Synthesis, structural characterization and C–H activation property of a tetra-iron(III) cluster

Article abstract of DOI:10.1016/j.molstruc.2018.11.062

A non-heme tetra-iron cluster, [Fe₄ ^III(μ-O)₂(μ-OAc)₆(2,2′-bpy)₂(H₂O)₂](NO₃ ^?)(OH^?) (1), [OAc = acetate; 2,2′-bpy = 2,2′-bipyridine] containing oxido- and acetato-bridges was synthesized and structurally characterized by different spectroscopic methods including single crystal X-ray diffraction studies. X-ray crystal structure analysis of 1 revealed that tetra-iron complex was crystallized in monoclinic system with C2/c space group. Each of the Fe centres in 1 was found to exist in octahedral geometry and interconnected by oxido- and acetato-bridges. Bond valence sum (BVS) calculation recommended the existence of iron centres in +3 oxidation state. Variable temperature magnetic measurement authenticated the dominating antiferromagnetic ordering among the iron centres in the solid state of 1. This tetra-iron cluster was also evaluated as an efficient catalytic system towards the oxidation of both linear & cyclic alkanes without production of primary C–H bond oxidation products. Oxidation of secondary C–H bonds attested the formation of both the corresponding alcohols & ketones in 27–900 TONs. The tetra-iron catalytic system with Alcohol/Ketone values 0.2–1.7 indicated the involvement of freely diffusing carbon-centered radicals rather than metal based oxidant.

Full text of DOI:10.1016/j.molstruc.2018.11.062

Accepted Manuscript
Synthesis, structural characterization and C H activation property of a tetra-iron(III)
cluster
Dhananjay Dey, Moumita Patra, Afnan Al-Hunaiti, Hare Ram Yadav, Afrah Al-mherat,
Sharif Arar, Milan Maji, Angshuman Roy Choudhury, Bhaskar Biswas
PII:
S0022-2860(18)31375-9
DOI:
https://doi.org/10.1016/j.molstruc.2018.11.062
MOLSTR 25891
Reference:
To appear in: Journal of Molecular Structure
Received Date: 26 May 2018
Revised Date: 10 November 2018
Accepted Date: 21 November 2018
Please cite this article as: D. Dey, M. Patra, A. Al-Hunaiti, H.R. Yadav, A. Al-mherat, S. Arar,
M. Maji, A.R. Choudhury, B. Biswas, Synthesis, structural characterization and C H activation
property of a tetra-iron(III) cluster, Journal of Molecular Structure (2018), doi: https://doi.org/10.1016/
j.molstruc.2018.11.062.
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ACCEPTED MANUSCRIPT
Synthesis, Structural Characterization and C-H Activation Property of a
Tetra-iron(III) Cluster
a
b
c,d*
e
Dhananjay Dey, Moumita Patra, Afnan Al-Hunaiti , Hare Ram Yadav, Afrah Al-
c
d
f
e
a,*
mherat, Sharif Arar, Milan Maji, Angshuman Roy Choudhury, Bhaskar Biswas
a
Department of Chemistry, Raghunathpur College, Raghunathpur, Purulia-723133, India
Department of Physics, Raghunathpur College, Raghunathpur, Purulia-723133, India
b
c
Department of Chemistry, University of Petra, Amman 11196, Jordan
d
Department of Chemistry, University of Jordan, Amman 111942, Jordan
e
Department of Chemical Sciences, Indian Institute of Science Education and Research, Mohali,
Sector 81, Knowledge City, S. A. S. Nagar, Manauli PO, Mohali, Punjab 140306, India
f
Department of Chemistry, National Institute of Technology, Durgapur 713209, India
III
4
This paper represents synthesis of a new oxido-acetato-bridged tetra-iron(III) complex, [Fe
(µ-O)
2
(µ-
-
-
OAc) (2,2′-bpy) (H O) ](NO )(OH ) which displays excellent catalytic activity towards C-H activation
6
2
2
2
3
property based on carbon centred radical in solution.

ACCEPTED MANUSCRIPT
Synthesis, Structural Characterization and C-H Activation Property of a
Tetra-iron(III) Cluster
a,e
b
c,d*
e
Dhananjay Dey, Moumita Patra, Afnan Al-Hunaiti , Hare Ram Yadav, Afrah Al-
c
d
f
e
a,*
mherat, Sharif Arar, Milan Maji, Angshuman Roy Choudhury, Bhaskar Biswas
a
Department of Chemistry, University of North Bengal, Darjeeling-734013, India
b
Department of Physics, Raghunathpur College, Raghunathpur, Purulia-723133, India
c
Department of Chemistry, University of Petra, Amman 11196, Jordan
d
Department of Chemistry, University of Jordan, Amman 111942, Jordan
e
Department of Chemical Sciences, Indian Institute of Science Education and Research,
Mohali, Sector 81, Knowledge City, S. A. S. Nagar, Manauli PO, Mohali, Punjab 140306,
India
f
Department of Chemistry, National Institute of Durgapur, Durgapur 713209, India
_
________________________________________________________________________________________________________________
Abstract
A non-heme tetra-iron cluster, [Fe (µ-O) (µ-OAc) (2,2′-bpy) (H O) ](NO )(OH ) (1),
III
-
-
4
2
6
2
2
2
3
[OAc = acetate; 2,2′-bpy = 2,2′-bipyridine] containing oxido- and acetato-bridges was
synthesized and structurally characterized by different spectroscopic methods including single
crystal X-ray diffraction studies. X-ray crystal structure analysis of 1 revealed that tetra-iron
complex was crystallized in monoclinic system with C2/c space group. Each of the Fe centres
in 1 was found to exist in octahedral geometry and interconnected by oxido- and acetato-
bridges. Bond valence sum (BVS) calculation recommended the existence of iron centres in
+
3 oxidation state. Variable temperature magnetic measurement authenticated the dominating
antiferromagnetic ordering among the iron centres in the solid state of 1. This tetra-iron cluster
was also evaluated as an efficient catalytic system towards the oxidation of both linear &
cyclic alkanes without production of primary C-H bond oxidation products. Oxidation of
secondary C-H bonds attested the formation of both the corresponding alcohols & ketones in
2
7-900 TONs. The tetra-iron catalytic system with Alcohol/Ketone values 0.2 ~1.7 indicated
the involvement of freely diffusing carbon-centred radicals rather than metal based oxidant.
Keywords: Iron; bridging ligand; X-ray crystal structure; Magnetic property; C-H activation
study

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2
*
*
Corresponding author. E-mail: mr.bbiswas@rediffmail.com / icbbiswas@gmail.com
Corresponding author. Tel.: +962 6 5799555; Fax: +962 6 5715570.
Email: aalhuniti@uop.edu.jo.
1
. Introduction
The selective transformations of different organic substrate remain a great challenge in the
development of effective catalytic system [1] and for this cause, considerable efforts have
been made to develop robust and selective homogeneous oxidation catalysts based on
different transition metal ions [2]. Nature has employed different heme- and non-heme iron
enzymes composed of oxido- & acetato-bridges to carry out important biological
transformations involving oxidation of substrates that use dioxygen as ultimate oxidant [3–7].
From the environmental and economic perspective, the selective and efficient
functionalization of aromatic hydrocarbons in large scale using oxidant like molecular
oxygen, hydrogen peroxide and tert-butyl hydroperoxide with different metal catalysts under
mild conditions through greener catalytic routes is still challenging for the industry [8-11].
Oxygenated hydrocarbons are important intermediates for chemicals, agrochemicals,
fragrances, pharmaceuticals, and polymers manufacture. Though industrial processes usually
use homogeneous catalysts as Cr(VI), Mn(VII) or Os(VII) complexes but inconvenient
formation of toxic metallic side products remain a difficulty. In this context, cleaner catalytic
systems that can operate at mild conditions are desirable [12]. With an aim to develop more
active catalytic systems and to gain new insight in the C-H activation, we have designed,
synthesized and structurally characterized an oxido-acetato tetra-iron cluster. Temperature
dependent magnetic susceptibility measurement authenticates the presence of
antiferromagnetic ordering in 1 at low temperature. This tetra-iron cluster acts as an efficient
catalytic system towards the oxidation of both linear & cyclic alkanes. Oxidation of secondary
C-H bonds leads to formation of both the corresponding alcohols & ketones with
Alcohol/Ketone value ~1.7 (after statistical correction) and TON of 900 is recommending the
involvement of freely diffusing carbon-centred radicals rather than metal based oxidant.
2
. Experimental Section
2
.1. Preparation of the complex
(a) Chemicals, solvents and starting materials

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High purity 2,2′-bipyridine (Lancaster, UK), iron(III) chloride hexahydrate (E. Merck,
India), triethylamine (E. Merck, India), tetraethyl ammonium chloride (Aldrich, USA) and
cyclohexane, cyclopentane, cyclooctane, adamantine, hexane, decane, octane, ethyl bezene, 1-
methylcyclo hexane, 1-isopropyl cyclohexane (Aldrich, USA) were purchased from
commercial sources and used as received. All the other reagents and solvents were used of
Analytical grade (A.R. grade).
(b) Synthesis of the diiron(III) precursor and tetra-iron(III) cluster
The previously reported [13] oxido-bridged diron(III) complex, tetraethylammonium[(µ-
oxo)bis[trichloroferrate(III)], (Et N) [Fe OCl ] was used as a precursor to synthesis this tetra-
4
2
2
6
iron(III) cluster. An aqueous solution (10 ml) of sodium acetate (0.164 g, 2.0 mmol) followed
by methanolic solution of 2,2′-bipyridine (0.312 g, 2.0 mmol) were added dropwise to an
aqueous solution of (Et N) [Fe OCl ] (0.632.7 g, 1.0 mmol, 20 ml) and mixed slowly on a
4
2
2
6
magnetic stirrer with slow stirring for 30 min. At that moment ceric ammonium nitrate was
added portion wise to the reaction mixture (0.550 g, 1.0 mmol) and kept on magnetic stirrer
for another 30 minutes. After that, the brown coloured solution was filtered. The supernatant
liquid was kept in air for slow evaporation. After 7-10 days, the fine microcrystalline brown
coloured compound (1) was separated out from mother liquor. The compound was washed
with hexane and dried in vacuo over silica gel indicator. Yield of 1: 0.374 g (81.3% based on
-1
metal salt) Spectroscopic results for C32
588 (vC=N), 1494, 1475(vM(µ-OAc)), 1384(v(-NO3)), 825(vM(µ-O)); UV-Vis (1×10 M, λmax(abs),
nm, MeCN): 246(0.53), 297(0.96), 350(0.33), 523(0.26).
39 5 4
H N O19Fe (1): IR (KBr pellet, cm ): 3367(vOH),
-4
1
2
.2. Physical measurements
Infrared spectrum (KBr) was recorded with
a
FTIR-8400S SHIMADZU
–
1
1
spectrophotometer in the range 4000–400 cm with KBr pellet. H NMR spectrum in DMSO-
d
6
was obtained on a Bruker Advance 300 MHz spectrometer at 25°C and was recorded at
2
99.948 MHz. Ground state absorption and all spectrophotometric analysis were recorded
with a JASCO V-730 UV-Vis spectrophotometer. Electrospray ionization (ESI) mass
spectrum was recorded using a Q-tof-micro quadruple mass spectrometer. The pH value of the
solutions was measured by Systronics pH meter at room temperature. Variable-temperature

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magnetic susceptibility measurements were performed on a Quantum Design SQUID MPMS-
XL magnetometer in the temperature range of 2-300 K.
2
.3. X-ray diffraction study
Single crystal X-ray diffraction data of the oxido-acetato-bridged tetra-iron complex was
collected using a Rigaku XtaLABmini diffractometer equipped with Mercury CCD detector.
The data were collected with graphite monochromated Mo-Kα radiation (λ=0.71073 Å) at
1
50.0(2) K using ω scans. The data were reduced using Crystal Clear suite 2.0. and the space
2
group determination was done using Olex [14]. The structure was resolved by direct method
and refined by full-matrix least-squares procedures using the SHELXL-2014/7 software
2
package using OLEX suite [15,16]. The tetra-iron complex was found to have six bridging
acetate ions and two bridging oxido ions. The bridging ions connected Fe(III) ions in dicubane
core. There were counter nitrate anion and hydroxide anion which balanced the cationic
charge of iron cluster. Some solvent molecules were also there but couldn’t be traced
properly. The counter anionic nitrate and solvate molecules were found in disordered state
while core structure remained perfectly ordered. Solvent masking method is used to remove
the disordered solvent from this structure during refinement. The crystallorgraphic data and
bond distance, bond angle has been calculated using PARST [17] and given in Table 1 and
Table S1 respectively.
2
.4. C-H activation analysis by the oxido-acetato-bridged tetra-iron complex (1)
Conversion of cyclooctane : Cyclooctane (228 mg, 268 µl, 2 mmol) was added to a
solution of 1 (0.04 mmol) in MeCN:acetic acid (1.5:0.5 ml) in over all catalyst 1: substrate:
oxidant 0.04:200:500). After the addition of H O (33% in H O; 454 µL, 5.0 mmol), the
2
2
2
o
reaction mixture was heated at 32 C for 3h. The mixture was then allowed to cool to room
temperature. The organic phase was extracted with Et O (3x1 ml), washed with brine and
2
dried (MgSO ). After filtration, the solvents of the filtrate were evaporated (rotary
4
evaporator). The remaining mixture was separated by column chromatography (silica gel;
diethyl ether: pentane=1:20 as eluent) and the product was analyzed by GC-MS using 1,2-
dichlorobenzene as an internal standard.
3
. Results and discussion

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3
.1. Synthesis and formulation of the tetra-iron(III) cluster (1)
The oxido-acetato-bridged brown coloured tetra-iron(III)-bipyridine complex was
synthesized and isolated in the form of single crystals by mixing oxido-bridged diiron(III)
precursor, tetraethylammonium[(µ-oxido)bis[trichloroferrate(III)], (Et N) [Fe OCl ] with
4
2
2
6
sodium acetate, CAN and 2,2'-bipyridine in aqueous methanolic medium. The structural
formulation and geometry of 1 was principally determined by single crystal X-ray diffraction
study along with different spectroscopic and analytical techniques.
3
.2. Proposed mechanistic pathway for the formation of the tetra-iron(III) cluster (1)
We have proposed a mechanism in the formation of the tetra-iron complex. The diiron
precursor was dissolved in aqueous medium and turned instantly brown after addition of
sodium acetate on magnetic stirrer. This colour change indicated the possible incorporation of
the symmetric bridging acetate ions into primary zone of coordination for iron centres with
simultaneous displacement of chloride ions. With the progress of time, this oxido-acetato iron
adduct presence of ceric ammonium nitrate (CAN) bound with another unit of diiron(III)
precursor to form a tetrameric iron complex in. It is well known that CAN is a versatile
oxidizing agent that helps to promote the metal ions in higher oxidation states and can supply
nitrate ion as a counter anion depending on the chemical environment. Here, the charges of
tetra-iron(III) core were internally compensated via two oxido- and six acetato-bridges in a
symmetric manner ; however total neutrality for the tetra-iron(III) cluster appeared from the
two anions, nitrate & hydroxide. The coordinative saturation for two iron centres was satisfied
by two bipyridine ligands and two aqua molecules in the tetra-iron cluster. The displaced
chloride ions were isolated and separated out as colorless crystalline compounds, NaCl and
Et NCl salt during the reaction. The chloride salts were separated and purified from the
4
reaction mixture through column chromatography using diethyl ether-water as an eluent
mixture. The isolated molecular salt, Et NCl was mainly characterized by FT-IR, UV-Vis and
4
1
H NMR spectroscopy (Fig. S1). Controlled experiments under the identical conditions in the
absence of NaOAc and CAN revealed that for successful production of this oxido-acetato-
bridged tetra-iron(III) cluster those are essential (Scheme 1).

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6
C
O
O
OW
O
2+
Ow
O
Cl
Fe
Cl
Cl
Fe
Cl
Fe
Fe
Fe
O
NaOAc
O-MeoH
O
NEt
4
Cl
Cl
2
C
C
H
2
O
O
Fe
O
O
O
Ow
Ow
C
N
N
CAN
N
C
OW
O
N
O
O
O
Fe
Fe
Fe
O
C
NaCl + N(Et) Cl +
O
O
4
C
O
C
O
O
O
C
Fe
Ow
O
O
O
N
C
N
Scheme 1. Proposed mechanistic pathway for the formation of 1
3
.3. Description of crystal structure
Single crystal X-ray diffraction analysis reveals that 1 crystallized in the monoclinic
system with space group C2/c. The compound 1 as an isolated neutral cluster was composed
2
-
of four iron(III) ions which were trigonally coordinated by two monoxide bridges (µ -O ;
3
O3, O7) and six acetate anions as end-to-end symmetric bridges, while two bipyridine ligands
behaved as terminal blockers and two water molecules were coordinated to complete the
III
primary zone of coordination for iron centres. The cationic charges (Fe ) of the molecule
were internally compensated by oxido and acetato bridging ligands along with two counter
anions, nitrate and hydroxide. The molecular structure of the complex is illustrated in Figure
1
. The iron cluster comprised an inner most core of two Fe atoms (Fe2 and Fe4) via doubly
oxido-linkages (Fe2…µ -O3…Fe4; Fe2…µ -O7…Fe4) and one acetate bridge
Fe2…O12O11…Fe4), which in turn connected outwards to the remaining Fe atoms (one
3
3
(
each), Fe1 and Fe3, through acetate linkges. To these outermost Fe atoms, the bpy ligands
chelate via pyridyl N1 and N2, and N3 and N4, atoms respectively. Adjacent Fe atoms were in
turn connected by a different number of acetate bridges. The Fe3/Fe4 were doubly bridged by
acetate ions, while the pairs Fe1/Fe2, Fe1/Fe4, Fe2/F4 and Fe2/Fe3 were singly bridged by
acetate ions. All four iron(III) environments were slightly distorted octahedral. The Fe–O–Fe
linkages were highly nonlinear, exhibiting an angle starting from 94.63° to 136.48°. The Fe–
µ3-O distances [1.831(3) Å to 1.976(3) Å] wasn’t accordance with those previously reported

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7
for (FeL) O (1.76–1.82 Å) [18]. The combination of the nonlinear bridge and the strong Fe–µ-
2
O bonding produced intermetallic (Fe…Fe) spacing from 3.248 to 3.488 Å. The bond lengths
of C–N and C–C in the bipyridine all fall within the range of the literature values [19] and the
two aromatic ring systems in each bpy were coplanar within the experimental error.
3
.4. Bond Valence Sum (BVS) calculation
Bond Valence Sum (BVS) [20] is a theoretical calculated quantity which is used to
determine the oxidation state of each of the metal ions in metal complex in solids from
crystallographically determined bond distance data. We applied this method to determine the
oxidation state for each of the iron ions in this tetranuclear iron cluster. All the iron centres in
iron cluster was found to have +3 oxidation state. The details calculation for the determination
of oxidation states for the iron centres is given in Table S2.
3
.5. Magnetic property
Temperature dependence magnetic susceptibility measurements were recorded for this
tetra-iron(III) cluster in solid state ranging from 2 to 300 K at H = 1000 Oe under zero field
cooling (ZFC) and field cooling (FC) condition. Figure 2 showed temperature variable
magnetic susceptibility diagram for the iron cluster. Close observation on the changes of
magnetic susceptibility with temperature displayed that χ T value gradually decreased from
Μ
3
-1
3
-1
3
.03 cm mol K at 300 K to 0.029 cm mol K at ~8K with an S=0 ground state where a
plateau was appeared. BVS calculation predicted the oxidation state for each of the iron
centres in tetra-iron(III) cluster as +3. Considering the presence of four non-interacting high
spin iron(III) centres, the magnetic susceptibility value for iron(III) cluster at room
3
-1
temperature should be 17.52 cm mol K, however the magnetic moment value was found
very low. So definitely, the magnetic data in solid state strongly suggested the existence of
antiferromagnetic interactions among the Fe(III) centres through different bridging units.
To view insights of the magnetic property, it was observed that few similar tetranuclear
iron clusters and their magnetic properties were previously reported by S.P. Perlepes et al.,
8
+
D.N. Hendrickson & G. Christiou [21]. It was seen that, most commonly [Fe (µ -O) ] core
4
3
2
in similar tetrairon(III) clusters existed in either bent (butterfly) or planar dispositions of four
3
+
Fe ions of types A & B (Scheme 2). In contrast, another highly unusual asymmetric
configuration of type C (Scheme 2) for four metal ions can possible and can be described as a
hybrid of types A & B. This unusual type C had been previously observed in two or three

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8
occasions only. Our synthesized tetrairon(III) cluster can actually referred to the type C
conformation and remains another example of unusual asymmetry.
M
M
M
M
O
O
O
O
O
O
M
M
M
M
M
M
M
M
Type A
Type B
Type C
8
+
Scheme 2. Different conformations of [Fe (µ -O) ] core
4
3
2
We have also determined the exchange parameters for the iron couples in 1. For a
centrosymmetric molecule like 1 (C ), a total of five exchange parameters, J are theoretically
i
ij
III
required for each possible pairwise exchange interaction between Fe ions i and j; a sixth
parameter J (Scheme 3; the numbering scheme is the same as that shown in Fig. 1) is treated
1
3
to be zero given the large distance between Fe(1) and Fe(3) [5.89A°]. The main exchange
2
-_
-
interaction pathway is via the O
groups, not CH CO2 groups, so that, from the
3
magnetochemical viewpoint, the core symmetry is C and only three values are required [21b]:
s
′
J
23 = J34 = Jwb; J12= J14 = J wb; J24 = Jbb, where b = body and w = wingtip. However,
consideration of the Fe–O and Fe–O–Fe distances and angles in Table S1 showed that the
slight difference in pyramidality between O(3) and O(7) had a little influence on these
structural parameters, and a 2J model (C2v symmetry), i.e. Jwb = J′wb appeared to be
reasonable.
Scheme 3. Tetra-nuclear iron(III) core exhibiting coordination connectivities

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9
This model was previously employed and discussed in detail elsewhere [22]. Thus, the
Hamiltonian formalism given in Eq. (1) was used to fit the data, considering S = 5/2 impurity
(ρ).
H = - Jbb
S
2
S
4
– Jwb[(S
1
+S
3
)(S
2
+S
4
)]
…(1)
The derived expression produced a good fit, shown as solid lines in Fig. 2, recommending the
-1
validation of the 2J model. The fitting exchange parameters were found as J = -13(5) cm ,
bb
-1
J
wb ¼ = -73 (1) cm , g = 2.0 and ρ = 0.58%. The determined J values were in close agreement
with previously determined exchange parameters for similar complexes [22,23].
3
.6. Oxidative C-H Activation Reactivity and reaction mechanism
Before going to study on C-H activation, we was recorded electronic spectrum for this
tetra-iron cluster in acetonitrile medium and observed the optical bands at 246, 297, 350, 523
nm (Fig. 3). Keeping in mind the factors that can affect the oxidation pathway, and the key
oxidizing species, the catalytic ability of 1 in oxidation of alkanes using H O as an oxidant
2
2
has been investigated. Depending on the conditions, the alcohol/ketone (A/K) ratio for the
benchmark substrate cyclohexane is 1.7. Oxidation of cyclohexane by H , using 1 as
catalyst under standard conditions ([1] = 0.4 mM and 1:H O : RH= 0.04: 500: 200) produced
2
O
2
2
2
cyclohexanol and cyclohexanone with A/K= 1.7, and a turnover number (TON) of 900 within
-1
2
h (TOF = 450 h ) based on the combined yield of products (Table 2). This catalyst also
demonstrated moderate activity in the oxidation of n-octane and other alkanes (Table 2,
-1
entries 2-9, Figure Supplementary) with TONs of (25-900) and TOF = 12.5-590 h ).
The system was quite efficient in oxidizing both linear and cyclic alkanes with no primary C-
H bond oxidation products being observed. Oxidation of secondary C-H bonds leads to the
formation of both the corresponding alcohols and ketones. Alcohol/ketone ratio of
cyclohexane was previously used to probe the nature of oxidation catalyst species. It was
observed that indiscriminate hydroxyl radicals typically afford selectivity ratios for A/K
oxidation less than 1, whereas more selective oxidants give substantially higher values [24]. In
this context, the present catalyst system with a A/K value around 1.7 (after statistical
correction) and TON of 900 was a positive indication about the involvement of freely
diffusing carbon-centered radicals rather than metal based oxidant (Table 2. entry 1).
Furthermore, the oxidation of adamantane and isopropyl cyclohexane into mainly the tertiary
alcohol products, suggested the ascendency of radical based oxidation mechanism.

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Interestingly, the catalyst was capable to oxidize linear alkanes, such as n-octane in secondary
C-H bond position in a A/K value 1.0 and TON of 109. The hydrocarbons conversion and
product yield were 29-89% (Table 2, entries 2-9) in the studied system, which, together with
some selectivity, recommended its potential use in preparative chemistry.
The elucidation of the reaction mechanism is outside the scope of this study. Nevertheless,
some observations may be noticed and some conclusions can be made. First, the reaction
patterns in Table 2 suggested the involvement of freely diffusing carbon-centered radical
processed in the course of catalysis, as the A/K ratio of 1 and the oxidation of adamantane into
adamantanol (mainly) in entry 9 formally indicated it. The low value of this ratio may also be
explained by the over oxidation of the initially formed alcohols to the corresponding ketones
in our system due to the relatively large concentration of oxidant and the prolonged reaction
times. It should be noted that all catalytic oxidation reactions were performed in the presence
of an equimolar amount of acetic acid relative to the catalyst (complex 1). The oxidation
results make us believed that the combination of hydrogen peroxide and acetic acid with
mononuclear and dinuclear non-heme iron and Fe O species in solution leads to the in situ
2
formation of peracetic acid, which was a relatively strong oxidant and an oxygen atom
donor.[25]
4
. Conclusions
The µ-oxido-acetato-bridged tetrairon complex containing the bipyridine ligand was
designed and synthesized using a µ-oxido-acetato-bridged tetrairon(III) precursor and
characterized by single crystal X-ray diffraction study including other analytical results. The
outcomes from BVS calculation truly satisfied the +3 oxidation state of iron centres in tetra-
iron(III) cluster. Temperature dependent magnetic susceptibility measurement recommended
the existence of antiferromagnetic ordering in 1 at low temperature. This tetrairon cluster
effectively behaved as an efficient catalytic system towards the oxidative C-H activation of
both linear & cyclic alkanes. This tetrairon catalytic system with low Alcohol/Ketone
values0.2 ~1.7 (after statistical correction) and TONs of 25-900 authenticated the involvement
of freely diffusing carbon-centered radical rather than metal based oxidant. The reactivity of
this catalyst toward alkane oxidation as well the easy preparation of the system makes it an
attractive strategy for alkane C-H activation.

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5
. Supplementary data
Crystallographic data are available free of charge from The Director, CCDC, 12 Union
Road, Cambridge, CB2 1EZ, UK (fax: +44-1223-336033; E-mail: deposit@ccdc.cam.ac.uk or
www: http://www.ccdc.cam.ac.uk) upon request, quoting deposition number CCDC 1541598
for the tetra-iron(III) complex (1).
Acknowledgement
BB thank DST India for financial support under the FAST TRACK SCHEME for
YOUNG SCIENTIST (No. SB/FT/CS-088/2013 dtd. 21/05/2014). BB sincerely
acknowledges Prof. T. K. Paine for recording variable temperature magnetic susceptibility
data for this complex. ARC thanks the X-ray facility of the Department of Chemical Sciences,
IISER Mohali for single crystal X-ray diffraction data collection.
References
[
[
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Table 1 Crystallographic data and structure refinement parameters for 1
Parameters
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
1
C H N O Fe
4
32
39
5
19
1021.08
150.0(2)
Monoclinic
C2/c
28.23(2)
12.625(10)
28.11(2)
108.719(9)
b (Å)
c (Å)
β
3
Volume (Å )
9487(12)
8
Z
–
3
ρ (gcm )
1.452
–
1
ꢀ
(mm )
1.273
F (000)
4240
θ ranges (º)
2.185-25.0
6.67
R
int
Number of unique reflections
Total number of reflections
Final R indices
10934
50231
0.0633, 0.1711
2.26, -1.24
-3
Largest peak and hole (eA˚ )

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Table 2. Catalytic oxidation results
Entry Substrate
Products (yield %)
cyclohexanol (53)
Total yield (%) Remarks
c
1
cyclohexane
83
TON=900
cyclohexanone (30)
cyclooctanol (13)
cyclooctanone (77)
Cyclopentanone(60)
2-octanol (5),3-
A/K=1.7
TON=290
A/K=0.2
c
2
cyclooctane
89
c
3
4
cyclopentan
e
60
33
TON = 200
TON=109
A/K=1.0
c
octane
octanol(3), 4-octanol(6)
2
-octanone (7), 3-
octanone(7) 4-octaone(5)
c
5
decane
2-deacnone (9)
30
TON=71
3
4
-decanone (13)
-decaone (8)
c
6
7
hexane
2-hexanol (15)
-hexanone(30)
45
TON=590
TON=77.5
2
c
1-methyl
1-methylcyclohexanol
(18)
31
cyclohexane
2
4
3
-methylcyclohexanol(4)
-methylcyclohexanol(6)
-methylcyclohexanol(3)
c
8
1-isopropyl
cyclohexane
2-cyclohexyl-2-
propanol(25)
29
TON=72
4
-isopropylcyclohexanol(4)
c
9
1
adamantane
1-adamantanol(7)
-adamantanone(3)
10
TON=25
2
b
0
ethylbenzene
1-phenylethanol (3)
acetophenone (51)
60
TON=169
2
-ethylphenol (6)
a
Reaction conditions (see Experimental Section): substrate (2.0 mmol, 0.16 M in CH CN), 1
3
(
0.04 mol %), CH CO H (0.5 equiv. relative to CH CN), H O (aqueous 30 wt %, 5mmol, in
3
2
3
2
2
o
b
CH CN, added dropwise over 5 min), 32 C, 180 min. Isolated yields based on an average of
3
c
3
runs. GC yields were determined using nitrobenzene as an internal standard for especially
volatile substrates and based on an average of 3 runs. *The remaining substrate remained
unreacted and no degradation of the substrate nor the products during the reaction.

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Fig 1. An ORTEP plot of tetra-iron (1) cluster with 30% ellipsoid probability
Fig 2. Plot of temperature dependence AC susceptibility for 1. Inset, the inverse susceptibility
data deviates from the red line below 240 K suggesting the starting temperature (Curie point)
of antiferromagnetic ordering.

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17
1.2
1
Abs
0.5
0
2
00
300
400 500
Wavelength[nm]
600
700
Fig 3. UV-Vis spectrum of the iron complex in MeCN complex

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Highlights
•
Synthesis and characterization of an oxido-acetato-bridged tetrairon(III)-
bipyridine complex
•
The iron(III) complex displays antiferromagnetic ordering of iron centres in the
solid state.
•
•
The iron complex efficiently activate C-H bond of both linear & cyclic alkanes
Probably generation of freely diffusing carbon-centered radical account for such
activity.