Photochemical Oxidation of a Manganese(III) Complex with Oxygen and Toluene Derivatives to Form a Manganese(V)-Oxo Complex

Abstract

Visible light photoirradiation of an oxygen-saturated benzonitrile solution of a manganese(III) corrolazine complex [(TBP₈Cz)Mn^III (1): [TBP₈Cz = octakis(p-tert-butylphenyl)corrolazinato³⁻] in the presence of toluene derivatives resulted in formation of the manganese(V)-oxo complex [(TBP₈Cz)Mn^V(O)]. The photochemical oxidation of (TBP₈Cz)Mn^III with O₂ and hexamethylbenzene (HMB) led to the isosbestic conversion of 1 to (TBP₈Cz)Mn^V(O), accompanied by the selective oxidation of HMB to pentamethylbenzyl alcohol (87%). The formation rate of (TBP₈Cz)Mn^V(O) increased with methyl group substitution, from toluene, p-xylene, mesitylene, durene, pentamethylbenzene, up to hexamethylbenzene. Deuterium kinetic isotope effects (KIEs) were observed for toluene (KIE = 5.4) and mesitylene (KIE = 5.3). Femtosecond laser flash photolysis of (TBP₈Cz)Mn^III revealed the formation of a tripquintet excited state, which was rapidly converted to a tripseptet excited state. The tripseptet excited state was shown to be the key, activated state that reacts with O₂ via a diffusion-limited rate constant. The data allow for a mechanism to be proposed in which the tripseptet excited state reacts with O₂ to give the putative (TBP₈Cz)Mn^IV(O₂^•−), which then abstracts a hydrogen atom from the toluene derivatives in the rate-determining step. The mechanism of hydrogen abstraction is discussed by comparison of the reactivity with the hydrogen abstraction from the same toluene derivatives by cumylperoxyl radical. Taken together, the data suggest a new catalytic method is accessible for the selective oxidation of C-H bonds with O₂ and light, and the first evidence for catalytic oxidation of C–H bonds was obtained with 10-methyl-9,10-dihydroacridine as a substrate.

INTRODUCTION

High-valent metal-oxo species are key oxidizing intermediates in a variety of biological oxidation reactions mediated by heme and non-heme metalloenzymes involved in respiration, metabolism, and photosynthesis.^1-4 Synthetic high-valent metal-oxo complexes (M⁽ⁿ⁺²⁾⁺(O)) are usually formed by reactions of metal complexes (Mⁿ⁺) with two-electron oxidants such as iodosylarenes, peroxy acids, and H₂O₂ [eqn (1)].^5-16 However, enzymes such as cytochrome

$${\mathrm{M}}^{n+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{M}}^{(n+2)+}\left(\mathrm{O}\right)+{\mathrm{H}}_2\mathrm{O}$$ (1)

P450 use dioxygen (O₂) together with the addition of two electrons and two protons, which is equivalent to H₂O₂, to generate a high-valent metal-oxo species [eqn (2)].^1,2 On the other hand,

$${\mathrm{M}}^{n+}+{\mathrm{O}}_2+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{M}}^{(n+2)+}\left(\mathrm{O}\right)+{\mathrm{H}}_2\mathrm{O}$$ (2)

high-valent manganese-oxo species are implicated in the water oxidation mechanism of Photosystem II, and may involve the formal two-electron oxidation of a H₂O molecule by a tetranuclear manganese complex to give high-valent Mn(O) intermediates.^4,5 Synthetic high-valent metal-oxo complexes have also been reported to be formed by two-electron oxidation of metal complexes with H₂O as the oxygen source [eqn (3)].^17-23

$${\mathrm{M}}^{n+}+{\mathrm{H}}_2\mathrm{O}-2{\mathrm{H}}^{+}-2{\mathrm{e}}^{-}\to {\mathrm{M}}^{(n+2)+}\left(\mathrm{O}\right)$$ (3)

There have been comparatively few reports on the formation of discrete, high-valent metal-oxo complexes using O₂.^24-26 The activation of O₂ and stabilization of high-valent metal-oxo species is challenging to carry out in a single ligand environment. Porphyrinoid ligands, and in particular corroles and corrolazines (Czs), are designed to stabilize high-valent species including high-valent metal-oxo complexes, and there are examples of Cr^V(O) corroles generated from O₂.²⁷ It has recently been reported that the oxidation of a Mn^III corrolazine complex with O₂ in cyclohexane or toluene as solvent under visible light irradiation leads to the quantitative production of a well-characterized, isolabe Mn^V(O) complex.²⁸ Relatively short-lived metal-oxo species have been generated by photo-initiated homolytic axial ligand cleavage reactions with metalloporphyrins, as well as photochemical splitting of metalloporphyrin μ-oxo dimers.^29,30 However, to our knowledge the former photo-initiated reaction involving Mn^III(Cz) is the first example of the production of a well-defined Mn^V(O) species from O₂.

In the former study, the combined data suggested a mechanism for the generation of Mn^V(O)(Cz) that involved solvent-assisted autoxidation, in which the solvent (cyclohexane or toluene) served as a sacrificial reductant for the activation and cleavage of a putative photactivated Mn-O₂ intermediate. The nature of the photoexcited state, and the product(s) of solvent oxidation were not identified in this study. The proposed mechanism suggested that, perhaps under conditions involving an inert solvent, the controlled oxidation of substrates such as toluene derivatives could be mediated by Mn^III(Cz)/O₂/light.

We report herein the use of benzonitrile (PhCN) as an inert solvent for the reaction of the Mn^III complex [(TBP₈Cz)Mn^III: TBP₈Cz = octakis(p-tert-butylphenyl)corrolazinato³⁻] with O₂ under visible light irradiation. The conversion of (TBP₈Cz)Mn^III to (TBP₈Cz)Mn^V(O) does not occur in PhCN in the absence of a proton/electron source, but with the addition of a series of toluene derivatives as substrates the smooth production of the Mn^V(O) complex is observed. The concommitant oxidation of the toluene derivatives occurs with high selectivity and efficiency, leading to monohydroxylation of a benzylic position. Femtosecond laser flash photolysis (LFP) measurements resulted in the spectroscopic observation of a short-lived, O₂-reactive (TBP₈Cz)Mn^III excited state. The LFP experiments, together with product analyses and kinetic measurements, including kinetic deuterium isotope effects, provide valuable insights into the mechanism of generation of the Mn^V(O) complex by the photochemical oxidation of the Mn^III complex using O₂ as an oxygen source and toluene derivatives as the source of protons and electrons.

EXPERIMENTAL SECTION

Materials

The starting material (TBP₈Cz)Mn^III was synthesized according to published procedures.³¹ The commercially available reagents (p-xylene, mesitylene, durene, pentamethylbenzene, and hexamethylbenzene) were purchased with the best available purity and used without further purification. Toluene and benzonitrile (PhCN) were dried according to literature procedures³² and distilled under Ar prior to use. The reagents 2,3,4,5,6-pentamethylbenzyl alcohol and pentamethylbenzaldehyde were also purchased with the best available purity and used without further purification from Wako Pure Chemical Industries, Ltd. and Tokyo Kasei Co., Ltd, respectively. Di-tert-butylperoxide was purchased from Nacalai Tesque Co., Ltd. and purified by chromatography through alumina, which removes traces of the hydroperoxide. 10-Methyl-9,10-dihydroacridine (AcrH₂) was prepared from 10-methylacridinium perchlorate (AcrH⁺ClO⁴⁻) by reduction with NaBH₄ in methanol and purified by recrystallization from ethanol.³³ Cumene was purchased from Tokyo Kasei Co., Ltd. and also purified by chromatography through alumina. Deuterated toluene and mesitylene were purchased from Cambridge Isotopes in the highest purity and used as received.

Product Analysis

A PhCN solution of (TBP₈Cz)Mn^III (5.0 × 10⁻⁴ M) was added with a microsyringe into an O₂-saturated PhCN solution containing hexamethylbenzene (2.0 × 10⁻² M) as a substrate in a quartz cell. A rather large concentration of (TBP₈Cz)Mn^III was employed to obtain detectable amounts of products by GC-MS. The mixture of the reaction was filled with O₂ and then irradiated for 24 h at room temperature, when the solution changed from the green-brown color indicative of (TBP₈Cz)Mn^III to the bright-green color of (TBP₈Cz)Mn^V(O). Complete conversion of (TBP₈Cz)Mn^III to (TBP₈Cz)Mn^V(O) was confirmed by UV-vis spectroscopy by sampling an aliquot of the reaction mixture. An aliquot was removed and injected directly into the GC-MS for analysis. All peaks of interest were identified by comparison of retention times and co-injection with authentic samples. Compounds were quantified by comparison against a known amount of detected products using a calibration curve consisting of a plot of mole versus area. Calibration curves were prepared by using concentrations in the same range as that observed in the actual reaction mixtures. Mass spectra were recorded with a JEOL JMS-700T Tandem MS station, and the GC-MS analyses were carried out by using a Shimadzu GCMS-QP2000 gas chromatograph mass spectrometer. GC-MS conditions in these experiments were performed as follows: an initial oven temperature of 60 °C was held for 1 min and then raised 30 °C min⁻¹ for 7.3 min until a temperature of 280 °C was reached, which was then held for further 10 min. This experiment was repeated three times, exhibiting error in yields within 10%.

Spectral and Kinetic Measurements

The photochemical oxidations of (TBP₈Cz)Mn^III (7.6 × 10⁻⁶ M) with excess O₂ were examined by monitoring UV-vis spectral changes in the presence of a large excess of toluene derivatives (2.0 × 10⁻² to 9.4 M) at 298 K using a Hewlett-Packard HP8453 diode array spectrophotometer under continuous irradiation of white light from a Shimadzu RF-5300PC fluorescence spectrophotometer at the same time. Typically, an aerated PhCN solution of (TBP₈Cz)Mn^III was added with a microsyringe to an aerated PhCN solution containing the toluene derivatives in a quartz cell (total vol. 2.0 mL). Rates of oxidation reaction of (TBP₈Cz)Mn^III to produce (TBP₈Cz)Mn^V(O) were monitored by the decrease in the absorption band due to (TBP₈Cz)Mn^III (λ_max = 695 nm, ε_max = 3.51 × 10⁴ M⁻¹ cm⁻¹). The concentration of toluene derivatives were maintained in large excess as compared to [(TBP₈Cz)Mn^III] for all kinetic measurements. The limiting concentration of O₂ in PhCN solution was prepared by a mixed gas flow of O₂ and N₂. The mixed gas was controlled by using a gas mixer (Kofloc GB-3C, KOJIMA Instrument Inc.), which can mix two or more gases at a certain pressure and flow rate.

Quantum Yield Determination

A standard actinometer (potassium ferrioxalate)³⁴ was used for the quantum yield determination of the photochemical oxidation of (TBP₈Cz)Mn^III with O₂ and toluene derivatives in O₂-saturated PhCN. Typically, a square quartz cuvette (10 mm i.d.), which contained an O₂-saturated PhCN solution (2.0 mL) of (TBP₈Cz)Mn^III (7.6 × 10⁻⁶ M) and a toluene derivative was irradiated with monochromatic light of λ = 450 nm from a Shimadzu RF-5300PC fluorescence spectrophotometer. Under the conditions of actinometry experiments, the actinometer and (TBP₈Cz)Mn^III absorbed essentially all of the incident light at λ = 450 nm. The light intensity of monochromatized light at λ = 450 nm was determined to be 3.7 × 10⁻⁸ einstein s⁻¹. The quantum yields were determined by monitoring the disappearance of absorbance at 695 nm due to (TBP₈Cz)Mn^III.

Photocatalytic Activity Measurements

The photocatalytic reactivity of (TBP₈Cz)Mn^III (1.7 × 10⁻⁴ M) with excess of O₂ was examined by monitoring the UV-vis spectral changes in the presence of a large excess of AcrH₂ (0.2 M) in a quartz cell (optical path length 1 mm) using a Xe lamp (500 W) for irradiation through a transmitting glass filter (λ > 480 nm) at room temperature. In a typical experiment, (TBP₈Cz)Mn^III (1.7 × 10⁻⁴ M) was dissolved in PhCN (0.5 mL) containing excess AcrH₂ (0.2 M). The solution was purged with O₂ gas for 10 min in the quartz cell, and then the reaction was initiated by irradiating the solution with a Xe lamp (500 W) transmitting through a glass filter (λ > 480 nm). The UV-vis spectra of the solution were measured using a Hewlett-Packard HP8453 diode array spectrophotometer every 1 h. The amount of 10-methyl-(9,10H)-acridinone (Acr=O) produced was quantified by an increase in the absorption band due to Acr=O (λ = 402 nm, ε_max = 8.6 × 10³ M⁻¹ cm⁻¹ in PhCN).

Femtosecond Laser Flash Photolysis Measurements

Measurements of transient absorption spectra in the oxidation reaction of (TBP₈Cz)Mn^III were performed according to the following procedures. An O₂- or N₂-saturated PhCN solution containing (TBP₈Cz)Mn^III (6.8 × 10⁻⁵ M) was excited using an ultrafast source, Integra-C (Quantronix Corp.), an optical parametric amplifier, TOPAS (Light Conversion Ltd.), and a commercially available optical detection system, Helios provided by Ultrafast Systems LLC. The source for the pump and probe pulses were derived from the fundamental output of Integra-C (λ = 786 nm, 2 mJ/pulse and fwhm = 130 fs) at a repetition rate of 1 kHz. 75% of the fundamental output of the laser was introduced into a second harmonic generation (SHG) unit: Apollo (Ultrafast Systems) for excitation light generation at λ = 393 nm, while the rest of the output was used for white light generation. The laser pulse was focused on a sapphire plate of 3 mm thickness and then white light continuum covering the visible region from λ = 410 nm to 800 nm was generated via self-phase modulation. A variable neutral density filter, an optical aperture, and a pair of polarizer were inserted in the path in order to generate stable white light continuum. Prior to generating the probe continuum, the laser pulse was fed to a delay line that provides an experimental time window of 3.2 ns with a maximum step resolution of 7 fs. In our experiments, a wavelength at λ = 393 nm of SHG output was irradiated at the sample cell with a spot size of 1 mm diameter where it was merged with the white probe pulse in a close angle (< 10 °). The probe beam after passing through the 2 mm sample cell was focused on a fiber optic cable that was connected to a CMOS spectrograph for recording the time-resolved spectra (λ = 410-800 nm). Typically, 1500 excitation pulses were averaged for 3 seconds to obtain the transient spectrum at a set delay time. Kinetic traces at appropriate wavelengths were assembled from the time-resolved spectral data. The decay rate of the tripquintet (⁵T₁) obeyed the first-order kinetics given by eqn (4) where A₁ and A₂ are pre-

$$\Delta \mathrm{Abs}=A_1\exp (-k_{1}t)+A_2$$ (4)

exponential factors for the absorbance changes and k₁ is the rate constant of the decay of the tripquintet (⁵T₁) after irradiation. The slower decay rate of the tripseptet (⁷T₁) also obeyed the first-order kinetics given by eqn (5), where A₃ is the final absorbance at 774 nm and k₂ is the rate

$$\Delta \mathrm{Abs}=A_1\exp (-k_{1}t)+A_2\exp (-k_{2}t)+A_3$$ (5)

constant of the decay of ⁷T₁. All measurements were conducted at room temperature, 298 K.

EPR Measurements

Photoirradiation of an oxygen-saturated PhCN solution containing di-tert-butylperoxide (1.0 M) and cumene (1.0 M) with a 1000 W mercury lamp (Ushio-USH1005D) through an aqueous filter resulted in formation of cumylperoxyl radical (g = 2.0156).³⁵ The EPR spectra were measured with a JEOL X-band spectrometer (JES-RE1XE). The ESR spectra were recorded under non-saturating microwave power conditions. The magnitude of modulation was chosen to optimize the resolution and the signal-to-noise (S/N) ratio of the observed spectra. The g value was calibrated by using a Mn²⁺ marker. Upon cutting off the light irradiated through a window directly, the decay of the EPR intensity was recorded with time. The decay rates were accelerated by the presence of toluene derivatives, indicating hydrogen atom transfer occurred. Rates of hydrogen atom transfer from toluene derivatives to cumylperoxyl radical were monitored by measuring the decay of the EPR signal in the presence of various concentrations of toluene derivatives in propionitrile at −80 °C. Pseudo-first-order rate constants were determined by a least-squares curve fitting procedure. The first-order plots of ln(I − I_∞) vs time (I and I_∞ are the EPR intensity at time t and the final intensity, respectively) were linear for three or more half-lives with the correlation coefficient, ρ > 0.99. Plot of the pseudo-first-order rate constants against the substrate concentrations gave linear lines.

RESULTS AND DISCUSSION

Photochemical Oxidation of (TBP₈Cz)Mn^III with O₂ and Toluene Derivatives

The photochemical generation of a Mn^V(O) complex using O₂ as an oxygen source was performed by photoirradiation of an aerated PhCN solution containing a Mn^III complex [(TBP₈Cz)Mn^III] and toluene derivatives (Scheme 1). Monitoring this reaction by UV-vis spectroscopy revealed the transformation of (TBP₈Cz)Mn^III (λ_max = 695 nm, ε_max = 3.51 × 10⁴ M⁻¹ cm⁻¹) to the Mn^V(O) complex [(TBP₈Cz)Mn^V(O)] (λ_max = 634 nm, ε_max = 2.0 × 10⁴ M⁻¹ cm⁻¹)²⁸ with isosbestic points as shown in Figure 1. No further reaction of (TBP₈Cz)Mn^V(O) with toluene occurred for another 1 hour (Figure S1 in Supporting Information (SI)), as expected from previous work.^28,36 No formation of (TBP₈Cz)Mn^V(O) was observed in the absence of O₂ or toluene derivatives, indicating both toluene derivatives and O₂ are certainly required to generate (TBP₈Cz)Mn^V(O) (Figure S2 and S3 in SI).

Scheme 1.

Formation of (TBP₈Cz)Mn^V(O) from (TBP₈Cz)Mn^III with O₂ and Toluene Derivatives under Photoirradiation (White Light)

Figure 1.

(a) UV-vis spectral changes and (b) time profiles of absorption changes at λ = 695 nm and 634 nm for the photochemical oxidation reaction of (TBP₈Cz)Mn^III (7.6 × 10⁻⁶ M) under irradiation (white light) in an aerobic solution of PhCN containing hexamethylbenzene (0.08 M) as a substrate at room temperature.

The photochemical oxidation of (TBP₈Cz)Mn^III with O₂ and hexamethylbenzene to produce (TBP₈Cz)Mn^V(O) is accompanied by the oxidation of hexamethylbenzene. Analysis of the reaction mixture by GC-MS reveals the major product of oxidation to be the pentamethylbenzyl alcohol in 87% yield (based on total Mn content), along with a small amount of pentamethylbenzaldehyde (8%) (see Figures S4 – S5, and Table S1 in SI). Thus the oxidation of hexamethylbenzene occurs with high selectivity, and the stoichiometry of the main reaction is given by eqn (6).

$$\left({\mathrm{TBP}}_8\mathrm{Cz}\right){\mathrm{Mn}}^{III}+{\mathrm{O}}_2+{\mathrm{C}}_6{\mathrm{Me}}_6\underrightarrow{nv}\left({\mathrm{TBP}}_8\mathrm{Cz}\right){\mathrm{Mn}}^V\left(\mathrm{O}\right)+C_6{\mathrm{Me}}_5{\mathrm{CH}}_2\mathrm{OH}$$ (6)

Kinetics

Rates of formation of (TBP₈Cz)Mn^V(O) in the photochemical oxidation of (TBP₈Cz)Mn^III with O₂ in the air and toluene derivatives were determined from the decay of absorbance at 695 nm due to (TBP₈Cz)Mn^III in aerated PhCN at 298 K. The zeroth-order rate constant (k_obs) was determined from the initial rate in Figure 1b in order to avoid the decrease in the light intensity absorbed by (TBP₈Cz)Mn^III in the course of the photochemical reaction. The k_obs values were derived from the initial slopes of the plots of [(TBP₈Cz)Mn^III] vs time for the different substrates, as shown in Figures S6 − S11 (see Supporting Information). The observed rate constants are proportional to concentrations of substrates, as shown in Figure 2 (see Supporting Information for separate Figures in Figure S12). The rates are also proportional to concentration of O₂ (Figure 3). Thus, the rate law is given by eqn (7), where k_ox is a second-order

$$-\mathrm{d}\left[\left({\mathrm{TBP}}_8\mathrm{Cz}\right){\mathrm{Mn}}^{III}\right]∕dt=k_{ox}\left[\mathrm{S}\right]\left[{\mathrm{O}}_2\right]$$ (7)

Supporting Information for separate Figures in Figure S12). The rates are also proportional to concentration of O₂ (Figure 3). Thus, the rate law is given by eqn (7), where k_ox is a second-order rate constant and [S] is substrates concentration. The slope of the best-fit lines for the k_obs values for the different substrates in Figure 2 yield first-order rate constants with respect to substrate concentration. These values can then be divided by the excess O₂ concentration (1.7 × 10⁻³ M) to give the second-order rate constants (k_ox) listed in Table 1. Interestingly, the second-order rate constants increase with an increasing number of methyl groups in the substrate. A dramatic 200- fold rate enhancement is observed in the presence of hexamethylbenzene over toluene.

Figure 2.

Plots of the observed zeroth-order rate constants (k_obs) of the oxidation reaction of (TBP₈Cz)Mn^III (7.6 × 10⁻⁶ M) under irradiation (white light) of an aerated PhCN solution containing toluene (red, 1.0 to 8.2 M), p-xylene (orange, 1.0 to 4.0 M), mesitylene (green, 0.5 to 2.0 M), durene (blue, 0.1 to 0.4 M), pentamethylbenzene (purple, 2.0 × 10⁻² to 1.0 × 10⁻¹ M) or hexamethylbenzene (black, 2.0 × 10⁻² to 1.0 × 10⁻¹ M) as a substrate at 298 K.

Figure 3.

Plot of the observed zeroth-order rate constants (k_obs) for the oxidation of (TBP₈Cz)Mn^III (7.6 × 10⁻⁶ M) with O₂ and hexamethylbenzene (0.10 M) under photoirradiation M) (white light) in PhCN vs the oxygen concentration (0 − 8.5 × 10⁻³ M)³⁷ at 298 K.

Table 1.

Second-Order Rate Constants for the Oxidation of (TBP₈Cz)Mn^III (k_ox) by O₂ with Toluene Derivatives in Aerated PhCN at 298 K and for the Hydrogen-Atom Transfer from Toluene Derivatives to Cumylperoxyl Radicals (k’_H) in Aerated Propionitrile at 193 K

substrate	kox, M−1 s−1	k’H, M−1 s−1
toluene	(4.0 ± 0.4) × 10−7	(6.5 ± 0.2) × 10−3
p-xylene	(1.1 ± 0.3) × 10−7	(8.3 ± 0.3) × 10−3
mesitylene	(2.7 ± 0.1) × 10−6	(9.2 ± 0.4) × 10−3
durene	(1.2 ± 0.4) × 10−6	(2.3 ± 0.1) × 10−2
pentamethylbenzene	(3.6 ± 0.1) × 10−5	(4.7 ± 0.2) × 10−2
hexamethylbenzene	(8.0 ± 0.2) × 10−5	(1.3 ± 0.1) × 10−1

When toluene (2.0 M) was replaced with its deuterated analog (C₆D₅CD₃), the reaction rate of formation of (TBP₈Cz)Mn^V(O) with O₂ became significantly slower as shown in Figure 4a (blue for C₆H₅CH₃ and red for C₆D₅CD₃). The deuterium kinetic isotope effect (KIE) for toluene was determined to be 5.4. A similar KIE was obtained for the case of mesitylene (KIE = 5.3) as shown in Figure 4b (blue for mesitylene and red for mesitylene-d₁₂). The bond-dissociation energy of the aryl C-H for benzene (109.8 kcal mol⁻¹) is over 20 kcal mol⁻¹ higher than the methyl C-H for toluene (87.2 kcal mol⁻¹).³⁸ Thus H-atom abstraction from the methyl C-H is dramatically favored, and it is reasonable to expect that H-atom transfer from the methyl C-H occurs rather than from the aryl C-H. In fact, the photochemical reaction of benzene showed negligible formation of (TBP₈Cz)Mn^V(O) (Figure S13 in SI). The fact that the methyl group of hexamethylbenzene was oxygenated to yield pentamethylbenzyl alcohol provides additional evidence for this mechanism. Thus the KIE value corresponds to that of H-atom transfer from the methyl C-H rather than the aryl C-H. Taken together, the findings of increasing reaction rate with increasing the number of abstractable hydrogen atoms along with the observed large KIEs suggest that hydrogen-atom abstraction from the toluene derivative is directly involved in the rate-determining step for the photochemical oxidation of (TBP₈Cz)Mn^III with O₂ and toluene derivatives.

Figure 4.

Time dependence of photochemical oxidation of (TBP₈Cz)Mn^III (8.0 × 10⁻⁶ M) with (a) M) C₆H₅CH₃ (blue, 2.0 M), C₆D₅CD₃ (red, 2.0 M), (b) mesitylene (blue, 2.0 M), or mesitylene-d₁₂ (red, 2.0 M) under irradiation (white light) in O₂-saturated PhCN at 298 K.

Femtosecond Transient Absorption Measurements

In order to detect the photoexcited state involved in the photochemical oxidation of (TBP₈Cz)Mn^III with O₂, the femtosecond laser flash photolysis measurements of (TBP₈Cz)Mn^III were performed in the absence and presence of O₂ in PhCN. The Mn^III metal ion has a high spin d⁴ ground state electronic configuration (S = 2) with only the high energy d_x2-y2 orbital unoccupied in the ground state.³⁹ Because of the coupling between unpaired electrons of the metal with the π electrons of the corrolazine ring, the ground state of (TBP₈Cz)Mn^III is a singquintet (⁵S₀). Upon femtosecond laser excitation, instantaneously formed minima at λ_min = 450 and 695 nm can be observed as shown in Figure 5a which closely mirror the ground-state absorption spectrum (Figure 1a). A new absorption maximum at λ_max = 530 nm is also formed and can be assigned to the tripquintet (⁵T₁) excited state. It is known that first row paramagnetic complexes, such as Mn^III complexes, undergo an extremely rapid intersystem crossing process from the singquintet (⁵S₁) excited state to the tripquintet (⁵T₁) excited state due to the presence of unpaired electrons.⁴⁰ In Mn^III porphyrins, for example, the existence of two tripmultiplet levels was suggested where a tripquintet (⁵T₁) relaxes to a long-lived tripseptet (⁷T₁), which requires a spin conversion to go back to the quintet ground state.^40,41 In the Mn corrolazine complex, we attribute the decay of the absorbance band at 530 nm to intersystem crossing from the ⁵T₁ state to the ⁷T₁ state, which has a small absorption band at 774 nm. The decay rate of the tripquintet (⁵T₁) obeyed first-order 7 ⁸ kinetics with a rate constant of 1.8 × 10¹⁰ s⁻¹ in deaerated PhCN (Figure 5b). The similar decay 9 ¹⁰ rate constant was observed in O₂-saturated PhCN (Figure 5c) showing no oxygen dependence on 11 ¹² the rate of intersystem crossing.

Figure 5.

(a) Transient absorbance spectral changes (purple after 1 ps, blue 10 ps, green 100 ps, III orange 1000 ps, and red 3000 ps) after photoexcitation of (TBP₈Cz)Mn in PhCN. Time profile of the generation and decay of [(TBP₈Cz)Mn^III_]* (⁵T ) at λ = 530 nm under (b) N and (c) O . The −3 red lines are exponential fitting given in eqn (4) (see Experimental Section). A₁ = 3.3 × 10 A₂ = 1.5 × 10⁻⁴, and k (c).

The absorbance at 774 nm due to ⁷T₁ remained unchanged up to 3000 ps in deaerated PhCN (Figure 6b), whereas the absorbance decays in O₂-saturated PhCN (Figure 6c), signaling a direct reaction between the excited state and O₂. The second-order rate constant of the decay of [(TBP₈Cz)Mn^III]^* (⁷T₁) in the presence of O₂ was determined to be 4.9 × 10⁹ M⁻¹ s⁻¹, which is comparable to the diffusion-limited rate constant in PhCN.⁴² Because the one-electron oxidation potential of [(TBP₈Cz)Mn^III]^* (⁷T₁) (−0.90 V vs SCE)^43-45 is more negative than the one-electon reduction potential of O₂ (−0.87 V vs SCE),⁴⁶ electron transfer from [(TBP₈Cz)Mn^III]^* (⁷T₁) to O₂ may occur efficiently to produce the Mn(IV)-superoxo complex, (TBP₈Cz)Mn^IV(O₂^•−).⁴⁷

Figure 6.

(a) Transient absorbance spectral changes (purple after 1 ps, blue 10 ps, green 100 ps, III orange 1000 ps, and red 3000 ps) after photoexcitation of (TBP₈Cz)Mn in PhCN. Decay time profiles of absorbace at 774 nm due to [(TBP₈Cz)Mn^III_]* (⁷T ) (b) in N-saturead PhCN and (c) in O₂-saturated PhCN. Best-fit lines (red) obtained from a double-exponential kinetic model given in eqn (5) (see Experimental Section). A₁ = 1.4 × 10, A₂ = 6.7 × 10 A₃ = 3.6 × 10 k₁ = 4.8 × 10⁹ k = 6.3 × 10⁷ for (b) and A = 1.0 × 10⁻³, A = 1.1 × 10⁻³, A = -4.5 × 10⁻⁴, k= 5.0 × 10⁹ and k = 1.1 × 10⁸ for (c).

An alternative mechanism could involve direct energy transfer from [(TBP₈Cz)Mn^III]^* (⁷T₁) to O₂ to produce singlet oxygen (¹O₂^*) and the ground state (TBP₈Cz)Mn^III (S = 2), which is a spin- allowed process. In the initial report on the photochemical oxidation of (TBP₈Cz)Mn^III to (TBP₈Cz)Mn^V(O), a significant role for singlet oxygen was ruled out by use of the ¹O₂^* trap, 9,10-dimethylanthracene.²⁸ However, to eliminate the possibility of ¹O₂^* as the major oxidant under inert solvent conditions as employed in the present study, we looked for the presence of ¹O₂^* by its phosphorescence spectrum (λ_max = 1270 nm).^37,48 The photoexcitation of (TBP₈Cz)Mn^III in O₂-saturated C₆D₆ (PhCN could not be used because of the short lifetime of ¹O₂) results in a much smaller phosphorescence signal at 1270 nm than that obtained by photoexcitation of C₆₀ under the same conditions.⁴⁹ (see Figure S12 in SI) Thus, the contribution of ¹O₂^* for the photochemical oxidation of (TBP₈Cz)Mn^III with O₂ may be negligible as compared with an electron-transfer pathway from [(TBP₈Cz)Mn^III]^* (⁷T₁) to O₂.

Reaction Mechanism

Based on the photodynamics of (TBP₈Cz)Mn^III, together with the effect of added substrate and the large observed KIEs, the mechanism of photochemical oxidation of (TBP₈Cz)Mn^III with O₂ and toluene as the substrate to produce (TBP₈Cz)Mn^V(O) is proposed as shown in Scheme 2. Upon photoexcitation of (TBP₈Cz)Mn^III, the produced tripquintet excited state ([(TBP₈Cz)Mn^III]^* (⁵T₁)) is converted rapidly by intersystem crossing to the triplet excited state ([(TBP₈Cz)Mn^III]^* (⁷T₁)). Electron transfer from [(TBP₈Cz)Mn^III]^* (⁷T₁) to O₂ occurs to produce the superoxo complex [(TBP₈Cz)Mn^IV(O₂^•−)]. Hydrogen-atom transfer from toluene to (TBP₈Cz)Mn^IV(O₂^•−) then follows as the rate-determining step, generating thehydroperoxo complex (TBP₈Cz)Mn^IV(OOH) and benzyl radical. This reaction is most likely in competition with the back-reaction of electron transfer to regenerate the ground state (TBP₈Cz)Mn^III and O₂. The subsequent homolytic O−O bond cleavage by benzyl radical may occur rapidly inside the reaction cage before the reaction of benzyl radical with O₂ to yield (TBP₈Cz)Mn^V(O) and benzyl alcohol (an oxygen rebound pathway). Alternatively, electron transfer may occur from benzyl radical derivatives to (TBP₈Cz)Mn^IV(OOH) to produce an ion pair {(TBP₈Cz)Mn^III(OOH)}⁻ and benzyl cation (PhCH₂)⁺, followed by heterolytic O−O bond cleavage to yield (TBP₈Cz)Mn^V(O) and benzyl alcohol derivatives. At present the homolysis vs heterolysis pathway has yet to be distinguished.

Scheme 2.

Mechanism of Photochemical Oxidation of (TBP₈Cz)Mn^III with O₂ and Toluene for Generation of (TBP₈Cz)Mn^v (O)

According to Scheme 2, the quantum yield of formation of (TBP₈Cz)Mn^V(O) is given by eqn (8), where Φ₀ is the quantum yield of photoinduced formation of (TBP₈Cz)Mn^IV(O₂^*−), k_H is the

$$\Phi ={\Phi }_0(k_{\mathrm{H}}{k}_{\mathrm{et}}∕)(k_{\mathrm{H}}+k_{-\mathrm{et}}))\left[\mathrm{S}\right]\left[{\mathrm{O}}_2\right]$$ (8)

rate of hydrogen-atom transfer from toluene derivatives (S) to (TBP₈Cz)Mn^IV(O₂^•−), k_et is the rate constant of electron transfer from [(TBP₈Cz)Mn^III]^* (⁷T₁) to O₂ to produce (TBP₈Cz)Mn^IV(O₂^•−), and k_-et is the back electron transfer from the O₂^•− moiety to the (TBP₈Cz)Mn^IV moiety to regenerate (TBP₈Cz)Mn^III and O₂. Eqn (8) agrees with the empirical rate law [eqn (7)], in which k_ox = Φ₀(k_Hk_et/(k_H + k_-et))In (In = light intensity absorbed by (TBP₈Cz)Mn^III).⁵⁰ Because significant KIEs were observed, as shown in Figure 4, the k_H value may be much smaller than the k_-et value: k_H << k_-et, suggesting the back electron transfer pathway is much more favored.

The mechanism in Scheme 2 is similar to O₂ activation by [(TMC)Fe^II]²⁺ (TMC = 1,4,8,11- tetramethyl-1,4,8,11-tetraazacyclotetradecane) with a hydrogen donor (RH) to produce [(TMC)Fe^IV(O)]²⁺. In this case, electron transfer from [(TMC)Fe^II]²⁺ to O₂ occurs thermally to produce [(TMC)Fe^III(O₂^•−)]²⁺. This is followed by hydrogen transfer from RH to [(TMC)Fe^III(O₂^•−)]²⁺ to give R^• and [(TMC)Fe^III-(OOH)]²⁺, which undergoes an oxygen rebound reaction via the homolytic O-O bond cleavage by R^• to yield [(TMC)Fe^IV(O)]²⁺ and ROH. ⁵¹

If a free radical (e.g., pentamethylbenzyl radical) were produced following hydrogen-atom transfer from a toluene derivative to (TBP₈Cz)Mn^IV(O₂^•−), the benzyl radical should react with O₂ to produce the peroxyl radical, which may act as a chain carrier for the autoxidation of hexamethylbenzene to produce pentamethylbenzyl hydroperoxide. The disproportionation of the peroxyl radical may yield equimolar pentamethylbenzyl alcohol and pentamethylbenzaldehyde. The fact that neither pentamethylbenzyl hydroperoxide nor pentamethylbenzaldehyde were produced and that pentamethylbenzyl alcohol was the major product, provides strong evidence for the conclusion that the rebound pathway to produce pentamethylbenzyl alcohol is faster than the rate of the reaction of pentamethylbenzyl radical with O₂. Judging from the reported rate constant of benzyl radical with O₂ (2.6 × 10⁹ M⁻¹ s⁻¹)⁵² to produce benzyl peroxyl radical, and the O₂ concentration (8.5 mM) of the reaction mixture, the lifetime of the radical is expected to be shorter than 45 ns = (2.6 × 10⁹ × 8.5 × 10⁻³)⁻¹. The lifetime of (TBP₈Cz)Mn^IV(OOH) is also expected to be shorter than 45 ns. In such a case, it is very difficult to detect (TBP₈Cz)Mn^IV(OOH), because this is a high energy species, which goes back to (TBP₈Cz)Mn^III, O₂ and H⁺.53,54

Comparison of Hydrogen Transfer Reactivity

In order to gain deeper insight into the rate-determining hydrogen transfer from toluene derivatives to (TBP₈Cz)Mn^IV(O₂^•−), the reactivity was compared with that of hydrogen transfer from the same toluene derivatives employed in this study to cumylperoxyl radical, which is regarded as an authentic hydrogen-transfer reaction.^35,55,56 The rates of hydrogen transfer from toluene derivatives to cumylperoxyl radical were determined by monitoring the reactions by EPR spectroscopy. Cumylperoxyl radicals were generated by photoirradiation of an aerated propionitrile solution containing di- tert-butylperoxide (^tBuOO^tBu) and cumene [PhCH(CH₃)₂] with a 1000 W Mercury lamp via a radical chain process as shown in Scheme 3.³⁵ The UV light irradiation of ^tBuOO^tBu results in the O−O bond cleavage to produce ^tBuO^•. This radical can then readily abstract a hydrogen from cumene to generate cumyl radical [PhC^• (CH₃)₂]. Rapid O₂ addition to the cumyl radical affords the cumylperoxyl radical (PhC(CH₃)₂OO^•). Once generated, this autoxidation process continues until cumylperoxyl radicals decay via β-scission producing acetophenone and CH₃O^•.⁵⁷

Scheme 3.

Generation of Cumylperoxyl Radical and Hydrogen Transfer from Toluene to PhCMe₂OO^*

After cutting off the light, the decay rates of cumylperoxyl radical were monitored by the decrease of EPR signal intensity at g = 2.0156 in the presence of a toluene derivative in propionitrile at 193 K, as shown in Figure 7. The decay rates of cumylperoxyl radical obeyed first-order kinetics under the conditions with large excess toluene derivatives.⁵⁸ The pseudo-first-order rate constants (k’_obs) were linearly proportional to concentrations of toluene derivatives as shown in Figure 8. The second-order rate constants (k’_H) of the hydrogen atom transfer from toluene derivatives to cumylperoxyl radicals were determined from the slopes of linear plots in Figure 8. The k’_H values are also listed in Table 1. The decay rates of cumylperoxyl radical in the presence of a toluene derivative become faster than in the absence of the toluene derivative.

Figure 7.

Time profiles of the EPR intensity change of cumylperoxyl radical in the absence (black line) and presence of toluene as a substrate (3 M, blue line) in aerated propionitrile at 193 K.

Figure 8.

Plots of k’_obs vs concentrations of toluene derivatives for hydrogen-atom transfer from toluene derivatives to cumylperoxyl radical in aerated propionitrile at 193 K.

These results suggest that hydrogen-atom transfer from the toluene derivative to cumylperoxyl radical readily occurs. Figure 9 shows plots of the logarithm of the k_ox values of the photochemical oxidation of (TBP₈Cz)Mn^III with O₂ in the presence of toluene derivatives and the k’_H values of hydrogen-atom transfer from toluene derivatives to cumylperoxyl radicals vs the one-electron oxidation peak potentials of the toluene derivatives (E^pox), which were reported previously.⁵⁹ There is a linear correlation between log k_ox and E^pox with a larger slope (3.2) than that between log k’_H and E^pox (1.7) in Figure 9. The slopes of 3.2 and 1.7 correspond to 19% (3.2/16.9) and 7% (2.2/26.0) of the difference in the free energy change of electron transfer, because the difference in 1 V of E_ox corresponds to the difference in terms of logarithm of the rate constant to be 2.3RT = 26.0 and 16.9 at 193 and 298 K, respectively. The k’_H value of hexamethylbenzene increases by a factor of 20 as compared with that of toluene, while the number of hydrogen atoms increases by a factor of 6. Thus, a small ca. 3-fold “net” increase was observed in the rates of cumylperoxyl radical decay rate in the presence of hexamethylbenzene compared to its reaction with toluene. In general, the k’_H value is expected to increase with decreasing the C−H bond dissociation energies of toluene derivatives.⁶⁰ However, the C-H bond dissociation energies of toluene derivatives have been reported to be only slightly affected by the electron-donating or –withdrawing subsitituents.⁶¹ The small “net” difference in the hydrogen-transfer reactivity of cumylperoxyl radical with hexamethylbenzene vs toluene results from the small change in the C-H bond dissociation energies of toluene derivatives. The larger slope (3.2) in the plot of log k_ox vs E^pox than the slope (1.7) in the plot of log k’_H and E^pox in Figure 9 indicates the higher degree of charge transfer in the transition state of the hydrogen-transfer reactions of (TBP₈Cz)Mn^IV(O₂^•−) as compared with those of cumylperoxyl radical, as reported for the linear relations between logarithm of the rate constants of hydrogen-transfer reactions of triplet excited state of acetophenone derivatives and the ionization potentials of hydrogen donors.⁶²

Figure 9.

Plots of log k and log k’ against the oxidation potential (E^p ) of toluene derivatives III for hydrogen-atom transfer with (a) (TBP₈Cz)Mn and (b) cumylperoxyl radical.

Photocatalytic Reactivity with AcrH₂

The photocatalytic reactivity of (TBP₈Cz)Mn^III was examined under irradiation (white light) of a reaction solution (0.5 mL) containing (TBP₈Cz)Mn^III (1.7 × 10⁻⁴ M) and AcrH₂ (0.2 M) which has a weaker C-H bond than toluene derivatives. The time course for formation of Acr=O as the product quantified by absorbance at 402 nm (λ = 402 nm, ε_max = 8.6 × 10³ M⁻¹ cm⁻¹)⁶³ is shown in Figure 10a. The turnover number was determined to be 11 in 5 hours based on the initial amount of (TBP₈Cz)Mn^III. A negligible amount of Acr=O was produced from a reaction solution without (TBP₈Cz)Mn^III as a control experiment.

Figure 10.

(a) UV-vis spectral changes and (b) time course of formed Acr=O under III photoirradiation (λ > 480 nm) of an aerobic solution (0.5 mL) containing (TBP₈Cz)Mn (1.7 × 10⁻⁴ M) and AcrH (blue, 0.2 M) or AcrD₂ (red, 0.2 M).

When AcrH₂ was replaced by the dideuterated compound AcrD₂, the photocatalytic reactivity dramatically decreased with a large kinetic deuterium isotope (KIE) value of 16 as shown in Figure 10b (blue for AcrH₂ and red for AcrD₂).⁶⁴ This KIE suggests that hydrogen-atom transfer from AcrH₂ to (TBP₈Cz)Mn^IV(O₂^•−) is involved in the rate-determining step as seen in the case of toluene derivatives in Scheme 2. This hydrogen transfer may also proceed via electron transfer from AcrH₂ to (TBP₈Cz)Mn^IV(O₂^•−), followed by rate-limiting proton transfer from AcrH₂^•+ to (TBP₈Cz)Mn^IV(O²²⁻) to produce AcrH^• and (TBP₈Cz)Mn^IV(OOH⁻). As seen for the toluene derivatives, oxygen rebound via homolytic O−O bond cleavage by AcrH^• yields (TBP₈Cz)Mn^V(O) and AcrHOH. The electron-donating OH group in AcrHOH may make it easier to be oxidized by (TBP₈Cz)Mn^V(O) as compared to AcrH₂ via electron and proton transfer, yielding Acr=O and regenerating (TBP₈Cz)Mn^III (Scheme 4). The stoichiometry of the photocatalytic reaction is given by eqn (9). The slow catalytic oxidation of AcrH₂ may result from the much faster back electron transfer in (TBP₈Cz)Mn^IV(O₂^•−) as compared with the hydrogen-transfer reaction with AcrH₂ in Scheme 4.

Scheme 4.

Mechanism of Photocatalytic Reaction of (TBP₈Cz)Mn^III with O₂ and AcrH₂

(9)

CONCLUSIONS

Photoirradiation of (TBP₈Cz)Mn^III with O₂ and toluene derivatives in the inert solvent PhCN results in the rare formation of a high-valent manganese(V)-oxo complex with O₂ as the oxidant. At the same time, the substrate hexamethylbenzene is selectively oxidized in good yield to a single, mono-hydroxylated benzyl alcohol product. The photochemical mechanism was interrogated by femtosecond laser flash photolysis measurements, leading to the first direct spectroscopic observation of a novel photo-excited state ([(TBP₈Cz)Mn^III]^* (⁷T₁)), which was found to be responsible for the initial reaction with O₂. The LFP experiments, together with kinetic studies involving substrate dependence and KIEs, indicate that the mechanism of photochemical oxidation involves the short-lived ⁷T₁ excited state reacting with O₂ via binding and electron-transfer to produce the proposed superoxo complex (TBP₈Cz)Mn^IV(O₂^•−). The superoxo complex then abstracts a hydrogen atom from the toluene derivatives in the rate- determining step to produce (TBP₈Cz)Mn^IV(O₂H), followed by oxygen rebound of the resulting benzyl radical derivatives with (TBP₈Cz)Mn^IV(OOH) to yield the corresponding benzyl alcohol derivatives and (TBP₈Cz)Mn^V(O) (Scheme 2). The hydrogen-atom abstracting reactivity of cumylperoxyl radical was examined with the same set of toluene derivatives, and comparison of the kinetic data confirmed that H-atom abstraction was the rate-determining step for the Mn complex, and indicated that the putative (TBP₈Cz)Mn^IV(O₂^•−) species exhibited more electrophilic character toward H-atom transfer than the cumylperoxyl radical. Our mechanistic findings suggested that catalytic oxidations should be possible with the appropriate substrate, and indeed, it was shown that carrying out the reaction with a stronger hydrogen donor such as AcrH₂ leads to the photocatalytic oxidation of this substrate by O₂ and (TBP₈Cz)Mn^III as catalyst. We have thus revealed a new method for the selective oxidation of benzylic C-H bonds involving only O₂, light and a discrete Mn^III complex. The mechanistic findings and accompanying model (Scheme 2) provide valuable insights into the generation of high-valent metal-oxo intermediates via O₂ activation in the presence of a hydrogen-atom (electron and proton) source.