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. 2026 Sep 7;11(37):55658–55664. doi: 10.1021/acsomega.6c04795

The Redox Activity of Two Identical Units on Xylylene-Bridged N‑Pyridinium-Substituted Diamino-1,3,5-triazine

Arumugam Thangavel 1,*, Justin Blake Hathaway 1, Nadia Hirbawi 1, Allen Ng 1, Ron Jacob Villanueva 1, Osvaldo Madera 1
PMCID: PMC13613398  PMID: 42798993

Abstract

The N-benzylpyridinium-substituted diamino-1,3,5-triazine (Bz-TZN) and its geometrically separated units on a xylene core (O-, M-, P-TZN) were synthesized. The cyclic voltammetry of these compounds shows two redox states, irrespective of the number and location on the xylene core. These compounds were characterized by 1H, 13C NMR, HRMS, CV, chronoamperometry, staircase voltammetry, and spectroelectrochemistry, which revealed that two units on a xylene core have negligible interaction with each other and mostly retain their independent redox states.

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Introduction

Redox-active compounds are an integral part of natural and artificial systems. In nature, they are involved in electron transfer, , DNA cleavage, and oxidative stress, among other processes. In artificial systems, notable examples include conducting polymers, supercapacitors, batteries, , sensors, and catalysts, among others.

In batteries, the processes of reduction and oxidation are utilized as tools to store electricity. For this process, molecules with stable and reversible redox states are a prerequisite. Currently used lithium-ion and vanadium-based redox flow batteries have their own disadvantages, including cost, fabrication, environmental, and safety factors. To overcome these issues, current research has focused on organic redox flow batteries, − which require the design and synthesis of organic molecules with stable redox states. In this regard, various organic molecules such as viologens, , TEMPO, , thianthrene, thienoviologens, and triquinoxalinylene to name a few, have been used as cathode and anode materials. The redox-active compounds are further studied as a function of through-space interactions to tune stability and redox properties. ,

Recently, a 1,3,5-triazine-based pyridinium compound was used to demonstrate stable multiredox activity in water; its redox activity was further tuned as a function of host–guest through-space interactions with cucurbit­[7,8]­urils. ,

In the same line, this research article describes the synthesis of pyridinium-substituted diamino-1,3,5-triazine and studies its redox activity as a function of the through-space interaction of two geometrically separated redox units. The target compounds were characterized by NMR, HRMS, CV, spectroelectrochemistry, staircase voltammetry, and chronoamperometry.

Experimental Conditions

All reagents and solvents were purchased from commercial sources and used without further purification. The 1H and 13C spectra were recorded on a JEOL 400 MHz spectrometer; high-resolution mass spectra were recorded on an Orbitrap Exploris 120. Cyclic voltammetry was recorded at room temperature in a one-compartment cell with a three-electrode setup using the VersaStat 4S instrument. A gold electrode (1.6 mm diameter), an Ag/AgCl, and a platinum electrode were used as the working, reference, and counter electrodes, respectively. The 0.1 M TBAPF6 was used as the supporting electrolyte; DMSO was dried with molecular sieves and used as the solvent. Before collecting the CV, the system was purged with nitrogen, and the working electrode was polished with diamond polishing paste, sonicated, cleaned with water, and dried. The reference electrode was periodically calibrated with a known standard. All electrochemical data were collected with automatic iR compensation by the instrument. Chronoamperometry was carried out using a gold ultramicroelectrode (radius = 5 μm). Spectroelectrochemistry was recorded with a three-electrode setup inside a transparent quartz cuvette (1 mm path length), using gold mesh as the working electrode, platinum wire as the counter electrode, and Ag/AgCl as the reference electrode. All electrodes were purchased from BASi Research Products.

The DFT calculations were carried out using Gaussian 09 software. The initial structures were optimized with the PM3 semiempirical method and were subsequently reoptimized using DFT (B3LYP/6-31G­(d)) with multiplicity 1, in vacuum.

Synthesis

The synthesis of all the compounds reported in this study is shown in Scheme , and the following general procedure was used: the free base, 2,4-diamino-6-(4-pyridyl)-1,3,5-triazine (TZN), was synthesized according to a previously reported method and then heated with the appropriate substituted aryl halide in acetonitrile. After 2 days under reflux, the reaction mixture was cooled; the solvent was removed using a rotary evaporator; and the resulting precipitate was washed thoroughly with dichloromethane. The product was further purified by recrystallization from methanol, which afforded analytically pure compounds. The purity and identity of the expected products were confirmed by HRMS, 1H and 13C NMR.

1. Synthesis of Target Compounds: Bz-, O-, M-, P-TZNs.

1

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Bz–TZN: Benzyl chloride (0.38 mL, 3.3 mmol, 1.25 equiv) and 2,4-diamino-6-(4-pyridyl)-1,3,5-triazine (TZN) (0.5 g, 2.65 mmol,1 equiv) in 30 mL CH3CN; yield: 0.82 g, 78%.1H NMR (400 MHz, DMSO-d 6) δ 9.31 (d, J = 6.4 Hz, 2H), 8.64 (d, J = 6.3 Hz, 2H), 7.53 (dd, J = 7.4, 2.1 Hz, 2H), 7.44 (d, J = 6.9 Hz, 3H), 7.16 (s, 4H), 5.92 (s, 2H). 13C NMR (101 MHz, DMSO-d 6) δ 63.12, 125.65, 128.97, 129.24, 129.40, 134.22, 145.70, 152.32, 165.74, 167.26. HRMS­(ESI): observed: 279.1354; expected: 279.1353.

O-TZN: ortho-Xylylene dichloride (0.28 g, 1.06 mmol, 1.0 equiv) and 2,4-diamino-6-(4-pyridyl)-1,3,5-triazine (TZN) (0.5 g, 2.65 mmol, 2.5 equiv) in 30 mL CH3CN; yield: 0.32 g, 36%; 1H NMR (400 MHz, DMSO-d 6) δ 9.30 (d, J = 6.6 Hz, 4H), 8.69 (d, J = 6.4 Hz, 4H), 7.51 (dd, J = 5.8, 3.3 Hz, 2H), 7.32–7.09 (m, 11H), 6.25 (s, 4H). 13C NMR (101 MHz, DMSO-d 6) δ 60.15, 125.82, 129.63, 130.17, 132.83, 146.10, 152.66, 165.72, 167.29. HRMS­(ESI): observed: 240.1119; expected: 240.1118

M-TZN: meta-Xylylene dichloride (0.28 g, 1.06 mmol, 1.0 equiv) and 2,4-diamino-6-(4-pyridyl)-1,3,5-triazine (TZN) (0.5 g, 2.65 mmol, 2.5 equiv) in 30 mL CH3CN; yield: 0.57 g, 64%; 1H NMR (400 MHz, DMSO-d 6) δ 9.47 (d, J = 6.6 Hz, 4H), 8.65 (d, J = 6.5 Hz, 4H), 7.88 (s, 1H), 7.61 (dd, J = 7.6, 1.8 Hz, 2H), 7.57–7.49 (m, 1H), 7.20 (s, 8H), 5.99 (s, 4H). 13C NMR (101 MHz, DMSO-d 6) δ 62.69, 125.69, 129.73, 129.86, 130.14, 135.09, 145.89, 152.39, 165.75, 167.28. HRMS­(ESI): observed: 240.1118; expected: 240.1118

P-TZN: para-Xylylene dichloride (0.28 g, 1.06 mmol, 1.0 equiv) and 2,4-diamino-6-(4-pyridyl)-1,3,5-triazine (TZN) (0.5 g, 2.65 mmol, 2.5 equiv) in 30 mL CH3CN; yield: 0.71 g, 80%; 1H NMR (400 MHz, DMSO-d 6) δ 9.39 (d, J = 6.6 Hz, 4H), 8.69–8.53 (m, 4H), 7.65 (s, 4H), 7.17 (s, 8H), 5.98 (s, 4H). 13C NMR (101 MHz, DMSO-d 6) δ 62.54, 125.72, 129.73, 135.29, 145.75, 152.43, 165.72, 167.27; HRMS­(ESI): observed: 240.1118; expected: 240.1118

Results and Discussion

The redox chemistry of N-benzylpyridinium-substituted diamino-1,3,5-triazine (Bz-TZN) in DMSO shows two well-separated redox states, the first is centered at E1/2 = −0.869 V and the second at E1/2 = −1.473 V vs Ag/AgCl at 50 mV/s. For the first and second redox states, the peak-to-peak separations were 76.93 and 73.0 mV, and the peak current ratios (ipc/ipa) were 1.20 and 1.06, respectively. The heterogeneous electron-transfer rate constant was calculated to be 2.07 ± 0.32 × 10–3 cm/s based on the Kochi method (Tables and S2).

1. Electrochemical and DFT Data of Bz-, O-, M-, and P-TZN.

  (V)
     (eV)
 Number of e transferred
  E11/2 E21/2 D × 106 (cm2/s) K × 10–3 (cm/s) LUMO HOMO 1st 2nd (cumulative),
Bz-TZN –0.87 –1.47 0.39 ± 0.13 2.07 ± 0.32 –0.219 –0.347 1.33 2.29
O-TZN –0.80 –1.50 0.45 ± 0.14 0.87 ± 0.17 –0.289 –0.398 1.92 3.96
M-TZN – 0.85 –1.47 1.16 ± 0.43 1.77 ± 0.22 –0.287 –0.401 2.03 4.16
P-TZN –0.82 –1.44 1.06 ± 0.43 2.05 ± 0.23 –0.282 –0.396 1.98 4.04
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a

Data from CV at 50 mV/S in DMSO with 0.1 M TBAPF6, Au electrode of 0.8 mm radius, Bz-TZN (3.5 mM), O-TZN (2.8 mM), M-TZN (2.55 mM), P-TZN (2.63 mM).

b

Average of diffusion coefficients obtained from nonlinear regression analysis of the Shoup–Szabo equation of chronoamperometry at 1.1 V and 1.7 V.

c

Heterogeneous electron-transfer constant based on the Kochi method.

d

Degenerate orbitals.

e

The number of electrons transferred for the first and second redox states was obtained from the nonlinear regression analysis of the Shoup–Szabo equation of chronoamperometry data.

f

The total number of electrons transferred which is the combination of the first and second reductions.

The DFT calculations were carried out using the B3LYP/6-31G­(d) basis set under vacuum to understand the redox activity of Bz-TZN. The optimized structure and its frontier orbitals are given in Supporting Information (Figure S21). The energy gap between the HOMO and LUMO provides information about the molecule’s electron-accepting preference. The LUMO is located on the pyridinium group at −0.22 eV, and the HOMO is on the triazine core at −0.35 eV. On this basis, the following redox scheme is proposed (Scheme ), in which the first electron is accepted by the pyridinium group, thereby generating a radical. The pyridinium-based radical delocalizes onto the triazine core, which then undergoes a second reduction to form an anionic compound.

2. Schematic Representation of Electron Transfer for First and Second Redox States of Bz-TZN.

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After one-electron reduction in DMSO, it forms a radical; the possibility of its dimerization was verified by spectroelectrochemistry. In the cationic form, before any reduction, the UV–vis spectrum shows λmax = 253 nm. After one-electron reduction by chronoamperometry at 1.1 V, the UV–vis spectrum shows the formation of a new absorption band at λmax = 466 nm, and no other bands between 466 and 1100 nm (Figure ). A well-studied viologen system, which undergoes dimerization after one-electron reduction, has λmax (nm) = ∼850, 533, and 360 for the monomer and dimer forms. ,

1.

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Left: The CV of Bz-TZN (3.5 mM, 0.1 M TBAPF6, Au electrode with a 0.8 mm radius, DMSO, scan rates (V/s): 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0), with an inset plot of peak current vs square root of scan rate (○ for the first and ● for the second redox states); Middle: Spectroelectrochemistry (3.5 mM, 1 mm path length, 0.1 M TBAPF6, DMSO) of Bz-TZN with an inset plot of chronoamperometry at 1.1 V (3.5 mM, Au mesh electrode, 0.1 M TBAPF6, DMSO); Right: DFT (B3LYP/6-31G­(d)) structure with frontier molecular orbitals (HOMO and LUMO) of Bz-TZN.

Spectroelectrochemistry shows no dimer formation in the radical state. This could be due to repulsion between the triazine core of one molecule and the pyridinium center of another in a dimerized state.

In the Bz-TZN compound, the following questions arise: will multiple units of TZN interact with each other if they are arranged together with flexible threads and conformational mobility but separated in space? If so, how would their electrochemistry respond to the interaction of multiple units? − To address these questions, the following three compoundsO-, M-, and P-TZNwere synthesized and characterized by NMR, HRMS, CV, DFT, chronoamperometry, staircase voltammetry, and spectroelectrochemistry. In these compounds (O-, M-, and P-TZN), two redox-active TZNs are connected to the xylene core at the ortho, meta, and para positions.

These three compounds, O-, M-, and P-TZN, irrespective of the different locations of the redox-active TZNs, show two well-separated redox states (Figure ), the same as Bz-TZN. This leads to the question of how many electrons are responsible for each redox state, which was addressed by chronoamperometry (Figures S11–S18) and staircase voltammetry (Table S1).

3.

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Left: Normalized current/concentration cyclic voltammetry of Bz-TZN (3.5 mM), O-TZN (2.81 mM), M-TZN (2.54 mM), and P-TZN (2.63 mM) using an Au electrode (0.8 mm radius) in DMSO with 0.1 M TBAPF6 at 50 mV/s; Middle: Spectroelectrochemistry of Bz-TZN (3.5 mM), O-TZN (3.09 mM), M-TZN (4.4 mM), and P-TZN (2.5 mM) in DMSO with 0.1 M TBAPF6 at 1.1 V. Right: DFT-optimized structure with frontier molecular orbitals (HOMO and LUMO) of O-TZN.

The number of electrons transferred for each reduction was verified by chronoamperometry and staircase voltammetry. Chronoamperometry was carried out at 1.1 and 1.7 V using a gold microelectrode (10 μm in diameter). The resulting current vs time data were processed with two different methods. The first method involves nonlinear regression analysis using the Shoup–Szabo equation. In this method, the diffusion coefficient and number of electrons were treated as variables, and the model was fitted to the experimental data. In the second method, the plot of diffusion current/steady-state current (Id/Iss) vs the inverse square of time (1/ t ) yields a straight line, and the resulting slope is used to calculate the diffusion coefficient. With knowledge of the diffusion coefficient, the number of electrons was calculated using the steady state current with the formula Iss = 4nFDCr (Figures S15–S18).

The results from chronoamperometry were further confirmed by staircase voltammetry (SCV). The peak current for each reduction was divided by the concentration, thereby giving the normalized peak current. The relative ratio of normalized peak current between Bz-TZN and O-, M-, and P-TZN matches the number of electrons transferred for each reduction (Table S1; Figure S19).

All three methods confirm that a single electron is transferred for Bz-TZN, and two electrons are transferred for O-, M-, and P-TZN, in the first and second reductions. The diffusion coefficients obtained from the two methods are comparable (Table ).

These methods confirm that the redox units, irrespective of ortho, meta, or para, behave almost independently. A DFT calculation (Figure S21) shows the presence of degenerate HOMO and LUMO orbitals, which further supports the uptake of two electrons in each reduction.

When compared with Bz-TZN, these compounds show differences of 65, 26, and 49 mV for the first, and −23, 1.0, and 32, mV for the second redox states of O-, M-, and P-TZNs, respectively, at 50 mV/s (Figure ). M-TZN shows a negligible effect, whereas O- and P-TZNs show noticeable changes compared with Bz-TZN.

Although 1H NMR shows the absence of any upfield protons for P-TZN, which implies the absence of any intramolecular π-stacking, it shows positive shifts of 49 and 32 mV for the first and second redox states, compared to Bz-TZN.

The DFT calculation shows that the LUMO is located at −0.28 eV for P-TZN, compared to −0.29 eV for the M- and O-TZN isomers. This could be a reason for a noticeable positive shift in the redox potential compared to Bz-TZN.

In the ortho isomer, the two redox-active units are close to each other geometrically and are expected to communicate with each other; ,− based on this, two modes of conformations, syn and anti, are expected , (Figure ). The 1H NMR spectra in DMSO-d6 show the absence of any upfield protons, indicating that they are not stacked via noncovalent interactions, and confirm that the two TZN units are not in close proximity to each other; this could be due to repulsion between the two positive charges of the pyridinium groups.

2.

2

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Two possible orientations of the TZN units in O-TZN: Left, syn; Right, anti conformation.

Compared to Bz-TZN, the CV of O-TZN shows a noticeable change in redox potential for both redox processes at 50 mV/s. The first redox wave, the formation of a radical, is shifted in the positive direction by 65 mV, and the opposite effect is observed for the second redox wave, forming an anion, by 23 mV. The positive shift of the first redox and the negative shift of the second redox indicate that it is easier to form the radical and more difficult to form the anion compared to Bz-TZN.

The same effect is observed with viologen , and anthracene-substituted systems. This could be due to repulsion between two positively charged pyridinium ions, which creates a favorable condition for the first reduction and shifts it to positive potential. After the first reduction, the resulting radicals attract each other and gain stability, thereby making the second reduction more difficult and shifting it to more negative potential.

The chronoamperometry indicates that O-, M-, and P-TZN take up two electrons for each reduction, indicating that two radicals are formed from two redox units (Scheme ). Since they are in close proximity to each other, at least for O-TZN, the possibility of its dimerization was verified by spectroelectrochemistry (Figure ) with chronoamperometry at 1.1 V. After reduction at 1.1 V by chronoamperometry, O-TZN forms two radicals, which were monitored by UV–vis. It shows the formation of a new absorption band similar to that of Bz-TZN, but the absence of any other absorption bands between 466 and 1200 nm indicates that dimerization is not favored by the two radicals, even though they are in close proximity to each other. This could be due to intramolecular repulsion of the NH2 groups, preventing dimer formation.

3. Schematic Representation of Electron Transfer for First and Second Redox States of O-TZN for Syn and Anti Conformations.

3

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The absence of dimer formation in the O-TZN isomer, even though two units are in close proximity, can be rationalized by DFT structures (Figure S21). The DFT structures show that the two redox-active units are oriented anti to each other, which keeps them far apart and diminishes dimer formation.

Conclusion

The amine- and N-benzylpyridinium-substituted-1,3,5-triazine derivative (Bz-TZN) was synthesized. In DMSO, Bz-TZN shows two well-separated redox states by forming a radical and an anion. When two TZNs are arranged at varying ortho, meta, and para positions on a xylene core with flexible mobility, the resulting compounds show the same redox activity as Bz-TZN without undergoing significant interaction with each other. The redox activity of these compounds was further studied for the number of electrons transferred and the possibility of radical dimerization. Chronoamperometry confirms that two electrons are transferred for O-, M-, and P-TZNs; spectroelectrochemistry, which shows the formation of a new absorption band at λmax = 466 nm and the absence of any other new bands at longer wavelengths, indicates a lack of preference for radical dimerization. This study concludes that, even though two redox-active TZN units are connected on a xylene core with conformational flexibility, they are largely independent and retain their individual redox properties under the conditions studied. Future work will focus on redox activity as a function of host–guest interactions under aqueous conditions and will explore potential applications in aqueous redox flow batteries.

Supplementary Material

ao6c04795_si_001.pdf (4MB, pdf)

Acknowledgments

A.T. thanks California State University, Dominguez Hills, for an RSCA grant. N.H. acknowledges support from the MARC U*STAR program funded by the NIH. The authors gratefully acknowledge the DoD grant (#79677-RT-REP) for generous support in purchasing the Orbitrap Exploris 120 instrument used in this study. The authors would like to thank the Center for Innovation in STEM Education (CISE) at CSUDH for generously covering the article processing charge for this publication.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c04795.

  • ESI-MS, 1H and 13C NMR, HRMS, CV, spectroelectrochemistry, staircase voltammetry, DFT calculations, heterogeneous electron-transfer constants of Bz-, O-, M-, and P-TZN (PDF)

The authors declare no competing financial interest.

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