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

Article abstract of DOI:10.1039/c9nj01381d

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 2:1 aqueous buffer (pH = 10.4):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 (logk₂′) 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.

Full text of DOI:10.1039/c9nj01381d

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DOI: 10.1039/C9NJ01381D
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A putative heme manganese(V)-oxo species in
the C-H activation and epoxidation reactions in
an aqueous buffer
Received 00th January
20xx,
Accepted 00th January 20xx
Dinesh S. Harmalkar,a G. Santosh,a,b Siddhi B. Shetgaonkar,a Muniyandi Sankaralingam,c*
DOI: 10.1039/x0xx00000x
Sunder N. Dhuria*
Dedicated to Prof. B. R. Srinivasan on the occasion of his 60th birthday.
www.rsc.org/
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
2:1 aqueous buffer (pH = 10.4): 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 linear correlation of logarithm of the
second rate constant (log 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 an H-atom abstraction as the rate determining step in the C-H activation reactions. On the
other hand, in contrast to C-H activation reaction cyclohexene which has weak C-H bond (BDE =
82.5 kcal mol-1) undergoes an epoxidation reaction.
macrocyclic ligand (TAML) was reported by Collins and
coworkers.4
Manganese–oxo
[Mn(IV)(O)] and [Mn(V)(O)] have been proposed as key
reactive intermediates in several oxidative
porphyrin
species,
Introduction
The high valent metal-oxo short-lived species have
been extensively studied and used in the oxidation of the
organic substrates into viable products.1-2 In iron-heme
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
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 a redox control while
maintaining the low molecular weight and tailored
lipophilicity.12 The high stability of metalloporphyrins
compared to nonheme metal macrocycles has led to
study Mn(III)-porphyrins in superoxide dismutase (SOD)
mimics.13 The synthetic water-soluble Mn(III)-porphyrins
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 nonheme manganese
compound viz [Mn(V)(O)(TAML)]- bearing a tetraamido
containing
pyridinium,
sulfonate
or
carboxylato
a.School of Chemical Sciences, Goa University, Taleigao Plateau, Panaji, Goa
403206, India. E-mail: sndhuri@unigoa.ac.in
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 reaction.16 The compounds such as
[Mn(III)-o-TMPyP] (where o-TMPyP = 5,10,15,20- tetra-
b.Divison of Chemistry, School of Advanced Sciences, Vellore Institute of
Technology, Chennai, Tamilnadu, 600127, India.
c.Department of Chemistry, National Institute of Technology Calicut,
Kozhikode, Kerala 673601, India. E-mail: msankaralingam@nitc.ac.in
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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unless required. The aqueous carbonate-bicarbVoienwaAtreticlbe Ounffliener
(pH ~ 10.4) was prepared by mixing 0.1DOMI:N10a.10C3O9/C(92N7J.0513m81LD)
(N-methylpyridinium-2-yl)porphyrin)
and
2
3
[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
and 0.1 M NaHCO3 (22.5 mL) and dilution with distilled water
in a 100 mL standard flask. pH measurements were
performed using Chemi Line Digital pH meter CL-110. PhIO
was prepared using a literature procedure (see in ESI†).21
TPPH, TPPS and [Mn(III)(TPPS)Cl] 2 were prepared as per
the literature methods22-24 (Scheme S1 in ESI†). The
deuterated xanthene was prepared by a procedure described
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1
in ESI†. H NMR spectra were recorded on a Bruker Avance-
on the reactivity of water-soluble [Mn(V)(O)] III 400 MHz NMR spectrometer. UV−vis spectra were
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+
measured on Agilent 8453 UV−vis spectrophotometer
attached with a water circulator and thermostatic control
(Morya Scientific make). 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 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
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.
(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 [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.
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 N2 atmosphere for about 5 min. To this solution a
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 30 % CH2Cl2-petether 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)
%. UV-vis spectrum in CHCl3 showed Soret band at 421 nm
and Q bands at 515, 550, 590, and 646 nm (see Fig. S1 in
ESI†).
Scheme 1. Water-soluble porphyrins reported in the
literature.
(i)
5,10,15,20-tetra-(N-methylpyridinium-4-
yl)porphyrin
methylpyridinium-3-yl)
5,10,15,20-tetra-(N-methyl pyridinium-2-yl)porphyrin (o-
TMPyP); (iv) 5,10,15,20-tetra-(4-carboxyphenyl)porphyrin
(p-TMPyP);
(ii)
5,10,15,20-tetra-(N-
(m-TMPyP); (iii)
TPPS: 10 mL of conc. H2SO4 (36 N) was added to the
TPPH (0.099 g, 0.162 mmol) and the reaction mixture was
porphyrin
o
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 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) %. UV-vis
spectrum in water showed Soret band at 410 nm and the Q
(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-
(viii) 5-(2,6-
sulfophenyl)porphyrin
(TAPPS);
diaminophenyl)-10,15,20-tri-(4-sulfophenyl)porphyrin
(TDAPPS); (ix) 5,10,15,20-tetrakis(4-sulfonatophenyl)-
21H,23H-porphine (TPPS).
1
bands at 516, 552, 581 and 635 nm (see Fig. S2 in ESI†). H
NMR spectrum of TPPS is shown in Fig. S3 in ESI†.
Materials and Methods
[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 N2
The chemicals used in this study were purchased from
commercial sources and used without any further purification
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atmosphere at 110 oC. The progress of the reaction was reaction. On addition of thioanisole (25 equiv) toVaiewsoArltuictleioOnnlionef
monitored by TLC (20 % CH3OH in CHCl3). After the 1, the Soret band at 422 nm and the Q bDaOnI:d1s0.1a0t359/2C09NaJn0d136861D0
completion of the reaction, the mixture was cooled to room nm decayed resulting in the simultaneous formation of a new
temperature and the unreacted solid was removed by Soret band at 466 nm and Q bands at 564 and 599 nm
filtration. The filtrate was evaporated to obtain corresponding to 2 in the UV-vis spectrum (Fig. S6a in ESI†).
[Mn(III)(TPPS)Cl] in good yields (52 mg, 80.0 %). The The appearance of Soret band and Q bands with the same
elemental analyses, calculated (found) are as follows: C 47.59 absorbance as that of starting [Mn(III)(TPPS4)Cl] 2 suggested
(46.93), H 2.18 (2.55), N 5.04 (5.12) and S 11.55(11.98) %. that the species 1 has undergone an oxygen atom transfer to
UV-vis spectrum in water showed Soret band at 466 nm and the thioanisole which is a two-electron process, in turn,
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the Q bands at 564 and 599 nm (see Fig. 1). IR spectra of converting the [Mn(V)(O)(TPPS)Cl]
TPPH, TPPS and [Mn(III)(TPPS)Cl] 2 are overlaid and shown [Mn(III)(TPPS)Cl] 2. Further, to confirm that the reaction
in Fig. S4 in ESI† for comparison. EPR spectrum of 2 was between 1 and thioanisole has really occurred we subjected
1
to the starting
silent and discussed in the reactivity section (vide infra).
the reaction mixture to a gas chromatographic (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 an
instant disappearance of Soret band at 422 nm and fast
appearance of a band at 466 nm due to Mn(III) species (Fig.
S6b in ESI†). This reaction also gave a nearly quantitative
oxygen atom transferred 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 Mn(V)(O) active core which is
electrophilic in nature and thus able to transfer an oxygen
atom to the substrate (Scheme 2).
Results and Discussion
Generation and characterization of [Mn(V)(O)TPPS)Cl] (1)
On addition of 1 equivalent of PhIO to 2 in 1:2 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,17c The X-band EPR spectrum of 1 was silent
suggesting low-spin state (S = 0) of Mn(V) in 1 (Fig. S5 in
ESI†).25 The species 1 was quite stable with t ~ 30 minutes
½
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
less (t = ~ 5 minutes) and decayed ultimately to regenerate
½
2.
Scheme 2. Oxygen atom transfer reaction between 1 and
thioanisole or PPh3 to regenerate the Mn(III) species 2.
C-H activiation 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 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
Fig. 1. UV-Vis spectra of 1 (blue line) and 2 (red line) in
(1:2) 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 1:2 CH3CN-buffer
solution of 2 at 298 K.
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 starting metal(II) species is well
documented.1 Such transformations are referred to as oxygen
atom transfer reactions and conclusively prove that 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
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Scheme 3. Alkyl hydrocarbons, cyclohexene and styrene used fluorene were performed under identical conditions. The
in the reactivity studies with 1.
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pseudo-first-order rate constants incrDeOaIs: e10d.103li9n/eCa9rNlyJ013w8i1tDh
increase in the concentration of xanthene and 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-1s-1
(Fig. 2b). The k2 values for DHA, CHD, and fluorene were 7.9
(±5), 4.1 (±3) and 6.6 x 10-1 (±2) M-1 s-1, respectively (Fig. S7
in 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-h2 vs xanthene-d2 (Fig. 2b). A linear correlation was
observed between the log 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 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
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 ESI†).25 Based on the yields of organic
products and formation of 2 in the reaction of alkyl
hydrocarbons and 1, we conclude that in the C-H activation
reaction, a 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 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
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Fig. 2.a) UV-Vis spectral changes on addition of 10 equiv
CHD to the 2 mL 1:2 CH3CN-aquous 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 KIE. (c) Plot of log
k2’ vs BDE of xanthene, DHA, CHD and fluorene.
Scheme 4. Proposed mechanism in the C-H bond activation of
DHA by 1 to form anthracene. H-atom abstraction is r.d.s in
this reaction.
On addition of CHD (10 equiv) to an aqueous buffer (pH =
10.4) : CH3CN (2:1) 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 same as
that of 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
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 an H-atom abstraction forming
2-cyclohexen-1-ol or epoxidation via insertion of an oxygen
atom to C=C double bond forming cyclohexene oxide.26 It has
been reported that the water present in the reaction micture
can enhance the rate of epoxidation reaction.29 To validate
such an observation we performed the reaction of
cyclohexene with 1 under the identical conditions used for the
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C-H activation of hydrocarbons. Upon addition of cyclohexene spectral changes similar to the that occurred in the reaction of
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(25 equiv) to the solution of 1, we observed the formation of cyclohexene with 1 (Fig. 3a). The secondDOoIr:d10e.r10r3a9t/eC9cNoJn0s1t3a8n1tD,
a new Soret band at 466 nm (Fig. 3a) accompanied by a k2D = 7.1 x 10-1 (±3) M-1s-1 was obtained by plotting the kobs
decay of the 422 nm peak. The band at 466 nm was identical values for the reaction of cyclohexene-d10 and 1. On
to that observed in the reaction of alkyl hydrocarbons with 1. comparing k2H and k2D values, a kinetic isotope effect, KIE =
The intensity and the absorbance of 466 nm peak were 1.0 was obtained. Such a KIE of 1 in these reactions suggest
matching with those of starting compound 2. Based on these that, the epoxidation is a likely occurence here instead of C-H
evidences we speculated that the reaction of cyclohexene with bond activation via an H-atom abstraction. The epoxidation
1 could be similar to the reaction of alkyl hydrocarbons with 1 reaction was further supported by the product analysis using
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giving a two electron manganese(III) compound 2. Looking at
GC which revealed 85 % (±3) yield of cyclohexene oxide
the spectral data, a question arose at this point in our mind
under argon atmosphere and a EPR silent behavior of the
was:
Does
cyclohexene
resulting Mn(III) solution (EPR spectrum similar to shown in
Fig. S6 in ESI†). Finally to confirm the nature of 1 which
undergoes epoxidation and not an H-atom abstraction in
cyclochexene, we performed the reaction of styrene with 1
under identical conditions. On addition of styrene to the 1:2
CH3CN: 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 ESI†). The pseudo-first-order rate
constants (kobs) increased linearly with increase in the styrene
concentration affording a second order rate constant, k2 = 5.5
x 10-1 (±2) M-1 s-1) (Fig. S9b in ESI†). When the reaction was
carried out using styrene-d8, we observed KIE = 1.0 (Fig. S9b
in 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 yield of 90 (±2). EPR spetcrum
of the resulting solution showed no signal suggesting that the
inorganic product formed is Mn(III) ion. Thus, the results
obtained from this study suggest that the use of
[Mn(V)(O)(TPPS)Cl] 1 species in aqueous buffer will result in
epoxidation of cyclohexene and styrene over C-H activation
reaction (Scheme S2 in ESI†).
Conclusions
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 1:2 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 methy 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 logk2’ with the bond dissociation energies of
alkyl hydrocarbons suggested a 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 products suggested that the mechanism
subsequent to a H-atom abstraction could be similar to
oxygen rebound pathway. Based on the KIE value of 1 for
the reaction of cyclohexene as well as styrene with 1 and
the quantitive 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
Fig. 3. (a) UV-Vis spectral changes observed in the
reaction of 1 (0.1 mM, red line) and cyclohexene (2.5 mM)
o
in 1:2 CH3CN: 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-1s-1) and the determination of the
kinetic isotope effect (KIE) value.
undergo an α-allylic C-H bond activation in aqueous buffer like
alkyl hydrocarbons or does it undergo an 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 422 nm band with the
increasing concentrations of cyclohexene-h10 and cyclohexene-
d10. By plotting the kobs values with the concentrations of
cyclohexene-h10, we determine a second-order rate constant,
k2H = 7.2 x 10-1 (±3) M-1s-1 (Fig. 3b). On addition of
cyclohexene-d10 to the aqueous solution of 1, we observed the
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(b) H. Chen, W. Lai, J. Yao, and S. Shaik, J. Chem.
Theory Comput., 2011, 7, 3D0O4I9: -130.01053V39ie;/wC9ArN(ticcJl)0e1O3n8lM1inDe.
Sankaralingam, Y.-M. Lee, W. Nam and S. Fukuzumi,
Inorg. Chem., 2017, 56, 5096-5104; (d) M. R. Mills, A.
C. Weitz, M. P. Hendrich, A. D. Ryabov and T. J. Collins,
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Bominaar, A. D. Ryabov and T. J. Collins, Chem. Eur.
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Vardhaman, W. Nam and S. Fukuzumi, Chem Commun.,
2017, 53, 8348-8351.
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.
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Acknowledgements
SND thanks Council of Scientific and Industrial Research
(CSIR) New Delhi, India for financial assistance 3. (a) S. Hong, Y.-M. Lee, M. Sankaralingam, A. K.
Vardhaman, Y. J. Park, K.-B. Cho, T. Ogura, R. Sarangi,
S. Fukuzumi and W. Nam, J. Am. Chem. Soc., 2016,
138, 8523-8532; (b) H. M. Neu, R. A. Baglia and D. P.
Goldberg, Acc. Chem. Res., 2015, 48, 2754-2764; (c)
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2015, 44, 1083-1100.
(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)
acknowledges the Department of Science and Technology
(DST), New Delhi, India for financial support through
DST-INSPIRE Faculty Award.
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A putative heme manganese(V)-oxo species in tDhOeI: 10.10C39-/CH9NJ01381D
activation and epoxidation reactions in an aqueous buffer
Dinesh S. Harmalkar,a G. Santosh,a,b Siddhi B. Shetgaonkar,a Muniyandi Sankaralingam,c* Sunder N.
Dhuria*
aSchool of Chemical Sciences, Goa University, Taleigao Plateau, Panaji, Goa 403206, India.
bDivison of Chemistry, School of Advanced Sciences, Vellore Institute of Technology, Chennai,
Tamilnadu, 600127, India.
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cDepartment of Chemistry, National Institute of Technology Calicut, Kozhikode 673601, Kerala,
India.
*corresponding author email: sndhuri@unigoa.ac.in; msankaralingam@nitc.ac.in
Table of Content (TOC)
Synthesis and reactivity studies of manganese(V)-oxo species in the C-H activation of alkyl
hydrocarbons and epoxidation of cyclohexene in aqueous conditions are investigated.