Manganese(III) acetate mediated catalytic oxidation of substituted dioxolene and phenols

Article abstract of DOI:10.1016/j.molcata.2014.08.014

Manganese(III) acetate is found to be an efficient catalyst to perform oxidation of 3,5-di-tert-butylcatechol (DTBC) to 3,5-di-tert-butylbenzoquinone (DTBQ) and 2-aminophenol (OAP) to 2-aminophenoxazinone (APX). The k _cat values are 1.72(2) × 10³ and 2.8(2) × 10² h^-1 for the formation of DTBQ and APX, respectively. The turnover number of APX formation is highest among the synthetic mimics. ESI-MS studies of DTBC oxidation suggest formation of a Mn^IV intermediate. Manganese(III) acetate is also capable of oxidative CC bond coupling in sterically hindered phenols to form biaryls in 65-94% yield.

Full text of DOI:10.1016/j.molcata.2014.08.014

Journal of Molecular Catalysis A: Chemical 395 (2014) 186–194
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Manganese(III) acetate mediated catalytic oxidation of substituted
dioxolene and phenols
∗
Suman Kr Dey, Arindam Mukherjee
Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur 741246, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 June 2014
Received in revised form 4 August 2014
Accepted 7 August 2014
Available online 14 August 2014
Manganese(III) acetate is found to be an efficient catalyst to perform oxidation of 3,5-di-tert-butylcatechol
(
(
DTBC) to 3,5-di-tert-butylbenzoquinone (DTBQ) and 2-aminophenol (OAP) to 2-aminophenoxazinone
APX). The kcat values are 1.72(2) × 10 and 2.8(2) × 10 h for the formation of DTBQ and APX, respec-
3
2
−1
tively. The turnover number of APX formation is highest among the synthetic mimics. ESI-MS studies
IV
of DTBC oxidation suggest formation of a Mn intermediate. Manganese(III) acetate is also capable of
oxidative C C bond coupling in sterically hindered phenols to form biaryls in 65–94% yield.
Keywords:
©
2014 Elsevier B.V. All rights reserved.
Manganese(III) acetate
Oxygen activation
Catechol oxidation
2
-Aminophenoxazinone
Oxidative C C bond coupling
1
. Introduction
are not always accessible for demonstrating a catalytic reaction of
the above type in the laboratory and hence, a commercially avail-
able efficient catalyst would be useful. Such cheaper systems may
also find use in various synthetic methods.
CO is an enzyme which has been extensively studied by mod-
elling the active site and incorporation of various metals viz.
copper [6–28], iron [29–32], cobalt [33–40], manganese [41–54],
and zinc [6,55] to probe catechol oxidase activity. PHS mimics
cannot be found as much as CO but yet are well studied using
Oxidation is a fundamentally important component of organic
synthesis. For economic and environmental reasons, the oxidation
processes of bulk chemical industries predominantly involve the
use of molecular oxygen as the primary oxidant [1–5]. But, direct
oxidation of organic substrates using molecular oxygen as the oxi-
dant under mild conditions is still difficult and also most of the time
the selectivity in the oxidation reaction is not always as controlled
as needed. However, nature has designed enzymes that can effi-
ciently and selectively oxidize substrates with the help of molecular
oxygen to produce desired products. Many of these enzymes, which
are metal based, are targets of bioinorganic chemists to design
mimics in order to gain insight into the mechanism of the reaction.
To understand the mechanism, the enzymes donor sites are mod-
elled by the syntheses of metal complexes to mimic the immediate
coordination environment. Such model complexes have provided
us with immense insights in to the mechanism of action of the orig-
inal enzymes and also showed new alternate pathways to form the
same products.
2
-aminophenol as substrate [46,56–59]. Apart from biomimetics,
quinone formation is also important since they are component of
many dyes and pigments [60–62] and quinones of lower molecu-
lar weight may have potential in host–guest liquid crystal displays
and optical recording [63]. On the other hand synthesis of the
phenoxazinone chromophore is an important step in biosynthe-
sis of the antibiotic actinomycin-D by Streptomyces antibioticus
using the enzyme Phenoxazinone Synthase (PHS) [64–68]. Oxida-
tive conversion of o-aminophenol (OAP) to 2-aminophenoxazinone
(2-Amino-3H-phenoxazine-3-one or APX) is a biologically rel-
evant reaction since APX has been used as a model for the
behaviour of actinomycin-D, which acts by inhibiting DNA-directed
RNA synthesis and is used clinically for the treatment of certain
types of cancer [69–71]. At the same time phenoxazinone con-
taining compounds are receiving increasing attention for various
industrial applications viz. as antifungal and antimicrobial agents,
dyes (Scheme S1) [60] and for the development of fluorescent
probes for the detection of hydroxy radicals and live cell imaging
[72–74].
Our quest was to probe if a simple metal complex which would
be rather easily available can perform the action of oxidative
enzymes viz. Catechol oxidase (CO) and Phenoxazinone Synthase
(
PHS). Many complex molecules have been designed so far which
∗ Corresponding author. Fax: +91 33 25873020.
E-mail address: a.mukherjee@iiserkol.ac.in (A. Mukherjee).
http://dx.doi.org/10.1016/j.molcata.2014.08.014
381-1169/© 2014 Elsevier B.V. All rights reserved.
1

S.K. Dey, A. Mukherjee / Journal of Molecular Catalysis A: Chemical 395 (2014) 186–194
187
2. Experimental
2.1. Materials and methods
All reactions were carried out using HPLC grade sol-
vents [methanol (Merck), acetonitrile (Merck)]. Manganese(III)
acetate dihydrate, 3,5-di-tert-butylcatechol, dimethylsulfoxide
(
2
DMSO), (± )-␣-tocopherol, probucol, D/L-p-chlorophenylalanine,
-aminophenol, potassium titanium oxide oxalate dihydrate, eth-
ylenediaminetetraacetic acid disodium salt dihydrate were all
purchased from Sigma and used without further purification.
3
,5-di-tert-butyl benzoquinone was also purchased from Aldrich
and used to calculate molar extinction coefficient (ε) in 90:1
acetonitrile, methanol mixture. L-Methionine, L-histidine [SRL
(India)] and sodium hydroxide [Merck (India)] were also used as
received. Methyl ester of methionine was synthesized according
to a previously reported literature procedure [101]. NMR spec-
tra were recorded on Bruker Avance III 500 MHz and also on
◦
Jeol ECS400 MHz spectrometer at room temperature (25 C). Elec-
tronic spectra were recorded using Varian Cary 300 Bio UV–vis
spectrophotometer. Electron-spray ionization mass spectra were
recorded using micromass Q-Tof microTM (Waters) by +ve mode
electrospray ionization. X-Band EPR spectra were recorded on a
Bruker BioSpin WinEPR spectrometer.
Scheme 1. MnIII acetate mediated oxidation reactions of different phenols.
In general to probe such catalysis ligands are designed to fit the
particular metal of choice and then the catalytic activities of the
metal complexes are probed. A quick review of the literature shows
that apart from complexes of transition metal [46,56,57,59,75–83],
salts of copper [84] and cobalt [85,86] are also known for their
capability to oxidize OAP to APX but the most common drawbacks
reported are (i) relatively more catalyst required (stoichiometric or
2.2. Catalytic oxidation of DTBC
UV–vis spectra for kinetic studies were recorded by using a
quartz cuvette (1.0 cm pathlength) and a Varian Cary 300 Bio
UV–vis spectrophotometer equipped with a Peltier thermostat-
ing accessory. All the kinetics measurements were conducted at
1
–10 mol%) and (ii) multiple product formation or polymerization
◦
a constant temperature of 25 C, monitored with a thermostat.
of product (APX). Efforts have been made to catalyze the reac-
tion with transition metal complexes immobilized on polymers
and mesoporous silica [59] but they too face the afore-mentioned
problems.
1
00 molar equiv of DTBC in acetonitrile were added to 1 ␮M solu-
III
tions of Mn -acetate dihydrate in acetonitrile-methanol mixture
under aerobic condition at room temperature (25 C). The final
ratio of acetonitrile:methanol in cuvette was 90:1 v/v. Absorbance
vs. wavelength plots were generated for these reaction mixtures,
recording spectrophotometric data at a regular time interval of
◦
Recently we found that an o-donor based MnIII complex of
9
-hydroxyphenalenone showed catechol oxidase activity with
highest turnover although it has no similarity to the enzyme
active site [87]. It is known that biomimetic oxidation of the
above type generally follow the radical based semiquinone type
5
min in the range 300−700 nm. To determine the substrate con-
centration dependence of the rate and various kinetic parameters,
␮M solutions of catalyst was treated with 100, 200, 300, 500, 600,
1
III
intermediates. Mn (OAc) ·2H O has an established rich role as
3
2
700, 800 molar equivalents of DTBC and the absorbance monitored
as mentioned above (Fig. S1). The kinetic parameters were deter-
mined by using Lineweaver-Burk plot as well as Michaelis-Menten
plot (Fig. S2). The product (DTBQ) was also characterized by ESI-
catalyst in various radical based organic synthesis [88–90] mainly
for its oxidative free radical chemistry [88,90–100]. Since the late
1
960s, Mn(OAc) -mediated radical reactions have attained signifi-
3
cant attention as single-electron-transfer reagent due to its ability
to form carbon, nitrogen, phosphorous or sulfur-centered radi-
cals from various compounds such as ␤-diketones, ␤-keto acid,
esters, phosphites, phosphine oxides, thiols, arylthioformanilides,
thioglycolic acid, mercaptoketones and azides. Hence, MnIII(OAc)3-
mediated catalysis provides alternative routes for the synthesis
of important organic compounds [90–98]. It appeared to us that
MS and NMR spectroscopy. 1H NMR (400 MHz, CDCl ): ı = 6.93 (d,
3
1
H, J = 2.28 Hz), 6.21 (d, 1H, J = 1.52 Hz), 1.26 (s, 9H), 1.22 (s, 9H)
13
ppm; C NMR (100 MHz, CDCl ): ı = 181.26 (C-1), 180.17 (C-2),
3
1
63.46 (C-3), 150.07 (C-4), 133.60 (C-5), 122.22 (C-6), 36.16 (C-7),
35.61 (C-8), 29.34 (C-9), 28.01 (C-10) ppm. ESI-MS (+ve ion mode):
m/z = 243.14 [(DTBC + Na )] (calc. 243.14) (Fig. S3–S4).
+
+
III
Mn (OAc) ·2H O might be performing oxidations of phenols, di-
3
2
2.3. Detection of hydrogen peroxide in the catalytic reaction of
oxidation of DTBC
oxolenes or aminophenols since they involve carbon centered
radicals.
Herein we present the catalytic studies of MnIII(OAc) ·2H O
3
2
Earlier studies indicated that either water or hydrogen perox-
ide could form as side product in catalytic oxidation of catechol.
Formation of hydrogen peroxide was probed by two methods
for the oxidation of DTBC and 2-aminophenol (OAP). The studies
III
showed that Mn (OAc) ·2H O has the highest turnover (kcat) for
3
2
oxidation of OAP to APX and the kcat for DTBC oxidation is signifi-
3
−1
cantly high, of the order of 10 h . The catalytic reactions showed
first order kinetics with respect to the substrates. These results
encouraged us to further probe the efficiency of the catalyst for oxi-
dation of sterically hindered phenols and the results of the studies
(
i) H O2 can be detected by the generation of characteristic peak
2
−
at 352 nm for I3 ion with potassium iodide. To detect hydro-
gen peroxide after the oxidation of DTBC, DTBC was oxidized
III
by 1 mol% Mn acetate for 2 h in acetonitrile and methanol
III
showed that Mn acetate is not only an efficient catalyst for oxi-
mixture. The formed DTBQ was then extracted three times
using dichloromethane. Water part was then acidified to pH
dation of DTBC and OAP but also for oxidative coupling of sterically
hindered phenols (Scheme 1).
2
using diluted H SO4 and one-third volume of KI solution
2

1
88
S.K. Dey, A. Mukherjee / Journal of Molecular Catalysis A: Chemical 395 (2014) 186–194
+
(
500 mg/10 mL) in water was added to it with 100 nM Horse
spectroscopy. ESI-MS (+ve ion mode): m/z = 409.30 [(BDPH)] (calc.
+
1
Radish Peroxidise and the appearance of the band at 352 nm
characteristic for I3 ion was monitored. Control experiments
409.31); m/z = 431.29 [(BDPNa)] (calc. 431.29). H NMR (500 MHz,
−
CDCl ): ı = 7.71 (s, 4H), 1.36 (s, 36H) ppm (Fig. S12–S13).
3
were performed using only H O solution. Since atmospheric
2
2
−
oxygen can also oxidize I blank experiments (without catalyst
or DTBC) were also performed (Fig. S5).
2.6. Oxidative C–C coupling of 2,4-di-tert-butylphenol
(conversion of 2,4-di-tert-butylphenol to
ꢀ
ꢀ
ꢀ
(
ii) H O formation can also be detected by formation of Ti(IV)-
2,2 -dihydroxy-3,3 -5,5 -tetra-tert-butylbiphenyl (BP)
2
2
peroxo species using potassium titanium(IV) oxalate. In this
experiment the catalytic reaction of DTBC oxidation and extrac-
tion procedure was same as that in method (i) stated above. The
isolated aqueous part was added 1 mM solution of potassium
titanium(IV) oxalate to monitor the band ca. 379 nm, due to the
formation of Ti(IV)-peroxo bond [102,103]. Control experiment
was also performed using hydrogen peroxide (Fig. S6).
Oxidative coupling of 2,4-di-tert-butylphenol (2,4-DTBP) was
performed by taking 2,4-DTBP in acetonitrile, Mn acetate (1 mol%)
III
in methanol and NaOH (10 mol%) in water, similar to above and
◦
the mixture was stirred at 25 C for 12 h. Product was analyzed
by ESI-MS and isolated by evaporating the reaction mixture and
extracting with water and dichloromethane. Combined organic
layers were then dried and collected. 1H-NMR of the dried prod-
uct indicates partial conversion (65%) of 2,4-di-tert-butylphenol to
2
2
.4. Catalytic oxidation of 2-aminophenol (synthesis of
-aminophenoxazinone or APX)
ꢀ
ꢀ
ꢀ
2,2 -dihydroxy-3,3 -5,5 -tetra-tert-butylbiphenyl with unreacted
+
reactant. ESI-MS (+ve ion mode): m/z = 433.32 [(BPNa)] (calc.
433.31). 1H NMR (500 MHz, CDCl3): ı = 7.39 (d, 2H), 7.11 (d, 2H),
Oxidation of o-aminophenol (OAP) was carried out by tak-
ing OAP (109.0 mg, 1.0 mmol) in acetonitrile and manganese(III)
acetate dihydrate (2.4 mg, 0.01 mmol) in methanol so that final
solvent mixture is 90:1 and then the reaction mixture was stirred
5.21 (br, s, 2H), 1.45 (s, 18 H), 1.32 (s, 18 H) ppm (Fig. S14–S15).
2.7. Mass spectrometry
for 12 h. Product precipitated out of the solution which was fil-
tered, collected and characterized by ESI-MS, 1H NMR and
13
C
ESI mass spectrometric data were recorded using Waters Q-TOF
micro mass spectrometer. The mass spectrometric studies of cate-
+
NMR. ESI-MS (+ve ion mode): m/z = 213.08 [(APXH)] (calc. 213.07);
m/z = 235.05 [(APXNa)] (calc. 235.05);m/z = 251.03 [(APXK)] (calc.
51.02). H NMR (500 MHz, Me SO-d ): ı = 7.71 (dd, 1H, J = 7.5 Hz,
+
+
chol oxidation were performed using (1:1) methanol: acetonitrile
1
III
2
mixture. The ESI-MS was performed with 1:50 mixture of Mn
-
2
6
ArH), 7.44 (m, 2H, ArH), 7.39 (m, 1H, ArH), 6.80 (br, s, 2H, NH ), 6.36
acetate dihydrate with DTBC. The ESI-MS data were performed in
presence of inhibitors at a molar concentration ratio of 1:10:50
(Mn:inhibitor:substrate). The ESI-MS of the catalytic products were
performed with 10 ␮M stock solutions.
2
1
3
(
s, 2H, ArH) ppm. C NMR (125 MHz, Me SO-d ) ı = 180.2 (C-3),
2 6
1
1
9
48.9 (C-10a), 148.2 (C-4a), 147.3 (C-2), 141.9 (C-5a), 133.7 (C-9a),
28.8 (C-7), 127.9 (C-9), 125.3 (C-8), 115.9 (C-6), 103.4 (C-1), and
8.3 (C-4) ppm (Fig. S7–S11).
Kinetics of the aerobic oxidation of OAP in the presence of MnIII
3. Results and discussions
acetate were measured by monitoring the change in absorbance
3
−1
−1
The redox chemistry of MnIII-acetate, using cyclic voltammetry,
in methanol and acetonitrile showed no significant redox events
in the range of −1.0 to +1.0 V. However, the oxidation of DTBC to
as a function of time at 430 nm (ε = 22 × 10 M cm ), which is
characteristic of 2-aminophenoxazin-3-one in methanol. All the
kinetics measurements were conducted at a constant temperature
◦
III
of 25 C, monitored with a thermostat. Initially, 100 molar equiv of
DTBQ by Mn -acetate was observed with quite high kcat compared
OAP in acetonitrile were added to 5 ␮M solutions of MnIII acetate
to many catechol oxidase mimics (Table 1). Detailed kinetic exper-
◦
III
in methanol under aerobic condition at room temperature (25 C).
iments to understand the effciency of Mn -acetate using 1 ␮M
III
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 sub-
strate concentration dependence on the rate and various kinetic
parameters, 5 ␮M solutions of catalyst was treated with 800, 1200,
solution of Mn -acetate and upto 800 equivalent of DTBC (sub-
strate) was performed. The experiments were done at a constant
◦
temperature of 25 C under aerobic condition in (90:1) v/v ace-
tonitrile and methanol mixture. The oxidation was monitored by
−
1
−1
the visible band appearing at 402 nm (ε = 1650 M cm ) due to
formation of DTBQ, using a UV–vis spectrophotometer (Fig. 1(A)).
Then the differences in absorbance at 402 nm were plotted against
time and rate constants of each case were determined by initial
rate method. We also calculated the average rate constants of each
of those experiments since we found complete conversion of the
reactant to product (saturation was observed) (Fig. S1). The plot of
rate constants vs. concentration of substrate was analyzed using
Lineweaver-Burk plot and all the important kinetic parameters
(Vmax, kM, kcat) were obtained (Fig. 2). The kcat value was found to be
1
500, 1800, 2600, 3200 and 3600 molar equivalents of OAP and
the absorbances monitored as mentioned above. The completion
of the reactions was determined spectrophotometrically by moni-
toring the increase in absorbance at 430 nm as a function of time.
The kinetic parameters were determined by using Lineweaver-Burk
plot.
2.5. Oxidative C–C coupling of 2,6-di-tert-butylphenol
3
−1
(
3
conversion of 2,6-di-tert-butylphenol to
,3 -5,5 -tetra-tert-butyldiphenoquinone)
1.72 (2) × 10 h . It should be noted here that such a high turnover
is obtained by a commercially available compound which is rela-
tively cheap and is active at a lower concentration than reported
for other functional mimics except the Mn complex of 9-hydroxy
phenalenone reported by us earlier [87].
ꢀ
ꢀ
Oxidative coupling of 2,6-di-tert-butylphenol (2,6-DTBP) was
performed by taking 2,6-DTBP in acetonitrile, sodium hydroxide
in water was added to it such that the overall concentration is
The literature shows that even transition metal salts of cop-
per [84] and cobalt [85] are capable to oxidize OAP to APX in
higher mol%. Hence when we obtained excellent kcat value for
III
1
0 mol%. To the above solution Mn acetate (1 mol%) in methanol
◦
was added and the mixture was stirred at 25 C for 4–12 h. Prod-
uct started forming immediately after addition of Mn acetate
III
III
oxidation of DTBC (Table 1) using Mn (OAc) ·2H O it prompted
3
2
solution. Reaction mixture was stirred for 4–12 h. Then the
product was collected by filtration, washed by methanol and
dried. The dried product was analyzed by ESI-MS and 1H NMR
us to study the oxidative conversion of o-aminophenol (OAP) to
2-aminophenoxazinone (2-Amino-3H-phenoxazine-3-one or APX)
due to its biological and industrial relevance. We found that the

S.K. Dey, A. Mukherjee / Journal of Molecular Catalysis A: Chemical 395 (2014) 186–194
189
Table 1
Comparison of DTBC oxidation parameters of Manganese(III) acetate with the earlier known Mn model complexes and the best dinuclear copper(II) model complex of catechol
oxidase.
Complexes[a]
kcat (103
h
−1
)
KM (10 M)
−4
Vmax (10 M s
−5
−1
)
Solvent [ref]
[
Cu2(H2L1)(OH)(H2O)(NO3)]3+
32.4
1320
21.6
18.0
14.4
10.8
7.20
7.2
23
–
18
83
13
60
42
67
65
19.0
9
22
1.76
1.00
37
24
7.00
20
24
31
5
7.0
13
64
80
27.8
12
15
23.2
1.149
127
4.170
49.9
0.826
6.265
1.57
1.370
6.38
90
18.3
60
50.0
40
30.0
20.0
20
10.0
10.0
10.0
9.19
0.12
6.85
6.79
6.70
b [19]
e [87]
b[44]
c [45]
b [44]
c [45]
b [45]
b [44]
c [45]
b [45]
c [45]
c [45]
c [104]
b [45]
c [45]
c [45]
c [45]
c [49]
c [49]
c [49]
c [50]
d [48]
b [52]
d [47]
b [52]
b [43]
b [52]
b [52]
b [43]
c [42]
d [46]
b [42]
b [43]
c [42]
c [42]
b [42]
b [42]
[f]
III
Mn (L2)2(OAc)(AcOH)
Mn(L3)(H2O)3]2
+
[
Mn(L4)Cl
Mn(L3)(SCN)2(H2O)]
[
Mn(L5)Cl
Mn(L4)Cl
[
Mn(L3) (H2O)2]+
Mn(L6)Cl
Mn(L5)Cl
Mn2L8Cl4
3.60
3.60
3.60
3.31
2.56
2.47
2.44
2.41
1.79
1.6(2)
1.5(2)
0.9(1)
0.336
0.27
Mn2L9Cl4
Mn 4MnIII4O2(pyz)2(C6H5CH2COO)10]
II
[
Mn(L6)Cl
Mn2(L10)Cl4
Mn2(L11)Cl4
Mn(L7)Cl
4.97
0.45
0.43
0.26
(
(
(
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
Ph4P)4[Mn2O2(L12)2]
Ph4P)4[Mn2O2(L13)2]
Ph4P)4[Mn2O2(L14)2]
III
Mn 2(ꢀ-oxo)2(L15)2]
Mn6(L16)6(CH3OH)3(H2O)3]
Mn(L17)(Cl)2]
Mn2(L18)2Cl4]
Mn(L19)(Cl)2]
(Ni(L20))2Mn(NCS)2]
Mn(L19)(OAc)(OCH3)]
Mn(L17)(OAc)(OCH3)]
–
0.075
0.639
0.427
0.361
0.290
0.281
0.239
0.213
0.0180
0.23
0.0124
0.0717
0.0057
0.0043
0.0032
0.0021
0.0477
+
0.23
0.168
0.13
+
0.105
0.101
0.086
0.077
0.065
0.049
0.045
0.026
0.021
0.016
0.012
0.008
1.72
+
+
({Ni(L20)}2Mn(NCO)2]
III
II
3+
3+
{Mn Mn (L21)(␮-O2CEt)(EtOH)}2(␮-O2CEt)]
2
+
Mn(L22)(H2O)2(CH3CN)]
III
II
{Mn Mn (L21)(␮-O2CEt)(EtOH)}2(␮-O2CEt)]
{Ni(L20)(EtOH)}2Mn(NO2)2]
III
II
2+
Mn Mn (L21)(␮-O2CMe)(H2O)2]
III
II
+
+
Mn Mn (L21)(␮-O2CPh)(MeOH)(ClO4)]
Mn Mn (L21)(␮-O2CMe)(H2O)2]
Mn Mn (L21)(␮-O2CPh)(MeOH)(ClO4)]
III
II
2+
III
II
III
Mn acetate dihydrate
[
(
a] Structure of the ligands L1–L22 are shown in Scheme S2; pyz = pyrazine; Solvent system [b] CH3OH; [c] CH3CN; [d] DMF; [e] CH3CN:DMF (90:1); [f] this work, CH3CN:CH3OH
90:1) mixture used as solvent.
relatively cost effective catalyst MnIII acetate, at only 1 mol%,
can catalyze the reaction in presence of molecular oxygen with
quantitative yield and selectively form APX from OAP in 5 h. In addi-
tion no polymeric product could be detected in solution showing
the reaction selectively gives one product. Detailed kinetic stud-
ies of the OAP oxidation reveal first order kinetics with respect
to substrate and show that only 5 ␮M of MnIII-acetate can cat-
alyze conversion of upto 3600 molar equivalent of OAP to APX.
The formation of APX was confirmed by NMR and ESI-MS stud-
ies (Fig. S7–S11). The UV–vis spectroscopic studies were done
in 90:1 v/v acetonitrile:methanol mixture. The kinetic parame-
ters of oxidation of OAP to APX have been derived from the
Lineweaver Burk plot (Fig. 3, Table 2). The results show that OAP
–1
can be converted to APX with a kcat value of 0.078 s . The kcat
–1
value of 0.078 s is the highest among the reported mimics of
PHS.
Fig. 1. (A) Increase in absorbance around 402 nm, after addition of 400 equivalent of DTBC to a 1 ␮M solution of MnIII-acetate. (B) Increase in absorbance around 430 nm,
III
after addition of 3200 equivalent of OAP to a 5 ␮M solution of Mn -acetate. Spectra were recorded every 5 min.

1
90
S.K. Dey, A. Mukherjee / Journal of Molecular Catalysis A: Chemical 395 (2014) 186–194
Table 2
A comparison of selected literature on conversion of 2-aminophenols to the corresponding phenoxazinones.
kcat (s 1
−
)
KM (mM)
kcat/KM (mM
−1 −1
s
)
Vmax (M s
1.91 × 10
−1
)
[ref]
Catalyst [a]
ꢀ
2+
−7
[
[
[
[
[
[
[
[
Co2(L23)2(␮-L23 )2Cl2]
Co(L24)(N3)3]
0.00382
0.0056
0.0092
0.0084
0.0064
0.00093
0.0018
0.0028
0.00061
0.00825
0.0365
0.00081
0.078
1.57
7.12 ± 0.72
8.58 ± 0.96
10.1
14.7
10.1
0.0024
0.0008
0.0011
0.0008
0.0004
0.00009
0.0001
0.0002
–
[56]
[75]
[75]
[76]
[76]
[76]
[76]
[76]
[58]
[59]
[59]
[46]
[b]
−
−
8
8
5.66 × 10
9.24 × 10
8.60 × 10
6.40 × 10
1.87 × 10
3.54 × 10
4.60 × 10
–
Co(L25)(N3)3]
Co2(L26)2(␮-O2)]4
+
+
−8
−8
−8
−8
−8
4
Co2(L27)2(␮-O2)]
Co(L28)(CH3CN)]
2
+
2
+
Co(L29)(H2O)]
12.8
13.2
Co(L30)(H2O)]2
+
H2O2 with modified cyclodextrin
Cu(L31)2]
10.7 ± 1.1
[
–
–
–
–
–
–
PS-[Cu(L31)2]
Mn(L22)(H2O)2(CH3CN)]2
+
5.13
55.3
0.00016
0.0014
1.34 × 10
−7
−7
[
III
Mn -acetate dihydrate
3.9 × 10
[a] Structure of the ligands L23–L31 are shown in Scheme S3; [b] This work.
Table 3
Oxidation of 2,6-DTBP to DPQ by Mn acetate.
III
Entry
[cat] (mol%)
Base (mol%)
Solvent
Time (h)
Yield[a]
1
2
3
4
5
6
7
8
9
1
1
0
0
1
1
1
1
0
1
0
1
No base
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
DMSO
12
04
12
12
12
12
04
0
90
0
75
0
NaOH(10)
NaOH(10)
NaOH(100)[
Et3N (100)
Pyridine (100)
NaOH(10)
NaOH(10)
NaOH(10)
Bu4NOH (100)
NaOH(100)[
NaOH(10)
b]
0
94
74
05
0
83
15
0.5
12
12
12
12
1
1
1
0
1
2
b]
[a] Isolated yield; [b] reaction was perfomed in presence of 2 mol% Na2EDTA to remove any adventitious Manganese.
We also found that the activity of the catalyst MnIII(OAc) .2H O
10 mol% sodium hydroxide and 1 mol% MnIII-acetate in methanol
was added to it. Reaction mixture was then stirred at 25 C. The
3
2
◦
decreases on aging of the solution (linearly with time, rate
−
1
0
.003 h . One representative study shown in Fig. S16). Hence, to
product started precipitating out within 5 min after starting the
reaction. As the reaction progressed DPQ was visibly witnessed as
reddish brown precipitate, which was finally collected by filtration
and dried and analyzed for purity by NMR spectroscopy and ESI-MS.
The results for several trials are shown in Table 3.
We observed while performing control reaction in presence of
stoichiometric amount of base without any catalyst, that alone
NaOH/KOH may provide oxidative C-C bond coupling in certain
get best efficiency a fresh solution of the catalyst should be used.
We further studied the oxidative C C bond coupling of sterically
hindered phenols [2,6-di-tert-butylphenol (2,6-DTBP) and 2,4-di-
III
tert-butylphenol (2,4-DTBP)] using Mn acetate. The catalytic
ꢀ
ꢀ
oxidation of 2,6-DTBP to 3,3 ,5,5 -tetra-tert-butyldiphenoquinone
DPQ) was attempted by dissolving 2,6-DTBP in acetonitrile with
(
Fig. 2. The Lineweaver-Burk plot for the oxidation of DTBC catalyzed by MnIII
acetate.
-
Fig. 3. The Lineweaver-Burk plot for the auto-oxidation of OAP to APX catalyzed by
Mn -acetate.
III

S.K. Dey, A. Mukherjee / Journal of Molecular Catalysis A: Chemical 395 (2014) 186–194
191
sterically hindered phenols viz. oxidative C–C coupling of 2,6-
DTBP to DPQ. However, strangely this observation has never been
reported in literature. An earlier report discussed that after 3 h there
is no reaction in presence of KOH which is true [105,106] but when
the reaction is allowed to continue longer, than after 6–7 h product
formation starts and can be observed by TLC. Using 1 molar equiv-
alent KOH we got 83% isolated yield after 12 h of reaction when we
used 2,6-DTBP as the substrate. In case of the relatively tougher sub-
strate 2,4-di-tert-butylphenol (2,4-DTBP) 1 mole equivalent KOH
ꢀ
ꢀ
ꢀ
or NaOH did not give the desired product 2,2 -dihydroxy-3,3 -5,5 -
tetra-tert-butylbiphenyl (BP) at room temperature even after 16 h.
However, the use of 1 mol% MnIII acetate with 10 mol% NaOH as
base gave 65% conversion of 2,4-DTBP to the desired BP in 12 h.
The formation of the phenolate in presence of the base may help
the propagation of the reaction. The added base is playing the role to
generate the corresponding phenolate anion of the 2,6-DTBP, which
is known to be more oxidizable than the original phenol because
of the difference between their redox potentials [107]. Yes, that
may be one of the reasons. However, the formation of phenolate
is not the only activating factor then the similar product would be
observed with other bases such as triethylamine (Et N) or pyridine
3
Fig. 4. UV–vis study of the inhibition of catechol oxidation probed by monitoring
A402. Reaction condition: 500 molar equivalent of DTBC and 1 ␮M catalyst in air; 10
molar equivalent of (± )-␣-tocopherol and probucol, 100 molar equivalent of methyl
ester of D/L-p-chlorophenylalanine, DMSO and benzoic acid with respect to catalyst.
Spectra were recorded every 5 min interval and the respective legends are provided
in the figure.
but using these bases we do not get any yield of DPQ even after 12 h.
During oxidation of 2,6-DTBP there is a possibility of formation
of another side product 2,6-di-tert-butyl-1,4-benzoquinone (2,6-
DTBQ). It was earlier shown that formation of 2,6-DTBQ required
peroxide or peroxo intermediates [108]. In our case we have selec-
tively got DPQ as the sole product. Hence, the reaction does not go
through any peroxo intermediate. Based on the proposed mech-
anism the reaction was initiated by the formation of phenoxyl
that the benzoic acid or the amino acids co-ordinate to the metal
centre. The ESI-MS data using histidine as an inhibitor supported
the metal histidine co-ordination since we a m/z of 519.13 which
III
radical from phenoxide via one electro oxidation prompted by Mn
III
+
(
Scheme S4A). The C–C coupling of a resonance form bearing a high
matches very well with the species [Mn (His) (OAc) (H O) ] ,
2 2 2 2
density of unpaired electron at para-position of the phenoxy rad-
ical will form an unstable dimer I (Scheme S4A). In our case, the
product DPQ can be formed from the dimer I via two different
(calc. 519.12) (Fig. S20). This strengthens the interpretation that
the possible cause of inhibition of the oxidation of DTBC is due to
coordination of the amino acid with the metal centre.
routes as shown earlier [108,109]. First one may be via H DPQ
intermediate which will form through tautomeric rearrangement
The ESI-MS studies were also performed for the reaction mix-
2
III
ture of DTBC oxidation by Mn (OAc) ·2H O. ESI-MS technique
3
2
(
Scheme S4A). Then H DPQ will sequentially oxidize to form DPQ.
is capable of providing mass spectral data of speciation rele-
vant to the solution condition [110–112]. The results provided
us with important insight about the solution speciation and pos-
2
The second route involves direct oxidative dehydrogenation of
intermediate I to DPQ and water.
We also performed some initial investigation on the possible
reactive oxygen species (ROS) involved with the catechol oxida-
tion reaction using UV-vis spectroscopy. The studies show that
there may be involvement of a hydroxyl radical (Fig. 4) since the
reaction is inhibited to a large extent (ca.70%) by DMSO but how-
ever, the reaction is not much affected by other ROS quenchers
like (± )-˛-tocopherol or probucol. DMSO is known to react with
hydroxyl radical to oxidize itself. Hence hydroxyl radical may be
generated from the reaction of molecular oxygen with the metal
centre. However, the reaction is not completely quenched by DMSO
sible mechanism of DTBQ formation. The mass spectrometric
III
data of the reaction mixture of DTBC and Mn (OAc) matches
3
IV
III
well with four species—Mn (OAc)(DTBC-2H), Mn (DTSQ)2 or
Mn (DTSQ)(DTBC-2H) and Mn (DTSQ) (Scheme 2, Fig. S21) most
IV
IV
3
of which may be formed during the catalytic cycle. Although the
ESI-MS data repeatedly generates the same features we could not
IV
obtain any spectroscopic evidence in support of generation of Mn
.
We attempted to find the semiquinone with X-band EPR at 77 K in
CH CN and CH OH mixture but could not find any evidence of radi-
3
3
cals suggesting that perhaps all these processes are faster than EPR
time scale (Fig. S18). The EPR spectrum indicates anisotropy and
the assignment is not straightforward. However, the giso is ca. 2.02
and the average A value is ca. 90 G (Fig. S18). Broad unresolved sig-
nals of less amplitude at g = 3.3–4.5 are also obtained (Fig. S18). The
spectrum is suggestive of a Mn(II) species [53,113,114]. It may be
that since we used a catalyst to substrate ratio of 1:2 in order to
get good signal to noise ratio, the substrate oxidized quickly much
before the instrument could be tuned to record data and most of
the catalyst disproportionated to Mn(II) and that is what we are
observing in the EPR data.
III
which we found with our earlier reported Mn complex (Table 1)
when hydroxyl radicals were involved [87]. Hence, formation of
alternative carbon centered radicals cannot be ruled out. We found
that benzoates and amino acids or esters of amino acids strongly
inhibit the oxidation reaction. Complete inhibition was observed
by histidine and methyl ester of methionine. Inhibition by amino
acid brings in the possibility of Schiff base formation between the
product quinone and the amino acid or formation of oxazole after
Schiff base condensation but however, we did not observe any such
product in the ESI-MS. Such products should be easily detected by
ESI-MS since they are pretty stable, easily ionized and hence, very
unlikely not to see if they have formed. In addition benzoic acid
and amino acids are used in 100 molar equivalents with respect to
the catalyst so unless they are deactivating the metal completely
through coordination there will be lot of catechol left for oxidation
and hence the band at 402 nm should increase after an initial delay.
But we see almost complete quenching which strongly suggest
One might argue that the speciation we see in ESI-MS may
not be happening under normal solution conditions. However,
in literature there exists strong evidences for similar speciation:
IV
III
Mn (DTSQ) (DTBC-2H) or Mn (DTSQ) , which was shown indi-
2
3
vidually by Pierpont et al. and Wieghardt et al. [115–117]. Hence
our ESI-MS data is in good agreement with the literature. The
proposed mechanism shows that in solution the catalytic reaction

1
92
S.K. Dey, A. Mukherjee / Journal of Molecular Catalysis A: Chemical 395 (2014) 186–194
Scheme 2. Proposed mechanistic pathway of DTBC oxidation by MnIII acetate.
may initiate through the formation of a [Mn (DTBC)(OAc)]+
IV
S4B) and the BQMI then couple with OAP to form APX as the final
product.
intermediate, which then is converted to [Mn (DTSQ)(OAc)]+
III
(Scheme 2). The m/z for both the species in ESI-MS would be
the same and the data shows a m/z of 334.10 (calc. 334.10)
4
. Conclusions
III
corresponding to the afore-mentioned species. The [Mn (DTSQ)
(
OAc)]+ may further react with another DTBC unit and molec-
We have shown that Manganese(III) acetate can efficiently per-
III
+
ular oxygen to form [Mn (DTSQ) (O H)] and then convert
2
2
form oxidation of DTBC and OAP to DTBQ and APX repsectively. It
seems to be a good, cheap, commercially available catalyst capa-
ble of producing quinone or phenoxazinone derivatives. It can also
form oxidatively coupled biaryls from sterically hindered phenols
at room temperature. The studies suggest that there is formation
of [Mn (DTBC)(DTSQ) ] during the oxidation of DTBC as per ESI-
MS data which is in strong agreement with earlier literature [117].
Oxidation of OAP to APX or DTBC to DTBQ did not show invovle-
ment of any radical in EPR studies. Although ESI-MS data of DTBC
oxidation suggests invovlement of semiquinone and Mn , the EPR
data reveals that the intermediates formed have very short life-
III
to [Mn (DTBC)(DTSQ)]. The ESI-MS data corresponds to
+
[
MnIV(DTBC)(DTSQ)] m/z of 495.22 (calc. 495.23) was found which
might be forming in mass spectral condition (Scheme 2, Fig. S21).
Further there is coordination of another DTBC unit as shown in
IV
Scheme 2 to form [Mn (DTBC)(DTSQ) ] similar to that found in
2
IV
2
earlier studies [117]. Two quinone molecules are hence produced
with two molecules of water and the catalytic cycle continues
(Scheme 2). The generation of DTBQ and water as a final product,
rather than H O is based on the fact that we could hardly detect the
2
2
IV
presence of any hydrogen peroxide using the Horse Radish perox-
idise experiment (details in Experimental section, Fig. S5) [27,118]
as well as using potassium titanium(IV) oxalate (Fig. S6) [102,103]
which indicates that even if any peroxide was generated was con-
sumed in the reaction and hence cannot be detected in the extracted
medium.
time and hence not detectable by EPR. The oxidative coupling of
III
2
,6-DTBP with only 1 mol% Mn (OAc) ·H O selectively forms DPQ
3
2
with more than 90% yield in less than 4 h and none of the side prod-
uct 2,6-DTBQ is observed. Oxidative coupling of 2,4-DTBP is only
III
possible with 1 mol% of Mn (OAc) ·H O and 10 mol% of base and
3
2
Probing the mechanistic pathway of oxidation of OAP to APX the
earlier literature described the involvement of organic radical inter-
mediates in their mechanistic pathways [46,56,59,75,78,81,83,84].
Although most of the cases they were unable to detect any organic
radical by EPR study, only in two cases organic radical involve-
ment was detected [80,82]. We have also carried out an X-band
EPR study to investigate any radial intermediate but similar to
most we too have not found any signal which corresponds to
organic radical, this may be due to the fact that the stability of
the radical intermediate is beyond the EPR time scale in presence
of our active catalyst. Hence, our proposed mechanistic pathway
is in line with the earlier proposed mechanisms. The OAP may
takes longer time (12 h). We have shown that although NaOH or
III
KOH alone without the Mn catalyst, cannot perform the oxida-
tive coupling of 2,4-DTBP but they perform the oxidative coupling
of 2,6-DTBP to the corresponding biaryl (DPQ) with 100 mol% tak-
ing a relatively longer period of 12 h which seemed to have gone
unnoticed earlier.
Acknowledgements
We acknowledge DST-India for funding vide project number
SR/S1/IC-36/2010. We are also thankful to IISER Kolkata for the
financial and infrastructural support. SKD wishes to acknowledge
CSIR, New Delhi, India for research fellowship.
III
first coordinate to Mn centre where it was oxidized to OAP rad-
ical which generate o-benzoquinone monoamine (BQMI) (Scheme

S.K. Dey, A. Mukherjee / Journal of Molecular Catalysis A: Chemical 395 (2014) 186–194
193
Appendix A. Supplementary data
[42] A. Jana, N. Aliaga-Alcalde, E. Ruiz, S. Mohanta, Inorg. Chem. 52 (2013)
732–7746.
43] P. Seth, L.K. Das, M.G.B. Drew, A. Ghosh, Eur. J. Inorg. Chem. 2012 (2012)
232–2242.
[44] A. Guha, K.S. Banu, A. Banerjee, T. Ghosh, S. Bhattacharya, E. Zangrando, D.
Das, J. Mol. Catal. A: Chem. 338 (2011) 51–57.
7
[
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.molcata.
2
2
014.08.014.
[
[
[
45] K.S. Banu, T. Chattopadhyay, A. Banerjee, M. Mukherjee, S. Bhattacharya, G.K.
Patra, E. Zangrando, D. Das, Dalton Trans. (2009) 8755–8764.
46] J. Kaizer, G. Barath, R. Csonka, G. Speier, L. Korecz, A. Rockenbauer, L. Parkanyi,
J. Inorg. Biochem. 102 (2008) 773–780.
47] J. Kaizer, R. Csonka, G. Barath, G. Speier, Transition Met. Chem. 32 (2007)
1047–1050.
References
[
[
[
[
[
[
1] K.M. Gligorich, M.S. Sigman, Chem. Commun. (2009) 3854–3867.
2] J. Piera, J.E. Backvall, Angew. Chem. Int. Ed. 47 (2008) 3506–3523.
3] S.S. Stahl, Science 309 (2005) 1824–1826.
4] T. Punniyamurthy, S. Velusamy, J. Iqbal, Chem. Rev. 105 (2005) 2329–2363.
5] S.S. Stahl, Angew. Chem. Int. Ed. 43 (2004) 3400–3420.
[48] A. Majumder, S. Goswami, S.R. Batten, M.S. El Fallah, J. Ribas, S. Mitra, Inorg.
Chim. Acta 359 (2006) 2375–2382.
[49] G. Blay, I. Fernandez, J.R. Pedro, R. Ruiz-Garcia, T. Temporal-Sanchez, E. Pardo,
F. Lloret, M.C. Munoz, J. Mol. Catal. A: Chem. 250 (2006) 20–26.
[50] Y. Hitomi, A. Ando, H. Matsui, T. Ito, T. Tanaka, S. Ogo, T. Funabiki, Inorg. Chem.
44 (2005) 3473–3478.
[51] S. Mukherjee, T. Weyhermueller, E. Bothe, K. Wieghardt, P. Chaudhuri, Dalton
Trans. (2004) 3842–3853.
[52] M.U. Triller, D. Pursche, W.-Y. Hsieh, V.L. Pecoraro, A. Rompel, B. Krebs, Inorg.
Chem. 42 (2003) 6274–6283.
6] A. Patra, G.C. Giri, T.K. Sen, L. Carrella, S.K. Mandal, M. Bera, Polyhedron 67
(2014) 495–504.
[
7] R.E.H.M.B. Osorio, R.A. Peralta, A.J. Bortoluzzi, V.R. de Almeida, B. Szpoganicz,
F.L. Fischer, H. Terenzi, A.S. Mangrich, K.M. Mantovani, D.E.C. Ferreira, W.R.
Rocha, W. Haase, Z. Tomkowicz, A. dos Anjos, A. Neves, Inorg. Chem. 51 (2012)
1
569–1589.
8] S. Mandal, J. Mukherjee, F. Lloret, R. Mukherjee, Inorg. Chem. 51 (2012)
3148–13161.
9] A. Biswas, L.K. Das, M.G.B. Drew, C. Diaz, A. Ghosh, Inorg. Chem. 51 (2012)
0111–10121.
[
[
[53] D. Kovala-Demertzi, S.K. Hadjikakou, M.A. Demertzis, Y. Deligiannakis, J.
Inorg. Biochem. 69 (1998) 223–229.
[54] Y. Gultneh, A. Farooq, K.D. Karlin, S. Liu, J. Zubieta, Inorg. Chim. Acta 211 (1993)
171–175.
1
1
[
[
[
[
10] M.A. Agotegaray, M. Dennehy, M.A. Boeris, M.A. Grela, R.A. Burrow, O.V. Quin-
zani, Polyhedron 34 (2012) 74–83.
11] D. Sadhukhan, A. Ray, R.J. Butcher, C.J. Gomez Garcia, B. Dede, S. Mitra, Inorg.
Chim. Acta 376 (2011) 245–254.
[55] A. Guha, T. Chattopadhyay, N.D. Paul, M. Mukherjee, S. Goswami, T.K. Mondal,
E. Zangrando, D. Das, Inorg. Chem. 51 (2012) 8750–8759.
[56] A. Panja, P. Guionneau, Dalton Trans. 42 (2013) 5068–5075.
[57] C. Mukherjee, T. Weyhermueller, E. Bothe, E. Rentschler, P. Chaudhuri, Inorg.
Chem. 46 (2007) 9895–9905.
[58] J. Bjerre, T.H. Fenger, L.G. Marinescu, M. Bols, Eur. J. Org. Chem. (2007)
704–710.
[59] M.R. Maurya, S. Sikarwar, T. Joseph, S.B. Halligudi, J. Mol. Catal. A: Chem. 236
(2005) 132–138.
[60] J. Jose, K. Burgess, Tetrahedron 62 (2006) 11021–11037.
[61] M. Matsuoka, Infrared Absorbing Dyes (1990) 35–43.
[62] S. Sabelle, in (L’Oreal, Fr.), Application: WO WO, 2013, 55 pp.; Chemical Index-
ing Equivalent to 159:109324 (FR).
[63] M. Matsuoka (Ed.), Infrared Absorbing Dyes (Topics in Applied Chemistry),
1990.
[64] A.W. Smith, A. Camara-Artigas, M. Wang, J.P. Allen, W.A. Francisco, Biochem-
istry 45 (2006) 4378–4387.
[65] J.C. Freeman, P.G. Nayar, T.P. Begley, J.J. Villafranca, Biochemistry 32 (1993)
4826–4830.
[66] C.E. Barry III, P.G. Nayar, T.P. Begley, Biochemistry 28 (1989) 6323–6333.
[67] C.E. Barry III, P.G. Nayar, T.P. Begley, J. Am. Chem. Soc. 110 (1988) 3333–3334.
[68] H.A. Choy, G.H. Jones, Arch. Biochem. Biophys. 211 (1981) 55–65.
[69] U. Hollstein, Chem. Rev. 74 (1974) 625–652.
[70] E. Frei 3rd, Cancer Chemother. Rep. 58 (1974) 49–54.
[71] M. Homma, A.F. Graham, Biochim. Biophys. Acta, Spec. Sect Nucleic Acids
Relat. Subj. 61 (1962) 642–644.
[72] F. Bruyneel, L. D’Auria, O. Payen, P.J. Courtoy, J. Marchand-Brynaert, Chem-
BioChem 11 (2010) 1451–1457.
[73] V.H.J. Frade, M.J. Sousa, J.C.V.P. Moura, M.S.T. Goncalves, Tetrahedron Lett. 48
(2007) 8347–8352.
12] M.K. Panda, M.M. Shaikh, R.J. Butcher, P. Ghosh, Inorg. Chim. Acta 372 (2011)
1
45–151.
13] S. Majumder, S. Sarkar, S. Sasmal, E.C. Sanudo, S. Mohanta, Inorg. Chem. 50
2011) 7540–7554.
(
[
[
14] R. Bakshi, M. Rossi, F. Caruso, P. Mathur, Inorg. Chim. Acta 376 (2011) 175–188.
15] A. Kupan, J. Kaizer, G. Speier, M. Giorgi, M. Reglier, F. Pollreisz, J. Inorg.
Biochem. 103 (2009) 389–395.
[16] K.S. Banu, T. Chattopadhyay, A. Banerjee, S. Bhattacharya, E. Zangrando, D.
Das, J. Mol. Catal. A: Chem. 310 (2009) 34–41.
[
[
17] E. Mijangos, J. Reedijk, L. Gasque, Dalton Trans. (2008) 1857–1863.
18] L. Gasque, V.M. Ugalde-Saldivar, I. Membrillo, J. Olguin, E. Mijangos, S. Bernes,
I. Gonzalez, J. Inorg. Biochem. 102 (2008) 1227–1235.
[
[
[
[
19] K.S. Banu, T. Chattopadhyay, A. Banerjee, S. Bhattacharya, E. Suresh, M. Nethaji,
E. Zangrando, D. Das, Inorg. Chem. 47 (2008) 7083–7093.
20] A. Banerjee, S. Sarkar, D. Chopra, E. Colacio, K.K. Rajak, Inorg. Chem. 47 (2008)
4
023–4031.
21] B. Sreenivasulu, F. Zhao, S. Gao, J.J. Vittal, Eur. J. Inorg. Chem. (2006)
656–2670.
22] I.A. Koval, K. Selmeczi, C. Belle, C. Philouze, E. Saint-Aman, I. Gautier-
Luneau, A.M. Schuitema, M. van Vliet, P. Gamez, O. Roubeau, M. Lueken,
B. Krebs, M. Lutz, A.L. Spek, J.-L. Pierre, J. Reedijk, Chem.-Eur. J. 12 (2006)
2
6138–6150.
[
[
[
[
23] M. Merkel, N. Moeller, M. Piacenza, S. Grimme, A. Rompel, B. Krebs, Chem.-Eur.
J. 11 (2005) 1201–1209.
24] C. Jocher, T. Pape, W.W. Seidel, P. Gamez, J. Reedijk, F.E. Hahn, Eur. J. Inorg.
Chem. (2005) 4914–4923.
25] R. Wegner, M. Gottschaldt, W. Poppitz, E.-G. Jager, D. Klemm, J. Mol. Catal. A:
Chem. 201 (2003) 93–118.
26] A. Neves, L.M. Rossi, A.J. Bortoluzzi, B. Szpoganicz, C. Wiezbicki, E. Schwingel,
W. Haase, S. Ostrovsky, Inorg. Chem. 41 (2002) 1788–1794.
27] J. Ackermann, F. Meyer, E. Kaifer, H. Pritzkow, Chem.-Eur. J. 8 (2002) 247–258.
28] R. Wegner, M. Gottschaldt, H. Gorls, E.-G. Jager, D. Klemm, Chem.-Eur. J. 7
[74] G.L. Newton, J.R. Milligan, Radiat. Phys. Chem. 75 (2006) 473–478.
[75] A. Panja, M. Shyamal, A. Saha, T.K. Mandal, Dalton Trans. 43 (2014) 5443–5452.
[76] A. Panja, Dalton Trans. 43 (2014) 7760–7770.
[77] A. Panja, Polyhedron 43 (2012) 22–30.
[
[
[78] S.H. El-Khalafy, M. Hassanein, J. Mol. Catal. A: Chem. 363–364 (2012) 148–152.
[79] M. Hassanein, M. Abdo, S. Gerges, S. El-Khalafy, J. Mol. Catal. A: Chem. 287
(2008) 53–56.
[80] I.C. Szigyarto, T.M. Simandi, L.I. Simandi, L. Korecz, N. Nagy, J. Mol. Catal. A:
Chem. 251 (2006) 270–276.
[81] T.M. Simandi, Z. May, I.C. Szigyarto, L.I. Simandi, Dalton Trans. (2005)
365–368.
[82] T.M. Simandi, L.I. Simandi, M. Gyor, A. Rockenbauer, A. Goemoery, Dalton
Trans. (2004) 1056–1060.
[83] L.I. Simandi, T. Barna, S. Nemeth, J. Chem. Soc., Dalton Trans. (1996) 473–478.
[84] T. Horvath, J. Kaizer, G. Speier, J. Mol. Catal. A: Chem. 215 (2004) 9–15.
[85] L.I. Simandi, S. Nemeth, N. Rumelis, J. Mol. Catal. 42 (1987) 357–360.
[86] S. Nemeth, L.I. Simandi, J. Mol. Catal. 14 (1982) 241–246.
[87] S.K. Dey, A. Mukherjee, ChemCatChem 5 (2013) 3533–3537.
[88] M. Mondal, U. Bora, RSC Adv. 3 (2013) 18716–18754.
[89] E. Bicer, M. Yilmaz, M. Karatas, A.T. Pekel, Helv. Chim. Acta 95 (2012) 795–804.
[90] A.S. Demir, M. Emrullahoglu, Curr. Org. Synth. 4 (2007) 321–351.
[91] K.V. Sajna, K.C.K. Swamy, J. Org. Chem. 77 (2012) 8712–8722.
[92] G.-W. Wang, F.-B. Li, J. Nanosci. Nanotechnol. 7 (2007) 1162–1175.
[93] R. Caliskan, M.F. Ali, E. Sahin, W.H. Watson, M. Balci, J. Org. Chem. 72 (2007)
3353–3359.
(
2001) 2143–2157.
29] M. Mitra, A.K. Maji, B.K. Ghosh, P. Raghavaiah, J. Ribas, R. Ghosh, Polyhedron
7 (2014) 19–26.
[
[
[
[
[
[
[
6
30] S.-I. Lo, J.-W. Lu, W.-J. Chen, S.-R. Wang, H.-H. Wei, M. Katada, Inorg. Chim.
Acta 362 (2009) 4699–4705.
31] T. Megyes, Z. May, G. Schubert, T. Grosz, L.I. Simandi, T. Radnai, Inorg. Chim.
Acta 359 (2006) 2329–2336.
32] L.I. Simandi, T.M. Simandi, Z. May, G. Besenyei, Coord. Chem. Rev. 245 (2003)
5–93.
33] S. Majumder, S. Mondal, P. Lemoine, S. Mohanta, Dalton Trans. 42 (2013)
561–4569.
34] A. Banerjee, A. Guha, J. Adhikary, A. Khan, K. Manna, S. Dey, E. Zangrando, D.
Das, Polyhedron 60 (2013) 102–109.
8
4
35] Y. Zhang, X.G. Meng, Z.R. Liao, D.F. Li, C.L. Liu, J. Coord. Chem. 62 (2009)
876–885.
[
[
36] L.I. Simandi, T.L. Simandi, J. Chem. Soc., Dalton Trans. (1998) 3275–3279.
37] L.I. Simandi, T. Barna, G. Argay, T.L. Simandi, Inorg. Chem. 34 (1995)
6337–6340.
[
[
38] A. Miyagi, S. Nishiyama, S. Tsuruya, M. Masai, J. Mol. Catal. 55 (1989) 379–390.
39] K. Kushioka, I. Tanimoto, K. Maruyama, Bull. Chem. Soc. Jpn. 62 (1989)
[94] C.-Y. Lin, Y.-C. Cheng, A.I. Tsai, C.-P. Chuang, Org. Biomol. Chem. 4 (2006)
1097–1103.
1147–1153.
[
[
40] S. Tsuruya, S. Yanai, M. Masai, Inorg. Chem. 25 (1986) 141–146.
41] M. Mitra, A.K. Maji, B.K. Ghosh, G. Kaur, A. Roy Choudhury, C.-H. Lin, J. Ribas,
R. Ghosh, Polyhedron 61 (2013) 15–19.
[95] J.-W. Huang, M. Shi, J. Org. Chem. 70 (2005) 3859–3863.
[96] D. Yang, X.-Y. Ye, M. Xu, K.-W. Pang, K.-K. Cheung, J. Am. Chem. Soc. 122 (2000)
1658–1663.

1
94
S.K. Dey, A. Mukherjee / Journal of Molecular Catalysis A: Chemical 395 (2014) 186–194
[
[
[
97] B.B. Snider, Chem. Rev. 96 (1996) 339–363. [109] I. Kohara, H. Fujiyama, K. Iwai, S. Nishiyama, S. Tsuruya, J. Mol. Catal. A: Chem.
98] J. Iqbal, B. Bhatia, N.K. Nayyar, Chem. Rev. 94 (1994) 519–564.
99] X.-Q. Pan, J.-P. Zou, W. Zhang, Mol. Diversity 13 (2009) 421–438.
153 (2000) 93–101.
[110] J.M. Daniel, S.D. Friess, S. Rajagopalan, S. Wendt, R. Zenobi, Int. J. Mass Spec-
trom. 216 (2002) 1–27.
[
[
100] G.G. Melikyan, Org. React. 49 (1997) 427–675.
101] C. Dubuisson, Y. Fukumoto, L.S. Hegedus, J. Am. Chem. Soc. 117 (1995)
[111] J.A. Loo, Int. J. Mass Spectrom. 200 (2000) 175–186.
[112] C.R. Blakley, J.J. Carmody, M.L. Vestal, J. Am. Chem. Soc. 102 (1980) 5931–5933.
[113] A. Bouchoucha, A. Terbouche, A. Bourouina, S. Djebbar, Inorg. Chim. Acta 418
(2014) 187–197.
[114] S.M.d.M. Romanowski, S.P. Machado, G.R. Friedermann, A.S. Mangrich, M.d.F.
Hermann, H.O. Lima, S. Nakagaki, J. Brazil. Chem. Soc. 21 (2010) 842–850.
[115] H. Chun, P. Chaudhuri, T. Weyhermueller, K. Wieghardt, Inorg. Chem. 41
(2002) 790–795.
3
697–3704.
[
[
[
102] M. Xu, B.R. Bunes, L. Zang, ACS Appl. Mater. Interfaces 3 (2011) 642–647.
103] R.M. Sellers, Analyst 105 (1980) 950–954.
104] P. Kar, R. Haldar, C.J. Gomez-Garcia, A. Ghosh, Inorg. Chem. 51 (2012)
4
265–4273.
105] H. Tadokoro, S. Nishiyama, S. Tsuruya, M. Masai, J. Catal. 138 (1992)
4–37.
[
2
[
[
106] S. Tsuruya, T. Takaki, M. Masai, J. Catal. 89 (1984) 511–519.
107] K. Iwai, T. Yamauchi, K. Hashimoto, T. Mizugaki, K. Ebitani, K. Kaneda, Chem.
Lett. 32 (2003) 58–59.
[116] F.N. Penkert, T. Weyhermueller, E. Bill, P. Hildebrandt, S. Lecomte, K.
Wieghardt, J. Am. Chem. Soc. 122 (2000) 9663–9673.
[117] A.S. Attia, C.G. Pierpont, Inorg. Chem. 37 (1998) 3051–3056.
[118] E. Monzani, L. Quinti, A. Perotti, L. Casella, M. Gullotti, L. Randaccio, S. Geremia,
G. Nardin, P. Faleschini, G. Tabbi, Inorg. Chem. 37 (1998) 553–562.
[
108] H. El-Hamshary, S. Al-Sigeny, M.M. Ibrahim, J. Macromol. Sci., Part A Pure
Appl. Chem. 47 (2010) 329–334.