Selective oxidation of benzyl alcohol to benzaldehyde, 1-phenylethanol to acetophenone and fluorene to fluorenol catalysed by iron (II) complexes supported by pincer-type ligands: Studies on rapid degradation of organic dyes

Article abstract of DOI:10.1002/aoc.4825

Hexacoordinated non-heme iron complexes [Fe^II(L¹)₂](ClO₄)₂ (1) and [Fe^II(L²)₂](PF₆)₂ (2) have been synthesized using ligands L¹?=?(E)-2-chloro-6-(2-(pyridin-2ylmethylene) hydrazinyl)pyridine and L²?=?(E)-2-chloro-6-(2-(1-(pyridin-2-yl)ethylidene)hydrazinyl) pyridine]. These complexes are highly active non-heme iron catalysts to catalyze the C (sp³)?H bonds of alkanes. These iron complexes have been characterized using ESI?MS analysis and molecular structures were determined by X-ray crystallography. ESI???MS analysis also helped to understand the generation of intermediate species like Fe^III?OOH and Fe^IV=O. DFT and TD?DFT calculations revealed that the oxidation reactions were performed through high-valent iron center and a probable reaction mechanism was proposed. These complexes were also utilized for the degradation of orange II and methylene blue dyes.

Full text of DOI:10.1002/aoc.4825

Received: 3 October 2018
DOI: 10.1002/aoc.4825
Revised: 19 November 2018
Accepted: 4 December 2018
F U L L P A P E R
Selective oxidation of benzyl alcohol to benzaldehyde,
1‐phenylethanol to acetophenone and fluorene to fluorenol
catalysed by iron (II) complexes supported by pincer‐type
ligands: Studies on rapid degradation of organic dyes
Ovender Singh | Priyanka Gupta | Anshu Singh | Ankur Maji | Udai P. Singh |
Kaushik Ghosh
Department of Chemistry, IIT Roorkee,
Hexacoordinated non‐heme iron complexes [FeII(L1)2](ClO4)2 (1) and
Roorkee 247667, Uttarakhand, India
[FeII(L2)2](PF6)2 (2) have been synthesized using ligands L1 = (E)‐2‐chloro‐6‐
Correspondence
Kaushik Ghosh, Department of
(2‐(pyridin‐2ylmethylene) hydrazinyl)pyridine and L2 = (E)‐2‐chloro‐6‐(2‐(1‐
(pyridin‐2‐yl)ethylidene)hydrazinyl) pyridine]. These complexes are highly
active non‐heme iron catalysts to catalyze the C (sp3)−H bonds of alkanes.
These iron complexes have been characterized using ESI−MS analysis and
molecular structures were determined by X‐ray crystallography. ESI − MS
analysis also helped to understand the generation of intermediate species like
FeIII−OOH and FeIV=O. DFT and TD−DFT calculations revealed that the oxi-
dation reactions were performed through high‐valent iron center and a proba-
ble reaction mechanism was proposed. These complexes were also utilized for
the degradation of orange II and methylene blue dyes.
Chemistry, IIT Roorkee, Roorkee ‐ 247667,
Uttarakhand, India.
Email: ghoshfcy@iitr.ernet.in
Funding information
Department of Science and Technology
(DST) New Delhi, and Science and Engi-
neering Research Board (DST‐SERB),
Grant/Award Number: SR/S1/IC‐47/2012;
MHRD
KEYWORDS
C‐H bond activation, iron oxo (FeIV=O) intermediate, organic dyes degradation eration gen, reactive
HO• radical
1 | INTRODUCTION
the efficiency and selectivity of the hydrocarbon oxidation
became the goal of industrial as well as academic
The oxygenation of non‐activated alkyl C‐H bonds is a
challenging area of research owing to their chemical
inertness towards chemical oxidation. Consequently,
highly reactive species, generated by a suitable catalyst,
are necessary to perform such chemistry.[1] In the chem-
ical industries, such catalytic oxidation and during the
oxidation process, desired selectivity of the organic mole-
cules produced, was extremely important.[2] It is well
known that alkanes are thermodynamically stable and
difficult to oxidize in a controlled and selective fashion.
Chromate and permanganate have already been used as
conventional oxidants however these oxidants are rather
inefficient and barely selective,[3] thus enhancement in
research. Nature has developed several metalloenzymes
(heme as well as non‐heme) which selectively catalyze
the oxidation of hydrocarbon.[4–7] Iron containing
metalloenzymes play an important role in these biologi-
cal processes. For example, cytochrome P450 selectively
oxidizes the aliphatic side chain of cholesterol, and the
same metalloprotein is involved in biosynthesis of the
female hormone progesterone.[8] Iron‐dependent cyto-
solic protein, soluble methane monooxygenase (sMMO)
activates C‐H bond and converts methane to methanol[9]
FeIII‐OOH species has been observed and was implicated
in the catalytic cycle of C‐H bond oxidation. Anticancer
drug bleomycin, naphthalene dioxygenase, and Rieske
Appl Organometal Chem. 2019;e4825.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4825

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SINGH ET AL.
oxygenase enzymes are also involved in such oxidation
processes.[10] The first non‐heme iron (IV)‐oxo complex
was characterized spectroscopically by Wieghardt and
co‐workers,[11] however, there has been considerable cur-
rent interest.[12] The present study stems from our inter-
est in such oxidation chemistry.[13] Stefano and co‐
workers recently reported oxidation of hydrocarbons by
using mononuclear iron complexes derived from
tridentate meridional ligands. In the mechanism of the
reaction hexacoordinated iron was found to be
pentacoordinated with the dissociation of one of the Fe‐
N bonds during the reaction with hydrogen peroxide
was reported and an intermediate iron complex having
FeIV=O moiety was generated.[14]
We have recently reported iron complex derived from
meridional tridentate ligand (LPh) having NNN donors
(Shown in Scheme 1).[15] Addition of H2O2 to this com-
plex did not provide us the indication of generation of sta-
ble hydroperoxo and oxo species in solution.
Investigation of literature revealed that Borovik and co‐
workers, Goldberg and co‐workers, and Fout and co‐
workers reported the stabilization of FeIV=O through
hydrogen bonding because the hydrogen atom present
in the ligand system showed proximity with the FeIV=O
species.[16] We have performed theoretical (DFT and
TD‐DFT) calculations for the complex derived from LPh
described in our previous report.[15] (Shown in Figure
S38) and we designed new ligands (L1 and L2 as shown
in Scheme 1) which possibly could stabilize hydroperoxo
and oxo species through hydrogen bonding during cata-
lytic activity. Costas and co‐workers described that some
iron complexes are capable of C‐H bond oxidation reac-
tion and selectively transform reactant(s) into the prod-
ucts.[17] On the other hand, the formation of high‐valent
iron hydroperoxo and oxo species could also be utilized
for the degradation of organic dye pollutants.[18,19]
Collins and co‐workers synthesized the iron complex
capable of generating high‐valent hydroperoxo and oxo
iron complexes and catalytic degradation of orange II
dye was investigated.[18] Hitomi and co‐workers used iron
(III) tetradentate monoamido complex as a non‐heme
iron‐based peroxidase mimetic for the catalytic oxidation
of organic dye.[20] With the view of above literature data,
herein, we have designed, synthesized and characterized
mononuclear iron (II) complexes [FeII(L1)2](ClO4)2 (1)
and [FeII(L2)2](PF6)2 (2) derived from a tridentate ligands
L1 and L2 (Shown in Scheme 1). Molecular structures of
[FeII(L1)2](ClO4)2 (1) and [FeIIL2 ](PF6)2, (2) were deter-
2
mined by X‐ray crystallography. Efficient and selective
oxidation of cyclohexane, benzyl alcohol, 1‐phenyl etha-
nol and fluorene in presence of catalytic amount
[FeII(L1)2](ClO4)2 (1) and [FeII(L2)2](PF6)2 (2) was investi-
gated. Complexes 1 and 2 were utilized for oxidative deg-
radation of orange II and methylene blue dyes. The
intermediate species FeIII‐OOH and FeIV=O were charac-
terized by UV–visible spectroscopy, ESI − MS and reso-
nance Raman analysis. A reaction model for oxidation
reaction and dye degradation will be scrutinized on the
basis of our observations in reactivity studies.
2 | RESULTS AND DISCUSSION
White solid of new tridentate ligands, (L1) and (L2) hav-
ing NNN type binding[21] have been synthesized by
reacting 2‐chloro‐6‐hydrazinylpyridine (1equivalent) with
2‐pyridinecarboxaldehyde
(1equivalent)
and
2‐
actetylpyridine respectively(1equivalent) followed by stir-
ring at room temperature. Ligand L1 reacted with Fe
(ClO4)2.xH2O in 2:1 equivalent (ligand and metal salt
ratio) and yielded red solid of [FeII(L1)2](ClO4)2 (1). For
complex 2, ligand L2 reacted with Fe (ClO4)2.xH2O in a
similar fashion as complex 1, after that NH4PF6 was
added to get the resultant product in good yield
(Scheme 1).
Complex 1 and 2 have been characterized by various
spectral and analytical techniques. Coordination of imine
N to iron metal center was indicated by the shifting of
azomethine (νC=N) stretching frequency from 1590 cm−1
in free ligand to 1605 cm−1. The characteristic peaks for
perchlorate anion were observed near 1090 and 625 cm
(b)
(a)
−1
in IR spectral studies in complex 1 (shown in
(c)
supporting information Figure S9). However we observed
the shifting of azomethine from 1599 cm−1 in free ligand
‐
to 1610 cm−1 in complex 2. The presence of PF6 was
expressed by the peak at 836 cm−1 (shown in Figure
S13). The UV–visible spectra of complexes were recorded
in acetonitrile solution. The absorption bands near 328
and 240 nm were assigned as n → π* and π → π*
SCHEME 1 Synthesis of complex 1 and complex 2 using
different substituted ligands

SINGH ET AL.
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transitions for ligand. Complex 1 showed two broad
bands near 460 and 503 nm due to ligand‐to‐metal charge
transfer transitions (LMCT) (shown in Figure S11). In the
case of complex 2, the peaks near 240, 272 and 324 nm
were assigned to intra‐ligand n → π* and π → π* transi-
tions. Complex 2 also showed a broad band near 454
and 509 nm due to ligand‐to‐metal charge transfer transi-
tion (LMCT) (Figure S14). Complexes 1 and 2 were found
to be low spin and provided 1H NMR spectra in
DMSO‐d6. As compared to the free ligands (L1 and L2),
the NMR signals were shifted towards high field after
complexation (Figure S12 and S15 for complexes 1 and
2). The ESI‐mass spectral study was performed for both
the complexes in acetonitrile solution. Complexes
[FeII(L1)2](ClO4)2 (1) and [FeII(L2)2](ClO4)2 (2) provided
a peak at m/z = 519.0317 (for [1‐2ClO4‐H+]+, and
m/z = 547.0655 (for [2‐2PF6‐H+]+, ion) respectively
(Figure S10 and Figure S16). The molecular structure of
the complex [FeII(L1)2](ClO4)2 (1) and [FeII(L2)2](PF6)2
(2) were determined by using X‐ray crystallographic stud-
ies (Table S7) and their ORTEP diagrams were displayed
in Figures 1. The coordination environment around the
metal center was described as hexacoordinated distorted
octahedral geometry. In complex [FeII(L1)2](ClO4)2 (1)
the tridentate ligand (L1) was coordinated to the Fe (II)
center in a meridional fashion via two imine nitrogen
(Nim) and four pyridine nitrogen (Npy) donors. The
Fe–Nim bond distances are 1.875 Å (Fe1—N3) and
1.912 Å Fe1—N7 and Fe–Npy distances have been found
to have slight differences 2.031 Å (Fe1—N8), 2.031 Å
(Fe1‐N5), 2.093 Å (Fe1‐N4), 2.122 Å (Fe1‐N1). A similar
pattern was observed in case of complex 2 (minor differ-
ences in Npy distances) Npy distances were found consis-
tent with the data reported in the literature,[15] however
the Nim distances were shorter in our case as compared
to the reported values. The bond parameters for com-
plexes 1 and 2 were quite similar to the data obtained
from theoretically optimized geometries of both the com-
plexes (shown in Table S8).
We have investigated the bond distances and bond
angles reported in the literature.[22] The selected bond
angles in complex 1 are N3—Fe1—N7 = 173.27°, N8—
Fe1—N5 = 160.92° and N4—Fe1—N1 = 160.99° and in
complex 2 are N3—Fe1—N7 = 172.81°, N8—Fe1—
N5 = 159.03° and N4—Fe1—N1 = 160.97°. These values
clearly indicated a distorted octahedral geometry in com-
plexes. The packing diagrams of complex 1 and 2 along
the ‘a’ axis exhibited the interaction between the counter
ions and complex cation (Figure S27 and S28). The elec-
trochemical properties of both the complexes 1 and 2
were investigated in dimethylsulfoxide. The E1/2 values
for complexes 1 and 2 were found to be 0.286 V and
0.123 V respectively. Complex 2 was oxidized at lower
potential due to electron donating methyl group present
at the azomethine bond that helps to oxidize the iron cen-
ter (FeII/FeIII) easily at lower potential. (As shown in
Figure 2 and Table S9).
FIGURE 2 Cyclic voltammograms of a 10‐3 M solution of
complex in dimethyl sulfoxide in presence of 0.1 M
tetrabutylammonium perchlorate (TBAP), using working electrode:
glassy‐carbon, reference electrode: Ag/AgCl; auxiliary electrode:
platinum wire, scan rate 0.1 Vs‐1
FIGURE 1 ORTEP diagram (50%
probability level) of complex 1 and
complex 2, All hydrogen atoms and
counter anions are omitted for clarity

4 of 12
SINGH ET AL.
2.1 | Catalytic oxidation of hydrocarbons
2.1.2 | Oxidation of benzyl alcohol
As a part of our ongoing research in the field of catalytic
oxidation of hydrocarbons,[13] we have synthesized and
characterized Fe (II) complexes which are capable of sta-
bilizing FeIV=O species in the presence of H2O2. This
FeIV=O species was probably responsible for the oxida-
tion of hydrocarbon to corresponding products. Hydrogen
bonding interaction between a ligand N‐H and oxygen of
FeIV=O species may be responsible for the stabilization of
iron‐oxo species.
Benzyl alcohol oxidation reaction was also performed
with both the complexes and we got the benzaldehyde
selectively with both the catalysts. The rate of conversion
with methyl substituted complex was found more as com-
pare to unsubstituted complex 1. (Shown in Table 3).
2.1.3 | Oxidation of 1‐phenyl ethanol
Secondary benzylic alcohol (1‐phenyl ethanol) oxidation
was also be carried out to its corresponding ketone is a
major concern in the oxidation chemistry. We have per-
formed the reaction with 1‐phenyl ethanol oxidation
using both the catalyst. Selectively we got acetophenone
with a good conversion rate. (Shown in Table 4) McGaf
and coworkers reported the oxidation of primary and sec-
ondary benzylic alcohols with hydrogen peroxide and
tert‐butyl hydroperoxide catalyzed with the good conver-
sion rate but with the complex 1 and complex 2 the con-
version rate was less.[24] (As shown in Table 5).
2.1.1 | Oxidation of cyclohexane
Oxidation of cyclohexane was performed using complexes
1 and 2 as catalysts. The percentage conversion and TON
has been shown in Table 1. On the basis of A/K ratio, it
was found that our complexes provided good catalytic
activity as compared to the results reported by Stefano
and coworkers[14] however less efficient than the catalysts
reported by Que and coworkers.[23] (As shown in Table 2).
TABLE 1 Oxidation of cyclohexanea
Complexes
Substrate
Substrate (mmol)
%Conversion of A and K
TONb
Product
1
2
1.42
1.42
70 and 18
55 and 34
50 (A), 13(K)
39(A), 24(K)
aAll reactions carried out in acetonitrile at 25 °C. The yields were measured by GC 24 hrs after the beginning of the reaction.
bTON defined as [TON = {(% conversion) × (mmol of substrate)/ (mmol of catalyst) × 100}].
TABLE 2 Comparative study with the literature for the oxidation of cyclohexane
Catalyst
A
K
Total
A/K
References
[14]
(L1)2Fe(OTf)2
(L2)2Fe(OTf)2
(L3)2Fe(OTf)2
(L4)2Fe(OTf)2
[(TPA)Fe (OTf)2]
Complex 1
14
14
9
1
10
1
1
1
24
28
23
21
3.2
63
63
1.40
1.00
0.64
0.50
5.00
3.84
1.62
[14]
[14]
[14]
[23]
1
14
14
0.5
1
14
‐
7.0 0.5
‐
50
39
1
1
13
24
1
1
This work
This work
Complex 2
TABLE 3 Oxidation of benzyl alcohola
Complexes
Substrate
Substrate (mmol)
% Conversion
TONb
Selectivity (%)
Product
1
2
1.10
1.10
85
88
47
48
100
100
aBenzyl alcohol oxidation reaction was carried out in acetonitrile at 25 °C.
bSee the footnote of Table 1.

SINGH ET AL.
5 of 12
TABLE 4 Oxidation of 1‐phenyl ethanol
Complexes
Substrate
Substrate (mmol)
% Conversion
TONa
Selectivity (%)
Product
1
2
0.60
0.60
35
95
11
28
100
100
aSee the footnote of Table 1.
TABLE 5 Comparative study with the literature for the oxidation of benzyl alcohol and 1‐phenylethanol
Catalyst
Substrate
Product
TON
References
[24]
Fe+3L
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
1‐phenyl ethanol
1‐phenyl ethanol
1‐phenyl ethanol
Benzaldehyde
Benzaldehyde
Benzaldehyde
Acetophenone
Acetophenone
Acetophenone
84
47
Complex 1
Complex 2
Fe+3L
This work
48
This work
[24]
230
11
Complex 1
Complex 2
This work
This work
28
Generation of FeIV=O intermediate can be further sup-
ported by UV–visible titration of [FeII(L1)2](ClO4)2 (1)
and [FeII(L2)2](PF6)2(2) with H2O2 at low temperature.
For the complex 1 the band at around 540 nm (ɛ
=1023 M−1 cm−1) in complex [FeII(L1)2](ClO4)2 (1)
decrease after addition of H2O2 (1.5 equivalent) and a
band around 662 nm (ɛ =484 M−1cm−1) appeared with
a fast color change from red to green at 0 °C (As shown in
Figure 3). ESI‐MS analysis of these complexes were done
with H2O2 as an oxidizing agent we got [FeIV(O)(L1)2]
(ClO4)2 (4) m/z = 535.0088 species in acetonitrile solvent.
(As shown in Figure 4).
2.1.4 | Oxidation of Fluorene
Fluorene oxidized to fluorenol in the presence of both the
catalysts. The reaction was done in acetonitrile solvent
system and hydrogen peroxide, selectively fluorenol was
obtained. The product was analyzed using GC‐Mass spec-
trometry analysis. The results obtained are shown in
Table 6.
2.2 | Characterization of reactive
intermediates
The EPR spectrum of complex 1 with 1.5 equivalent
H2O2 was recorded in acetonitrile solvent and we got
EPR signal at g = 2.01. The spectra at g = 2.01 show
one line EPR which is not similar as reported by Que
and coworkers[25] however, this spectra may be due to
the production of hydroxyl (HO•) radical species gener-
ated in solution. (Shown in Figure S21) The resonance
Raman spectra of both the complexes were recorded to
analyze the reactive intermediates on adding H2O2 in
both the complexes. The vibrational modes were generat-
ing from the stable iron oxo species due to the iron oxy-
gen stretching mode. In complex 1 it was 825 cm−1 and
for complex 2 at 831 cm−1. (As shown in Figure 5 and
Generation of reactive species after addition of H2O2 in
the acetonitrile solution of complex 1 and complex 2
was authenticated by the generation of FeIII‐OOH and
concomitant production of FeIV=O in solution. The addi-
tion of hydrogen peroxide molecule in complex 1 gener-
ated the active [FeIII‐OOH(L1)2](ClO4)2 (3) species
•
which undergo homolytic cleavage and OH radical spe-
cies and complex having FeIV=O moiety were produced.
Visible changes in the color of the complex [FeII(L1)2]
(ClO4)2 (1) (red) upon addition H2O2, the formation of
[FeIIIOOH(L1)2](ClO4)(3) light green and [FeIV(O)(L1)2]
(ClO4)2 (4) dark green. (As shown in the Figure S17)
TABLE 6 Oxidation of fluorenea
Complexes
Substrate
Substrate (mmol)
% Conversion
TONd
Selectivity (%)
Product
1
2
0.40
0.40
38
42
8
8
100
100
aSee the footnote of Table 1.

6 of 12
SINGH ET AL.
FIGURE 3 UV‐visible spectral changes
observed during the reaction [FeII(L1)2]
(ClO4)2 (1) (1.25 x 10‐4 M) with 1.5 equiv.
of H2O2 in acetonitrile at 0 °C
TABLE 7 Spectroscopic characterization data of FeIV=O
intermediates
λmax (nm),
νFe = O
Complexes
ε (M−1 cm−1) (cm−1) References
[27]
[FeIV(O)(TMC)(NCMe)]2+ 824 (400)
839
826
831
841
818
825
831
[28]
[29]
[30]
[31]
FeIV(O)(TMC‐Py)]2+
[FeIV(O)(TMCSO2)]+
[FeIV(O)(15cyclam)]2+
[FeIV(O)(TPEN)]2+
[FeIV(O)(L1)2]2+
834 (260)
830 (170)
750 (500)
730 (380)
662(484)
725(670)
This work
This work
[FeIV(O)(L2)2]2+
2.2.1 | DPPH (2,
2‐diphenyl‐1‐picrylhydrazine) radical
quenching assay
FIGURE 4 ESI‐MS spectra of complex 1 on adding H2O2,
recorded in acetonitrile for the formation of [4‐2ClO4‐H+]+
,
m/z=535.0088
This assay was performed for the detection of reactive
•OH radical formation. Hydroxyl radical was formed dur-
ing the homolytic cleavage of hydroperoxo species when
H2O2 was added in complex 1. (As shown in scheme 2).
The color of the DPPH solution was disappeared when
the complex 1 and H2O2 were added simultaneously in
the DPPH solution and electronic spectra of this radical
was recorded by UV–visible spectroscopy. In UV‐ visible,
the band at 520 nm was responsible for this radical and it
gradually decreased when it reacted with •OH radical. (As
shown in the Figure S30). Similar reaction was performed
with the complex 2. The concern spectrum was depicted
in the supporting information S31.
2.3 | Proposed mechanism for oxidation
FIGURE 5 Resonance Raman spectra of complex [FeII(L1)2]
(ClO4)2 (1) and on adding H2O2 formation of FeIV=O intermediate
The probable mechanism for the oxidation of hydrocar-
bons using complex 1 and complex 2 have been shown
in Scheme 2. By the addition of hydrogen peroxide one
nitrogen opened by removing the H+ and generates an
active species [Fe(L1)2OOH]2+ which was supported with
the visibly and UV–visible spectroscopy. The hydroperoxo
Figure S20) The data obtained from the UV–visible and
resonance was confirmed the formation of stable FeIV=O
species as compared with literature[26] and it was found
quite similar. The comparison data shown in Table 7.

SINGH ET AL.
7 of 12
species of both the complexes upon homolysis generate
•OH radical and formed a FeIV=O intermediates, as
shown in step “C”. Generation of FeIV=O intermediates
for both complexes further supported by UV–visible titra-
tion (As shown in Figure 3 and Figure S18) and ESI‐MS
spectrum (As shown in Figure 4 and S19). This FeIV=O
species further upon addition of substrate molecule gen-
eco‐environment.[33,34] Orange II, and methylene blue
(MB) are the typical pollutants, highly toxic and difficult
to degrade, have become one of the most serious global
environmental issues today.[35,36] Therefore, an effective
and economical technique needs to be developed to
reduce the concentration of these organic pollutants
before releasing the wastewater into the aquatic environ-
ment. The degradation experiment was carried out in ace-
tonitrile solvent and it was monitored using UV–visible
absorbance at different wavelength and GC–MS spectro-
photometer for corresponding dyes.
•
erate R'• species (step C). Further OH radical leads to
alcohol formation in step “D” and water elimination hap-
pen in step “E” followed by [Fe (Ln)2]2+ (n = 1, 2) species
formed. In this mechanism substrate oxidized in the pres-
ence of high‐valent oxo‐species.[17,32]
Investigation of catalytic oxidation of cyclohexane,
benzyl alcohol, 1‐ phenyl ethanol, and fluorene clearly
indicated that both the complexes selectively converted
benzyl alcohol, 1‐phenyl ethanol, and fluorene to benzal-
dehyde, acetophenone, and fluorenol. Since FeIV=O was
the key intermediate for the oxidation of alkane because
it abstract the hydrogen from the alkane and same time
reactive oxygen species (•OH) which was consumed in
oxidation reaction. From the literature[18] it was clear that
these reactive oxygen species utilize for the degradation
of different organic dyes.
2.4.1 | Degradation of orange II dye
This dye was degraded in the presence of both the com-
plexes, and degraded products were determined on the
bases of GC–MS analysis. (shown in Figure S39‐S42)
Since the high‐valent hydroperoxo and oxo species are
more stable in both the complexes so degradation mech-
anism was proposed on the bases of that intermediates.
(As shown in Figure 6 and scheme 3).
The degradation of orange II was found much faster
with complex 2 as compare to complex 1. This is probably
due to methyl group present on the azomethine bonds
which stabilize the metal center at higher oxidation state
which correspond to stabilize the intermediates such as
hydroperoxo and oxo which increase the oxidation
reaction.
2.4 | Dye degradation
Organic dyes are chemically stable and not biodegradable
in water, which makes them potentially harmful to the
SCHEME 2 Proposed reaction
mechanism for oxidation of hydrocarbons

8 of 12
SINGH ET AL.
FIGURE 6 Oxidative degradation of
orange II dye (D) (1x10‐3M). In the
presence of complex 1 (a) and complex 2
(b) (1x10‐6M ) with H2O2 (20 equivalent)
in acetonitrile
electron density plot over the molecule calculated using
B3LYP level. (As shown in Figure S24 and S26) A TD‐
DFT calculation was also performed for major transition
along with their orbital contribution shown in Table S3
and S6. From the orbital contribution it was found out
that HOMOs of complexes 1(Fe −77%) and 2(Fe −78%)
were mainly metal center. In the case of LUMOs we
found orbital contribution mainly due to ligand center
for complex 1(L − 86%) and 2(L − 88%). Both the com-
plexes were found redox active in nature which was sup-
ported by theoretical calculation during oxidation
electron was released from HOMOs it is mainly due to
Fe (II) to Fe (III) oxidation potential. For the complex 1
the transition occurs with the oscillator strength at differ-
ent wavelength, at 240 nm the oscillator strength was
found 0.0168 due to different transition like HOMO‐
8 → LUMO+1 (32%), HOMO‐2 → LUMO+7 (8.5%) at
328 nm with oscillator strength(0.0018) the major transi-
tion was HOMO‐2 → LUMO+9 (30%), at 457 nm with
oscillator strength (0.0058) the major transition HOMO
→LUMO (64.6%) and 503 nm with oscillator
strength(0.0002) HOMO‐3 → LUMO+6 (25%) was the
major transition. For the complex 2 the transition along
with oscillator strength was found to be H‐5 → LUMO
(70%) (0.007) at 270 nm, H‐1 → L + 2 (84%) (0.0107) at
307 nm, HOMO → LUMO (71%) (0.0048) at 449 nm, H‐
3 → L + 9 (23%) (0.0002) at 508 nm. The energy gaps
between HOMO and LUMO in both the complexes were
found almost similar, so both the complexes showed sim-
ilar kind of activities only the rate performance of the
complex 2 was found to be greater than the complex 1.
In HOMO the electron density existed over the metal cen-
ter and in LUMO the major electron density was found
over the ligand center so metal based oxidation reaction
should be more favorable in these types of complexes.
Theoretical calculations were employed to develop a
deeper understanding of the FeIV=O stabilization. We
have performed DFT calculation for the intermediate
FeIII‐OOH, FeIV=O, we have found out that FeIV=O
was more stable because of (FeIV=O•••H‐N) hydrogen
bonding (shown in the Figure S32 for complex 1 and
SCHEME 3 Degraded products of orange II dye
2.4.2 | Degradation of methylene blue
Oxidative degradation of methylene blue was also done in
the presence of both the complexes. It was found out that
the degradation in the presence of complex 2 happened
faster than complex 1. Same condition of hydroperoxo
and oxo were responsible for the faster degradation of
methylene blue dye. The oxidative degraded products
were analysed and determined with the help of UV–
visible (As shown in Figure S29 and scheme S2) and
GC–MS analysis. (shown in Figure S43‐S45).
2.5 | Theoretical calculations
The geometry optimizations for both the complexes were
performed at DFT level (Figure S22 and S25).[37] Theoret-
ical experiments for both the complexes have been car-
ried out using the density functional theory (DFT)
employing the LANL2DZ basis set. The molecular orbital
diagram and significant contribution of iron and ligand in
HOMOs and LUMOs have been shown in Figure S36.
The percentage contribution of particular atom in the
complex also calculated shown in Table S2 and S5. Total

SINGH ET AL.
9 of 12
Figure S33 for complex 2). The Gibbs free energy (ΔG) for
the complex 1, 2, FeIII‐OOH (3) and FeIV=O(4) were cal-
culated. The relative ΔG value for FeIV=O species was
found more negative, it indicated the stabilization
FeIV=O reactive intermediate as compared to the other
species. With the view of this indication of ΔG value for
FeIV=O stabilization helps in the oxidation of hydrocar-
bons (shown in Figure 7 for complex 1 and Figure S34
for complex 2).
Mulliken spin density calculations were also done for
both intermediates. It was found out that maximum spin
density over the metal center, for that metal based oxida-
tion reaction involved such type of complexes. The Mulli-
ken spin density of both the intermediates shown in
Figure 8 for complex 1 and Figure S35 for complex 2.
The electronic configuration of [FeIV(O)(L1)2](ClO4)2
(4) was calculated using DFT and displayed in Figure S37.
3 | CONCLUSIONS
Following are the conclusions of the present studies.
First, inspired by literature reports[14,17] we have designed
new ligands and synthesized iron complexes. Second, the-
oretical data clearly indicated the stabilization of
hydroperoxo and oxo intermediates through hydrogen
bonding. Third, complexes were characterized by differ-
ent spectroscopic methods and redox properties were
examined. Molecular structures of complexes 1 and 2
were determined by X‐ray crystallography. Fourth, these
complexes were found to be efficient as mild oxidizing
agents during catalytic activity. Complex 1 and complex
2 selectively converted benzyl alcohol to benzaldehyde,
fluorene to fluorenol, 1‐phenylethanol to acetophenone.
Fifth, UV–vis. ESI‐MS and resonance Raman data clearly
authenticated the formation of hydroperoxo and oxo
intermediates. Sixth, these complexes were found to be
efficient in the degradation of orange II and methylene
blue dyes through oxidation chemistry. The probable
mechanism also supported the formation of high‐valent
iron hydroperoxo and oxo intermediates. Details of these
works and studies on extensive oxidation chemistry are
under progress.
4 | EXPERIMENTAL SECTION
Materials all the solvents used were reagent grade. 2‐
chloro‐6‐hydrazinylpyridine was prepared by the reported
procedure.[38] Pyridine 2‐aldehyde was purchased from
Acros organics, USA. Solvents used for spectroscopic
studies were HPLC grade and purified by standard proce-
dure before use.
FIGURE 7 Profile of the calculated relative ΔG for the reaction
of hydrogen peroxide and complex 1, the formation of
4.1 | Physical measurements
•
hydroperoxo species and by removal of OH radical stabilization of
Elemental analyses were carried micro analytically at Ele-
mental Vario EL III. Infra red spectra were obtained as
KBr pellets with Thermo Nikolet Nexus FT‐IR spectrom-
eter, using 16 scans and were reported in cm−1. Elec-
tronic absorption spectra were recorded with an UV/Vis
FeIV=O intermediate. The relative Gibbs free energies are given in
kcal mol−1
spectrophotometer,
Shimadzu
(UV‐2401,).
A
PerkinElmer Clarus
680‐ClarusSQ8T
Gas
chromatograph‐Mass spectrometer was used to record
the GC–MS spectra. Cyclic voltammetry measurements
were carried out using a CH‐600 electroanalyzer. A con-
ventional three‐electrode arrangement was using
consisting a platinum wire as auxiliary electrode, glassy
carbon as working electrode and the Ag(s)/AgCl as refer-
ence electrode. These measurements were performed in
the presence of 0.1 M tetrabutylammonium perchlorate
(TBAP) as the supporting electrolyte, using complex
FIGURE 8 Spin density plots for [FeIIIOOH(L1)2](ClO4)(3) and
[FeIV(O)(L1)2](ClO4)2 (4) intermediates (isosurface cutoff value=
0.002)

10 of 12
SINGH ET AL.
concentration
10−3
M
in
acetonitrile.
The
cm−1): 1605, νC=Nimine, 1090, 627, ν ClO4‐, UV–visible
ferrocene/ferrocenium couple was found at E1/2 = +0.40
scan rate 0.1 V/s, with respect to Ag/AgCl under the same
experimental conditions. Most of the experiments were
performed at room temperature and solutions were thor-
oughly degassed with nitrogen prior to beginning the
experiments. The X‐ray data collection and processing
for the complexes were performed on Bruker Kappa
Apex‐II CCD diffractometer by using graphite
monochromated Mo‐Kα radiation (λ = 0.71070 Å) at
296 K. Crystal structures were solved by direct methods.
Structure solution, refinement and data output were car-
ried out with the SHELXL program.[39,40] All non‐
hydrogen atoms were refined anisotropically. Hydrogen
atoms were placed in geometrically calculated positions
and refined using a riding model. Images were created
with the DIAMOND program.[41] (ESI‐MS) experiments
were performed on a Brüker micrOTOFTM‐Q‐II mass
spectrometer. EPR of the reactive intermediate on adding
H2O2 in complex 1 was performed on 9.5 GHz JEOL spec-
trometer operated at X‐band frequency. Resonance
Raman for both the complexes on adding H2O2 was ana-
lyzed on Renishaw inVia Raman spectrometer (serial no.
021R88) using a diode laser (785 nm).
[CH3CN
(31032), 457(8260), 503 (5891). ESI‐MS (CH3CN):
m/z = 519.0317 for [Fe (L1)2]2+
λ
max/nm (ε/M−1cm−1)]: 240 (24902), 328
.
4.3.2 | Synthesis of complex [FeII(L2)2]
(PF6)2 (2)
The ligand (L2) (2 mmol) was used to synthesize complex 2
following the same procedure as for complex 1 in THF.
Moreover, a solution of NH4PF6 (0.2 mmol) with methanol
was added to the reaction mixture in this case to replace
the counter anion (ClO4−). Single crystals of red color were
grown by the layering of methanol onto dichloromethane
solution of complex 2. Yield 82%. Anal. Calcd for
C24H22Cl2F12FeN8P2 (839.15) C, 34.35; H, 2.64; N, 13.35.
1
Found: C, 34.03; H, 2.60; N, 13.03. H NMR (400 MHz,
DMSO‐d6): 10.40(s, 2H); 8.58–8.56(d, 3H); 8.15–8.13(d,2H);
7.87–7.85(t, 3H); 7.73–7.70(t, 3H); 7.39–7.34(m, 4H);
6.90–6.88(d, 2H) 3.16(s, 6H). IR data (KBr, νmax/cm−1):
1610 νC=Nimine,1455, 1298, 1122, 836 ν =
−, 538. UV–
PF6
visible [CH3CN λmax/nm (ε/M−1cm−1)]: 240 (31,925), 272
(22,915), 324 (56,250), 455 (7,925), 509 (8,225). ESI‐MS
(CH3CN): m/z = 547.0655 for [Fe (L2)2]2+.
4.2 | Synthesis of ligand
4.4 | Catalytic oxidation procedure
2‐chloro‐6‐hydrazinylpyridine has been synthesized fol-
lowing the procedure reported in literature.[38] Details of
their synthesis are described in the supporting
information.
In the of oxidation of cyclohexane, benzyl alcohol, 1‐
phenyl ethanol and fluorene catalyst were taken
0.02 mmol for complex 1 and 2, and 1 ml H2O2 was added
at 25 °C temperature after that the reaction was stirred
for 24 hours for all experiments. The solution was eluted
with ethyl acetate through Al2O3 and analyzed by GC.
4.3 | Synthesis of metal complexes
4.3.1 | Synthesis of complex [FeII(L1)2]
(ClO4)2 (1)
ACKNOWLEDGEMENTS
K.G. is thankful to Department of Science and Technol-
ogy (DST) New Delhi, and Science and Engineering
Research Board (DST‐SERB), New Delhi of India for
financial assistance (no. SR/S1/IC‐47/2012) dated 13
May 2014. OS, PG, AM and AS are thankful to MHRD
for financial support.
A batch of ligand L1 (0.2 mmol) was dissolved into 3 ml of
tetrahydrofuran. A solution of Fe (ClO4)2.xH2O (0.1 mmol)
with 2 ml of THF was added dropwise to the above
solution. The resulting red solution was stirred for
15 mins; a red color precipitate was obtained. The solid
was filtered out and washed with diethyl ether. The
crystallization of the complex 1 was done by dissolving
the complex 1 into dichloromethane in a vial and
layered with methanol, red color crystals were obtained
within two days. Yield 74%. Anal. Calcd for
C22H18Cl4FeN8O8 (720.084): C, 36.70; H, 2.52; N, 15.56.
ORCID
Kaushik Ghosh https://orcid.org/0000-0002-3792-2848
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DMSO‐d6): 11.45 (s, 2H), 8.47 (d, 2H), 7.96 (s, 2H),
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Additional supporting information may be found online
in the Supporting Information section at the end of the
article.
How to cite this article: Singh O, Gupta P, Singh
A, Maji A, Singh UP, Ghosh K. Selective oxidation
of benzyl alcohol to benzaldehyde, 1‐phenylethanol
to acetophenone and fluorene to fluorenol
catalysed by iron (II) complexes supported by
pincer‐type ligands: Studies on rapid degradation of
organic dyes. Appl Organometal Chem. 2019;e4825.
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