described problems, in this work on the synthesis of phenylene
oligomers, we have used this general method of C–C bond
formation in the presence of nickel complex. We describe below
the synthesis of p-sexiphenyl (I) and p-octiphenyl (II), using
4-bromo-p-terphenyl (III) and 4-bromo-p-quaterphenyl (IV)
respectively (Fig. 1).
The study of semi-conducting polymers like poly(p-phenylene)
(PPP), poly(p-thiophene) (PPT), poly(p-phenylene vinylene)
(PPV) and polyaniline (PAn) has dramatically increased after
the discovery of the high conductivity of doped polyacetylene
(PA) by Shirakawa et al. in 1977.1 However, the complexity of
the polymer usually formed, hindered the completion of the
study. Studies on model molecules, usually oligomers,2–5 allow a
good understanding of the characteristics of the corresponding
polymer, since they possess a defined degree of polymerization
and consequently uniformity.
But the key question is: from which size is the oligomer
representative of the polymer? For PPP it is believed that
these characteristics begin to be observed starting from
p-quaterphenyl (oligomer with four aromatic rings), for the
vibrational characteristics at least.
Another very important point in research using oligomers
is their preparation, because of the high degree of purity
required. There are a lot of methods in the literature proposed
for the coupling of aromatic rings. Among the classical
methods, are found the reactions of Grignard, Fittig, Ullmann
or Wurtz, but these well known methods are not favorable for
the synthesis of phenyl oligomers due to several limitations
such as: low yield, drastic conditions of synthesis, tedious
purification, non-reproduceable results and multiple step
synthesis.6–11 For example:
Fig. 1 Synthesis of p-sexiphenyl (I) and p-octiphenyl (II).
The synthesis of the monobromo compound is an important
factor in the formation of the oligomers. The starting product
should have only one bromine, located at the 4-position of the
aromatic ring, in order to form the desired oligomer. The point
in this type of synthesis is to avoid the insertion of more than
one bromine. For p-terphenyl, the substitution of the first
bromine atom at the 4-position happens rather easily, but the
substitution of a second bromine in the 4Ј-position is more
complicated.
In the case of p-quaterphenyl, the situation is quite different.
The presence of a bromine atom at one of the extremities
(4-position) is not able to hinder the substitution of the second
bromine, because of the molecule length, therefore the final
product results in a mixture of 4-bromo-p-quaterphenyl and
4,4Ј-dibromo-p-quaterphenyl. In the case of terphenyl even
if there is, in the final product, the presence of 4,4Ј-dibromo-
p-terphenyl it is always possible to separate the compounds by
sublimation. For the quaterphenyl this is not so easy. When an
certain quantity of dibromo compound is mixed with the
monobromo, we observed that these two materials formed a
composite crystal, preventing the separation of the two com-
pounds. The solution was to reduce the ratio reactant : bromine
(1 : 0.8), even if that results in a smaller yield of the desired
compound. In this way, although the synthesis is incomplete
and an amount of starting material (p-quaterphenyl) remains, it
is easily recovered. The monobromo compound and a small
quantity of dibromo product can be separated through
sublimation.
᭿ In the work developed by Cadê and Pilbeam7 the butyl-
lithium was reacted with 4-bromo-p-terphenyl in the presence
of cobalt chloride and butyl bromide. In this synthesis the
p-sexiphenyl, whose yield was not mentioned, was obtained
after distillations, washing, extractions with hydrochloric acid,
water and then benzene followed by sublimation. But, the yields
mentioned for the products are given before final rigorous
purification.
᭿ In 1964, Doss and Solomon10 presented a p-sexiphenyl
synthesis in which this oligomer was obtained with a yield of
1% after irradiation electrochemistry of the p-terphenyl at
320 ЊC for 14 hours.
᭿ Kovacic and Lange12 synthesized p-sexiphenyl by chemical
polymerization of p-terphenyl, using AlCl3/CuCl2 as a catalyst.
The process, under optimised conditions, lead to a mixture of
oligomers with 35% yield. However, although their UV-Vis
absorption spectrum and melting points correspond to the
literature values, the infrared absorption spectra present some
shifted bands, for example the one corresponding to the C–H
in-plane vibration is found at 811 cmϪ1 whereas it should
be at 814 cmϪ1, which could be due to a contamination of the
obtained material.
The infrared spectra of these three compounds show import-
ant differences in the low-frequency range between 900 and
600 cmϪ1. Strong absorption bands come from the out-of-plane
(oop) bending of the C–H bonds. For p-quaterphenyl three
bands can be seen at 823, 750 and 682 cmϪ1 (Fig. 2). When a
bromine atom is inserted (4-bromo-p-quaterphenyl) a shift of
the bands is observed (808, 762 and 687 cmϪ1). However for a
product with two bromines (4,4Ј-dibromo-p-quaterphenyl) the
IR spectra show a strong band at 804 cmϪ1. The intensity of this
band is so strong, that it hides the others, only a small band
appears at 822 cmϪ1 that could come from some unreacted
p-quaterphenyl.
The use of nickel complex to promote a C–C bond was
extensively studied13–17 in the case of dehalogenative conden-
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 4 5 2 – 4 5 4
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
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suitable for C–C coupling reactions promoted by Ni(COD)2
and was already used for reactions of polymers, but they did
not used COD in excess to stabilize the catalyst. Semmelhack
and co-workers21 have reported similar dimerization of aryl
halides using Ni(COD)2 without addition of the auxiliary
ligand (2,2Ј-bipyridine). However their yields were lower
than those obtained by Yamamoto. In the case of the syntheses
of p-sexiphenyl and p-octiphenyl, the reaction involved is more
complicated due to certain factors such as the solubility of the
used compounds and their lower reactivities. Because of this
it is also necessary to use 1,5-(cyclooctadiene) to form the
oligomers.17
The utilization of nickel complex with COD ligands and
an excess of 2,2Ј-bipyridine and COD to promote a C–C bond
was shown to be favorable for the synthesis of p-sexiphenyl
and p-octiphenyl. The obtained oligomers are 100% pure
after a simple washing, thus avoiding sublimation and tedious
further purification.
Fig.
2
Infrared spectra of (a) p-quaterphenyl, (b) 4-bromo-p-
quaterphenyl and (c) 4,4Ј-dibromo-p-quaterphenyl (100 scans, 4 cmϪ1).
In Fig. 3 the results of DSC show melting points of
p-quaterphenyl, 4-bromo-p-quaterphenyl and 4,4Ј-dibromo-
p-quaterphenyl. It can be seen that the values increase as a
function of the quantity of bromine, 337 ЊC for the 4-bromo-
p-quaterphenyl and 377 ЊC for the 4,4Ј-dibromo-p-quater-
phenyl. The presence of only one endothermic peak for these
two materials also proves the purity of the synthesized
products.
Experimental
The Fourier Transform Raman Scattering spectra were
recorded on a Bruker RFS100 spectrometer with an excitation
wavelength (λexc) of 1064 nm. For infrared absorption spectra a
Nicolet 20SXC with an Attenuated Total Reflection (ATR)
accessory was used. The two measurements were obtained with
a resolution of 4 cmϪ1. The DSC measurements were carried
out with a TG-DSC equipment model 111 from Setaram, in the
temperature range 20–500 ЊC (5 ЊC minϪ1) under argon.
All reagents were purchased from commercial sources
(Aldrich or Fluka), without further purification except when
specified. The 4 bromo-p-terphenyl (III) was prepared as
described in the literature22.
4-Bromo-p-quaterphenyl (IV): in a 50 mL three-necked flask,
equipped with a pressure-equalizing dropping funnel, a reflux
condenser and a stirring bar, were placed 1.008 g (3.29 mmol)
of p-quaterphenyl and 0.13 mL (2.61 mmol) of bromine in 30
mL of 1,2-dichlorobenzene at 85 ЊC for 24 hours under argon.
The solution was filtered at room temperature, washed with
ethanol and dried. The material was sublimated in a four-zone
oven, η 28%.
p-Sexiphenyl (I): inside the dry-box, in a 50 mL three-necked
flask, a mixture of 1.11 g (3.59 mmol) of 4-bromo-p-terphenyl,
0.41 g (2.63 mmol) of 2,2-bipyridine, 0.61 g (2.22 mmol) of
bis(1,5-cyclooctadiene)nickel() and 0.74 mL (6.85 mmol) of
1,5 cyclooctadiene was added to DMF (30 mL). The mixture
was stirred at 70–75 ЊC for 40 h under argon, filtered, washed
with water, HCl solution in ethanol for elimination of Ni2ϩ and
then ethanol. After drying, the material was washed with hot
toluene for elimination of eventual p-terphenyl and 4-bromo-
p-terphenyl (η 34%).
Fig. 3 DSC spectra of (a) p-quaterphenyl, (b) 4-bromo-p-quater-
phenyl and (c) 4,4Ј-dibromo-p-quaterphenyl.
The p-sexiphenyl synthesized from monoterphenyl with the
nickel compound is 100% pure and a sublimation step is not
required because any unreacted material (p-terphenyl) and/or
4-bromo-p-terphenyl can be eliminated by washing with hot
toluene and filtration. Because of this, the synthesis becomes
an easy preparation, relatively fast without sublimation, which
usually reduces the yield of the obtained oligomer.
During the synthesis it was possible to follow the change
of coloration for the reaction mixture: from blue to yellow,
when using a large quantity of catalyst. In the work developed
by Yamamoto et al.18 on the formation of biphenyl with
Ni(COD)2, they first saw a purple color (formation of Ni(BPY)-
(COD)), then red for a short time and a green solution
(formation of NiBr2(BPY)).
In the infrared and Raman spectra of p-sexiphenyl, all the
bands show the same position and intensity as described for this
phenylene oligomer obtained by electrochemical reaction after
extensive purification.19 This is a signature of the good purity
of the obtained material. Besides, we could not see the char-
acteristic bands of p-terphenyl (839, 745 cmϪ1) or 4-bromo-
p-terphenyl (818, 762 cmϪ1) in the infrared spectrum. In the
same way, a characteristic spectrum was also observed for
p-octiphenyl, with the C–H out-of-plane vibrations located at
810 cmϪ1 for di-substituted rings and 762 cmϪ1 for mono-
substituted rings, as already described in the literature.20
Yamamoto et al.18 promote various reactions using
Ni(COD)2 as a catalyst to obtain biphenyl in order to determine
the mechanism of C–C coupling of aromatic halides. They used
an excess of phenyl bromine in the presence of 2,2Ј-bipyridine
at 50–70 ЊC and the yields were in the range of 75–100%, using
DMF as a solvent under argon or nitrogen. This polar solvent is
For the p-octiphenyl (II) the method used was the same as
that described above for p-sexiphenyl (I), but the temperature
was set at 70–75 ЊC for 40 hours and the product was washed
with boiling THF for elimination of eventual p-quaterphenyl
and 4-bromo-p-quaterphenyl.
Acknowledgements
The authors thank Dr Claude Chevrot (Laboratoire de
Physicochimie des Polymères et des Interfaces – LPPI –
Université Cergy Pontoise – France) for very helpful discussion
and suggestions. Laura O. Péres thanks the French “Ministère
de la Recherche” for the financial support.
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