Highly electron-deficient 1-propyl-3,5-dinitropyridinium: evaluation of electron-accepting ability and application as an oxidative quencher for metal complexes

Abstract

Impacts of the nitro groups on the electron-accepting and oxidizing abilities of N-propylpyridinium were evaluated quantitatively. A 3,5-dinitro derivative has efficiently quenched emission from photosensitizing Ru(II) and Ir(III) complexes owing to the thermodynamically-favored electron transfer to the pyridinium whose LUMO is greatly lowered by the presence of electron-withdrawing nitro groups.

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Introduction

Pyridines are typical electron-deficient heterocyclic compounds that can be seen all around us as substructures of functional materials such as pharmaceuticals, ligands, and optical and electronic devices.¹ The nucleophilic ring nitrogen, furthermore, undergoes N-alkylation, resulting in pyridinium salt. Owing to their highly electron-deficient aromatic character, pyridinium skeletons have been utilized in a variety of natural/artificial systems as electron acceptors as represented by NAD⁺,² methyl viologen³ and so forth.⁴ On the other hand, a nitro group exhibits strong electron-withdrawing ability due to both resonance and inductive effects, with the latter effect equivalent to two chloro groups.⁵ Therefore, a combination of the highly electron-deficient pyridinium and strong electron-withdrawing nitro group is expected to significantly increase the oxidation or electron-accepting abilities.

In our previous work, we have demonstrated that 1-propyl-3,5-dinitropyridinium salt 2a·OTs is formed in situ upon treatment of N-propyl-β-formyl-β-nitroenamine 1a (R = Pr) with p-toluenesulfonic acid (TsOH), and formation of the salt 2a·OTs is confirmed by trapping as 4-arylated 1,4-dihydropyridine derivatives 3 with electron-rich benzenes.⁶ On the other hand, 3,5-dinitropyridine 4 was obtained when N-tert-butylenamine 1b (R = t-Bu) was subjected to the same reaction, which is because a stable tert-butyl cation is readily eliminated (Scheme 1). The easy access to 3,5-dinitropyridine 4 facilitates the N-modification to afford versatile N-alkyl-3,5-dinitropyridinium salts 2. Indeed, treatment of 4 with propyl triflate (PrOTf) proceeded at room temperature to furnish 2a·OTf. This easily modifiable feature prompted us to evaluate the electron-acceptability of its N-alkylated form (2a⁺) by comparing with 3-nitro and unsubstituted pyridinium relatives (5⁺ and 6⁺) that are prepared from commercially available pyridines by N-propylation.

Scheme 1 Generation of 1-alkyl-3,5-dinitropyridinium salts 2·OTs in situ from N-alkyl-β-formyl-β-nitroenamines 1.

Experimental

Preparation of 2a·OTf

A mixture of 3,5-dinitropyridine 4 (86 mg, 0.5 mmol) and PrOTf (122 mg, 0.63 mmol) was stirred without solvent at room temperature for 2 d. Colorless precipitates were collected and washed with CH₂Cl₂ to afford 2a·OTf (95 mg, 0.28 mmol, 56%) as colorless powder, mp 217.0–217.8 °C. ¹H NMR (400 MHz, CD₃CN) δ 1.03 (t, J = 7.6 Hz, 3H), 2.12 (tq, J = 7.6, 7.6 Hz, 2H), 4.80 (t, J = 7.6 Hz, 2H), 9.78 (t, J = 2.0 Hz, 1H), 9.94 ppm (d, J = 2.0 Hz, 2H); ¹³C NMR (100 MHz, CD₃CN) δ 10.2 (CH₃), 25.3 (CH₂), 66.5 (CH₂), 121.8 (q, J = 318.0 Hz, CF₃), 135.9 (CH), 147.3 (CH), 147.9 ppm (C); ¹⁹F NMR (376 MHz, CD₃CN) δ 79.36 ppm; HRMS (ESI-TOF) calcd for C₈H₁₀N₃O₄ (M⁺): 212.0666, found: 212.0676.

Characterization of 5·OTf

mp 102.3–102.7 °C. ¹H NMR (400 MHz, CD₃CN) δ 1.00 (t, J = 7.2 Hz, 3H), 2.06 (tq, J = 7.2, 7.2 Hz, 2H), 4.66 (t, J = 7.2 Hz, 2H), 8.30 (dd, J = 8.4 Hz, 6.2 HZ, 1H), 9.01 (dd, J = 6.2, 2.0 Hz, 1H), 9.18 (ddd, J = 8.4, 2.0, 1.2 Hz, 1H), 9.63 ppm (d, J = 1.2 Hz 1H); ¹³C NMR (100 MHz, CD₃CN) δ 10.3 (CH₃), 25.2 (CH₂), 65.2 (CH2), 125.1 (q, J = 319 Hz, CF₃), 130.4 (CH), 141.0 (CH), 142.7 (CH), 147.8 (C), 150.1 ppm (CH); ¹⁹F NMR (376 MHz, CD₃CN) δ 79.31 ppm; HRMS (ESI-TOF) calcd for C₈H₁₁N₂O₂ (M⁺): 167.0815, found: 167.0819.

Characterization of 6·OTf

¹H NMR (400 MHz, CD₃CN) δ 1.00 (t, J = 7.6 Hz, 3H), 2.06 (tq, J = 7.6, 7.6 Hz, 2H), 4.49 (t, J = 7.6 Hz, 2H), 8.03 (br, 2H), 8.51 (t, J = 7.6 Hz, 1H), 8.70 ppm (d, J = 5.6 Hz, 2H); ¹³C NMR (100 MHz, CD₃CN) δ 9.3 (CH₃), 24.0 (CH₂), 62.9 (CH₂), 120.9 (q, J = 318 Hz, CF₃), 128.1 (CH), 144.2 (CH), 145.5 ppm (CH); ¹⁹F NMR (376 MHz, CD₃CN) δ 79.30 ppm; HRMS (ESI-TOF) calcd for C₈H₁₂N (M⁺): 122.0964, found: 122.0966.

Other chemicals

[Ru(bpy)₃](PF₆)₂ (bpy = 2,2′-bipyridine) is the same sample which has been used in the earlier literatures.⁷ [Ir(ppy)₂(bpy)]PF₆ (ppyH = 2-phenylpyridine) was synthesized and purified similarly to the reported procedure.⁸ Tetra-n-butylammonium hexafluorophosphate (TBAPF₆, Wako Pure Chemical Industries) was purified by repeated recrystallizations from ethanol. Ferrocene (Wako Pure Chemical Industries) was used as supplied. Anhydrous or spectroscopic-grade CH₃CN (Wako Pure Chemical Industries) was used without further purification for the electrochemical or spectroscopic measurements, respectively.

Electrochemical measurements

Cyclic voltammetry of the complexes in CH₃CN at 298 K was performed by using a BAS ALS-1202A electrochemical analyzer with a three-electrode system using glassy-carbon working, Ag auxiliary, and Ag/AgNO₃ reference electrodes (∼0.01 mol dm⁻³ (=M) in CH₃CN containing ∼0.1 M TBAPF₆) supplied by BAS Inc. The sample solutions containing a pyridinium salt or metal complex (∼1.0 mM) and TBAPF₆ as a supporting electrolyte (∼0.1 M) in the absence or presence of ferrocene as an internal standard were deaerated by purging an argon-gas stream over 20 min prior to measurements. The potential sweep rate was 100 mV s⁻¹.

Emission quenching study

Emission spectra were recorded and emission quantum yields (Φ_em) were determined by the absolute method using a Hamamatsu Photonics Quantaurus-QY Plus C13534-02. Emission intensity at each wavelength was corrected for system spectral response so that the vertical axis of a spectrum corresponds to the photon number at each wavelength. Emission decay profiles of [Ir(ppy)₂(bpy)]PF₆ was measured by using a Hamamatsu C4334 streak camera with a C5094 polychromator by exciting at 400 nm using second harmonics of a femtosecond-pulse mode-locked Ti:sapphire laser (MKS Instruments Spectra-Physics Tsunami^® 3941-M1BB and 3980 frequency doubler/pulse selector, 1 MHz) and analyzed by a single exponential decay function. Sample solutions were deaerated by purging with an argon-gas stream for over 30 min.

Free energy changes for the electron-transfer processes (−ΔG) were calculated by:⁹

−ΔG = nF[E_1/2(Q^+/0) − E_1/2(M*)] + Z_QZ_Me²/D_sd = nF[E_1/2(Q^+/0) − E_1/2(M)] + E₀(M*) + Z_QZ_Me²/D_sd (1)

In eqn (1), E_1/2(Q^+/0) is the reduction potential of Q⁺, and E_1/2(M) is the oxidation potential of the complex (1.32 and 1.63 V vs. SCE for [Ru(bpy)₃]²⁺ and [Ir(ppy)₂(bpy)]⁺, respectively, see Fig. S1†). E₀(M*) is the excited-state zeroth energy and has been determined to be 16 360 and 16 850 cm⁻¹ for [Ru(bpy)₃]²⁺ and [Ir(ppy)₂(bpy)]⁺, respectively, by the Franck–Condon analysis.¹⁰ Z_Q and Z_M are the charges of Q⁺ and complex. d is the sum of effective radii of Q⁺ and complex estimated for the optimized geometries by DFT calculations (4.7, 4.6, 4.4, 6.2 and 6.2 Å for 2a⁺, 5⁺, 6⁺, [Ru(bpy)₃]²⁺ and [Ir(ppy)₂(bpy)]⁺, respectively). D_s, n, F and e are the static dielectric constant of the solvent (relative dielectric constant of CH₃CN: 37.5), the number of electrons transferred, the Faraday constant and the formal charge, respectively. It should be noted that, in eqn (1), an electrostatic work term for the electron-transfer products was omitted since the reduced pyridiniums are charge-neutral.

Theoretical calculations

Theoretical calculations for the compounds were conducted with Gaussian 09W software (Revision C.01).¹¹ The ground-state geometries of the pyridinium cations were optimized by using density functional theory (DFT) using the restricted B3LYP functional with 6-31+G(d,p) basis set. All the optimized geometries did not gave any negative frequencies under identical methodologies. Lowest-energy unoccupied molecular orbitals were plotted using GaussView 5.¹² All the calculations were carried out as in acetonitrile by using a polarizable continuum model (PCM).

Results and discussion

Down-field shifts of the ring protons in the ¹H NMR spectra in CD₃CN were observed as the number of nitro groups increased (Fig. 1), indicating a decrease in the electron density of the pyridine ring. All the pyridiniums 2a⁺, 5⁺ and 6⁺ in CH₃CN exhibited an irreversible reduction wave as shown in Fig. 2. Half reduction potential (E_1/2) was shifted to a positive potential region with increasing the nitro group (E_1/2 = −0.061 (2a⁺), −0.41 (5⁺) and −0.80 V (6⁺) vs. saturated calomel electrode (SCE)). These tendencies were supported by DFT calculations, as the LUMO energy of 2a⁺ was lowered to −4.4 eV (see Fig. 1). It is, furthermore, worth emphasizing that the E_1/2 value of 2a⁺ is surprisingly positive even by comparing with that of methyl viologen (−0.44 V vs. sodium saturated calomel electrode (SSCE)¹³). In contrast to fully reversible redox behavior of methyl viologen and resulting applications as a redox shuttle,³ the highly-positive reduction potential of 2⁺ is advantageously utilizable as a sacrificial electron acceptor in the various photochemical systems.

Fig. 1 Chemical shifts of ¹H NMR in CD₃CN (given in ppm) and LUMO distributions/energies of pyridiniums 2a⁺, 5⁺ and 6⁺.

Fig. 2 Cyclic voltammograms of 2a·OTf (blue), 5·OTf (pink) and 6·OTf (red) in deaerated CH₃CN containing 0.1 M TBAPF₆. Reversible waves at around +0.43 V represent redox couples of ferrocene as an internal standard.

The strong electron-accepting ability of 2a⁺ is utilizable as an oxidative quencher in photoinduced electron-transfer reactions. As shown in Fig. 3(a), emission from a famous photosensitizer [Ru(bpy)₃]²⁺ (ref. 14) in CH₃CN (3.8 × 10⁻⁵ M) was reduced upon addition of 2a⁺ ((0.0–4.0) × 10⁻³ M), and emission quantum yield (Φ_em) of [Ru(bpy)₃]²⁺ was decreased from 0.096 to 0.014 in the presence of 2a⁺ (4.0 × 10⁻³ M). Emission from a cyclometalated iridium(III) complex [Ir(ppy)₂(bpy)]⁺ in CH₃CN (3.8 × 10⁻⁴ M) was also quenched when 2a⁺ coexisted in a solution (Φ_em = 0.085 and 0.017 in the absence and presence (4.0 × 10⁻³ M) of 2a⁺, respectively) as shown in Fig. 3(b). Stern–Volmer plots for emission quenching of the complexes by 2a⁺ are shown in Fig. 4, together with those by 5⁺ and 6⁺ (emission spectra are shown in Fig. S2–S5†). The plots exhibited good linear dependences, irrespective of the complex and pyridinium, as expressed by the Stern–Volmer equation: Φ_em,0/Φ_em = 1 + k_qτ₀[Q⁺] with Φ_em,0 and Φ_em the emission quantum yields in the absence and presence of the quencher (i.e., 2a⁺, 5⁺ or 6⁺), respectively, k_q the quenching rate constant, τ₀ the excited-state lifetime of the complex in the absence of the quencher (890 and 300 ns for [Ru(bpy)₃]^{2+ 15} and [Ir(ppy)₂(bpy)]⁺, respectively), and [Q⁺] the quencher concentration. As clearly seen in Fig. 4 and Table 1, emission quenching by 2a⁺ (k_q = 1.6 × 10⁹ and 3.2 × 10⁹ M⁻¹ s⁻¹ for [Ru(bpy)₃]²⁺ and [Ir(ppy)₂(bpy)]⁺, respectively) was more efficient than those by 5⁺ and 6⁺ (k_q ≤ 1.0 × 10⁹ M⁻¹ s⁻¹), indicating that the more nitro group, the stronger the quenching ability.

Fig. 3 Emission spectra of [Ru(bpy)₃](PF₆)₂ (a, 3.8 × 10⁻⁵ M, λ_ex = 500 nm) and [Ir(ppy)₂(bpy)]PF₆ (b, 3.8 × 10⁻⁴ M, λ_ex = 470 nm) in the absence and presence of dinitropyridinium salt 2a·OTf ((0.0–4.0) × 10⁻³ M: orange/green → black) in deaerated CH₃CN.

Fig. 4 Stem–Volmer plots for [Ru(bpy)₃]²⁺ (top panel) and [Ir(ppy)₂(bpy)]⁺ (bottom panel) quenching by 2a⁺ (blue), 5⁺ (pink) and 6⁺ (red) in deaerated CH₃CN. Solid lines represent linear regressions with the intercept fixed at 1.

Table 1 Reduction potentials, driving forces for electron transfer and quenching rate constants of pyridiniums 2a⁺, 5⁺ and 6⁺

Q^+	E1/2/V	[Ru(bpy)3]^2+		[Ir(ppy)2(bpy)]^+
		−ΔG/eV	kq/10^9 M^−1 s^−1	−ΔG/eV	kq/10^9 M^−1 s^−1
2a^+	−0.061	+0.72	1.6	+0.43	3.2
5^+	−0.41	+0.37	0.99	+0.084	1.0
6^+	−0.80	+0.016	0.048	−0.30	<0.01

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The efficient emission quenching by 2a⁺ can be discussed in terms of a driving force of the electron-transfer process (−ΔG, Table 1), which is calculated from the reduction potentials of the pyridinium, the oxidation potentials of the excited-state complexes and so forth. The k_q value correlates well with the −ΔG value, suggesting that the observed emission quenching originates in the electron transfer from the metal complex (i.e., [Ru(bpy)₃]²⁺ or [Ir(ppy)₂(bpy)]⁺) in the excited state to pyridinium (i.e., 2a⁺, 5⁺ or 6⁺). It is worth noting that the electron transfer between [Ir(ppy)₂(bpy)]⁺* to 6⁺ is highly an endergonic process (−ΔG = −0.30 eV) and, therefore, no emission quenching has been observed. Thus, an introduction of a nitro group(s) into a pyridinium skeleton improves the electron-accepting ability.

Conclusions

A combination of an electron-deficient pyridinium and electron-withdrawing nitro group(s) enhanced electron-accepting and oxidizing abilities. Each nitro-group introduction lowered the LUMO by several tenths of an electron volt, and dinitropyridinium 2a⁺ especially served as an excellent oxidative quencher in photoinduced electron-transfer reactions. Since pyridinium derivatives have attracted increasing interest and utilized in a variety of photochemical systems such as natural/artificial photosynthesis, these nitropyridiniums are possible candidates as a new class of electron acceptors.

Author contributions

A. Ito: conceptulization, data curation, writing – original draft, and supervision. Y. Kuroda, K. Iwai and S. Yokoyama: investigation. N. Nishiwaki: supervision and writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Cyclic voltammograms of the metal complexes and emission quenching data by 5⁺ and 6⁺. See DOI: https://doi.org/10.1039/d4ra00845f

‡ Present address: The Institute of Scientific and Industrial Research (SANKEN), Osaka University, Ibaraki, Osaka 567-0047, Japan.

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