Photocatalytic oxygenation of 10-methyl-9,10-dihydroacridine by O₂ with manganese porphyrins.

Photocatalytic oxygenation of 10-methyl-9,10-dihydroacridine (AcrH2) by dioxygen (O2) with a manganese porphyrin [(P)Mn(III): 5,10,15,20-tetrakis-(2,4,6-trimethylphenyl)porphinatomanganese(III) hydroxide [(TMP)Mn(III)(OH)] (1) or 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinatomanganese(III) acetate [(TPFPP)Mn(III)(CH3COO)] (2)] occurred to yield 10-methyl-(9,10H)-acridone (Acr═O) in an oxygen-saturated benzonitrile (PhCN) solution under visible light irradiation. The photocatalytic reactivity of (P)Mn(III) in the presence of O2 is in proportion to concentrations of AcrH2 or O2 with the maximum turnover numbers of 17 and 6 for 1 and 2, respectively. The quantum yield with 1 was determined to be 0.14%. Deuterium kinetic isotope effects (KIEs) were observed with KIE = 22 for 1 and KIE = 6 for 2, indicating that hydrogen-atom transfer from AcrH2 is involved in the rate-determining step of the photocatalytic reaction. Femtosecond transient absorption measurements are consistent with photoexcitation of (P)Mn(III), resulting in intersystem crossing from a tripquintet excited state to a tripseptet excited state. A mechanism is proposed where the tripseptet excited state reacts with O2 to produce a putative (P)Mn(IV) superoxo complex. Hydrogen-atom transfer from AcrH2 to (P)Mn(IV)(O2(•-)) generating a hydroperoxo complex (P)Mn(IV)(OOH) and AcrH(•) is likely the rate-determining step, in competition with back electron transfer to regenerate the ground state (P)Mn(III) and O2. The subsequent reductive O-O bond cleavage by AcrH(•) may occur rapidly inside of the reaction cage to produce (P)Mn(V)(O) and AcrH(OH), followed by the oxidation of AcrH(OH) by (P)Mn(V)(O) to yield Acr═O with regeneration of (P)Mn(III).

Introduction

High-valent metal–oxo complexes are the reactive oxidants in the oxidation of various substrates with heme and nonheme iron enzymes.[1−6] Synthetic high-valent metal–oxo complexes have been prepared using oxidants such as peroxy acids, iodosylarenes, and hydrogen peroxide, and the mechanisms of oxidation of substrates by high-valent metal–oxo complexes have been studied extensively.[7−12] High-valent metal–oxo complexes have also been produced by using dioxygen (O2) with reductants.[13−18] Among various high-valent metal–oxo complexes, high-valent manganese–oxo complexes have attracted special attention because they are postulated as important intermediates for water oxidation in the oxygen-evolving center (OEC) of photosystem II.[19−27] A well-characterized manganese(V)–oxo complex has been prepared by oxidation of a manganese(III) corrolazine [(TBP8Cz)MnIII; TBP8Cz3– = octakis(p-tert-butylphenyl)corrolazinato3–] with O2 in the presence of toluene derivatives under visible light irradiation.[28,29] The (TBP8Cz)MnIII complex also acts as a photocatalyst for oxidation of 10-methyl-9,10-dihydroacridine (AcrH2) by O2.[29] However, there has been no report on photocatalytic oxidation of substrates by O2 using manganese porphyrins.

We report herein the photocatalytic oxidation of AcrH2 by O2 with manganese(III) porphyrins [(P)MnIII: (TMP)MnIII(OH) (1: TMP2– = dianion of tetramesitylporphyrin) and (TPFPP)MnIII(CH3COO) (2: TPFPP2– = dianion of 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin)] (Chart 1) under visible light irradiation in O2-saturated benzonitrile (PhCN) at room temperature. The photocatalytic reactivity of manganese porphyrin complexes is compared with that of (TBP8Cz)MnIII. The photocatalytic mechanism of oxidation of AcrH2 by O2 with (P)MnIII is investigated based on kinetic studies and laser flash transient absorption measurements to clarify the photoinduced reaction mechanism of (P)MnIII with O2.

Chart 1

Manganese(III) Porphyrins Used in This Study

Experimental Section

Materials

The (TMP)MnIII(OH) was synthesized according to published procedures.[30] The (TPFPP)MnIII(CH3COO) complex was prepared by the following procedures. First, 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (0.10 g) from Sigma-Aldrich Co., manganese(II) acetate (0.96 g) from Wako Pure Chemical Industries, Ltd. and DMF (25 mL) were placed in a round-bottomed flask (50 mL). After the mixture was refluxed and stirred for 6 h, the solvent was removed under reduced pressure to yield 0.245 g of crude (TPFPP)MnIII(CH3COO) as a dark solid that was used without further purification. 10-Methyl-9,10-dihydroacridine (AcrH2) was prepared by reduction of 10-methylacridinium perchlorate (AcrH+ClO4–, Tokyo Chemical Industry Co., Ltd.) with NaBH4.[31] PhCN was distilled over P2O5 in vacuo and stored under an argon atmosphere prior to use.[32]

Photocatalytic Reaction

The reactivity of photocatalytic oxygenation of manganese(III) complexes, 1 or 2 (1.0 × 10–5 M), with AcrH2 and O2 was evaluated by monitoring the UV–vis absorption spectral change in a quartz cuvette (10 mm × 10 mm). Visible light irradiation was carried out by a xenon lamp (500 W) through a transmitting glass filter (λ < 480 nm). In a typical experiment, 1 or 2 (1.0 × 10–5 M) was dissolved in PhCN (2.0 mL) containing AcrH2 (0–2.0 × 10–1 M) in the quartz cuvette. The mixture was degassed by bubbling O2 gas for 10 min, and then, the reaction was initiated by irradiating the solution with a xenon lamp transmitting through a color glass filter (λ > 480 nm). The electronic absorption spectral changes during photocatalytic oxygenation of 1 or 2 were monitored using a Hewlett–Packard HP8453 diode array spectrophotometer. The yield of Acr=O produced was calculated by an increase in the absorption band at 402 nm (εmax = 8.6 × 103 M–1 cm–1) in PhCN due to Acr=O.[33]

Quantum Yield Determination

A standard actinometer (potassium ferrioxalate)[34] was used to estimate the quantum yield of the photochemical oxidation of 1 (1.0 × 10–5 M) with O2 and AcrH2 (0–1.5 × 10–1 M) in PhCN (2.0 mL). Typically, a square quartz cuvette (10 mm × 10 mm) that contained an O2-saturated PhCN solution (2.0 mL) of 1 (1.0 × 10–5 M) and AcrH2 was irradiated with a Panther OPO pumped Nd:YAG laser (Continuum, SLII-10, 4–6 ns fwhm) at λ = 476 nm. Typical pulse energies for the photoexcitation of the sample solution were in the range of 10 mJ per pulse. Under the conditions of actinometry experiments, the actinometer and 1 absorbed essentially all of the incident monochromatized light at 476 nm. The light intensity at 476 nm was 1.8 × 10–9 einstein s–1. The quantum yields were estimated by monitoring the appearance of absorbance at 402 nm (εmax = 8.6 × 103 M–1 cm–1) due to Acr=O.

Femtosecond Laser Flash Photolysis Measurements

Measurements of transient absorption spectra of 1 and 2 were carried out according to the following procedures. An O2- or N2-saturated PhCN solution containing 1 (4.0 × 10–5 M) or 2 (8.0 × 10–5 M) was excited by a femtosecond laser pulse at 393 nm using an ultrafast laser source, Integra-C (Quantronix Corp.), and a second harmonic generation (SHG) unit, Apollo (Ultrafast Systems, U.S.A). The optical detection system Helios was also provided by Ultrafast Systems. The detailed instrumental setting is shown in the Supporting Information.

Kinetic analyses were assembled from the time-resolved spectral data. The decay rate of the tripquintet (5T1) of manganese(III) porphyrin obeyed the first-order kinetics given by eq 1where A1 is the pre-exponential factor for the absorption changes, A2 is the final absorbance, and k1 is the rate constant of the decay of the tripquintet (5T1) after femtosecond laser pulse irradiation. The slower decay rate of the tripseptet (7T1) also obeyed the first-order kinetics given by eq 2, where A3 is the final absorbance at 565 nm and k2 is the rate constant of the decay of 7T1.

Results and Discussion

Photocatalytic Oxidation Reaction of AcrH2 by O2 with (P)MnIIIX

The photocatalytic oxidation of 10-methyl-9,10-dihydroacridine (AcrH2) by O2 with (P)MnIIIX was performed by photoirradiation of an O2-saturated PhCN solution containing AcrH2 and (TMP)MnIII(OH) (1) or (TPFPP)MnIII(CH3COO) (2) using a xenon lamp (500 W) with a transmitting glass filter (λ > 480 nm). The absorption spectral changes in the photocatalytic oxidation of AcrH2 by O2 with 1 and 2 are shown in Figures 1a and b, respectively. The absorption spectra of 1 and 2 remained during the photocatalytic reaction, whereas the absorption band at λmax = 402 nm (εmax = 8.6 × 103 M–1 cm–1) due to 10-methyl-(9,10H)-acridone (Acr=O) appeared and the concentration of Acr=O increased linearly with photoirradiation time. The turnover numbers were determined to be 17 and 6 for 1 and 2 at 5 h of photoirradiation. It was confirmed that no formation of Acr=O was observed in the absence of O2 or the light source [see Figures S1 and S2 in the Supporting Information (SI)]. It should be noted, however, that the direct photo-oxidation of AcrH2 by O2 occurred under photoirradiation without a glass filter.[35] The stoichiometry of the photocatalytic oxidation of AcrH2 by O2 is given by eq 3.

Figure 1

UV–vis absorption spectral changes and time course of formed Acr=O (inset) in an O2-saturated PhCN solution (2.0 mL) containing (a) (TMP)MnIII(OH) (1.0 × 10–5 M) or (b) (TPFPP)MnIII(CH3COO) (1.0 × 10–5 M) and AcrH2 (2.0 × 10–1 M) under visible light irradiation (λ > 480 nm).

Kinetics

Rates of formation of Acr=O in the photocatalytic oxidation of AcrH2 with 1 in O2-saturated PhCN were monitored by an appearance in the absorbance at 402 nm due to Acr=O under photoirradiation using a xenon lamp with a transmitting glass filter (λ < 480 nm) at 298 K. The initial reaction rate of the photocatalytic oxidation of AcrH2 by O2 in Figure 2a was determined to avoid the effects of changes in the light intensity absorbed by 1 and also in the concentration of AcrH2.

Figure 2

(a) Plots of the formation of Acr=O during the photocatalytic oxygenation of AcrH2 under irradiation (λ > 480 nm) for an O2-saturated PhCN solution containing (TMP)MnIII(OH) (1.0 × 10–5 M) in the presence of 5 (red), 10 (green), 20 (blue), or 30 mM (black) of AcrH2 as a substrate at 298 K. (b) Plot of the initial reaction rate versus concentration of AcrH2.

The initial reaction rate increased with increasing concentration of AcrH2, as shown in Figure 2b. The initial reaction rates are proportional to the concentration of O2 (Figure 3). Thus, the rate law is given by eq 4, where kox is the second-order rate constant and [S] is the substrate concentration. The kox value was determined from the slope in Figure 2b to be 1.5 × 10–5 M–1 s–1 for 1 and 8.5 × 10–6 M–1 s–1 for 2 (Figures S3 in the SI).

Figure 3

Plot of the initial reaction rate for the oxidation of (TMP)MnIII(OH) (1.0 × 10–5 M) with O2 (0–8.5 × 10–3 M)[36,37] and AcrH2 (2.0 × 10–2 M) under photoirradiation by a xenon lamp (λ > 480 nm) versus the concentration of O2 in PhCN at room temperature (298 K).

The reaction solution of 1 (1.0 × 10–5 M) with O2 and AcrH2 (0–1.5 × 10–1 M) in O2-saturated PhCN (2.0 mL) was irradiated to determine the quantum yield by changing the light source from a xenon lamp with a transmitting glass filter (λ > 480 nm) to nanosecond pulse where typical pulse energies at the sample were in the range of 10 mJ per pulse laser at 476 nm. The absorption band for Acr=O appeared, and the concentration of Acr=O increased linearly with excitation numbers. The zeroth-order rate constant of the photocatalytic oxidation of AcrH2 by O2 was also observed from the initial rate in Figure 4a. The quantum yield was determined to be 0.14% when 1 (1.0 × 10–5 M) with O2 and AcrH2 (1.5 × 10–1 M) in O2-saturated PhCN (2.0 mL) was irradiated 1.0 × 105 times.

Figure 4

(a) Plots of the formation of Acr=O versus the laser excitation number for the photocatalytic oxygenation of AcrH2 with an O2-saturated PhCN solution containing (TMP)MnIII(OH) (1.0 × 10–5 M) in the presence of 5.0 (red), 10 (green), 30 (blue), or 50 (black) of AcrH2 as a substrate under irradiation by a nanosecond laser at 298 K. (b) Plot of the initial reaction rate versus concentration of AcrH2.

When AcrH2 (0.20 M) was replaced by the dideuterated compound, AcrD2, the reaction rate of formation of Acr=O became significantly slower with 22 or 6 as the deuterium kinetic isotope effect (KIE) values for 1 or 2, as shown in Figure 5. The KIE value shows that the hydrogen-atom transfer (HAT) from AcrH2 is directly involved in the rate-determining step for the photocatalytic reaction of (P)MnIII with O2 and AcrH2.

Figure 5

Time courses of Acr=O formed under photoirradiation (λ > 480 nm) of an O2-saturated PhCN solution (0.5 mL) containing (a) 1 or (b) 2 (1.7 × 10–4 M) and AcrH2 (blue, 0.20 M) or AcrD2 (red, 0.20 M).

Femtosecond Transient Absorption Measurements

Femtosecond transient absorption spectroscopy was employed to investigate the photochemical processes involved in the photocatalytic oxygenation of AcrH2 with (P)MnIII and O2. Time-resolved transient absorption measurements by femtosecond laser flash photolysis of (P)MnIII were performed in the absence and presence of O2 in PhCN. Figure 6a shows the transient absorption spectral change of 1 in N2-saturated PhCN. Femtosecond laser excitation at 393 nm resulted in an instantaneous appearance of an absorption maximum at λmax = 638 nm, which is assigned to the tripquintet excited state (5T1). It has been reported that the MnIII metal ion has a d4 ground-state electronic configuration (S = 2).[29,38] Therefore, the (π,π*) states of (P)MnIII are not the normal singlets or triplets because of coupling of the unpaired metal electrons with the ring π electrons of the corrolazine ring. The ground state is a quintet (5S0), and a quintet excited state (5S1) is derived from the lowest excited ring (π,π*) singlet.[29,38] The excited states of first-row paramagnetic complexes, such as MnIII complexes, undergo an extremely rapid intersystem crossing (ISC) process from the quintet (5S1) excited state to the tripquintet (5T1) excited state.[38−42] The transient absorption spectra in Figure 6a show the decay of the absorption band at 638 nm, indicating ISC from the tripquintet (5T1) state to the long-lived tripseptet (7T1) state, which has an absorption band at 565 nm. From the decay profile of the tripquintet (5T1), a rate constant of 2.3 × 1010 s–1 obeying first-order kinetics in degassed PhCN (Figure 6b) was determined. The rate constant of 2.5 × 1010 s–1 was also determined in aerated PhCN (Figure 6c), indicating no oxygen dependence on the rate of ISC.

Figure 6

(a) Transient absorption spectral changes (red after 10 ps, blue 50 ps, and black 3000 ps) after photoexcitation of (TMP)MnIII(OH) in PhCN. Decay time profiles of absorbance at 638 nm due to [(TMP)MnIII(OH)]* (5T1) (b) in N2-saturated PhCN and (c) in O2-saturated PhCN. Decay time profiles of absorbance due to [(TMP)MnIII(OH)]* (7T1) at λ = 565 nm under (d) N2 and (e) O2.

The absorbance at 565 nm due to 7T1 was diminished faster in O2-saturated PhCN (Figure 6e) rather than in deaerated PhCN (Figure 6d), providing a direct reaction between the excited state and O2. The second-order rate constant of the decay of [(TMP)MnIII(OH)]* (7T1) in the presence of O2 was determined to be (8.1 ± 2.0) × 109 M–1 s–1, which is comparable to the diffusion-limited rate constant in PhCN (5.6 × 109 M–1 s–1).[43] The fast absorbance decay for 7T1 also occurred when (P)MnIII replaced from 1 to 2 (Figure S4 in the SI) with 2.0 × 107 M–1 s–1 of the second-order rate constant of the decay of [(TPFPP)MnIII(CH3COO)]* (7T1). Together with the second-order rate constant of the decay of [(TBP8Cz)MnIII]* (7T1), which is 4.9 × 109 M–1 s–1,[29] the second-order rate constants of the decay of [MnIII]* (7T1) corresponding to the rate constant of bimolecular reaction between [MnIII]* (7T1) and O2 increased in the order of (TMP)MnIII(OH), (TBP8Cz)MnIII, and (TPFPP)MnIII(CH3COO). This order agrees with the order of the maximum turnover number of photocatalytic oxidation of AcrH2 in the presence of MnIII in O2-saturated PhCN under photoirradiation.

Porphyrins and metalloporphyrins are well-known to be effective triplet-state photosensitizers and to be capable of producing singlet oxygen (1O2*) due to direct energy transfer from the porphyrin excited triplet state to molecular oxygen. To examine a role for singlet oxygen in this photochemistry, the possibility of generation of 1O2* has been examined by comparison of 1O2* phosphorescence spectra in the presence of (P)MnIII or C60. The photoexcitation of (P)MnIII with light of λ = 532 nm in O2-saturated deuterated benzene (C6D6) results in a negligible phosphorescence signal at 1270 nm,[44] which is the phosphorescence spectrum of 1O2*, whereas that obtained by photoirradiation of C60 under the same conditions shows significantly intense spectra, as shown in Figure S5 in the SI. Thus, it was assessed that the contribution of 1O2* for the photochemical oxidation of (P)MnIII with O2 may be negligible as compared with an electron-transfer pathway from [(P)MnIII]* (7T1) to O2. It has been revealed that a significant role for 1O2* was ruled out upon the photochemical oxidation of (TBP8Cz)MnIII to (TBP8Cz)MnV(O) by use of a 1O2* trap reagent, 9,10-dimethylanthracene.[29]

Photocatalytic Mechanism

The mechanism of photocatalytic oxidation of AcrH2 by O2 with (P)MnIII to give Acr=O as the product is proposed according to the photodynamics of (P)MnIII, together with the large KIE values, as shown in Scheme 1. After photoexcitation of (P)MnIII, the tripquintet excited state ([(P)MnIII]* (5T1)) is produced and converted rapidly to the triplet excited state ([(P)MnIII]* (7T1)) by ISC. Generated [(P)MnIII]* (7T1) undergoes electron transfer from [(P)MnIII]* (7T1) to O2 to produce the superoxo complex [(P)MnIV(O2•–)], followed by HAT from O2•– moiety to the (P)MnIV to produce the hydroperoxo complex (P)MnIV(OOH) and acridinyl radical (AcrH•), which is the rate-determining step of the overall reaction. This reaction is most likely in competition with the back electron transfer from O2•– moiety to the (P)MnIV to regenerate the ground state of (P)MnIII and O2. The subsequent O–O bond cleavage by AcrH• may occur rapidly inside of the reaction cage before the reaction of AcrH• with O2 to yield (P)MnV(O) and 9-hydroxy-10-methyl-9,10-dihydroacridine [AcrH(OH)]. This is followed by subsequent facile oxidation of AcrH(OH) by (P)MnV(O) to yield Acr=O, accompanied by regeneration of (P)MnIII (Scheme 1).

Scheme 1

Mechanism of Photocatalytic Oxidation of AcrH2 in the Presence of (P)MnIII and O2

When the hydrogen transfer from AcrH2 to (P)MnIV(O2•–) is the rate-determining step in the catalytic cycle in Scheme 1, the rate of formation of Acr=O is given by eq 5, where kH is the rate of HAT from AcrH2 to (P)MnIV(O2•–).The rate of formation and decay of (P)MnIV(O2•–) is given by eq 6where ket is the rate constant of electron transfer from [(P)MnIII]* (7T1) to O2 to produce (P)MnIV(O2•–) and k–et is the back electron transfer from the O2•– moiety to the (P)MnIV moiety to regenerate (P)MnIII and O2. The rate of formation and decay of 7T1 is given by eq 7where Φ0 is the quantum yield of formation of 7T1, In is the light intensity absorbed by 1, and k2 is the decay rate constant of 7T1 without O2. By applying the steady-state approximation, the steady-state concentration of 7T1 is derived from eq 7, as shown by eq 8The steady-state concentration of MnIV(O2•–) is also derived from eqs 6 and 8, as given by eq 9Then, the quantum yield for photocatalytic oxidation of AcrH2 is derived from eqs 5 and 9, as given by eq 10Equation 10 is rewritten by eq 11where there are linear correlations for Φ–1 versus [AcrH2]−1 and Φ–1 versus [O2]−1. Linear plots of Φ–1 versus [AcrH2]−1 and Φ–1 versus [O2]−1 are shown in Figure 7a and b, respectively. From the slopes and intercepts of the linear plots, the kH/k–et and ket/k2 values were obtained as 53 and 7, respectively.

Figure 7

Linear plots of Φ–1 versus [AcrH2]−1 and Φ–1 versus [O2]−1.

Conclusions

Photocatalytic oxidation of AcrH2 by O2 with (P)MnIII (1 and 2) in PhCN occurs to produce Acr=O as the sole oxidation product. The kinetic and laser flash photolysis measurements revealed the photocatalytic mechanism, as shown in Scheme 1, where electron transfer from the excited state ([(P)MnIII]* (7T1)) to O2 occurs to produce the superoxo complex [(P)MnIV(O2•–)], which oxidizes AcrH2 to Acr=O via hydrogen-atom transfer from AcrH2 to [(P)MnIV(O2•–)] and formation of (P)MnV(O). The photocatalytic reactivity of (P)MnIII agrees with the rate constants of electron transfer from [(P)MnIII]* (7T1) to O2 in the order (TMP)MnIII(OH) > (TBP8Cz)MnIII > (TPFPP)MnIII(CH3COO). The present study paves the way for development of new photocatalytic oxidation of substrates by O2 using manganese porphyrins.