Self pH regulated iron(II) catalyst for radical free oxidation of benzyl alcohols

Article abstract of DOI:10.1016/j.apcata.2019.117292

Selective oxidation of various aromatic benzyl alcohols to benzaldehydes was found to be catalyzed with 90% conversion and 99% selectivity by an iron (II) catalyst herein designated as Fe-DDPA [DDPA = 3′-disulfanediyldipropionic acid]. The Fe-DDPA catalyst was prepared by a small loading of FeCl₂ into a 2D sheet formed by the supramolecular assembling of DDPA. From both solid and liquid state nuclear magnetic resonance (NMR) spectroscopic study it was evident for the stabilization of the Fe(II) center through Fe-S interaction with the disulfide (S-S) unit of DDPA. DDPA was found to serve as an excellent support to maintain a pH that was required for a radical free oxidation of benzyl alcohol to aldehydes. The catalytic oxidation of benzyl alcohols was found to occur with excellent conversion and selectivity in acetonitrile (CH₃CN) solvent in comparison to various other solvents. From various spectroscopic studies viz UV-vis, FT-IR and ESI-MS it was ascertained that the CH₃CN interacted with Fe-DDPA to form a [(DDPA)₂Fe(CH₃CN)₂]²⁺ species which then reacted with H₂O₂ to form an intermediate Fe-hydroxoperoxo, Fe^III-OOH species. The Fe-OOH further got oxidized to the active Fe^IV=O species and was responsible for bringing the high selectivity in the oxidation reaction. The generation of highly unstable Fe-OOH species was further confirmed by electrochemical study, UV–vis, Raman and ESR spectroscopic analysis.

Full text of DOI:10.1016/j.apcata.2019.117292

Journal Pre-proof
Self pH Regulated Iron(II) Catalyst for Radical Free Oxidation of Benzyl
Alcohols
Biraj Das, Manash J. Baruah, Mukesh Sharma, Bipul Sarma, Galla
V. Karunakar, Lanka Satyanarayana, Subhasish Roy, Pradip K.
Bhattacharyya, Kamala Kanta Borah, Kusum K. Bania
PII:
S0926-860X(19)30447-8
DOI:
https://doi.org/10.1016/j.apcata.2019.117292
APCATA 117292
Reference:
To appear in:
Applied Catalysis A, General
Received Date:
Revised Date:
Accepted Date:
15 August 2019
18 September 2019
6 October 2019
Please cite this article as: Das B, Baruah MJ, Sharma M, Sarma B, Karunakar GV,
Satyanarayana L, Roy S, Bhattacharyya PK, Borah KK, Bania KK, Self pH Regulated Iron(II)
Catalyst for Radical Free Oxidation of Benzyl Alcohols, Applied Catalysis A, General (2019),
doi: https://doi.org/10.1016/j.apcata.2019.117292
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Self pH Regulated Iron(II) Catalyst for Radical Free Oxidation of Benzyl Alcohols
a
a
a
a
b
Biraj Das, Manash J. Baruah, Mukesh Sharma, Bipul Sarma, Galla V. Karunakar, Lanka
c
d,e
f
g
Satyanarayana, Subhasish Roy, Pradip K. Bhattacharyya, Kamala Kanta Borah , Kusum K.
a,*
Bania
a
Department of Chemical Sciences, Tezpur University, Assam, 784028, India.
Division of Fluro and Agro Chemicals, CSIR- Indian Institute of Chemical Technology, Hyderabad,
b
Telangana 500007, India.
c
Center for NMR and Structural Chemistry, Indian Institute of Chemical Technology, Uppal Road,
Tarnaka, Hyderabad, Telangana 500007, India.
d
Department of Chemistry, Indian Institute of Technology, Guwahati 791039, India.
e
Department of Chemistry, School of Applied Sciences, University of Science and Technology,
Meghalaya, Techno City, Kling Road, Baridua 9th Mile, Ri-Bhoi, Meghalaya, India 793101
f
Department of Chemistry, Arya Vidyapeeth College, Guwahati, Assam, India, 781016
Department of Chemistry, Mangaldai College, Guwahati, Assam, India, 784125
g
*Corresponding Author
kusum@tezu.ernet.in or bania.kusum8@gmail.com
Graphical abstract
Selective Oxidation of benzyl alcohol with Fe-DDPA using H
2
2
O as oxidant
1

Highlights





Formation of non-heme iron (II) catalyst through Fe-S interaction.
Square pyramidal Fe(II)-DDPA complex formation.
High activity of Fe-DDPA in benzyl alcohol oxidation.
Mechanistic study through UV-vis, Raman and ESR study.
Non-free radical benzyl alcohol oxidation under non-Fenton like condition.
Abstract
Selective oxidation of various aromatic benzyl alcohols to benzaldehydes was found to be catalyzed
with 90% conversion and 99% selectivity by an iron (II) catalysts herein designated as Fe-DDPA
[
DDPA= 3-disulfanediyldipropionic acid]. The Fe-DDPA catalyst was prepared by a small loading
of FeCl into a 2D sheet formed by the supramolecular assembling of DDPA. From both solid and
2
liquid state nuclear magnetic resonance (NMR) spectroscopic study it was evident for the
stabilization of the Fe(II) center through Fe-S interaction with the disulfide (S-S) unit of DDPA.
DDPA was found to serve as an excellent support to maintain a pH that was required for a radical
free oxidation of benzyl alcohol to aldehydes. The catalytic oxidation of benzyl alcohols was found
to occur with excellent conversion and selectivity in acetonitrile (CH CN) solvent in comparison to
3
various other solvents. From various spectroscopic studies viz UV-Vis, FT-IR and ESI-MS it was
]2+ species
ascertained that the CH
3
CN interacted with Fe-DDPA to form a [(DDPA)
2
Fe(CH
3
CN)
2
III
which then reacted with H
2
O
2
to form an intermediate Fe-hydroxoperoxo, Fe -OOH species. The Fe-
IV
OOH further got oxidized to the active Fe =O species and was responsible for bringing the high
selectivity in the oxidation reaction. The generation of highly unstable Fe-OOH species was further
confirmed by electrochemical study, UV-Vis, Raman and ESR spectroscopic analysis.
2

Keywords: Fe-DDPA, Hydrogen Peroxide, Benzyl Alcohol Oxidation, Non-Free Radical
Mechanism
1
. Introduction
Iron-sulfur clusters or such high potential iron proteins (like ferredoxins, rubredoxins) are one
of the important classes of metalloenzymes playing a pivotal role in various electron transfer
processes [1]. Their ability to access variable oxidation states under different biological conditions
makes them suitable redox-active centers [2]. Because of which they are known to participate in
different electron-transfer processes and are also involved in various enzymes like carbon monoxide
dehydrogenase, hydrogenases, nitrogenases, etc. activating the small molecules like carbon monoxide
(
CO), hydrogen (H
2
) and nitrogen (N ) [3, 4 5, 6]. Inspired by the switchable redox behavior of such
2
metal clusters, different iron-sulfur model systems have been developed to mimic the structural,
electronic, electron-transfer mechanism and catalytic behaviors [7]. In this regard, thiols and their
derivatives have been extensively used to synthesize different iron-sulfur complexes with varied
structural diversity and reactivity. For example, Wang et al. [8] reported for synthesis of disulfide
bridged co-ordinated dinuclear Fe(II) species and its interconversion to Fe(III) thiolate mononuclear
complex in different solvents. Lippard and his co-worker synthesize tetrathiolato Iron(II) complexes
for nitrous oxide (NO)-binding [9]. Chen et al. [10] prepared tetranuclear iron complex with
benzenetetrathiolate for hydrogen production by proton reduction process. Lee et al. [11] also
synthesized iron-thiolate complex for NO binding. The reported Fe-S clusters are mostly been
studied either for NO binding, nitrogen fixation reaction, dehydrogenation reaction and to understand
the electronic properties [9,11]. Catalytic oxidation of organic substrates like benzyl alcohol or the
application of such iron-sulfur complexes in other organic transformation reaction is less known in
literature [9-11]. The one best known is the benzoin oxidation to benzil by Sun et al. [12] using iron
3

(
II) thiolate complex as catalyst. Subramanian et al. Fe(II)-thioglycolate system for complete
oxidation of organic contaminants in water [13].
Selective oxidation of benzyl alcohol to benzaldehyde is one of the important reactions due to
the high applicability of benzaldehydes in pharmaceutical products [14, 15 16]. But to the best of our
knowledge iron-sulfur complexes has so far been not known to catalyze such reactions. The main
problem with Fenton-like catalyst is the free radical generation and chain propagation reaction that
hampers the selectivity by producing various side products [17]. Another problem is the regulation of
pH that could suppress the free radical generation. Shi et al. [18] has demonstrated that at low pH
(~1) the reactivity of Fe(III)-catalyst could be controlled for selective oxidation of BA with H
2 2
O .
Due to the several disadvantages associated with Fenton-like catalyst for selective oxidation
reactions, the interest has been driven towards the design of non-heme iron-catalysts [19, 20 21].
Non-heme iron complexes having axially co-ordinated labile groups like water (H
2
O), triflate (OTf),
acetonitrile (CH CN) are often preferred as they avoid the free radical mechanism [22, 23 24]. The
3
IV
catalytic reactions with such complexes were supposed to proceed via catalytically active Fe =O or
V
Fe (O)OH species giving highly selective oxidized product [22-24]. However, most of these reported
non-heme iron complexes bear the N-co-ordinations sites [22-24]. The applicability of non-heme
iron-sulfur complexes having axially co-ordinated labile groups has so far been not explored for
similar kinds of reactions. Therefore, intrigued by the redox activity and the catalytic ability of iron-
sulfur complexes herein we tried to develop a new iron-sulfur catalyst having an axially co-ordinated
labile group and can have control over the pH under Fenton-like conditions.
2
2
. Experimental Sections
.1. Materials. Anhydrous iron chloride (FeCl ) was purchased from E-MERCK. 3-
3
marcaptopropanoic acid (C
3
H
6
2
O S, 3-MPA) was brought from Sigma-Aldrich. Sodium hydroxide
(NaOH) and potassium chloride (KCl) were received from E-MERCK. HPLC grade acetonitrile
4

(
CH
3
CN) and all the solvent used were purchased from E-Merck. Benzyl alcohol and the other
substituted relevant alcohols were used as received from Sigma-Aldrich. Hydrogen peroxide (H
2
2
O )
and tetrabutylammonium bromide (TBAB) were purchased from E-MERCK. Milli-Q water was used
in all the reaction mention in the text.
2
.2. Preparation of 3, 3'-disulfanediyldipropionic acid, DDPA. The compound was prepared by
treating 1 mL of 3-MPA with NaOH in 5 mL of water at room temperature and the reaction mixture
was stirred for 6 h in open air, Scheme 1. The white precipitate was filtered, dried under air and
finally crystallized in CH
3
CN for further characterization. The same white material was obtained
(3-4 mL) in 10 mL of water.
within 10-15 min by reacting 3-MPA directly with 30% of H
2
O
2
Scheme 1. Schematic representation of the formation of DDPA from 3-MPA.
2
.3. Synthesis of iron-loaded DDPA, Fe-DDPA. An aqueous solution of NaOH was added to 1 mL
of 3-MPA taken in a 20 mL round bottom flask by maintaining the pH of the reaction mixture in the
range of 4-5. The aliquot of FeCl prepared by dissolving 1 g of anhydrous FeCl in 10 mL distilled
3
3
water was added slowly (100 L at an instant) to the above solution. The resultant white precipitate
so obtained was filtered, washed with water and dried in an open atmosphere. The material was
vacuum dried before characterization. From CHN analysis the %C and %H content was found to be
3
4.19 and 4.52, respectively. The material was white, sparingly soluble in water (H
acetonitrile (CH CN) and maintains a pH level of 3.8-3.9 in H O and 3.3-3.4 in CH CN.
.4. Catalytic Oxidation of Benzyl Alcohols with Hydrogen Peroxide as Oxidant. In a general
process, the benzyl alcohol oxidation reaction was performed by stirring a reaction mixture
containing 10 mL of CH CN, 1 mmol of BA and 3 mmol of H . The reaction was performed at
2
O) and
3
2
3
2
3
2
O
2
5

room temperature under air for 3h. The progress of the reaction was monitored by thin-layer
chromatography, TLC. The benzaldehydes were isolated in good yield using column chromatography
1
13
and the products were characterized using H and C NMR spectroscopy. The % conversion and
selectivity were obtained by GC-MS analysis.
3
3
. Results and discussion
.1. Characterization of DDPA and Fe-DDPA
Both the materials, DDPA and Fe-DDPA were characterized using different spectroscopic
and physicochemical techniques. The structure of DDPA was confirmed from X-ray crystallographic
analysis and indicated for the oxidation of 3-MPA forming the disulfide (S-S) bond. The structure of
the compound was found to be similar to that reported by Prout et al. [25] The S-S bond distance was
found to be 2.03 Å and C-S-S bond angle was 104.4. The crystal packing of the molecule (Fig. 1a
and Fig. 1b), represented a zigzag ribbon-like structure by interlinking with the neighboring unit
through the carboxylate unit of DDPA via intermolecular facial C-O---H interaction (1.82 Å) that
usually appears in carboxylic acids (Fig. 1a) [26]. In addition, the parallel structural motifs were
found to be interconnected by weak C-H---O and C-H---S interaction forming a 2D sheet-like
structure. The O-H and S-H bond distances in these two weak intermolecular interactions were in the
range of 2.71 and 3.16 Å, respectively. The presence of H-bonding interaction between the
methylene proton and oxygen atom of the carboxylic groups leads to the creation of voids similar to
that of the crown ethers (Fig. 1c). The ORTEPs of 3,3'-disulfanediyldipropionic acid with 50%
probability ellipsoid is shown in Fig. S1 and the details of the crystallographic parameters are
provided in Table S1 as supporting information.
The crystals of iron incorporated Fe-DDPA was found to be fragile and hence was unable to
obtain the crystal structure. This was also evident from the thermogravimetric analysis (TGA)
showing ~90% weight loss of Fe-DDPA in comparison to ~60% of the same in case of DDPA at
6

similar temperature ~250 C (Fig. S2a). Polarizing microscopic images further revealed for the fact
that the crystals of DDPA were flat, glassy and harder than the Fe-DDPA (Fig. S2b-S2e). The
Scanning Electron Microscopy (SEM) images of DDPA (Fig. S2f) and Fe-DDPA (Fig. S2g) were
almost identical suggesting the absence of unwanted species like iron-oxides or hydroxides over the
surface of DDPA. The low thermal stability of Fe-DDPA in comparison to DDPA was attributed to
the presence of iron (Fe) interfering the weak intermolecular interactions like C-H---S and C-H---O
in DDPA [27]. Howie et al. [28] also reported for the reduction of hydrogen bonding interactions on
+
+
the introduction of cations like Cs , Na into the supramolecular aggregation of dithiocarbamate.
II
Probably in the present case the sulfur (S) atoms were involved in the Fe ---S interactions and
thereby affected the original crystal packing of DPPA.
Fig. 1. The structure of DDPA showing a) 2D-sheet like arrangement, b) zig-zag ribbon-like
motif and c) creation of voids via weak intermolecular interactions.
Powder X-ray diffraction pattern of Fe-DDPA (Fig S3a, black line) was found to be slightly
shifted from that of DDPA (Fig S3a, red line) implying the interaction of Fe with DDPA (Fig. S3a).
7

No additional peak was observed for pure FeCl
FeCl on DDPA. The absence of sharp signal especially at 2 values of 32.14 and 35.76 further
confirmed for the non-existence of iron-oxide materials like Fe and Fe [29]. The crystallinity
2
signifying the absence of surface adsorbed or bulk
2
2
O
3
3
O
4
of the Fe-DDPA material was found to retain up to 120 °C without any loss of characteristic signal
and was evident by comparing the XRD pattern of the same before, (Fig. S3b, black line) and after
heating, (Fig. S3b, blue line). Fe-DDPA however on heating above 150 °C transformed to amorphous
black material as confirmed from XRD analysis (Fig. S4) [29]. These results were in accordance with
our TGA analysis.
II
13
The presence of Fe ---S interaction was evident from solid-state C-NMR spectrum or cross-
1
3
polarization magic-angle spinning nuclear magnetic resonance ( CPMAS-NMR) spectroscopy.
13CPMAS-NMR of DDPA shown in Fig. 2a, black line exhibited three sharp signals at 181.4, 44 and
2
9.2 ppm corresponding to –COOH and two –CH
attached to carboxylate carbon and the remaining one as–CH
bonded to S-atom of disulfide, respectively. The same in Fe-DDPA was observed at 180.1, 25.0 and
0.7 ppm (Fig. 2a, red line). The upfield shifting of –CH -C carbon by 19 ppm and downfield shift –
CH -S carbon by 1.5 ppm clearly suggested for the symmetrical interaction of disulfide group with
2
carbon, here represented as CH
2
-C (44 ppm)
2
-S (29.2 ppm) for carbon directly
3
2
2
Fe [30]. This was probably possible due to the presence of interstices spaces (see Fig. 1c) caused by
the supramolecular assembling of DDPA. The interaction was considered similar to that recently
reported by Wang et al [8]. The lowering of thermal stability was thus attributed to the weakening of
the C-S bond caused by the interaction between S and Fe. Moreover, the DDPA molecules might
have organized themselves to hold the Fe-atom resulting in a less stable and distorted conformation.
The peak at ~100 ppm in both the spectra was due to spinning side bands (SSB). As there was only
minimal change in the chemical shift value of –COOH so this functional group was not considered to
1
3
interact with Fe. The solution NMR analysis also suggested for similar interactions. The C-NMR of
8

DDPA was recorded in DMSO-d
6
while the same for Fe-DDPA was recorded in DMSO-d , methanol
6
(
CD
3
OD) and acetonitrile (CD
3
CN) (Fig. 2b-2f). As observed in 13CPMAS, the two methylene
1
3
carbon in DDPA recorded in DMSO-d
6
gave two highly separated C-NMR signal at 47.9 ppm and
2
3
6.9 ppm for –CH
4.1 and 33.6 ppm (Fig. 2b). The C-NMR signal of–CH
CN and at 34.8 and 34.5 ppm in CD
-C carbon and downfield shifting of–CH
2
-C and –CH
2
-S, respectively (Fig. 2b). The same in Fe-DDPA was observed at
1
3
2
-C and-CH
2
-S carbon of Fe-DDPA was
OD (Fig. 2c).
-S carbon
found at 34.2 and 30.9 ppm, respectively in CD
3
3
The lowering of chemical shift value for –CH
2
2
clearly suggested for the retention of similar interaction in solution as well in solid-state. The proton
1H-NMR of Fe-DDPA was also recorded in the above-mentioned solvents (Fig. 2d-2f). As expected,
the proton signals of the –CH
due to SFe charge transfer transition in comparison to DDPA recorded in DMSO-d
CD CN and CD OD. The chemical shift () values obtained from both solid and solution phase NMR
analysis are provided in Table 1 and also compared with MPA. The H-NMR and C-NMR spectra
of MPA recorded in CD CN are provided in supporting information Fig. S5-S6. The comparison of
2
- proton attached to S-atom was observed at lower chemical shift value
6
and also in
3
3
1
13
3
the NMR spectrum of MPA with DDPA are shown in Fig. S7-S8. The absence of S-H proton signal
in DDPA and the difference in chemical shift value was a clear indication for the dimerization of
MPA through S-S bond formation. It can be seen from Table 1 that the peak separation values ()
between CH
2
-C and CH -S was significantly low in Fe-DDPA compared to DDPA. This implied the
2
existence of strong interaction between the S atom of disulfide bond and Fe. The NMR- spectra of
DDPA and Fe-DDPA with proper integration are given in SI, Fig. (S9-S16).
9

1
3
13
Fig. 2. a) C PMAS spectra NMR of Fe-DPPA (red line) and DDPA (black line), b) C NMR
1
3
spectra Fe-DPPA (red line) and DDPA (black line) recorded in DMSO-d , c) C NMR spectra Fe-
6
1
DPPA recorded in CD
3
OH (blue line) and CD
red line) and DDPA (black line) recorded in DMSO-d
red line) and CD CN(black line), f) shows the peak separation in proton signals recorded in different
, red, black and blue are those for Fe-DDPA in CD OD,
CN and DMSO-d6. The broad signals are from solvent water.
3
CN (brown line) and d) H NMR spectra Fe-DPPA
1
(
(
6
, e) H NMR spectra Fe-DPPA in CD OD
3
3
solvent, pink is for DDPA in DMSO-d
CD
6
3
3
1
0

1
3
13
Table 1. Chemical shift values of DDPA and Fe-DDPA from CPMAS and solution phase C and
H NMR in different solvent.
1
13C  in ppm
Phase
Compound Solid/liquid Solvent
COOH -CH
2
-C -CH -S  in ppm
2
DDPA
Fe-DDPA
MPA
solid
solid
-
-
3
181.4
180.1
175.1
173.1
172.2
175.4
173.4
44
29.2
30.7
19.7
26.9
33.6
34.5
30.9
14.8
-5.7
18.6
21
0.5
0.3
25.0
38.3
47.9
34.1
34.8
34.2
liquid
liquid
liquid
liquid
liquid
CD
CN
DPPA
DMSO-d
DMSO-d
CD OD
CN
6
Fe-DPPA
Fe-DPPA
Fe-DPPA
6
3
CD
3
3.3
1
H-NMR  in ppm
Compound
MPA
Phase
liquid
liquid
liquid
liquid
liquid
Solvent
CD CN
DMSO-d
DMSO-d
CD OD
CD CN
COOH -CH
2
2.61
2.6
2.5
2.7
2.7
-C -CH -S  in ppm
2
3
-
-
2.68
3.3
2.8
2.9
2.9
0.07
0.7
0.3
0.2
0.2
DPPA
6
Fe-DPPA
Fe-DPPA
Fe-DPPA
6
12.3
-
-
3
3
*
=[(CH
2
-C)-[CH
2
-S)]; for carbon; *=[(CH
2
-S)-[CH -C)]; for proton
2
The non chelation of Fe with the carboxylic group of DDPA was also clearly evident from
FTIR and Raman analyses (Fig. S17a-b and Fig. S18). The -COOH vibrational bands were found to
remain intact in Fe-DDPA without any significant shift when compared to those of DDPA and MPA,
(Figure S17b). In case of both DDPA and Fe-DDPA the a(COO) and s(COO) vibrations
-1
respectively were observed at 1698 and 1424 cm and were found to be slightly shifted from those in
-1
MPA appearing at 1714 and 1426 cm . The details of FTIR and Raman analysis are provided in
supporting information. Literature reports predicted that MPA can either interact covalently or by
ionic interaction with metal like Fe [31]. Some earlier studies also found the co-ordination of MPA
with iron by bridging between two Fe-centers [32]. However, in the present case, no such bridging
mode of –COOH unit was observed in the FTIR or in the Raman spectra. But from our study, it was
revealed that the MPA undergoes an oxidation process to form disulfide bond and thereby reduced
the Fe(III) to Fe(II). The Fe(II) species was then trapped by DDPA through Fe---S interaction. The
1
1

presence of Fe---S interaction was also evident from diffuse reflectance spectroscopy (DRS) analysis
Fig. S19). In the DRS spectra of DDPA a single electronic transition was found at ~210 nm
(
originated from carbonyl group (>C=O) of carboxylate moiety (Fig. S19, blue line). In Fe-DDPA
three prominent bands were found at 215, 252 and 350 nm (Fig. S19, red line). The peak at ~350 nm
was due to a charge transition from SFe.
Based on the above spectroscopic analyses a possible binding mode of Fe with DDPA is
proposed herein (Fig. 3a). The coordination sphere around Fe was supposed to be surrounded by four
-
sulfur atoms of DDPA while one of axial coordination sites was probably occupied by the Cl ligand
-
forming a square pyramidal geometry. One of the Cl ion satisfied the secondary valency maintaining
the +2 state of Fe, Fig 3a. The square pyramidal geometry was proposed as most of the complexes of
Fe(II) with disulfides and thiol coordinated through sulfur atom are reported to adopt such
geometrical arrangement [8,33, 34 35]. In order to substantiate and to understand the possibility of
interaction between Fe and S, density functional theory calculation was performed. The initial
coordinates of the DDPA was taken from the crystallographic data. The geometry of Fe-DDPA was
optimized using triple zeta basis set with sufficiently polarized and diffused function, 6-311++G(d,p)
combined with Becke Three Parameter Exchange and Lee, Yang and Parr correlation functional
(B3LYP) [36] implemented in Gaussian 16 suit programme [37]. From the Density Functional
Theory (DFT) calculations also the square pyramidal geometry with the four S-atoms occupying the
-
four corners of the plane and the Cl ligand at the axial positions (Fig. 3b) was found to be much
energetically favorable and stable one. and the details are provided in the supporting information.
The calculated Fe-S bond distances were found to be in the range of 2.28-2.30 Å and the Fe-Cl bond
distance was found to be 2.23 Å. The calculated bond distances were found to match with those
observed by Wang et al. [8]. The highest occupied molecular orbital (HOMO), Fig. 3c was mostly
located at the Fe and Cl while the lowest unoccupied molecular orbital (LUMO) was concentrated at
1
2

the DDPA and Fe (Fig. 3d). This type of mononuclear Fe pentacoordinate thiol based systems are
already reported in literature [33-35,38, 39]. However, to the best of our knowledge, the disulfide co-
ordinated complexes of iron (II) are very less [8]. Hence, Fe-DDPA might find application similar to
those of other Fe-S compounds
Fig. 3. a) Plausible structure of Fe-DDPA, b) optimized geometry of Fe-DDPA, c) and d) represents
the HOMO and LUMO orbitals of Fe-DDPA. Grey colour is for carbon atoms and red for oxygen
atoms, yellow for S atoms.
1
3

Fig. 4. XPS spectra of Fe-DDPA a) iron (Fe), b) chlorine (Cl), c) oxygen (O), d) sulfur (S) and e)
carbon (C).
The +2 oxidation state of Fe in Fe-DDPA was predicted from XPS analysis. As shown in Fig.
4
a on curve fitting the Fe (2P3/2) region, multiple peaks were observed with binding energies (B.E.)
of 708.8, 710.7, 712.5 eV and a shakeup satellite peak around ~714.1 eV [40, 41]. These binding
energy values were characteristics of Fe-compounds in the +2 oxidation state. The peak at 708.8 eV
2
+
was attributed to the electron-deficient Fe sites caused by the breaking of Fe-S bonds. Similar B.E.
values were also observed by Uhlig et al. [42] in marcasite. Based on XPS analysis the oxidation
state of Fe in Fe-DDPA was found to be +2. Yamashita et al. [43] also observed similar B.E. values
2
+
for Fe(2p3/2) region for Fe in Fe
2
SiO4. They found the satellite peak nearly at 718.80.1 eV for Fe
SiO ) was observed at 714.60.2
in +3 oxidation (in Fe ) while the same for Fe in +2 state (in Fe
2
O
3
2
4
2
+
eV. Mullet also found the B.E. for Fe in green rust (GR) at 708.40.1 and 710.50.2 eV [1]. As the
B.E. values of Fe(2p3/2) region in Fe-DDPA was mostly matching with those of the reported values
for +2 oxidation of iron so it was confirmed that the Fe in Fe-DDPA was in +2 oxidation state. The
1
4

XPS spectrum of Cl2p shown in Fig. 4b represented spin-orbit doublets for Cl atom. The sharp and
intense peak at 197.8 eV for Cl(2p3/2) and 199.4 eV (2p1/2) with a separation of 1.6 eV was attributed
-
to inorganic Cl anions [40]. The splitting of both the energy state giving rise to satellite peak at
higher binding energy region of 198.4 eV (2p3/2) and 200.02 eV (2p1/2) suggested for the sharing of
-
-
the Cl electron density with Fe(II). The binding energies values of Cl were found to match with
those reported by Wang et al. [40] for FeCl intercalated into graphite layers. The binding energy for
2
O1s at 531.6 eV was for oxygen atom of –COO (-C-O) and that at 532.7 eV was for C=O of
carboxylic acid (Fig. 4c) [45]. The binding energy for oxygen was not observed below 530 eV
confirming the absence of any iron oxide material or co-ordination of Fe with carboxylate ligand.
The S(2P3/2) and S(2P1/2) energy levels were found at 162.8 eV and 163.8 eV (Fig. 4d) [46]. The
carbon (C1s) binding energy was found at 284.4 eV for C-C and 288.6 eV for O-C=O (Fig. 4e) [42].
1
5

Fig. 5. a) Cyclic voltammetry of the DDPA in water by purging N
DDPA recorded without N flow (black line) and absence of nitrogen (red line) in water, c) cyclic
voltammetry of the Fe-DDPA material recorded in CH CN by purging N with TBAB as a
2
, b) cyclic voltammetry of the Fe-
2
3
2
supporting electrolyte and d) UV-vis spectra of the Fe-DDPA material in presence of CH
TBAB.
3
CN and
The presence Fe(II) species in Fe-DDPA was also ascertained from electrochemical analysis.
The cyclic voltammogram (CV) of DDPA exhibited two oxidation peaks at 0.15 V and 0.38 V and a
reduction peak at 0.23 V (Fig. 5a). The oxidation peak at 0.38 V was matching exactly with that
reported for cystine oxidation [47]. The CV of the Fe-DDPA recorded with and without purging N is
2
shown in Fig. 5b. The anodic peak at -0.04 (-0.07) was for oxidation of Fe(II) to Fe(III) while its
corresponding cathodic peak was observed at -0.27 (-0.29) V [48]. The peak at 0.32 V and 0.17
(0.29) V were for DDPA and got shifted to lower value due to the presence of Fe(II) chlorides (Fig.
-
-
5
a). The sharp irreversible peak at 0.66 V was for Cl /ClO
3
couple (Fig. 5b) [47]. On purging N the
2
peak at 0.66 V almost disappeared completely and the new feeble peak was observed at 1.03 V. The
other peaks although got shifted but were less influenced by air. This change in potential values
-
-
allowed us to suggest that the redox behavior of Fe-DDPA especially the Cl /ClO couple was highly
3
sensitive to the presence of atmospheric oxygen. The results of the CV-study were in good
-
accordance with the Pourbaix diagram for Fe and Cl species at acidic pH (3-4) [49]. Based on the
CV result it was found that Fe in Fe–DDPA was mostly in the +2 oxidation state. On performing the
CV in CH
Br couple (Fig. 5c) [50]. The UV-Vis spectrum of the light yellowish solution resulted from the
reaction of Fe-DDPA with TBAB in CH CN also showed three bands at 386, 423 and 464 nm. The
peak at 386 nm was due to Br species and the other two bands were attributed to charge transfer
transition of [C16 [FeBr ] (Fig. 5d) [51, 52 53]. Based on this analysis in CH CN in presence of
TBAB the following reactions might take place in solution [51].
3
CN with TBAB as supporting electrolyte a strong signal at 0.84 V was observed due to Br-
/
2
3
2
H36N]
2
4
3
1
6

In addition to the XPS and CV analysis, the +2 oxidation state of Fe in Fe-DDPA was also
confirmed from the phenanthroline test. Usually, Fe (III) compounds do not show this until it is
reduced to Fe(II) by hydroxyl-ammine hydrochloride. But in the present case, the phenanthroline test
was found to be positive giving deeply colored red-complex in absence of external reducing agent.
This also helped us to estimate the Fe-content in Fe-DDPA by colorimetric estimation. The aqueous
solution of Fe-DDPA also gave positive silver nitrate (AgNO
) test indicating the presence of Cl-
3
ions. The amount of Fe in 20 mg of Fe-DDPA dissolved in 50 mL water was estimated by iron-
-
detection test with 1, 10-phenanthroline following the reported procedure [54]. The amount of Cl ion
was calculated with respect to silver chloride (AgCl) precipitation with AgNO from 20 mg of Fe-
3
DDPA dissolved in 10 mL of water. The calibration curve and the UV-Vis spectrum used for iron
estimation are provided in Fig. S20. Results predicted the presence of 0.05 mg of Fe(II) and 0.037 mg
-
of Cl giving an approximate 1:1 mole ratio of Fe(II):Cl. This implied that probably there was one
chloride ion out of the coordination sphere of Fe.
3
.3. Catalytic Study
Oxidation of benzyl alcohol (BA) to benzaldehyde without the formation of benzoic acid still
.
appears to be one of the challenging reactions [55]. Generation of hydroxyl free radical ( OH) from
leading to uncontrolled oxidation of organic products like BA is one of the major drawback
16]. The problem associated with such free radical assisted BA oxidation has been overcome by the
H
2
O
2
[
use of precious metals like Pd, Au, Ag that participated in benzylic C-H bond activation without
following the free radical path [17,56, 57]. Recently, it was also found that metals like Ag can
1
7

stabilize the transition state of BA oxidation by participating in Ag-π interaction [14]. Despite being
gaining the controlled over the radical mechanism, the fate of the reaction is still doubtful as
sometimes the small leaching of Pd, Ag or Au nanoparticles retards the catalytic reaction [17,56,57].
Apart from this, the use of such costly metals or catalyst is also not beneficial for large scale
productions [54,55]. Therefore, there still remains scope for low-cost catalyst or material that could
overcome the problem associated with BA oxidation with H
2
2
O [58, 59, 60]. In this regard, Fe is
considered to be one of the cheapest metals that can be employed for various organic transformation
reactions [18-21]. Non-heme iron(II) catalyst are although superior but difficult to synthesize [23,24].
Hence, designing of low valent Fe(II) catalyst without going for ligand synthesis can be a new
approach in the field of homogeneous catalysis. Commonly available iron salts have been known to
catalyze many organic reactions in presence of some costly reagents or additives [61]. But the same
iron salts lead to polymeric tar formation in presence of H
2
2
O leading to uncontrolled oxidation [18].
The Fenton-like systems are therefore limited to organic waste management treatment or so-called
advanced oxidation process (AOP) [62, 63].
Another problem in BA oxidation is the requirement of high temperature (˃60 °C). The
oxidation of BA was also reported with H
2
2
O above 80 °C [64]. Therefore, it is reasonable to find
catalysts that still operate at room temperature retarding the non-catalytic and non-selective BA
oxidation. Only a few reports are available that accounts for the room temperature oxidation of BA
V
[
65]. Both Ghosh and Panda et al. synthesized Fe (O) complex with biuret modified tetra-
amidomacrocyclic ligands (Fe-TAML) for room temperature selective oxidation of benzyl alcohol to
IV
•+
aldehyde proceed via the formation of Fe (O)-porphyrin intermediate [66, 67]. Liu et al. reported
that conversion of BA to benzaldehyde with 100% selectivity in the presence of Pt/BiOCl-Al
catalyst at room temperature in 3h [68].
2
O
3
1
8

As from the above analysis it was evident that Fe-DDPA had a pH within the range which is
suitable for non-free radical Fenton-like reaction and hence the material was explored for BA
oxidation in presence of H
temperature conditions as the iron (II) species readily gets oxidized under high thermal condition. Fe-
DDPA was soluble in both CH CN and H O that are known to be a suitable solvent for BA oxidation.
Therefore, without going for other solvents, the amount of H required for BA oxidation was
optimized at room temperature using 10 mL CH CN or H O as the solvents and 10 mg Fe-DDPA.
From the graph shown in Fig. 6a with 3 mmol of H 2, 44% of BA was converted to benzaldehydes
with 91% selectivity in H O. The conversion and selectivity were found to be 75% and 97%,
respectively in CH CN with 2 mmol of H after running the reaction for 3 h. On increasing the
concentration above 4 mmol there occurred a substantial decrease in selectivity with almost
constant conversion. Thus 2 and 3 mmol of H was considered to be beneficial for the BA
2
O
2
as oxidant. The catalytic reactions were performed at room
3
2
2
O
2
3
2
2
O
2
3
2
O
2
H
2
O
2
2
O
2
oxidation in respective solvents. Catalyst amount also plays a vital role in improving the catalytic
conversion of BA. The catalyst amount was varied from 5 mg to 25 mg in both the solvents. It was
found that on using 15 mg of Fe-DDPA in H
0% selectivity. But no improvement was achieved in CH
Fig. 6b). This indicated that the amount oxidant and catalyst was dependent on the solvent. The
2
O the conversion was improved from 44% to 62% with
9
3
CN on increasing the catalyst amount
(
dependence of the reaction on catalyst amount was also monitored through UV-vis study (Fig. S21).
It was found that with the increase in catalyst concentration the formation of benzaldehyde increases
linearly showing the first-order rate kinetics. The significant outcome of this analysis was that there
was no peak corresponding to benzoic acids (225 nm and 275 nm) clearly demonstrating the high
selectivity of the reaction. Usually benzyl alcohol gives an absorption band at 247 nm while
benzaldehyde peak appears at 244 nm and 282 nm [22].
1
9

The optimum time required for selective and controlled oxidation of BA was also monitored
keeping all other parameters fixed, (Fig. 6c). Performing the reaction more than 3h did not influence
the selectivity and conversion in both the solvents. So, this amount of period was considered to be the
optimal reaction time. To see the effect of solvent on the oxidation of BA to benzaldehyde few other
solvents like methanol, ethanol, DCM and chloroform were also examined and the reactions were
performed taking 10 mg of Fe-DDPA and 2 mmol of H
d. The selectivity and conversion were found to be less in these solvents in comparison to H
CH CN. This might be due to the compatibility of Fe-DDPA in H O and CH CN acting as true
homogeneous Fe(II) catalyst. Further, Fe-DDPA was highly immiscible in dicloromethane (DCM)
and chloroform and thereby sluggish the reaction. Thus, from the analysis, CH CN was found to the
2
O
2
. The results are shown graphically in Fig.
6
2
O and
3
2
3
3
most suitable solvent in terms of conversion and selectivity and also required less amount catalyst
and oxidant. As the formation of over oxidized product like benzoic acid is one of the drawbacks of
this kind of oxidation reaction, the possibility of formation of such minor products was checked
1
3
through C-NMR after an interval of 1h (Fig. S22). Absence of peak near 170-175 ppm for-COOH,
appearance of peak around 192 ppm for –CHO group and vanishing of signal at 64.3 ppm for -CH
2
benzylic carbon in the 13C-spectra confirmed for the selective oxidation of benzyl alcohol to
benzaldehyde.
2
0

Fig. 6. Optimization parameters of benzyl alcohol oxidation with Fe-DDPA at room temperature a)
amount of H , b) catalyst amount, c) time required in CH CN (solid line) and H O (dotted line)
2
O
2
3
2
and d) in different solvent. Blue line for (%) selectivity and black line for (%) conversion. The (%)
selectivity and (%) conversion was calculated by GC-analysis.
On optimization of the reaction condition with BA various other substrates were successfully
oxidized to their corresponding benzaldehyde (Table 2). All the reactions were performed CH CN as
3
some of the substituted BA’s were highly insoluble in water. The substituted benzyl alcohols were
found to provide high yield and selectivity in comparison to simple BA. In order to see the high
selectivity of the reaction, cinnamyl alcohol having both –CH
2
OH and C=C unit was considered. In
the presence of H 2, the alkenes have high proximity to undergo epoxidation reaction forming
2
O
epoxides or diol [69]. There are also several studies on the oxidation of C-H and C=C oxidation using
IV
non-heme Fe(II) complex proceeding via the formation of active Fe-OOH and Fe =O species [70,
7
1]. However, in the present case no such epoxidation reaction occurred and the cinnamaldehydes as
well as the chloro substituted cinnamaldehyde were selectively formed (99%) without effecting
aliphatic double bond (Scheme 2a, entry 10 Table 2). This was also evident from the crystal structure
2
1

of 4-chloro-cinnamaldehyde as shown in Scheme 2b. The ORTEPs of 4-chlorocinnamaldehyde with
0% probability ellipsoid is provided in Fig. S23 and the crystallographic information are in Table S1
5
1
13
of SI. The H and C NMR of the same are also shown in Fig. S24 and Fig. S25, respectively.
Table 2. Oxidation of various substituted BA’s using 10 mg Fe-DDPA and 3 mmol H
CH CN at room temperature.
2
2
O in 10 ml
3
Sl.No Reactant
Product
% Conv % Sel
Yield(in mg) TON TOF/h
1
8
9
8
6
3
5
99
99
96
110
131
102
323
385
300
116
128
100
2
3
4
5
6
9
8
8
1
1
8
95
97
97
110
88
323
258
317
107
86
108
105
7
8
9
9
8
9
4
6
3
98
97
99
120
147
114
352
432
335
117
144
111
1
0
9
5
99
145
426
142
TON= mg of product/mg of Fe content in 10 mg Fe-DDPA, TOF=TON / time (3h).
2
2

Scheme 2. a) Oxidation of cinnamyl alcohol without effecting C=C bond, b) crystal structure of 4-
chloro-cinammaldehyde isolated by oxidation of the corresponding alcohol with 99% selectivity.
3
.4. Role of Water and Acetonitrile: The above analysis stated that CH
suitable solvent for both the catalyst. In order to understand the influence of two solvents, we first
considered the change in pH on addition of H to Fe-DDPA (10 mg) dissolved in CH CN (black
line) and H O (red line). The pH was found to decrease with the addition of H (Fig. 7a). This
3
CN and H
2
O served as a
2
O
2
3
2
2
O
2
indicated the presence of high concentration of proton in the solution. Shi et al. [18] also
demonstrated that at low pH (~1), the oxidation of alcohol occurs via the non-radical Fenton
mechanism giving high selectivity in the conversion of BA to benzaldehyde. When BA was added to
the solution containing 10 mg of Fe-DDPA and 2-3 mmol of H
and 0.92 in CH CN and H O, respectively. This indicated that in CH
maintained a pH that was suitable for a non-free radical Fenton like mechanism giving high
selectivity in the BA oxidation [62,72]. The influence of H to Fe-DDPA was also compared with
FeSO .7H O. The addition of H also leads to a decrease in pH in case of FeSO .7H O (Fig. 7a,
blue line) but the iron got precipitated out as yellow iron (III) sulfate, Fe (SO (Fig. 7a, inset). This
2
O
2
the pH was found to drop to 0.81
3
2
3
CN and H O both the catalyst
2
2
O
2
4
2
2
O
2
4
2
2
)
4 3
further implied the viability of the two catalysts under the prevailing condition. The absence of
polymeric tar formation during BA oxidation with Fe-DDPA also signified for the non-free radical
pathway (Fig. 7a, inset) [18]. It was observed that other Fe salts like ammonium ferrous sulfate,
(
NH
4
)
2
Fe(SO
4
)
2
·6H
2
O, ferric nitrate, Fe(NO
3
)
3
.7H
2
O and anhydrous FeCl
3
proceeded with
2
3

immediate formation of polymeric tar in presence of H
2
2
O reducing the conversion and selectivity
(Fig. 7b) [18]. DDPA thus helped in maintaining the pH as well as the oxidation state of Fe
restricting the shuttling between Fe(II) and Fe(III) and thereby favored the non-free radical
mechanism. This statement was validated from the CV study, the CV of Fe-DDPA in water in a
saturated H
2
2
O solution resulted in one oxidation peak characteristic of DDPA and sharp cathodic
peak at -0.12 V for reduction of Fe(III) to Fe(II) (Fig. 8a) [73]. The absence of any oxidation peak for
Fe(II) to Fe(III) implied the inhibition of catalytic cycle between Fe(II) and Fe(III) that allows the
production of hydroxyl free radical [70]. On addition of BA, the reduction of Fe(III) to Fe(II) signal
was further shifted to -0.28 without any corresponding anodic signal indicated for the non-radical
Fenton mechanism [70]. The peak at 0.49 V was due to the oxidation of BA (Fig. 8a). The CV results
were in accordance with those reported by Shi el al. [18] under acidic condition.
Fig. 7. a) pH variation of Fe-DDPA with different mmol of H
red line) and FeSO .7H O in water (blue line) and the solution shown in the inset after addition
, b) Tar formation with (NH Fe(SO ·6H O, Fe(NO .7H O and anhydrous in presence of
and BA.
2
O
2
in CH
3
CN (black line) and H O
2
(
4
2
H
H
2
O
O
2
4
)
2
4
)
2
2
3
)
3
2
2
2
The UV-vis study of Fe-DDPA in CH
3
CN solvent predicted for the formation of CH
3
CN
2+
solvated Fe-complex herein we designated as [(DDPA)
2
Fe(CH
3
CN)
2
] complex. This was evident
2
4

from the presence of three signals at 239 nm, 312 nm and 361 nm (Fig. 8b, red line). The peak at 239
nm was from the carboxyl group of DDPA (Fig. 8b, black line) while the other two transitions were
for * transition of CH
Fig. 8b) [74, 75]. On addition of H
characteristic peak of Fe-DDPA in water (Fig. 8b, blue line) implying the very fast exchange between
3
CN and ligand to metal charge transfer transition d(Fe)*NCCH
3
(
2
O, the peaks slowly got vanished and ultimately showed the
2
+
H
2
O and CH CN (Fig. 8c). This transition proceeded via the formation of intermediate Fe(OH) or
3
+
FeCl
2
species having an absorption band at ~337 nm (Fig. 8c, blue line) [76, 77]. The formation of
Fe(CH CN)
]2+ complex was also evident from FTIR analysis with the presence of (-
CN) stretching frequency at 2293 cm for lattice co-ordinated CH
CH CN (Fig. 8d) [78]. The stretching frequencies were exactly matching with those reported by
Rajadurai et al. [78] and they also proposed for the lattice solvation of CH CN with Fe(II) complex.
The FTIR spectrum was compared with that of pure CH CN i.e. the one containing no solute.
Although there was no significant shifting in vibrational frequencies but the intensity of the signals at
[
(DDPA)
2
3
2
-1
-1
3
CN and 2256 cm for free
3
3
3
-
1
2
293 and 2254 cm were comparatively less than that of CH
of Fe-DDPA containing CH CN was slightly higher than that of neat CH
that Fe-DDPA in CH CN preceded through the formation of [(DDPA)
while in water probably it formed a [Fe(H O)
]2+ complex [22]. The lattice
3
CN. Further, the % transmittance value
3
3
CN. The results indicated
]2+ complex
3
2
Fe(CH
3
CN)
2
2+
2
O)
6
] or [(DDPA)
2
Fe(H
2
2
coordination of CH
3
CN with Fe was also evident from the ESI-mass analysis with the presence of
]+.H
O. The mass
strong intense signal at m/z value of 577 corresponding to [(DDPA)
2
Fe(CH
3
CN)
2
2
spectrum recorded in CH
3
CN is given in Fig. S26 of SI. Recent studies have shown that the redox
2
+
CN)
]2+ species was more favorable for such
potential associated with [Fe(CH
3
CN)
2
] or [Fe(CH
3
6
oxidation process in contrast to the Fe(H
2
O) 2+
6
complex [22]. Probably this might be the reason for
giving the better catalytic conversion in CH
3
CN in comparison to H O.
2
2
5

Fig. 8. a) CV of Fe-DDPA recorded in water after addition of H
2
O
2
(black line) and H
CN (red line ) and H
CN (black line), c) UV-vis spectra of Fe-DDPA (black line) and after addition of H
2
O
2
and BA
(blue line), b) UV-vis spectra of Fe-DDPA in CH
3
2
O (blue line) and DDPA in
CH
3
2
O in the
same solution and d) FTIR spectra of Fe-DDPA recorded after dissolved in CH
pure CH CN (red line).
3
CN (black line) and
3
3
.5. Influence of Chloride ion Concentration: Various studies on Fenton-like reactions using
-
Fe(III)- chloride salts predicted that the presence of Cl ion retards the H
2
2
O decomposition and
.
sometimes produces chloro-substituted product. As the Cl radical competes with OH radical, the rate
of decomposition of organic waste was found to decrease dramatically with increase in Cl-
concentration [77]. In the present case, however, no chlorinated product was found but on addition of
NaCl (0.5 mL of 0.01M) the reaction became sluggish and up to 24 h, no conversion was observed in
TLC. The impact of NaCl was also monitored through UV-vis study. On addition of NaCl solution
1
0L after an interval of 5 min to Fe-DDPA in CH
3
CN, the UV-Vis band corresponding to
2
+
[
(DDPA)
2
Fe(CH
3
CN)
2
] got vanished with the appearance of a very weak d-d transition at 741 nm
2
6

-
for some unidentified Fe-species (Fig. S27). This truly implied that at high [Cl ] concentration
2+
retarded the formation of [(DDPA)
2
Fe(CH
3
CN)
2
] and influenced the catalytic ability of Fe-DDPA.
+
or FeCl +
species on the addition of NaCl.
In water, no additional peak was observed for FeCl
2
2
.

Further, the generation of Cl or Cl
2
was nullified as no oxidation peak corresponding to the couple
2Cl was observed at 2.41 or 2.09 V (Vs SHE) respectively in CV study [77]. A feeble
peak at 0.89 V (1.27 vs SHE) was however observed in the cyclic voltammetry after addition of BA




Cl /Cl or Cl
2
-
-
-
to a mixture Fe-DDPA +H
2
O
2
corresponding to oxidation of Cl for Cl /ClO
3
[48]. Thus it was
-
concluded that under the present condition the amount of [Cl ] concentration present in Fe-DDPA did
not hamper the catalytic oxidation of benzyl alcohol by competing with H
2
2
O or OH radical.
3
.6. Mechanism
From the above physiochemical and spectrochemical analysis, it was clear that the conversion
of BA to benzaldehydes occurred via a non-free radical Fenton mechanism in the presence of Fe-
DDPA and H . It is most expected that Fe(II) species will result in the formation of active
2
O
2
hydroxyl free radical making the reaction to be non-selective. But in the present case no such
retardation in the selectivity was observed. The addition of free radical scavenging agent did not
hamper the catalytic conversion suggesting for non-hydroxyl free radical mechanism. Based on these
evidences, a probable mechanism is provided in Scheme 3. It was assumed that probably the axially
co-ordinated Cl-ligands in Fe-DDPA got exchanged with CH
3
CN to forms a redox-active and labile
[
(DDPA)
2
Fe(CH
3
CN)
2
]2+ species. Therefore, the present reaction system was compared to that
reported by Bataineh et al. [22] using iron(II)bis(acetonitrile)bis(triflate), Fe(CF
3
SO
3
)
2
(CH
3
CN) as
2
the source of [Fe(CH
3
CN)
6
]2+ species. As CH
3
CN was associated as a solvated molecule so they
were easily exchangeable with H
2
O as evident from our UV-vis study [77]. On addition of 30%
CN ligands immediately undergo an
O to form a Fe-OOH species as shown in Scheme 3. Prat et al
H
2
O2, a proton abstraction process took place and the CH
3

exchange process with HOO and H
2
2
7

[
79] also predicted for similar type of exchange of CH
3
CN and triflate (OTf) ligand with H
2
2
O and
water in a non-heme Fe(II) complex. As proposed by Prat et al. [79] and many other studies, [22-24]
IV
V
at this stage probably the cleavage of OOH occurs forming a highly reactive Fe =O or Fe (O)OH
species (Scheme 3) which then reacted with BA to form benzaldehyde. The interaction of H with
Fe-DDPA in CH CN was also monitored by UV-vis study (Fig. 9a). Slow addition of H (1L at a
time) initially showed disappearance of peak I (black line) at 239 nm originated from the ligand
DDPA. On further addition of H he other two peaks II and III at 312 and 361 nm originated from
the charge transfer of FeNCCH started to vanish consecutively one after the other (II followed by
2
O
2
3
2
O
2
2
O
2
3
III, coloured lines) and finally a new peak having a very short lifetime appeared at ~440 nm (peak
IV, blue dotted line) corresponding to the OFe charge transfer transition in Fe-hydroperoxo (Fe-
OOH) species (Fig. 9a). The disappearance of peaks at 312 and 361 nm and the appearance of short
lifetime peak at 440 nm clearly suggested the replacement of CH
3
CN by H O and co-ordination of
2

HOO . Laat. et al. also reported for appearance of similar band for Fe-OOH species in a similar range
[
76]. Chen et al. [80] also observed the formation of Fe-OOH species after addition of large excess of
(>50equivalent) and characterized by UV-Vis and Raman studies. The generation of Fe-OOH
species was also confirmed from the Raman spectroscopy with the appearance of sharp and intense
H
2
O
2
-1
band at 792 cm for O-O vibration of Fe-OOH species (Fig. 9b) [80]. The less intense band for O-O
vibration of H further implied the small concentration of unreacted H . From this
spectrochemical analysis, it can be assessed that the H first got stabilized by the dimeric layer of
2
O
2
2
O
2
2
O
2
2+
DDPA which than slowly interacted with [Fe(CH
3
CN) ] unit forming a reactive Fe-OOH species
2
IV
IV
and finally formed Fe =O species. The active Fe =O species further catalyzed the selective
conversion of BA to benzaldehyde as shown in Scheme 3.
2
8

Scheme 3. Plausible mechanism of benzyl alcohol oxidation with Fe-DDPA
2
9