Approaches to a polypyrrole model

Synthetic Metals, 58 ( 1 9 9 3 ) 2 3 3 - 2 4 2

233

Approaches to a polypyrrole model Stephen J. van Eyk* and Herbert Naarmann Kunststofflaboratorium, BASF AG, D6700 Ludwigschafen (Germany) (Received October 20, 1992; in revised form November 2, 1992; accepted November 4, 1992)

Abstract A m o d e l of t h e p r o p o s e d m a c r o c y c l i c p o l y p y r r o l e h a s b e e n s y n t h e s i z e d a n d is discussed. Also p r o p o s e d is a n i n t e r e s t i n g f o r m of polypyrrole b o r n e f r o m t h e s e s t u d i e s with a fullerene-like s t r u c t u r e . A p o s s i b l e s y n t h e t i c m e t h o d to it is discussed.

Introduction

Polypyrrole was known as far back as 1888 [1] and its electrical conductivitywas first demonstrated in 1964 [2 ]. The conductivity of polypyrrole and its relative stability have led to the development of several products including rechargeable batteries using a polypyrrole electrode, conductive coatings for plastic surfaces, sensors and their use for ELMI shielding and in the manufacture of printed circuit boards. Polypyrrole has further potential applications as piezoceramics, optical information storage systems and many other uses still under development. Polypyrrole that is stable under normal laboratory conditions for indefinite periods and which has a good conductivity has been produced [3]. Various substituted polypyrroles have also been made [4] but the conductive properties are generally not as good as unmodified polypyrroles. As is typical of polymers the exact structure of the polypyrrole is difficult to determine [5]. As a result of this the process of producing a new polypyrrole with a specific conductivity or other required properties is largely an empirical one. The generally accepted mechanism for the electrochemical polymerization of pyrrole is shown in Fig. 1. The resulting polypyrrole is a linear chain of pyrroles joined by 5,2' linkages. It has recently been proposed [6] that, at least under certain polymerization conditions, polypyrrole with a macrocyclic structure (4, 5) is formed. Evidence for this includes the lack of C-H signals in 13C NMR, the lack of C-D Fr-IR signals for deuterated films and the films sequester metal ions [7]. This macrocyclic pyrrole might be formed via benzotripyrrole subunits (Fig. 2). It would be helpful to be able to prove or disprove this theory on the basis of model studies. *Present address: Institute of Environmental Health and Forensic Sciences, Mt. Albert Science Centre, Private Bag 92-021, Auckland, New Zealand.

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Fig. 1. Electropolymerization of pyrrole.

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Fig. 2. Electropolymerization of pyrrole: alternative mechanism. This w o r k is a i m e d at p r o d u c i n g m o d e l c o m p o u n d s of the p r o p o s e d m a c r o c y c l i c p o l y p y r r o l e . O t h e r interesting p r o p o s a l s b o r n e f r o m t h e s e studies are i n t r o d u c e d , including a fullerene-like f o r m o f p o l y p y r r o l e a n d a possible synthetic m e t h o d to it.

235 Discussion

The benzotripyrrole units which are proposed can be either symmetric (2, 1H-benzo[1,2-b:3,4-b':5,6-b"]tripyrrole) or asymmetric (3, 1H-benzo[2,1b:3,4-b ':5,6-b"]tripyrrole). These trimers can further polymerize to different forms of a macrocyclic polymer (4, 5). The trimers also seem a very satisfactory model to test the macrocyclic theory of polypyrrole (6). If the trimers polymerize to form polymers with similar properties to polypyrrole it is likely that they are indeed true precursors, or at least sub-units, of polypyrrole. On the other hand, since these trimeric units have not been isolated in the preparation of polypyrrole, it is expected that if they exist at all they will also be highly reactive toward polymerization and, hence, difficult to isolate.

The driving force for forming these trimeric units is not known and not observed for pyrrole, but has been seen at work in the formation of thiophene trimers [8], benzothiophene trimers [9], benzofuran trimers [10] and indole trimers [ 11 ], amongst others. The unambiguous synthesis of a protected form of the tripyrrole is described in Scheme 1 [ 12 ]. The basic skeleton was functionalized according to reagents that were easily available and to the need to protect the pyrrole from further reactions, hence the phenyl group and the N-methyl groups. The reaction was straightforward and used known chemistry; yields were often low however. The final product, 2-(2-(1-methylpyrrol-3-yl)-l-methyl-5phenylpyrrole-3-yl) pyrrole (12), was recovered as a mottled blue solid, thinlayer chromatography (TLC) indicated that the blue material was a highly polar impurity, easily separated by chromatography from the product. Cyclic

236

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Scheme 1.

voltammetry was performed in three different solvents, dichloromethane, acetonitrile and nitromethane; the voltammograms and potentials are shown in Fig. 3. It is not possible to detect reversible peaks in nitromethane or acetonitrile. If new products are being formed (e.g. cyclization products or polymers) then they are at least not conductive and they do not form on the electrode surface. Attempts to form the corresponding benzotripyrrole by cyclization of 12 using 1,2-dichloro-5,6-dicyano-p-benzoquinone [13] were unsuccessful, only a black polymer was recovered. Polymerization of 12 using potassium peroxidisulfate also yielded a black insoluble polymer. The elemental analysis and the infrared spectrtun showed little about the structure of this polymer, but is included here for completeness. Note the appearance of the infrared absorbance at 1705 c m - 1 and the retention of the absorbance at 699 cm-1. 2-(2-(1-Methylpyrrol-3-yl)-l-methylpyrrol-3-yl)-l-methylpyrrole (17) is a closer model to the required benzotripyrroles than 12. The proposed synthesis (Scheme 2), following the first, was unsuccessful. 1,3-Dioxolan-2-yl-methyltributylphosphonium bromide (14) was used as the synthetic equivalent of acetaldehyde [14] to form the enal (15) resulting in much better yields than when acetaldehyde itself was used. Reaction of 15 with 10 was not possible under these conditions. Although 16 remains a viable intermediate for the synthesis of 17 it is clear that an alternative synthesis to 16 is required. An interesting observation arising from this work was the possibility of the easy synthesis of a fullerene-like molecule (21) constructed from pyrrole. If polypyrrole is indeed a macrocyclic structure formed via benzotripyrrole

237 intermediates then one would expect that 1-(1-pyrrolyl) pyrrol (18) should form the trimer 19. This might form via intramolecular reaction a part-sphere ( 2 0 ) and further polymerization o f 18 or 19 o n t o this part-sphere might produce the fullerene-type molecule 21. If 21 was produced f r o m 18 under

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Scheme 2.

t h e s a m e p o l y m e r i z a t i o n c o n d i t i o n s as m a c r o c y c l i c p o l y p y r r o l e is p r o d u c e d , t h e n this w o u l d n o t only r e s u l t in the i n t e r e s t i n g ' f o o t b a l l ' f r o m p y r r o l e b u t w o u l d also p r o v i d e a n e l e g a n t p r o o f o f t h e m a c r o c y c l i c p o l y m e r i z a t i o n mechanism.

239

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Work on making a suitable model for polypyrrole and producing the pyrrole-fullerene is continuing.

Experimental Preparation o f 2-(3-phenyl-l-prop-l-en-3-onyO pyrrole (9) Acetophenone (9.6 g) was added with stirring to a solution of sodium hydroxide (5 g) in ethanol (30 ml) and water (45 ml). Pyrrole-2-aldehyde (9.5 g) was added immediately and the mixture stirred for 48 h, cooled, filtered and the solid washed with water until neutral and with a small quantity of cold ethanol. The crude product was recrystallized several times with ethanol to give 2-(3-phenyl-l-prop-l-en-3-onyl) pyrrole (9) as a yellow crystalline solid (9.12 g). IH NMR (CDC13, D6DMSO) ~ : 6.25, 6.65, 7.0, 7.3-8.1, 10.95. 13C NMR (CDCI3, D6DMSO) ~c: 110.9, 115.4, 116.0, 123.7, 128.3, 128.4, 129.6, 132.2, 135.0, 138.9, 190.3. Preparation o f 1-(1-methyl-3-pyrrolyl)-2-(2-pyrrolyl)-4-phenyl-l , 4butadione (11) 2-(3-Phenyl-l-prop-l-en-3-onyl) pyrrole (9, 4.1 g), freshly distilled 1methyl pyrrole 3-aldehyde (2.25 g), triethylamine (4 g) and 3,4-dimethyl-5(2-hydroxyethyl) thiazolium iodide (1.14 g) were dissolved in very dry ethanol (CaO, then Mg/I2) (30 ml) and gently refluxed under argon for 8 h and left

240 to cool overnight. The solution was further cooled in ice and crystals (3.9 g) of the product (11) were collected. The starting material and product are not easily distinguishable by TLC. Impurities can be washed out with acetone. 1H NMR (CDCI3, D6DMSO) ~ : 3.3, d of d, 1H; 3.65, 3H; 4.15 d of f, 1H; 5.0, d of d, 1H; 6.0, 6.5-6.7, 7.35-7.6, 7.95; 9.8, NH. 13C NMR (CDCla, D6DMSO) tic: 36.5, q, Me; 41.9, t, CH2C--O; 42.9, d, _CHC=O; 106.2, d, 1H-pyca; 108.1, d, 1Me-pyc4; 109.8, d, 1H-pyc4; 117.5, d, 1H-pycs; 123.3, d, 1Me-pycs; 124.7, s, 1Me-pyc3; 127.6, d, 1Me-pyc2; 128.0, d, Arc2,c6; 128.5, d, Arca.cs; 129.2, s, 1H-pyc,; 133.1, d, Arc4; 136.7, s, Arcl; 193.0, s, py_C=O; 198.4, s, PhC_--O.

Preparation of 2-(2-(1-methylpyrrol-3-yO-l-methyl-5-phenylpyrrol-3-yO pyrrole (12) 1-(1-Methyl-3-pyrrolyl)-2-(2-pyrrolyl)-4-phenyl-l,4-butadione(11, 0.5 g) in n-propanol (15 ml) was refluxed and methyl amine hydrochloride (1 g) in 40% aqueous methyl amine (10 ml) was added. Reflux was continued for 1 h, the reaction mixture cooled and left overnight. A product (12) as a mottled blue solid formed (410 mg) and was collected. C20H,gNa requires 301; found 301. vm~: 3406, 3354, 1600, 1476, 795, 768, 750, 720, 701 cm-1.1H NMR (DsDMSO) 6H: 3.35, 3.65, 5.8--7.5, 10.3. laC NMR (D6DMSO) tic: 24.1, 32.9, 35.7, 103.6, 106.4, 107.5, 110.1, 113.9, 115.2, 121.9, 122.3, 126.2, 126.3, 127.8, 128.4, 132.9, 133.3.

Polymerization of 2-(2-(1-methylpyrrol-3-yO-l-methyl-5-phenylpyrrol-3yl) pyrrole (12) 2 -(2-(1 -Methylpyrrol-3-yl)-1-methyl-5-phenylpyrrol-3-yl)pyrrole (12, 100 mg) was dissolved in methanol (20 ml) and water was added until the product began to precipitate. More water was added and potassium peroxydisulfate (90 mg) was added and the mixture shaken periodically over three days during which time a black precipitate formed. The mixture was diluted with 100 ml water, filtered through fine filter paper and washed with more water. The solid collected was dissolved in ethanol and again filtered, the black solid (35 mg) was collected and dried. Anal. Found: C, 69.0; H, 5.2; N, 11.8; S, 1.2%. Vm~: 1705, 1599, 1575, 1482, 1450, 1384, 1315, 1237, 1179, 699.

Preparation of 2-(1-prop-l-en-3-al) pyrrole 2-Bromomethyl-l,3-dioxolane (97%, 37.7 g) and tri-n-butylphosphine (95%, 42.6 g) were stirred together at 90 °C for three days. A salt, 1,3dioxolan-2-yl-methyltributylphosphonium bromide, was obtained as a glass. No further purification was necessary. A solution of sodium ethoxide (150 ml, 1 M, 50% excess) in absolute alcohol was added dropwise to a solution of 1-methylpyrrole-2-aldehyde(10.9 g) and 1,3-dioxan-2-yl-methyltributylphosphoniumbromide (1.1 equiv., 31.84 g) in 250 ml DMF at 90 °C. The resulting solution was stirred for 16 h at 90 °C and poured into 1 l water. The product (and tributylphosphine oxide)

241 w a s e x t r a c t e d i n t o e t h e r ( 3 × 3 0 0 m l ) , w a s h e d w i t h s a t u r a t e d b r i n e (2 × 2 0 0 ml) and dried (MgSO4). After filtration the ether was removed. The residue w a s d i s s o l v e d in T H F ( 2 5 0 m l ) a n d a 1 0 % a q u e o u s s o l u t i o n o f HCI ( 2 5 0 ml) was then added rapidly. The mixture was stirred for two hours at room temperature and poured into 1 1 of water. The product was extracted, washed and dried as above, the solvent removed and the product chromatographed on silica eluting with petroleum ether/ether gradient elutions and obtained a s a s o l i d (5 g). 13C N M R (CDCIa): 3 4 . 5 9 , 1 1 0 . 2 1 , 1 1 4 . 7 2 , 1 2 3 . 5 3 , 1 2 9 . 0 5 , 1 2 9 . 2 5 , 1 3 9 . 8 8 , 1 9 3 . 0 5 . 1H N M R : 3 . 7 , 3H; 6 . 2 - 7 . 4 , 5 H ; 9 . 5 , 1H.

Acknowledgements T h i s c o n t r i b u t i o n is p a r t o f t h e B M F T P r o j e c t 0 3 M 4 0 4 5 - 8 , P o l y m e r e mit aussergew6hnlichen Eigenschaften. Many thanks to Ronald Reuter of the M P I P o l y m e r f o r s c h u n g in M a i n z f o r c a r r y i n g o u t t h e c y c l i c v o l t a m m e t r y experiments.

References 1 M. Dennstedt and J. Zimmermann, Ber. Dtsch. Chem. Ges., 21 (1888) 1478. 2 H. Naarmann and F. Beck, Neuartige Polymerisation aus aromatischen und heterocyclischen Verbindungen und ihre elektrophysikalischen Eigenschaflen, GeseUschafl deutscher Chemiker Meet., Munich, Germany, Oct. 12, 1964; H. Naarmann, F. Beck and E. G. Kastning (BASF AG), DB Patent No. 1 178 529 (Apr. 11, 1964/May 20, 1965); DB Patent No. 1 105 497 (Feb. 1, 1963/June 24, 1965); GP Patent No. 1 092 137 (Mar. 12,

1964/Nov. 22, 1967). 3 H. Naarmann and P. Strohriegel, in H. R. Kricheldorf (ed.), Handbook of Polymer Synthesis, Part B, 1992, pp. 1353-1435. 4 For example: A. F. Diaz, J. I. Castillo, J. A. Logan and W.-Y. Lee, J. Electroanal. Chem., 120 (1981) 115-132; M. Salmon, A. Diaz and J. Goitia, ACS Syrup. Ser., 102 (1982) 65-70; A.F. Diaz, J. Castillo, K. K. Kanazawa and J. A. Logan, J. Electroanal. Chem., 133 (1982) 233-239; J. P. Travers, P. Audebert and G. Bidan, Mol. Cryst. Liq. Crgst., 118 ( 1 9 8 5 ) 149-153. 5 G. B. Street, S. E. Lindsey, A. I. Nazzal and K. J. Wynne, Mol. Cryst. Liq. Cryst., 118 (1985) 137-148; A. I. Nazzal, G. B. Street and K. J. Wynne, Mol. Cryst. Liq. Cryst., 125 (1985) 303-307. 6 D. Schraeisser, H. Naarmann and W. G6pel, Proc. Conf. on Science and Technology of Synthetic Metals (ICSM '92), GSteborg, Sweden, A~g. 12-18, 1992; Synth. Met., 55-57

(1993) in press. 7 H. Naarmann, Synth. Met., 41 (1991) 1. 8 R. Proetzsch, D. Bieniek and F. Korte, Tetrahedron Lett., (1972) 543-544. The first step in the trimerization of pyrrole may be formation of pyrrolinone, as the report referenced above uses a thiolactone as precursor. R. Proetzsch, D. Bieniek and F. Korte, Z. Naturforsch., Teil B, 31 (1976) 520-530. 9 R. Cayuela, H. T. Nguyen, C. Destrade and A. M. Levelut, Mol. Cryst. Liq. Cryst., 177 (1989) 81-91; Nguyen Huu Tin_h, R. Cayela and C. Destrade, Mol. Cryst. Liq. Cryst., 122 (1985) 141-149; C. Destrade, Nguyen Huu Tinh, L. Mamlok and J. Malthete, Mol. Cryst. Liq. Cryst., 114 (1984) 139-150.

242 10 J. Bergman, B. Egestad and D. Ra[]apaksa,Acta Chem. Scand., Set. B, 33 (1979) 405-409; C. Destrade, Nguyen Huu Tinh, H. Gasparoux and L. Mamlok, Liq. Cryst., 2 (1987) 229-233. 11 V. Bocchi and G. Palla, Tetrahedron, 42 (1986) 5019-5024. 12 D. M. Perrine, J. Kagan, De-Bin Huang, Ke Zeng and Boon-Keng Teo, J. Org. Chem., 52 (1987) 2213-2216. 13 J. Bergman and N. Eklund, Tetrahedron, 36 (1980) 1445-1450. 14 C. W. Spangler and R. K. McCoy, Synth. Commun., 18 (1988) 51-59.