Investigation of 3d-transition metal acetates in the oxidation of substituted dioxolene and phenols

Article abstract of DOI:10.1016/j.molcata.2015.06.020

Abstract Enzymatic reactions have inspired many chemists to design small molecule mimics that would perform the function of the enzymes in aqueous and or non-aqueous medium. Catechol oxidase (CO) and phenoxazinone synthase (PHS) are two multi-copper enzymes in nature, which has led to model complexes of Mn, Fe, Co, Ni, Cu. Based on our earlier work in this area we have probed the commercially available metal acetates of the above metals to establish a trend in reactivity for catalytic conversions similar to those of the two enzymes. The results show that Mn is the best 3d transition metal for similar catalysis. Mn^II acetate was found to convert 3,5-di-tert-butylcatechol (DTBC) to 3,5-di-tert-butylquinone (DTBQ) with a k_cat of 1.3(1) × 10³ h^-1and for o-aminophenol (OAP) to 2-aminophenoxazinone (APX) conversion the k_cat = 111(2) h^-1, demonstrating efficient CO and PHS like activity. Kinetic studies show that DTBC oxidation follows a first order kinetics with respect to the substrate for each of those metal(II) acetates with activity order of Mn >> Co > Cu > Fe ≥ Ni. Through mechanistic investigation we found that the reactive oxygen species detected during the oxidation of DTBC is mostly hydroxyl radical for Mn, Fe and Co whereas Cu and Ni generate H₂O₂.

Full text of DOI:10.1016/j.molcata.2015.06.020

Journal of Molecular Catalysis A: Chemical 407 (2015) 93–101
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Investigation of 3d-transition metal acetates in the oxidation of
substituted dioxolene and phenols
∗
Suman Kr. Dey, Arindam Mukherjee
Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, 741246, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 April 2015
Received in revised form 12 June 2015
Accepted 15 June 2015
Available online 20 June 2015
Enzymatic reactions have inspired many chemists to design small molecule mimics that would perform
the function of the enzymes in aqueous and or non-aqueous medium. Catechol oxidase (CO) and phe-
noxazinone synthase (PHS) are two multi-copper enzymes in nature, which has led to model complexes
of Mn, Fe, Co, Ni, Cu. Based on our earlier work in this area we have probed the commercially available
metal acetates of the above metals to establish a trend in reactivity for catalytic conversions similar to
those of the two enzymes. The results show that Mn is the best 3d transition metal for similar catalysis.
Keywords:
Oxygen activation
Catechol oxidation
II
Mn acetate was found to convert 3,5-di-tert-butylcatechol (DTBC) to 3,5-di-tert-butylquinone (DTBQ)
3
−1
with a kcat of 1.3(1) × 10 h and for o-aminophenol (OAP) to 2-aminophenoxazinone (APX) conversion
−1
the kcat = 111(2) h , demonstrating efficient CO and PHS like activity. Kinetic studies show that DTBC
oxidation follows a first order kinetics with respect to the substrate for each of those metal(II) acetates
with activity order of Mn » Co > Cu > Fe ≥ Ni. Through mechanistic investigation we found that the reac-
tive oxygen species detected during the oxidation of DTBC is mostly hydroxyl radical for Mn, Fe and Co
whereas Cu and Ni generate H2O2.
2
-Aminophenoxazinone
Oxidative C–C bond coupling
Metal acetates
Hydroxyl radical
©
2015 Elsevier B.V. All rights reserved.
1
. Introduction
(CO) and phenoxazinone synthase (PHS), two copper contain-
ing metalloenzymes in nature that inspired us and many other
bio-inorganic chemists to design model complexes. CO oxidizes
catechols/dioxolenes to corresponding quinones. Quinone based
molecules are found to be useful to design compounds used to
suppress bone marrow function, inhibit proteases, and generate
anticancer agents. Naturally occurring quinones like metachronin,
bolinaquinone are potent anticancer agents [11,12]. Quinones
are also useful scavengers of mercaptans, sulfides, cyanides and
amines [13]. PHS catalyzes the oxidative coupling of substituted 2-
aminophenol to 2-aminophenoxazinone chromophore (Scheme 1)
known as actinomycin, a well known antineoplastic agent [14].
Among various approaches to utilize the properties of enzymes in
different ways, attempt to fix the enzymes on surfaces and perform
organic transformations through heterogeneous catalysis is one of
them [15–19]. However, binding enzymes to a solid state support
are tedious and may not always work in organic solvents for simi-
lar transformations. We attempt to involve commercially available
metal acetates to perform organic transformations similar to CO
and PHS (Scheme 1) in laboratory conditions in organic solvents
due to their potential in pharmaceutical, dyes and other industrial
purposes.
Bio-oxidation reactions have inspired the design of a library of
metal complexes, which led to many successful functional models
of enzymes over the past few decades [1–5]. These complexes have
gained importance for their ability to shed insight on the mech-
anistic pathways of the original enzymes. Many of these are also
considered as efficient catalysts in spite of their activity being much
less in comparison to the enzymes. The advantage of the small
molecule mimics are that they are capable of doing the reactions
in organic solvents. Metalloenzymes can catalyze the conversion of
organic compounds with molecular oxygen under physiologically
mild conditions [6–10] but they are not capable of performing in
organic solvents unless specially modified. Therefore, the design
and synthesis of transition metal complexes and having ability to
activate molecular oxygen is an attractive approach to green oxi-
dation catalyst based on bio-inspired concept.
Among the various catalytic conversions carried by enzymes
we chose the ones similar to that carried by catechol oxidase
∗ Corresponding author. Fax: +91 33 25873020.
E-mail address: a.mukherjee@iiserkol.ac.in (A. Mukherjee).
http://dx.doi.org/10.1016/j.molcata.2015.06.020
1
381-1169/© 2015 Elsevier B.V. All rights reserved.

9
4
S.Kr. Dey, A. Mukherjee / Journal of Molecular Catalysis A: Chemical 407 (2015) 93–101
ꢀ
ꢀ
II
CONHR
bond coupling of 2,6-di-tert-butylphenol (2,6-DTBP) to 3,3 -5,5 -
tetra-tert-butyldiphenoquinon (DPQ). We found that although Cu
happens to be the metal of choice in the enzymes CO and PHS. In
-acetate is the most
efficient. Co - and Cu -acetate seems to be next in line with sim-
NH
2
OH
OH
II/III
laboratory among the metal acetates, Mn
II
II
OH
ilar kcat for oxidizing DTBC to DTBQ but the ROS species involved
in the processes are different. In conversion of OAP to APX, Mn
3
/2 O
2
II/III
1
/2 O
2
again seems to be the best among the 3d-transition metal ions and
PHS
II
II
II
CO
the activity of Cu -acetate is comparable with Fe - and Co -acetate.
Probing the active species during DTBC oxidation showed that the
DTBC also act as ligand and the acetates are all stripped of from the
metal ion during catalysis.
3
2
H O
2
H O
O
O
CONHR
CONHR
N
O
NH
O
2
2. Experimental
2.1. Materials and methods
R = Thr-DVal-Pro-Sar-MeVal
All reactions were carried out using HPLC grade solvents
methanol (Merck), acetonitrile (Merck)]. All metal(II) acetate
e.g., manganese(II) acetate tetrahydrate, iron(II) acetate,
Scheme 1. Oxidation reactions catalysed by catechol oxidase (CO) and phenoxazi-
none synthase (PHS).
[
[
cobalt(II) acetate tetrahydrate, nickel(II) acetate tetrahydrate,
copper(II) acetate monohydrate], 3,5-di-tert-butylcatechol,
dimethylsulfoxide (DMSO), (± )-␣-tocopherol, probucol, d/l-
p-chlorophenylalanine, 2-aminophenol, potassium titanium
oxide oxalate dihydrate were all purchased from Sigma and
used without further purification. 3,5-di-tert-butylbenzoquinone
was also purchased from Aldrich and used to calculate molar
extinction coefficient (ꢀ) in 90:1 acetonitrile, methanol mixture.
L-methionine, L-histidine [SRL (India)] and sodium hydroxide
Researchers around the world have put significant effort to
model CO and PHS through design of metal complexes of com-
plicated ligand systems [20–23] and provided insight about the
mechanistic pathway [21,22,24–26] of these enzymes. The metal
salts, especially the acetates which are used as precursor in syn-
theses of many such biomimetic complexes are scarcely probed
for their efficiency in such catalysis [27–30]. At first it seems quite
reasonable, since the coordinating atoms and the geometry of the
enzyme active site are not related to the structure of metal acetates
in any way. However, since the metal salts have been ignored, the
role and influence of the metal ion as standalone remained unex-
plored. One may also argue that probing metal acetates is essential
since it would inform us that how much the mechanistic pathway
or the ROS generated may be influenced by the ligand when com-
plexed with the metal. During our efforts in this area we happened
[
Merck (India)] were also used as received. Methyl ester of methi-
onine and histidine were synthesized according to a previously
reported literature procedure [96]. NMR spectra were recorded on
Bruker Avance III 500 MHz or on Jeol ECS400 MHz spectrometer
◦
at room temperature (25 C). Electronic spectra were recorded
using Varian Cary 300 Bio UV–vis spectrophotometer. Electrospray
ionisation mass spectra were recorded using micromass Q-Tof
TM
micro (Waters) by positive and also negative mode electrospray
III
to obtain a Mn complex, which provided the best turnover for
ionization. X-Band EPR spectra were recorded on a Bruker BioSpin
WinEPR spectrometer.
DTBC oxidation and it had OR type oxygen donors [31]. The above
result led us to think that metal acetates needs to be investigated
for their efficiency in DTBC oxidation. To begin this chase, since
III
III
our success was with a Mn complex, hence Mn -acetate was our
obvious first choice to probe for CO and PHS activity. The results
2.2. Catalytic oxidation of DTBC
III
were promising. Mn acetate demonstrated itself to be one of the
UV–vis spectra for kinetic studies were recorded by using a
quartz cuvette (1.0 cm path length) and a Varian Cary 300 Bio
UV–vis spectrophotometer equipped with a Peltier thermostat-
ing accessory. All the kinetics measurements were conducted at
best small molecule mimics for PHS using 2-aminphenol (OAP) as
substrate to form 2-aminophenoxazinone (APX) and an efficient
3
−1
CO mimic with a turnover of the order of 10 h for oxidation of
,5-di-tert-butylcatechol (DTBC) to 3,5-di-tert-butylbenzoquinone
◦
3
a constant temperature of 25 C, monitored with a thermostat.
II
(
DTBQ) [32].
DTBC in acetonitrile were added to the solutions of M -acetate in
methanol under aerobic condition at room temperature (25 C).
◦
However, we still did not know about the activity of the rest of
the metal ions frequently used for the syntheses model complexes
of CO or PHS. Hence, we probed a series of 3d-transition metal
acetates for DTBC and OAP oxidation. The metals were chosen from
the list of 3d-metals used so far to design mimics of CO or PHS using
different ligand systems viz. copper [24,33–54], iron [25,55–57],
cobalt [58–65], manganese [66–79], nickel [26,68,80–83] and zinc
The final ratio of acetonitrile:methanol in cuvette was 90:1 v/v.
Absorbance vs. wavelength plots were generated for these reac-
tion mixtures, recording spectrophotometric data at a regular
time interval of 5 min in the range 300–700 nm. To determine
the substrate concentration dependence of the rate and various
−
4
−6
kinetic parameters, 10 –10 M solutions of catalyst was treated
with 100–800 M equivalents of DTBC and the absorbance moni-
[
33,84]. PHS mimics cannot be found as much as CO but yet are
well studied with 3d-metals viz. Co [85–92], Mn [71,89,91,93], Cu
94], Fe [91,95] complexes using 2-aminophenol as substrate. We
II
tored as mentioned above. For Mn -acetate kinetic experiments
−
6
[
were performed using 10 M catalyst concentration while other
metal acetates are not active at such low concentration. Hence
investigated the possible reactive oxygen species (ROS) involved for
each of the metal ions. We chose metal acetates over chlorides or
nitrates since the acetates are labile ligands, commercially available
and solvent compatible with DTBC and OAP.
II
II
−5
for Co and Cu acetate we used 10 M concentration while for
Fe and NiII acetate even higher concentration (10 M) of cata-
lyst were used. The kinetic parameters were determined by using
Michaelis–Menten, Lineweaver–Burk and Eadie–Hofstee plot. The
product (DTBQ) was also characterized by ESI–MS and NMR spec-
II
−4
The comparative study of Mn , Fe , Co , Ni and CuII is pre-
II
II
II
II
sented here for three type of oxidation reactions which are
oxidation of DTBC to DTBQ, OAP to APX and oxidative C
C
troscopy. 1H NMR (400 MHz, CDCl ): ı = 6.93 (d, 1H, J = 2.28 Hz),
3

S.Kr. Dey, A. Mukherjee / Journal of Molecular Catalysis A: Chemical 407 (2015) 93–101
95
Table 1
6
(
.21 (d, 1H, J = 1.52 Hz), 1.26 (s, 9H), 1.22 (s, 9H) ppm; 13C NMR
100 MHz, CDCl ): ı = 181.26 (C-1), 180.17 (C-2), 163.46 (C-3),
Kinetic parameters for the oxidation of DTBC to DTBQ.
3
Vmax [M min−1] Std. error KM [M]
Std. error kcat [h
−1
]
1
8
50.07 (C-4), 133.60 (C-5), 122.22 (C-6), 36.16 (C-7), 35.61 (C-
), 29.34 (C-9), 28.01 (C-10) ppm. ESI–MS positive ion mode:
[Cat]
II
−5
−5
−6
−5
−6
−5
3
Mn acetate 2.2 × 10
1.67 × 10 0.00066 9.25 × 10 1.3(1) × 10
II
−7
−4
−3
−4
−5
+
+
Fe acetate
1.4 × 10
5.6 × 10
1.4 × 10
9.86 × 10 0.00714 8.54 × 10 8.4(2)
m/z = 243.14 [(DTBC + Na )] (calc. 243.14). The data showed that
only DTBQ is formed from DTBC. No other side product is obtained.
II
−6
2
Co acetate
1.96 × 10 0.010
5.39 × 10 0.32(1) × 10
II
−7
Ni acetate
4.46 × 10 0.00302 2.25 × 10 8.4(3)
II
−6
−7
2
Cu acetate
4.03 × 10
9.79 × 10 0.00015 1.26 × 10 0.23(1) × 10
2
.3. Detection of hydrogen peroxide in the catalytic reaction of
2
.5. Oxidative CC coupling of 2,6-di-tert-butylphenol (conversion
oxidation of DTBC
of 2,6-di-tert-butylphenol to
ꢀ
ꢀ
3,3 -5,5 -tetra-tert-butyldiphenoquinone)
The formation of hydrogen peroxide during the catalytic oxida-
tion of DTBC was probed by two different methods-
Oxidative coupling of 2,6-di-tert-butylphenol (2,6-DTBP) was
H O can be detected by the generation of characteristic peak at
2
2
performed by taking 2,6-DTBP in methanol, sodium hydroxide in
water was added to it such that the overall concentration is 10 mol%.
To the above solution M acetate (0.01 mmol) (M = Mn , Fe , Co ,
Ni , Cu ) in methanol was added and the mixture was stirred at
2
−
3
52 nm for I3 ion with potassium iodide. To detect hydrogen per-
II
oxide after the oxidation of DTBC, DTBC was oxidized by 1 mol% M
acetate for 2 h in acetonitrile and methanol mixture. The formed
DTBQ was then extracted three times using dichloromethane.
Water part was then acidified to pH 2 using diluted H SO4 and
one-third volume of KI solution (500 mg/10 mL) in water was added
to it with 100 nM Horse Radish Peroxidase and the appearance of
the band at 352 nm characteristic for I3 ion was monitored. Con-
trol experiments were performed using only H O2 solution and
atmospheric oxygen (without catalyst or DTBC).
II
II
II
II
II
II
II
◦
5 C for 4–24 h. Product started forming as red precipitate after an
2
II
hour of addition of M acetate solution. On completion of the reac-
tion the product was collected by filtration, washed by methanol
and dried. The product was found to be pure by TLC and hence the
−
dried product was analyzed by ESI–MS and 1H NMR spectroscopy.
2
+
ESI–MS (positive ion mode): m/z = 409.30 [(DPQH)] (calc. 409.31);
m/z = 431.29 [(DPQNa)]+ (calc. 431.29). 1H NMR (500 MHz, CDCl ):
3
H O formation can also be detected by formation of TiIV-peroxo
2
2
ı = 7.71(s, 4H), 1.36 (s, 36H) ppm.
species using potassium titanium(IV) oxalate. In this experiment
the catalytic reaction of DTBC oxidation and the extraction proce-
dure was same as the above method. The isolated aqueous part was
added 1 mM solution of potassium titanium(IV) oxalate to moni-
tor the band ca. 379 nm, due to the formation of TiIV-peroxo bond
2.6. Mass spectrometry
ESI mass spectrometric data were recorded using Waters Q-Tof
micro mass spectrometer. The mass spectrometric studies of cate-
chol oxidation were performed using (1:1) acetonitrile, methanol
mixture. The ESI–MS was performed with 1:50 mixture of M -
[
97,98]. Control experiment was also performed using hydrogen
peroxide.
II
II
acetate with DTBC having M -acetate in 10 ␮M concentration.
Higher catalyst ratio was used to obtain good signal to noise ratio for
the various metal bound species formed. The ESI–MS of the catalytic
products were also performed with 10 ␮M stock solutions.
2
2
.4. Catalytic oxidation of 2-aminophenol (synthesis of
-aminophenoxazinone or APX)
Oxidation of o-aminophenol (OAP) was carried out by taking
3
. Results and discussion
II
II
II
OAP (109.0 mg, 1.0 mmol) and M acetate (0.01 mmol) (M = Mn ,
II
II
II
II
Fe , Co , Ni , Cu ) in methanol and then the reaction mixture
was stirred for 12 h. Pure product precipitated out of the solution
which was filtered, collected. The product was characterized by
3.1. Catecholase activity studies
Catechol oxidase (CO) is a dinuclear CuII containing enzyme
1
13
ESI–MS, H NMR and C NMR. ESI–MS (+ve ion mode): m/z = 213.08
having a type-3 active site [99], which catalyzes the oxidation
of catechol to quinone. The structural and functional aspects of
CO have been elucidated with the help of several model sys-
tems [24,49,68,71,81,82,84,100–102]. In addition the search for
the mimics leads to several alternate mechanisms of oxidation
[20–22,46,103–105]. In model studies on catecholase activity, DTBC
is usually used as the substrate since the bulky groups prevent over
oxidation such as ring opening [106–108] and also show prominent
increase in absorbance at ca. 400 nm with increase in the formation
of oxidized species (DTBQ).
+
[
(APXH)]+ (calc. 213.07); m/z = 235.05 [(APXNa)] (calc. 235.05);
m/z = 251.03 [(APXK)] (calc. 251.02). 1H NMR (500 MHz, Me SO-
+
2
d ): ı = 7.71 (dd, 1H, J = 7.5 Hz, ArH), 7.44 (m, 2H, ArH), 7.39 (m, 1H,
6
13
ArH), 6.80 (br, s, 2H, NH ), 6.36 (s, 2H, ArH) ppm. C NMR (125 MHz,
2
Me SO-d ) ı = 180.2 (C-3), 148.9 (C-10a), 148.2 (C-4a), 147.3 (C-
2
6
2
1
), 141.9 (C-5a), 133.7 (C-9a), 128.8 (C-7), 127.9 (C-9), 125.3 (C-8),
15.9 (C-6), 103.4 (C-1), and 98.3 (C-4) ppm.
Kinetics of the aerobic oxidation of OAP to APX in presence
II
of Mn acetate were measured by monitoring the change in
3
−1
−1
absorbance as a function of time at 430 nm (ꢀ= 22 × 10 M cm ),
which is characteristic of 2-aminophenoxazin-3-one. All the kinet-
ics measurements were conducted at a constant temperature of
We performed kinetic and mechanistic studies of DTBC oxida-
−4
−6
tion using 10 –10 M solutions of six transition metal acetates
as mentioned earlier. We found that among six metal acetates we
tried, only Zn acetate was inactive whereas the other five (Mn ,
◦
2
5 C, monitored with a thermostat. To determine the substrate
II
II
concentration dependence on the rate and various kinetic param-
eters, 5 ␮M solutions of catalyst was treated with 500, 1000, 1500,
000, 2400, 3000, 3200 and 3400 M equivalents of OAP and the
absorbance monitored as mentioned above. Absorbance vs. wave-
length plots were generated for these reaction mixtures, recording
spectrophotometric data at a regular time interval of 5 min in
the range 300–700 nm. The final ratio of acetonitrile:methanol in
cuvette was 90:1 v/v. The kinetic parameters were determined by
using Michaelis–Menten plot and Lineweaver–Burk plot.
II
II
II
II
II
Fe , Co , Ni and Cu -acetate) were active. The redox inactive Zn
acetate is supposed to be catalytically inactive, which corroborates
our result. For a particular catalyst–substrate mixture the rates
were calculated from the initial slope of ꢁA vs. time plots (change
in absorbance at 400 nm), using up to 800 M equivalent of DTBC and
analyzed by Michaelis–Menten equation, Lineweaver–Burk and
Eadie–Hofstee plot [109] (Table S1 and Fig. S1) and the purity of the
product DTBQ was further confirmed by NMR and ESI–MS studies
2
(Figs. S2–S4). In all the three cases the kinetic parameters obtained

9
6
S.Kr. Dey, A. Mukherjee / Journal of Molecular Catalysis A: Chemical 407 (2015) 93–101
Table 2
Percentage (%) of inhibition of DTBC oxidation catalysed by M acetates (MII = MnII,
II
II
II
II
II
Fe , Co , Ni , Cu ). The mean of two independent experiments are tabulated (less
than 10% inhibition is neglected).
Inhibitor
MnII
FeII
CoII
NiII
CuII
acetate
acetate
acetate
acetate
acetate
(
± )-␣-Tocopherol
None
None
65(3)
100
None
15(3)
22(2)
100
None
16(4)
29(3)
100
None
None
None
100
None
None
None
100
probucol
DMSO
methionine ester
tively performing a reaction similar to enzyme and may help us
choose the metal to perform a similar reaction in the laboratory.
The metal acetates also provide more insight about the rational-
ity of the choice of metal at the enzyme active site based on their
redox activity for a particular reaction. This reactivity knowledge
may be used to design better bioinspired catalyst to perform similar
reactions. A point to note is that not all proteins/enzymes are selec-
tive in vitro. Enzymes without specific condition and coordination
environment around them might be non-selective viz. xanthane
oxidase can generate superoxide in presence of oxygen [124,125],
haloperoxidase can catalyze sulfoxidation [126], tyrosinase can
perform catecholase activity [127,128] and phenoxazinone syn-
thase activity [127,128], laccase can also perform phenoxazinone
synthase activity [129,130], nitrogenase can reduce unsaturated
hydrocarbon substrates, cyanate, thiocyanate [131,132]. Although
most model complexes of the enzyme CO are copper complexes
but a close look at the Mn complexes show that the Mn complexes
exhibit good catalytic efficiency which is overall better than any
other probed metal complexes suggesting that Mn is the best for
similar activity in laboratory or in industry.
Fig. 1. Plot of initial rate vs. substrate concentration for the oxidation of DTBC catal-
ysed by Mn acetate. The inset shows Eadie–Hofstee plot.
II
are in good match with each other. Based on the three fitting data
the turnover number (kcat) and other kinetic parameters are shown
II
in Table 1 which reflects that Mn acetate is the most efficient cat-
3
−1
alyst among them with a kcat of 1.3(1) × 10 h (Fig. 1). The kcat of
II
Mn -acetate is better than majority of designed mimics and of the
III
3
−1
same order as that of Mn -acetate (kcat = 1.72(2) × 10 h ) [32]
reported by us earlier. This suggests that Mn is the most efficient
among 3d-transition metals for this particular purpose. Nature did
not redeem Mn fit for this purpose and rather chose Cu although
the abundance of Mn is more than copper in earth crust or in sea
bed [110,111]. One reason might be that it is difficult for nature to
incorporate Mn in that ligand environment and direct its function
to specifically oxidize catechol.
3.2. Inhibition of catecholase activity
II/III
In fact the Mn
acetate is known to perform other oxidation
reactions as well [112–115]. Hence the differences in electronic
properties of the two metal ions viz. redox potential, stabilization of
a certain geometry rendered by the protein, ability to perform other
reactions led to the choice of Cu. The results emphasize that being
efficient as a metal ion for a certain purpose and mere abundance
does not necessarily render it suitable by nature for incorporation
The mechanistic pathway for DTBC oxidation was probed for all
the active metal acetates used. An important part of the mechanis-
tic pathway is the type of ROS involved in the catalytic process.
We used different inhibitors in the reaction mixture to check
their effect on the reaction rate which in-turn provided insight
into the possible ROS involved. In general we have used 10 M
equivalent of the inhibitor with respect to the catalyst concen-
tration. The inhibitors used in the catalytic oxidation of DTBC
are (± )-␣-tocopherol (singlet oxygen quencher) [133,134], probu-
col (peroxide and superoxide quencher) [135,136] and DMSO
(hydroxyl radical inhibitor). The effects of those inhibitors are
shown in Table 2 (Fig. S5). One common result for all the metal
acetates is that amino acids inhibit the catechol oxidation abil-
ity viz. use of 10 M equivalent of methyl ester of methionine or
methyl ester of histidine showed complete inhibition of oxidation
of DTBC. This is in good agreement with our earlier studies with
other CO model complexes which suggested that the amino acids
or methyl esters of amino acids are potential inhibitors of catechol
oxidation as they compete with the substrate (DTBC) and or oxy-
gen for binding to the catalyst and inhibit the oxidation process
[31,32,137].
II
into a proteins active site. Nature has chosen Cu for the active site
II
2
−1
of CO although Cu acetate (kcat = 0.2(1) × 10 h ) is less active
II
3
−1
II
than Mn acetate (kcat = 1.3(1) × 10 h ). On the other hand Cu
II
2
−1
acetate and Co acetate (kcat = 0.3(1) × 10 h ) have similar kcat
for the CO activity but the KM values are different. The ratio of
kcat/KM actually predicts the best catalyst for a bio-mimetic reac-
II/III
tion [35,45,52]. In our case the calculation shows that Mn
has
6
−1 −1
h and the next best one is
the nature’s choice Cu with a kcat/KM of 10 M h . The other 3d-
kcat/KM value of the order of 10 M
II
5
−1 −1
metals (Fe, Co, Ni) probed are at least two order of magnitude less
II
than Cu -acetate. It appears that although there are many excel-
II
II
lent Ni catalysts for CO activity but in case of Ni the importance
II
of the ligand is very much appreciated since Ni acetate by itself
has a very low turnover number even at 10 M concentration
Table 1). Fe -acetate also has a low kcat even at 10 M concentra-
−
4
II
−4
(
tion and it appears from the literature that there has not been much
II
attempts to use Fe based complexes as mimics of CO [25,55–57].
Fe
3.3. Evaluation of the ROS species
II/III
seems to be a better choice for catechol dioxygenase (which
leads to intradiol or extradiol cleavage) based on the literature data
116–123]. Hence, the two metal ions Mn and Cu may provide a
good example to demonstrate how the ‘rule of fitness’ may lead to
rejection of a metal ion in an enzyme active site.
In this work, we do not intend to say that the metal acetates are
specific to a catalytic reaction. In fact we know that metal acetates
used in this work are also known to catalyze many other reactions.
Our results demonstrate that how metal acetates can be effec-
During oxidation of DTBC when the metal is reduced, the
presence of molecular oxygen would lead to oxidation of the
metal ion where the oxygen is converted to an ROS. However,
depending on the number of electrons available for the ROS it
may be converted to water or stay as hydrogen peroxide. Out of
the major mechanistic pathways established the one which pro-
duces two molecules of quinone and water is the most accepted
one for the enzyme catechol oxidase. However, there are only
[

S.Kr. Dey, A. Mukherjee / Journal of Molecular Catalysis A: Chemical 407 (2015) 93–101
97
Suggested by UV-vis
DTBQ
1
/2O
2
HO
DTBC
O
O
O
O
O
O
O
O
MII
MIII
O
Suggested by ESI-MS
OH
H
2
MII = Mn , Fe and CoII
II
II
1
/2O
2
O
O
O
O
1
/2H
2
O
2
MII
2
H O + DTBQ
+
DTBQ
O
2
Suggested
by UV-vis
H O
2 2
1/2O
2
Suggested
by UV-vis
O
O
O
O
MII = NiII and CuII
MII
O
MII
O
O
O
Fig. 2. UV–vis spectra of hydrogen peroxide detection test showing characteristic
peak at around 379 nm for the generation of TiIV-peroxo complex by H2O2 in pres-
ence of potassium titanium(IV) oxalate. The spectrum for control using H2O2 as
standard is also shown.
O
O
Suggested by ESI-MS
DTBC
1/2O2
O
O
O
few model systems which follow this pathway [138–141]. Most
designed complexes that catalytically perform catechol oxidation
are known to perform the oxidation through alternate pathway
which involves production of quinone along with H O rather than
MII
2 2
1/2H O
+
DTBQ
O
DTBC
Suggested
by UV-vis
Suggested by ESI-MS
2
2
water [24,34–36,52,53,76,140,142–145]. Presence or absence of
hydrogen peroxide as end product in our cases were tested by using
KI solution and Horse Radish Peroxidase [53,140] and also using
potassium titanium(IV) oxalate [97,98].
Scheme 2. Plausible mechanism of DTBC oxidation.
signal is characteristic of formation of organic radical species
semiquinone of DTBC) as the reaction intermediate during the cat-
alytic process. The literature data is also in good agreement with
our studies, suggesting formation of a semiquinone intermediate
20,26,61,62,67,80,137,146,151] in presence of the 3d-metals used
in those complexes.
In order to get further insight into the nature of possible inter-
mediates, electrospray ionization mass spectral (ESI–MS) studies in
negative ion mode were performed with the catalytic reaction mix-
(
II
II
For Cu and Ni acetate hydrogen peroxide was detected as
the side product of DTBC oxidation, while for other three acetates
II
II
II
(
Mn , Fe and Co ) H O was not detected as an end product
2 2
[
(
Fig. 2 and S6). Literature survey showed that formation of H O2
2
largely depends on the metal centre involved in the catalytic pro-
cess. In most of the model studies the ROS detection tests were
not performed as per the literature data but in cases where it has
been performed, a trend appears. MnII/III/IV systems always pro-
II
ture of M acetates and DTBC (1:50) in methanol. ESI–MS spectra
duce water not H O . It may be argued that Mn would decompose
II
2
2
were recorded within 5 min of mixing a M acetate and DTBC. The
H O [76] hence it cannot be detected but then we have detected
2
2
observed and simulated patterns are shown in Figs. S8–S13. Forma-
tion of similar species were observed in all cases, where two DTBC
II/III
peroxide for Mn based catalyst earlier [31]. For Ni
CO mimics,
H O is always found to be the end product of DTBC oxidation
II
−
2
2
units were attached to the metal centre as [M (DTBC)(DTSQ)] . The
data suggests that in solution the metal acetates render monomeric
metal-DTBC complexes which help catalyze the DTBC to DTBQ.
Hence the substrate also acts as ligand in solution. ESI–MS being
a soft technique when used carefully seems to provide useful data
on the reaction solutions. We opted for ESI–MS negative mode since
the substrate was a di-negative catechol which can provide mul-
tiple binding to a metal ion of di-positive charge and hence the
resultant species is supposed to be negative.
[
26,80–82,146] and the same is true for model complexes of Cu
also [24,53]. Co and Fe complexes are also known to decompose
H O [147–150]. Although we do not get H O2 as an end product
2
2
2
but our mechanistic studies using UV–vis spectroscopy in presence
of inhibitors show that probucol inhibits the conversion of DTBC to
II
II
DTBQ by Fe and Co by ca. 15%, suggesting that peroxo interme-
diate is generated which may get consumed by the metal center.
II
Though there are a few Cu model complexes, which report the
production of water during oxidation of DTBC but the predictions
were made without any experimental evidences [103,138]. Hence,
Once we combine the evidences obtained from UV–vis studies,
ESI–MS and EPR, the results suggest that Mn, Fe and Co oxidizes
DTBC in presence of oxygen through generation of hydroxyl radical
whereas Ni and Cu oxidizes DTBC through generation of hydrogen
peroxide. Hence we propose the mechanistic pathway where the
metal form a bis/tris chelate with the substrate followed by which
the substrate gets oxidized to quinone and then leaves the metal
centre. In this way the substrate also acts as a ligand and forms the
active catalytic species by replacing the labile acetates (Scheme 2).
EPR studies provide evidence of organic radical, a semiquinone
intermediate and hence our proposition in all the cases, except Ni.
II
it seems that as per our studies and the literature evidences, Cu
II
and Ni generate H O . Mn generates H O and Fe and Co generates
2
2
2
H O which decomposes fast in presence of the metals and can-
2
2
not be detected. The results indicate the importance of the protein
coordination in changing the mechanistic pathway viz. although
II
Cu generates H O in most of the CO model complexes and as
2
2
acetate [24,34–36,52,53,140], in the protein active site it is known
to generate water [21,103].
II
3.4. Speciation during catalysis
Ni is very slow reacting due to which the amount of semiquinone
intermediate at any time may not have been enough to obtain a
good signal in EPR. Manganese appears to be different from the
other metal ions investigated in this work.
The EPR studies show weak signal at g ≈ 2.00 during oxi-
II
II
II
II
dation of DTBC for Mn , Fe , Co , Cu acetates (Fig. S7). The

9
8
S.Kr. Dey, A. Mukherjee / Journal of Molecular Catalysis A: Chemical 407 (2015) 93–101
Table 3
Oxidation of OAP to APX using M acetates (MII = Mn , Fe , Co , Ni , Cu ).
II
II
II
II
II
II
[cat]
Solvent
Time [h]
% Yielda
II
Mn acetate
CH3OH
H2O
12
12
12
12
12
12
24
60(4)
35(5)
32(2)
28(2)
00
II
Mn acetate
II
Fe acetate
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
II
Co acetate
II
Ni acetate
II
Cu acetate
26(2)
00
No catalyst
◦
Reaction conditions: OAP (1 mmol), catalyst (1 mol%) under air, 25 C.
a
Isolated yield.
We earlier found the involvement of MnIV [32], which was
strongly supported by the literature evidences [152–155]. Now in
II
this work when we started with Mn acetate it seems that the
II
III
Mn ↔ Mn couple is active during the redox process as per the
obtained evidences showing the versatility of Mn in oxidizing DTBC
due to the accessible oxidation states.
Fig. 3. Plot of initial rate vs. substrate concentration for the oxidation of OAP catal-
ysed by Mn acetate. The inset shows Lineweaver–Burk plot.
II
one). Detailed kinetic studies reveal first order kinetics with respect
3
.5. Oxidative coupling of 2-aminophenol (OAP)
−1
to substrate (OAP) (Fig. 3) with a kcat value of 111(2) h . We also
performed some initial investigation on the possible reactive oxy-
gen species (ROS) involved with the OAP oxidation using UV–vis
spectroscopy. The studies show that hydroxyl radical may not be
involved in this case, unlike the DTBC oxidation, since the reaction
rate is not much affected by DMSO, which is a strong inhibitor of
hydroxyl radical. The reaction also remains almost unaffected by
other ROS quenchers viz. (± )-␣-tocopherol or probucol (Fig. S19),
which suggests that superoxide, peroxide and hydroxyl radical may
be excluded. Methionine ester inhibits the reaction completely sug-
gesting that substrate coordination with catalyst is essential for
activity. Our earlier work shows that the amino acids or their esters
coordinate to the metal centre preventing the reaction with DTBC
or oxygen [31,32,137,168].
Synthesis of the phenoxazinone chromophore gained impor-
tance in catalysis due to its occurrence as an intermediate in
biosynthesis of the antibiotic actinomycin-d by Streptomyces
antibioticus using the enzyme phenoxazinone synthase (PHS)
[
156–160]. Actinomycin-d, acts by inhibiting DNA-directed RNA
synthesis and is used clinically for the treatment of certain types of
cancer [161–163]. Phenoxazinone containing compounds are also
receiving increasing attention for various industrial applications
viz. as antifungal and antimicrobial agents, dyes [164] and for the
development of fluorescent probes for the detection of hydroxy
radicals and live cell imaging [130,165–166].
In laboratory oxidative conversion of o-aminophenol (OAP) to
-aminophenoxazinone (APX) has been used as a model for the
2
The results of this work suggest that Mn may be the best with
even oxygen donor ligands for CO or PHS activity. However, nature
chose ‘Cu’ over ‘Mn’ probably due the ‘rule of fitness’ for an enzyme
site. The environment and suggested geometry around the metal
behaviour of the enzyme PHS. The native enzyme is known to
require 4–5 copper centres for maximum activity and less number
of copper centres reduces the activity [156]. However, the model
complexes of PHS are mostly rather simple mononuclear com-
plexes of transition metal [71,85–95,167]. There is only one report
centres in CO or PHS probably did not support the incorporation
II/III
II
of Mn
due to reduced stability of the bound Mn and its possi-
of a tetranuclear Cu complex as model of phenoxazinone synthase
ble ability to switch between at least three oxidation states (II–IV)
and perform other reactions using the available articulate ligand
environment of the protein.
by Mukherjee et al. In that report the ligand is also attributed to be
important for the activity due to its redox non-innocent nature [94].
Based on the literature data, it appears that mononuclear metal
complexes can model the functional activity of PHS, which is a
pentanuclear copper(II) enzyme. In this report we show that with-
out designing of any new ligand and rather using commercially
available metal acetates at room temperature we can perform the
conversion of OAP to APX or DTBC to DTBQ.
3.6. Oxidative CC bond coupling of 2,6-di-tert-butylphenol
(2,6-DTBP)
Carbon–hydrogen bond activation protocols allow the utiliza-
tion of a CH bond as a functional group, offering a direct route for
the creation of carbon–carbon and carbon–heteroatom bonds. The
success of the metal acetates in formation of semiquinone in DTBC
and the OAP oxidation and the fact that DTBC attaches to the metal
acting as ligand and substrate suggested that it worth making an
effort to probe the oxidative coupling of sterically hindered 2,6-
DTBP. In general, 3d-transition metals are relatively less popular
for activation of carbon–hydrogen bonds compared to 4d and 5d
transition metal catalysts (viz. Pd, Ru) [169–177]. First-row (3d)
transition metals such as Fe, Co, Ni, Mn, or Cu are used relatively
rarely [178–184].
II
II
II
II
We examined the OAP oxidation ability of Mn , Fe , Co , Ni ,
CuII metal acetates. The oxidation reactions were performed using
◦
1
mol% of catalyst in methanol at 25 C (Table 3) and the end prod-
uct was characterized and confirmed by NMR and ESI–MS (Figs.
II
S14–S18). Again the activity of Mn acetate is better than the other
III
metal acetates used in the study. Earlier we found that Mn -acetate
showed the highest turnover among the model complexes of PHS.
Now after probing a series of 3d-transition metals which have been
used for modelling PHS we find that Mn tops the list. Nature’s choice
Cu seems to be less active than Mn for the PHS activity than Mn.
II
Mn -acetate has the ability to convert OAP to APX even in water
at room temperature although the yield almost reduces to half
Various research groups have studied the oxidation of 2,6-
t
(Table 3).
DTBP using molecular oxygen [185–189]. H O2 or BuOOH as
2
ꢀ
ꢀ
The best performance of Mn led us to perform detailed
oxidant to give two different oxidation products 3,3 -5,5 -tetra-tert-
butyldiphenoquinone (DPQ) and 2,6-di-tert-butylbenzoquinone
(BQ) [190,191]. Among the 3d-transition metals, there are more
examples of copper [185,186,190,192] and cobalt salts and
kinetic studies of its aerobic OAP oxidation to APX, by monitor-
ing the change in absorbance as a function of time at 430 nm
3
−1
−1
(
ꢀ= 22 × 10 M cm , characteristic of 2-aminophenoxazin-3-

S.Kr. Dey, A. Mukherjee / Journal of Molecular Catalysis A: Chemical 407 (2015) 93–101
99
Table 4
Oxidation of 2,6-DTBP to DPQ using M acetates (MII = Mn , Fe , Co , Ni , Cu ).
Acknowledgements
II
II
II
II
II
II
[
cat]
Solvent
Time [h]
% Yielda
We acknowledge CSIR-India for funding vide project no.
01(2475)/11/EMR-II. We are also thankful to IISER Kolkata for the
financial and infrastructural support. SKD wishes to acknowledge
C.S.I.R, New Delhi, India for research fellowship.
II
Mn acetate
Fe acetate
Co acetate
Ni acetate
Cu acetate
No catalyst
CH3OH/NaOH
CH3OH/NaOH
CH3OH/NaOH
CH3OH/NaOH
CH3OH
04
24
04
24
04
24
90(2)
0
II
II
46(3)
0
50(2)
II
II
b
CH3OH/NaOH
0
Appendix A. Supplementary data
Reaction conditions: 2,6-DTBP (1 mmol), catalyst (1 mol%) and NaOH (10 mol%),
◦
under air, 25 C.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molcata.2015.06.
a
Isolated yield.
Without NaOH.
b
0
20
complexes [188,193–202] as catalysts but use of iron and man-
ganese are relatively less [76,187,203]. Organic ligands have also
been found to be effective catalyst to form DPQ as major prod-
uct [204]. It was also found that hydrogen peroxide in presence of
iodine leads to selective formation of BQ [205].
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