Diferric oxo-bridged complexes of a polydentate aminopyridyl ligand: Synthesis, structure and catalytic reactivity

Article abstract of DOI:10.1007/s11243-014-9875-0

The catalytic reactivity of a group of diferric oxo-bridged complexes (1-3) of a tetradentate ligand (bpmen = N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)-1,2-diaminoethane) toward alkane hydroxylation has been evaluated. Among the three complexes, the μ-oxo diiron(III) complex [Fe(bpmen)(μ-O)FeCl₃] (1) has been synthesized for the first time. The complex 1 has been characterized by spectroscopic analysis and X-ray crystallography. At room temperature, the μ-oxo diiron(III) complexes 1-3 have been found to be useful catalysts in hydroxylation of alkanes with m-chloroperbenzoic acid as oxidant. [Fe(bpmen)(μ-O)FeCl₃] (1) has been found to be the most active catalyst. Moreover, the catalytic ability of the complexes in the oxidation of alcohols to ketones with hydrogen peroxide at room temperature has also been investigated.

Full text of DOI:10.1007/s11243-014-9875-0

Transition Met Chem
DOI 10.1007/s11243-014-9875-0
Diferric oxo-bridged complexes of a polydentate aminopyridyl
ligand: synthesis, structure and catalytic reactivity
•
Ambica Kejriwal Achintesh Narayan Biswas
•
•
Amitava Choudhury Pinaki Bandyopadhyay
Received: 26 June 2014 / Accepted: 16 August 2014
ꢀ Springer International Publishing Switzerland 2014
Abstract The catalytic reactivity of a group of diferric
oxo-bridged complexes (1–3) of a tetradentate ligand
(bpmen = N,N0-dimethyl-N,N0-bis(2-pyridylmethyl)-1,2-
diaminoethane) toward alkane hydroxylation has been
evaluated. Among the three complexes, the l-oxo diiron(III)
complex [Fe(bpmen)(l-O)FeCl3] (1) has been synthesized
for the first time. The complex 1 has been characterized by
spectroscopic analysis and X-ray crystallography. At room
temperature, the l-oxo diiron(III) complexes 1–3 have been
found to be useful catalysts in hydroxylation of alkanes with
m-chloroperbenzoic acid as oxidant. [Fe(bpmen)(l-
O)FeCl3] (1) has been found to be the most active catalyst.
Moreover, the catalytic ability of the complexes in the oxi-
dation of alcohols to ketones with hydrogen peroxide at room
temperature has also been investigated.
among bioinorganic chemists over the past two decades [1–
3]. Given the high stability of the l-oxo diiron(III) motif, it
is not surprising that nature has chosen this motif as the
active sites of several proteins [4–9]. The l-oxo diiron(III)
unit has been found in several nonheme enzymes, which
include methane monooxygenase (MMO) [5, 6], ribonu-
cleotide reductase (RNR) [7, 8] and fatty acid desaturases
[9]. These enzymes carry out diverse biological reactions
such as hydroxylation of methane (MMO), toluene (toluene-
4-monooxygenase), generation of tyrosine radical (RNR)
and desaturation of saturated fatty acids. This fascinating
pattern of reactivity shown by the l-oxo diiron(III) motif
present in the enzymes has stimulated efforts to develop
analogous in model systems to provide a chemical basis for
understanding the reactivity of the metalloenzymes. Over
the last two decades, several complexes with the l-oxo
diiron(III) unit have been synthesized and characterized [1,
10–16]. Depending upon the bite angle of the nitrogen-rich
ligands employed, l-oxo diferric complexes with Fe–O–Fe
angles ranging from 180ꢁ to 114ꢁ have been obtained [10].
In this regard, tetradentate tripodal ligands such as TPA
(TPA = tris(2-pyridylmethyl)amine) and linear tetraden-
tate ligands such as bpmen (bpmen = N,N0-dimethyl-N,N0-
Introduction
Nonheme diiron centers bridged by oxo (or hydroxo) and
carboxylato groups have attracted a great deal of interest
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-014-9875-0) contains supplementary
material, which is available to authorized users.
bis(2-pyridylmethyl)-1,2-diaminoethane)
have
been
explored in designing a variety of dinuclear metal com-
plexes [1, 10, 14–17]. In particular, doubly bridged oxo/
hydroxo diferric complexes based on TPA have emerged as
excellent structural models for compound Q of sMMO [14].
On the other hand, l-oxo diferric complexes based on
bpmen ligand [11, 15, 16] have received much less attention
in this regard. Indeed a few well-characterized complexes
were reported but the investigation of their catalytic ability
is only limited to the epoxidation of alkenes [31]. So far, no
report on the catalytic behavior of l-oxo diferric complexes
of bpmen ligand toward hydroxylation of alkanes is
A. Kejriwal ꢀ P. Bandyopadhyay (&)
Department of Chemistry, University of North Bengal,
Raja Rammohunpur, Siliguri 734013, India
e-mail: pbchem@rediffmail.com
A. N. Biswas
Department of Chemistry, Siliguri College, Siliguri 734001,
India
A. Choudhury
Department of Chemistry, Missouri University of Science and
Technology, Rolla, MO 65409-0010, USA
123

Transition Met Chem
Table 1 Crystallographic data and processing parameters for com-
plex 1
N
N
N
A hyd. F
l
3
C
e
n
O
F l
C
e
3
Chemical formula
C16 H22 Cl4 Fe2 N4 O
N
N
N
N
e
F
Et H, RT
O
l
C
Formula weight
539.88
N
Crystal system, space group
Unit cell dimensions
Monoclinic, P(2)1/c
(
1. Fe ClO4)3
2. Na Ac
O
bp
men
(1)
Me
O
H
˚
a (A)
31.551(6)
(
F
e
l
C O )
4 3
˚
b (A)
10.670(2)
˚
c (A)
14.130(3)
3+
+
3
a (ꢁ)
b (ꢁ)
c (ꢁ)
90
N
F
N
102.226(3)
N
N
O
O
N
N
N
90
e
N
N
F
e
N
N
Fe
Fe
N
N
O
H
3
˚
Volume (A )
Z, Calculated density (mg m-3
4648.8(16)
O N
O
N
N
)
8, 1.543
Absorption coefficient (mm-1
Crystal size (mm)
)
1.721
0.25 9 0.16 9 0.11
1.98–25.00
(2)
(3
)
h range (ꢁ) for data collection
Reflections collected/unique
Scheme 1 Synthetic routes to complexes 1–3
44314/8190 [R(int) = 0.1837]
99.9 %
Completeness to theta = 25.00
Data/restraints/parameters
F(000)
Final R indices [I [ 2r (I)]a, b
R indices (all data)
8190/0/491
available in the literature. This prompted us to undertake a
detailed study on the catalytic behavior of l-oxo diferric
complexes of bpmen ligand toward room-temperature
hydroxylation of alkanes. A group of l-oxo (1), (l-oxo) (l-
hydroxo) (2) [15] and (l-oxo) (l-acetato) (3) [11] bridged
diiron(III) complexes based on the bpmen ligand has been
chosen to examine their catalytic ability toward room-tem-
perature alkane hydroxylation with m-chloroperbenzoic
acid. The l-oxo diiron(III) complexes (1–3) differ in their
coordination environment around the iron(III) centers as
well as the Fe–O–Fe angles (Scheme 1). The oxo-bridged
diferric complex [Fe(bpmen)(l-O)FeCl3] (1) has been
synthesized for the first time and structurally characterized.
Moreover, the catalytic efficacy of the above complexes has
been further compared in oxidation of benzyl alcohols at
room temperature with hydrogen peroxide.
2192
R1 = 0.0420, wR2 = 0.1092
R1 = 0.0518, wR2 = 0.1130
1.012
GOF (on F2)
a
R1 = (R||F0|-|Fc||)/(R|F0|)
wR2 = [Rw(F20-F2c)2/Rw(F02)2]1/2
b
(GC) analyses were performed on a Clarus-500 (Perkin-
Elmer) GC instrument equipped with an FID detector.
Synthesis of the complex [Fe(bpmen)(l-O)FeCl3] (1)
To a stirring solution in ethanol (2.0 mL) of the ligand
bpmen (0.17 g, 0.629 mmol), a 2.0 mL solution of anhy-
drous FeCl3 (0.102 g, 0.629 mmol) in 2.0 mL ethanol was
added. After the addition, immediate precipitation of
brown solid was observed. The precipitate was filtered off
and washed with diethyl ether (Yield 0.152 g, 45 %).
Anal.Calc. C, 34.54; H, 4.33; N, 10.02. Found C, 34.58; H,
4.37; N, 10.32 %. kmax/nm (e/M-1 cm-1) = 269 (25,850),
339 (12,170), 400 (2,580).
Experimental section
Materials
All chemicals were of reagent grade and were used without
further purification. The ligand N,N0-dimethyl-N,N0-bis(2-
pyridylmethyl)-1,2-diaminoethane) (bpmen) and the diiron
complexes, viz., [Fe2(l-O)(l-OH)(bpmen)2] (2) and
[Fe2(l-O)(l-CH3CO)(bpmen)2] (3) were prepared accord-
ing to the published procedures [11, 15, 17]. Elemental
microanalyses (C, H and N) were obtained with either a
Perkin-Elmer (Model 240C) or Heraeus Carlo Erba 1108
elemental analyzer. Electronic spectra were recorded on an
Agilent-5843 spectrophotometer. Gas chromatography
X-ray crystallography
Single crystals of 1 were grown by slow diffusion of die-
thyl ether into an acetonitrile solution of the complex.
Selected crystal data and data collection parameters are
given in Table 1. Data on the crystals were collected on a
Bruker SMART 1000 CCD area-detector diffractometer
123

Transition Met Chem
˚
using graphite monochromated MoKa (k = 0.71073 A)
radiation by x scan. The structure was solved by direct
methods using SHELXS-97 [18] and difference Fourier
syntheses and refined with the SHELXL97 package
incorporated in WinGX 1.64 crystallographic collective
package [19]. All the hydrogen positions for the compound
were initially located in the difference Fourier map, and for
the final refinement, the hydrogen atoms were placed
geometrically and held in the riding mode. The last cycles
of refinement included atomic positions for all the atoms,
anisotropic thermal parameters for all nonhydrogen atoms
and isotropic thermal parameters for all the hydrogen
atoms. Full-matrix-least-squares structure refinement
against |F2|. Molecular geometry calculations were per-
formed with PLATON [20], and molecular graphics were
prepared using ORTEP-3 [21].
Fig. 1 Ortep plot of the complex 1 [Fe(bpmen)(l-O)FeCl3] showing
50 % probability ellipsoids
CCDC-989145 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
ethanol under aerobic conditions afforded the l-oxo diir-
on(III) complex 1. The other two complexes (2 and 3) have
been synthesized following a procedure already reported in
the literature [11]. All the complexes have been charac-
terized by elemental analysis and ESIMS studies. More-
over, the solid state structure of complex 1 has been
confirmed by single-crystal X-ray diffraction. The molec-
ular structure of the complex 1 is shown in Fig. 1. The
crystal structure of 1 reveals two chemically identical
complexes with almost identical geometries per asym-
metric unit. The differic complex is comprised of two iron
atoms with nonequivalent coordination spheres, in which
Fe1 is six-coordinated in a pseudo-octahedral environment
and Fe2 is tetra-coordinated. The two iron atoms are
bridged by the oxo donor. Two pyridyl nitrogen atoms (N1
and N4), the two nitrogen atoms from the tertiary amines of
the ligand bpmen (N2 and N3) and a chloride ligand con-
stitute the coordination sphere of Fe1. In contrast, Fe2 is
coordinated to the oxo donor and three chlorides. The
presence of a linear l-oxo diiron(III) unit in Complex 1 is
reflected by the Fe–O–Fe bond angle of 171.21(15)ꢁ with
Catalytic reactions
Homogeneous catalytic oxidation reactions were carried
out in small screw-capped vials fitted with PTFE septa. In a
typical reaction, 0.7 mM concentration of catalyst and
700 mM concentration of substrate were taken in 2 mL of
acetonitrile. The oxidation reaction was initiated by adding
H2O2 (final concentration: 7.0 mM), and the contents were
stirred using a magnetic bar. A standard solution of iod-
opentaflurobenzene was added to the reaction mixture as
internal standard, and an aliquot was injected into a pre-
heated GC. The identification and the quantization of the
products were done from the response factors of standard
product samples as usual. In the case of oxidation with m-
CPBA, an acetonitrile solution (1.0 mL) of this oxidant
(70 mM) was added to an acetonitrile solution (1.0 mL) of
catalyst (0.7 mM) and alkane (700 mM) under nitrogen
atmosphere with vigorous stirring. After 1 h, the iod-
opentaflurobenzene was added to the reaction mixture as an
internal standard and directly analyzed by GC. In the case
adamantane, CH3CN–CH2Cl2 (2:3, v/v) was used as the
solvent due to the poor solubility of adamantane in
acetonitrile.
˚
an Fe–Fe distance of 3.531 A, which is considerably longer
than that obtained for the (l-oxo) (l-hydroxo)diiron(III)
complex (2) (Table 2) [17]. The Fe–l-O bond lengths in 1
are typical of such bonds reported in the literature
˚
(1.788(2) and 1.752(2) A, respectively). As expected, the
˚
Fe–Npy bond lengths (average 2.17 A) are shorter than the
˚
Fe–Namine distances (average 2.26 A).
Results and discussion
The visible range electronic spectra of complexes 1, 2
and (l-oxo) (l-acetato)diiron(III) complex (3) in acetoni-
trile at room temperature are shown in Fig. 2. The spectral
absorptions of l-oxo diiron(III) complexes have been
shown to be dependent on their Fe–O–Fe angles. Com-
plexes containing a linear Fe–O–Fe unit normally show
Synthesis and characterization
The synthetic procedures for the l-oxo diiron(III) com-
plexes (1–3) have been outlined in Scheme 1. Reaction of
equimolar amounts of bpmen and anhydrous FeCl3 in
123

Transition Met Chem
˚
Catalytic performance of l-oxo diiron(III) complexes
Table 2 Selected bond lengths (A) and bond angles (ꢁ) for the
complexes 1–3
1a
2b
3c
Hydroxylation of alkanes
˚
Bond lengths (A)
The catalytic efficacy of the l-oxo diiron(III) complexes
(1–3) has been evaluated toward activation of unactivated
C–H bonds, such as those in cycloalkanes. Given the
higher bond dissociation energies of cycloalkanes
(99.3 kcal/mol for cyclohexane), activation of C(sp3)–H
bonds is extremely challenging [23, 24]. The performance
of the complexes in alkane hydroxylation at room tem-
perature with m-CPBA as the oxidant has been compiled in
Table 3. As evident from Table 3, the unsymmetric l-oxo
diiron(III) complex 1 emerges as a modest catalyst in
alkane hydroxylation at room temperature. With a catalyst:
oxidant: substrate molar ratio of 1:100:1000, the oxidation
of cyclohexane has been found to be completed within an
hour. Catalytic work-up and GC analysis showed the
generation of cyclohexanol and cyclohexanone as the
oxidation products. Conversion around 22 % was observed
with a higher selectivity for the alcohol product (A/
K = 3.4). Under identical reaction conditions, complex 3
has shown similar activity in cyclohexane oxidation
affording 21 % oxygenates (A/K = 3.0) in 1 h. In contrast,
complex 2 has been found to be a much less reactive in
cyclohexane oxidation (Entry 2, Table 3). Similar results
are obtained in case of cyclooctane oxidation. It is inter-
esting to note that complex (2) was found to be a much
superior catalyst to complex (3) in olefin epoxidation [31].
In order to gain further insight, the reaction of complex 2
with m-CPBA in presence of cyclohexane has been mon-
itored by UV–Vis spectroscopy. Addition of m-CPBA to
the reaction mixture containing complex 2 (1.0 mM) and
cyclohexane in acetonitrile leads to the appearance of a
band around 615 nm. Identical spectral changes have been
observed in the absence of substrate. The observation is
indicative of the formation of the salicylate bound iron(III)
complex formed via ortho-hydroxylation of m-CPBA akin
to what observed in reactions of mononuclear nonheme
iron(II) complexes with m-CPBA [25]. Indeed, after acet-
ylation of the bluish green reaction mixture with acetic
anhydride and 1-methyl imidazole [25], GC analysis con-
firms the presence of 3-chlorosalicylic acid (0.4 TN) in the
present case. Thus, catalytic hydroxylation and ortho-
hydroxylation of m-CPBA have been found to be com-
petitive in this case, which in turn justifies the lower cat-
alytic efficiency of 2 in alkane hydroxylation.
Fe(1)…Fe(2)
Fe(1)–l-O(1)
Fe(2)–l-O(1)
Fe(1)–N(1)
Fe(1)–N(4)
Fe(1)–N(2)
Fe(1)–N(3)
Bond angles (ꢁ)
Fe1–(l-O1)-Fe2
a
3.531 (4)
2.8212 (19)
1.884 (4)
1.892 (4)
2.115 (5)
2.124 (5)
2.215 (5)
2.226 (5)
3.253 (1)
1.804 (3)
1.804 (3)
2.156 (6)
2.143 (6)
2.205 (6)
2.260 (6)
1.784 (2)
1.755 (2)
2.145 (2)
2.159 (2)
2.277 (3)
2.239 (3)
171.21 (15)
96.68 (19)
128.7 (4)
Each asymmetric unit contains two independent molecules
b
c
Ref. [15]
Ref. [17]
Fig. 2 UV–Vis spectra of the complexes (1–3) in acetonitrile at room
temperature (concentration of the complexes = 1.0 mM)
bands in the 600–700 nm region and are assignable to oxo-
to-iron(III) charge transfer transitions. As the Fe–O–Fe
angles decrease, such bands shift toward lower wave-
lengths [22]. Moreover, the intensities of bands in the
400–550 nm regions increase when the Fe–O–Fe angles
become more and more acute. This trend is well main-
tained in the symmetrical diiron(III) complexes 2 and 3.
However, no absorption corresponding to oxo-to-iron(III)
charge transfer is obtained in the case of complex 1, which
can be attributed to the asymmetric coordination environ-
ment around the two iron(III) centers. The UV–Vis spec-
trum of complex 1 (ESI, Fig. S1) in acetonitrile exhibits an
intense feature at 250 nm, which is assigned to a pyridyl p–
p* transition. Moreover, it shows characteristic bands at
339 and 400 nm, assignable to the transitions arising from
ligand to metal charge transfer.
The catalytic property of the complexes has also been
studied in the oxidation of adamantane (Entry 3, Table 3).
The catalytic activity of the diiron(III) complexes toward
oxidation of adamantane follows the trend 1 [ 3 [ 2. In the
case of catalyst 1, adamantane has been found to get oxi-
dized to a mixture of 1-adamantanol (25 %), 2-adamantanol
123

Transition Met Chem
O
O
IV
Table 3 Oxidation of alkanes by m-CPBA catalyzed by complexes 1,
2 and 3 at room temperature
O
FeIII
O
FeIII
O
FeIII
O
Fe
O
FeIII
FeIII
O-O homolysis
m-CPBA
Entry Substrate
1
Catalyst Products (% yield)a
A/K
3ꢁ/2ꢁ
O
X
O
Ar
Ar
1
2
3
1
2
3
1
Cyclohexanol (17)
Cyclohexanone (05)
Cyclohexanol (09)
Cyclohexanone (03)
Cyclohexanol (17)
Cyclohexanone (04)
Cyclooctanol (25)
3.4
–
X
-
OH
OAc
Complex
O-O hoterolysis
1
2
3
2
3
4
5
6
7
3.0
4.3
3.5
2.75
3.3
–
–
O
O
OH
FeIII
O
V
Fe
–
O
O
–
Ar
Cyclooctanone (07)
Cyclooctanol (11)
Scheme 2 Plausible mechanism for alkane hydroxylation by the l-
oxo diiron(III)complexes (1–3) and m-CPBA
–
Cyclooctanone (04)
Cyclooctanol (20)
–
Cyclooctanone (06)
Adamantan-1-ol (25)
Adamantan-2-ol (02)
Adamantan-2-one (04)
Adamantan-1-ol (12)
Adamantan-2-ol (02)
Adamantan-2-one (01)
Adamantan-1-ol (22)
Adamantan-2-ol (03)
Adamantan-2-one (02)
transient in the present case is also consistent with high A/
K ratio during cycloalkane oxidation and moderately high
3ꢁ/2ꢁ ratio during oxidation of adamantane.
12.5
8
9
2
3
–
–
12.0
13.2
Oxidation of alcohols
The reactivity of the l-oxo diiron(III) cores toward alkanes
with m-CPBA encouraged us to use mild and environ-
mentally benign hydrogen peroxide as the oxidant. Akim-
ova et al. [31] have previously studied catalytic olefin
epoxidation by the complexes 2 and 3 with H2O2. Complex
3 has been found to be catalytically inactive while complex
2 afforded epoxides and trace amount of cis-diols. There-
fore, in order to evaluate their ability toward catalytic
hydroxylation, we attempted reactions of cycloalkanes with
H2O2 in the presence of complexes (1–3). But apart from
complex 2, the other two complexes turn out to be inactive
as \1 % conversion of H2O2 into products has been
obtained. However, the l-oxo diiron(III) complexes have
been found to catalyze oxidation of alcohols at room
temperature with hydrogen peroxide. Their catalytic per-
formance has been compared using benzyl alcohol as the
substrate. Interestingly, complex 1 has been found to pro-
mote oxidation of benzyl alcohol with significantly higher
yield than 2 or 3 (Table 4). While the catalytic perfor-
mance of complex 3, the so called ‘thermodynamic sink’
[32] is not surprising, somewhat lesser yield observed in
the oxidation of benzyl alcohol to benzaldehyde catalyzed
by complex 2 with H2O2 is unusual. However, when this
reaction is followed by UV–Vis spectroscopy, formation of
a band at 615 nm is observed and the solution turns blue-
green (Fig. 3). The spectroscopic feature is indicative of
the formation of an iron(III) complex bearing a 2-hydroxy
benzyl alcohol ligand derived from the aromatic ring
hydroxylation of benzyl alcohol at the ortho-position
(Scheme 3). Similar changes in electronic spectra have
previously been observed in reactions between the non-
heme iron(II) complex [FeII(bqen)]2? (bquen = N,N0-
Reaction condition: 0.7 mM catalyst, 700 mM substrate, 70 mM m-CPBA in
acetonitrile (in case of adamantane, 2:3 acetonitrile/dichloromethane, v/v was
used as solvent) at room temperature
a
Yields are reported with respect to the initial concentration of the oxidant
(02 %) and 2-adamantanone (04 %). The normalized 3ꢁ/2ꢁ
ratio in this case has been found to be 12.5. It is noteworthy
that an average 3ꢁ/2ꢁ value of 2.7 has been reported in ‘gif
type’ oxidation of adamantane, [26]. In contrast, hydroxyl
radical mediated oxidation of adamantane typically yields
3ꢁ/2ꢁ values close to 2.0 [27]. The normalized 3ꢁ/2ꢁ values
obtained in the present study are close to that obtained for a
structurally similar l-oxo diiron(III) complex [Fe2-
O(bipy)4(H2O)2](ClO4)4 [28]. The normalized 3ꢁ/2ꢁ ratios
for the Fe(II)TPA (CH3CN)2/H2O2 system and PhIO cata-
lyzed P450 mimics are 15–33 [29] and 11–48 [30], respec-
tively. The enhanced regioselectivity with this catalyst using
m-CPBA indicates the involvement of a metal-centered
oxidant as an active intermediate in the oxidation of the C–H
bonds of adamantane.
The exact mechanism for alkane hydroxylation with m-
CPBA by the l-oxo diiron(III) motifs is not completely
clear to us. However, the sequence shown in Scheme 2
seems probable. We propose that addition of m-CPBA to
the diferric complexes generates a benzoylperoxodi-
iron(III) species, [Fe2O(L)2(OOCOC6H4Cl)]3?. This puta-
tive species can undergo either O–O bond homolysis or
heterolysis leading to the formation of high-valent iro-
n(IV)-oxo or iron(V)-oxo species, which in turn oxidizes
the substrate. The formation of high-valent oxoiron
dimethyl-N,N0-bis(8-quinolyl)ethane-1,2-diamine)
and
123

Transition Met Chem
Table 4 Oxidation of alcohols to carbonyl compounds by hydrogen peroxide catalyzed by complexes 1, 2 and 3 at room temperature
Entry
Substrate
Catalyst
Products (% yield)a
1
2
3
4
5
6
1
2
3
1
2
3
Benzaldehyde (90)
Benzaldehyde (65)
Benzaldehyde (60)
4-Nitro benzaldehyde (80)
4-Nitro benzaldehyde (100)
4-Nitro benzaldehyde (75)
7
8
9
1
2
3
4-Methoxy benzaldehyde (70)
4-Methoxy benzaldehyde (60)
4-Methoxy benzaldehyde (50)
Reaction condition: 0.7 mM catalyst, 700 mM substrate, 7 mM H2O2 in acetonitrile at room temperature
a
Yields are reported with respect to the initial concentration of the oxidant
observed for 4-nitro benzyl alcohol with 2/H2O2 and the
corresponding benzaldehyde is obtained quantitatively.
Conclusion
The Catalytic activity of a group of l-oxo-bridged diferric
complexes of the bpmen ligand (1–3) differing in the Fe–O–Fe
angles and the coordination environment around iron(III)
toward alkane hydroxylation has been studied. For the first
time, it has been demonstrated that diferric oxo-bridged
complexes of the bpmen ligand can catalyze the hydroxyl-
ation of alkanes at room temperature using m-CPBA. Among
the three diferric complexes, complex 1 is hitherto unknown.
The complex (1) featuring a linear l-oxo bridge was char-
acterized by single-crystal X-ray crystallography. Inert
alkanes such as cyclohexane (BDE = 99 kcal/mol) and
cyclooctane (BDE = 96 kcal/mol) have been oxidized to
the corresponding hydroxyalkanes with moderate selectivity
(A/K = 3.0–3.5). The Catalytic reactivity of complex 1 in
alkane hydroxylation has been found to be greater than the
other two l-oxo diiron(III) complexes (2 and 3). The diir-
on(III) complexes (1–3) are alsoeffective in catalytic alcohol
oxidation with environmentally benign H2O2 as the oxidant
at room temperature. The catalytic activity of the diiron(III)
complexes toward oxidation of benzylic alcohols follows the
trend 1 [ 3 [ 2. The results clearly demonstrate the cata-
lytic potential of suitable l-oxo diiron(III) complexes toward
oxyfunctionalization of hydrocarbons under mild condition.
Further studies aimed at developing practical and syntheti-
cally useful catalytic systems based on oxo-bridged diir-
on(III) complexes are in progress in our laboratory.
Fig. 3 UV–Vis spectral changes in the reaction of 1.0 mM [Fe2(l-
O)(l-OH)(bpmen)2] (2) and H2O2 (10 mM) in presence of 100 mM
benzyl alcohol in acetonitrile at 298 K
Scheme 3 Ortho-hydroxylation of benzyl alcohol by 2/H2O2
benzyl alcohol with peracetic acid [33]. Complete absence
of such iron(III) species in case of complexes 1 and 3
suggests that only complex 2 is capable of hydroxylating
the aromatic ring of benzyl alcohol. This is further sup-
ported by the fact that no aromatic hydroxylation is
Acknowledgments The financial support from SERB (SR/S1/IC-
53/2010), DST, Government of India and CSIR (1/2498/11/EMR-II),
123

Transition Met Chem
New Delhi, is gratefully acknowledged. We thank CSIR (India) for
the award of fellowship to AK.
17. Okuno T, Ito S, Ohba S, Nishida Y (1997) J Chem Soc Dalton
Trans 19:3547
¨
18. Sheldrick GM (1997) SHELX97, University of Gottingen,
Germany
19. Farrugia LJ (2003) WinGX Version 1.64, An Integrated System
of Windows Programs for the Solution, Refinement and Analysis
of Single-Crystal X-ray Diffraction Data, Department of Chem-
istry, University of Glasgow
References
1. Que L Jr, Tolman WB (2002) Angew Chem Int Ed 41:1114
2. Kurtz DM Jr (1990) Chem Rev 90:585
3. Tshuva EJ, Lippard SJ (2004) Chem Rev 104:987
4. Wallar BJ, Lipscomb JD (1996) Chem Rev 96:2625
5. Merkx M, Kopp DA, Sazinsky MH, Blazyk JL, Mu¨ller J, Lippard
SJ (2001) Angew Chem Int Ed 40:2782
6. Rosenzweig AC, Fredrick CA, Lippard SJ, Nordlund P (1993)
Nature 366:537
7. Jordan A, Reichard P (1998) Annu Rev Biochem 67:71
20. Spek AL (2003) PLATON. J Appl Crystallogr 36:7
21. Farrugia LJ (1997) J Appl Crystallogr 30:565
22. Poussereau S, Blondin G, Cesario M, Guilhem J, Chottard G,
Gonnet F, Girerd JJ (1998) Inorg Chem 37:3127
23. White MC (2012) Science 335:807
24. Chen MS, White MC (2010) Science 327:566
25. Makhlynets OV, Das PP, Taktak S, Margaret F, Ruben MB,
Rybak-Akimova EV, Que L Jr (2009) Chem Eur J 15:13171
26. Barton DHR, Boivin J, Motherwell WB, Ozabalik N, Schwarz-
entruber KM, Jankowski K (1986) New J Chem 10:387
27. Singh B, Long JR, De Biani FF, Gatteschi D, Stavropoulos P
(1997) J Am Chem Soc 119:7030
28. Menage S, Vincent JM, Lambeaux C, Fontecave M (1994) J
Chem Soc Dalton Trans 2081
29. Kim C, Chen K, Kim J, Que L Jr (1997) J Am Chem Soc
119:5964
30. Kitajima N, Fukui H, Moro-oka Y (1988) J Chem Soc Chem
Commun 7:485
˚
8. Logan DT, Su XD, Aberg A, Regnstrom K, Hajdu J, Eklund H,
¨
Nordlund P (1996) Structure 4:1053
9. Broadwater JA, Ai J, Loehr TM, Sanders-Loehr J, Fox BG (1998)
Biochemistry 37:14664
´
10. Norman RE, Holz RC, Menage S, O’Conor CJ, Zhang JH, Que L
Jr (1990) Inorg Chem 29:4629
11. Hazell R, Jensen KB, McKenzie CJ, Toftlund H (1995) J Chem
Soc Dalton Trans 5:707
´
12. Esmelindro MC, Oestreicher EG, Marquez-Alvarez H, Dariva C,
Egues SMS, Fernandes C, Bortoluzzi AJ, Drago V, Antunes OAC
(2005) J Inorg Biochem 99:2054
31. Taktak S, Kryatov SV, Hass TE, Rybak-Akimova EV (2006) J
Mol Catal A Chem 259:24
13. Khattar R, Hundal MS, Mathur P (2012) Inorg Chim Acta
390:129
14. Xue G, De Hont R, Mu¨nck E, Que L Jr (2010) Nat Chem 2:400
15. Taktak S, Kryatov SV, Rybak-Akimova EV (2004) Inorg Chem
43:7196
32. Norman RE, Yan S, Que L Jr, Backes G, Ling J, Loehr JS, Zhang
JH, O’Connor JC (1990) J Am Chem Soc 112:1554
33. Yoon J, Wilson SA, Jang YK, Seo MS, Nehru K, Hedman B,
Hodgson KO, Bill E, Solomon EI, Nam W (2009) Angew Chem
Int Ed 48:1257
16. Duban EA, Drebushchak TN, Bryliakov KP, Talsi EP (2007)
Mendeleev Commun 17:291
123