A putative heme manganese(V)-oxo species in the C–H activation and epoxidation reactions in an aqueous buffer

Dinesh S. Harmalkar a, G. Santosh ab, Siddhi B. Shetgaonkar a, Muniyandi Sankaralingam *c and Sunder N. Dhuri *a
aSchool of Chemical Sciences, Goa University, Taleigao Plateau, Panaji, Goa 403206, India. E-mail: sndhuri@unigoa.ac.in
bDivison of Chemistry, School of Advanced Sciences, Vellore Institute of Technology, Chennai, Tamilnadu 600127, India
cDepartment of Chemistry, National Institute of Technology Calicut, Kozhikode, Kerala 673601, India. E-mail: msankaralingam@nitc.ac.in

Received 16th March 2019 , Accepted 8th June 2019

First published on 10th June 2019

A water-soluble manganese(V)-oxo species 1 was generated in the reaction of [Mn(III)(TPPS)Cl] 2 (TPPS = 5,10,15,20-tetrakis(4-sulfonatophenyl)-21H,23H-porphine) and iodosylbenzene (PhIO) in a 2[thin space (1/6-em)]:[thin space (1/6-em)]1 aqueous buffer (pH = 10.4)[thin space (1/6-em)]:[thin space (1/6-em)]acetonitrile (CH3CN) mixture. The formula of the EPR silent species 1 is proposed as [Mn(V)(O)(TPPS)Cl] based on the Soret band (422 nm) and Q bands (520, 660 nm) in its UV-vis spectrum and its reaction with thioanisole, regenerating 2 and methyl phenyl sulfoxide. The reactivity of 1 was investigated in the C–H activation of alkyl hydrocarbons and epoxidation of cyclohexene. Based on the observation of the linear correlation of the logarithm of the second rate constant (log[thin space (1/6-em)]k2′) and the bond dissociation energy (BDE, kcal mol−1) of alkyl hydrocarbons along with a large kinetic isotope effect (KIE = 8.5) for xanthene vs. xanthene-d2, we propose H-atom abstraction as the rate determining step in the C–H activation reactions. On the other hand, in contrast to the C–H activation reaction, cyclohexene, which has a weak C–H bond (BDE = 82.5 kcal mol−1), undergoes an epoxidation reaction.


High valent metal-oxo short-lived species have been extensively studied and used in the oxidation of organic substrates into viable products.1,2 In heme-iron chemistry, several researchers have investigated two distinct species, namely compound I [Fe(IV)(O)(porp˙+)] and compound II [Fe(IV)(O)(porp)] (porp is porphyrin).1,2 Likewise, a large number of reports are available on the biomimetic chemistry of heme and nonheme high valent manganese-oxo species.3–7 Nam and coworkers reported the first single crystal structure of a nonheme iron compound viz. [Fe(IV)(TMC)(O)(CH3CN)]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane),8 while the first crystal structure of a nonheme manganese compound viz. [Mn(V)(O)(TAML)] bearing a tetraamido macrocyclic ligand (TAML) was reported by Collins and coworkers.4 Manganese-oxo porphyrin species [Mn(IV)(O)] and [Mn(V)(O)] have been proposed as key reactive intermediates in several oxidative transformations.9 The oxidation of olefins and alkanes by catalytic amounts of Mn(III)-porphyrin compounds in the presence of additional oxidants such as iodosylbenzene (PhIO), hydrogen peroxide (H2O2), tert-butyl hydroperoxide (t-BuO2H) and m-chloroperbenzoic acid (m-CPBA) has been previously reported.10,11

The chemistry of Mn(III)-porphyrin compounds is fascinating as these compounds represent a class of new molecular designs that can achieve redox control while maintaining low molecular weight and tailored lipophilicity.12 The high stability of metalloporphyrins compared to nonheme metal macrocycles has led to study of Mn(III)-porphyrins in superoxide dismutase (SOD) mimics.13 Synthetic water-soluble Mn(III)-porphyrins containing pyridinium, sulfonate or carboxylato substituents are known to be effective radiosensitizers.14 It has been reported that these water-soluble porphyrins accumulate in tumor cells.15 The water-soluble [Fe(III)(TPPS)Cl] (where TPPS = 5,10,15,20-tetrakis(4-sulfonatophenyl)-21H,23H-porphine) is known to catalyze N-dealkylation reactions.16 Compounds such as [Mn(III)(o-TMPyP)] (where o-TMPyP = 5,10,15,20-tetra-(N-methylpyridinium-2-yl)porphyrin) and [Mn(III)(TPPS)Cl] are reported to form high valent manganese(V)-oxo species on addition of external oxidants.17 [Mn(III)(TPPS)Cl] was used in the dye degradation process and other reactions using H2O2 as an oxidant.17 Several other stable water-soluble Mn(III) porphyrins are known in the literature (Scheme 1).9,18,19 Although the [Mn(IV)(O)] and [Mn(V)(O)] species are now well known, only a handful of reports are available on the reactivity of water-soluble [Mn(V)(O)] species.9,11,17,20 Nam and coworkers have reported the rate determining H-atom abstraction in the C–H activation reactions of alkyl hydrocarbons by [Mn(V)(tf4tmap)(O)2]3+ (tf4tmap = meso-tetrakis(2,3,5,6-tetrafluoro-N,N,N-trimethyl-4-aniliniumyl)porphyrinato dianion) species in aqueous basic buffer solution.9 The resulting species [Mn(IV)(OH)] was reported to be sluggish in the H-atom abstraction as compared to the [Mn(V)(O)2] species. Herein, we report on the characterization and the reactivity of a water-soluble high valent [Mn(V)(O)] species (namely [Mn(V)(O)(TPPS)Cl] 1) in the C–H activation of alkyl hydrocarbons. This reaction is compared with the reactivity of cyclohexene and styrene.

image file: c9nj01381d-s1.tif
Scheme 1 Water-soluble porphyrins reported in the literature. (i) 5,10,15,20-Tetra-(N-methylpyridinium-4-yl)porphyrin (p-TMPyP); (ii) 5,10,15,20-tetra-(N-methylpyridinium-3-yl)porphyrin (m-TMPyP); (iii) 5,10,15,20-tetra-(N-methyl pyridinium-2-yl)porphyrin (o-TMPyP); (iv) 5,10,15,20-tetra-(4-carboxyphenyl)porphyrin (TCPP); (v) 5,10,15,20-tetra-(4-N-trimethylanilinium)porphyrin (TAPPH2); (vi) 5,10,15,20-tetra-(2,3,5,6-tetrafluro-4-N-trimethylanilinium)-porphyrin (TAFPP); (vii) 5-(4-amino phenyl)-10,15,20-tri-(4-sulfophenyl)porphyrin (TAPPS); (viii) 5-(2,6-diaminophenyl)-10,15,20-tri-(4-sulfophenyl)porphyrin (TDAPPS); (ix) 5,10,15,20-tetrakis(4-sulfonatophenyl)-21H,23H-porphine (TPPS).

Materials and methods

The chemicals used in this study were purchased from commercial sources and used without any further purification unless required. The aqueous carbonate–bicarbonate buffer (pH ∼ 10.4) was prepared by mixing 0.1 M Na2CO3 (27.5 mL) and 0.1 M NaHCO3 (22.5 mL) and dilution with distilled water in a 100 mL standard flask. pH measurements were performed using a Chemi Line Digital pH meter CL-110. PhIO was prepared using a literature procedure (see the ESI).21 TPPH, TPPS and [Mn(III)(TPPS)Cl] 2 were prepared as per literature methods22–24 (Scheme S1 in the ESI). The deuterated xanthene was prepared by a procedure described in the ESI.1H NMR spectra were recorded on a Bruker Avance-III 400 MHz NMR spectrometer. UV-vis spectra were measured on an Agilent 8453 UV-vis spectrophotometer attached to a water circulator and thermostatic control (Morya Scientific). Infrared (IR) spectra in the region of 4000–400 cm−1 were recorded on a Shimadzu (IR-Prestige-21) FTIR spectrometer by diluting the powdered samples in KBr. Elemental analyses were performed on an Elementar Variomicro Cube CHNS analyzer. EPR spectra were measured at 77 K on a JEOL X-band spectrometer (JESFA100). The g value was calibrated using the Mn(II) marker. The organic product analysis was performed on Shimadzu GC 2014 equipped with an HP capillary column and FID detector. The retention time and peak areas of the products were compared with authentic samples using n-decane as an internal standard.

Synthesis of TPPH, TPPS and [Mn(III)(TPPS)Cl]22–24

TPPH. A round bottom flask containing 100 mL CHCl3 and freshly distilled benzaldehyde (1.0 mL, 9.84 mmol) was stirred under a N2 atmosphere for about 5 min. To this solution freshly distilled pyrrole (0.683 mL, 9.84 mmol) was added followed by addition of boron trifluoride-diethyl ether (BF3·OEt2) (0.1 mL, 2.5 M). After two hours, 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) (0.975 mL, 7.30 mmol) was added to the mixture and kept under stirring for a day. The progress of the reaction was monitored by thin layer chromatography (TLC). At the end of the reaction, the solvent was evaporated to obtain a solid compound that was purified on a silica gel column using a 30% CH2Cl2–pet ether eluent. Yield: 540 mg. The elemental analyses, calculated (found), are as follows: C 85.96 (85.19), H 4.93 (5.37) and N 9.11 (9.38) %. The UV-vis spectrum in CHCl3 showed the Soret band at 421 nm and the Q bands at 515, 550, 590, and 646 nm (see Fig. S1 in the ESI).
TPPS. 10 mL of conc. H2SO4 (36 N) was added to TPPH (0.099 g, 0.162 mmol) and the reaction mixture was stirred at 60 °C for a day.22 The cooled mixture was then neutralized with sodium hydroxide (1.0 M) and TPPH was then extracted using CHCl3 (20 mL). The solid product obtained after evaporation of CHCl3 on a rotary evaporator was dissolved in dimethylformamide (DMF). The solution was filtered to remove traces of insoluble impurities and DMF was then removed to give TPPS in good yields (90 mg). The elemental analyses, calculated (found), are as follows: C 51.66 (51.02), H 2.56 (2.83), N 5.47 (5.68) and S 12.53 (12.01) %. The UV-vis spectrum in water showed the Soret band at 410 nm and the Q bands at 516, 552, 581 and 635 nm (see Fig. S2 in the ESI). The 1H NMR spectrum of TPPS is shown in Fig. S3 in the ESI.
[Mn(III)(TPPS)Cl] 2. TPPS (0.06 g, 0.058 mmol) and MnCl2·4H2O (0.9 g, 4.55 mmol) were taken in 10 mL DMF in a round bottom flask and heated for two days under a N2 atmosphere at 110 °C. The progress of the reaction was monitored by TLC (20% CH3OH in CHCl3). After the completion of the reaction, the mixture was cooled to room temperature and the unreacted solid was removed by filtration. The filtrate was evaporated to obtain [Mn(III)(TPPS)Cl] in good yields (52 mg, 80.0%). The elemental analyses, calculated (found), are as follows: C 47.59 (46.93), H 2.18 (2.55), N 5.04 (5.12) and S 11.55 (11.98) %. The UV-vis spectrum in water showed the Soret band at 466 nm and the Q bands at 564 and 599 nm (see Fig. 1). The IR spectra of TPPH, TPPS and [Mn(III)(TPPS)Cl] 2 are overlaid and shown in Fig. S4 in the ESI for comparison. The EPR spectrum of 2 was silent and is discussed in the reactivity section (vide infra).
image file: c9nj01381d-f1.tif
Fig. 1 UV-vis spectra of 1 (blue line) and 2 (red line) in (1[thin space (1/6-em)]:[thin space (1/6-em)]2) CH3CN–aqueous basic buffer solution (pH = 10.4). Inset shows the Q bands of 1 and 2. 1 was formed by adding PhIO (1 equiv., CH3OH) to the 1[thin space (1/6-em)]:[thin space (1/6-em)]2 CH3CN–buffer solution of 2 at 298 K.

Results and discussion

Generation and characterization of [Mn(V)(O)(TPPS)Cl] (1)

On addition of 1 equivalent of PhIO to 2 in 1[thin space (1/6-em)]:[thin space (1/6-em)]2 CH3CN–aqueous buffer (pH = 10.4) solution at 298 K, a new Soret (422 nm) and the Q bands (520, 660 nm) appeared in the UV-vis spectrum corresponding to species 1 (Fig. 1). The observed Soret and Q bands assigned to 1 were also reported earlier.17b,c The X-band EPR spectrum of 1 was silent, suggesting a low-spin state (S = 0) of Mn(V) in 1 (Fig. S5 in the ESI).25 The species 1 was quite stable with t1/2 ∼ 30 minutes and decayed slowly to give the starting compound 2. The species 1 was also generated using 1.5 equivalent of m-CPBA. However, the lifetime of 1 formed using m-CPBA was quite a lot lower (t1/2 = ∼5 minutes) and it decayed ultimately to regenerate 2.

The reaction of high valent metal(IV)-oxo species with thioanisole and triphenylphosphine affording quantitative yields of methyl phenyl sulfoxide and triphenylphosphine oxide with formation of the starting metal(II) species is well documented.1 Such transformations are referred to as oxygen atom transfer reactions and conclusively prove that the high valent metal(IV)-oxo species has undergone two-electron reduction. We therefore chose this method for the characterization of species 1. We investigated the reaction of 1 with thioanisole and analyzed the products after the reaction. On addition of thioanisole (25 equiv.) to a solution of 1, the Soret band at 422 nm and the Q bands at 520 and 660 nm decayed, resulting in the simultaneous formation of a new Soret band at 466 nm and Q bands at 564 and 599 nm corresponding to 2 in the UV-vis spectrum (Fig. S6a in the ESI). The appearance of a Soret band and Q bands with the same absorbance as that of the starting [Mn(III)(TPPS)Cl] 2 suggested that species 1 has undergone an oxygen atom transfer to the thioanisole, which is a two-electron process, in turn converting the [Mn(V)(O)(TPPS)Cl] 1 to the starting [Mn(III)(TPPS)Cl] 2. Further, to confirm that the reaction between 1 and thioanisole has really occurred we subjected the reaction mixture to gas chromatography (GC) analysis. This revealed the quantitative formation of methyl phenyl sulfoxide (85 (±5)%) in the above reaction of 1 with thioanisole. Similarly, a reaction of 1 with PPh3 was performed. This reaction was very fast as evidenced by the instant disappearance of the Soret band at 422 nm and fast appearance of a band at 466 nm due to the Mn(III) species (Fig. S6b in the ESI). This reaction also gave a nearly quantitative oxygen atom transfer product i.e. triphenylphosphine oxide (O = PPh3) and the starting Mn(III) compound 2. Thus, based on the quantitative regeneration of 2 and the formation of methyl phenyl sulfoxide or O = PPh3 it was confirmed that the intermediate species 1 has a Mn(V)(O) active core, which is electrophilic in nature and thus able to transfer an oxygen atom to the substrate (Scheme 2).

image file: c9nj01381d-s2.tif
Scheme 2 Oxygen atom transfer reaction between 1 and thioanisole or PPh3 to regenerate the Mn(III) species 2.

C–H activation reactions by [Mn(V)(O)(TPPS)Cl] (1)

The species 1 generated using PhIO was quite stable compared to the species generated by m-CPBA. Hence, we decided to explore the reactivity of 1 in C–H bond activation reactions of a few alkyl hydrocarbons which were selected based on their C–H bond dissociation energies (BDE). In this investigation, xanthene, 1,10-dihydroanthracene (DHA), 1,4-cyclohexadiene (CHD) and fluorene with BDE values of 75.5, 77.0, 78.0 and 80.0 kcal mol−1 respectively were chosen as the substrates (Scheme 3).26
image file: c9nj01381d-s3.tif
Scheme 3 Alkyl hydrocarbons, cyclohexene and styrene used in the reactivity studies with 1.

On addition of CHD (10 equiv.) to an aqueous buffer (pH = 10.4)[thin space (1/6-em)]:[thin space (1/6-em)]CH3CN (2[thin space (1/6-em)]:[thin space (1/6-em)]1) solution of 1, the Soret band at 422 nm of 1 slowly decays, forming a new band at 466 nm corresponding to starting compound 2 with distinct isosbestic points at 390, 485, 535 and 610 nm (Fig. 2a). The absorbance of the resulting solution after the reaction with 1 was the same as that of the starting [Mn(III)(TPPS)Cl] (0.1 mM, 2.0 mL) suggesting that the quantitative formation of Mn(III) via two electron reduction of Mn(V) to Mn(III) has occurred. The reaction of 1 and CHD followed pseudo-first-order kinetics as evidenced from the time trace of the reaction at 422 and 466 nm (Fig. 2a). Similarly, the reactions of xanthene, DHA and fluorene were performed under identical conditions. The pseudo-first-order rate constants increased linearly with an increase in the concentration of xanthene and the other substrates. The pseudo-first-order rate constants (kobs) thus obtained were plotted against the concentration of xanthene to obtain a second order rate constant, k2 = 15.3 (±2) M−1 s−1 (Fig. 2b). The k2 values for DHA, CHD, and fluorene were 7.9 (±5), 4.1 (±3) and 6.6 × 10−1 (±2) M−1 s−1, respectively (Fig. S7 in the ESI). Further, we determine the kinetic isotope effect (KIE) value using xanthene-d2 as the substrate. The addition of xanthene-d2 to a solution of 1 gave k2 = 1.8 (±4) M−1 s−1. Based on this, we determined a KIE = 8.5 for the reaction of xanthene-h2vs. xanthene-d2 (Fig. 2b). A linear correlation was observed between the log[thin space (1/6-em)]k2′ and C–H bond dissociation energies (BDE) of the four hydrocarbons, thus suggesting that the H-atom abstraction is the rate-determining step in their reactions with 1 (the k2 values were divided by the number of equivalent target C–H bonds of the substrates to obtain the k2′ values) (Fig. 2c). When the reaction solutions of xanthene, DHA and CHD were subjected to GC analysis, only xanthone (42 ± 5%), anthracene (84 ± 5%) and benzene (90 ± 4%) were obtained as the products. Since the final solution gave the same UV-vis bands corresponding to 2, it was subjected to the X-band EPR measurement, which gave an EPR silent signal of Mn(III) (Fig. S8 in the ESI).25 Based on the yields of the organic products and formation of 2 in the reaction of the alkyl hydrocarbons and 1, we conclude that in the C–H activation reaction, rate determining H-atom abstraction could be similar to the oxygen rebound pathway as proposed in the reactions of several high valent heme metal-oxo compounds and alkanes (Scheme 4).27 Unlike the present work, Nam and co-workers have shown an alternate pathway i.e. oxygen non-rebound in the C–H activation by a trans-dioxo Mn(V) porphyrin species, [Mn(V)(tf4tmap)(O)2]3+.28

image file: c9nj01381d-f2.tif
Fig. 2 (a) UV-vis spectral changes on addition of 10 equiv. CHD to the 2 mL 1[thin space (1/6-em)]:[thin space (1/6-em)]2 CH3CN–aqueous buffer solution (pH = 10.4) of 1 (0.1 mM, 2.0 mL) at 298 K. (b) Plot of pseudo-first order rate constants vs. concentration of xanthene and xanthene-d2, to obtain the KIE. (c) Plot of log[thin space (1/6-em)]k2vs. BDE of xanthene, DHA, CHD and fluorene.

image file: c9nj01381d-s4.tif
Scheme 4 Proposed mechanism in the C–H bond activation of DHA by 1 to form anthracene. The H-atom abstraction is the r.d.s in this reaction.

Epoxidation of cyclohexene by [Mn(V)(O)(TPPS)Cl] (1)

Cyclohexene has been used as the substrate to test the paradigm of C–H activation via H-atom abstraction forming 2-cyclohexen-1-ol or epoxidation via insertion of an oxygen atom into the C[double bond, length as m-dash]C double bond forming cyclohexene oxide.26 It has been reported that water present in the reaction mixture can enhance the rate of an epoxidation reaction.29 To validate such an observation we performed the reaction of cyclohexene with 1 under identical conditions to those used for the C–H activation of hydrocarbons. Upon addition of cyclohexene (25 equiv.) to the solution of 1, we observed the formation of a new Soret band at 466 nm (Fig. 3a) accompanied by a decay of the 422 nm peak. The band at 466 nm was identical to that observed in the reaction of the alkyl hydrocarbons with 1. The intensity and the absorbance of the 466 nm peak were matching with those of starting compound 2. Based on this evidence we speculated that the reaction of cyclohexene with 1 could be similar to the reaction of alkyl hydrocarbons with 1 giving a two electron manganese(III) compound 2. Looking at the spectral data, the question that arose at this point in our mind was: does cyclohexene undergo α-allylic C–H bond activation in aqueous buffer like alkyl hydrocarbons or does it undergo O-atom transfer like thioanisole and triphenylphosphine? To address this dilemma in the reactivity pattern, we performed the reaction of 1 with varying concentrations of cyclohexene-h10 and cyclohexene-d10. The pseudo-first-order rate constants were obtained by fitting the kinetic data for the decay of the 422 nm band with increasing concentrations of cyclohexene-h10 and cyclohexene-d10. By plotting the kobs values with the concentrations of cyclohexene-h10, we determine the second-order rate constant, k2H = 7.2 × 10−1 (±3) M−1 s−1 (Fig. 3b). On addition of cyclohexene-d10 to the aqueous solution of 1, we observed spectral changes similar to those that occurred in the reaction of cyclohexene with 1 (Fig. 3a). The second order rate constant k2D = 7.1 × 10−1 (±3) M−1 s−1 was obtained by plotting the kobs values for the reaction of cyclohexene-d10 and 1. On comparing the k2H and k2D values, a kinetic isotope effect KIE = 1.0 was obtained. Such a KIE of 1 in these reactions suggest that epoxidation is a likely occurrence here instead of C–H bond activation via H-atom abstraction. The epoxidation reaction was further supported by the product analysis using GC, which revealed an 85% (±3) yield of cyclohexene oxide under an argon atmosphere and EPR silent behavior of the resulting Mn(III) solution (an EPR spectrum similar to that shown in Fig. S6 in the ESI). Finally, to confirm the nature of 1, which undergoes epoxidation and not H-atom abstraction in cyclohexene, we performed the reaction of styrene with 1 under identical conditions. On addition of styrene to the 1[thin space (1/6-em)]:[thin space (1/6-em)]2 CH3CN[thin space (1/6-em)]:[thin space (1/6-em)]buffer (pH = 10.4) solution of 1, the Soret band at 422 nm decayed with concomitant formation of a new Soret band at 466 nm (Fig. S9a in the ESI). The pseudo-first-order rate constants (kobs) increased linearly with an increase in the styrene concentration, affording a second order rate constant, k2 = 5.5 × 10−1 (±2) M−1 s−1 (Fig. S9b in the ESI). When the reaction was carried out using styrene-d8, we observed KIE = 1.0 (Fig. S9b in the ESI). The KIE value of unity suggests that the reaction of styrene and 1 does not occur via H-atom abstraction and instead follows an oxygen atom transfer pathway forming styrene oxide.26 The product analysis by GC gave styrene oxide as the sole product with a yield of 90% (±2). The EPR spectrum of the resulting solution showed no signal, suggesting that the inorganic product formed is the Mn(III) ion. Thus, the results obtained from this study suggest that the use of the [Mn(V)(O)(TPPS)Cl] 1 species in aqueous buffer will result in epoxidation of cyclohexene and styrene over the C–H activation reaction (Scheme S2 in the ESI).
image file: c9nj01381d-f3.tif
Fig. 3 (a) UV-vis spectral changes observed in the reaction of 1 (0.1 mM, red line) and cyclohexene (2.5 mM) in 1[thin space (1/6-em)]:[thin space (1/6-em)]2 CH3CN[thin space (1/6-em)]:[thin space (1/6-em)]buffer (pH = 10.4) at 25 °C. Inset shows the time course monitored at 422 and 466 nm due to the decay of 2 and formation of 1. (b) Plot of the pseudo-first-order rate constants, kobs, against cyclohexene-h10 and cyclohexene-d10 concentrations to determine second-order rate constants, k2 (M−1 s−1), and the determination of the kinetic isotope effect (KIE) value.


The present study is on the paradigm reactivity of [Mn(V)(O)(TPPS)Cl] 1 with alkyl hydrocarbons and cyclohexene. A reaction of [Mn(III)(TPPS)Cl] 2 and PhIO in a 1[thin space (1/6-em)]:[thin space (1/6-em)]2 CH3CN–buffer solution (pH = 10.4) resulted in the formation of a new species 1, which is formulated as [Mn(V)(O)(TPPS)Cl] based on the appearance of new Soret and Q bands in its UV-vis spectrum and its reaction with thioanisole forming methyl phenyl oxide and 2. The reactions of 1 with alkyl hydrocarbons, cyclohexene and styrene were investigated. A large KIE value and a linear correlation of log[thin space (1/6-em)]k2′ with the bond dissociation energies of alkyl hydrocarbons suggested H-atom abstraction as the rate determining step in the C–H activation reactions. The formation of starting Mn(III) compound 2 and the yields of the products suggested that the mechanism subsequent to H-atom abstraction could be similar to the oxygen rebound pathway. Based on the KIE value of 1 for the reaction of cyclohexene as well as styrene with 1 and the quantitative yields of cyclohexene oxide and styrene oxide, an H-atom abstraction step is ruled out in the reaction of cyclohexene and styrene. Efforts are currently underway in our laboratory to stabilize species 1 in different solvent systems and then investigate its reactions with biologically relevant substrates.

Conflicts of interest

There are no conflicts of interest to declare.


SND thanks Council of Scientific and Industrial Research (CSIR) New Delhi, India for financial assistance (01(2923)/18/EMR-II). DH thanks Goa University for his Research Studentship. SND acknowledges the support from DST-FIST (DST, New Delhi) and UGC-SAP (UGC, New Delhi) to the Department of Chemistry, Goa University. GS (IFA-12-CH-71) and MS (IFA17-CH286) acknowledge the Department of Science and Technology (DST), New Delhi, India for financial support through a DST-INSPIRE Faculty Award.


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Dedicated to Prof. B. R. Srinivasan on the occasion of his 60th birthday.
Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nj01381d

This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019