Electrosynthesis of poly-2,5-pyridine promoted by nickel complexes

Synthetic Metals, 25 (1988) 365 - 373

365

ELECTROSYNTHESIS OF POLY-2,5-PYRIDINE PROMOTED BY NICKEL COMPLEXES GILBERTO SCHIAVON and GIANNI ZOTTI

C.N.R. 1.P.E.L.P., Corso Stati Uniti 4, 35100 Padua (Italy) GINO BONTEMPELLI and FILIPPO LO COCO

Institute o f Chemistry, University o f Udine, viale Ungheria 43, 33100 Udine (Italy) (Received February 2, 1988; accepted April 21, 1988)

Abstract A nickel-catalysed electrochemical method for the synthesis of poly2,5-pyridine is suggested. It is based on the reduction of 2,5-dibromopyridine carried out b y generating electrochemically and continuously recycling nickel(0) from nickel(II) in the presence of b o t h bipyridyl and triphenylphosphine. The simultaneous presence of these ligands is required to avoid the abstraction of the nickel promoter from the electrocatalytic cycle, thus allowing a very satisfactory yield (89%) to be achieved. Bipyridyl is in fact a ligand suitable for binding the metal in the 2+ oxidation state, so that it is able to compete with the nitrogen-coordination sites present in the polymer originated in the process, thus maintaining nickel(II) in the solution, while triphenylphosphine is able to stabilize nickel(0), thus preventing side reactions of this very reactive species with the solvent. The conductivity of poly2,5-pyridine after doping has also been tested. Doping with electron-acceptor species such as iodine or bromine leads to ch~irge-transfer complexes displaying semiconducting properties, while n-doping with sodium naphthalenide causes a larger increase in the electrical conductivity, which suggests that a conducting material is obtained.

Introduction Polyconjugated organic polymers represent an emerging class of important conductive materials whose main interest arises from their peculiar electrical properties [1]. They present great opportunities for truly novel contributions and important new applications are being proposed continuously [2, 3]. The basic routes followed to synthesize polyaromatics are b o t h oxidative cationic coupling (Friedel-Kraft syntheses [4]) and the reductive dehalogenation of dihaloaromatics (Wurtz, Ullmann or Grignard syntheses [4]). In 0379-6779/88/$3.50

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366 particular, this latter procedure has been adopted [5] to prepare poly-2,5pyridine, with a yield of 60%, by reducing 2,5-bibromopyridine with magnesium in the presence of catalytic amounts of nickel complexes. Both these routes are well suited for electrochemical versions. In particular, anodic coupling is a straightforward and profitable method now widely adopted to prepare the most thoroughly investigated polyaromatics such as polypyrrole and polythiophene [6, 7]. Unfortunately, this m e t h o d cannot be employed in basic media [7], thus preventing its use to achieve pyridine coupling. We have recently suggested an alternative route for achieving the deposition of polypyridine films on electrodes [8, 9], which is based on the nickel-catalysed cathodic dehalogenation of 2,5-dibromopyridine (PyBr2) according to the following abridged scheme: PyBr 2 -- [--py--].

+

Ni °

, Br--Nin--PyBr

•

/2

These investigations have shown that the solid deposit consists of a polypyridine framework coordinating Ni 2+ ions, whose coordination shell is completed b y the ligands available in the solution. Thus the metal catalyst is abstracted b y the reaction product, so that the process is self-inhibited. In view of the ability of 2,5-polypyridine to behave after doping as an organic conductor as well as being able to bind b o t h cations and anions (these last after protonation of the pyridine nitrogen in acid medium), such a polymer appears interesting not only as a film b u t also as a massive material. Consequently, we have attempted to improve the above-mentioned cathodic procedure by opposing the self-inhibition with the use of two different ligands for the nickel promoter. The former, bipyridyl (bipy), is preferred b y the 2+ oxidation state and is able to compete with the coordination sites present in the polymer originated in the process, thus maintaining nickel(II) in the solution, while the latter, triphenylphosphine (PPh3) , is able to stabilize nickel(0) thanks to its ~-acceptor properties, thus preventing the abstraction of this promoter species as a consequence of parasitic reactions with the solvent. In this paper a method based on these statements is proposed for the nickel-catalysed electrosynthesis of 2,5-polypyridine, which is characterized b y high turnover numbers. Some tests on the conductivity of this material after doping with b o t h electron-donor and electron-acceptor species are reported. Experimental

Chemicals

All the chemicals employed were of reagent-grade quality. Reagentgrade acetonitrile was further purified by distilling repeatedly from phos-

367 phorus pentoxide and was stored on molecular sieves (0.4 nm) under a nitrogen atmosphere, while reagent-grade, 1,2-dimethoxyethane (DME) was distilled from N a - K alloy directly into the test vessel. The supporting electrolyte, t e t r a e t h y l a m m o n i u m perchlorate (TEAP), as well as the ligands triphenylphosphine (PPh3) and 2,2'-bipyridyl (bipy), were recrystallized from methanol and dried under vacuum at 50 °C. Stock solutions of anhydrous nickel(II) perchlorate were prepared by anodic oxidation of metallic nickel in acetonitrile containing the appropriate a m o u n t of supporting electrolyte, in agreement with a previous report [10]. Acetonitrile solutions of the complex [Ni(bipy)3] 2+ were prepared according to the literature [11] by simple addition of the ligand directly to the anhydrous Ni 2+ solutions in a 3:1 molar ratio. Nitrogen used to remove dissolved oxygen from the working solutions before all electrochemical tests was previously passed through concentrated sulphuric acid to remove traces of water and then equilibrated to the vapour pressure of the medium.

Apparatus Voltammetric, coulometric and preparative experiments were carried out in an H-shaped three-electrode cell with cathodic and anodic compartments separated by a sintered glass disc. A nickel spiral was used as the counter electrode, while the working electrode was mercury, platinum or glassy-carbon with different areas, depending on the experimental requirements. In all cases a silver/0.1 M silver perchlorate electrode in acetonitrile was used as the reference electrode and the concentration of the supporting electrolyte was 0.1 M. The electroanalytical unit was a three-electrode system assembled with an Amel 552 potentiostat in conjunction with an Amel 568 digital logicfunction generator and an Amel 558 digital integrator. The recording device was a Hewlett-Packard 7090 A measurement plotting system. Ultraviolet-vis spectra were obtained in hydrochloric acid solutions on a Perkin-Elmer Lambda 15 spectrophotometer, while absorbance and reflectance i.r. spectra were run on a Perkin-Elmer 682 spectrophotometer. Reflectance i.r. spectra were obtained on filmed platinum foils mounted on a Specac (Analytical Accessories Ltd., England) multiple attenuated total reflectance unit. Conductivity measurements were performed on compressed pellets of the polymer with a four-point probe equipped with a Keithley 195 digital multimeter and a Keithley 220 programmable current source.

Polymerization procedure The polymer was typically prepared in acetonitrile solutions (30 ml) containing TEAP (0.1 M) in which a sacrificial nickel anode was oxidized at first, until a Ni 2÷ concentration of 6 × 10 -3 M (36 C) was achieved. Afterwards, bipy, PPh 3 and 2,5-dibromopyridine (PyBr2) were added in molar

368 ratios of 6:1, 2:1 and 10:1, respectively, with respect to the nickel content and then the electrolyses were started. The electrolyses were performed at --1.7 V versus Ag/Ag + 0.1 M in CH3CN at a mercury electrode in order to allow the electrode surface to be renewed continuously by stirring. They were stopped when the cathodic current decreased to about one half of its initial value, after which 360 C were passed (2 mol of electrons per mol of PyBr2). At the end of these electrolyses, an a b u n d a n t brown precipitate was obtained in which appreciable amounts of metallic mercury were apparently present, these coming from the cathode shuttering. Consequently, the solid product was filtered off and dissolved in concentrated hydrochloric acid. After a further filtration to eliminate the insoluble mercury, the solution was diluted with water (5:1) until complete reprecipitation occurred and the suspension obtained was treated with EDTA and ammonia to eliminate possible nitrogen-coordinated Ni 2+ ions. The solid was then recovered by centrifugation, washed with NH3 containing ethanol + water mixtures and dried under vacuum. In such a way, 130 mg of dry yellow product could be obtained, with a 89% yield referred to the PyBr 2 employed. The elemental analysis data fit quite well those expected for poly-2,5pyridine (--CsH3N--),. (Found: C, 76.7; H, 3.9; N, 17.9. Calcd: C, 77.8; H, 3.9; N, 18.3.) In addition, they point out the absence of both Br and Ni. Moreover, as shown in Fig. l(a), the i.r. spectrum of the solid in KBr shows a strong band at 820 cm -1 (in agreement with an aromatic paradisubstitution [12], consistent with a chain of conjugated pyridine rings), while u.v.-vis spectra recorded on solutions of the polymer in concentrated hydrochloric acid display an absorption band at 360 nm (attributable to conjugated poly-2,5-pyridine [8] ).

Doping procedures P
Results and discussion

Optimization of the polymerization procedure for poly-2,5-pyridine Figure 2 (curve a) shows the cyclic voltammetric behaviour displayed by the complex [NiII(bipy)3] 2+ in acetonitrile at a glassy-carbon electrode. Its reduction occurs in an appreciably reversible two-electron process leading to [Ni°(bipy)2], which is re-oxidized in the associated anodic reaction,

369

J

8C T%40(a)

/

80-

T%40-

(b)

1800

16()0

14()0 1200 Wavenumber

1000 (cm -~)

800

600

Fig. 1. I n f r a r e d s p e c t r a r e c o r d e d o n KBr pellets of: (a) t h e u n d o p e d p o l y m e r ; (b) the p o l y m e r d o p e d w i t h b r o m i n e .

~1'0

E/V

0.0

-~A

-0.4

Fig. 2. Cyclic v o l t a m m e t r i c curves r e c o r d e d w i t h a glassy-carbon e l e c t r o d e on 0.1 M T E A P - C H a C N s o l u t i o n s c o n t a i n i n g : (a) 2 × 10 - 3 M [Ni(bipy)a]2+; (b) 2 × 10 - 3 M [Ni(bipy)3] 2+ and 2 × 10 - 2 M PyBr2; (c) 2 × 10 - a M [Ni(bipy)3] 2+, 2 × 10 -2 M PyBr2 and 4 X 10 - 3 M PPh 3. Scan rate: 0.1 V s -1.

370 according to previous reports [11, 13]. After the addition of excess 2,5dibromopyridine (in a molar ratio of 10:1 with respect to the nickel content), the nickel(II) reduction peak increases appreciably, while the associated anodic peak due to the nickel(0) re-oxidation disappears completely (see Fig. 2, curve b). Such behaviour can be explained taking into account that nickel(0) undergoes an oxidative addition by the organic substrate to give the organometallic adduct [Ni(bipy)(PyBr)Br], which is in turn reducible at the working potential [9] (about - - 1 . 6 V), thus accounting for the increase observed for the cathodic peak. As a result of the combination of these electrode and chemical steps, the overall reduction process occurs through the following reaction sequence [9] : [NiII(bipy)3] 2+ + 2e-

~

[Ni°(bipy)2] + BrPyBr

~ [Ni°(bipy)2] + bipy

(1)

> [BrNiII(PyBr)(bipy)] + b i p y

[BrNiII(PyBr)(bipy)] + 2e-

(2) 1 ~ [Ni°(bipy)2] + 2Br- + - - [ - - P y - - ] , ( 3 ) I

n

The nickel(0) obtained in reaction (3) of course reacts with the excess PyBr 2 present in the solution, thus renewing the organo-nickel adduct and allowing an electrocatalytic cycle to take place. On the basis of this scheme, it is expected that 2,5
371 To overcome this drawback, triphenylphosphine, together with bipy, has been added to the nickel(II) solutions before reduction in order to take advantage of the ~-ability of this ligand for stabilizing nickel(0) with respect to its undesired reaction with the solvent. Under these experimental conditions, the abstraction of nickel from the reaction pathway ( 1 ) - ( 3 ) is no longer observed and the complete conversion of PyBr 2 into poly-2,5-pyridine is indeed accomplished b y transferring the theoretical charge expected for this process. Moreover, our results point out that by carrying out these controlled potential electrolyses in the presence of a large excess of PyBr2, high turnover numbers are achieved for the Ni-mediated polymerization process. In typical 24 h experiments, more than 50 electrocatalytic cycles were in fact feasible without any appreciable consumption of the nickel promoter being detected voltammetrically at the end of the electrolyses. However, the higher nickel(0) stability, conditioned b y the presence of PPh3, causes its reaction with the organic substrate (reaction (2)) to b e c o m e slower, as is clearly apparent in Fig. 2, curve c. Consequently, a lower current flows during electrolyses, thus implying a slackening of the entire electropolymerization process. However, this is more than balanced by the mentioned increase of yield referred to the charge transferred. This is the reason why the PPh 3 content must be kept at as low a level as possible, compatible with its role in the process.

Electrical conductivity of the doped polymer In the course of previous investigations dealing with the voltammetric behaviour of poly-2,5-pyridine films deposited on glassy-carbon electrodes [8, 9], we found that cathodic doping could be accomplished in correspondence with a reversible reduction process occurring at --2.1 V, accompanied b y an electrochromic effect, in which the uptake of 1 electron per 3 pyridine moieties was involved. Conversely, unexpectedly no anodic doping was observed in the positive-going scans before the film dissolution, probably because it required such a high potential that aromatic rings and/or traces of water were involved in anodic reactions, thus causing an immoderate local increase in acidity and the consequent dissolution of the polymeric material. Therefore, we attempted the chemical p ~ o p i n g of the polymer with bromine and iodine b y following the procedure reported in the Experimental Section. The results obtained in four-probe electrical conductivity measurements performed on pellets of the doped material are presented in Table 1, where they are compared with the conductivity of the undoped polymer. These figures shows unambiguously that poly-2,5-pyridine is able to give charge-transfer complexes displaying semiconducting properties with electron-acceptor species. The higher conductivity exhibited b y the brominedoped material in comparison with the iodine-doped sample may be related to both the higher oxidation potential of this dopant an/] the lower doping level acquired with 12 under our experimental conditions (see the minimal formulae in Table 1). This different interaction is confirmed by i.r. spectra

372 TABLE 1 Electrical conductivities measured on pellets of doped and undoped polymer by the fourprobe approach Dopant

Minimal formula

Electrical conductivity (~ 1 cm-1)

-12 Br2 NaCmHs

CsH3N CsH3NI0.39a CsH3NBr0.92a _b

<10 -l° 2.0 x 10-8 1.5 X 10-s ~3.0 X 10 2

aEstimated by the weight increase due to the doping process. bUndetermined. recorded on the p o l y m e r prior to and following the doping process. In the case of iodine as the d o p a n t , no appreciable change is found, while the appearance of new absorption bands at 1270 and 1430 cm -1 and the concomitant disappearance of a strong band at 1450 cm -1 (all due to ring deformation in an aromatic beckbone) are observed when poly-2,5-pyridine is doped with bromine. Figure l ( b ) reports t he relevant i.r. absorption spectrum, which is found to be identical to the i.r. reflectance recorded on platinum foils filmed with the doped material. In c o n n e c t i o n with this last point, it is w o r t h noting that similar spectral changes {attributed to the partial conversion o f the aromatic f r a m e w o r k in a quinoid t ype) have been observed re c e ntly for o th er doped polyaromatics [15]. For the sake of comparison, Table 1 also reports the electrical cond u c t i vity measured on t he p o l y m e r d o p e d with sodium naphthalenide. In this case a value typical of conducting materials is obtained, thus pointing o u t (in agreement with our previous electrochemical findings m ent i oned above) that poly-2,5-pyridine is m o r e prone to n-doping than to p-doping, conceivably as a consequence of the electron-withdrawing effect of the nitrogen a t o m present in the pyridine ring. Conclusions The results obtained in this investigation point out that the electrochemical r e d u c t i o n of d i b r o m o p y r i d i n e carried out in the presence of nickel complexes represents a synthetic r o u t e for poly-2,5-pyridine characterized b y a higher yield t han that of the Grignard m e t h o d [5], thanks to a suitable choice of t he ligands aimed at stabilizing b o t h the reduced and oxidized f o r m of the nickel p r o m o t e r . Consequently, this paper emphasizes once again that the electrosynthetic p r o c e d u r e based on the nickel-catalysed electrochemical r e d u c t i o n of d i b r o m o a r o m a t i c s , previously proposed by us [8, 9, 1 6 - 19], is an effective and profitable m e t h o d suitable for quite general application. As to the p o l y m e r i c material obtained here, it can undergo doping by b o t h electron-donor and electron-acceptor species, thanks to its high

373

degree of conjugation. This fact, together with the presence of nitrogen atoms in the organic chain, suggests that such a material exhibits an interesting combination of electroactive and coordinative properties, making it particularly attractive for further investigations.

Acknowledgements We wish to thank Mr. S. Sitran of C.N.R. for experimental assistance.

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