Naphthalene Oxidation of a Manganese(IV)-Bis(Hydroxo) Complex in the Presence of Acid

Abstract

Naphthalene oxidation by metal-oxygen intermediates is one of difficult reactions in environmental and biological chemistry. Herein, we report that a Mn^IV-bis(hydroxo) complex, which was fully characterized by various physicochemical methods, such as UV-vis, ESI-MS, EPR, X-ray and XAS, shows the naphthalene oxidation in the presence of acid to afford 1,4-naphthoquinone. Redox titration of the Mn^IV-bis(hydroxo) complex exhibits one electron reduction potential of 1.09 V, which is the most positive potential for the previously reported nonheme Mn^IV-bis(hydroxo) species as well as Mn^IV-oxo analogues. Kinetic studies including kinetic isotope effect suggest that the naphthalene oxidation by the Mn^IV-bis(hydroxo) complex in the acid-promoted reaction occurs via a rate-determining electron transfer process.

COMMUNICATION

A Mn^IV-bis(hydroxo) complex, which was fully characterized by various physicochemical methods, shows the reactivity in the naphthalene oxidation in the presence of acid to afford 1,4-naphthoquinone via an acid-promoted rate-determining electron transfer process.

The metabolic conversion of polycyclic aromatic hydrocarbons (PAHs) has received a lot of interests in environmental and biological chemistry because many PAHs are genotoxic, mutagenic, and carcinogenic pollutants.^[1,2] Three main degradation pathways are known for microbial catabolism of PAHs; (i) the insertion of dioxygen molecule into the aromatic ring to form cis-dihydrodiols by dioxygenases,^[3] (ii) hydroxylation or epoxidation of aromatic compounds by cytochrome P450 via arene oxides,^[4] and (iii) ligninases oxidize PAHs to quinones followed by ring cleavage.^[5] Importantly, these reactions are proposed to be carried out by metal-reactive oxygen species such as metal-hydroxo, -peroxo and -oxo intermediates.

Naphthalene, which is the most stable PAH, is known as a major component of the coal and tar-based industries and a highly toxic xenobiotic compound.^[6] However, naphthalene oxidation has been challenging reaction due to the low reactivity of aromatic C-H bond and notoriously high oxidation potential. High-valent metaloxo species have been studied on the oxidation of relatively weak PAHs such as anthracene.^[7] Only one example of the Mn^IV-oxo complex exhibited sluggish oxidation of naphthalene.^[8] Some Fe catalysts have also investigated the naphthalene oxidation under catalytic reaction conditions.^[9,10] Recently, high-valent metal-hydroxo intermediates have been suggested as reactive species in lipoxygenase and ligninolytic enzymes.^[11–16] In biomimetic studies, a few nonheme high-valent metal-hydroxo complexes have been prepared and exhibited the C-H bond activation reactivity towards organic substrates,^[16–23] whereas there is no example of their reactivity in aromatic molecules. Herein, we report the very first example of the oxidation reaction of naphthalene in the presence of acid by a mononuclear Mn^IV complex with terminal hydroxide ligands, [Mn^IV(TBDAP)(OH)₂]²⁺ (2) [TBDAP = N,N-di-tert-butyl-2,11-diaza[3.3](2,6)-pyridinophane],^[24] together with kinetic studies in detail. To the best of our knowledge, it is the first report on the acid-promoted oxidation by the high-valent metal-hydroxo species.

The starting manganese(II) complex, [Mn^II(TBDAP)(OTf)₂] (1-(OTf)₂) was prepared and characterized using UV-vis absorption spectroscopy, electrospray ionization mass spectrometry (ESI-MS), electron paramagnetic resonance (EPR) spectroscopy, and X-ray crystallography (Figures 1a and 2a; Supporting Information (SI), Figure S1). Addition of 1.5 equiv of iodosylbenzene (PhIO) to a solution of 1 resulted in the generation of a green intermediate 2 in CF₃CH₂OH/Acetone (v/v = 1:3) at −30 °C showing a characteristic absorption band at 834 nm with ε = 380 M⁻¹ cm⁻¹ (Figure 2a). The stoichiometry of PhIO to 1 has been examined by absorption spectral titration (Figure S2).^[25] Complex 2 was metastable at −30 °C with a half-life (t_1/2) of 30 min, allowing us to use it for spectroscopic characterization and reactivity studies. The intermediate 2 was also synthesized by adding 8 equiv of cerium(IV) ammonium nitrate (CAN) to 1 in CH₃CN/H₂O (v/v = 9:1) at −30 °C, showing the same absorption spectrum (Figure S3) and enhanced stability (t_1/2 = 10 h). The latter method is inspired by the similar synthetic strategy of Nam and coworkers reported previously.^[26,27] The ESI-MS of 2 exhibits a prominent peak at m/z = 220.7 (Figure 2b), whose mass and isotope distribution pattern correspond to [Mn^IV(TBDAP)(OH)₂]²⁺ (2-¹⁶O) (calculated m/z = 220.6). Upon substitution of PhI¹⁶O with PhI¹⁸O, a mass peak corresponding to [Mn^IV(TBDAP)(¹⁸OH)₂]²⁺ (2-¹⁸O) is obtained at m/z = 222.7 (calculated m/z = 222.6) (Figure 2b, inset), demonstrating 2 contains two oxygen atoms. The X-band EPR spectrum of 2 reveals a broad signal at g_∥ = 4.32 and a six-line hyperfine pattern (A_⊥ = 10 mT) centered at g_⊥ = 1.99, indicating S = 3/2 Mn^IV ground state (Figure 2a, inset).^[23,28,29] In contrast, 1 shows the intense resonance at g_∥ = 6.48 and weak hyperfine pattern (A_⊥ = 9 mT) at g_⊥ = 2.02, which means 1 possesses S = 5/2 Mn^II center. These contrasting spectral intensities of 1 and 2 are originated from the different axial zero-field splitting term D.^[28–30]

Figure 1.

ORTEP diagrams of (a) the starting complex 1 and (b) the intermediate 2 with thermal ellipsoids drawn at the 30% probability level. Hydrogen atoms are omitted for clarity except for hydrogen atoms on hydroxo groups.

Figure 2.

(a) UV-vis absorption spectra of 1 (black) and 2 (red) in CF₃CH₂OH/acetone (v/v = 1:3) at −30 °C. Inset shows X-band EPR spectra of 1 (4.0 mM, black line) and 2 (4.0 mM, red line), which was generated in CH₃CN/H₂O (v/v = 4:1) at 293 K. The spectra were recorded at 113 K. (b) ESI-MS spectrum of 2 (0.10 mM) in CF₃CH₂OH/acetone (v/v = 1:3). Inset shows isotope distribution patterns for 2-¹⁶O (red) and 2-¹⁸O (blue).

The X-ray crystal structure of [Mn(TBDAP)(OH)₂][Ce(NO₃)₆] (2-[Ce(NO₃)₆]) revealed the mononuclear Mn-bis(hydroxo) complex in distorted octahedral geometry (Figure 1b). Structural parameters of the [Ce(NO₃)₆] moiety is quite similar to those of [(NH₄)₂Ce^IV(NO₃)₆] complex, which supports the formal oxidation state of cerium is 4+.^[31] The presence of one [Ce^IV(NO₃)₆]²⁻ counter anion per Mn in the crystal lattice indicates that the Mn is of the 4+ oxidation state, which is consistent with the results of EPR (vide supra) and XAS (vide infra). Notably, the Mn-O bond lengths (1.804 and 1.807 Å) and successful refinement of the hydroxo groups clearly establish the Mn(OH)₂ moiety (Figure S4). The Mn-O bond distances in 2 are comparable to those in the previously reported Mn^IV-bis(hydroxo) complexes, [Mn^IV(Me₂EBC)(OH)₂]²⁺ (1.811 Å) and [Mn^IV(^H,MePytacn)(OH)₂]²⁺ (1.799 Å).^[23,32]

Mn K-edge X-ray absorption spectroscopy was performed to further analyze Mn centers of 1 and 2 (Figure 3). The Mn rising edges exhibit a noticeable shift toward higher energies in the series of Mn^II (1, 6547.6 eV), Mn^III ([Mn^III(Me₂EBC)(O₂)]⁺, 6550.3 eV)^[33] and Mn^IV(2, 6551.4 eV). The pre-edge of 2 has an intensity of 6 units, consistent with a centrosymmetric complex, which is quite similar to [Mn^IV(Me₂EBC)(OH)₂]²⁺.^[34] 2 showed variability in the rising edge (Figure S5). However, the pre-edge intensity and edge shift remained consistent between the samples, showing they are all centrosymmetric Mn^IV species. On the basis of the structural and spectroscopic data, 2 is assigned as a manganese(IV) bis(hydroxo) complex.

Figure 3.

Normalized Mn K-edge XAS spectra of 1 (black), 2 (red) and [Mn^III(Me₂EBC)(O₂)]⁺ (blue).^[33] Inset shows expanded pre-edge region from 6536 to 6546 eV.

The one electron reduction potential of 2 was determined from redox titration. The electron transfer (ET) reactions of 2 with [Fe^II(phen)₃]²⁺ (E_ox = 1.08 V vs. SCE, Figure S6) occurred in CF₃CH₂OH/acetone (v/v = 1:3) at −30 °C and were followed by monitoring decay of the absorption band at 834 nm of 2, with the concomitant formation of a broad absorption band at 620 nm due to [Fe^III(phen)₃]³⁺ (Figure S7). ET reduction of 2 were found to be in equilibrium with [Fe^II(phen)₃]²⁺, where the final concentration of [Fe^III(phen)₃]³⁺ produced in the ET of 2 increases with increasing the initial concentration of [Fe^II(phen)₃]²⁺. The equilibrium constant (K_et) was determined to be 1.38 and the one electron reduction potential (E_red) of 2 was then determined to be 1.09 V vs. SCE using the Nernst equation (Figures S8 and S9). Importantly, to the best of our knowledge, the value of 1.09 V for one electron reduction potential of 2 is the most positive potential for the present nonheme Mn^IV-bis(hydroxo) as well as Mn^IV-oxo intermediates.^[35]

With the highly positive reduction potential, the aromatic oxidation reactivity of 2 was investigated in the oxidation reaction of naphthalene. Upon addition of the large excess of naphthalene to 2 (1.0 mM) in a solvent mixture of CF₃CH₂OH/acetone (v/v = 1:3) at −30 °C, however, 2 remained intact and product analysis of the reaction solution confirms that no oxidized products of naphthalene were obtained.

Interestingly, upon addition of HOTf (10 mM) to the reaction solution, 2 became reactive with naphthalene affording the remarkable absorption band changes with a first-order decay profile (Figure 4a). It should be noted that spectroscopic characterization of 2 in the presence of HOTf reveals that 2 remains intact (Figure S10). It is well known that the presence of acid induces the enhancement for electrophilic reactivity of high-valent nonheme Fe- and Mn-oxo complexes by the positive shift of their one-electron reduction potential, resulting in the change of reaction mechanism to proton-coupled electron transfer (PCET).^[36–41] Thus, the redox titration of 2 in the presence of HOTf (10 mM) was performed by using [Ru^II(Cl-phen)₃]²⁺ (E_ox = 1.4 V vs. SCE; see Figure S11), which is in equilibrium with 2 under the reaction conditions (Figures S12 and S13). The equilibrium constant (K_et) of 1.73 in the electron transfer reaction between 2 and [Ru^II(Cl-phen) ₃]²⁺ was obtained (Figures S13 and S14). The one electron reduction potential of 2 in CF₃CH₂OH/acetone (v/v = 1:3) at −30 °C was then determined to be 1.41 V vs. SCE from Nernst equation in the presence of HOTf (10 mM). Thus, the E_red value of 2 is significantly shifted in the positive direction from 1.09 V vs. SCE in the absence of HOTf to 1.41 V vs. SCE in the presence of HOTf (10 mM). Therefore, the oxidation of naphthalene (E_ox = ~1.7 V vs. SCE)^[42,43] could be made possible by the positive shift in reduction potential of 2 in the presence of HOTf via proton-coupled electron transfer (PCET).

Figure 4.

Reactions of 2 with naphthalene in CF₃CH₂OH/acetone (v/v = 1:3) at −30 °C in the presence of HOTf (10 mM). (a) UV-Vis spectral changes observed in the reaction of 2 (1.0 mM) with 50 equiv of naphthalene. Inset shows the time courses monitored at 380 nm in the absence (black circles) and presence (red circles) of HOTf (10 mM). (b) Plots of pseudo-first-order rate constants (k_obs) against the concentrations of naphthalene (black circles) and naphthalene-d₈ (red circles) to determine second-order rate constants (k₂). (c) Plot of log k₂ against E_pc of X-naphthalene. X = OH, OMe, Me and H.

The pseudo-first-order rate constants (k_obs) increased linearly with an increase of the concentration of naphthalene (Figure 4b), giving a second-order rate constant (k₂) of 7.3(6) × 10⁻¹ M⁻¹ s⁻¹ at −30 °C. The observed k₂ values show the first-order dependence on [HOTf] at lower concentrations and the second-order dependence at higher concentrations (Figure S15).^[37] From such a mixture of the first- and second-order dependence of k₂ values on [HOTf], the reaction rate equation is reproduced as Eq. (1),

$$k_2={k\text{'}}_1[\text{HOTf}]+{k\text{'}}_2{[\text{HOTf}]}^2$$ (1)

where k'₁ and k'₂ are the rate constants for the first- and second-order dependence on [HOTf], respectively. The linear plot of k₂ against to [HOTf]² indicates the involvement of two protons in the naphthalene oxidation via PCET reaction (Figure S16).

We confirmed that the final product formed in the oxidation of naphthalene is 1,4-naphthoquinone as the sole oxidized product (90% yield based on naphthalene) by taking ¹H NMR spectra of the reaction solution (Figure S17). Further, when naphthalene is oxidized by 2-¹⁸O, the oxygen atoms in the 1,4-naphthoquinone were found to be derived from the Mn^IV-(OH)₂ species (Figure S18). After the reaction, 2 decayed to a Mn^III species (Scheme 1), which was proved by ESI-MS and EPR (Figures S19 and S20).

Scheme 1.

Proposed mechanisms of the naphthalene oxidation reaction by 2 in the presence of acid.

To gain mechanistic insights, the kinetic isotope effect (KIE) was investigated in the hydroxylation of undeuterated and deuterated naphthalenes by 2 in the presence of HOTf (Figure 4b). A k₂ value of 7.4(4) × 10⁻¹ M⁻¹ s⁻¹ was obtained in the reaction of 2 and naphthalene-d₈, giving the calculated k_H/k_D value of 1.0(3). Thus, reaction rates of the oxidation of naphthalene and naphthalene-d₈ by 2 are irrelevant to the C-H bond dissociation energy of aromatic substrates. This result is consistent with the electron transfer reaction of metal-oxo species.^[37,41] The reactivity of 2 was also investigated with substituted naphthalenes, X-naphthalene (X = OH, OMe, Me, H) to evaluate the electronic effect of substituents on naphthalene oxidation by 2 (Figure S21 and Table S3). A good linear correlation was obtained from the plotting of the electron transfer rates against oxidation peak potentials (E_pc) of naphthalenes, affording the negative slope of −3.7 (Figure 4c). These kinetic studies suggest that the acid-promoted oxidation of naphthalene by 2 occurs via a rate-determining electron transfer pathway (Scheme 1, pathway a).

It has been documented that the oxidation of polyaromatics to quinones requires totally six-electron oxidation.^[44,45] The mechanistic proposal for the oxidation of naphthalene by 2 is shown in Scheme 1. First, the rate-determining electron transfer from naphthalene to protonated 2 triggers the six-electron oxidation of naphthalene affording a naphthalene radical cation with [Mn^III(TBDAP)(OH)(OH₂)]²⁺ (Scheme 1, pathway a), followed by the further oxidations (Scheme 1, pathways b – f). 1-naphthol which is two-electron oxidized product of naphthalene is supposed to undergo faster four-electron oxidation, possessing lower oxidation potential than naphthalene. Indeed, product analysis revealed that 1-naphthol was fully oxidized to 1,4-naphthoquinone with 95% yield upon reaction with four equiv of 2 in CF₃CH₂OH/acetone (v/v = 1:3) at −30 °C. The k₂ value (1.9(2) × 10² M⁻¹ s⁻¹) of the oxidation of 1-naphthol by 2 is much larger than those of naphthalene oxidation by 2 (Table S3). This result is consistent with the hypothesis that the initial oxidation of naphthalene to 1-naphthol is rate-determining step.

Thus, the presence of acid enhances the oxidizing power of Mn^IV-bis(hydroxo) species to afford an electron transfer from naphthalene, as shown similarly in high-valent metal-oxo chemistry.^[36–39] On the basis of our results, we suggest that high-valent nonheme metal-hydroxo species can also be a putative intermediate in naphthalene oxidation, though the product is more oxidzed than that of naphthalene dioxygenase.^[46]

In summary, we have demonstrated the generation of a unique nonheme mononuclear bis(hydroxo)manganese(IV) complex bearing the macrocyclic TBDAP ligand, [Mn^IV(TBDAP)(OH)₂]²⁺ (2) with full characterization using various spectroscopic methods and X-ray crystallography. 2 exhibits a prominent ability to oxidize naphthalene in the presence of acid to afford naphthoquinone in CF₃CH₂OH/acetone (v/v = 1:3) at −30 °C. Based on the kinetic and isotope labeling experiments, we suggest that 2 reacts with naphthalene to give a Mn^III-bis(hydroxo) species and naphthalene cation radical via the PCET reaction, and then produces a Mn^II-(hydroxo) species and 1-naphthol, which undergoes faster four electron oxidation. Further detailed studies, including the density functional theory calculations as well as the electron transfer property of Mn^IV-bis(hydroxo) species in light of Marcus theory of outer-sphere electron transfer, to understand the intrinsic properties of arene hydroxylation by metal-hydroxo species in the presence of acid are under investigation.