Synthesis and characterization of chromium(III) complexes derived from tridentate ligands: Generation of phenoxyl radical and catalytic oxidation of olefins

Article abstract of DOI:10.1016/j.inoche.2012.07.028

The mononuclear chromium complexes [Cr(Phimp)₂](ClO₄) (1), [Cr(N-Phimp)₂](ClO₄) (2), and [Cr(Me-Phimp) ₂](ClO₄) (3) (where PhimpH = (2-((2-phenyl-2-(pyridin-2- yl)hydrazono)methyl)phenol), N-PhimpH = (2-((2-phenyl-2-(pyridin-2-yl)hydrazono) methyl)napthalen-1-ol), Me-PhimpH = (2-(1-(2-phenyl-2-(pyridine-2-yl)hydrazono) ethyl)phenol) where H stands for dissociable proton) were synthesized and characterized spectroscopically. Electrochemical studies were investigated for the stabilization of Cr(III) complexes and electrochemically generation of phenoxyl radical complexes. Phenoxyl radical complexes were also generated by ceric ammonium nitrate (CAN) and characterized by UV-visible spectra. The complexes were evaluated for their activity as catalysts for the epoxidation of olefins. The chromium complexes catalyzed epoxidation of olefins, viz. styrene, cis-cyclooctene and trans-stilbene with hydrogen peroxide (H₂O ₂) as terminal oxidant, to give corresponding epoxides and oxidative products at ambient temperature.

Full text of DOI:10.1016/j.inoche.2012.07.028

Inorganic Chemistry Communications 24 (2012) 81–86
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis and characterization of chromium(III) complexes derived from tridentate
ligands: Generation of phenoxyl radical and catalytic oxidation of olefins
⁎
Kaushik Ghosh , Pramod Kumar, Isha Goyal
Department of Chemistry, Indian Institute of Technology, Roorkee, Roorkee-247667, Uttarakhand, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The mononuclear chromium complexes [Cr(Phimp)2](ClO4) (1), [Cr(N–Phimp)2](ClO4) (2), and [Cr(Me–
Phimp)2](ClO4) (3) (where PhimpH=(2-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)phenol), N–PhimpH=
(2-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)napthalen-1-ol), Me–PhimpH=(2-(1-(2-phenyl-2-(pyridine-
2-yl)hydrazono)ethyl)phenol) where H stands for dissociable proton) were synthesized and characterized spectro-
scopically. Electrochemical studies were investigated for the stabilization of Cr(III) complexes and electrochemically
generation of phenoxyl radical complexes. Phenoxyl radical complexes were also generated by ceric ammonium ni-
trate (CAN) and characterized by UV–visible spectra. The complexes were evaluated for their activity as catalysts for
the epoxidation of olefins. The chromium complexes catalyzed epoxidation of olefins, viz. styrene, cis-cyclooctene
and trans-stilbene with hydrogen peroxide (H2O2) as terminal oxidant, to give corresponding epoxides and oxida-
tive products at ambient temperature.
Received 17 May 2012
Accepted 17 July 2012
Available online 24 July 2012
Keywords:
Chromium complexes
Electrochemistry
Phenoxyl radical complexes
Oxidation catalyst
© 2012 Elsevier B.V. All rights reserved.
Catalysis is known to play a key role in modern chemical technologies.
Nowadays it is generally agreed that the key role in catalytic reactions is
played by intermediate chemical interaction of reactive molecules with
definite functional groups (active sites) of homogeneous, heterogeneous
or biological catalysts (enzymes) [1]. Asymmetric oxidation of hydrocar-
bons under mild and environmental friendly conditions is an important
research field, since industrial processes, especially in the pharmaceutical
industry, for synthesizing synthons from readily available olefins [2].
However, selective oxidation of alkenes under mild conditions is a diffi-
cult task due to their chemical inertness. In this regard, use of coordina-
tion complexes of transition metals as catalysts is of abiding importance
as it offers an effective possibility for synthesis of pure compounds [3,4].
Although transition metal Schiff base complexes of bear resemblance to
enzymatic catalysts and are eye-catching since they provide advan-
tages due to their relatively easy synthesis and versatile coordination
structures [5–9]. In this regard, first row transition metals namely
vanadium, chromium, iron and copper complexes are preferred be-
cause of their versatile coordination chemistry in different oxidation
states [10]. Moreover, these transition metal-catalyzed oxidation re-
actions are chemical models for the monooxygenase, cytochrome
P-450 enzymes [11,12].
and lipid metabolism [14,15]. Schiff base complexes of chromium(III)
such as N,N′-ethylene bis(salicylidene-iminate)diaquochromium(III)
chloride, [Cr(salen)(H2O)2]Cl were used as a new model of GTF [16,17].
Chromium–salen complexes are well-known catalysts, both in hetero-
geneous and homogeneous systems. Other applications of theses com-
plexes are reported, such as, the stereoselective alkene epoxidations
[18,19] and alcohol oxidations [20]. There is a wide range of chromium
complexes that are known to be capable of catalyzing asymmetric oxi-
dation of unfunctionalized olefins as well as other organic molecules
in presence of terminal oxidants, reports on the use of chromium com-
plexes are sparse in the literature [3,4,21–24].
Herein we report the synthesis and characterization of chromium
complexes [Cr(Phimp)2](ClO4) (1), [Cr(N–Phimp)2](ClO4) (2), and [Cr
(Me–Phimp)2](ClO4) (3) derived from tridentate ligands PhimpH, N–
PhimpH and Me–PhimpH respectively having NNO donors (Scheme 1).
Electrochemical studies were also investigated for the stabilization of
Hence chromium is an important metal for catalytic studies because
chromium exhibited variable oxidation states, spin states, coordination
numbers and redox properties in different chromium complexes [13].
However, chromium(III) is an essential nutrient that is involved in the
glucose tolerance factor (GTF) in maintenance of normal carbohydrate
⁎ Corresponding author. Fax: +91 1332 273560.
E-mail address: ghoshfcy@iitr.ernet.in (K. Ghosh).
Scheme 1. Schematic drawing of tridentate Schiff's base ligands and their abbreviations.
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2012.07.028

82
K. Ghosh et al. / Inorganic Chemistry Communications 24 (2012) 81–86
Scheme 2. Schematic synthetic procedure of complexes 1–3.
Cr(III) oxidation states. Phenoxyl radical complexes are important inter-
mediates in the catalytic oxidation of primary alcohol to aldehyde and al-
dehyde to carboxylic acid by galactose oxidase (GO) [25,26] and
glyoxal oxidase [27] enzymes respectively. Due to the presence of
phenolato function in the ligand frame we have explored the gener-
ation of phenoxyl radical complexes. The catalytic oxidation chemis-
try of olefins by the chromium complexes was examined.
The tridentate ligands PhimpH, N-PhimpH and Me-PhimpH were
synthesized by the reported procedure [28]. Complexes were synthe-
sized by the stirring of ligands with chromium(III) salt CrCl3·6H2O in
ethanol for 6 h. Bis complexes were obtained even though reaction
was started by 1:1 ratio of ligand and salts or 2:1 ratio. Details of
the syntheses are summarized in Scheme 2.
The characteristic band of azomethine (ν–HC=N) group in IR spectra of
the ligands were observed near 1607–1611 cm−1 [29]. Coordination of
the nitrogen to the metal center reduced the electron density in the
azomethine moiety and thus lowered the (ν–HC=N) frequency. Decrease
in stretching frequencies for (ν–HC=N) in complexes 1, 2 and 3 clearly in-
dicated the ligation of azomethine nitrogen (−HC_N–) to metal center
[30,31]. The bands near 1090 cm−1 together with a band at 623 cm−1
were found in all chromium(III) complexes 1–3 which showed the pres-
ence of perchlorate ion. The lack of splitting of these two bands suggested
the presence of non-coordinated perchlorate ion to the metal center
[30,31]. A high intensity band near 1300 cm−1 in the Schiff's bases can
be assigned as phenol C\O (νC–OPh) stretching frequency. Shift of νC–O
to higher frequency supported the deprotonation of phenolate function
and the formation of metal oxygen bond [32]. It was further supported
by disappearance of the broad νO–H band in IR spectra of all metal com-
plexes. The absorption spectra of complexes were recorded in acetonitrile
at room temperature (Fig. S1). The transition band near 235–245 nm was
designated as π–π* transition in ligand (Table 1). The band near 420 nm
for complexes 1, 2 and 3 was assigned as ligand-to-metal charge transfer
(LMCT) transition of phenolato oxygen to chromium(III) [33]. How-
ever, in addition 2 possesses one absorption band ~450 nm which
was also of charge transfer type. The molar conductivity measure-
ments in dimethylformamide at ca.10–3 M determined at 298 K for
complexes were found to be 48–59 Ω−1cm2mol−1. These values
confirmed the uni-univalent (1:1) electrolyte behavior in solution
[34]. For complexes, magnetic moments were measured at room
temperature (300 K) (Table S1). Complexes were paramagnetic in
nature and stabilized the 3d3 system [35].
Electrospray ionization of methanolic solution of [Cr(Phimp)2](ClO4)
(1), [Cr(N–Phimp)2](ClO4) (2) showed intense signals corresponding to
[Cr(Phimp)2]+ (m/z=628) and Cr(N–Phimp)2]+(m/z=728) respec-
tively after losing the ClO4− appeared as the leading cationic species.
Representative ESI-MS spectra for [Cr(Phimp)2](ClO4) (1), [Cr(N–
Phimp)2](ClO4) (2) are presented in Figs. S2 and S3.
The redox potentials of complexes 1–3 are described in Table 1 and
representative voltammograms are shown in Fig. 1 and Figs. S4–S5. The
neutral uncomplexed ligands did not show any cyclic voltammogram
over the range from 0.0 to 1.1 V; hence all the curves were attributed
to the redox activity of our complexes. Complexes 1 and 2 showed
quasi-reversible peaks and from the Table 1, it has been shown that
one electron is involved in this redox process. The wave detected at
0.12–0.92 V vs. Ag+/AgCl has been considered as one-electron-redox
process attributed to the reduction of [CrIIIL2]+ to [CrIIL2] (where L=
Phimp−, N-Phimp−) indicating Cr(III)/Cr(II) redox couple. On the other
hand, complex 3 showed irreversible peak at −1.3 V (Fig. S5). We want
to mention here that for complex 2, we found small peaks at ~−0.75 V
and +0.5 V during our scan from −1.4 V to +1.4 V vs. Ag/AgCl. If we
20
III II
15
Cr /Cr
Table 1
Electronic spectra and cyclic voltammetric redox potentials of complexes 1–3 and their
phenoxyl radical species.
500
400
300
200
10
[2]/[2]
Complexa
λ
max/nm (ε/M−1 cm−1)]b
[Cr(III)/Cr(II)]cE1/2d/V
(ΔEe/V)
100
5
1
427(8700), 338 (18,200), 302 (21,400),
244 (37,900)
601(420), 551(720), 422(8800), 336(18,280),
300(22,600), 246(36,800)
484 (4300), 445 (7900), 426 (7400),
350 (16,600), 257 (32,400), 244 (34,600)
805(600), 672(360), 444(12,020),
424(11,940), 349(24,040)
409 (7300), 354 (10,300), 308 (15,800),
235 (42,300)
0.123(0.064)
1.0
1.1
1.2
1.3
[1]•+
2
–
0
-5
0.92(0.112)
[2]•+
3
1.20(0.093)
–
–
-10
[3]•+
740(100), 560(420), 399(6,100), 311(14,140),
268(23,960)
a
All complexes in parentheses are radical species.
Solvent: acetonitrile.
Conditions: solvent dichloromethane; supporting electrolyte, TBAP (0.1 M); work-
-1.0
-0.5
0.0
0.5
1.0
b
c
Potential (V)
ing electrode glassy carbon; reference electrode, Ag/AgCl; scan rate 0.1 V/s, concentra-
tion ~10−3M at 298 K.
Fig. 1. Cyclic voltammograms of a 10−3 M solution of complex 2: using working elec-
trode: glassy-carbon, reference electrode: Ag+/AgCl; auxiliary electrode: platinum
wire, scan rate 0.1 V/s. Inset: Cyclic voltammograms at100–500 mV/s scan rates.
d
E1/2=0.5(Epa+Epc).
e
ΔE=(Epa−Epc) where Epa, Epc are anodic and cathodic peak potential.

K. Ghosh et al. / Inorganic Chemistry Communications 24 (2012) 81–86
83
3
2
1
0
Generation of phenoxyl radical complexes through the ligand cen-
tered oxidation of phenolato chromium(III) complexes 1–3 by addition
of two equivalents of the CAN afforded the corresponding phenoxyl
radical complexes at 0 °C. The formation of the phenoxyl radical com-
plexes of 1–3 showed dark reddish brown color and have been con-
firmed by the characteristic peaks in UV–visible spectra for phenoxyl
radical complexes [36–39]. Upon addition of CAN, the LMCT band for
the complex 2 near 484 nm readily disappeared together with the con-
comitant appearance of the new bands at 805(600) nm, 672(360) nm
which indicated the formation of phenoxyl radical complex (Fig. 2). A
similar absorption spectrum (601(420) nm, 551(720) nm, 422(8,800)
nm) of 1 and 740(100) nm, 560(420) nm, 399(6,100) nm) of 3 were
also obtained during the oxidation in the same procedure given as
above (Figs. S7 and S8). Reddish brown color and characteristics peaks
in UV–visible spectra (Fig. 2, Figs. S7 and S8 and Table 1) showed the
generation of phenoxyl radical complexes of 1–3.
0.10
0.05
0.00
349 nm
0
400
800
Time(sec.)
1200
1600
424 nm
444 nm
805 nm
During our studies on the synthesis of phenoxyl radical com-
plexes, we found that the radicals were unstable and gradually
decomposed within a very short time (2–5 min) because of the par-
ticipation of oxygen or solvent for the regeneration of the precursor
complex [28,39,40]. The decomposition rate of radical complexes of 1–3
was monitored by a time dependent study at λ values of 601 nm
(Fig. S9), 805 nm (inset of Fig. 2) and 740 nm (Fig. S10). It is
reported in the literature that stability of the phenoxyl radical com-
plexes increases with the increase in λ value of the lowest energy
transition [39,40], hence, the stability order of the phenoxyl radical
complexes was 2>3>1. The higher stability of phenoxyl radical
complex of 2 may be due to engagement of ortho and para position
of phenolato function in naphthol ring. We have generated and charac-
terized the phenoxyl radical complexes, however the catalytic activity
of these radical complexes (conversion of primary alcohol to aldehydes)
are under progress.
In the present investigation, the heating rates were suitably con-
trolled at 10 °C min−1 under nitrogen atmosphere and the weight
loss was measured from ambient temperature up to ~800 °C. The re-
sults of the thermogravimetric analysis of the complexes are not very
conclusive in terms of the loss of specific group(s) on increasing the
temperature under nitrogen atmosphere. However, it is clear from
the thermogravimetric profiles that these complexes are stable up
to 260 °C (Table S2). Complex 1 loses weight in three steps. A weight
400
600
800
1000
Wavelength (nm)
Fig. 2. UV–visible spectra of generated phenoxyl radical complex of 2 (50 μM) in ace-
tonitrile by CAN (50–400 μM) at 0 °C. Inset: decomposition of generated phenoxyl rad-
ical complex of 2 monitored by 805 nm at 0 °C.
measure the cyclic voltammogram in the range of −1.3 to 0 V and
0 to −1.35 V, we did not get those peaks (Fig. S6). So these small
peaks are not due to impurities. These may be due to some species
generated during the scan from −1.4 V to +1.4 V vs. Ag/AgCl. Inter-
estingly, cyclic voltammogram of 2 afforded a reversible peak at
~1.2 V due to ligand centered oxidation (Fig. 1). We measured the
cyclic voltammograms for generation of phenoxyl radical with dif-
ferent scan rates of 100–500 mV/s (Inset of Fig. 1). Repeated scans
as well as different scan rates clearly expressed the stability and gen-
eration of a phenoxyl radical complex of 2 [36,37]. The E1/2 and ΔE
values were found to be +1.20 V and 0.093 V respectively vs. Ag+
/
AgCl and was also described as phenoxyl radical redox couple [36].
Recently, we have reported copper complex [Cu(tBuPhimp)(Cl)] de-
t
rived from tridentate ligand BuPhimpH, showed a quasi-reversible
redox couple near +1.0 V, which clearly indicated ligand-centered
oxidation, i.e., the generation of a phenoxyl radical [37].
O
A
B
HO
OH
H2O2 /Cat.
H2O2 /Cat.
O
cis-Cyclooctene
Epoxycyclooctane
(EC)
Styreneoxide
(SO)
1-phenylethane-1,2-diol
(PhED)
Styrene
O
HO
O
Benzoic acid
(BzAc)
Benzaldehyde
(BzA)
O
HO
O
C
O
O
H2O2 /Cat.
+
+
+
O
Benzil
(Bzl)
Benzoic acid
(BzAc)
Benzaldehyde
(BzA)
trans- Stilbene
trans- Stilbeneoxide
(TSO)
Scheme 3. Various oxidation products of (A) styrene, (B) cis- cyclooctene and (C) trans-stilbene.

84
K. Ghosh et al. / Inorganic Chemistry Communications 24 (2012) 81–86
Table 2
A
200
180
160
140
120
100
80
Effect of time on the oxidation of styrene and the product selectivity.
Complex 1
Complex 2
Complex 3
Catalyst Time (h) Conversion% Product selectivity%
SOb BzAc BzAcd PhEDe PhAcf
TON
1
2
3
1
2
6
1
2
6
1
2
3
34. 5
38.1
96. 5
29.4
47.0
86.0
39.6
50.4
92.7
15.1 80.7
3.1 91.9
2.7 47.3 28.6
1.7 95.0
2.1 93.2
3.6 57.8 25.4
1.2 90.5
1.4 88.5
2.2 41.5 36.7
–
–
4.2
5.0
4.4
3.2
4.6
3.6
–
–
–
–
–
16.5
–
192.5
–
–
–
–
–
9.6
5.7
6.6
10.0
170.0
–
2.6
2.5
1.0
9.7
–
60
192.3
40
aReaction conditions: styrene (0.208 g, 2.0 mmol), catalyst (0.01 mmol), oxidant
(0.453 g, 4.0 mmol), temperature (90 °C) and acetonitrile (2.0 mL).bStyrene oxide,
cBenzaldehyde, dBenzoic acid, e1-Phenylethane-1,2-diol, fPhenylacetaldehyde.
20
0
% Conversion %SO %BzA %BzAc %PhET %Others TON
Oxidation of styrene has been investigated using homogeneous as
well as heterogeneous catalysts and the major oxidation products gen-
erally obtained are styreneoxide, benzaldehyde, benzoic acid, phenyl
acetaldehyde and 1-phenylethane-1,2-diol [12,18,42]. We have found
that Cr(III) complexes 1, 2 and 3 satisfactorily catalyze the oxidation of
styrene using H2O2 as oxidant and gives styreneoxide, benzaldehyde,
1-phenylethane-1,2-diol and benzoic acid along with some unidentified
products in homogeneous conditions (Scheme 3A) [12,43]. In order to
get the maximum oxidation of styrene, effect of time on the oxidation
of styrene reaction has been studied in detail. Carrying out this reaction
in 2.0 mL acetonitrile at 90 °C with 30% aqueous H2O2 under these reac-
tion conditions (i.e. 1:2, substrate to oxidant ratio) gave 80–90% conver-
sion after 6 h but the recovery of the catalyst from the reaction mixture
became very difficult due to partial decomposition as well as blending
of the complexes with solvent. Thus, under the above reaction condi-
tions (i.e. styrene (0.208 g, 2.0 mmol), catalyst (0.01 mmol) and 30%
aqueous H2O2 (0.453 g, 4.0 mmol), acetonitrile (2.0 mL), temperature
(90 °C) and time (6 h)), catalytic activity of all the three complexes 1,
2 and 3 was analyzed (Fig. 3A).
The percent conversion of styrene follows the order: 96. 5%
[Cr(Phimp)2](ClO4)>92.7% [Cr(Me-Phimp)2](ClO4)>86.0% [Cr(N-
Phimp)2](ClO4), where formations of styreneoxide are 2.7, 2.2 and
3.6%, respectively. The oxidation of styrene after 6 h of the reaction time
is illustrated in Table 2. Thus, turns over number (TON: moles of substrate
converted per mole of metal in the solid catalyst) of the three catalysts
are ~192.2 (for 1and 3) and 170.0 (for 2). However, in all cases, the se-
lectivity of various products follows the order: benzaldehyde>benzoic
acid>Phenylacetaldehyde>1-phenylethane- 1,2-diol>styreneoxide.
The highest formation of benzaldehyde is probably due to a nucleophilic
attack of H2O2 to styreneoxide. Benzaldehyde formation may also be fa-
cilitated by direct oxidative cleavage of the styrene side chain double
bond via radical mechanism. Other products, e.g. benzoic acid formation
200
B
Complex 1
Complex 2
160
120
80
40
0
Complex 3
% Conversion % EC
% Others
%TON
C
140
120
100
80
Complex 1
Complex 2
Complex 3
60
40
20
Table 3
0
Effect of time on the oxidation of cis-cyclooctene and the product selectivity.
%Conversion %TSO %Bzl
%BzA %BzAc %Others TON
Catalysta
Time (h)
Conversion%
Product
TON
Fig. 3. Bar diagram showing total conversion of (A) styrene, (B) cis-cyclooctene and
(C) trans-stilbene and selectivity of their various products in 6 h respectively.
selectivity%
ECb
Others
1
2
3
1
2
6
1
2
6
1
2
3
5.7
56.8
68.7
34.4
74.5
91.8
40.8
58.0
95.0
100
–
–
loss of 31.5% was observed up to 310 °C in the first two overlapping
steps followed by a weight loss of 68.3% in a sharp and narrow tem-
perature range of 310–420 °C. The total weight loss of complex 1 is
99.8%. Similarly, complexes 2 and 3 loss the maximum weight of
63.6% and 45.7% in the third, a sharp narrow step (between 360 °C
and 440 °C) and in the second step (between 257 and 267 °C) re-
spectively. Finally, the remaining loss of 4.6% for 2 and 12.4% for 3
is in the range of 400–770 °C. The initial decomposition and inflec-
tion temperatures have been used as an indication on the thermal
stability of complexes [41].
92.7
90.4
92.1
86.9
76.4
88.6
85.4
82.4
7.3
9.6
7.9
–
137.0
–
13.1
23.5
11.4
14.6
17.6
–
186.3
–
–
188.1
a
Reaction conditions: cis-cyclooctene (0.22 g, 2.0 mmol), catalyst (0.01 mmol), ox-
idant (0.453 g, 4.0 mmol), temperature (90 °C) and acetonitrile (2 mL).
b
Epoxycyclooctane.

K. Ghosh et al. / Inorganic Chemistry Communications 24 (2012) 81–86
85
through benzaldehyde is rather slow in all reactions. Water present in
H2O2 as well as formed during the reaction if any, is probably responsi-
ble for partial hydrolysis of styreneoxide to 1-phenylethane-1,2-diol.
Oxidation of cis-cyclooctene catalysed by complexes 1, 2 and 3 gave
epoxycyclooctane (Scheme 3B) [44] along with some unidentified prod-
ucts. Catalytic activity of all the three complexes 1, 2 and 3 was analyzed
at reaction conditions (i.e. cis-cyclooctene (0.220 g, 2.0 mmol), catalyst
(0.01 mmol) and 30% aqueous H2O2 (0.453 g, 4.0 mmol), acetonitrile
(2.0 mL), temperature (90 °C) and time (6 h)), (Fig. 3B). As 90 °C was
found an ideal temperature to run the homogeneous catalytic reaction,
achieve the maximum oxidation of cis-cyclooctene using complexes 1,
2 and 3 as a representative catalyst. The oxidation of cis-cyclooctene
after 6 h of the reaction time is illustrated in Table 3.
The maximum percent conversion of cis-Cyclooctene is in the
order of 95.0% for [Cr(Me-Phimp)2](ClO4)>91.8% for [Cr(N-
Phimp)2](ClO4)>68.7% for [Cr(Phimp)2](ClO4) where formations
of epoxycyclooctane are 82.4, 76.4 and 90.4%, respectively. Thus,
turns over number of the three catalysts are ~188.1 (for 2and 3) and
137.0 (for 1). However, in all cases, the selectivity of epoxycyclooctane
is appreciable.
of Cr(III) complexes already reported in the literature. The results of
the catalytic studies revealed that oxidation of various unfunctionalized
alkenes is achievable in the presence of either 1, 2 or 3 using with 30%
H2O2 as a precursor oxidant, though 2 is comparatively less efficient.
We examined the ligand centered oxidation of complexes 1–3 in rele-
vance with the active site modeling of GO and phenoxyl radical com-
plexes were generated in solution at 0 °C.
Acknowledgments
KG is thankful to IIT Roorkee for Faculty Initiation Grant Scheme B
for the financial support. PK is thankful to CSIR, India for financial as-
sistance. IG is thankful to MHRD, India for financial assistance. We are
thankful to Chanchal Haldar and Priyanka Saini for their help.
Appendix A. Supplementary material
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.inoche.2012.07.028.
Complexes were also tested for the oxidation of trans-stilbene
where trans-stilbeneoxide, benzaldehyde, benzoic acid and 1,2-
diphenylethanedione (benzil) were obtained as major oxidation
products (Scheme 3C) [45]. All reactions were carried out by the
above procedure which followed in the oxidation of styrene and
cis-cyclooctene. Complex complexes 1, 2 and 3 were used as a repre-
sentative catalyst.
Results presented in Table 4 shows that time of reaction plays an
important role in that increasing the oxidation of trans-stilbene. How-
ever, the selectivity of epoxide increases from ~0.4 to 23%. A consid-
erably lower formation of trans-stilbeneoxide is probably due to
further reaction of trans-stilbeneoxide with H2O2 to give benzyl
which in turn converted into benzaldehyde due to hydrolysis. The
percentage conversion of all three catalysts along with the selectivity
of oxidation products are presented as bar diagrams in Fig. 3C. Thus,
turns over number of the three catalysts are ~127.0 (for 1 and 2)
and 142.0 (for 3). However, unidentified products are further oxida-
tion products of benzil as indicated by GCMS, but these are only in
trace amounts and in the frame of the present study, no efforts
were undertaken to separate them.
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Table 4
Effect of time on the oxidation of trans-stilbene and the product selectivity.
Catalysta Time (h) Conversion % Product selectivity %
TSOb Bzlc BzAd BzAce Others
TON
1
2
3
1
2
6
1
2
6
1
2
3
14.6
15.3
64.0
19.5
40.7
62.9
16.2
34.8
71.3
0.4 12.8 83.7
13.0 11.8 56.2 10.7
–
3.1
8.3
3.8
9.8
2.5
2.1
5.3
5.6
3.2
–
–
22.9
2.6
3.7
3.9 33.3 36.2
8.9 78.7
4.0 88.3
127.6
–
–
1.5
–
19.0
2.7
8.5
3.4 48.7 26.8
9.5 81.0
7.6 75.1
124.0
–
1.6
3.3
–
17.0
3.3 58.3 18.2
141.8
a
Reaction conditions: trans-stilbene (0.364 g, 2.0 mmol), catalyst (0.01 mmol), ox-
idant (0.453 g, 4.0 mmol), temperature (90 °C) and acetonitrile (2 ml).
b
trans-stilbene oxide.
Benzil.
Benzaldehyde,
Benzoic acid.
c
d
e

86
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