Preparation of head-to-tail φ-conjugated poly(thiophenepyridine) and polypyrimidine by organometallic polycondensation

Article abstract of DOI:10.1246/cl.2011.992

The metalation of BrThPyBr (Th: thiophene-2,5-diyl; Py: pyridine-2,5-diyl) and 2-iodo-5-bromopyrimidine with i-PrMgCl proceeds regioselectively. The cross-coupling polycondensation of regioselectively metalated intermediates gives head-to-tail φ-conjugated poly(thiophenepyridine) and polypyrimidine, respectively, and their packing structures in the solid state and optical data are discussed.

Full text of DOI:10.1246/cl.2011.992

doi:10.1246/cl.2011.992
992
Published on the web September 5, 2011
Preparation of Head-to-tail ³-Conjugated Poly(thiophene-pyridine)
and Polypyrimidine by Organometallic Polycondensation
Hiroki Fukumoto, Yoshiki Fujiwara, and Takakazu Yamamoto*
Chemical Resources Laboratory, Tokyo Institute of Technology,
4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa 226-8503
(Received May 31, 2011; CL-110463; E-mail: tyamamot@res.titech.ac.jp)
The metalation of Br-ThPy-Br (Th: thiophene-2,5-diyl;
and no Br-Th-Py-H or 2-iodopyrimidine was detected in the
hydrolysis products by 1H NMR spectroscopy. The selective
formation of 3 was also supported by the production of the
following boron compound 5 (Chart 2) from 3 in good yield.5
These results support the selective metalation of 1 and 2 by
i-PrMgCl. The use of 2,5-dibromopyrimidine, instead of 2, did
not lead to the selective formation of 4.
Py: pyridine-2,5-diyl) and 2-iodo-5-bromopyrimidine with
i-PrMgCl proceeds regioselectively. The cross-coupling poly-
condensation of regioselectively metalated intermediates gives
head-to-tail ³-conjugated poly(thiophene-pyridine) and poly-
pyrimidine, respectively, and their packing structures in the solid
state and optical data are discussed.
The addition of diphosphine complexes of nickel to 3 and 4
in THF gave Polymer-1 and Polymer-2, respectively.5 On the
other hand, the zerovalent-nickel-complex ([Ni(0)Lm])-promoted
dehalogenative polycondensation of dihaloaromatic compounds
usually gives head-to-head-rich regiorandom polymers,6 and
such polymers, i.e., Polymer-1¤7 and Polymer-2¤,8 were
previously prepared by the dehalogenative polycondensation
of 1 and 2,5-dibromopyrimidine, respectively.
The synthesis of regioregular ³-conjugated heteroaromatic
polymers by cross-coupling polymerization is important for
controlling the solid structures and electrical and optical
properties of ³-conjugated polymers.1,2 Head-to-tail-type
poly(3-alkylthiophene-2,5-diyl) (HT-P3RTh)1,2 and poly(6-
alkylpyridine-2,5-diyl) (HT-P6RPy)3 (Chart 1) have been pre-
pared via the regioselective metalation1-4 of dihalo starting
materials (e.g., via XMg-C4HS(R)-X, X = halogen) and cross-
coupling polymerization using Ni(II) catalysts.
Polymer-1 was soluble in HCOOH and showed an intrinsic
¹1
viscosity [©] of 0.58 dL g (dL = 100 mL) in HCOOH. The
Suzuki cross-coupling polycondensation of 5 using [Pd(PPh3)4]
and K2CO3 in THF gave Polymer-1 with a smaller [©] of
0.16 dL g¹1. Polymer-2 was soluble in H2SO4 and partly soluble
in HCOOH; however, it was insoluble in the other solvents
tested. The MALDI-TOF-Mass spectrum of Polymer-2 showed
peaks in the range from m/e = 450 corresponding to n = 6 in
(C4H2N2)n to m/e = 1200 (n = 15).
Here, we report on the selective metalation of the following
dihalo compounds with i-PrMgCl and the utilization of the
metalated intermediates for the synthesis of new head-to-tail-
type ³-conjugated polymers.
Reactions of 1 and 2 with i-PrMgCl4 gave the intermediates
3 and 4 (Scheme 1). The hydrolysis of the formed intermediates
gave only H-Th-Py-Br and 5-bromopyrimidine, respectively,
The 1H NMR spectra of Polymer-1 and Polymer-1¤ (cf.,
Figure S1 in Supporting Information5) clearly differ from each
other owing to the difference in microstructure between the
two polymers. Because Polymer-1 has only aromatic protons,
R
R
N
S
1
detailed analysis of its H NMR spectrum was difficult. How-
S
N
n
n
ever, the following hexyl polymer (Polymer-3a (Scheme 2)5)
synthesized analogously gives rise to a simple 1H NMR
spectrum (Figure 1), supporting the formation of a regioregular
polymer. In particular, the appearance of the ¡-CH2 signal,
which is often used for evaluating the regioregularity of ³-
R
R
HT-P3RTh
HT-P6RPy
Chart 1.
Br
Br
ClMg
Br
S
S
N
N
O
1
3
B
Br
THF
S
+
i-PrMgCl
O
N
N
N
5
Br
I
Br
MgCl
N
N
2
4
Chart 2.
4
3
3 4
5
5
2
2
[NiCl2(dppp)] for 3
1S
R
R
1 N
or
n
6
Polymer-1
i-PrMgCl, [NiCl2(dppp)]
[NiCl2(dppe)] for 4
Br
Br
3
N
4
S
S
N
N
THF
2
dppp: 1,3-bis(diphenylphosphino)propane
dppe: 1,2-bis(diphenylphosphino)ethane
n
5
6a, 6b
Polymer-3a, 3b
N
n
6
1
a: R = hexyl
b: R = dodecyl
Polymer-2
Scheme 2.
Scheme 1.
Chem. Lett. 2011, 40, 992-994
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

993
e
f
HCOOD
f
(a)
(b)
e
d
c
c d
a
b
4
3
2
S
N
1
n
Polymer-3a
1
(c)
4
a
b
2
3
10
20
30
40
50
2
θ
/degree
Figure 3. Powder XRD patterns of (a) Polymer-3b, (b)
Polymer-3a, and (c) Polymer-1.
Figure 1. 1H NMR spectrum of Polymer-3a in DCOOD.
(a)
27.3°
3.27 Å
(210)
HT-PPym
17.5°
d = 5.07 Å
(110)
23.7°
ca.3.8 Å
(200)
159.2
4
3
N
5
2
128.1
5-C
N
2, 4, 6-C
ran-PPym
23.7°
ca.3.8 Å
(200)
17.6°
26.5°
3.36 Å
(210)
6
1
n
d = 5.04 Å
Polymer-2
(110)
*
*
5
10
15
20
25
30
*
*
2 /degree
θ
(b)
a
Figure 2. CP/MAS solid 13C NMR spectrum of Polymer-2.
Peaks with * are spinning side bands.
b
conjugated polymers with an alkyl side chain,1-3,9 as a single
peak supports the regioregular structure of Polymer-3a; regio-
irregular polymers usually show split ¡-CH2 signals. The
1H NMR spectrum of Polymer-3a is compared with that of a
regioregular alternating ³-conjugated copolymer of alkylthia-
zole (Tz(R)) and thiophene.9 Polymer-3b also gives rises to a
simple 1H NMR spectrum, which indicates its regioregular
structure.
Polymer-3a and Polymer-3b are soluble in o-dichloroben-
zene and 1,1,2,2-tetrachloroethane. Polymer-3a and Polymer-
3b showed Mw (weight-average molecular weight) of 11000 and
56000 with polydispersity indexes of 3.6 and 3.1, respectively,
in GPC analysis at 140 °C (vs. polystyrene standards, eluent:
1,2,4-trichlorobenzene).
Figure 4. (a) Powder XRD patterns of Polymer-2 and
Polymer-2¤. (b) Postulated herringbone packing of Polymer-2.11
Figure 3 shows the powder X-ray diffraction (XRD)
patterns of Polymer-1, Polymer-3a, and Polymer-3b. As
shown in Figure 3, the XRD patterns show distinct peaks
supporting the notion that the polymers assume ordered
structures in the solid state. The XRD pattern of Polymer-1 is
considerably different from that of Polymer-1¤ shown in
Figure S2.5
The XRD patterns of Polymer-3a and Polymer-3b resem-
ble those of regioregular ³-conjugated aromatic polymers with
long alkyl side chains,1-3 including the regioregular alternating
copolymer of Tz(R) and Th (see above).9 The d space of the
peak in a low-angle region is usually assigned to the distance
between polymer chains separated by alkyl side chains. By
increasing the length of an alkyl side chain from hexyl (2ª =
6.3°, d = 14 ¡) to decyl (2ª = 3.8°, d = 23 ¡), the d space
increases reasonably.
Figure 2 shows the CP/MAS solid 13C NMR spectrum of
Polymer-2. A sharp signal at ¤ 128.1 is assigned to the 5-C
carbon of the pyrimidine ring. 2-C, 4-C, and 6-C carbons show
a single peak at ¤ 159.2. The 13C NMR spectrum pattern of
Polymer-2 is similar to that of Polymer-2¤.8 However, the 5-C
peak of Polymer-2¤ is broader (half width = 376 Hz) than that
of Polymer-2 (half width = 312 Hz), owing to the regio-irreg-
ular structure of Polymer-2¤. On the other hand, for the head-to-
head (HH)-type poly(pyrimidine-2,5-diyl) (HH-PPyrim) sepa-
rately prepared from 2,2¤-dichloro-5,5¤-bipyrimidine,10 the two
13C NMR peaks are shifted upfield (155 and 126 ppm, respec-
tively).
The comparison of the XRD patterns of Polymer-2 and
Polymer-2¤ shows differences, as shown in Figure 4. The XRD
pattern of Polymer-2 shows sharper XRD peaks than that of
Polymer-2¤. Linear ³-conjugated polymers without side chains,
e.g., poly(p-phenylene) and poly(thiophene-2,5-diyl), often
Chem. Lett. 2011, 40, 992-994
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

994
References and Notes
329
(a)
344
1.0
0.5
0.0
1
a) Handbook of Conducting Polymers, 3rd ed., ed. by T. A.
Skotheim, J. R. Reynolds, CRC Press, Boca Raton, Florida,
2007. b) Conjugated Polymer Synthesis: Methods and
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K. Bechgaard, B. M. W. Langeveld-Voss, A. J. H. Spiering,
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J. Kline, I. McCulloch, M. Heeney, J. Am. Chem. Soc. 2007,
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(b)
(c)
300
400
500
Wavelength / nm
2
3
a) R. D. McCullough, S. Tristram-Nagle, S. P. Williams,
R. D. Lowe, M. Jayaraman, J. Am. Chem. Soc. 1993, 115,
4910. b) T.-A. Chen, X. Wu, R. D. Rieke, J. Am. Chem. Soc.
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Figure 5. UV-vis spectra of (a) Polymer-2, (b) HH-PPyrim,
and (c) Polymer-2¤ in concd H2SO4.
H+
N
H+
N
H+
N
H+ H+
H+
N
H
H
H
H
H H
N
N
N
N
N
N
N
N
H+
H+
H+
H+ H+
H+
H
H
H
H
H H
4
5
F. Trécourt, G. Breton, V. Bonnet, F. Mongin, F. Marsais, G.
Quéguiner, Tetrahedron 2000, 56, 1349.
Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
protonated Polymer-2
protonated HH-PPyrim
Figure 6. Postulated protonation of Polymer-2 and HH-
PPyrim.
6
7
T. Yamamoto, T. Maruyama, Z.-H. Zhou, T. Ito, T. Fukuda,
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assume herringbone packing in the solid state,1 and the packing
of Polymer-2 may also be explained by the herringbone pack-
ing in view of its XRD pattern and density, as shown in
Figure 4.11
Both Polymer-1 and Polymer-1¤ showed a UV-vis peak at
about 475 nm in HCOOH. Polymer-1 obtained from 5 and with
a smaller [©] gave rise to a UV-vis peak at 455 nm in HCOOH.
Polymer-3a showed a UV-vis peak at 438 nm in HCOOH.
Figure 5 shows the UV-vis spectra of Polymer-2 and related
polymers in concd H2SO4. The ³-³* transition peaks of
polypyrimidines appear at approximately 335 nm, which is
considerably shorter than that (ca. 370 nm) of poly(p-phenyl-
ene).1
8
9
10 a) T. Yamamoto, N. Hayashida, T. Maruyama, K. Kubota,
Chem. Lett. 1998, 1125. b) N. Hayashida, T. Yamamoto,
Bull. Chem. Soc. Jpn. 1999, 72, 1153.
The protonation of the polymers seems to cause the twisting
of the main chain, leading to the shift to a lower wavelength
(Figure 6).12 Polymer-1 and Polymer-3a were photolumines-
cent. Polymer-1 showed emission peaks at 525 and 552 nm with
a quantum yield of 7% in HCOOH, and Polymer-3a gave rise to
an emission peak at 518 nm with a quantum yield of 5% in
HCOOH.
As described above, i-PrMgCl is useful for the regioselec-
tive metalation of dihalides of thiophene-pyridine compounds
and pyrimidine, and new regiocontrolled ³-conjugated polymers
have been obtained by polycondensation utilizing the formed
intermediate. Detailed analysis of functionalities of the obtained
polymers is under way.
11 If one assumes a and b dimensions of 7.4 and 6.9 ¡,
respectively, for the herringbone packing, d spaces of (110),
(200), and (210) diffractions are calculated as 5.05, 3.7, and
3.26 ¡, respectively, which agree with the observed diffrac-
tion peaks. If Polymer-2 has a repeating height of 4.2 ¡
along the polymer chain, similarly to poly(p-phenylene),1
the density of Polymer-2 is calculated as dcalcd = 1.21
g cm¹3, which roughly agrees with the observed density
d
obs = 1.15 g cm¹3. Polymers often give a lower observed
density than the calculated density because they contain
amorphous parts. The postulated a and b dimensions or the
cross section (S = 51 ¡2) of the pyrimidine unit is larger than
those of poly(p-phenylene) (PPP) (a = 7.8 ¡ and b = 5.5 ¡;
S = 43 ¡2)1 and poly(2,2¤-bipyridine-5,5¤-diyl) (PBpy) (a =
8.1 ¡ and b = 5.7 ¡; S = 46 ¡2),6 thus giving the order: S
(Polymer-2) > S (PBpy) > S (PPP). The importance of the
presence of a lone pair of electrons for the determination of
the effective cross section of the aromatic unit is suggested.
12 The repulsion between a lone pair of electrons11 and o-H
may also cause the shift to a shorter wavelength.
The authors are grateful to Dr. Yoshiyuki Nakamura of our
institute for experimental supports.
This paper is in celebration of the 2010 Nobel Prize
awarded to Professors Richard F. Heck, Akira Suzuki, and
Ei-ichi Negishi.
Chem. Lett. 2011, 40, 992-994
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/