Highly selective hydroxylation of alkanes catalyzed by (μ-oxo)bis(μ- carboxylato)-bridged diiron(iii) complexes: Involvement of mononuclear iron(iii) species in catalysis

Article abstract of DOI:10.1039/b901508f

A few diiron(iii) complexes [Fe₂(O)(OAc)₂(L1) ₂](ClO₄)₂1, [Fe₂(O)(OBz) ₂(L1)₂](ClO₄)₂2, [Fe ₂(O)(OAc)₂(L2)₂](ClO₄)₂3 and [Fe₂(O)(OBz)₂(L2)₂](ClO₄) ₂4, where L1 = N,N-bis(pyrid-2-ylmethyl)-iso-butylamine, L2 = N,N-bis(pyrid-2-ylmethyl)benzylamine, AcO = acetate and BzO = benzoate, have been isolated and characterized by means of elemental analysis and spectral and electrochemical methods. The molecular structures of the complexes 2 and 4 have been determined by single-crystal X-ray diffraction analysis and they possess a distorted bioctahedral geometry in which each iron atom is coordinated to the oxygen atom of the μ-oxo bridge, two oxygen atoms of the μ-benzoato bridges and three nitrogen atoms of L1 and L2 ligands capping the two ends of the diiron(iii) cluster. The ESI-MS spectral data of the complexes reveal that the complexes remain intact in dichloromethane (DCM) solution. Upon adding one equivalent of Et₃N to a mixture of one equivalent of the diiron(iii) complexes and excess of m-chloroperbenzoic acid (m-CPBA) in DCM, an intense absorption band (λ_max, 670-700 nm) appears, which corresponds to the species [Fe₂(O)(OAc)(m-CPBA)(L)₂]²⁺ (ESI-MS, m/z 466) suggested as the intermediate involved in the oxygenation reactions. All the present complexes show efficient alkane hydroxylation with 300-400 turn over numbers and good selectivities for cyclohexane (A/K, 10-14) and adamantane (3°/2°, 9-11). Interestingly, the formation of monoiron(iii) species has been discerned in the alkane hydroxylation reactions beyond ～50 turnovers. The mononuclear 1: 1 iron(iii) complexes of L1 and L2 ligands generated in situ are also found to catalyze the oxygenation reactions with high selectivity and efficiency for cyclohexane (A/K, 10-14). Upon their reaction with m-CPBA in DCM, a characteristic absorption band (λ_max, 600 nm, ε_max, 355 M^-1 cm^-1) appears and decays at room temperature. This spectral feature is consistent with the mononuclear high-valent iron-oxo species suggested as an intermediate in the oxygenation reactions.

Full text of DOI:10.1039/b901508f

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PAPER
www.rsc.org/dalton | Dalton Transactions
Highly selective hydroxylation of alkanes catalyzed by
l-oxo)bis(l-carboxylato)-bridged diiron(III) complexes: involvement of
mononuclear iron(III) species in catalysis†‡
(
a
b
a
Kusalendiran Visvaganesan, Eringathodi Suresh and Mallayan Palaniandavar*
Received 23rd January 2009, Accepted 2nd March 2009
First published as an Advance Article on the web 24th March 2009
DOI: 10.1039/b901508f
A few diiron(III) complexes [Fe
Fe (O)(OAc) (L2) ](ClO 3 and [Fe
ylmethyl)-iso-butylamine, L2 = N,N-bis(pyrid-2-ylmethyl)benzylamine, AcO = acetate and
2
(O)(OAc)
2
(L1)
2
](ClO
4
)
2
1, [Fe
2
(O)(OBz)
2
(L1)
2
](ClO
4
)
2
2,
[
2
2
2
4
)
2
2
(O)(OBz)
2
(L2)
2
4 2
](ClO )
4, where L1 = N,N-bis(pyrid-2-
BzO = benzoate, have been isolated and characterized by means of elemental analysis and spectral and
electrochemical methods. The molecular structures of the complexes 2 and 4 have been determined by
single-crystal X-ray diffraction analysis and they possess a distorted bioctahedral geometry in which
each iron atom is coordinated to the oxygen atom of the m-oxo bridge, two oxygen atoms of the
m-benzoato bridges and three nitrogen atoms of L1 and L2 ligands capping the two ends of the
diiron(III) cluster. The ESI-MS spectral data of the complexes reveal that the complexes remain intact in
dichloromethane (DCM) solution. Upon adding one equivalent of Et
of the diiron(III) complexes and excess of m-chloroperbenzoic acid (m-CPBA) in DCM, an intense
absorption band (lmax, 670–700 nm) appears, which corresponds to the species [Fe (O)(OAc)(m-
(ESI-MS, m/z 466) suggested as the intermediate involved in the oxygenation reactions.
All the present complexes show efficient alkane hydroxylation with 300–400 turn over numbers and
3
N to a mixture of one equivalent
2
2
+
CPBA)(L) ]
2
◦
◦
good selectivities for cyclohexane (A/K, 10–14) and adamantane (3 /2 , 9–11). Interestingly, the
formation of monoiron(III) species has been discerned in the alkane hydroxylation reactions beyond
~
50 turnovers. The mononuclear 1 : 1 iron(III) complexes of L1 and L2 ligands generated in situ are also
found to catalyze the oxygenation reactions with high selectivity and efficiency for cyclohexane (A/K,
0–14). Upon their reaction with m-CPBA in DCM, a characteristic absorption band (lmax, 600 nm,
1
e
max, 355 M- cm ) appears and decays at room temperature. This spectral feature is consistent with the
1
-1
mononuclear high-valent iron-oxo species suggested as an intermediate in the oxygenation reactions.
Introduction
and also study the mechanism of O activation involved in the
2
7,8
oxygenation of alkanes. The active site of reduced form of
sMMO is composed of a diiron center containing four glutamate
and two histidine residues, in which the iron atoms are bridged by
two carboxylate ligands from glutamate residues while the oxidized
form contains only one carboxylate bridge and two hydroxo
The catalytic and selective functionalization of hydrocarbons
under mild conditions is a subject of great current interest
in bioinorganic and synthetic chemistry. The iron-containing
1
biological molecules such as soluble methane monooxygenases
(
sMMO), cytochrome P450 and bleomycin oxidize an aliphatic
9
bridges (sMMOox, Scheme 1) in its active site. It is fascinating that
2
C–H bond of alkanes to give an alcohol product. The non-
heme sMMO enzyme shows remarkable catalytic activity to-
wards the oxidation of a variety of alkanes including methane
several non-heme diiron proteins such as ribonucleotide reductases
10
11
(
RNR) and methemerythrin (Hr) also contain at least two
carboxylate bridges in the active site of their reduced forms. So, the
3–6
using dioxygen. The selectivity, efficiency and mild reaction
conditions of these reactions indicate a methodology different
from traditional synthetic processes, which usually require high
temperatures and pressures. Efforts have been made during the
last two decades to develop synthetic analogues for sMMO
isolation and study of synthetic models containing the structural
2+
core [Fe
2
(O)(O
2
CR)
2
]
have received significant interest among
a
School of Chemistry, Bharathidasan University, Tiruchirappalli, 620 024,
India. E-mail: palaniandavarm@gmail.com, palanim51@yahoo.com
b
Analytical Science Discipline, Central Salt and Marine Chemicals Research
Institute, Bhavnagar, 364 002, India
th
†
Dedicated to Prof. C. N. R. Rao on the occasion of his 75 birthday.
‡
Electronic supplementary information (ESI) available: ESI-MS spec-
trum, product analysis. CCDC reference numbers 712228 and 712232.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/b901508f
Scheme 1 Active site structures of sMMO in its reduced and oxidized
forms.
3
814 | Dalton Trans., 2009, 3814–3823
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1
2–14
◦
◦
the chemical and biochemical communities.
containing the structural motif [Fe (O)(O CR)
a tridentate 3N ligand, have been isolated and studied as models
Synthetic models
over numbers and excellent selectivity (A/K ratio, 10–14; 3 /2 ,
9–11). Also, in the hydroxylation reactions the diiron(III) com-
plexes decompose to form a catalytically-active mononuclear
iron(III) species after ~50 turn-overs.
2
+
2
2
2
(L) ] , where L is
2
15–19
for metHr and sMMO.
Christou et al. used the triply-bridged
derived from bipyridine for alkane hy-
20,21
diiron(III) complexes
droxylation using tert-butylhydroperoxide (tert-BuOOH) as the
oxygen source while Kitajima et al. isolated the triply-bridged
diiron(III) complexes derived from trispyrozolylborate ligand for
alkane hydroxylation using molecular oxygen (in the presence of
an electron source) but lower selectivities and poor yields were ob-
served for these reactions. Later, Kodera et al. found that the com-
Experimental
22
Materials
Pyridine-2-carboxaldehyde,
iso-butylamine,
benzylamine,
AgClO , iron(III) perchlorate hydrate, adamantane, sodium triace-
4
plex [Fe
bis(pyrid-2-ylmethyl))-6-pyridyl]ethane, exhibits a high activity
with notable alcohol/ketone ratio (A/K, 2.4) for the hydroxylation
2
(O)(O
2
CCH
3
)
2
(hexpy)](ClO
4
)
2
, where hexpy is 1,2-bis[2-
toxyborohydride, m-chloroperbenzoic acid (Aldrich), triethyl-
amine, benzoic acid, acetic acid glacial, dichloromethane,
diethylether, tetrahydrofuran (Merck, India), cyclohexane
(
◦
◦
of cyclohexane and tertiary/secondary carbon ratio (3 /2 , 4.2)
(Ranbaxy) and ethanol (Hayman Limited, England) were
for monooxygenation of adamantane using m-chloroperbenzoic
23–25
26
used as received. Acetonitrile was distilled before use. The
supporting electrolyte tetrabutylammonium perchlorate (TBAP)
was prepared in water and recrystallized twice from aqueous
ethanol.
acid (m-CPBA).
Very recently, Itoh et al. have isolated the
triply-bridged diiron(III) complex [Fe
where L is the dinucleating carboxylate ligand 1,2-bis(N-benzyl-
-aminomethyl-6-pyridyl)ethane-N¢,N¢-diacetic acid, and found
higher selectivities for the hydroxylation of cyclohexane (A/K, 10)
2
(O)(O
2
CCH
3
)
2
(L)](ClO
4
)
2
,
2
◦
◦
and for monooxygenation of adamantane (3 /2 , 13.6) using
hydrogen peroxide. However, the reasons for the high selectivity
and the mode of action of these complexes in alkane hydroxylation
remain unclear. Also, many oxo-bridged diiron(III) complexes of
Synthesis of ligands
The ligand N,N-bis(pyrid-2-ylmethyl)-iso-butylamine (L1) and
N,N-bis(pyrid-2-ylmethyl)benzylamine (L2) were prepared by a
method reported previously.
lamine and sodium triacetoxyborohydride (3.75 g, 17.7 mmol) in
dichloromethane (100 mL) was added pyridine-2-carboxaldehyde
27–32
tetradentate ligands have been isolated
as synthetic models for
36,37
To a mixture of (4.5 mmol) alky-
the diiron biosites, but the catalytic efficiency and selectivity of
these systems are still lower than those of the enzymes.
All the above observations prompted us to embark on
the isolation of diiron(III) complexes of tri- and tetraden-
tate ligands with various carboxylate bridging ligands and
study of their use as models for the benzene/alkane hy-
(1.45 g, 13.5 mmol) and stirred for 18 h. The reaction was quenched
with saturated sodium hydrogencarbonate and extracted with
ethylacetate (3 ¥ 150 mL). The organic fractions were combined,
dried (Na
2
SO ) and the solvent was removed under reduced
4
33–35
droxylation reactions catalyzed by sMMO.
In this study,
pressure. The residue was redissolved in tetrahydrofuran (50 mL)
and treated with NaH (0.22 g, 9.1 mmol). After the mixture was
stirred for 2 h, the solvent was removed and the residue was
extracted with dichloromethane. The extracts were combined and
the solvent was evaporated under reduced pressure to obtain the
we have isolated the (m-oxo)bis(m-carboxylato)diiron(III) com-
plexes [Fe
2
(O)(OAc)
2
(L1)
2
](ClO
4
)
2
1, [Fe
2
(O)(OBz)
(O)(OBz)
2
2
(L1)
(L2)
2
2
](ClO
](ClO
4
)
)
2
2
2, [Fe
2
(O)(OAc) (L2)
2
2
](ClO
4 2
)
3 and [Fe
2
4
4
, where L1 = N,N-bis(pyrid-2-ylmethyl)-iso-butylamine, L2 =
N,N-bis(pyrid-2-ylmethyl)benzylamine (Scheme 2), AcO = ac-
etate and BzO = benzoate, aiming at constructing more efficient
synthetic analogues for sMMO and studying the intermediate
species responsible for the highly selective hydroxylation. The
product. N,N-Bis(pyrid-2-ylmethyl)-iso-butylamine (L1). Yield:
1
0
.91 g (79%). H NMR (200 MHz, CDCl
3
): d 8.48–7.29 (m, 8H),
3
.88 (s, 4H), 2.38 (d, 2H), 1.87 (m, 1H), 1.05 (d, 6H). EI-MS
+
m/z = 255 C16
H
21
N
3
. N,N-bis(pyrid-2-ylmethyl)benzylamine
3
N ligands chosen contain two heterocyclic pyridine nitrogen
donors to mimic the imidazolyl nitrogen donors in the enzymes.
The single-crystal X-ray structures of [Fe (O)(OBz) (L1) ](ClO
and [Fe (O)(OBz) (L2) ](ClO 4 have been determined to
(L2). Yield: 1.11 g (85%). 1H NMR (200 MHz, CDCl3): d 8.50–
7
C
.02 (m, 13H), 3.85 (s, 4H), 3.21 (s, 2H). EI-MS m/z = 289
2
2
2
4
)
2
+
19
H
19
N
3
.
2
2
2
2
)
4 2
show that in all of these diiron(III) complexes each iron atom is
coordinated to one oxygen atom of the m-oxo bridge, two oxygen
atoms of the m-benzoato bridges and three nitrogen atoms of
the ligands. We have observed that all the four complexes show
efficient alkane hydroxylation with approximately 300–400 turn
Synthesis of diiron(III) complexes
An ethanol solution (8 mL) of ligand (1.0 mmol) was added to a
mixture of (2 mL H
2
O) 354 mg of Fe(ClO
4
)
3
·6H
2
O (1.0 mmol),
1.0 mmol of carboxylic acid and 1.0 mmol of triethylamine and
stirred at room temperature. The resulting dark red solution was
turned reddish brown. Reddish brown microcrystals of diiron(III)
complexes were formed within 24 h upon slow cooling and
standing. The microcrystals were collected by suction filtration,
washed by a small amount of ethanol and dried in vacuo.
Single crystals of 2 and 4 suitable for X-ray diffraction were
obtained by the slow evaporation of an acetonitrile solution of
the corresponding complexes.
Scheme 2 Tridentate 3N ligands used in the study.
This journal is © The Royal Society of Chemistry 2009
Dalton Trans., 2009, 3814–3823 | 3815

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[
Fe
calcd for C36
C 45.78, H 5.32, N 8.54.
2
(O)(OAc)
2
(L1)
2
](ClO
4
)
2
(1). Yield: 0.73 g (76%) Anal.
tained by a cryo-circulator (HAAKE D8-G). The E1/2 observed
+
H
48Cl
2
Fe
2
N
6
O
13: C 45.26, H 5.06, N 8.80. Found
under identical conditions for Fc/Fc couple in acetonitrile was
+
0.100 V with respect to the Ag/Ag reference electrode. The
experimental solutions were deoxygenated by bubbling research
grade nitrogen and an atmosphere of nitrogen was maintained over
the solution during measurement. The products were analyzed by
using Hewlett Packard (HP) 6890 GC series Gas Chromatograph
equipped with a FID detector and a HP-5 capillary column
(30 m ¥ 0.32 mm ¥ 2.5 mm). GC-MS analysis was performed
on a Perkin-Elmer Clarus 500 GC-MS instrument using a PE-5
(HP-5 equivalent) capillary column under conditions that are
identical to that used for GC analysis.
[
Fe (O)(OBz) (L1) ](ClO
2
2
2
4
)
O
2
(2). Yield: 0.86 g (80%) Anal.
51.18, 4.86, 7.78.
calcd for Fe
C
46
H
52Cl
2
2
N
6
13
:
C
H
N
Found C 51.58, H 4.89, N 7.75. ESI-MS, m/z = 880.85
2
+
[
Fe
Fe
calcd for C42
2
(O)(OBz)
2
(L1)
(L2)
Fe
2
] .
[
2
(O)(OAc)
2
2
](ClO
4
)
2
(3). Yield: 0.81 g (79%) Anal.
H
44Cl
2
2
N O13: C 49.29, H 4.33, N 8.21. Found
6
C 49.23, H 4.26, N 8.42.
Fe (O)(OBz) (L2) ](ClO
calcd for Fe
Found C 54.23, H 4.18, N 7.38. ESI-MS, m/z = 971.65
Fe (O)(OBz) (L2) + Na .
Caution! Perchlorate salts of compounds are potentially explo-
[
2
2
2
4
)
O
2
(4). Yield: 0.83 g (72%) Anal.
54.42, 4.22, 7.32.
C
52
H
48Cl
2
2
N
6
13
:
C
H
N
Crystallographic data collection and structure refinement
2
+
+
[
2
2
2
]
Suitable crystals of 2 and 4 selected from the mother liquor was
immersed in paraffin oil, then mounted on the tip of a glass fiber
and cemented using epoxy resin. Intensity data for the crystals
sive. Only small quantities of these compounds should be prepared
and suitable precautions should be taken when they are handled.
˚
were collected using MoKa (l = 0.71073 A) radiation on a
Bruker SMART APEX diffractometer equipped with a CCD area
detector at 293 and 100 K. The crystallographic data were collected
Reactivity studies
38
in Table 1. The SMART program was used for collecting frames
of data, indexing the reflections, and determination of lattice
In a typical reaction, a CH
was added to a CH Cl (400 mL) solution of alkane and diiron(III)
complex (100 mL in CH
at room temperature. The solution was stirred for another 30 min
and the diiron(III) complex was removed by passing the solution
through silica gel followed by elution with diethylether. An internal
standard (bromobenzene) was added at this point and the solution
was subjected to GC analysis. The organic products were identified
by GC and GC-MS comparison with authentic compounds.
All of the products were quantified using GC (FID) with the
following temperature program: injector temperature 130 C;
initial temperature 60 C, heating rate 10 C min to 130 C, then
increasing at a rate of 2 C min to 160 C, and then increasing at
a rate of 5 C min to 260 C; FID temperature 280 C. GC-MS
analysis was performed under conditions identical to those used
for GC analysis.
2
Cl
2
solution of 179 mg of m-CPBA
2
2
38
parameters; SAINT program for integration of the intensity
-
3
3
CN, 1 mmol dm ) with vigorous stirring
39
of reflections and scaling; SADABS program for absorption
40
correction, and the SHELXTL program for space group and
2
structure determination and least-squares refinements on F . The
structure was solved by the heavy-atom method. Other non-
hydrogen atoms were located in successive difference Fourier
syntheses. The final refinement was performed by full-matrix
least-squares analysis. Hydrogen atoms attached to the ligand
moiety were located from the difference Fourier map and refined
isotropically.
◦
◦
◦
-1
◦
◦
-1
◦
◦
-1
◦
◦
Table 1 Crystallographic data of 2 and 4
2
4
Empirical formula
C
46
H
52Fe
2
N
6
O
13Cl
2
C
52 2 6 2
H48Fe N O13Cl
Formula weight/g mol- 1079.55
1
1147.58
Needle, reddish brown Needle, reddish brown
Monoclinic
Physical measurements
Crystal habit, colour
Crystal system
Triclinic
P1
Electronic spectra were recorded on a Diode Array Spectropho-
tometer Agilent 8453. Low temperature spectra were obtained on
a Diode Array Spectrophotometer Agilent 8453 equipped with an
¯
Space group
P2 /n
1
a/A˚
12.2855(17)
19.283(3)
21.682(3)
90
103.9(3)
90
4986.1(12)
4
1.438
12.1543(10)
14.4925(12)
15.7642(13)
82.999(1)
87.506(1)
68.929(1)
2571.8(4)
2
1.482
1184
100
22 194
b/A
˚
1
UNISOKU USP-203 cryostat. H NMR spectra were recorded
c/A˚
◦
on a Bruker 200 MHz NMR spectrometer. Cyclic voltammetry
a/
◦
b/
(
CV) and differential pulse voltammetry (DPV) were performed
◦
g /
◦
at 25 ± 0.2 C using a three-electrode cell configuration. A
V/A˚
3
platinum sphere, a platinum plate and Ag(s)/AgNO
3
were used
Z
r
calcd/g cm-
F(000)
T/K
3
as working, auxiliary and reference electrodes, respectively. The
platinum sphere electrode was sonicated for two minutes in dilute
nitric acid, dilute hydrazine hydrate and in double distilled water
to remove the impurities. The reference electrode for non-aqueous
2240
293
29 731
No. of reflections
collected
+
No. of unique
reflections
11 534
11 641
solution was Ag(s)/Ag , which consists of a Ag wire immersed
in a solution of AgNO (0.01 M) and tetra-N-butylammonium
3
Radiation (MoKa)/A˚
0.71073
0.0667/0.1523
0.1241/0.1778
0.71073
0.0452/0.1048
0.0549/0.1096
perchlorate (0.1 M) in acetonitrile placed in a tube fitted with a
vycor plug. The instruments utilized included an EG & G PAR
a
R
R
1
/wR
/wR
2
[I > 2s(I)]
(all data)
1
2
2
&
73 Potentiostat/Galvanostat and P-IV computer along with EG
G M270 software to carry out the experiments and to acquire
the data. The temperature of the electrochemical cell was main-
ꢀ
ꢀ)/ꢀ|F
{[ꢀ(w(F
a
2
2 2
R
1
=
[
(ꢀF
o
| - |F
c
o
|]; wR
2
=
o
c
- F ) )/
ꢀ
4
1/2
(
wF
o
)] }.
3
816 | Dalton Trans., 2009, 3814–3823
This journal is © The Royal Society of Chemistry 2009

View Article Online
Results and discussion
Synthesis of ligands and diiron(III) complexes
The linear tridentate 3N ligands N,N-bis(pyrid-2-ylmethyl)-iso-
butylamine (L1) and N,N-bis(pyrid-2-ylmethyl)benzylamine (L2)
were (Scheme 2) synthesized according to known procedures,
36,37
which involve reductive amination.
The ligands L1 and L2 were
prepared by reductive amination of iso-butylamine and benzy-
lamine, respectively, with two moles of pyridine-2-carboxaldehyde
using sodium triacetoxyborohydride as reducing agent and char-
1
acterized by H NMR spectroscopy and mass spectrometry.
The diiron(III) complexes 1–4 of these ligands were prepared
by adding one equivalent of L1/L2 to an aqueous ethanol
solution containing one equivalent each of Fe(ClO
acid/benzoic acid and triethylamine. All of them are formulated
as [Fe (O)(O CR) (L) ](ClO on the basis of elemental analysis
4
)
3
·6H
2
O, acetic
2
2
2
2
)
4 2
Fig. 2 ORTEP diagram of complex 4 showing 50% probability thermal
ellipsoids and the labelling scheme for selected atoms. All hydrogen atoms
are omitted for clarity.
and ESI-MS, which is supported by the X-ray crystal structures
of 2 and 4. These complexes with N-alkyl substituents on the
ligands are expected to mimic the active site environment of
the substrate-bound protein sMMO and have been chosen to
model the benzene/alkane hydroxylation reactions of the enzyme.
The carboxylate bridging ligands with different electron donating
abilities are expected to play an important role in determining the
reactivity.
nitrogen atoms of the ligand L1. The tridentate ligand capping
the two ends of the (m-oxo)bis(m-carboxylato)diiron(III) cluster
coordinates to iron(III) in a least strained facial rather than a
meridional fashion. Also, the tertiary amine nitrogen atom of the
˚
3
N ligand is weakly coordinated (Fe–Namine, 2.286(3), 2.304(3) A)
trans to the strongly coordinated m-oxo bridge. The Fe–Namine
2
+
Description of the structures of [Fe
2
(O)(OBz) (L1)] 2 and
2
˚
bonds are longer than the Fe–Npy bonds (2.111–2.138 A) located
2
+
[Fe
2
(O)(OBz) (L2)] 4
2
3
2
cis to the m-oxo bridge obviously due to sp and sp hybridizations,
respectively, of the amine and pyridine nitrogen atoms. The bond
lengths and bond angles in 2 are comparable to those in complexes
with a similar (m-oxo)bis(m-carboxylato)diiron(III) core structure
The molecular structures of 2 and 4 have been determined by
single-crystal X-ray diffraction and the structures of the complex
cations together with atom numbering scheme are shown in
Fig. 1 and 2, respectively, and the bond lengths and bond angles
are listed in Table 2. The coordination geometry around each
iron atom in the diiron(III) complex 2 is distorted octahedral
constituted by the oxygen atom of m-oxo bridge, two oxygen
atoms one each from the two m-benzoato bridges and three
17,41–45
reported in the literature.
The Fe–Ooxo distances (Fe(1)–O(1),
˚
˚
1.794(2); Fe(2)–O(1), 1.795(3) A) fall within the range 1.73–1.82 A
17,41–45
found for oxo-bridged diiron(III) complexes reported earlier
The Fe ◊ ◊ ◊ Fe separation (3.094 A) and Fe–
O–Fe bond angle (119.11 ) are very close to those observed for
2
- 46–49
˚
and for Fe
2
OCl
6
.
◦
2
+
the analogous complex [Fe
2
(O)(OBz)
2
(L) ] , where L is N-alkyl
2
18
substituted bis(pyrid-2-ylmethyl)amine, reported earlier. The
Fe–O–Fe bond angle is well outside the range for single and
double-bridged complexes (140–180 ) and is closer to 120 as
◦
◦
+
50–52
observed in the planar Fe
3
O
7
unit
and this leads to an
˚
unusually short Fe ◊ ◊ ◊ Fe separation (3.094 A). The Fe ◊ ◊ ◊ Fe
separation (3.094 A) is shorter than those in singly- and doubly-
˚
˚
bridged m-oxodiiron(III) complexes (3.4–3.6 A).
The molecular structure of 4 is very similar to the analogous
2
+
complex cation [Fe
2
(O)(OBz)
2
(L1)
2
] 2 with very similar Fe–Ooxo
˚
(
(
Fe(1)–O(1), 1.7963(16); Fe(2)–O(1), 1.7904(17) A), Fe–Npy
˚
˚
2.121–2.141 A) and Fe–Namine (2.270(19), 2.255(2) A) bond dis-
˚
tances, Fe ◊ ◊ ◊ Fe separation (3.060 A) and the Fe–O–Fe bond angle
◦
(
117.13(8) ). The most notable structure difference between 2 and
˚
4
is the Fe–Namine bond distances. The Fe ◊ ◊ ◊ Fe separation (3.060 A)
◦
and Fe–O–Fe bond angle (117.13(8) ) are very close to those ob-
2
+
served for the analogous complex [Fe
N-alkyl substituted bis(pyrid-2-ylmethyl)amine, reported earlier.
2
(O)(OBz) (L)] , where L is
2
18
An analysis of the bond parameters indicates that on replacing
N-iso-butyl group in 2 by N-benzyl group to obtain 4 almost all
the Fe–O and Fe–N bond distances decrease while the Fe ◊ ◊ ◊ Fe
distance increases (0.034 A) indicating the lower donor ability of
the bulky N-iso-butyl group. A similar effect has been observed
Fig. 1 ORTEP diagram of complex 2 showing 50% probability thermal
ellipsoids and the labelling scheme for selected atoms. All hydrogen atoms
are omitted for clarity.
˚
This journal is © The Royal Society of Chemistry 2009
Dalton Trans., 2009, 3814–3823 | 3817

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◦
Table 2 Selected bond lengths ( A˚ ) and bond angles ( ) for 2 and 4
complexes containing two carboxylate bridges. Four weak spin-
forbidden d–d transitions are expected for octahedral high-spin
iron(III) complexes but only one band around 700 nm is readily
observed for all the complexes and the remaining bands appear
to be obscured by the more intense LMCT bands. The molar
absorptivity of this band is much higher than that observed for
high-spin mononuclear octahedral iron(III) complexes due to the
lower symmetry of the dimeric complex with the oxo-bridge and
2
4
Bond lengths/A˚
Fe(1)–O(1)
Fe(1)–O(2)
Fe(1)–O(5)
Fe(1)–N(1)
Fe(1)–N(2)
Fe(1)–N(3)
Fe(2)–O(1)
Fe(2)–O(3)
Fe(2)–O(4)
Fe(2)–N(4)
Fe(2)–N(5)
Fe(2)–N(6)
1.794(2)
2.031(3)
2.032(2)
2.128(3)
2.286(3)
2.137(3)
1.795(3)
2.015(3)
2.046(2)
2.111(3)
2.304(3)
2.138(3)
1.7963(16)
2.0285(17)
2.0154(16)
2.121(2)
2.2696(19)
2.124(2)
1.7904(17)
2.0539(16)
2.0106(16)
2.1322(19)
2.255(2)
44
also spin–spin interactions between the iron(III) centers. The oxo-
bridge of all the complexes remains intact in DCM solutions
as revealed by electrospray-ionization mass-spectrum (ESI-MS)
(
Fig. S1, ESI).‡ The reaction of the diiron(III) complexes 1–4 with
the oxidants H , tert-BuOOH and m-CPBA was monitored
by employing electronic absorption spectroscopy as a convenient
tool. The addition of H and tert-BuOOH to the present
2
O
2
2.141(2)
2
O
2
diiron(III) complexes dissolved in DCM at room temperature did
not show any spectral change even in the presence triethylamine.
However, on adding the strong oxidant m-CPBA to the diiron(III)
complexes in DCM at room temperature, a small shift in the visible
spectral bandis observedandwhen one equivalentoftriethylamine
was added to this reaction mixture a stable and intense visible band
◦
Bond angles/
O(1)–Fe(1)–O(2)
O(1)–Fe(1)–O(5)
O(1)–Fe(1)–N(1)
O(1)–Fe(1)–N(2)
O(1)–Fe(1)–N(3)
O(2)–Fe(1)–O(5)
O(2)–Fe(1)–N(1)
O(2)–Fe(1)–N(2)
O(2)–Fe(1)–N(3)
O(5)–Fe(1)–N(1)
O(5)–Fe(1)–N(2)
O(5)–Fe(1)–N(3)
N(1)–Fe(1)–N(2)
N(1)–Fe(1)–N(3)
N(2)–Fe(1)–N(3)
O(1)–Fe(2)–O(3)
O(1)–Fe(2)–O(4)
O(1)–Fe(2)–N(4)
O(1)–Fe(2)–N(5)
O(1)–Fe(2)–N(6)
O(3)–Fe(2)–O(4)
O(3)–Fe(2)–N(4)
O(3)–Fe(2)–N(5)
O(3)–Fe(2)–N(6)
O(4)–Fe(2)–N(4)
O(4)–Fe(2)–N(5)
O(4)–Fe(2)–N(6)
N(4)–Fe(2)–N(5)
98.56(11)
97.07(7)
99.54(7)
98.43(7)
174.70(8)
97.46(7)
97.53(7)
162.11(7)
85.90(7)
87.40(7)
88.68(7)
84.37(7)
161.55(7)
78.00(7)
81.78(8)
78.24(7)
98.96(7)
100.39(7)
96.05(7)
170.27(7)
97.43(8)
89.84(6)
164.26(7)
88.84(7)
80.94(7)
92.08(6)
85.35(6)
161.02(8)
75.76(7)
98.12(11)
95.73(11)
170.91(11)
100.08(12)
90.48(10)
165.12(11)
89.75(10)
82.92(11)
91.58(10)
85.41(9)
161.37(11)
75.74(11)
90.55(11)
77.19(11)
99.04(11)
97.64(11)
100.34(13)
170.47(11)
94.69(11)
90.75(10)
160.46(12)
83.68(11)
89.72(11)
84.23(12)
91.43(10)
167.43(10)
77.60(12)
(
670–700 nm) eventually develops for 1–4 (Fig. 3). The ESI-MS
spectrum of the later reaction mixture shows a peak at m/z 466
Fig. 4), the mass and isotope pattern of which corresponds to
(
2+
the intermediate species [Fe
intermediate species has been proposed to be formed initially
in alkane hydroxylation reactions catalysed by the complex
2
(O)(OAc)(m-CPBA)(L) ] . Such an
2
23
[
Fe
2
(O)(O
2
CCH
3
)
2
(hexpy)](ClO
4
2
) using m-chloroperbenzoic acid
(
m-CPBA).
by us in mononuclear iron(III) complexes of the type [Fe(L)Cl
3
],
where L is N-alkyl substituted bis(pyrid-2-ylmethyl)amine), where
˚
the Fe–Namine bond distance increases by 0.018 A upon changing
Fig. 3 Reaction of complex 4 with m-CPBA (5 equiv.) and triethylamine
1 equiv.) followed by UV-vis at room temperature.
37
the N-alkyl group from methyl to n-propyl. Obviously, the lone-
pair orbital on the nitrogen atom of the sterically hindering N-alkyl
group is not oriented exactly toward the iron(III) orbital, rendering
the Fe–Namine bond longer. This illustrates the importance of the
steric rather than the electron-releasing effect of the alkyl group
in dictating the coordination environment around iron(III) center.
Hence on replacing the sterically hindering N-iso-butyl group in
(
Electrochemical properties
The electrochemical properties of the diiron(III) complexes were
investigated in dichloromethane by employing cyclic (CV) and
differential pulse voltammetry (DPV) on a stationary platinum
electrode. All of the complexes show a cathodic reduction current
in the range -0.536 to -0.612 V but not any coupled oxidation wave
in the CV (Fig. 5). The cathodic current functions (6.73–7.19 ¥
2
by the less sterically hindering N-benzyl group to obtain 4, the
triply-bridged diiron(III) core becomes more compact.
-
6
2
-1
10
cm s ) calculated (n = 1) by substituting the slope obtained
Spectral studies
1
/
2
5
4
from the linear ipc vs. n plot in Randles–Sevcik’s equation are
The three electronic spectral bands located in the range 400–
of the same order as those observed for mononuclear iron(III)
complexes undergoing a one-electron reduction process and so
the reduction process in the diiron(III) corresponds to:
-
1
-1
23,44,53
55
5
50 nm (emax, 1150–1650 M cm , Table 3) are characteristic
2
-
of O → iron(III) LMCT bands in oxo-bridged diiron(III)
3
818 | Dalton Trans., 2009, 3814–3823
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Table 3 Electronic spectral data (lmax in nm; emax in M- cm-1 in parenthesis) and electrochemical data for diiron(III) complexes in an acetonitrile–
1
dichloromethane mixture
-1
-1
b
b
Complex
l
max/nm (emax/M cm )
E
p,c (CV)
E
1/2 (DPV)
Redox process
2
+
III
III
II
III
III
III
III
[
[
[
[
Fe
2
Fe
2
Fe
2
Fe
2
(O)(OAc)
2
(L1)]
460 (1650)
-0.612
-0.570
-0.602
-0.536
-0.530
-0.525
-0.537
-0.469
Fe Fe → Fe Fe
5
7
00 (1250)
06 (185)
a
2+
III
III
II
(O)(OBz)
2
(L1)]
466 (1480)
Fe Fe → Fe Fe
5
7
06 (1205)
13 (190)
a
2+
III
III
II
(O)(OAc)
2
(L2)]
459 (1480)
Fe Fe → Fe Fe
5
7
04 (1150)
00 (180)
a
2+
III
III
II
(O)(OBz)
2
(L2)]
461 (1510)
Fe Fe → Fe Fe
5
7
05 (1165)
03 (190)
a
a
6
4
4
b
d–d transition corresponds to A1g → T2g ( G). Potential measured vs. Ag/AgNO
3
(0.001 M, 0.1 M TBAP); add 0.544 V to convert to NHE.
Fig. 5 Cyclic voltammogram (CV) and differential pulse voltammogram
◦
(
DPV) of 1 mM complex 4 in a DCM–acetonitrile mixture at 25 C.
-1
Supporting electrolyte: 0.1 M TBAP. Scan rate: 50 mV s for CV and
mV s- for DPV.
1
5
potential is shifted to slightly more negative values suggesting
that the FeN coordination sphere becomes more compact
cf. above) to stabilize the iron(III) oxidation state. The X-ray
3
O
3
(
structural data indicate that upon replacement of the coordinated
ligand L1in 2 by L2 to obtain 4, the diiron(III)center becomes more
compact, as understood from the decrease in distances of most of
the coordinate bonds, and hence the redox potential is expected to
be shifted to more negative values. However, interestingly, on going
from 2 to 4, the redox potential becomes slightly more positive
implying that the complex undergoes intricate structural changes
upon adding an electron to the diiron(III) core in 2.
Fig. 4 Electron spray ionization-mass spectra of complex 2 + 1 equiv.
m-CPBA + 1 equiv. triethylamine.
-
Fe(III)-O-Fe(III) + e → Fe(II)-O-Fe(III).
Functionalization of alkanes
III
II
The E1/2 values of the Fe /Fe redox couples (-0.469 to
-
0.537 V, Table 3) fall in the range observed for similar type of
The experimental conditions and the results of catalytic oxidation
of alkanes into alcohols for the complexes 1–4 are summarized
in Table 4. The conversion of alkanes into hydroxylated products
was quantified based on gas chromatographic analysis by using
authentic samples and an internal standard. All the complexes
display efficient alkane hydroxylation with approximately 300–
400 turn over numbers (TON) with an excellent selectivity for
the hydroxylation of cyclohexane (A/K, 10–14) and adamantane
43,44
triply-bridged diiron(III) complexes.
mainly due to the strong coordination of the bridging carboxylate
groups and follow the trend 1 < 2 > 3 < 4. On replacing the bridg-
ing acetates in 1 by the bridging benzoates to obtain 2, the Fe /Fe
redox potential is shifted to more positive values due to the weaker
coordination (pK , AcOH, 4.76; BzOH, 4.19) of benzoate anion
to iron(III). A similar but larger shift in redox potential is observed
on moving from 3 to 4. Also, on decreasing the bulkiness of the
tridentate ligand backbone in 1 to obtain 3, the Fe /Fe redox
They are highly negative
III
II
a
◦
◦
(3 /2 , 9–11; Table 4). Formation of small amount of capro-
lactone has been observed in cyclohexane hydroxylation which
III
II
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a
Table 4 Products of oxygenation of alkanes catalysed by diiron(III)
Interestingly, the amount of cyclohexanol formed for 2 increases
A/K, 2.7–13.7) with that of cyclohexanone formed being con-
complexes
(
stant (Fig. 6 and Table 5) beyond ~50 turnovers on increasing
the concentration of m-CPBA. A similar enhancement in the
yield of 1-adamantanol with the yield of 2-adamantanol being
constant (Fig. S3, S5 and S7‡) is observed for adamantane. All
the other three complexes show a similar trend in the selective
formation of cyclohexanol (ESI, Fig. S2, S4 and S6‡) and
bc
c
Cyclohexane
Adamantane
◦
◦
Complex
-ol
-one
A/K Ratio
1-ol
2-ol
3 /2
1
2
3
4
353
373
335
277
28
27
33
27
12.6
13.8
10.2
10.3
340
375
345
295
35
37
32
34
9.7
10.1
10.8
8.7
1
-adamantanol, after ~50 turnovers illustrating that on increasing
◦
◦
a
-3
the concentration of m-CPBA, the A/K value and 3 /2 ratio
increase. Interestingly, the present diiron(III) complexes show
higher selectivity in the hydroxylation of cyclohexane (A/K,
Reaction conditions: [complex] = 1 mmol dm , [m-CPBA] = 0.8 mol
-3
-3
dm , [alkane] = 3 mol dm for cyclohexane and [complex] = 1 mmol
-3
-3
-3
dm , [m-CPBA] = 0.6 mol dm , [alkane] = 1 mol dm for adamantane
in a mixture of CH Cl (0.9 mL) and CH CN (0.1 mL) stirred for 30 min.
2
2
3
◦
◦
b
c
10–14) and adamantane (3 /2 , 9–11; Table 4) demonstrating that
at higher concentrations of m-CPBA metal-based oxidants rather
than non-selective radical-based species are involved in the C–H
hydroxylation. Also, no change in the product selectivity and
yield is observed upon performing the hydroxylation under inert
condition, which ruled out the formation of cyclohexylperoxide
species. This is also strongly supported the involvement of metal-
based oxidants. In contrast, Kodera et al. have reported a lower
selectivity in the hydroxylation of cyclohexane (A/K, 2.4) and
Formation of small amount of ester observed for all the complexes. Turn
over number = no. of mol of product per no. of mol of catalyst.
is produced by the reaction of formed ketone with m-CPBA.
Obviously, a variation in the carboxylate bridges and the N-alkyl
substituents on the tridentate ligands do not affect the catalytic
conversion very much. In the case of complex 2 as catalyst and
cyclohexane as substrate, 373 turnovers of cyclohexanol (A) and
◦
◦
2
7 turnovers of cyclohexanone (K) are obtained, corresponding to
adamantane (3 /2 , 4.2) using a similar type of m-oxodiiron(III)
complex but derived from a dinucleating ligand and m-CPBA as
an oxidant.
a 50% conversion of the oxidant into organic products. Also, the
conversion efficiency is maintained for the subsequent addition
of the oxidant into the reaction mixture demonstrating that the
active species is quite robust as a catalyst for alkane hydroxylation.
The complexes 1–3 display turn over numbers (360–400 for
cyclohexane; 370–410 for adamantane) higher than 4 does (304
for cyclohexane; 329 for adamantane) and this may be traced
to the more negative Fe /Fe redox potentials of the former
complexes (cf. above). The stability of the high-valent iron-oxo
species generated from certain mononuclear iron(II) complexes
has been correlated with the number of pyridine donors present
in the primary ligand and on increasing the number of pyridine
23,24
The UV-vis spectral studies of the present diiron(III) complexes
show that the spectral bands characteristic of triply-bridged
diiron(III) complexes disappear after 50 turnovers revealing that
the diiron(III) core of mononucleating ligands degrade slowly
during catalysis. This suggests that after approximately 50 turn-
overs a new active metal-based mononuclear intermediate
III
II
2
+
[Fe(L)(RCO
2
)(Sol)
2
]
is formed (cf. above), which is responsible
for the highly selective hydroxylation of alkanes to alcohols. To
gain further insight into the nature of the active species, the
3
+
complex species [Fe(L2)(CH
3
CN)
3
]
generated in acetonitrile
56
III
II
donors in the ligand, the Fe /Fe redox potential is shifted
to more negative values suggesting that the negative charge built
on iron(II) center stabilizes the high-valent iron-oxo intermediate.
Such high-valent iron-oxo complexes are invoked as the key
solution was used for catalytic studies. In a typical reaction, a
DCM solution of 9.0 mg of m-CPBA was added to a DCM
solution (400 mL) of cyclohexane (324 mL), and a 1 : 1 mixture
3
+
of [Fe(L2)(CH
3
CN)
3
]
and benzoic acid (100 mL in CH CN,
3
57
-3
turn over intermediates, in the C–H bond functionalization by
1 mmol dm ) with vigorous stirring at room temperature. After
30 min the reaction mixture was analyzed by GC. The turn
over number is 24.0 for the formation of cyclohexanol and 2.3
for the formation of cyclohexanone. In contrast to diiron(III)
complexes, the in situ prepared mononuclear iron(III) species
show a high selectivity (A/K, 10.6) at lower concentrations of
8
,58–60
enzymes and their synthetic analogues.
complexes with more negative iron(III)/iron(II) redox potentials
Table 3) are expected to stabilize the high-valent iron-oxo active
species more so that they can activate hydrocarbon molecules with
strong C–H bonds at room temperature.
Thus, m-oxodiiron(III)
(
a
Table 5 Results obtained for the oxygenation of cyclohexane employing complex 2
Catalyst/mol dm-
3
m-CPBA/mol dm-3
Cyclohexanol
b
Cyclohexanone
b
A/K ratio
Exp.
-
3
3
3
3
3
3
3
3
3
-3
1
2
3
4
5
6
7
8
9
1 ¥ 10-
20 ¥ 10
10.2
18.8
29.0
38.3
46.0
3.2
7.0
8.7
10.2
14.8
20.1
23.4
26.6
27.2
3.2
2.7
3.3
3.8
3.1
3.9
8.4
11.0
13.7
-3
1 ¥ 10
40 ¥ 10
-
-3
1 ¥ 10
60 ¥ 10
-
-3
1 ¥ 10
80 ¥ 10
-
-3
1 ¥ 10
100 ¥ 10
-
-3
1 ¥ 10
200 ¥ 10
78.4
-
-3
1 ¥ 10
400 ¥ 10
197.4
293.4
373.2
-
-3
1 ¥ 10
600 ¥ 10
-
-3
1 ¥ 10
800 ¥ 10
a
-3
-3
-3
-3
Reaction conditions: [complex] = 1 ¥ 10 mmol dm , [m-CPBA] = 0.02–0.8 mol dm , [alkane] = 3 mol dm in a mixture of CH
2
Cl
2
(0.9 mL) and
b
CH
3
CN (0.1 mL). Turn over number = no. of mol of product per no. of mol of catalyst.
3
820 | Dalton Trans., 2009, 3814–3823
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Fig. 6 Product yields as a function of equivalents of m-CPBA added into an acetonitrile–dichloromethane (v/v, 1 : 9) solution of complex 2 and
cyclohexane in air. (a) 0–100 equiv. of m-CPBA; (b) 200–800 equiv. of m-CPBA.
m-CPBA indicating the significant participation of mononuclear
high-valent iron-oxo active species in catalysis. This is further
supported by the formation of a stable intense band (600 nm;
1
-1
e
max, 355 M- cm , Fig. 7), which is characteristics of high-valent
56
iron-oxo species when five equiv. of m-CPBA were added to a
: 1 mixture of [Fe(L2)(CH
3
+
◦
1
3
CN)
3
]
and benzoic acid at -40 C.
This band disappears slowly (Fig. 8) upon increasing the tempera-
ture and the decay of the iron-oxo intermediate has been followed
◦
-3 -1
at 25 C (k, 4.15 ¥ 10 s ). Hence, the high selectivity in alkane
hydroxylation observed for the present triply-bridged diiron(III)
complexes of mononucleating ligands is ascribed to the formation
of the active mononuclear iron(III) species (Scheme 3) during
hydroxylation. It is evident that the reaction of mononuclear
iron(III) species with m-CPBA gives benzoylperoxoiron(III) species,
Fig. 8 Time course of decay of high-valent iron-oxo species generated
3+
◦
3 3
from [Fe(L2)(CH CN) ] and benzoic acid at 25 C.
which would undergo O–O bond heterolysis to afford iron(V)-oxo
intermediate and m-chlorobenzoic acid as byproduct or O–O bond
homolysis to yield iron(IV)-oxo intermediate and chlorobenzene as
byproduct. Very recently, Que et al. have reported that complex
II
2+
cation [Fe (N4Py)(CH
3
III
CN)] promotes O–O bond heterolysis,
3+
while the cation [Fe (N4Py)(CH CN)] favours O–O bond
3
homolysis and concluded that the nature of the O–O bond
61
cleavage is dependent on the iron oxidation state. In contrast,
we have observed that the m-chlorobenzoic acid (byproducts of
heterolytic cleavage) as the major and chlorobenzene (byproducts
of homolytic cleavage) as the minor byproduct in the catalysis
illustrating that most of the benzoylperoxoiron(III) species undergo
O–O bond heterolysis rather the O–O bond homolysis to afford
iron(V)-oxo species and m-chlorobenzoic acid. The strong coor-
dination of pyridine nitrogen of the 3N ligands L1 and L2 and
3
+
3 3
Fig. 7 Electronic spectra of reaction of [Fe(L2)(CH CN) ] and benzoic
acid with 5 equiv. of m-CPBA in a mixture of acetonitrile–dichloromethane
◦
solvent at -40 C.
This journal is © The Royal Society of Chemistry 2009
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Acknowledgements
We sincerely thank the Council of Scientific and Industrial
Research, New Delhi, for a Senior Research Fellowship to K. V. We
thank Department of Science and Technology, New Delhi, for the
Award of Ramanna Fellowship to M. P. and also for supporting
this research [scheme no. SR/S5/BC-05/2006]. We thank Prof. K.
Natarajan, Bharathiar University, Coimbatore, for CHN analysis.
References
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M. Costas, K. Chen and L. Que, Jr., Coord. Chem. Rev., 2000, 200–202,
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H. Dalton, Adv. Appl. Microbiol., 1980, 26, 71.
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9
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Rosenzweig, C. A. Frederick, S. J. Lippard and P. Nordlund, Nature,
Scheme 3 Proposed mechanism of alkane hydroxylation.
1
993, 366, 537; (c) L. Westerheide, M. Pascaly and B. Krebs, Curr. Opin.
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◦
◦
A/K and 3 /2 ratios for the hydrocarbon oxidation catalyzed
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