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Article

Synthesis and Redox Activity of Polyenaminones for Sustainable Energy Storage Applications

by
Tomaž Kotnik
1,
Svit Menart
1,2,
Žan Adam
1,2,
Jan Bitenc
2,
Luka Ciber
1,
Uroš Grošelj
1,
Nejc Petek
1,
Bogdan Štefane
1,
Jurij Svete
1,* and
Boštjan Genorio
1,*
1
University of Ljubljana, Faculty of Chemistry and Chemical Technology, Večna pot 113, 1000 Ljubljana, Slovenia
2
National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia
*
Authors to whom correspondence should be addressed.
Polymers 2024, 16(19), 2700; https://doi.org/10.3390/polym16192700
Submission received: 22 July 2024 / Revised: 9 September 2024 / Accepted: 18 September 2024 / Published: 24 September 2024
(This article belongs to the Special Issue Utilizing Polymers for the Construction of Lithium-Ion Battery)
Browse Figures
Versions Notes

Abstract

:
In the search for novel polymeric molecules that could be used as electroactive materials, seven novel polyenaminones were prepared in high yields by the transaminative polymerization of resorcinol-derived bis-enaminones with m- and p-phenylenediamine and with 2,5-diaminohydroquinone. The obtained polymers show very low solubility in organic solvents and absorb UV light and visible light at wavelengths below 500 nm. All the obtained polymeric products were tested for redox activity in a Li battery setup. The 2,5-diaminohydroquinone-derived compound showed the best redox activity, with a maximum capacity of 86 mAh/g and relatively good capacity retention, thus confirming the hydroquinone group as the primary redox-active group. Other potential redox-active groups, such as resorcinol and conjugated carbonyls, showed limited activity, while variations in the phenylene groups and the substitution of phenolic groups in the resorcinol residue did not impact the electrochemical activity of the polymers. Their electrochemical properties, together with their previously established chemical recyclability, make polyenaminones promising scaffolds for the development of materials for sustainable energy storage applications.

1. Introduction

The development of synthetic polymers began relatively late on the chemistry timeline, with the first reported examples at the end of 19th century [1,2]. Considering this, it is fascinating that in such a brief time, modern life without synthetic polymers became unimaginable. As human civilization progresses rapidly, novel materials with specific properties are constantly needed to meet the requirements of advanced technologies [1,2]. Because of the growing demands for novel materials and to prevent future ecological crises, great efforts are put into development of novel synthesis routes for degradable polymers with desired properties [3,4] that are compliant with the requirements of sustainable development [5,6].
Carbon-based energy commodities (coal, natural gas, and petroleum) are the basis for producing most of the world’s energy today. Although various policies advocate energy from renewable sources, the use of renewable energy is still quite limited today [7,8]. In this regard, the scientific community is developing renewable-energy-based technologies and technologies that reduce the impact of greenhouse gases on climate change. Energy storage systems and, in particular, Li-ion batteries, are being accepted worldwide as alternatives to reduce the carbon footprint. However, the drawbacks of redox-active inorganic materials used in batteries are mainly related to the use of critical raw materials such as cobalt, lithium, nickel, graphite, etc. Numerous studies suggest that inorganic redox-active materials could be replaced by renewable organic materials. Today, organic semiconductors, and especially redox-active materials, are the basis for the transition to sustainable renewable energy sources [9,10,11]. Among different redox-active organic molecules, quinone (and related structural elements) or its reduced form, hydroquinone, offer interesting possibilities due to their high capacity and redox potential [12,13,14,15].
Enaminones are vinylogous amides in which a C=C double bond is inserted into a C–N bond of the amide [16], which are used as versatile difunctional reagents in organic synthesis [17,18,19,20]. Recently, there has been a notable increase in research interest in the enaminone motif in the field of materials science, particularly for the synthesis of covalent organic frameworks (COF)s [21]. Enaminone-linked COFs were employed for photocatalytic applications, methane storage, and chiral induction. However, the polymeric analogs, i.e., polyenaminones, also known as β-keto enamine-linked conjugated polymers, have been little studied [22]. Polyenaminones are generally available by polycondensation between diamines and various dielectrophiles, such as 1,3-dicarbonyls and analogues [23,24,25,26,27,28,29], diynones [30,31,32], and bis(N,N-disubstituted) enaminones [33,34,35,36]. Being conjugated enaminone push–pull systems, polyenaminones absorb UV and visible light. They also exhibit redox activity [36] and film-forming properties [34], and they are degradable and recyclable under mild conditions [29,35].
In the continuation of our research on polyenaminones [34,35,36], we recently focused our attention on redox-active polyenaminones that could be useful in energy storage and conversion systems. After initial encouraging results on cyclohexanedione-based polyenaminones [36], the studies were extended to resorcinol- and hydroquinone-based polyenaminones that could potentially improve the stability and increase the capacity of cathode materials for Li-ion and “beyond lithium” battery technologies (Na, Mg, Al, Ca batteries). Herein, we report on the synthesis, optical properties, and redox activity of a new family of polyenaminones, which are easily accessible from resorcinol and phenylenediamines (Figure 1).

2. Materials and Characterizations

2.1. Materials

All solvents and reagents were used as received. Resorcinol (1, ≥99.0%), m-phenylenediamine dihydrochloride (4a) (≥99.0%), p-phenylenediamine dihydrochloride (4b) (≥99.0%), 2,5-diaminohydroquinone dihydrochloride (4c) acetic anhydride (≥98.0%), zinc chloride (≥99.0%), potassium carbonate (99%), methyl iodide (99.5%), benzyl bromide (98%), 1-butyl bromide (99%), and N,N-dimethylformamide dimethylacetal (DMFDMA, for synthesis, ≥96%) are commercially available from Sigma-Aldrich (St. Louis, MO, USA).
Melting points (mps) were determined on a Kofler micro hot stage and on a Mettler Toledo MP30 automated melting point system (Mettler Toledo, Columbus, OH, USA). The NMR spectra were recorded in DMSO-d6 as deuterated solvent using Me4Si as the internal standard on Bruker Avance DPX 300, Bruker Avance III UltraShield 500, and Bruker Avance Neo 600 instruments (Bruker, Billerica, MA, USA) at 300, 500, and 600 MHz for 1H and at 75.5, 126, and 151 MHz for 13C nucleus, respectively. Chemical shifts (δ) are given in ppm relative to Me4Si as internal standard (δ = 0 ppm) and vicinal coupling constants (J) are given in hertz (Hz). UV-vis spectra were recorded in MeOH using a Varian Cary Bio50 UV-Visible Spectrophotometer (Agilent Technologies, Santa Clara, CA, USA). Fourier-transform infrared (FT-IR) spectra were obtained on a Bruker FTIR Alpha Platinum spectrophotometer (Bruker, Billerica, MA, USA) using attenuated total reflection (ATR) sampling technique. HRMS spectra were recorded on an Agilent 6224 time-of-flight (TOF) mass spectrometer equipped with a double orthogonal electrospray source under atmospheric pressure ionization (ESI) coupled to an Agilent 1260 high-performance liquid chromatograph (HPLC) (Agilent Technologies, Santa Clara, CA, USA). Microanalyses for C, H, and N were obtained on a Perkin-Elmer CHNS/O Analyzer 2400 Series II (PerkinElmer, Waltham, MA, USA).
Scanning electron microscopy (SEM) was used to investigate the morphological and topological features of the materials. Micrographs were taken on a Zeiss ULTRA plus SEM (Zeiss, Jena, Germany) instrument. Samples were adhered to the conductive carbon tape placed on the aluminum SEM holder and Au/Pd (80/20) coated, with a nominal thickness of 20 nm, using a Qurum Q150T ES turbomolecular pumped coater (AGC, Charleroi, Belgium). SEM images were taken at 2 kV using SE2 detector at WD 5.5 mm. SEM images were analyzed for particle size distribution with AxioVision 4.8 software (Informer Technologies, Inc., Los Angeles, CA, USA). Nitrogen physisorption isotherms were obtained on an ASAP 2020 Micromeritics instrument. Specific surface areas (SSA)s were determined using the Brunauer–Emmett–Teller theory. Samples were degassed under vacuum at 30 °C for 1 h. Organic cathodes were prepared by mixing polymers with Printex XE2 carbon black and PTFE binder (60 wt.% water dispersion) in a 60:30:10 weight ratio. All of the components and isopropanol (Sigma-Aldrich, St. Louis, MO, USA) were added into a stainless steel jar and homogenized for 30 min on a Retsch PM100 (Retsch, Haan, Germany) ball mill at 300 rpm. Composite matrices were then rolled in between a glass plate and a sheet of baking paper to give self-standing electrodes. Subsequently, 12 mm self-standing electrodes were cut, dried, and transferred into the Ar-filled glovebox. The average loading on the electrode was around 2.5 mg of active material per cm2. Battery cells were assembled in an argon-filled glovebox with water and oxygen levels below 1 ppm. Swagelok-type battery cells were assembled using the aforementioned electrodes, a 13 mm glass fiber separator (Whatman GF/A, Whatman plc, Maidstone, Kent, UK), and freshly rolled lithium (12 mm diameter, Sigma-Aldrich, St. Louis, MO, USA). The electrolyte used was a 1 M solution of bis(trifluoromethane)sulfonimide lithium salt in a mixture of dry 1,3-dioxolane and dimethoxyethane (1 M LiTFSI in DOL + DME, Sigma-Aldrich, St. Louis, MO, USA). Electrochemical measurements were performed at room temperature (25 °C) using a potentiostat/galvanostat VMP3 (Bio-Logic, Seyssinet-Pariset, France). The batteries were galvanostatically cycled between 1.5 and 3.5 V vs. Li/Li+ at a current density of 50 mA/g based on the mass of the tested polymer.

2.2. Synthesis of 4,6-Diacetylresorcinol (2a) [37,38]

This compound was prepared according to a slightly modified procedure from the literature [37]. A mixture of resorcinol (1) (10 g, 91 mmol), acetanhydride (37 mL, 182 mmol), and ZnCl2 (10 g, 73 mmol) was placed in a 250 mL flask, equipped with a reflux condenser. The mixture was heated at 140 °C for 30 min, and then cooled to room temperature. Water (20 mL) was added slowly (exothermic reaction) through a reflux condenser and the reaction mixture was heated to boiling point. Next, ethanol was added until complete dissolution of the precipitate (approx. 4 mL). The reaction mixture was cooled to room temperature and the precipitate was collected by filtration to give 2a. Yield: 30.5 g (88%) of white solid; mp = 179–181 °C, mp = 178–179 °C [37], mp = 182 °C [38]. FT-IR (ATR): νmax 3407, 3218, 3085, 1621 (C=O), 1488, 1336, 1254, 1046, 951, 833, 660 cm−1. 1H NMR (500 MHz; CDCl3): δ 2.63 (6H, s), 6.43 (1H, s), 8.21 (1H, s), 12.93 (2H, s). 13C NMR (126 MHz; CDCl3): δ 202.5, 169.0, 136.3, 113.7, 105.0, 26.1. m/z (HRMS): 195.0651 (MH+). C10H11O4 requires m/z 195.0652. Spectral data are in agreement with the literature data [37].

2.3. General Procedure for the Synthesis of Dialkylated 4,6-Diacetylresorcinols 2b and 2c [39]

Compounds 2b and 2c were prepared according to a modified procedure from the literature [39]. While cooling in an ice-bath, alkyl halide (20 mmol) was slowly (portion-wise) added to a mixture of diketone 2a (1.94 g, 10 mmol), anh. DMF (20 mL), and K2CO3 (2.76 g, 20 mmol) and the mixture was stirred at 20–80 °C for 5–24 h. The precipitated product was collected by filtration to give 2b and 2c. The compounds described below were prepared in this manner.

2.3.1. 1,1′-(4,6-Dibutoxy-1,3-phenylene)bis(ethan-1-one) (2b)

Prepared from 2a (780 mg, 4 mmol), butyl bromide (1.2 mL, 1.5 g, 11 mmol), and K2CO3 (3 g, 22 mmol), 80 °C, 24 h. Yield: 725 mg (72%) of brownish solid; mp = 85–88 °C. FT-IR (ATR): νmax 2956, 2870, 1663 (C=O), 1581, 1459, 1355, 1277, 1224, 1192, 1061, 1020, 956, 817 cm−1. 1H NMR (500 MHz; CDCl3): δ 1.00 (6H, t, J = 7.4 Hz), 1.54 (4H, br quintet, J = 7.4 Hz), 1.87 (4H, br quintet, J = 7.0 Hz), 2.57 (6H, s), 4.10 (4H, t, J = 6.4 Hz), 6.39 (1H, s), 8.33 (1H, s). 13C NMR (126 MHz; CDCl3): δ 13.9, 19.5, 31.2, 31.9, 68.9, 95.9, 120.9, 134.9, 163.3, 197.3. m/z (HRMS): 307.1902 (MH+). C18H27O4 requires m/z 307.1904.

2.3.2. 1,1′-(4,6-Dibenzyloxy-1,3-phenylene)bis(ethan-1-one) (2c) [39]

Prepared from 2a (2.0 g, 10.4 mmol), benzyl bromide (2.4 mL, 3.55 g, 21 mmol), and K2CO3 (2.875 g, 21 mmol), 20 °C, 5 h. Yield: 3.12 mg (81%) of light brownish solid; mp = 124–126 °C, mp = 126–127 °C [39]. FT-IR (ATR): νmax 3061, 3003, 2996, 1656 (C=O), 1579, 1558, 1500, 1427, 1387, 1353, 1274, 1223,1188, 1055, 1005, 961, 822, 735, 695,605 cm−1. 1H NMR (500 MHz; CDCl3): δ 2.54 (6H, s), 5.15 (4H, s), 6.54 (1H, s), 7.37–7.41 (10H, m), 8.34 (1H, s). 13C NMR (126 MHz; CDCl3): δ 31.9, 71.2, 97.8, 121.6, 127.6, 128.7, 129.0, 134.9, 135.5, 162.5, 197.2. m/z (HRMS): 375.1586 (MH+). C24H23O4 requires m/z 375.1591.

2.4. General Procedure for the Synthesis of Bis-Enaminones 3a–c

Enaminones 3a–c were prepared following slightly modified procedure from the literature for the synthesis of compound 3a [40]. A mixture of diketone 2 (10 mmol), anh. xylene (30 mL), and DMFDMA (3.5 mL, 25 mmol) was heated under reflux for 24 h. The reaction mixture was cooled to 0 °C (ice bath) and the precipitate was collected by filtration to give 3. The compounds described below were prepared in this manner.

2.4.1. (2E,2′E)-1,1′-(4,6-dihydroxy-1,3-phenylene)bis [3-(dimethylamino)prop-2-en-1-one] (3a) [40]

Prepared from 2a (4.1 g, 21 mmol) and DMFDMA (6.1 mL, 44 mmol). Yield: 5.20 g (82%) of brown solid; mp = 196–199 °C, mp = 198–200 °C [40]. FT-IR (ATR): νmax 2902, 2805, 1615 (C=O), 1528, 1432, 1366, 1279, 1187, 1104, 891, 785, 712, 602 cm−1. 1H NMR (600 MHz; CDCl3): δ 2.95 and 3.16 (12H, 2 br s, 1:1), 5.63 (2H, d, J = 12.1 Hz), 6.35 (1H, s), 7.82 (2H, d, J = 12.1 Hz), 8.04 (1H, s), 14.73 (2H, s). 13C NMR (151 MHz, CDCl3) δ 190.5, 168.6, 154.5, 130.4, 113.1, 104.6, 89.3, 45.5, 37.4. m/z (HRMS): 305.1493 (MH+). C16H21N2O4 requires m/z 305.1496. Spectral data are in agreement with the literature data [40].

2.4.2. (2E,2′E)-1,1′-(4,6-dibutoxy-1,3-phenylene)bis [3-(dimethylamino)prop-2-en-1-one] (3b)

Prepared from 2b (602 mg, 2.4 mmol) and DMFDMA (0.8 mL, 6 mmol). Yield: 800 mg (80%) of beige solid; mp = 145–147 °C. FT-IR (ATR): νmax 2947, 2868, 1637 (C=O), 1589, 1534, 1414, 1342, 1257, 1226, 1163, 1096, 1058, 1023, 968, 810, 782, 748, 666, 609 cm−1. 1H NMR (300 MHz; CDCl3): δ 0.96 (6H, t, J = 7.4 Hz), 1.51 (4H, br sextet, J = 7.3 Hz), 1.79 (4H, br quintet, J = 7.0 Hz), 2.95 (12H, br s), 4.03 (4H, t, J = 6.4 Hz), 5.67 (2H, d, J = 12.7 Hz), 6.42 (1H, s), 7.56 (2H, d, J = 12.7 Hz), 7.90 (1H, s). 13C NMR (75.5 MHz; DMSO-d6): δ 13.9, 19.4, 31.3, 45.0, 68.6, 97.4, 98.2, 123.4, 126.0, 132.3, 153.9, 159.6 (found: C, 68.81; H, 8.62; N, 6.27. C24H36N2O4·¼H2O requires C, 68.46; H, 8.74; N, 6.65%). m/z (HRMS): 417.2744 (MH+). C24H37N2O4 requires m/z 417.2748.

2.4.3. (2E,2′E)-1,1′-(4,6-dibenzyloxy-1,3-phenylene)bis [3-(dimethylamino)prop-2-en-1-one] (3c)

Prepared from 2c (1.88 g, 5 mmol) and DMFDMA (1.75 mL, 11.7 mmol). Yield: 2.06 g (84%) of beige solid; mp = 171–174 °C. FT-IR (ATR): νmax 3484, 2938, 2877, 1635 (C=O), 1587, 1532, 1418, 1349, 1251, 1169, 1098, 989, 909, 845, 793, 730, 698, 658, 625 cm−1. 1H NMR (300 MHz; CDCl3): δ 2.72 and 2.98 (12H, 2s, 1:1), 5.09 (4H, s), 5.65 (2H, d, J = 12.7 Hz), 7.28–7.45 (10H, m), 7.54 (2H, d, J = 12.4 Hz), 7.95 (1H, s). 13C NMR (126 MHz; CDCl3): δ 35.0, 37.2, 45.0, 71.0, 98.2, 99.4, 124.3, 127.6, 128.1, 128.7, 132.4, 154.4, 159.0. m/z (HRMS): 485.2430 (MH+). C30H33N2O4 requires m/z 485.2435.

2.5. Synthesis of Compounds 5aa–5ac, 5ba, 5bb, 5ca, and 5cb

General Procedure. A mixture of bis-enaminone monomer 3 (1 mmol), diamine dihydrochloride 4 (1 mmol), and methanol (10 mL) was shaken on an orbital shaker at room temperature for 24 h. The precipitate was collected by filtration, washed with methanol (2 mL), and air-dried to give 5. The compounds described below were prepared in this manner.

2.5.1. Poly{(EZ)-3-{[3-(λ2-azaneyl)phenyl]amino}-1-[2,4-dihydroxy-5-(3λ3-prop-2-enoyl)phenyl]prop-2-en-1-one} (5aa)

Prepared from 3a (305 mg, 1 mmol) and 1,3-phenylenediamine dihydrochloride (4a) (182 mg, 1 mmol). Yield: 306 mg (95%) of dark ochre solid. FT-IR (ATR): νmax 3067, 1628 (C=O), 1593, 1538, 1481, 1267, 1183, 1161, 993, 838, 713 cm−1. 1H NMR (500 MHz; DMSO-d6): δ 6.02–8.65 (10H, m), 10.39–10.74 and 11.38–11.63 (2H, 2m, 2:1), 13.67–15.71 (2H, m). 13C NMR (126 MHz; DMSO-d6): δ 190.4, 190.1, 189.8, 175.6, 166.4, 159.4, 157.8, 156.8, 147.1, 146.5, 141.9, 141.4, 131.3, 130.3, 126.6, 124.0, 119.2, 116.6, 116.6, 113.2, 112.2, 112.0, 112.0, 104.6, 39.5 (multiple peaks for each type of carbon nuclei are due to slow isomerization around the C=C double bonds). Found: C, 62.59; H, 4.39; N, 7.70. C18H14N2O4·⅔HCl requires C, 62.37; H, 4.27; N, 8.08%. λmax (MeOH)/nm 420.

2.5.2. Poly{(EZ)-3-{[4-(λ2-azaneyl)phenyl]amino}-1-[2,4-dihydroxy-5-(3λ3-prop-2-enoyl)phenyl]prop-2-en-1-one} (5ab)

Prepared from 3a (305 mg, 1 mmol) and 1,4-phenylenediamine dihydrochloride (4b) (182 mg, 1 mmol). Yield: 322 mg (100%) of red-brown solid. FT-IR (ATR): νmax 3233, 1605 (C=O), 1536, 1485, 1267, 1178, 817, 714 cm−1. 1H NMR (300 MHz; DMSO-d6): δ 5.98–8.52 (10H, m), 10.21–10.80 and 11.53–11.73 (2H, 2m, 3:2), 13.83–15.83 (2H, m). 13C NMR (126 MHz; DMSO-d6): δ 39.5, 104.6, 106.7, 112.0, 117.6, 119.2, 125.4, 126.6, 126.9, 133.0, 133.4, 140.8, 145.6, 159.3, 166.4, 169.1, 175.6 (multiple peaks for each type of carbon nuclei are due to slow isomerization around the C=C double bonds). Found: C, 63.20; H, 4.34; N, 7.38. C18H14N2O4·½HCl requires C, 63.49; H, 4.29; N, 8.24%. λmax (MeOH)/nm 455.

2.5.3. Poly{(EZ)-3-{[4-(λ2-azaneyl)-2,5-dihydroxyphenyl]amino}-1-[2,4-dihydroxy-5-(3λ3-prop-2-enoyl)phenyl]prop-2-en-1-one} (5ac)

Prepared from 3a (76 mg, 0.25 mmol) and 2,5-diaminohydroquinone dihydrochloride (4c) (53 mg, 0.25 mmol). Yield: 96 mg (100%) of red-brown solid. FT-IR (ATR): νmax 3344, 3153, 1614 (C=O), 1526, 1493, 1364, 1269, 1177, 1105, 839, 784, 711 cm−1. 1H NMR (500 MHz; DMSO-d6): δ 6.00–6.47 (2H, m), 6.80–7.25 (3H, m), 7.54–8.51 (5H, m), 9.62–10.20 (2H, m), 11.56–11.90 (1H, m), 13.16–14.38 (1H, m), 14.98–15.80 (1H, m). 13C NMR (151 MHz; DMSO-d6): δ 189.2, 178.4, 175.9, 168.0, 159.2, 157.8, 156.7, 155.4, 153.6, 153.3, 151.3, 130.5, 126.6, 124.0, 112.3, 112.2, 112.0, 107.5, 104.7, 103.5, 101.1, 94.6, 92.9, 89.0 (multiple peaks for each type of carbon nuclei are due to slow isomerization around the C=C double bonds). Found: C, 57.41; H, 3.80; N, 7.13. C18H14N2O6·⅔HCl requires C, 57.10; H, 3.90; N, 7.40%. λmax (MeOH)/nm 340, 380.

2.5.4. Poly{(EZ)-3-{[3-(λ2-azaneyl)phenyl]amino}-1-[2,4-bis(butoxy)-5-(3λ3-prop-2-enoyl)phenyl]prop-2-en-1-one} (5ba)

Prepared from 3b (208 mg, 0.5 mmol) and 1,3-phenylenediamine dihydrochloride (4a) (91 mg, 0.5 mmol). Yield: 189 mg (88%) of ochre solid. FT-IR (ATR): νmax 2954, 2929, 2869, 1630 (C=O), 1576, 1458, 1253, 1177, 1156, 991, 785, 734, 681 cm−1. 1H NMR (500 MHz; DMSO-d6): δ 0.77–1.06 (6H, m), 1.17–1.37 (2H, m), 1.40–1.60 (3H, m), 1.68–1.90 (3H, m), 3.93–4.32 (4H, m), 6.09–6.47 (2H, m), 6.60–8.37 (10H, m), 9.81–9.97 and 11.83–12.01 (2H, 2m, 2:3). 13C NMR (126 MHz; DMSO-d6): δ 13.3, 13.7, 18.2, 18.8, 18.9, 30.6, 31.9, 68.4, 68.7, 77.2, 83.3, 95.8, 96.2, 97.8, 98.7, 106.7, 108.6, 108.9, 110.8, 119.5, 121.5, 130.9, 141.7, 141.9, 153.8, 157.0, 157.5, 157.9, 170.8, 178.3 (multiple peaks for each type of carbon nuclei are due to slow isomerization around the C=C double bonds). Found: C, 69.74; H, 7.13; N, 6.07. C26H30N2O4·⅔H2O requires C, 69.93; H, 7.07; N, 6.27%. λmax (MeOH)/nm 395.

2.5.5. Poly{(EZ)-3-{[4-(λ2-azaneyl)phenyl]amino}-1-[2,4-bis(butoxy)-5-(3λ3-prop-2-enoyl)phenyl]prop-2-en-1-one} (5bb)

Prepared from 3b (208 mg, 0.5 mmol) and 1,4-phenylenediamine dihydrochloride 4b (91 mg, 0.5 mmol). Yield: 199 mg (92%) of brown solid. FT-IR (ATR): νmax 2954, 2929, 2889, 1622 (C=O), 1584, 1520, 1462, 1351, 1253, 1164, 963, 814, 784, 685 cm−1. 1H NMR (500 MHz; DMSO-d6): δ 0.77–1.06 (6H, m), 1.17–1.37 (2H, m), 1.40–1.60 (3H, m), 1.68–1.90 (3H, m), 3.93–4.32 (4H, m), 6.09–6.47 (2H, m), 6.60–8.37 (10H, m), 9.81–9.97 and 11.83–12.01 (2H, 2m, 2:3). 13C NMR could not be recorded due to extremely low solubility of compound 5bb in DMSO-d6 (found: C, 70.03; H, 6.88; N, 6.22. C26H30N2O4·⅔H2O requires C, 69.93; H, 7.07; N, 6.27%). λmax (MeOH)/nm 425.

2.5.6. Poly{(EZ)-3-{[3-(λ2-azaneyl)phenyl]amino}-1-[2,4-bis(benzyloxy)-5-(3λ3-prop-2-enoyl)phenyl]prop-2-en-1-one} (5ca)

Prepared from 3c (480 mg, 1 mmol) and 1,3-phenylenediamine dihydrochloride 4a (182 mg, 1 mmol). Yield: 420 mg (84%) of brown solid. FT-IR (ATR): νmax 3337, 3029, 1629 (C=O), 1579, 1494, 1454, 1255, 1156, 997, 729, 693 cm−1. 1H NMR (500 MHz; DMSO-d6): δ 5.15–5.43 (4H, m), 6.10–6.44 (2H, m), 6.68–7.59 (14H, m), 7.72–8.91 (4H, m), 9.91–10.05 and 11.84–11.93 (2H, 2m, 1:1). 13C NMR (126 MHz; DMSO-d6): δ 39.5, 70.1, 70.4, 70.8, 98.7, 99.0, 99.5, 127.1, 127.3, 127.7, 127.9, 128.0, 128.1, 128.1, 128.5, 128.6, 128.6, 128.7, 136.3, 141.6, 144.2, 144.5, 161.0, 161.2, 161.6, 188.4 (multiple peaks for each type of carbon nuclei are due to slow isomerization around the C=C double bond). Found: C, 74.28; H, 5.08; N, 5.43. C32H26N2O4·¾H2O requires C, 74.48; H, 5.37; N, 5.43%. λmax (MeOH)/nm 395.

2.5.7. Poly{(EZ)-3-{[4-(λ2-azaneyl)phenyl]amino}-1-[2,4-bis(benzyloxy)-5-(3λ3-prop-2-enoyl)phenyl]prop-2-en-1-one} (5cb)

Prepared from 3c (480 mg, 1 mmol) and 1,4-phenylenediamine dihydrochloride 3b (182 mg, 1 mmol). Yield: 455 mg (91%) of ochre solid. FT-IR (ATR): νmax 3031, 2917, 2850, 1622 (C=O), 1585, 1522, 1468, 1349, 1257, 1201, 1161, 1109, 1003, 815, 878, 730, 695 cm−1. 1H NMR (500 MHz; DMSO-d6): δ 5.22–5.38 (4H, m), 5.66–5.73, 6.10–6.18, and 6.24–6.33 (2H, 3m, 19:47:34), 6.91–7.12 (2H, m); 7.21–8.21 (14H, m), 7.72–8.91 (4H, m), 9.84–10.01 and 11.97–12.05 (2H, 2m, 1:1). 13C NMR (126 MHz; DMSO-d6): δ 39.5, 70.1, 70.1, 70.4, 98.0, 98.2, 98.7, 99.1, 99.1, 117.4, 117.5, 117.6, 127.4, 127.7, 127.9, 128.0, 128.1, 128.5, 128.7, 135.9, 136.3, 136.4, 136.4, 136.5, 136.5, 136.7, 144.3, 144.4, 144.4, 158.9, 159.2, 159.3, 159.5, 159.9, 187.4, 187.9, 188.0, 188.2, 189.2 (multiple peaks for each type of carbon nuclei are due to slow isomerization around the C=C double bond). Found: C, 71.79; H, 5.69; N, 5.85. C32H26N2O4·¼Me2NH·¾H2O requires C, 72.13; H, 5.31; N, 5.82%. λmax (MeOH)/nm 425.

3. Results

3.1. Synthesis

The starting diketones 2a–c were prepared in 2–3 steps from resorcinol (1). The acetylation of 1 in the presence of anh. ZnCl2, following the procedure in the literature [37], gave 4,6-diacetylresorcinol (2a) in an 88% yield. The O-alkylation of 2a with 1-butyl bromide and benzyl bromide in the presence of K2CO3 gave the diketones 2b and 2c in 72% and 81% yields, respectively (Scheme 1, Table 1, Entries 1–3). Heating diketones 2a–c with 1.2 equiv. DMFDMA in xylene for 24 h then produced the respective bis-enaminone monomers 3a–c in 62–84% yields (Scheme 1, Table 1, Entries 4–6). The bis-enaminones 3a–c were then treated with equimolar amounts of phenylenediamines dihydrochlorides 4a–c in methanol at room temperature for 24 h and the precipitated products 5aa–5ac, 5ba, 5bb, 5ca, and 5cb were obtained by filtration in 84–100% yields (Scheme 1, Table 1, Entries 7–13). The reaction mechanism is explainable by step-growth polymerization through dimeric, trimeric, and oligomeric intermediates in the same manner as before for closely related transaminative polymerizations (Scheme 1) [20,34,35,36].

3.2. Characterization

The intermediates 2a–c and 3a–c and the title compounds 5aa, 5ab, 5ac, 5ba, 5bb, 5ca, and 5cb were characterized by spectroscopic methods (1H NMR, 13C NMR, FTIR, HRMS, and UV-vis) and by elemental analysis for C, H, and N.

3.2.1. Characterization by NMR

The 1H NMR and 13C NMR data for bis-enaminones 3a–c and polyenaminones 5aa–5ac, 5ba, 5bb, 5ca, and 5cb are in agreement with the data for closely related bis-enaminones and polyenaminones [34,35,36]. In the 1H NMR spectra of polyenaminones 5aa, 5ab, 5ba, 5ca, and 5cb, the cumulative relative intensity of signals within the δ chemical shift frame 6–9 ppm corresponds to a total of ten aromatic and enamine CH=CH protons, while the cumulative relative intensity of signals within the δ chemical shift frame 10–12 ppm correspond to two NH protons. In the 1H NMR spectra of products 5aa and 5ab, the signals within the δ chemical shift frame 13.5–15 ppm correspond to the phenolic protons. Finally, the 1H NMR spectra of the O-alkylated compounds 3b,c, 5ba, 5ca, and 5cb exhibit the expected signals for the alkyl residues. Unfortunately, characterization of compound 5bb by NMR was not possible due to its insolubility in DMSO-d6 and other organic solvents. For more details, see the Supporting Information.

3.2.2. Characterization by FTIR

The FTIR spectra of compounds 2 (~1660 cm−1), 3 (~1635 cm−1), and 5 (~1615 cm−1) show absorption bands in a region between 1600 cm−1 and 1660 cm−1, which are characteristic for the C=O group of a conjugated ketone. The absorption bands around 2900 cm−1 in the spectra of compounds 2, 3, and 5 are in line with typical C–H and N–H absorption bands [32,33,34,35,36]. The broad absorption bands around 3000 cm−1 in the spectra of compounds 5 are in agreement with the N–H···O=C hydrogen bonding of compound 5 in the solid state.

3.2.3. Characterization by SEM

The morphological properties (Figure 2) and particle size distribution (Figures S2–S7) of the title compounds 5aa, 5ab, 5ac, 5ba, 5bb, and 5cb were determined by scanning electron microscopy (SEM). Polymer 5aa consists of agglomerated irregular particles approximately 100 nm in size (Figure S2). Polymer 5ab exhibits a similar morphology, but with a wide particle size distribution, ranging from sub-100-nanometer particles to irregular chunks several micrometers in size (Figure S3). Polymer 5ac also has a broad size distribution, predominantly featuring sharp-edged particles between 1 and 5 μm (Figure S4). Notably, polymer 5ba consists of round-shaped merged structures approximately 10 μm and larger (Figure S5). Similarly shaped particles are observed in polymer 5cb, but with smaller sizes and a wider distribution (Figure S7). Polymer 5bb has the most porous structure, with agglomerates 5 μm and larger (Figure S6) consisting of interconnected small particles forming a highly porous network. For electrochemical applications, high-surface-area porous materials are desired to allow the electrolyte to access the polymer surfaces and utilize surface-redox-active functional groups. Polymers 5aa, 5ab, 5ac, 5ba, 5bb, and 5cb were tested for their specific surface area by N2 adsorption (BET experiment). The results revealed 68.3 m2/g, 3.8 m2/g, 0.1 m2/g, 1.4 m2/g, and 0.3 m2/g for 5aa, 5ab, 5ac, 5bb, and 5cb. The specific surface area of 5ba could not be measured due to its extremely low size. In this respect, materials 5aa, 5ab, and 5bb exhibit the most promising morphologies in terms of specific surface area.

3.2.4. Characterization by UV-Vis Spectroscopy

The UV-vis spectra of compounds 5 were measured in MeOH at 250–800 nm (Figure S1). The normalized UV-vis spectra of compounds 5 are shown in Figure 3. Compounds 5 showed absorption maxima between 338 nm (5ac ▬) and 455 nm (5ab ▬). The strongest absorption maxima of the resorcinol-phenylenediamine-derived polymers 5aa, 5ab, 5ba, 5bb, 5ca, and 5cb were at the borderline of visible light between 395 nm (5ba ▬, 5ca ▬) and 455 nm (5ab ▬). Compounds 5ab, 5bb, 5cb had para-substituted diamine moieties absorbed at wavelengths that were 30–40 nm longer (455, 425, and 425 nm, respectively) than those in compounds 5aa, 5ba, 5ca, which had meta-substituted diamine moieties (420, 395, and 395 nm, respectively) (Figure UV-vis). These absorption spectral data were consistent with the data for closely related polymers [34,35,36], thus indicating interesting UV-light-shielding properties that might find use in various optical applications.

3.2.5. Determination of Redox Activity

Polyenaminones 5aa, 5ab, 5ac, 5ba, 5bb, and 5cb were electrochemically characterized in a Li electrolyte to probe their redox activity, which is also the starting point for the development of other monovalent (Na, Li, K) and multivalent (Mg, Ca, Al) batteries [9]. Specifically, we aimed to elucidate the potential redox-active centers of 5ac, 5ab, and 5bb. Polymer 5ac has three possible redox-active centers: hydroquinone, resorcinol, and conjugated carbonyl. In contrast, 5ab contains only resorcinol and conjugated carbonyl, while 5bb has only conjugated carbonyl. Despite these differences, the basic backbone structure of the polymers is quite similar.
The redox activity of all the materials was investigated in a Li-battery setup, with the polymers serving as cathode materials and Li metal as the anode. From the galvanostatic tests, the capacity versus the cycle number was extracted, as shown in Figure 4. Compound 5ac exhibited an initial capacity of 86 mAh/g, which gradually faded to approximately 70 mAh/g after several cycles before stabilizing. In contrast to 5ac, compound 5ab, lacking a hydroquinone redox center, showed almost no redox activity, with a maximum capacity of 37 mAh/g. This suggests that the resorcinol group undergoes irreversible oxidation, rendering it ineffective for reversible cycling [41]. A similar trend was observed for 5aa, which also contains a resorcinol center but has a 1,3-phenylene moiety instead of 1,4-phenylene, with an initial specific capacity of 62 mAh/g that rapidly drops to around 30 mAh/g. The final polymer in this sequence, 5bb, has a resorcinol center protected by butyl groups and no hydroquinone redox center. It contains only a potentially redox-active conjugated carbonyl sequence. The electrochemical results showed a maximum capacity of 27 mAh/g, which indicates that the material has limited redox activity. Our previous testing of plain Printex carbon electrodes showed that the capacitance contribution of conductive carbon black inside the electrode should be around 15 mAh/g [42]. Similar electrochemical results were also observed for 5ba, which delivered a maximum capacity of 32 mAh/g. Interestingly, 5cb delivered the highest initial specific capacity of all the tested materials (158 mAh/g), which rapidly faded to 35 mAh/g. This behavior could be explained by initial redox-mediated cleavage of the benzyl ether groups, which afterwards show no redox activity [43]. Our results clearly indicate that the hydroquinone group in 5ac is the primary redox-active group, while electrodes containing other redox-active groups such as resorcinol and conjugated carbonyls exhibit no significant redox activity, instead showing capacitive charge/discharge behavior of the carbon black additive and pseudocapacitance of the polymer. Furthermore, variations in the phenylene structure do not impact the electrochemical activity of the polymers. This conclusion is further supported by the Coulombic efficiency of compounds 5ac, 5ab, and 5bb, which vary significantly and suggest possible side electrochemical reactions (Figure 4).
The galvanostatic curves shown in Figure 5 further support the rationalization above. All of the tested polymers, except 5ac and 5cb, show almost no redox activity, and most of the exhibited capacity is probably attributable to the capacitance of the carbon black conductive additive. The comparison between 5cb and 5bb, which differ in resorcinol-protecting groups (benzyl vs. butyl, respectively), showed additional plateaus of 5cb in initial cycles, which rapidly transformed into a single sloping curve. As mentioned earlier, we attributed its additional redox process to the redox-mediated deprotection of the benzyl groups (Figure 5) [43].
On the other hand, 5ac demonstrated high capacity (Figure 5c), although this was still lower than the theoretical value of 146 mAh/g, which is common for organic cathodes due to factors such as active particle morphology and electrolyte accessibility. Compound 5ac displayed reversible redox activity, with a sloping discharge plateau at around 2.4 V vs. Li/Li+. It reached a maximum capacity of 86 mAhg−1, with an average discharge voltage of 2.30 V vs. Li/Li+ and a capacity retention of 71% after 180 cycles. The relatively high reversibility observed is most likely due to the presence of hydroquinone redox centers.
This was further supported by the dQ/dE vs. potential (E) curves presented in Figure 6. In these curves, only 5ac exhibited a redox peak, which appeared at a lower potential than the regular benzoquinone/hydroquinone (BQ/HQ) redox peak. This shift may be attributed to the donating effect of the -NH group on the phenylene ring [44]. The other two polymers, 5ab and 5bb, did not show any redox peaks in the dQ/dE curves.
Furthermore, when comparing the morphology (Figure 2) to the electrochemical results, the slightly less porous morphologies of 5ac and 5ab demonstrate good reversibility, although the specific capacities of both polymers are lower than their theoretical values (146 mAh/g). This is particularly noticeable for 5ac, which showed redox activity through the hydroquinone center. Such low material utilization is common in Li–organic batteries and is typically due to restricted ionic and electronic transport within the organic materials [45]. This issue could be addressed through polymer engineering, in which the morphology can be optimized, for example, by the in situ addition of conductive nanofillers like CNTs [46].

4. Conclusions

Like other related polyenaminones, 5aa, 5ab, 5ac, 5ba, 5bb, 5ca, and 5cb are also available through the acid-catalyzed transaminative polymerization between resorcinol-derived bis-enaminones 2a–c and 1,3- and 1,4-phenylenediamines 3a and 3b. Monomer building blocks 2 and 3 are also accessible from biomass-derived cyclohexanediones, thus complying with the requirements of sustainable chemistry. Compounds 5 are optically active and exhibit strong absorption of UV-vis light below 500 nm, which makes them suitable for optical applications. Testing polyenaminones 5aa, 5ab, 5ac, 5ba, 5bb, 5ca, and 5cb for redox activity revealed that the highest activity was by 2,5-diaminohydroquinone-derived compound 5ac, with a maximum capacity of 86 mAh/g and relatively good capacity retention. This is in line with the premise that the hydroquinone moiety is the primary redox active group, while a significant drop in capacity after the first cycle suggests irreversible oxidation of the resorcinol moiety. The variation of other structural elements, such as the substitution of the phenolic hydroxy groups and the variation of the phenylenediamine moiety, did not affect the redox activity.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/polym16192700/s1, copies of 1H NMR, 13C NMR, IR, and UV-vis spectra of compounds 2a–c, 3a–c, 5aa, 5ab, 5ac, 5ba, 5bb, 5ca, and 5cb. Figure S1: UV-Vis spectra of compounds 5aa (▬), 5ab (▬), 5ba (▬), 5bb (▬), 5ca (▬), 5cb (▬), and 5ac (▬). Figure S2: Particle size distribution histogram determined from SEM image for polymer 5aa. Figure S3: Particle size distribution histogram determined from SEM image for polymer 5ab. Figure S4. Particle size distribution histogram determined from SEM image for polymer 5ac. Figure S5. Particle size distribution histogram determined from SEM image for polymer 5ba. Figure S6. Particle size distribution histogram determined from SEM image for polymer 5bb. Figure S7. Particle size distribution histogram determined from SEM image for polymer 5cb.

Author Contributions

The individual contributions of the authors are as follows: conceptualization, T.K., B.G. and J.S.; methodology, T.K., Ž.A., J.B., L.C., U.G., S.M., N.P., B.Š., B.G. and J.S.; software, T.K., J.B., B.G. and J.S.; validation, T.K., Ž.A., J.B., L.C., U.G., S.M., N.P., B.Š., B.G. and J.S.; formal analysis, T.K., Ž.A., J.B., L.C., U.G., S.M., N.P., B.Š., B.G. and J.S.; investigation, T.K., J.S., B.G., L.C., N.P. and J.B.; resources, B.G. and J.S.; data curation, T.K., J.B., B.G. and J.S.; writing—original draft preparation, T.K., J.B., B.G. and J.S.; writing—review and editing, T.K., Ž.A., J.B., L.C., U.G., S.M., N.P., B.Š., B.G. and J.S.; visualization, J.S., B.G., J.B. and S.M.; supervision, B.G., J.B. and J.S.; project administration, J.S. and B.G.; funding acquisition, J.S. and B.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Slovenian Research Agency (ARRS), research core funding No. P1-0179 and P2-0423 and research grant J2-4462.

Data Availability Statement

The data presented in this study are available in the main manuscript and in the Supplementary Materials of this manuscript.

Acknowledgments

NMR and LC-HRMS characterizations of compounds were performed at the Centre for Research Infrastructure at the Faculty of Chemistry and Chemical Technology, University of Ljubljana (IC UL FCCT).

Conflicts of Interest

The authors declare no conflicts of interest.

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Polymers 16 02700 g001
Figure 1. Preparation of polyenaminones from resorcinol (1)-derived bis-enaminones and phenylenediamines.
Figure 1. Preparation of polyenaminones from resorcinol (1)-derived bis-enaminones and phenylenediamines.
Polymers 16 02700 g001
Polymers 16 02700 sch001
Scheme 1. Synthesis of polymers 5aa–5ac, 5ba, 5bb, 5ca, and 5cb by transamination of enaminones 3a–c with diamine dihydrochlorides 4a–c [37].
Scheme 1. Synthesis of polymers 5aa–5ac, 5ba, 5bb, 5ca, and 5cb by transamination of enaminones 3a–c with diamine dihydrochlorides 4a–c [37].
Polymers 16 02700 sch001
Polymers 16 02700 g002
Figure 2. SEM images of the polymers (a) 5aa, (b) 5ab, (c) 5ac, (d) 5ba, (e) 5bb, and (f) 5cb were obtained using secondary electrons with an SE2 detector to examine their morphology.
Figure 2. SEM images of the polymers (a) 5aa, (b) 5ab, (c) 5ac, (d) 5ba, (e) 5bb, and (f) 5cb were obtained using secondary electrons with an SE2 detector to examine their morphology.
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Polymers 16 02700 g003
Figure 3. Normalized UV-vis spectra of compounds 5aa (▬), 5ab (▬), 5ba (▬), 5bb (▬), 5ca (▬), 5cb (▬), and 5ac (▬).
Figure 3. Normalized UV-vis spectra of compounds 5aa (▬), 5ab (▬), 5ba (▬), 5bb (▬), 5ca (▬), 5cb (▬), and 5ac (▬).
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Polymers 16 02700 g004
Figure 4. Specific capacity (a) and Coulombic efficiency (b) of the polymers 5aa, 5ab, 5ac, 5ba, 5bb, and 5cb.
Figure 4. Specific capacity (a) and Coulombic efficiency (b) of the polymers 5aa, 5ab, 5ac, 5ba, 5bb, and 5cb.
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Polymers 16 02700 g005
Figure 5. Galvanostatic charge/discharge curves of compounds of the polymers (a) 5aa, (b) 5ab, (c) 5ac, (d) 5ba, (e) 5bb, and (f) 5cb.
Figure 5. Galvanostatic charge/discharge curves of compounds of the polymers (a) 5aa, (b) 5ab, (c) 5ac, (d) 5ba, (e) 5bb, and (f) 5cb.
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Polymers 16 02700 g006
Figure 6. dQ/dE curves of compounds (a) 5ac, (b) 5ab, and (c) 5bb.
Figure 6. dQ/dE curves of compounds (a) 5ac, (b) 5ab, and (c) 5bb.
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Table 1. Experimental data for compounds 2a–c, 3a–c, 5aa–5ac, 5ba, 5bb, 5ca, and 5cb.
Table 1. Experimental data for compounds 2a–c, 3a–c, 5aa–5ac, 5ba, 5bb, 5ca, and 5cb.
EntryTransformationRR′Yield (%)
11 → 2a- 88
22a → 2bn-Bu 72
32a → 2cCH2Ph 81
42a → 3aH 82
52b → 3bn-Bu 62
62c → 3cCH2Ph 84
73a + 4a → 5aaHH95
83a + 4b → 5abHH100
93a + 4c → 5acHOH100
103b + 4a → 5ban-BuH88
113b + 4b → 5bbn-BuH92
123c + 4a → 5caCH2PhH84
133c + 4b → 5cbCH2PhH91
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Kotnik, T.; Menart, S.; Adam, Ž.; Bitenc, J.; Ciber, L.; Grošelj, U.; Petek, N.; Štefane, B.; Svete, J.; Genorio, B. Synthesis and Redox Activity of Polyenaminones for Sustainable Energy Storage Applications. Polymers 2024, 16, 2700. https://doi.org/10.3390/polym16192700

AMA Style

Kotnik T, Menart S, Adam Ž, Bitenc J, Ciber L, Grošelj U, Petek N, Štefane B, Svete J, Genorio B. Synthesis and Redox Activity of Polyenaminones for Sustainable Energy Storage Applications. Polymers. 2024; 16(19):2700. https://doi.org/10.3390/polym16192700

Chicago/Turabian Style

Kotnik, Tomaž, Svit Menart, Žan Adam, Jan Bitenc, Luka Ciber, Uroš Grošelj, Nejc Petek, Bogdan Štefane, Jurij Svete, and Boštjan Genorio. 2024. "Synthesis and Redox Activity of Polyenaminones for Sustainable Energy Storage Applications" Polymers 16, no. 19: 2700. https://doi.org/10.3390/polym16192700

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