Bio-inspired Cu(II) amido-quinoline complexes as catalysts for aromatic C–H bond hydroxylation†

Monika ^a, Aniruddha Sarkar ^b, Naiwrit Karmodak ^a, Basab Bijayi Dhar *^a and Sanjay Adhikari *^c
aDepartment of Chemistry, Shiv Nadar IoE, U.P. 201314, India. E-mail: basabbijayi@gmail.com
^bDepartment of Chemical Sciences, IISER Kolkata, Mohanpur 741246, India
^cFaculty of Basic and Applied Sciences, Madhav University, Rajasthan 307026, India

First published on 5th December 2022

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Direct functionalization of aromatic C–H bonds (bond energy > 460 kJ mol⁻¹) is a challenging task in organic chemistry and has received considerable attention in synthetic organic chemistry.^1,2 In the industrial process, phenol is synthesized by a three-step process at 250 °C and high oxygen pressure, where the synthesis of cumyl hydroperoxide from benzene and propylene is followed by acid-mediated decomposition.^3,4 The overall yield of phenol in this process is ∼5%. Thus, it is very important to develop cheaper and sustainable methods that can effectively transform benzene into phenol in a one-step process using environmentally free benign oxidants such as O₂ and H₂O₂.⁵

Nature possesses several monooxygenase copper enzymes that activate the aromatic C–H bonds under physiological conditions.⁶ Inspired by the reactivity of these metalloenzymes, several bioinspired first-row transition metal complexes have been developed for intramolecular aromatic hydroxylation and the reactive intermediates were extensively studied using various spectroscopic techniques.⁷ The oxidation of benzene to phenol by hydrzonato vanadium(V) and vanadium(V) peroxo-picolinic acid complexes has been investigated.^8a,b Biologically inspired manganese complexes successfully achieved aromatic C–H bond oxidation in fluorinated alcohol solvents via the formation of an Mn(V)–oxo intermediate.^8c Photoexcitation of a Mn^IV–oxo complex binding Sc²⁺ ions in a solvent mixture of trifluoroethanol and acetonitrile (v/v = 1:1) resulted in the formation of the long-lived photoexcited state, which can hydroxylate benzene to phenol.^8d In 2014, Kuhn et al. reported a Fe^II NHC complex for the catalysis of benzene hydroxylation, and the reaction proceeds through an electrophilic attack by high valent Fe(IV)O species.^8e Using [Ru^II(Me₂phen)₃]²⁺ as a photocatalyst, photocatalytic hydroxylation of benzene to phenol by O₂ occurs using [Co^III(Cp*)(bpy)(H₂O)]²⁺ as an efficient catalyst in the presence of Sc²⁺.^8f Based on density functional theory (DFT), Pedro and his co-workers mentioned that Cu–Oxyl species were formed during catalytic benzene hydroxylation.⁹ A dicopper(II) complex was reported by Kodera et al. where a Cu–O˙ species was the reactive intermediate.¹⁰ Liu et al. and Ghosh et al. reported a radical-based mechanism for benzene oxidation by copper complexes.¹¹ Bioinspired Cu(II) complexes of N₄-tripodal diazepane ligands were reported by Mayilmurugan et al. where a Cu(II)–OOH species was proposed to be the reactive intermediate.¹² Itoh and co-workers proposed dinickel(III) bis(μ-oxo) as the reactive intermediate for the [Ni^II(tepa)]²⁺ complex.¹³ Our observation revealed that in the majority of the reported complexes, tertiary nitrogen or pyridinic nitrogen-based ligand frameworks were used to stabilize 3d transition metal centres for aromatic C–H activation in the presence of an organic base using H₂O₂ as an oxidant. Here, our idea is to design a ligand framework in such a way that we can avoid using an extra base and make the process greener. This report describes the Cu(II) center stabilized by a tetradentate amido-quinoline ligand that effectively catalyzes aromatic C–H hydroxylation using H₂O₂ as the oxidant with high selectivity (∼90% phenol for benzene).

The copper complexes were synthesized by the reaction of CuCl₂·2H₂O with the respective amido-quinoline ligands, as shown in (Scheme 1). Ligands were synthesized according to a reported method.¹⁴ The reaction of L1 with CuCl₂·2H₂O in CHCl₃/MeOH yielded the Cu-complex (1A) in which L1 acted as a mono-anionic tridentate ligand, whereas when the same reaction was carried out in DMF using Et₃N as a base, L1 acted as a di-anionic tetradentate ligand towards the Cu centre (1B). However, in the case of L2, we could isolate only a tetradentate Cu(II) complex using triethylamine as a base. These Cu(II) complexes have been isolated as dark green crystals which were further characterized by single-crystal X-ray diffraction (Table S1†) and various spectroscopic techniques (Fig. S1–S5† and Fig. 1). The structure of 1A contains a Cu(II) ion with tridentate coordination of the amido-quinoline ligand through deprotonated amido nitrogen (N2), quinoline nitrogen (N1), carbonyl oxygen (O1), and a labile chloride (Cl1), whereas in 1B and 2 (Fig. S1†) the Cu(II) centre is coordinated by a ligand in a tetradentate manner through two deprotonated N_amido and two N_quinoline groups. The geometry around the Cu centre appears to be a square planar; however, it is significantly distorted with tetrahedral distortion parameters τ₄ in the range of 0.2–0.3 (Table S2†); τ₄ values for perfect square planar and tetrahedral geometries are 0 and 1 respectively.¹⁵ The Cu–N_amide bond lengths are shorter than the Cu–N_quinoline bond length, thus suggesting a significant delocalization of the π-electron density around the six-membered chelate ring. The N_quinoline–Cu–N_amide five-membered chelate angles were smaller than 90°, thus suggesting tight chelation around the copper centre. L1 affords two coordination isomers due to the allowed rotation at sp³ carbon of the amido-quinoline ligand, which is less feasible in the case of L2.¹⁶

EPR spectra of all these complexes were recorded in frozen DMF (80 K) (Fig. S3†). Rhombic symmetry with g anisotropy values (g_xx, g_yy, g_zz) was derived from the simulation of the EPR spectrum of 1A and 1B. However, the simulation study supports the axial symmetry of complex 2. 1A showed its molecular ion peak at m/z = 481.0592, corresponding to the [LCu − Cl]⁺ ion, whereas 1B and 2 showed their peaks at 446.0782 and 433.0591 attributed to the [LCu + H]⁺ ion peak (Fig. S2†). All the complexes exhibited a similar type of voltammogram consisting of an irreversible one-electron oxidation wave attributed to Cu^II/Cu^III (vs. NHE) (Fig. S4†). For (1A), an irreversible oxidation peak at 1.25 V was observed; similarly, for (1B), the redox potential was 1.19 V. However, the oxidation peak was observed at a more positive value of 1.63 V for complex (2). The UV-vis spectra of Cu(II) complexes were recorded in CHCl₃ and are shown in Fig. S5†. These complexes exhibit a similar pattern consisting of two intense bands in the UV region around 230–350 nm and a broad band in the visible region 370–400 nm. The band in the UV region can be assigned to an intra ligand (IL) π–π* transition within the quinoline ring, whereas the lower energy band may be assigned to the LMCT transition from amido nitrogen of the ligand framework to the p/d orbital of Cu(II). UV-vis spectra for the conversion of 1A to 1B and vice versa were studied using Et₃N and HCl at RT. From HR-MS data, we found that at first Et₃N replaced the chloride from 1A (Fig. S6†). The successive addition of triethylamine (0.2 to 1 equiv.) to a CHCl₃ solution of 1A led to a decrease of the band at λ_max = 315 nm along with the enhancement of the band at λ_max = 405 nm through an isosbestic point at λ_max = 289 nm and 375 nm. The subsequent addition of HCl (0.2 to 1 equiv.) to a solution of 1B led to an increase of the band at λ_max = 308 nm along with the decrease of the band at λ_max = 385 nm through an isosbestic point at 285 and 354 nm. This result establishes the reversible nature of the two coordination isomers (Fig. 2).

All these complexes were found to be effective in the hydroxylation of benzene, its derivatives, and anthracene (Schemes S2–S7 and Fig. S11–S18†). The reaction conditions were optimized using 1B as the catalyst and benzene as a model substrate (Fig. S7 and S8†) by examining the effect of the concentration of H₂O₂ and temperature on the catalysis. No effect was observed when the reaction was carried out in the presence of a N₂ atmosphere. In a typical reaction, the concentration of the catalyst (Cu-L), benzene and H₂O₂ was respectively 0.1 mM, 1 M and 3 M in the CH₃CN–CH₂Cl₂ mixture (8:2 v/v) and this solution was stirred at 60 °C for 5 hours (Scheme 2). Then, a fraction of the reaction mixture was analyzed by GC/FID and the products were identified by GC-MS. Quantitative analysis of both the substrate and products was performed using the calibration curve of authentic compounds (Fig. S9†) and a TON value of 820 was achieved (90% selectivity). The catalytic performances of 1A and 2 were also investigated under the same reaction conditions (Table 1). Complex 1A showed a TON value of 770 with 92% selectivity for phenol. However, in the case of 2, the selectivity of phenol was reduced to 86% and a TON value of 720 was achieved (Fig. S11†). A similar observation was found in the case of our Ni complexes reported earlier for the catalysis of alkane chlorination using the same ligand framework.¹⁴ Additionally, phenol was taken as the substrate, which provided 1,4-benzoquinone (1,4-BQ) and catechol as the products (Scheme S2 and Fig. S12†) with 1B.

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Furthermore, to check the selectivity of aromatic vs. aliphatic C–H functionalization, the reaction of toluene, ethylbenzene, and cumene was carried out using 1B as the catalyst (Table S3†). The bond dissociation energy (BDE) for the aromatic C–H bond (Csp²–H) was much greater than that for the benzylic Csp³–H bond. The use of toluene led to the formation of o- and p-cresol (70% selectivity) and benzaldehyde, suggesting that oxidation occurs preferentially at the sp² aromatic carbon (Scheme S3 and Fig. S13†). Similarly, in the case of ethylbenzene, o-, p-hydroxylated products were produced as a major product (96%) with m- and benzaldehyde as minor products, and the ratio of aromatic to aliphatic oxidation was 24:1 (Scheme S4 and Fig. S14†). Cumene afforded o-, m-, and p-substituted products (collected selectivity > 94%) along with a trace yield of acetophenone (Scheme S5 and Fig. S15†).

To prove the potential abilities of these Cu(II) complexes towards the hydroxylation of the aromatic C–H bond, we tried the oxidation of anthracene. The current synthetic methods at the industry level for anthraquinone are not based on anthracene, and a strongly acidic medium is required in the presence of a metal catalyst which is a major limitation of the current process.¹⁷ Using our Cu(II) complexes as catalysts, oxidation of anthracene was carried out in the absence of acids (Scheme 3). The sole product 9,10-anthraquinone was observed which was confirmed by GC-MS (Fig. S16†).

After establishing the catalytic activity of the copper complexes, we focused our attention to investigate the reaction mechanism. The kinetic analysis supports the existence of a pre-equilibrium process before the oxidation reaction and the turnover limiting step of the catalytic reaction is the substrate oxidation step by a reactive species generated from 1A/1B/2 and H₂O₂ (Scheme S8†). The reaction rate can be expressed as eqn (1). Fitting rate (V) to the concentration of 1A/1B, benzene, and H₂O₂ gave the equilibrium constants (K) of the pre-equilibrium processes and the rate constant (k) of the subsequent oxidation reactions. The value of K (M⁻¹) and k (M⁻¹ min⁻¹) was determined to be 1.07 ± 0.05 and 1.67 ± 0.03, respectively for 1B (Table S4 and Fig. S19†). For 1A, the K (M⁻¹) value was found to be 0.59 ± 0.08; however, k (M⁻¹ min⁻¹) was determined to be 1.76 ± 0.08 (Table S4†).

V (M min−1) = = kK[1B][substrate][H2O2]/(1 + K(H2O2)	(1)

At first, we checked whether the reaction is going through an oxygen-based non-metal radical intermediate or not. No biaryl product was detected by GC/GC-MS (Fig. S11†), confirming that no phenyl radical was generated.¹⁸ No 18-labelled phenol was obtained as a product when the reaction was carried out in the presence of H₂O.¹⁸ Furthermore, a reaction was carried out in the presence of CCl₄ (3 M, 1:1 with respect to the oxidant) and 1B as a catalyst. However, no difference in the phenol yield% was observed, and chlorobenzene was not obtained as the co-product (Scheme S1†). Kinetic isotope effect (KIE) values of 1.1 ± 0.1 (Fig. S20†) were determined from the peak intensity ratios of phenol to phenol-d₅ by GC-MS for all these Cu complexes (benzene hydroxylation). According to previous literature, the KIE value of the Fenton-type mechanism and metal insertion-type reaction was respectively ∼1.7 and ∼2.0–4.8.¹⁹ On the other hand, the KIE value ∼1.0 was reported for arene hydroxylation involving metal-bound oxygen species as the key intermediate via the electrophilic aromatic substitution reaction. In the presence of radical scavengers like TEMPO and DMPO, no change was observed in the conversion of benzene. However, an over-oxidized product of phenol, i.e.1,4-BQ, was observed only in the presence of TEMPO. In the control reaction, we observed that phenol is converted to 1,4-BQ in the presence of TEMPO and Cu(II) complexes (Fig. S21†). Earlier, Semmelhack and co-workers proposed the TEMPO⁺-mediated mechanistic pathway in which Cu(II) oxidizes TEMPO-H to TEMPO⁺, which then serves as the reactive intermediate in the alcohol oxidation reactions.²⁰ We are assuming that a similar kind of pathway might be operating for overoxidation of phenol in the presence of TEMPO (Scheme S9†). Now, based on our experimental result and literature review, we have concluded that the reaction is not going through reactive oxygen species. Then, various spectroscopic techniques were used to identify the metal-based reactive intermediate and reaction pathway. When 10 equiv. of H₂O₂ were added to the solution of 1B at room temperature (RT), a spectral shift occurs at 389 nm in the UV-vis spectrum (Fig. S22†) which may correspond to O(π*σ)→ Cu LMCT. Ramasamy et al. found a similar kind of observation for the Cu(II)-OOH species.¹² The formation of Cu-OOH (1-OOH) species was also confirmed by HR-MS analysis (Fig. S23†) which showed a prominent peak at m/z 480.0455 with isotopic distribution when 1B was treated with 10 equiv. of H₂O₂. Further insight comes from examining the EPR spectra obtained after adding 10 equiv. of H₂O₂ in 1A/1B (Fig. 3). The intensity change of the EPR spectra at different reaction times shows that the reaction happened and a new EPR-active species was formed.²¹

We further confirmed the stability, the preferred structure of 1-OOH and the formation free energy of 1-OOH from 1B using density functional theory (DFT) calculation. We performed the geometry optimization and frequency calculations of both 1B and 1-OOH at B3LYP/6-311+G(d,p) in the presence of an implicit acetonitrile solvent medium (Computational details in the ESI†).²² The calculations show that both the compounds were minima along the potential energy surface. The optimized geometries and the reaction free energy value for the 1B to 1-OOH step are shown in Fig. 4. The thermodynamic free energy of formation of 1-OOH was found to be 7.1 kcal mol⁻¹. The implicit solvent calculations²³ with the entropic corrections performed at room temperature suggest that the reaction free energy for this step is slightly endothermic in nature. Due to the complexity involved in modelling the explicit solvent interactions in the reaction pathway, we used an implicit solvent medium to understand the effect of solvent on the reaction free energy. We expect that this lower thermodynamic barrier could be easily surmounted under the experimental reaction conditions.

Based on our observation and the related mechanism reported in the literature,^9,10 we are claiming the formation of Cu^II–OOH as the reaction intermediate. Kodera et al. proposed that copper-bound oxyl and peroxyl species generated from the Cu^II–OO intermediate and the copper-bound oxyl reacts with benzene in the rate-limiting step.¹⁰ Pedro and co-workers also suggested that Cu–O˙ is capable of aromatic C–H activation either by electrophilic aromatic substitution or hydrogen abstraction followed by a rebound route.⁹ To gain insight into the mechanism in our case, hydroxylation reactions for the substituted benzene-bearing electron-donating and electron-withdrawing groups were performed. We observed that benzene with an electron-donating group is more susceptible to oxidation compared to substituted benzene with an electron-withdrawing group. The negative ρ value (−0.167) obtained from the Hammett plot (Fig. 5 and Table S5†) can be interpreted as the involvement of an electrophilic intermediate that may be stabilized by the electron donating group present in substituted benzene.⁹

Now, we propose a plausible reaction mechanism as shown in Fig. 6. During the formation of 1-OOH, one of the quinoline moieties of the ligand framework is acting as a base (path I). Then according to path IIa, aromatic oxidation is expected to form the cage pair species as the reactive intermediate via homolytic cleavage. The hydroxyl radical first attacks the aromatic ring to form a transient hydroxycyclohexadienyl radical, which is further oxidized by the encaged Cu(II)–O˙ to yield a cationic intermediate and then a NIH shift in the ketonization step is proposed.²⁴ In path IIb, Cu(II)–O˙ could be generated from 1-OOH along with the formation of Cu(II)–OO˙ species (1-OO˙) and release of a water molecule. In the previous report by Kodera et al.,¹⁰ it has been shown that the formation of Cu(II)–O˙ is taking place along with the formation of Cu(II)–OO˙ (similar to path IIb). Therefore, we expect that in our study, path IIb would be more preferable than path IIa. Then Cu(II)–O˙ would be involved in an electrophilic aromatic substitution.

In summary, three new Cu(II) complexes of amido-quinoline ligands were reported as catalysts for one-step aromatic hydroxylation using H₂O₂ as an oxidant without the presence of an external base. In the case of benzene, more than 90% selectivity for phenol with a TON value of 820 for 1B was obtained. The KIE value, spectroscopic studies and DFT calculation supported the involvement of Cu(II)–OOH species and oxidation likely proceeded via an electrophilic substitution pathway.

Author contributions

Investigation, analysis, writing, and draft preparation – Monika; EPR data – A. S.; Crystallography and writing – S. A.; DFT calculation – N. K. M.; Idea, funding acquisition, and writing, review and editing – B. B. D.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

B. B. D acknowledges CSIR, New Delhi (Grant no EMR-II 80(0086)/17) for funding. Monika acknowledges SNIoE for a fellowship. A. S. and S. A. acknowledge CSIR, New Delhi, for an SRF and RA fellowship. All the authors acknowledge Dr Yang Liu from the University of Goettingen for EPR simulation.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2184057–2184059. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2dt03242b

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