Photocatalytic oxidation of iron(II) complexes by dioxygen using 9-mesityl-10-methylacridinium ions

Abstract

Photocatalytic oxidation of iron(II) complexes by dioxygen occurred using the organic photocatalysts, 9-mesityl-10-methylacridinium ions (Acr⁺-Mes) and 2-phenyl-4-(1-naphthyl) quinolinium ions (QuPh⁺-NA), in the presence of triflic acid in acetonitrile under visible light irradiation. The electron-transfer state of Acr⁺-Mes produced upon photoexcitation oxidized the iron(II) complexes, whereas it reduced dioxygen with protons to produce iron(III) complexes and H₂O₂.

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Metal complexes are usually oxidized by inorganic oxidants such as cerium ammonium nitrate and lead dioxide.^1–5 In such cases, stoichiometric amounts of inorganic oxidants are required to obtain oxidized metal complexes, producing inorganic wastes which cause environmental problems. The ideal oxidant, which is environmentally benign, is dioxygen (O₂), producing only hydrogen peroxide or water as the reduced product. However, the oxidation of metal complexes by O₂ is often endergonic even in the presence of an acid. Thus, an appropriate photocatalyst is required for the oxidation of metal complexes by O₂ in the presence of an acid. Ruthenium(II) complexes, such as [Ru(bpy)₃]²⁺ (bpy = 2,2′-bipyridine) are known to be oxidized by O₂ in the presence of an acid to yield the corresponding Ru(III) complexes.^6,7 On the other hand, organic photocatalysts have merited increasing attention for a variety of oxidation reactions.^8–14 However, there has been no report on photocatalytic oxidation of metal complexes by O₂ using organic photocatalysts.

We report herein the photocatalytic oxidation of iron(II) complexes by O₂ using 9-mesityl-10-methylacridinium ions (Acr⁺-Mes)¹⁵ and 2-phenyl-4-(1-naphthyl)quinolinium ions (QuPh⁺-NA)¹⁶ as organic photocatalysts in the presence of triflic acid (HOTf) in acetonitrile (MeCN) under visible light irradiation [eqn (1)]. Visible light irradiation of O₂-saturated acetonitrile is shown in Fig. 1, where the absorption band at 520 nm due to [Fe^II(bpy)₃]²⁺ decreased, accompanied by the increase in absorption at 650 nm due to [Fe^III(bpy)₃]³⁺. [Fe^II(bpy)₃]²⁺ was not oxidized without Acr⁺-Mes under irradiation (Fig. S1 and S2 in the ESI†). The reduced product of O₂ was H₂O₂, which was detected by spectral titration with the use of the oxo[5,10,15,20-tetra(4-pyridyl)porphyrinato]titanium(IV) complex (see the Experimental section in the ESI†).¹⁷

Fig. 1 Visible absorption change in photocatalytic oxidation of [Fe^II(bpy)₃]²⁺(PF₆⁻)₂ (2.0 mM) by O₂ in O₂-saturated MeCN in the presence of [Acr⁺-Mes](ClO₄⁻) (0.20 mM) and HOTf (0.10 M) at 298 K under visible light irradiation using a xenon lamp with a cut filter (λ < 390 nm).

The photocatalytic oxidation of other iron(II) complexes by O₂ was examined in the presence of HOTf in O₂-saturated MeCN (Fig. S3–S6 in the ESI†). The quantum yields of formation of iron(III) complexes were determined using a ferrioxalate actinometer (see Fig. S7–S9 and the Experimental section in the ESI†).¹⁸ The quantum yields (Φ) of photocatalytic oxidation of iron(II) complexes by O₂ in the presence of HOTf in MeCN are listed in Table 1 together with the concentrations of iron(II) complexes, HOTf and O₂, the one-electron oxidation potentials of iron(II) complexes (E_ox)¹⁹ and the free energy change of the oxidation (ΔG_ox). The ΔG_ox values were evaluated from the E_ox values and the E_red value of O₂ in the presence of an acid in MeCN (0.75 V vs. SCE) [eqn (2)].²⁰

ΔG_ox = −2e(E_red − E_ox) (2)

Table 1 Product and quantum yields (Φ) of iron(III) complexes for photocatalytic oxidation of iron(II) complexes by O₂ in the presence of HOTf together with the one-electron oxidation potentials of iron(II) complexes (E_ox) and the free energy change of the oxidation (ΔG_ox)

Entry	Fe^II complex	E ox vs. SCE^a, V	ΔGox, eV	Acr^+-Mes yield^d, %	TON	Φ , %	QuPh^+-NA yield^e, %	TON	Φ , %
Clphen = 5-chloro-1,10-phenanthroline, bpy = 2,2′-bipyridine, Me2bpy = 4,4′-dimethyl-2,2′-bipyridine, BrC5H4 = bromocyclopentadienyl, C5H5 = cyclopentadienyl, reaction conditions: [photocatalyst] = 0.20 mM; [HOTf] = 0.10 M; [[Fe^II(Clphen)3]^2+(PF6^−)2] = 1.0 mM, [[Fe^II(bpy)3]^2+(PF6^−)2] = 2.0 mM, [[Fe^II(Me2bpy)3]^2+(PF6^−)2] = 2.0 mM, [Fe^II(BrC5H4)2] = 1.0 mM, [Fe^II(BrC5H4)(C5H5)] = 2.0 mM.a Taken from ref. 19.b MeCN (0.40 mL), cell path length 0.1 cm.c MeCN (3.0 mL), cell path length 1 cm.d Photoirradiation (λ > 390 nm).e (λ > 300 nm).f See the Experimental section in the ESI.
1^b	[Fe^II(Clphen)3]^2+	1.20	0.90	5	0.25	0.11	30	1.5	0.34
2^b	[Fe^II(bpy)3]^2+	1.06	0.62	42	4.2	0.32	32	3.5	0.73
3^b	[Fe^II(Me2bpy)3]^2+	0.88	0.26	22	2.1	1.6	30	3.0	5.2
4^c	Fe^II(BrC5H4)2	0.72	−0.06	81	4.1	7.2	87	4.2	13
5^c	Fe^II(BrC5H4)(C5H5)	0.53	−0.44	81	4.1	19	60	3.0	26

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Judging from the ΔG_ox values, the photocatalytic oxidation of [Fe^II(Clphen)₃]²⁺, [Fe^II(bpy)₃]²⁺ and [Fe^II(Me₂bpy)₃]²⁺ is endergonic, whereas that of Fe^II(BrC₅H₄)₂ and Fe^II(BrC₅H₄)(C₅H₅) is exergonic. The Φ values increased with the decreasing the E_ox values of iron(II) complexes as the free energy change of the reaction in eqn (1) decreased to be thermodynamically more favourable. Dependence of Φ on concentrations of [Fe^II(bpy)₃]²⁺(PF₆⁻)₂, HOTf and O₂ is shown in Fig. 2 (parts a, b and c, respectively). The Φ value became constant with the increasing concentrations of [Fe^II(bpy)₃]²⁺(PF₆⁻)₂, HOTf and O₂, respectively. The photocatalytic oxidation of iron(II) complexes is enhanced by using QuPh⁺-NA instead of Acr⁺-Mes as shown in Table 1. (Fig. S10–S14 in the ESI†).

Fig. 2 Dependence of Φ on concentrations of (a) [Fe^II(bpy)₃]²⁺, (b) HOTf and (c) O₂. Standard conditions: [[Acr⁺-Mes](ClO₄⁻)] = 1.0 mM; [[Fe^II(bpy)₃]²⁺(PF₆⁻)₂] = 2.0 mM; [HOTf] = 0.15 M; [O₂] = 2.6 mM; dehydrated MeCN (0.40 mL); cell path length 0.1 cm; excitation wavelength: 420 nm.

Nanosecond laser flash photolysis measurements were performed in order to clarify the catalytic mechanism for photocatalytic oxidation of iron(II) complexes by O₂ using Acr⁺-Mes in the presence of HOTf in MeCN. Transient absorption spectra were taken after the nanosecond laser excitation at 355 nm of a deaerated MeCN solution of Acr⁺-Mes in the absence and presence of [Fe^II(bpy)₃]²⁺(PF₆⁻)₂ as shown in Fig. 3. The transient absorption band at 490 nm is due to the electron-transfer state of Acr⁺-Mes. In the presence of [Fe^II(bpy)₃]²⁺ the absorption at 490 nm decayed more rapidly and the decay rate increased with the increasing concentration of [Fe^II(bpy)₃]²⁺. The decay rate obeyed pseudo-first-order kinetics and the pseudo-first order rate constant increased linearly with the increasing concentration of [Fe^II(bpy)₃]²⁺(PF₆⁻)₂. From the slope the rate constant (k_ox) of electron transfer from [Fe^II(bpy)₃]²⁺ to the electron-transfer state of Acr⁺-Mes was determined to be 3.7 × 10⁸ M⁻¹ s⁻¹ as shown in the inset of Fig. 3. Similarly the k_ox values of other iron(II) complexes were determined (see Fig. S15–S23 in the ESI†) as listed in Table 2 together with the E_ox values. The k_ox value increases with the decreasing E_ox values. In the presence of O₂, electron transfer from the electron-transfer state of Acr⁺-Mes to O₂ is known to occur with a rate constant of 6.8 × 10⁸ M⁻¹ s⁻¹.²¹ The rate constant of electron transfer from the electron-transfer state of QuPh⁺-NA to O₂ was determined to be 6.3 × 10⁸ M⁻¹ s⁻¹ (see Fig. S24 in the ESI†). Thus, the photocatalytic oxidation of iron(II) complexes (Fe^II) by O₂ in the presence of HOTf proceeds as shown in Scheme 1. Photoexcitation of Acr⁺-Mes results in the formation of the electron-transfer state of Acr⁺-Mes, which oxidizes iron(II) complexes to iron(III) complexes and reduces O₂ with protons to produce HO₂˙, which disproportionates to yield H₂O₂.

Fig. 3 Transient absorption decay at 490 nm due to the electron-transfer state of [Acr⁺-Mes](ClO₄⁻) with various concentrations of [Fe^II(bpy)₃]²⁺(PF₆⁻)₂. Inset: Decay rate constant versus concentrations of [Fe^II(bpy)₃]²⁺(PF₆⁻)₂.

Table 2 Rate constants of electron transfer from iron(II) complexes to the electron-transfer state of the organic photocatalyst and the one-electron oxidation potentials of iron(II) complexes (E_ox)

Entry	Fe^II complex	E ox vs. SCE, V	Acr^+-Mes ket, M^−1 s^−1	QuPh^+-NA ket, M^−1 s^−1
1	[Fe^II(Clphen)3]^2+	1.20	1.4 × 10^8	4.9 × 10^8
2	[Fe^II(bpy)3]^2+	1.06	3.7 × 10^8	4.5 × 10^8
3	[Fe^II(Me2bpy)3]^2+	0.88	4.5 × 10^8	5.8 × 10^8
4	Fe^II(BrC5H4)2	0.72	7.6 × 10^9	7.2 × 10^9
5	Fe^II(BrC5H4)(C5H5)	0.53	8.5 × 10^9	7.9 × 10^9

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Scheme 1 Photocatalytic cycle for oxidation of iron(II) complexes to iron(III) by O₂ with Acr⁺-Mes.

In conclusion, iron(II) complexes are oxidized to iron(III) complexes by O₂ using Acr⁺-Mes as an organic photocatalyst in the presence of HOTf in MeCN under visible light irradiation via electron-transfer oxidation of iron(II) complexes and reduction of O₂ by the electron-transfer state of Acr⁺-Mes produced upon photoexcitation of Acr⁺-Mes, respectively. The present study provides an environmentally benign approach for oxidation of metal complexes by O₂ to obtain the oxidised metal complexes and hydrogen peroxide (H₂O₂). Because there are many synthetically useful oxidation reactions using H₂O₂,²² this study has paved a new way for photocatalytic oxidation of substrates by O₂ with organic photocatalysts and iron(II) complexes.

This work was supported by Grants-in-Aid (no. 26620154 and 26288037 to K. O.) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT); ALCA and SENTAN projects from JST, Japan (to S. F.).

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Footnote

† Electronic supplementary information (ESI) available: Experimental and kinetic details. See DOI: 10.1039/c6cc00359a

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