Polyphenylenepyridines Based on Acetylaromatic Compounds

Abstract

Nitrogen-containing polyphenylene type polymers containing pyridine rings were synthesized. The polymer-forming reaction is based on the interaction of diacetylarylene and triethylorthoformate with the formation of a pyrylium salt and subsequent treatment of the intermediate product with ammonia. The optimal ratios of the reagents for the formation of the pyridine fragment were determined. The mechanism of the main reaction is discussed. The formation of the pyridine ring and phentriyl (1,3,5-triphenylsubstituted benzene) fragments was confirmed using ¹H NMR data of the example of model reactions. After heating at a temperature of 450 °C, when a more complete polycondensation process occurs, the polymers reach high values of thermal characteristics—10% weight loss in an inert atmosphere corresponds to 600 °C. The structure of the synthesized polymers was confirmed using elemental analysis, IR, XPS, and EPR spectroscopy. The conjugation length in cross-linked polyphenylene pyridines can be controlled by varying the arylene bridge groups between the phentriyl fragments, which opens up opportunities for the development of new composite materials for electrical applications.

1. Introduction

The dramatic climate change occurring during last decades connected with greenhouse gas and, accordingly, the need to reduce these emissions, as well as the global energy crisis, have contributed to the rapid development of renewable energy sources. However, the uneven production of energy by these sources requires the use of energy storage devices [1,2,3]. Among energy storage devices, supercapacitors have unique advantages: high power density and long service life (compared to batteries) and high energy storage density (compared to dielectric capacitors), which lead to their wide application [4,5]. According to the energy storage mechanism, supercapacitors can be divided into electrostatic double-layer capacitors and pseudocapacitors [6]. Charge conservation in electrostatic double-layer capacitors is carried out by electrostatic adsorption, which means that ions in solution accumulate either on the positive or negative electrode [7]. The electrode material in electrostatic double-layer capacitors is usually carbon-based materials with a large specific surface area and a suitable porous structure, which can provide high power density [8,9]. Conductive polymeric materials [10,11,12] have previously been developed as electrode materials with a high pseudocapacitance, but their practical application was limited by a rapid capacitance decay and a narrow operating voltage (0–1 V). Carbon materials, due to their low cost and wide voltage range, have shown great potential in electrochemical energy storage applications [13,14,15,16]. At present, an urgent task in terms of improving the properties of supercapacitors is the creation of new electrode materials based on carbon with increased energy storage density and power density [17,18]. It was previously reported that doping with heteroatoms (in particular, nitrogen) is one of the most promising ways to improve the surface properties of carbon electrode materials [19,20,21,22]. First, doping with heteroatoms can promote an increase in capacitance active centers [23]. Second, doping with heteroatoms can also increase the hydrophilicity of the carbon electrode surface, which will naturally increase the adsorption of anions/cations from the electrolyte solution by the carbon electrode material [24,25]. In addition, the introduction of nitrogen atoms into the structure of a carbon material also greatly increases its electrical conductivity [26], which makes it possible to drastically improve the efficiency of supercapacitors based on such electrodes. The most promising in terms of using electrode materials for supercapacitors as precursors are conjugated polymers built by covalently bonded aromatic rings into π-conjugated skeletons [27]. Carbon materials based on such polymers have nanoporous structures, large surface areas, and high chemical stability. Such nitrogen-containing polymers are obtained mainly by metal complex catalysis, that is, using Suzuki, Yamamoto, Heck, Sonogashira, and Buchwald–Hartwig reactions. These reactions require ultrapure conditions and the use of expensive catalysts [28,29,30,31]. Similar shortcomings of the synthesis are inherent in the reaction of azide–alkyne cycloformation [32]. This work is devoted to the synthesis of conjugated nitrogen-containing polymers by a simpler and more technological method, when the polymer-forming reaction is the formation of pyridine rings.

2. Materials and Methods

Triethylorthoformate (1,1,1-triethoxymethane, TEOF) was distilled in argon over potash (K₂CO₃), collecting a fraction boiling at 143–145 °C. Toluene was distilled in argon over sodium, collecting a fraction boiling at 109–110 °C. Benzene was distilled in argon over sodium, collecting a fraction boiling at 79–80 °C. After drying over CaCl₂, dichloroethane was distilled over P₂O₅ in an argon flow, collecting a fraction with a boiling point of 82–83 °C. Acetyl chloride (Aldrich, St. Louis, MO, USA), anhydrous aluminum chloride (Aldrich, St. Louis, MO, USA), 4,4’-diacetylbiphenyl (abcr GmbH, Karlsruhe, Germany), 1,4-diacetylbenzene (abcr GmbH), and biphenyl (Vektor, Novosibirsk, Russia) were used without purification.

X-ray photoelectron spectroscopy (XPS) spectra were acquired with an Axis Ultra DLD (Kratos, Manchester, UK,) spectrometer using monochromatized Al Kα. The energy scale was calibrated to provide the following values for reference metal surfaces freshly cleaned by ion bombardment: Au 4f_7/2—83.96 eV, Cu 2p_3/2—932.62 eV, and Ag 3d_5/2—368.21 eV. The electrostatic charging effects were compensated by using an electron neutralizer. Sample charging was corrected by referencing to the C-C/C-H group identified in the C 1s spectrum (284.8 eV). After charge referencing, a Shirley-type background with inelastic losses was subtracted from the high-resolution spectra.

The IR spectra of the samples were recorded on IR Fourier spectrometer Bruker “Tensor-37” (Bruker, Billerica, MA, USA).

¹H NMR spectra were recorded on a Bruker Avance-400 pulse spectrometer (Bruker, Billerica, MA, USA) with an operating frequency of 400.13 MHz. Sample solutions in deuterated chloroform were used. Chemical shifts are given in ppm, and the calibration was carried out according to the residual protons of the solvent.

The thermal properties of the synthesized polymers were studied using thermogravimetric (TGA) and differential thermal analysis (DTA) methods on a Q-1500 derivatograph (MOM, Budapest, Hungary). The sample heating rate was 20 °C/min (in an inert argon gas flow, 180 mL/min).

X-band EPR spectra were recorded on a Bruker EMX spectrometer (Bruker, Karlsruhe, Germany). The microwave power was no more than 2 mW, and the modulation amplitude of 100 kHz did not exceed 1 G. To determine the magnetic resonance parameters of the EPR signals, the WINEPR and SIMFONIA programs (Bruker, Karlsruhe, Germany) were used.

4-Acetylbiphenyl. To dichloroethane cooled to 0 °C (25 mL), 7.5 g (0.056 mol) of aluminum chloride and 4.6 mL (0.066 mol) of acetyl chloride were added with stirring in a stream of argon. The resulting complex was added dropwise to a solution of 7.0 g (0.045 mol) of biphenyl in 50 mL of dichloroethane cooled to 0 °C. The reaction solution was stirred at room temperature for 3 h, after which it was poured into acidified ice water. The aqueous layer was separated from the organic layer and extracted with chloroform. The combined organic layer was extracted with distilled water, dried over CaCl₂. After removal of volatile components under reduced pressure, the product was purified using column chromatography (silica gel–chloroform). Yield 6.8 g (76%). ¹H NMR spectrum (CDCl₃, δ, ppm, J/Hz): 2.67 (s, 3H, Me); 7.42 (t, 1 H, Ph, J = 7.3); 7.50 (t, 2H, Ph, J = 7.3); 7.65 (d, 2H, Ph, J = 7.6); 7.71 (d, 2H, C₆H₄, J = 8.2); 8.06 (d, 2H, C₆H₄, J = 8.2). The spectral characteristics are close to those in the literature [33,34].

1,3,5-Tris(phenyl)benzene (TPB). Hydrogen chloride was bubbled through a solution of acetophenone (11.7 mL, 0.1 mol), triethylorthoformate (TEOF) (20 mL, 0.12 mol), and benzene (68 mL) at room temperature and stirred for 6 h. Then, the precipitate was filtered off, washed with cold benzene, then ethanol, and dried. Yield 7 g (69%). 1H NMR spectrum (CDCl₃, δ, ppm, J/Hz): 7.43 (t, 3H, Ph, J = 7.2); 7.52 (t, 6 H, Ph, J = 7.4); 7.74 (d, 6H, Ph, J = 7.6); 7.83 (s, 3 H, C₆H₃). The spectral characteristics are close to those in the literature [35].

1,3,5-Tris(4-diphenyl)benzene. Gaseous hydrogen chloride was passed through a mixture consisting of 0.392 g (2 mmol) of 4-acetylbiphenyl, 0.4 mL (2.4 mmol) of TEOF, and 10 mL of benzene with stirring at room temperature for 70 min. Then, the contents of the flask were poured into 50 mL of ethanol, after which the volatile components were removed under reduced pressure. The residue was dissolved in chloroform and the product was purified using column chromatography (silica gel–chloroform, Rf = 0.8). Yield 0.217 g (60%). ¹H NMR spectrum (CDCl₃, δ, ppm, J/Hz): 7.40 (t, 3H, Ph, J = 7.4); 7.51 (t, 6 H, Ph, J = 7.7); 7.70 (d, 6H, Ph, J = 7.2); 7.76 (d, 6H, C₆H₄, J = 8.3); 7.84 (d, 6H, C₆H₄, J = 8.3); 7.92 (s, 3 H, C₆H₃). The spectral characteristics are close to those in the literature [35].

2,6-Diphenylpyridine. Hydrogen chloride was bubbled through a solution of acetophenone (1.2 mL, 0.01 mol) and TEOF (24 mL, 0.144 mol) at room temperature and stirred for 6 h. Then, this solution was added with stirring to a saturated solution of ammonia in chloroform (200 mL). The mixture was kept in a closed flask for 65 h. The precipitated salt was filtered off and washed with chloroform. The filtrate was evaporated under reduced pressure. The product was purified using column chromatography (silica gel–chloroform, Rf = 0.8). Yield 0.6 g (52.1%). ¹H NMR spectrum (CDCl₃, δ, ppm, J/Hz): 7.50–7.63 (m, 6H, Ph); 7.74 (d, 2H, C₅H₃N, J = 7.2); 7.83 (t, 1 H, C₅H₃N, J = 6.6); 8.27 (d, 4H, Ph, J = 7.2). The spectral characteristics are close to those in the literature [36].

2,6-Bis(4-diphenyl)pyridine. Gaseous hydrogen chloride was passed through a solution of 0.4 g (2 mmol) of 4-acetylbiphenyl and 20 mL (0.12 mol) of TEOF with constant stirring at room temperature for 3 h. A 60 mL volume of a saturated alcoholic solution of ammonia was added to the resulting mixture with vigorous stirring, and the resulting mixture was stirred for one hour. Then, the precipitate was filtered using a Schott filter and washed with methylene chloride. The filtrate was evaporated under reduced pressure. The product was separated using column chromatography (silica gel–chloroform, Rf = 0.7). Yield 0.16 g (43%). ¹H NMR spectrum (CDCl₃, δ, ppm, J/Hz): 7.41 (t, 2H, Ph, J = 7.3); 7.51 (t, 4 H, Ph, J = 7.3); 7.71 (d, 4H, Ph, J = 7.3); 7.74-7.82 (m, 6 H, C₆H₄ + C₅H₃N); 7.88 (t, 1 H, C₅H₃N, J = 7.0); 8.28 (d, 4H, C₆H₄, J = 8.5). MS, m/z (I (%)): 191 (11), 382 (27), 383 (100).

General procedure for the synthesis of polymers. Hydrogen chloride was bubbled through a solution of 5 mmol of the monomer in 55 mL of a mixture of TEOF and toluene (see

Table 1. Synthesis conditions and properties of polymers. 1. Monomer—5 mmol, reaction medium (TEOF + toluene)—55 mL, HCl gas, 24 h; 2. NH₃, 65 h.

Synthesis Conditions				Properties of the Polymers
№	Monomer	The Ratio of TEOF and a Monomer mol/mol	Percentage of TEOF in the Initial Mixture, TEOF-Toluene, %	Polymer	C, %	H, %	N, %	Yield, %
1	8a	2.4	3.6	9a	91.84	5.20	0	70
2	8a	3.6	5.5	10a	91.09	5.27	0.99	86
3	8a	7	11	10a	90.26	5.08	2.2	78
4	8a	12	18	10a	88.72	4.94	2.7	68
5	8a	16.8	25	10a	88.02	5.08	3.96	60
6	8a	33	50	10a	87.06	4.96	3.86	54
7	8b	2.4	3.6	9b	92.34	5.37	0	76
8	8b	3.6	5.5	10b	91.76	5.29	0.76	88
9	8b	7	11	10b	91.06	5.29	1.26	84
10	8b	12	18	10b	90.14	5.17	2.22	83
11	8b	16.8	25	10b	89.71	5.17	2.25	67
12	8b	33	50	10b	89.67	5.05	1.87	62

) at room temperature and stirring for 6 h. Then, the reaction mass was kept in a closed flask for 18 h, after which it was placed under stirring in 200 mL of a saturated solution of ammonia in chloroform. The mixture was kept in a closed flask for 65 h. The gel was filtered off, washed with chloroform, ethanol, and hot water and again with ethanol, and dried under reduced pressure. The heat treatment of the gels was carried out at 450 °C in an argon atmosphere for 3 h.

3. Results and Discussion

Previously, using a model reaction of the interaction of acetophenone and triethylorthoformate (TEOF) in the presence of an acid catalyst, we studied the conditions for the formation of compounds that are potential polymer-forming fragments [37]. The latter were phentriyl (1,3,5-trisubstituted benzene ring), phenylene, and disubstituted pyrylium salt, which, after treatment with ammonia, was converted into substituted pyridine. It was found that the formation of one or another fragment depends on the ratio of toluene, as a solvent, and TEOF. Depending on the reaction conditions, either phentriyl or pyridine polymer-forming fragments were dominant. In the case when toluene prevailed in the reaction sphere compared to TEOF, the main product was the compound with the phentriyl fragment. When using small amounts of toluene or in its absence, the main product was the pyrylium salt, which was transformed into a substituted pyridine. Thus, on the basis of acetophenone, it would seem that under similar conditions, various products are formed, and this, in our opinion, can be explained by different reaction mechanisms.

The first act of the reaction is the formation of a diethoxycarbonium cation (1) under the action of an acid catalyst on TEOF (Scheme 1).

The ketone in a nonpolar solvent is mainly in the keto form (2), which determines the corresponding mechanism for the preparation of 1,3,5-tris(phenyl)benzene (TPB). Under these conditions, the diethoxycarbonium cation, which has a strong alkylating effect, O-ethylates molecule 2, converting it into a carbocation (3), which can be reversibly converted into α-ethoxystyrene (4). Next, 3 reacts with 4 at the methylene carbon of the vinyl ether, which is polarized due to the presence of ether oxygen in the vicinity of the double bond. The intramolecular elimination of ethanol leads to the formation of a more stable form of the carbocation, stabilized by conjugation. Then, another molecule of α-ethoxystyrene is added, which leads to a trimeric carbocation, which, after the loss of an ethanol molecule, forms an intermediate complex. The latter, losing a proton and one more ethanol molecule, turns into TPB.

The presence of an acid catalyst, especially in molar amounts, intensifies the process of ketoenol tautomerism (Scheme 2). When the reaction is carried out in an TEOF medium, the enol form of the ketone is stabilized by hydrogen bonding with TEOF molecules.

Therefore, the interaction of the enol form of the ketone with TEOF leads to the formation of the pyrylium salt. The enol reacts with the diethoxycarbonium cation to produce the corresponding carbocation. The latter in two stages turns into another carbocation, which reacts with another enol molecule. The resulting carbocation, losing an alcohol molecule, forms an intermediate complex, which gives the pyrylium salt.

To facilitate the interpretation of the spectral data of the polymers, the compounds that model polymer chain fragments, i.e., 2,6-bisphenylpyridine (6a), 2,6-bis(4-diphenyl)pyridine (6b), 1,3,5-tris(phenyl)benzene (7a), and 1,3,5-tris(4-diphenyl)benzene (7b), were synthesized according to Scheme 3.

Trimerization cyclocondensation of 4-acetylbiphenyl (Scheme 4) produced a model compound, 1,3,5-tris(4-diphenyl)benzene (7b), a compound simulating the branching center of the polyphenylene chain. The NMR spectrum of the product (Figure 1) shows a singlet at 7.97 ppm, which is responsible for the protons of the 1,3,5-trisubstituted benzene ring, and there is no signal from the protons of the acetyl group. In addition, the proton signals of p-substituted benzene rings in the product and the starting compound differ in chemical shifts.

In the ¹H NMR spectrum of 2,6-bis(4-diphenyl)pyridine (6b) (Figure 2), compared to the spectrum of 1,3,5-tris(4-diphenyl)benzene (Figure 1), instead of a singlet from protons 1,3,5-trisubstituted benzene ring, there is a triplet signal of the pyridine ring protons at 7.88 ppm.

In addition, there is an overlap of doublet signals of the pyridine ring protons and the protons of the phenylene fragments located in the m-position to the pyridine ring (7.89 ppm). The signal of the protons of the phenylene fragments located in the o-position to the pyridine ring is significantly shifted to a weaker field (8.27 ppm).

When comparing the IR spectra of compounds 6a and 7a, it becomes obvious that the spectrum of the disubstituted pyridine-containing model compound has two intense absorption bands that are absent in the spectrum of the phenylene model (Figure 3).

These bands are located in the region of 818 cm⁻¹ and 1565 cm⁻¹ and refer to C-H bending vibrations and C=C and C=N stretching vibrations of the pyridine ring, respectively. When comparing the spectra of compounds 6b and 7b (Figure 4), a similar picture is observed, and the absorption band of the bending vibrations of C-H of the pyridine ring (for 6b) shifts by 11 cm⁻¹ towards lower frequencies up to 807 cm⁻¹ due to more effective conjugation in elongated substituent chain.

The band of stretching vibrations C=C and C=N groups of the pyridine ring at 1565 cm⁻¹ looks like a shoulder of the absorption band at 1579 cm⁻¹, which refers to planar stretching vibrations C=C of conjugated aromatic systems.

The polymers were obtained by the reaction of diacetylaromatic compounds (p-diacetylbenzene (8a) or 4,4’-diacetylbiphenyl (8b)) and triethylorthoformate (TEOF) in the presence of toluene as a nonpolar solvent and gaseous HCl as a catalyst (Scheme 4). Moreover, the concentration of monomers remained constant in the TEOF/toluene mixture and amounted to 0.1 mol/l, while the synthesis temperature and synthesis time were also the identical. The synthesized polymers were insoluble in organic solvents.

When comparing the IR spectra of polymers based on monomer 8a, obtained at different ratios of TEOF and toluene (Figure 5), it can be seen that as the proportion of TEOF in the initial mixture of solvents increases, the intensity of the absorption bands at 1565 and 803 cm⁻¹ (after treatment of polymers with ammonia) also increases.

For the absorption band at 803 cm⁻¹, the tendency to shift towards lower frequencies is preserved due to the conjugation effect when the substituent chain is extended [38]. A similar trend is observed in the IR spectra of polymers based on monomer 8b. The synthesized polymers contained residual terminal acetyl groups (absorption bands at 1680 cm⁻¹ and 1270 cm⁻¹) and fragments of dimerization condensation of acetyl groups (absorption band at 1655 cm⁻¹).

In order to increase the conversion of reactive groups and fragments, as well as to increase the degree of cyclocondensation, by the analogy with previously obtained polyphenylenes of various structures [39,40,41,42,43], the synthesized polymers were thermally treated in argon at 450 °C.

Synthesis conditions and polymers properties after heat treatment at 450 °C are presented in Table 1.

The yields of polyphenylenes 9a and 9b, obtained at a low concentration of TEOF, are lower than those of the other polyphenylenepyridines 10a and 10b. At a higher concentration of TEOF, when its proportion in the TEOF/toluene mixture was 25% (experiments 5 and 11) and 50% (experiments 6 and 12), a decrease in the yield of polymers is also observed. A similar situation was observed during the model condensation reaction of TEOF and acetophenone [37] at the same TEOF concentrations. Apparently, with an increase in the concentration of TEOF, the proportion of the keto form and, accordingly, its derivatives 3 and 4 increases. In this case, the rate of the TPB formation reaction is significantly reduced. For this reason, in the case of obtaining polymers, the reaction time for some polymers turned out to be insufficient, and therefore, the yield of the gel fraction decreased.

The elemental analysis data confirm the trend that, with an increase in the percentage of TEOF in the reaction mixture, the amount of nitrogen in the polymer increases, and, consequently, of pyridine fragments too. Thus, when the percentage of TEOF was 3.6% (experiments 1 and 7), nitrogen was not found in the polymers at all. The amount of nitrogen in the final polymers reached its maximum values at the TEOF amount of 25% (experiments 5 and 11).

The presence of pyridine rings in polymer was also confirmed using X-ray photoelectron spectroscopy. Figure 6 shows the N 1s photoelectron spectrum of polymer 10a (experiment 5). The binding energy of 398.7 eV corresponds to pyridine-like nitrogen [44].

When studying the thermal characteristics, the 10a (5) sample showed high thermal stability—the weight loss in the range of 200–900 °C amounted to about 15% (Figure 7). At the same time, in the region of 600–700 °C, a significant endo effect is detected on the thermogram, apparently associated with a partial structural rearrangement of a material [43].

According to elemental analysis data, the synthesized polyphenylenepyridines contain several percentages of oxygen atoms (Table 1), the presence of which, apparently, is associated not only with the presence of residual terminal acetyl groups, but also with side reactions of the triethylorthoformate with delocalized positive intermediates (intermediate 3 and similar ones, Scheme 1 and Scheme 2). In this case, after thermolysis at 450 °C, preconditions are created for the formation of phenolic fragments, which is also confirmed using EPR spectroscopy data (Figure 8).

The EPR spectra of samples 10a and 10b (exp.5 and exp.10, respectively) show symmetrical singlets of the Lorentzian line shape with widths of 6.5 G and 7.2 G, respectively, and a g-factor of 2.0043 ± 0.0002 in both cases. The magnetic resonance parameters of these signals are close to the values characteristic of radicals of the semiquinone structure [45]. It is noteworthy that for the polymer based on the monomer 4,4’-diacetylbiphenyl (sample 10, 6.1 × 1016 spin/g), a 24.4 times higher concentration of paramagnetic centers is observed compared to the polymer sample synthesized from p-diacetylbenzene (sample 5, 2.5 × 1015 spin/g). Thus, the extension of the conjugated system leads to the stabilization of semiquinone radicals formed from phenolic hydroxyls. On the other hand, the obtained results indicate a weakening of the effect of conjugation length in 1,3,5-triarylbenzene fragments, which opens up possibilities for controlling the properties of the resulting polymers over a wide range, changing the number of para-conjugated rings, and varying the substituents contained in the bridging groups X (Scheme 4).

4. Conclusions

Acid-catalyzed polycondensation of diacetylaromatic compounds with triethylorthoformate followed by ammonolysis yielded a number of cross-linked polyphenylenepyridines. It is shown that the reaction proceeds with the intermediate formation of pyrylium rings, which, after interaction with ammonia, form pyridine fragments in macromolecules. The formation of the pyridine ring and phentriyl fragments was confirmed by the data of ¹H NMR spectroscopy with the participation of low molecular weight model compounds.

The reaction mechanism is considered, and optimal conditions for the maximum formation of pyridine rings are found. It has been established that to achieve the maximum yield of cross-linked polyphenylene pyridines, at least an eight-fold molar excess of triethylorthoformate is required in terms of reactive acetyl groups.

It was found that after heating the samples at a temperature of 450 °C, a more complete polycondensation process occurs, and the synthesized polymers have high thermal stability in an inert atmosphere. The structure of the obtained cross-linked polyphenylene pyridines was confirmed using elemental analysis, IR, and XPS spectroscopy.

According to the data of EPR spectroscopy and elemental analysis, the probable formation of a small amount of phenolic hydroxyls as a result of side reactions accompanying cyclopolycondensation was shown. It was found that the conjugation efficiency in the chain increases when phenyl fragments are replaced by biphenyl ones in the bridging groups, and the phentriyl fragments decrease the length of conjugation.

Thus, a simple and technological approach for the synthesis of cross-linked pyridine-containing polyconjugated systems of various architectures, which are of interest as components of composite polymeric materials for electrical purposes, has been proposed.