Novel structural units for electropolymerizable compounds: Pyrimidiniumolate-functionalized thiophenes

Article abstract of DOI:10.1039/a704922f

The synthesis of pyrimidiniumolates with a thiophene unit (3a-c) is reported. The electrooxidation of 3a leads to a conductive polymer. Some quantum chemical data for 3a-c and the parent system 5 are given [semiempirical calculations (AM1, PM3), ab initio

Full text of DOI:10.1039/a704922f

Novel structural units for electropolymerizable compounds: pyrimidiniumolate-
functionalized thiophenes1
Peter Laackmann, Astrid-Christina Koch and Willy Friedrichsen*
Institute of Organic Chemistry, University of Kiel, Otto-Hahn-Platz 4, D-24098 Kiel, Germany
The synthesis of pyrimidiniumolates with a thiophene unit (3a–c) is reported. The electrooxidation of 3a leads to a conductive
polymer. Some quantum chemical data for 3a–c and the parent system 5 are given [semiempirical calculations (AM1, PM3),
ab initio data and density functional theoretical results].
Research on conducting polymers has increased enormously
in the last decade.2–4 Despite these efforts, the control of the
long-range order of these materials constitutes a challenge in
this field, and improving the order and linear assembly of the
monomers during electrooxidation is still a major task.5,6
Thiophenes and derivatives thereof especially have been investi-
gated quite intensively.7 Recently, a liquid crystalline thiophene
derivative has been reported9 that leads to a highly conductive
polymer upon electrooxidation. Fluorinated alkyl side chains
and electron withdrawing groups have also been introduced
for this reason.9 It seemed to us that dipolar heterocycles
(mesoionic systems, mesomeric betaines etc.)10,11 are especially
suitable for this purpose, because it is likely that a large dipole
moment would increase the order of the monomeric units. We
now report the synthesis of pyrimidiniumolates 3a–c and the
electropolymerisation of 3a.
3a: Pale yellow crystals (180 mg, 60%) with mp 203 °C. MS
m/z (rel intensity) 298 (64), 271 (7), 270 (36), 255 (17), 124 (9),
119 (9), 118 (100), 92 (9), 77 (41); n
1646 (vs), 1332 (w), 1254 (m), 1026 (w), 845 (m), 798 (m, sh),
(KBr)/cm−1 3078 (w),
max
781 (m), 723 (w), 631 (w), 448 (w); UV (acetonitrile) l /nm
max
(log e) 208 (4.398), 223 (4.313), 268 (3.62), 354 (3.705); d
H
(500 MHz, CDCl ) 3.25 (s, 6H), 7.38 (dd, 1H, J 5.12, J 3.11),
3
7.68–7.80 (m, 5H), 8.18 (dd 1H, J 3.16, J 1.15), 8.23 (dd, 1H,
J 5.11, J 1.18); d (CDCl ) 34.41, 93.35, 120.52, 121.77, 126.67,
129.03, 129.50, 129.70, 131.26, 135.42, 157.18, 157.87 (Found:
C, 63.8; H, 4.5%. Calc. for C H N O S: C, 64.41; H, 4.73%).
C
3
16 14
2 2
3b: Pale yellow crystals (268 mg, 71%) with mp 325 °C
(decomp.). MS m/z (rel intensity) 378 (22), 376 (21), 350 (12),
348 (12), 198 (29), 196 (28), 140 (10), 126 (16), 125 (20), 124
(17), 123 (16), 120 (10); n
1526 (w), 1473 (m), 1425 (w), 1333 (w), 1255 (m), 1012 (m),
(KBr)/cm−1 1643 (vs), 1550 (w),
max
841 (m), 790 (m), 697 (w), 630 (w); UV (acetonitrile) l /nm
max
(log e) 225 (4.267), 270 (3.526), 358 (3.553); d (500 MHz,
H
[2H ]DMSO) 3.25 (s, 6H), 7.39 (dd, 1 H, J 5.10, J 3.16), 7.73
6
Experimental
(d, 2 Aryl-H), 7.90 (d, 2 Aryl-H), 8.17 (dd, 1 H, J 5.10, J 1.22),
Mp values were determined using a Tottoli apparatus. IR
spectra were measured using a Paragon 1000 FTIR spec-
trometer (Perkin-Elmer). Mass spectra: MAT 8230 (Finnigan).
NMR spectra were recorded on AM 500 (Bruker) and EM390
(Varian) instruments using tetramethylsilane as an internal
standard. J Values are in Hz. UV: DMR 10 (Zeiss).
8.23 (dd, 1 H, J 3.16, J 1.22). Calc. for C H N O SBr:
377.98605. Found: 377.98600.
16 13
2 2
3c: Pale yellow crystals (257 mg, 68%) with mp 338 °C
(decomp.). MS m/z (rel intensity) 378 (M+, 90), 377 (20), 376
(90), 351 (10), 350 (58), 349 (14), 348 (59), 335 (17), 333 (16),
198 (98), 196 (100), 157 (22), 155 (22), 124 (35), 123 (10), 117
(33), 102 (20); n
1470 (w), 1383 (m), 1332 (w), 1287 (w), 1251 (m), 853 (m), 789
(KBr)/cm−1 1648 (vs), 1555 (m), 1516 (w),
Bis(2,4,6-trichlorophenyl)2-(3-thienyl)malonate (1b)
max
A mixture of 2-(3-thienyl)malonic acid 1a (Aldrich; 0.95 g,
5 mmol), 2,4,6-trichlorophenol (1.96 g, 10 mmol) and phos-
phorus oxychloride (1.80 g, 20 mmol) was heated to 90 °C for
1 h. The reaction mixture was cooled to room temp., poured on
50 g of ice–water and extracted with diethyl ether. The organic
layers were washed with water, dried (Na SO ), and the solvent
(m), 685 (w), 628 (w); UV (acetonitrile) l /nm (log e) 228 (sh,
max
4.142), 272 (3.401), 362 (3.468); d (500 MHz, [2H ]DMSO)
H
6
3.14 (s, 6H), 7.39 (dd, 1 H, J 5.11, J 3.17), 7.67 (t, 1 Aryl-H),
7.80 (d, 1 Aryl-H), 7.92 (d, 1 Aryl-H), 8.09 (s, 1 Aryl-H), 8.17
(dd, 1H, J 5.11, J 1.23), 8.24 (dd, 1 H, J 3.17, J 1.23). Calc. for
C H N O SBr: 377.98605. Found: 377.98670.
2
4
16 13
2 2
evaporated. Recrystallization from isopropyl alcohol–pentane
gave 1b (2.47 g, 90%) as unstable dark yellow crystals with mp
103.5 °C. MS m/z (rel. intensity) 320 (M+−C H Cl O +H, 2),
Electropolymerization of 3a
7 2
3 2
A solution of 100 mg of 3a and 100 mg of LiClO in 20 ml of
281 (0.6), 196 (11.1), 167 (3.1), 160 (2.6), 124 (28.2), 97 (100);
4
acetonitrile was electrolysed for 30 min (on 0.8 cm2 platinum
C H O 35SCl
(calc.
319.92325,
(calc. 321.92029, found 321.92020),
found
319.92330),
12 3
7
2
electrodes, U=1.80 V). After this process the electrode was
rinsed with acetonitrile. The polymer was removed mechan-
ically, washed with hot acetonitrile and dried. Yield: 20 mg of
grey polymer material, aspect similar to graphite powder. The
polymer is insoluble in most common solvents at room temp.
(slightly soluble in hot acetonitrile). The polymerisation and
additionally the conductivity measurements have also been
carried out in the BASF laboratories.
C H O 35S37ClCl
12
12
7
7
2
2
2
C H O 35S37Cl Cl (calc. 323.91733, found 323.91920);
d
2
3
H
(90 MHz, CDCl ) 5.45 (s, 1H), 7.25 (m, 1H), 7.35 (s, 4H), 7.6 (m,
2 H); n
(KBr)/cm−1 3080, 1790, 1765, 1565, 1445.
General procedure for the synthesis of pyrimidiniumolates 3
max
A mixture of the malonate 1b (0.55 g, 1 mmol) and the
corresponding N,N∞-dimethylamidine 2a–c (1 mmol) in 5 ml
of dry anisole was heated to 80 °C for 3 min. After cooling to
room temp., 20 ml of diethyl ether was added. After stirring
for 6 h at room temp., the precipitate was filtered and recrys-
tallized from ethanol–diethyl ether.
Results and Discussion
Pyrimidiniumolates have been known since 1971.12–14 The
dipolar structure of this system leads to a high dipole moment15
J. Mater. Chem., 1997, 7(11), 2205–2208
2205

in the direction of the longitudinal molecular axis, calculated
by semiempirical methods16 as 11.35 D (PM3; AM1: 10.83 D)
for 3a (Scheme 1). Pyrimidiniumolates can be prepared quite
easily12–14 by treatment of N,N∞-disubstituted amidines with
reactive malonic acid derivatives like acid chlorides or trichlor-
ophenyl esters (magic esters; TCPM procedure by Kappe and
co-workers), using solvents such as bromobenzene or anisole.
Thiophene derivatives can undergo anodic polymerization in
acetonitrile to form electrically conducting polymers. By vary-
ing the groups R1 and R2, the resulting total dipole strength
of the monomer can be adjusted over a wide range. In this
paper, the synthesis and electropolymerization of pyrimidin-
iumolate-functionalized thiophene monomers (3a–c) are
described. Using thienylmalonic acid (1a) as starting material,
the ester 1b was available by treatment with 2,4,6-trichloro-
phenol–POCl in 90% yield. Amidines17 can be prepared by
3
a variety of methods. N,N∞-Dimethylamidines of type 2 are
synthesized most conveniently18 either from the corresponding
imidoyl chlorides19 on treatment with methylamine or from
thiuronium salts20,21 on treatment with methylamine. Heating
ester 1b with amidines 2a–c in anisole gives pyrimidin-
iumolates 3a–c as pale yellow crystals in satisfactory yields.
The electropolymerization of 3a was carried out on a gold disc
electrode using acetonitrile/LiClO as solvent/electrolyte. As is
4
shown in Fig. 1, a 100 mV s−1 potential scan exhibits a strong
anodic oxidation wave at +1.50 V. Microscopic analysis of
the polymer-coated gold electrodes showed the formation of
small dendritic crystals on an amorphous base layer of poly-
meric material. A cyclic voltammogram of the monomer-free
polymer with increasing scan rates is also shown in Fig. 1.
Preliminary measurements indicate that this polymer exhibits
Fig. 1 (a) Cyclic voltammogram of 3a on Au disc electrode, in 0.1 
LiClO MeCN. Scan rate 100 mV s−1. (b) Cyclic voltammogram of a
4
film of poly-3a on Au, in 0.1  LiClO MeCN. Scan rate 20–340
mV s−1 by increments of 20 mV s−1.
4
a
conductivity of approximately 10−3 S cm−1. Further
improvements of this methodology may give rise to materials
which could be of interest in the field of electroconductive/
electroluminescent polymer applications.
The structure of the polymer is—as in other related cases—
not known with certainty. If the polymerisation takes place at
position 2 and 5 of the monomer, several structures are of
course possible (2–5∞–2∞–5◊–2◊ … etc., 2–2∞–5∞–5◊–2◊ … etc.; ∞
and ◊ refer to polymerisation sites of the monomer). To gain
some insight into the geometry of compounds 3, semiempirical
optimizations22 have been performed, resulting in the following
geometry (for 3a, see Fig. 2). The phenyl ring is nearly perpen-
dicular to the pyrimidiniumolate system (AM1: 90.5°, PM3:
90.2°) whereas the thiophene ring is twisted by 19.8° (AM1,
PM3: 46.2°). The rotational barriers for the phenyl ring and
the thiophene ring have been investigated in some detail
(Fig. 3, AM1 values). Whereas for the former rotation a barrier
of 17.2 kcal mol−1 (1 cal=4.184 J) was found—irrespective of
the fact of whether v(a–b–c–d) was fixed or not—the rotational
Fig. 2 Calculated geometry of 3a (AM1)
barrier of the thiophene ring is 2.8 kcal mol−1 [v(e–f–g–h) was
fixed to 90°]. The electrochemical properties of the resulting
polymer should not be affected significantly by the twisting
angle of the phenyl group. As expected the dipole moment of
compounds 3 depends to a large extent upon substituents.
Semiempirical calculations show that on changing the substitu-
ent R1 there is an increase of the dipole strength from approxi-
mately 5.1 (5.7) to 12.9 (13.4) Debye (Table 1). The geometrical
features of 3 (bond lengths, bond angles) are in line with
expectations and with X-ray data.23 Ab initio calculations for
the parent system 5 (see Table 2) with density functional
methods24,25—especially when Becke’s non-local three-param-
eter exchange and correlation functional26 in conjunction with
the Lee–Yang–Parr correlation functional27 is used—give an
excellent picture of this molecule. These results are in agreement
with DFT studies on other heterocyclic systems with unusual
structure elements (nonclassical furoxans,28 furoxans,29,30
benzofuroxans,29,30 furazans, benzofurazans and related
compounds30,31).
O
Me
OR
OR
HN
N
R1
+
S
Me
R2
O
1a R = H
2a R1 = R2 = H
b R1 = Br, R2 = H
c R1 = H, R2 = Br
PhOMe
heat
b R = TCP
O
Me
Me
N
anodic
oxidation
+
R1
polymer 4
–
S
N
O
R2
3a R1 = R2 = H
b R1 = Br, R2 = H
c R1 = H, R2 = Br
2206
J. Mater. Chem., 1997, 7(11), 2205–2208

O
CH3
e
N
d
h
a
g
f
b
c
+
–
S
N
O
CH3
3a
10
H
2
1
N
3
N
4
H
H
+
–
5
7
O
O
12
H
5
In conclusion it can be stated that dipolar systems of type
3 (5) may serve as potentially useful starting materials for
conductive polymers.
The continued support of our work by the Fonds der
Chemischen Industrie is gratefully acknowledged. Thanks
are due to Dr H. Naarmann, BASF, for conductivity
measurements.
References
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barrier
for
(a)
thienyl
ring
rotation
[0°∏v(a–b–c–d)∏90°, v(e–f–g–h)=90°] and (b) phenyl ring rotation
in 3a [0°∏v(e–f–g–h)∏180°, v(a–b–c–d)=19.8°] (AM1 values)
Table 1 Calculated dipole moments of pyrimidiniumolates of
type 3
9
dipole moment/
Debye (AM1 values)
dipole moment/
Debye (PM3 values)
R1 (R2=H)
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-NO
2
5.09
9.05
10.83
11.50
12.85
5.74
10.15
11.35
12.03
13.39
-Br
-H
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2
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Table 2 Calculated geometry of compound 5; distances r in A
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r
r
r
r
r
r
W. Friedrichsen, T. Kappe and A. Bottcher, Heterocycles, 1982,
19, 1083.
¨
1,2
3,4
4,5
4,7
2,10
5,12
AM1a
PM3a
1.343
1.352
1.296
1.317
1.317
1.313
1.33
1.465
1.492
1.466
1.481
1.491
1.493
1.48
1.412
1.406
1.402
1.409
1.411
1.409
1.41
1.243
1.223
1.194
1.231
1.222
1.215
1.22
1.116
1.104
1.075
1.086
1.086
1.085
—
1.096
1.094
1.070
1.083
1.082
1.080
—
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2207

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Paper 7/04922F; Received 9th July, 1997
2208
J. Mater. Chem., 1997, 7(11), 2205–2208