Non-heme μ-Oxo- and bis(μ-carboxylato)-bridged diiron(III) complexes of a 3N ligand as catalysts for alkane hydroxylation: Stereoelectronic factors of carboxylate bridges determine the catalytic efficiency

Article abstract of DOI:10.1039/c6dt01059h

A series of non-heme (μ-oxo)bis(μ-dicarboxylato)-bridged diiron(iii) complexes, [Fe₂(O)(OOCH)₂(L)₂]²⁺1, [Fe₂(O)(OAc)₂(L)₂]²⁺2, [Fe₂(O)(Me₃AcO)₂(L)₂]²⁺3, [Fe₂(O)(OBz)₂(L)₂]²⁺4, [Fe₂(O)(Ph₂AcO)₂(L)₂]²⁺5 and [Fe₂(O)(Ph₃AcO)₃(L)₂]²⁺6, where L = N,N-dimethyl-N′-(pyrid-2-ylmethyl)ethylenediamine, OAc^- = acetate, Me₃AcO^- = trimethylacetate, OBz^- = benzoate, Ph₂AcO^- = diphenylacetate and Ph₃AcO^- = triphenylacetate, have been isolated and characterized using elemental analysis and spectral and electrochemical techniques. They have been studied as catalysts for the selective oxidation of alkanes using m-chloroperbenzoic acid (m-CPBA) as the oxidant. Complexes 2, 3, and 4 possess a distorted bioctahedral geometry in which each iron atom is coordinated to an oxygen atom of the μ-oxo bridge, two oxygen atoms of the μ-carboxylate bridge and three nitrogen atoms of the 3N ligand. In an acetonitrile/dichloromethane solvent mixture all the complexes display a d-d band characteristic of the triply bridged diiron(iii) core, revealing that they retain their identity in solution. Upon replacing electron-donating substituents on the bridging carboxylates by electron-withdrawing ones the E_1/2 value of the one-electron Fe^IIIFe^III → Fe^IIIFe^II reduction becomes less negative. On adding one equivalent of Et₃N to a mixture of one equivalent of the complex and an excess of m-CPBA in the acetonitrile/dichloromethane solvent mixture an intense absorption band (λ_max, 680-720 nm) appears, which corresponds to the formation of a mixture of complex species. All the complexes act as efficient catalysts for the hydroxylation of cyclohexane with 380-500 total turnover numbers and good alcohol selectivity (A/K, 6.0-10.1). Adamantane is selectively oxidized to 1-adamantanol and 2-adamantanol (3°/2°, 12.9-17.1) along with a small amount of 2-adamantanone (total TON, 381-476), and interestingly, the sterically demanding trimethylacetate bridge around the diiron(iii) centre leads to high 3°/2° bond selectivity; on the other hand, the sterically demanding triphenylacetate bridge gives a lower 3°/2° bond selectivity. A remarkable linear correlation between the pK_a of the bridging carboxylate and TON for both cyclohexane and adamantane oxidation is observed, illustrating the highest catalytic activity for 3 with strongly electron-releasing trimethylacetate bridges.

Full text of DOI:10.1039/c6dt01059h

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Communicated to: Dalton Trans.
Ms Type : Article
ꢀꢀOxoꢀ and Bis(ꢀꢀcarboxylato)ꢀbridged Diiron(III) Complexes of a 3N Ligand
as Catalysts for Alkane Hydroxylation: Steroelectronic Factors of
Carboxylate Bridge Determine the Catalytic Efficiency
Mani Balamurugan,a Eringathodi Suresh,b and Mallayan Palaniandavara*
aSchool of Chemistry, Bharathidasan University, Tiruchirappalli ꢀ 620024, Tamil Nadu, India.
bAnalytical Science Discipline, Central Salt and Marine Chemicals Research Institute,
Bhavnagar ꢀ 364 002, India.
*To whom correspondence should be addressed, eꢀmail: palanim51@yahoo.com and
palaniandavarm@gmail.com
1

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Abstract
A
series of nonꢀheme (ꢀꢀoxo)bis(ꢀꢀdicarboxylato)ꢀbridged diiron(III) complexes
[Fe2(O)(OOCH)2(L)2]2+ [Fe2(O)(OAc)2(L)2]2+ [Fe2(O)(Me3AcO)2(L)2]2+
1, 2, 3,
[Fe2(O)(OBz)2(L)2]2+ 4, [Fe2(O)(Ph2AcO)2(L)2]2+ 5 and [Fe2(O)(Ph3AcO)3(L)2]2+ 6, where L =
N,NꢀdimethylꢀN’ꢀ(pyridꢀ2ꢀylmethyl)ethylenediamine, OAc¯ acetate, Me3AcO¯
=
=
trimethylacetate, OBz¯ = benzoate, Ph2AcO¯ = diphenylacetate and Ph3AcO¯ = triphenylacetate,
have been isolated and characterized using elemental analysis and spectral and electrochemical
techniques. They have been studied as catalysts for selective oxidation of alkanes using mꢀ
choloroperbenzoic acid (mꢀCPBA) as the oxidant. The complexes 2, 3, and 4 possess a distorted
bioctahedral geometry in which each iron atom is coordinated to the oxygen atom of the ꢀꢀoxo
bridge, two oxygen atoms of the ꢀꢀcarboxylate bridge and three nitrogen atoms of the 3N ligand.
In acetonitrile/dichloromethane solvent mixture all the complexes display a dꢀd band
characteristic of triply bridged diiron(III) core revealing that they retain their identity in solution.
Upon replacing electronꢀdonating substituent on the bridging carboxylates by electronꢀ
withdrawing one the E1/2 value of the oneꢀelectron FeIIIFeIII
FeIIIFeII reduction becomes less
negative. On adding one equivalent of Et3N to a mixture of one equivalent of the complex and
excess of mꢀCPBA in acetonitrile/dichloromethane solvent mixture an intense absorption band
(λmax, 680ꢀ720 nm) appears, which corresponds to the formation of a mixture of complex species.
All the complexes act as efficient catalysts for hydroxylation of cyclohexane with 380 ꢀ 500 total
turnover numbers and good alcohol selectivity (A/K, 6.0ꢀ10.1). Adamantane is selectively
oxidized to 1ꢀadamantanol and 2ꢀadamantanol (3°/2°, 12.9ꢀ17.1) along with small amount of 2ꢀ
adamantanone (Total TON, 381ꢀ476), and interestingly, the sterically demanding
trimethylacetate bridge around the diiron(III) centre leads to high 3º/2º bond selectivity; on the
other hand, sterically demanding triphenylacetate bridge gives lower 3º/2º bond selectivity. A
remarkable linear correlation between the pKa of the bridging carboxylate and TON for both
cyclohexane and adamantane oxidation is observed illustrating the highest catalytic activity for 3
with strongly electronꢀreleasing trimethylacetate bridges.

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Introduction
In nature iron containing enzymes like methane monooxygenases, cytochrome P450 and
bleomycin play a vital role in catalyzing many biologically essential organic transformations.
Particularly, the soluble methane monooxygenases (sMMO), which catalyze the oxidation of
methane to methanol using dioxygen, are the widely investigated metalloenzymes.1ꢀ10 Also,
selective hydroxylation of hydrocarbons under mild conditions is a challenging scientific goal in
synthetic organic chemistry,11 because industrial hydroxylation processes typically involve high
temperatures and high pressures, while enzymes catalyze these reactions under mild reaction
conditions and follow different methodologies from those of industrial processes. Thus the active
site of the reduced form of sMMO possesses a diiron(II) center containing four glutamate and
two histidine residues, and the iron atoms are bridged by two carboxylate ligands from glutamate
residues. On the other hand, the oxidized form contains only one carboxylate and two hydroxo
bridges (sMMOox, Scheme 1) in its active site.12 The interesting fact that several nonꢀheme
diiron proteins such as ribonucleotide reductases (RNR)13 and hemerythrin (Hr)14 also contain at
least two carboxylate bridges in the active site of their reduced forms, but they are different in
their functions. Interestingly, the bridging units also play a vital role in dictating the function of
the enzymes. So, the isolation and study of synthetic models containing the structural core
[Fe2(O)(O2CR)2]2+ have received greater attention among the bioinorganic communities.15–17
Also, a wide variety of coordination environments and bridging modes around the iron center in
nonꢀheme enzymes generate distinct oxidizing intermediates, which are supposed to result in
their intrinsic catalytic transformations.18,19 Inspired by these enzymes efforts have been made
during the last two decades to develop synthetic models for sMMO and study the mechanism of
O2 activation involved in the oxygenation of alkanes.9,20 The study of synthetic diiron
Glu243
Glu243
O
O
H
H
H
H
O
O
O
O
O
O
O
O
H
O
O
O
O
O
Glu114
His147
Glu209
His246
Glu114
His147
Glu209
His246
FeII
FeII
FeIII
FeIII
O
O
H
H2
N
N
N
N
O
O
O
O
His246
N
N
N
N
Glu144
Glu144
H
H
H
H
sMMOred
sMMOox
Scheme 1: Active site structures of soluble methane monooxygenase

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compounds with different carboxylate motifs has mainly served as benchmarks for illustrating
the oxidizing intermediates and catalytic pathway of the enzymes, and efforts to make use of
them as functional mimics are scarce.
Many synthetic models have been isolated and studied as structural and functional
models for metHr and sMMO containing the structural motif [Fe2(O)(O2CR)2(L)2]2+, where L is a
tridentate 3N ligand.21–25 Christou et al. have isolated triplyꢀbridged diiron(III) complexes26,27
derived from bipyridine and used them as catalysts for alkane hydroxylation using tertꢀ
butylhydroperoxide (tꢀBuOOH) as oxidant. Kitajima et al. studied the triplyꢀbridged diiron(III
)
complex28 [Fe2(O)(O2CCH3)2(HB(pz)3)]2+ as catalyst for alkane hydroxylation using molecular
oxygen (in the presence of an electron source), but low selectivity and poor yields were observed
for
these
reactions.
Later,
Kodera
et
al.
have
isolated
the
complex
[Fe2(O)(O2CCH3)2(hexpy)](ClO4)2, which catalyses the oxidation of cyclohexane with notable
alcohol/ketone ratio (A/K, 2.4) and that of adamantane with moderate tertiary/secondary carbon
ratio (3°/2°, 4.2) using mꢀchloroperbenzoic acid (mꢀCPBA) as oxidant.29–31 Recently, Itoh et al.32
have isolated the triplyꢀbridged diiron(III) complex [Fe2(O)(O2CCH3)2(L)](ClO4)2, where L is 1,2ꢀ
bis(Nꢀbenzylꢀ2ꢀaminomethylꢀ6ꢀpyridyl)ethaneꢀN’,N’ꢀdiacetic
acid,
which
catalyses
hydroxylation of cyclohexane (A/K, 10.0) and adamantane (3°/2°, 13.6) with selectivity using
hydrogen peroxide. We have also recently reported33 triplyꢀbridged diiron(III) complexes of the
i
type [Fe2O(iBuꢀbpa/Bzꢀbpa)2(RCOO)2]2+, where Buꢀbpa = N,Nꢀbis(pyridꢀ2ꢀylmethyl)ꢀisoꢀ
butylamine, Bzꢀbpa = N,Nꢀbis(pyridꢀ2ꢀylmethyl)benzylamine (Scheme 2), R = ꢀCH3, ꢀC6H5,
which act as efficient catalysts (A/K, 10.2ꢀ13.8) towards alkane hydroxylation.33 Also, many
oxoꢀbridged diiron(III) complexes of tetradentate ligands have been isolated34–40 as synthetic
models for the diiron(III) biosites, but the catalytic efficiency and selectivity of these systems are
lower than those of the enzymes. However, the basis for the high selectivity, efficiency and the
mechanistic pathway of alkane hydroxylation reaction catalyzed by small molecule analogues
remains unclear.
All the above observations encouraged us to isolate diiron(III) complexes of tridentate
(3N) ligands with various carboxylate bridging ligands, such as [Fe2(O)(OOCH)2(L)2](ClO4)2 1,
[Fe2(O)(OAc)2(L)2](ClO4)2 2, [Fe2(O)(Me3Ac)2(L)2](ClO4)2 3, [Fe2(O)(OBz)2(L)2](ClO4)2 4,
[Fe2(O)(Ph2Ac)2(L)2](ClO4)2 5 and [Fe2(O)(Ph3Ac)3(L)2](ClO4)2 6, where L = N,NꢀdimethylꢀN’ꢀ
(pyridꢀ2ꢀylmethyl)ethylenediamine (Scheme 3), and study their use as functional models for the

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H
H
HN
N
N
N
N
NH
N
N
H
H
N
N
N
H
N
N
N
Me3-TACN
bipy
Me3TACN
TACN
H
H
N
N
N
N
N
N
N
N
N
N
HO
O
O
OH
hexpy
R
N
N
N
N
N
N
N
N
N
N
N
R = ꢀCH3
R = ꢀC2H5
R = ꢀCH2C6H5
R = ꢀCH2C4H3O
R = ꢀCH2C4H7O
N
Mepea
RBPA
TPEN
H
H
N
H
N
N
P
N
R
R
N
N
N
N
N
N
N
N
N
N
BH N
N
R
N
N
N
N
R = ꢀCH2C6H3(OCH3)2
BBA
HB(pz)3
R3TACN
TMIP
Scheme 2. Structures of biꢀ and tridentate 3N ligands of diiron(III) complexes reported already
alkane hydroxylation reactions catalyzed by sMMO. We intend to study the steric and electronic
effects of different carboxylate bridges on the highly selective hydroxylation of alkanes. The 3N
H
COO
H
COO
COO
CH3
COO
COO
COO
N
H3C
CH3
CH3
N
N
Scheme 3. Structures of tridentate 3N and bridging carboxylate ligands employed in the study

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nitrogen donors in the enzymes and the weakly coordinating ꢀNMe2 donor is expected51 to
enhance the Lewis acidity of the diiron(III) core and hence the coordinating ability of the
carboxylate bridges. The singleꢀcrystal Xꢀray structures of 2, 3 and 4 have been determined to
show that each iron atom in the complexes is coordinated to one oxygen atom of the µꢀoxo
bridge and two oxygen atoms of both the µꢀcarboxylate bridges apart from three nitrogen atoms
of the 3N ligand. We have observed that all the six complexes show efficient hydroxylation of
cyclohexane and adamantane with approximately 380ꢀ500 turn over numbers and good
selectivity (A/K ratio, 6.0ꢀ10.1; 3°/2°, 12.9ꢀ17.1), as tuned, interestingly, by the varying electronꢀ
donating and electronꢀwithdrawing nature of substituents on the carboxylate bridging ligands.
Experimental
Materials
Pyridineꢀ2ꢀcarboxaldehyde, N,Nꢀdimethylethylenediamine, iron(III) perchlorate hydrate,
adamantane, cumene, trimethylacetic acid, diphenylacetic acid, tripheneylacetic acid, mꢀ
chloroperbenzoic acid, sodium borohydride, tetraꢀNꢀbutylammonium bromide (Aldrich),
triethylamine, benzoic acid, acetic acid glacial, formic acid (Merck, India), cyclohexane
(Ranbaxy) and ethanol (Hayman Limited, England) were used as received. Dichloromethane,
diethylether, tetrahydrofuran, acetonitrile (Merck, India) and methanol (Sisco Research
Laboratory, Mumbai) were distilled before use. The supporting electrolyte tetraꢀNꢀ
butylammonium perchlorate (TBAP) was prepared in water and recrystallized twice from
aqueous ethanol. The ligand N,NꢀdimethylꢀN’ꢀ(pyridꢀ2ꢀylmethyl)ethaneꢀ1,2ꢀdiamine was
prepared as reported41 earlier.
Synthesis of Complexes
The general procedure involves the addition of carboxylic acids (2.0 mmol) previously
neutralized with 2.0 mmol of triethylamine/sodium hydroxide to a mixture of 924 mg of
Fe(ClO4)3·6H2O (2.0 mmol) in methanol (5 mL) containing H2O (0.1 mL) and stirred for 30 min
at room temperature. To this, a methanol (8 mL) solution of the ligand (2.0 mmol) was added
and the resulting dark red solution was turned reddish brown or green upon stirring for an hour.
Reddish brown or green precipitate or microcrystals of diiron(III) complexes were formed within

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24 hr upon standing. The precipitates were collected by suction filtration, washed with small
quantities of cold methanol and dried in vacuo.
[Fe2(O)(O2CH)2(L)2](ClO4)2 (1). Yield: 0.56 g (72%). ESIꢀMS, m/z = 677.0 [(M−ClO4)+]; 289.0
[(M−2ClO4)2+]. Anal. Calcd. for C22H38Cl2Fe2N6O13: C 34.00, H 4.93, N 10.81. Found C 34.04,
H 4.90, N 10.75.
[Fe2(O)(OAc)2(L)2](ClO4)2 (2). Yield: 0.69 g (86%). ESIꢀMS, m/z = 705.0 [(M−ClO4)+]; 303.0
[(M−2ClO4)2+]. Anal. Calcd. for C24H42Fe2N6O13Cl2: C 35.80, H 5.26, N 10.44. Found C 35.79,
H 5.28, N 10.39. Single crystals of 2 suitable for Xꢀray diffraction were obtained by the slow
evaporation of methanol/acetonitrile solution of the complex.
[Fe2(O)(Me3AcO)2(L)2](ClO4)2·CH3CN (3). Yield: 0.72 g (77%). ESIꢀMS, m/z = 787.1
[(M−ClO4−CH3CN)+]; 344.1 [(M−2ClO4−CH3CN)2+]. Anal. Calcd. for C32H55Fe2N7O13Cl2: C
41.40, H 5.97, N 10.56. Found C 41.42, H 5.95, N 10.50. Single crystals of 3 suitable for Xꢀray
diffraction were obtained by the slow evaporation of methanol/acetonitrile solution of the
complex.
[Fe2(O)(OBz)2(L)2](ClO4)2·CH3OH·H2O (4). Yield: 0.79 g (81%). ESIꢀMS, m/z = 827.0
[(M−ClO4−CH3OH−H2O)+]; 364.2 [(M−2ClO4−CH3OH−H2O)2+]. Anal. Calcd. for
C35H50Fe2N6O15Cl2: C 43.01, H 5.16, N 8.60. Found C 43.09, H 5.12, N 8.55. Single crystals of
4 suitable for Xꢀray diffraction were obtained by slow evaporation of methanol/acetonitrile
solution of the complex.
[Fe2(O)(Ph2AcO)2(L)2](ClO4)2 (5). Yield: 0.84 g (76%). ESIꢀMS, m/z = 1007.2 [(M−ClO4)+];
454.1 [(M−2ClO4)2+]. Anal. Calcd. for C48H56Cl2Fe2N6O13: C 52.05, H 5.10, N 7.59. Found C
52.09, H 5.17, N 7.58.
[Fe2(O)(Ph3AcO)2(L)2](ClO4)2 (6). Yield: 0.88 g (70%). ESIꢀMS, m/z = 1159.2 [(M−ClO4)+];
530.1 [(M−2ClO4)2+]. Anal. Calcd. for C60H64Cl2Fe2N6O13: C 57.20, H 5.12, N 6.67. Found C
57.23, H 5.15, N 6.69.
Caution: Perchlorate salts of the compounds are potentially explosive! Only small quantities of
these compounds should be prepared and suitable precautions should be taken when they are
handled.

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Catalytic Oxidations
The oxidation of alkanes was carried out at room temperature under an atmosphere of air.
3
In a typical reaction, the oxidant mꢀCPBA (0.8 mol dm− ) was added to a mixture of diiron(III)
3
3
3
complex (1 × 10− mmol dm− ) and alkanes (3 mol dm− ) in CH2Cl2:CH3CN mixture (4:1 v/v).
After 30 min (Figure S5) the reaction was quenched with triphenylphosphine, the reaction
mixture was filtered over a silica column and then eluted with diethylether. An internal standard
(bromobenzene) was added at this point and the solution was subjected to GC analysis. The
mixture of organic products were identified by GCꢀMS and quantitatively analyzed by HP 6890
series GC equipped with HPꢀ5 capillary column (30 m × 0.32 mm × 2.5 ꢀm) using a calibration
curve obtained 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 to 130 °C min−1, increasing the temperature to 160 °C at a rate of 2 °C min−1, and
then increasing the temperature to 260 °C at a rate of 5 °C min−1; FID temperature 280 °C. GCꢀ
MS analysis was performed under conditions identical to those used for GC analysis. The
averages of three measurements are reported.
Physical Measurements
Elemental analyses were performed on a Perkin Elmer Series II CHNS/O Analyzer 2400.
1H NMR spectra were recorded on a Bruker 400 MHz NMR spectrometer. Electronic spectra
were recorded on an Agilent 8453 Diode Array Spectrophotometer. Low temperature spectra
were obtained on an Agilent 8453 Diode Array Spectrophotometer equipped with an UNISOKU
USPꢀ203 cryostat. Electrosprayꢀionization massꢀspectrum (ESIꢀMS) analyses were recorded on a
Micromass Quattro II triple quadrupole mass spectrometer. Cyclic voltammetry (CV) and
differential pulse voltammetry (DPV) were performed at 25±0.2 °C using a threeꢀelectrode cell
configuration. A platinum sphere, a platinum plate and Ag(s)/AgNO3 were used 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 nonꢀaqueous solution was Ag(s)/Ag+, which consists
of a Ag wire immersed in a solution of AgNO3 (0.01 M) and tetraꢀNꢀbutylammonium
perchlorate (0.1 M) in acetonitrile placed in a tube fitted with a vycor plug. The instruments
utilized included an EG & G PAR 273 Potentiostat/Galvanostat and PꢀIV computer along with

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EG & G M270 software to carry out the experiments and to acquire the data. The temperature of
the electrochemical cell was maintained by a cryoꢀcirculator (HAAKE D8ꢀG). The E1/2 values
observed under identical conditions for Fc/Fc+ couple in acetonitrile was 0.102 V with respect to
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
measurements. The products were analyzed by using Hewlett Packard (HP) 6890 GC series Gas
Chromatograph equipped with a FID detector and a HPꢀ5 capillary column (30 m × 0.32 mm ×
2.5 ꢀm). GCꢀMS analysis was performed on an Agilent GCꢀMS equipped with 7890A GC series
(HPꢀ5 capillary column) and 5975C inert MSD under conditions that are identical to that used
for GC analysis.
Crystallographic Data Collection and Structure Refinement
The diffraction experiments were carried out on a Bruker SMART APEX diffractometer
equipped with a CCD area detector. High quality crystals, suitable for Xꢀray diffraction was
chosen after careful examination under an optical microscope. Intensity data for the crystals were
collected using MoKα (λ = 0.71073 Å) radiation on a Bruker SMART APEX diffractometer
equipped with a CCD area detector at 100 and 293 K. The structure was solved by direct
methods using the SHELXSꢀ97 and OLEX 2.42 The refinement and all further calculations were
carried out using SHELXLꢀ97.42 The SMART43 program was used for collecting frames of data,
indexing reflection and determination of lattice parameters; SAINT44 program for integration of
the intensity of reflections and scaling; SADABS45a program for absorption correction, and the
SHELXTL42 program for space group and structure determination, and leastꢀsquares refinements
on F2. The disordered solvent molecule present in the lattice of 4 is removed by SQUEEZ
function in PLATON.45b,c The structure was solved by heavy atom method and other nonꢀ
hydrogen atoms were located in successive difference Fourier syntheses. Crystal data and
additional details of the data collection and refinement of the structure are presented in Table 1.
The selected bond lengths and bond angles are listed in Table 2.
Results and Discussion
Synthesis and characterization of 3N ligand and diiron(III) complexes. The tridentate 3N
ligand was synthesized according to known procedures, which involve Schiff base condensation

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of N,Nꢀdimethylethylenediamine with pyridineꢀ2ꢀcarboxaldehyde followed by reduction with
NaBH4 and characterized by 1H NMR and mass spectrometry.41 The diiron(III) complexes 1 – 6
were prepared by adding one equivalent of the 3N ligand to an aqueous methanol solution
containing one equivalent each of Fe(ClO4)3·6H2O, the carboxylic acid and triethylamine. All of
them are formulated as [Fe2(O)(O2CR)2(L)2](ClO4)2 on the basis of elemental analysis, UVꢀVis
spectroscopy, ESIꢀMS and Xꢀray crystal structures of 2, 3 and 4. All the complexes with
different bridging carboxylate groups are expected to mimic the active site environment of the
substrateꢀbound enzyme sMMO and have been chosen to model the alkane hydroxylation
reactions of the enzyme.
Description of Xꢀray Crystal Structures of [Fe2(O)(OAc)2(L)2](ClO4)2 2,
[Fe2(O)(Me3AcO)2(L)2](ClO4)2 3 and [Fe2(O)(OBz)2(L)2](ClO4)2 4
The molecular structure of the dimeric cation of [Fe2(O)(OAc)2(L)2]2+ 2 is shown in
Figure 1 together with the atom numbering scheme and the selected bond lengths and bond
angles are collected in Table 2. The molecule contains an inversion centre and the coordination
geometry around each iron atom in the diiron(III) complex 2 is distorted octahedral constituted
by the oxygen atom of the µꢀoxo bridge, two oxygen atoms one each from the µꢀacetate bridges
and three amine nitrogen atoms of the 3N ligand. The coordination geometry of the complex
cation is very similar to those of complexes23,40,46ꢀ50 with (µꢀoxo)bis(µꢀcarboxylato)diiron(III)
core structures. The tridentate ligand capping the two ends of the (µꢀoxo)bis(µꢀcarboxylato)ꢀ
diiron(III) cluster coordinates to iron(III) in a least strained facial rather than meridional fashion
(Scheme 4). The FeꢀNpy (2.202 Å) and FeꢀNamine (2.170(3), 2.234(3) Å) bond distances fall
respectively in the ranges 2.100(10) ꢀ 2.227(4) Å and 2.160(5) ꢀ 2.334(7) Å observed for monoꢀ51
and diiron(III)25,30,40,52,53 complexes reported in the literature. The FeꢀNamine bond (2.234(3) Å)
formed by the terminal amine nitrogen of the tridentate ligand is longer than the FeꢀNpy bond
(2.202(2) Å) obviously due to sp3 and sp2 hybridizations respectively of the tertiary amine and
pyridyl nitrogen atoms. However, the FeꢀNpy bond is longer (cf. below, 3) than the FeꢀNamine
bond (2.170(3) Å) involving sp3 hybridized central amine nitrogen due to the trans effect exerted
by the strongly coordinated µꢀoxo bridge. Also, the terminal FeꢀNamine bond is longer than the
central FeꢀNamine bond, which is expected. The FeꢀOacetate bond distances (1.988(2), 2.069(2) Å)

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fall within the range 1.98 ꢀ 2.142 Å observed for similar (µꢀoxo)bis(µꢀcarboxylato)diiron(III)
complexes.29ꢀ40,54
The Xꢀray crystal structure of the dimeric cation of [Fe2(O)(Me3AcO)2(L)2]2+ 3 is shown
in Figure 2 together with the atom numbering scheme and the selected bond lengths and bond
angles are collected in Table 2. The coordination geometry of 3 is similar to that of 2 but no
inversion centre is present. Each iron(III) centre possesses a distorted octahedral coordination
geometry with small differences in bond lengths and bond angles of the other centre. The FeꢀNpy
bond in 3 (2.145(3), 2.159(3) Å) is shorter than the FeꢀNamine bonds formed by the central
(2.192(3), 2.181(3) Å) and terminal (2.216(3), 2.225(3) Å) tertiary amine nitrogen atoms, which
is expected of the sp2 and sp3 hybridizations respectively of the pyridyl and tertiary amine
nitrogen atoms. Also, the FeꢀNamine bond formed by the terminal amine nitrogen atom is longer
than the central amine nitrogen, as expected. Interestingly, the FeꢀNpy bond in 2 is much longer
than that in 3 confirming the trans effect of µꢀoxo bond in the former. The presence of electronꢀ
releasing methyl groups in 3 is expected to render the coordination of trimethylacetate anion
stronger (cf. below) than that of acetate anion in 2. However, the FeꢀOcarboxylate bond distances in
3 (2.021 ꢀ 2.069 Å) are longer than those (1.988, 2.069 Å) in 2. So it is evident that the steric
bulk rather than electronic effect of trimethylacetate anion is important.
The Xꢀray crystal structure of the dimeric cation of [Fe2(O)(OBz)2(L)2]2+ 4 is shown in
Figure 3 together with the atom numbering scheme and the selected bond lengths and bond
angles are collected in Table 2. The coordination geometry of 4 is similar to that of 3 and each
iron(III) center possesses a distorted octahedral coordination geometry with small differences in
bond lengths and bond angles. As for 3, the FeꢀNpy bonds in 4 (2.148(4), 2.158(5) Å) are shorter
than the FeꢀNamine bonds formed by the central (2.219(4), 2.198(5) Å) and terminal (2.203(4),
2.192(4) Å) tertiary amine nitrogen atoms, as expected. Interestingly, the FeꢀNamine bond formed
(a)
(b)
N
Npy
Npy
N
Me2
N
Me2
N
Fe
Fe
O
O
O
O
Ooxo
Ooxo
Scheme 4. Mode of coordination of 3N ligand with change in 2 (a) and 3 and 4 (b)

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by the central amine nitrogen atom in 4, unlike in 3, is longer than the FeꢀNamine bonds formed by
the terminal amine nitrogen atom. The replacement of the acetate anion in 2 by the electronꢀ
withdrawing benzoate anion as in 4 renders one of the FeꢀOcarboxylate bonds weaker (2.001 – 2.057
Å), causing the terminal FeꢀNamine and FeꢀNpy bonds to become stronger by trans effect and
consequently, the FeꢀNamine bond formed by the central amine nitrogen becomes longer. Similar
trends in bond lengths have been observed on replacing the acetate bridges in
[Fe2(O)(OAc)2(mbpa)2]2+,55a where mbpa is N,Nꢀbis(pyridꢀ2ꢀylmethyl)methylamine, by benzoate
bridges to give [Fe2(O)(OBz)2(mbpa)2]2+.55b
Interestingly, the central amine nitrogen atom of the capping ligand in 3 and 4 is axial to
Fe(1)O(2)O(3)N(1)N(3) plane and is coordinated trans to the µꢀoxo bridge (Scheme 4), which is
in sharp contrast to the terminal pyridyl nitrogen in 2 located trans to the µꢀoxo bridge. Upon
replacing the acetate bridges in 2 by trimethylacetate (3) or benzoate (4) bridge the coordination
mode of 3N ligand changes and is similar to that found in the analogous 3N ligand complexes
[Fe2(O)(OAc/OBz)2(mbpa)2]2+,55 and [Fe2(O)(OBz)2(bba)2]2+,56 where bba is bis(benzimidazolꢀ
2ꢀylmethyl)amine. A weaker coordination of the bridging carboxylate, caused by steric or
electronꢀwithdrawing effect, requires a stronger FeꢀN bond, which is fulfilled by the more
strongly coordinating pyridine rather than weakly coordinating amine nitrogen atom, leading to
the observed change in mode of coordination of capping ligand.
The Fe–Ooxo bond distance in 2 (Fe(1)–O(1), 1.791(2) Å), 3 (1.796(2), 1.792 (2) Å) and
4 (1.788(3), 1.794(3) Å) falls within the range 1.73–1.82 Å found for oxoꢀbridged diiron(III)
complexes reported23,40,46ꢀ50 in the literature and for [Fe2OCl6]2ꢀ.57ꢀ59 The Fe–O–Fe bond angle (2,
120.27(16); 3, 116.96(11); 4, 119.32(16)°) is very close to those observed for the analogous
complexes [Fe2(O)(OAc/OBz)2(mbpa)2]2+,55 and is well outside the range for the singlyꢀ60 and
doublyꢀbridged61 complexes (140–180°). The FeꢀꢀꢀFe separation (2, 3.107; 3, 3.059; 4, 3.091 Å)
is shorter than those in singlyꢀ and doublyꢀbridged µꢀoxoꢀdiiron(III) complexes (3.4–3.6 Å),
which is expected of the presence of three ligand bridges. The FeꢀꢀꢀFe separation in 2 (3.107 Å)
is slightly longer than those in 3 (3.059 Å) and 4 (3.091 Å) due to the steric bulk of
trimethylacetate and benzyl groups in the carboxylate bridges. For 2, 3 and 4 the NꢀFeꢀN, OꢀFeꢀ
N and OꢀFeꢀO bond angles fall in the range 74.6(2)ꢀ 102.55(9)° and the NꢀFeꢀO bond angle in
the range 159.49(10) ꢀ 173.69(8)°, which deviate from the ideal octahedral angles of 90° and

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180° respectively, revealing the presence of significant distortion in the iron(III) coordination
geometry. A molecular model building study shows that the carboxylate bridges are very
flexible.
Electronic Spectral and Electrochemical Studies
In acetonitrile solution, all the present diiron(III) complexes 1 ꢀ 6 exhibit an intense band
in the range 460–520 nm (εmax, 830–1640 M−1 cm−1, Table 3, Figure 4), which are
characteristic29,49,56 of O2− → iron(III) LMCT bands in oxoꢀbridged diiron(III) complexes
containing two carboxylate bridges. The less intense band around 700 nm corresponds to the spin
forbidden ligand field (LF) transition, which is expected for the mononuclear highꢀspin
octahedral iron(III) complexes.49 The oxoꢀbridge of all the complexes remains intact in
dichloromethane/acetonitrile solvent mixture as revealed by ESIꢀMS analysis.
The electrochemical properties of the diiron(III) complexes were investigated in
acetonitrile/dichloromethane solvent mixture by employing cyclic (CV) and differential pulse
voltammetry (DPV) on a stationary platinum electrode. All the complexes show a cathodic peak
in the range −0.633 to −0.841 V in the CV, which corresponds to one electron reduction in the
Fe(III)ꢀOꢀFe(III) + e−
Fe(II)ꢀOꢀFe(III)
diiron(III) unit, and no coupled oxidation wave is discernible (Figure 5). The E1/2 values of the
FeIII/FeII redox couple (−0.595 to −0.770 V, Table 3) fall in the range observed for similar type
of triplyꢀbridged diiron(III) complexes.40,48,49 They are highly negative mainly due to the strong
coordination of the bridging carboxylate and oxo groups and follow the trend 1 < 2 < 3 > 4 > 5 >
6 revealing the importance of the bridging carboxylates in stabilizing iron(III) oxidation state.
Thus on replacing both the bridging formates (pKa, HCOOH, 3.77) in 1 by the bridging acetates
(pKa, AcOH, 4.76) to obtain 2, the FeIII/FeII redox potential is shifted from ꢀ0.665 V to a more
negative value (ꢀ0.757 V), which is expected of the acetate anion coordinating more strongly to
iron(III) center. A similar shift in redox potential is observed on moving from 2 to 3 (E1/2, ꢀ0.770
V) revealing that the stronger coordination of trimethylacetate anion (pKa, Me3AcOH, 5.0)
renders the iron(III) center difficult to reduce. However, the Xꢀray crystal structures of 2 and 3
show that the trimethylacetate anion binds more weakly than the acetate anion (cf. above)
possibly due to the higher steric bulk of the former. On replacing the electronꢀreleasing methyl
groups in the bridging acetates in 2 by electronꢀwithdrawing phenyl group to obtain 4, the

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FeIII/FeII redox potential is shifted to a less negative value (E1/2, ꢀ0.743 V) implying weaker
coordination of benzoates (pKa, BzOH, 4.19) to iron(III). Upon replacing both the bridging
benzoates in 4 by diphenylacetates (pKa, Ph2AcOH, 3.94) to obtain 5, the FeIII/FeII redox
potential is shifted to a less negative value (E1/2, ꢀ0.630 V) revealing that diphenylacetate ion is
coordinated to iron(III) more weakly than the benzoate anion. A similar shift in redox potential is
observed on moving from 5 to 6 (E1/2, ꢀ0.595 V) revealing that the triphenylacetate anion (pKa,
Ph3AcOH, 3.96) coordinates less strongly to iron(III) making it easier to reduce. Thus upon
replacing the electronꢀreleasing substituents by the less electronꢀreleasing ones, the FeIII/FeII
redox potential becomes progressively less negative revealing that a variation in the substituents
on the bridging carboxylate tunes well the redox potential. In fact, interestingly, a linear
correlation between the pKa of the bridging carboxylate and E1/2 value is obtained (Figure 6).
Functionalization of alkanes
The experimental conditions and results of catalytic oxidation of alkanes for the
diiron(III) complexes 1ꢀ6 are summarized in Tables 4 and 5. The conversion of alkanes into
hydroxylated products was quantified based on gas chromatographic analysis by using authentic
samples and an internal standard. The catalytic ability of diiron(III) complexes towards oxidation
of alkanes like cyclohexane and adamantane was investigated by using mꢀCPBA as oxidant in
dichloromethane/acetonitrile solvent mixture (4:1 v/v) at room temperature. Control reactions
performed in the absence of diiron(III) complexes with mꢀCPBA as oxidant yielded only very
small amounts of oxidized products (Cyclohexane, 3 TON; Adamantane, 5 TON), revealing that
all the diiron(III) complexes act as catalysts towards oxidation of alkanes to alcohols. In the
presence of diiron(III) complexes the oxidation of cyclohexane proceeds to give cyclohexanol
(A) as the major product along with cyclohexanone (K) and εꢀcaprolactone as minor products.
The latter is the over oxidized product of cyclohexanone in the presence of excess or unreacted
mꢀCPBA. All the complexes display efficient alkane hydroxylation with 300ꢀ500 turn over
numbers (TON) with good selectivity for the hydroxylation of cyclohexane (A/K, 6.0ꢀ10.1;
Table 4). Also, no significant change in both product selectivity and yield is observed upon
performing the reaction under nitrogen atmosphere revealing the involvement of metal based
oxidants rather than radical species in the catalytic reactions. The catalytic activity of diiron(III)
complexes with the same 3N capping ligand towards hydroxylation of cyclohexane follows the

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trend 1 (Total TON, 401; A/K, 8.5) < 2 (442; 9.0) < 3 (492; 7.6) > 4 (411; 10.1) > 5 (398; 7.2) >
6 (387; 6.0) illustrating the importance of the bridging carboxylates. Very interestingly, the same
order of TON is observed for adamantane oxidation also, 1 (Total TON, 381; 3°/2°, 15.1) < 2
(466; 14.9) < 3 (476; 17.1) > 4 (437; 15.7) > 5 (416; 13.8) > 6 (399; 12.9) (Table 5) revealing the
involvement of a mechanism same as that for cyclohexane oxidation. Thus, all the present
diiron(III) complexes show high selectivity in the hydroxylation of cyclohexane (A/K, 6.0–10.1;
Table 4) and adamantane (3°/2°, 12.9–17.1) as well, signifying the involvement of metalꢀbased
oxidants rather than nonꢀselective freely diffusing radical species in the alkane
hydroxylation.33,37,62,63 Also, recently several iron and nickel complexes have been reported
involving metalꢀbased oxidants in hydroxylation of alkanes with mꢀCPBA as oxidant.64
The complex 1 catalyses the oxidation of cyclohexane to give 408 total TON with good
A/K ratio (8.5) corresponding to 51.0% conversion of the oxidant into organic products. Upon
replacing both the formate bridges in 1 by acetate bridges to give 2, the catalytic activity is
enhanced to 442 total TON with the A/K ratio of 9.0 corresponding to 55% conversion. It is clear
that the stronger coordination of the acetate group in 2 (cf. above) enhances the electronic charge
on the iron(III) center and hence the efficiency of putative highꢀvalent intermediate species
(Scheme 5) leading to a better catalytic activity. Also, previously it has been reported that the
Fe(V)=O species have been stabilized by injecting more electronic charge into the 3d shell by the
strong σꢀdonating capacity of the TAML tetraanion.65 Interestingly, the activity of 2 towards
cyclohexane oxidation is higher than those of [Fe2O(OAc)2(iBubpa/Bzbpa)2](ClO4)2 (Total TON:
iBubpa, 380; Bzbpa, 348),33 suggesting that a change in nitrogen donors of the capping ligand
plays a vital role in dictating the yields of hydroxylation products of alkanes. The complex 3
catalyses the oxidation of cyclohexane with 492 total TON and A/K value of 7.6 corresponding
to 62% conversion of the oxidant into the organic products, which is the highest among the
present complexes and also higher than those for the previously reported triplyꢀbridged
diiron(III) complexes.29ꢀ33 Upon replacing the bridging acetates in 2 by bridging
trimethylacetates to obtain 3 the electron density on the iron(III) center increases (cf. above),
which enhances the stability of putative highꢀvalent intermediate species leading to higher
catalytic activity. Thus, it is inferred that electronꢀreleasing substituents on the bridging
carboxylates plays a vital role in stabilizing the putative highꢀvalent ironꢀoxo species formed
during the catalytic cycle.

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Upon replacing electronꢀreleasing bridging acetates in 2 by electronꢀwithdrawing
benzoate groups to give 4, a decrease in the TON (411) as well as extent of conversion (51%)
but, interestingly, with a higher selectivity (A/K, 10.1) are observed. Obviously, the electronꢀ
withdrawing benzoate groups in 4 decreases the stability of putative highꢀvalent FeIV/V=O
reactive intermediate species leading to a decrease in catalytic activity. Also, a change in the
i
capping ligand from Bubpa/Bzbpa as in [Fe2O(OBz)2(iBubpa/Bzbpa)2](ClO4)2 (total TON:
iBubp, 400; Bzbpa, 329) to L in 4, a higher total TON (411) is observed, supporting the above
observation. Upon replacing both the benzoate bridging groups in 4 by diphenylacetate anions to
give 5, slightly lower total TON (398) and A/K ratio (7.3) are observed due to the incorporation
of two electronꢀwithdrawing phenyl groups in the carboxylate bridge. The diiron(III) complex 6
containing three electronꢀwithdrawing phenyl groups in the carboxylate bridge catalyses the
oxidation of cyclohexane to give total a TON of 387 with the A/K value of 6.0, which are lower
than those for 5. The electronic rather than steric effect of substituents on the bridging
carboxylates play a major role in dictating the stability of intermediate species (Scheme 5) and
hence the catalytic efficiency of diiron(III) complexes and in particular, the electronꢀreleasing
trimethylacetate anion in 3 makes the complex the most efficient catalyst for alkane
hydroxylation reaction.
All the diiron(III) complexes catalyse the oxidation of adamantane efficiently to give 1ꢀ
adamantanol and 2ꢀadamantanol as the major products along with 2ꢀadamantanone as the minor
product. Complex 1 catalyzes the oxidation of adamantane with a total TON of 381 with a good
selectivity (3°/2°, 15.1). Upon replacing the bridging formates in 1 by acetates to give 2, the
adamantane oxidation occurs with increased catalytic activity giving a total TON of 466, which
is higher than that for 1, as for cyclohexane oxidation (cf. above). The total TON for 3 is 476,
which is the highest among the present complexes, as observed for cyclohexane oxidation.
Interestingly, the replacement of methyl group on the carboxylate bridge in 2 by electronꢀ
withdrawing benzoates as in 4 leads to a total TON of 437, which is lower than that for 2. The
total TON observed for 5 is 416, which is lower than that for 4 obviously due to the higher
electronꢀwithdrawing effect of diphenylacetate bridges in the former. A still lower catalytic
activity is obtained for 6 (total TON, 399), due to the presence of three electronꢀwithdrawing
phenyl groups on the bridging triphenylacetate. Also, the selectivity observed for 5 (3°/2°: 13.8)
and 6 (3°/2°: 6, 12.9) is lower than that for 4. Thus, a variation in the substituents on the bridging

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carboxylates around the diiron(III) centre leads to remarkable tuning of the catalytic efficiency
towards adamantane oxidation.
Interestingly, a linear correlation is observed between the pKa of the bridging
carboxylates and total TON (Figure S2 and S3) revealing that the coordinating ability of the
bridging carboxylates as determined by the substituents on them plays a vital role in determining
the catalytic ability of the diiron(III) complexes. Also, there is a linear correlation between the
E1/2 value and total TON (Figure S4), which is expected of the observed linear correlation
between the pKa of the bridging carboxylates and the E1/2 value of the diiron(III) complexes (cf.
above). So, it is evident that the E1/2 value may be considered as a measure of stabilization of
iron(III) oxidation state and hence that of putative highꢀvalent intermediate species. Also, it has
been previously shown that, on increasing the number of pyridyl nitrogen donors in some
mononuclear iron(II) complexes, the FeIII/FeII redox potential is shifted to more negative values
and the negative charge built on the iron center stabilizes the highꢀvalent ironꢀoxo intermediate
species.62 Similarly, the present diiron(III) complexes with more negative FeIII/FeII redox
potentials facilitate the formation of putative highꢀvalent intermediate species so as to act as
efficient turn over catalysts for alkane hydroxylation.
Reaction of diiron(III) complexes with mꢀCPBA
When the oxidant H2O2 or tꢀBuOOH is added to the diiron(III) complexes dissolved in
dichloromethane/acetonitrile solvent mixture at room temperature no spectral change is observed
even after adding triethylamine. Also, the addition of the strong oxidant mꢀCPBA to acetonitrile
solution of the diiron(III) complexes at room temperature produces no significant spectral
changes. However, addition of one equivalent of triethylamine to the reaction mixture leads to
the appearance of a new band in the region of 680ꢀ720 nm (Figure 7). Previously it was reported
that that the band corresponds to the formation of [Fe2O(iBubpa/Bzbpa)2(OBz)ꢀ
(OOCOC6H4Cl)]2+,33 formed upon replacing one of the bridging carboxylates by mꢀCPBA. For
the present adducts also no intermediate was discerned characteristic of highꢀvalent intermediate
species during their decay in the absence of any substrate. However, such an intermediate species
has been proposed to be formed initially in alkane hydroxylation reactions catalyzed by the
33
29
complexes [Fe2(O)(OBz)2(iBubpa/Bzbpa)2](ClO4)2 and [Fe2(O)(OAc)2(hexpy)](ClO4)2 using
mꢀCPBA as oxidant. On the other hand, the electrospray ionization mass (ESIꢀMS) spectrum of

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the reaction mixture of 4 with 1 eq. of mꢀCPBA and 1 eq. of TEA at RT in acetonitrile shows
five peaks at m/z = 827.0, 860.9, 882.8, 894.9 and 916.8 corresponding to
III
III
III
{[Fe2 (O)(L)2(OBz)2]ClO4}+, {[Fe2 (O)(L)2(OBz)2]ClO4}+, {[Fe2 (O)(L)2(OBz)(OBzCl)]ꢀ
III
III
ClO4}+, {[Fe2 (O)(L)2(OBz)2]ꢀ(OBzCl)}+ and {[Fe2 (O)(L)2(OBz)(OBzCl)](OBzCl)}+
respectively. Due to the addition of mꢀCPBA the diiron(III) complex forms different cations
under ESI condition. Also, the electrospray ionization mass (ESIꢀMS) spectrum of the reaction
mixture of 4 with 5 eq. of mꢀCPBA and 1 eq. of TEA at RT in acetonitrile shows four intense
III
peaks at m/z = 860.9, 894.8, 916.8 and 950.7 corresponding to {[Fe2 (O)(L)2(OBz)2]ClO4}+,
III
III
III
{[Fe2 (O)(L)2(OBz)2](OBzCl)}+, {[Fe2 (O)(L)2(OBz)(OBzCl)](OBzCl)}+ and {[Fe2 (O)ꢀ
(L)2(OBzCl)2](OBzCl)}+ and three less intense peaks at 876.8, 882.8 and 910.8 (Figure S1).
From the observed isotope distribution patterns, the peak at 876.8 is formulated as
III
{[Fe2 (O)(L)2(OBz)(OOCOC6H4Cl)]ClO4}+ and is derived from the diiron(III) complex
resulting from the replacement of one of the benzoate bridging ligand. Such a species formation
has also been reported previously for the [Fe2O(iBubpa/Bzbpa)2(OBz)2]2+.33 The ESIꢀMS
spectrum also features other less intense band at 910.8 is formulated as
{[Fe2 (O)(L)2(OBzCl)(OOCOC6H4Cl)]ClO4}+ is derived from the diiron(III) complex resulting
III
from the replacement of one of the benzoate bridging ligand by mꢀchlorobenzoic acid and the
other benzoate by mꢀchloroperbenzoic acid. Also, we propose that all the other peaks are formed
from the diiron(III) complex in the absence of a substrate, under the ESIꢀMS conditions.
So, we now propose that the acyloxo adduct species [Fe2O(L)2(RCO2)(OOCOC6H4Cl)]2+
(Scheme 5) either undergoes OꢀO bond homolysis or OꢀO bond heterolysis leading to the
formation respectively of putative highꢀvalent FeIIIꢀOꢀFeIV=O or FeIIIꢀOꢀFeV=O intermediate
species, which are involved in the selective hydroxylation of alkanes. For all the present
diiron(III) complexes we have observed the formation of chlorobenzene up to 90% based on total
TON. This observation strongly supports the proposed formation of the adduct species
[Fe2O(L)2(RCO2)(OOCOC6H4Cl)]2+, undergoes OꢀO bond homolysis leading to generate FeIIIꢀOꢀ
FeIV=O species and chlorobenzoate radical. The chlorobenzene is formed from the
decarbonylation of chlorobenzoate radical. This observation also resembles the recent
observation by Que and coworkers that the mononuclear [(S,SꢀPDP)FeIII(κ2ꢀperacetate)] species
undergo OꢀO bond hemolysis to form the [(S,SꢀPDP)FeIV(O)(AcO·)] species, that performs the
efficient alkane hydroxylation.63 As chlorobenzene is produced up to 90% based on total TON, it

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has been suggested that up to 90% of the oxidized products are formed from the involvement of
putative highꢀvalent FeIIIꢀOꢀFeIV=O species. Also, recently Que and coworkers characterized the
similar intermediate species, derived from the diiron(III) complex [FeIII O(L)2]2+, where L is
2
N,Nꢀbisꢀ(3′,5′ꢀdimethylꢀ4′ꢀmethoxypyridylꢀ2′ꢀmethyl)ꢀN′ꢀacetylꢀ1,2ꢀdiaminoethane.66
Also,
Wieghardt and coꢀworkers have reported the formation of FeIIIꢀOꢀFeIV=O species derived by one
electron oxidation of (µꢀoxo)bis(µꢀcarboxylato)diiron(III) complexes.67 In addition,
chlorobenzene is only produced up to 90% based on the total TON. So, it is clear that some other
intermediate species is also involved in the hydroxylation reaction to produce the remaining
R
O
H
N
R
O
H
N
N
N
O
O
O
O
O
O
FeIII
FeIII
N
III
mꢀCPBA
N
N
FeIII
N
Fe
ꢀRCOO
O
O
N
N
N
N
O
R
H
H
OꢀO bond
heterolysis
OꢀO bond
homolysis
Cl
R
H
N
R
H
N
N
O
N
O
O
O
FeV
N
N
N
FeIII
IV
O
O
N
Fe III
N
Fe
O
O
N
N
N
O
O
mꢀCPBA
O
H
O
H
Cl
Cl
R
H
N
R
O
H
N
N
N
O
O
O
FeIII
O
O
FeIII
FeIII
N
FeIII
N
O
N
N
Sol Sol
Sol Sol
N
N
N
H
N
H
HO
O
OH
O
Cl
OH
CO2
Cl
Scheme 5. Proposed mechanism for alkane hydroxylation

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hydroxylated products. So, we also propose the involvement of putative highꢀvalent FeIIIꢀOꢀ
FeV=O species responsible for the hydroxylation of the remaining hydroxylated products, which
may be formed from the OꢀO bond heterolysis of the acyloxo adduct species, favored by the
electronꢀreleasing bridging carboxylates.
Conclusions
A series of nonꢀheme ꢀꢀoxoꢀ and bis(ꢀꢀcarboxylato)ꢀbridged diiron(III) complexes of a
linear 3N ligand has been isolated and all of them possess distorted bioctahedral geometry
around the diiron(III) centre. The good selectivity observed for cyclohexane hydroxylation using
mꢀCPBA as oxidant (A/K, 6.0ꢀ10.1) and adamantane (3°/2°, 12.9ꢀ17.1) suggests the involvement
of putative highꢀvalent ironꢀoxo species rather than freely diffusing radicals in the catalytic
reaction. Interestingly, the total TON observed for hydroxylation of cyclohexane for the
diiron(III) complexes with trimethylacetate bridges is the highest among the present complexes
and is also higher than those observed for the previously reported triplyꢀbridged diiron(III)
complexes. Also, interestingly, the incorporation of electronꢀreleasing substituents on the
carboxylate bridges makes the diiron(III) complex to an efficient catalyst towards alkane
hydroxylation whereas that of electronꢀwithdrawing substituents makes the complex a relatively
poor catalyst. Thus, an interesting linear correlation between the TON of the diiron(III)
complexes and pKa of the bridging carboxylates has been observed, which reveals that
electronic rather than steric effect of the substituents on bridging carboxylates plays a remarkable
role in dictating the efficiency as well as selectivity of catalysts towards alkane hydroxylation.
Acknowledgments
We sincerely thank the Department of Science and Technology, New Delhi for DST a Nano
Mission Project [Scheme No. SR/NM/NSꢀ110/2010(G) for funding with a Research
Associateship (M.B.) and SERB, New Delhi for a research project EMR/2015/002222.

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Table 1. Crystal Data and Structure Refinement Details for [Fe2O(OAc)2(L1)2](ClO4)2 2,
[Fe2O(Me3Ac)2(L2)2](ClO4)2⋅CH3CN 3 and [Fe2O(OBz)2(L1)2] (ClO4)2⋅CH3OH⋅H2O 4
2
3
4
Empirical formula
C24H42Fe2N6O13Cl2
C32H55Fe2N7O13Cl2
C35H50Fe2N6O15Cl2
Formula weight /
g mol–1
805.22
928.43
977.41
Crystal habit, colour
Crystal system
Crystal size
Space group
a, Å
Blocks, Wine Red
Monoclinic
Blocks, Brown
Orthorhombic
Prism, Brown
Monoclinic
0.50 x 0.43 x 0.34 mm 0.41 x 0.38 x 0.22 mm 0.10 x 0.10 x 0.12 mm
C2/c
P212121
10.9602(7)
18.5001(12)
20.8260(14)
90.00
C2/c
25.674(10)
7.876(3)
20.102(8)
90.00
34.8958(8)
11.1865(2)
28.5616(7)
90.00
b, Å
c, Å
α
, deg
, deg
, deg
123.068(10)
90.00
90.00
127.463(2)
90
β
90.00
γ
V/Å3
3407(2)
8
4222.8(5)
4
8849.8(3)
8
Z
ρcalcd/ g cmꢀ3
F(000)
T/K
1.566
1.460
1.395
1664
1944
3856
293
100
296
No. of Reflections
collected
13280
25416
73976
No. of unique
reflections
3897
9827
8138
0.71073
0.71073
1.061
524
0.71073
1.034
559
Radiation(MoKα)/ Å
Goodnessꢀofꢀfit on F2 1.019
Number of refined
Parameters
256
R1/ w R2[I>2σ(I)]a
0.0465/0.1243
0.0685/0.1423
0.0456/0.1096
0.0508/0.1143
0.0613/ 0.1957
0.1035
R1/ w R2 (all data)
2
4
aR1 = [Σ(||Fo|ꢀ|Fc||)/Σ|Fo|]; wR2 = {[Σ(w(Fo ꢀFc2)2)/Σ(wFo )]1/2}

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Table 2. Selected bond lengths [Å] and bond angles [°] for 2, 3 and 4
2
3
4
Bond lengths/ Å
Fe(1)ꢀN(1)
Fe(1)ꢀN(2)
Fe(1)ꢀN(3)
Fe(1)ꢀO(1)
Fe(1)ꢀO(2)
Fe(1)ꢀO(3)
2.202(2)
2.170(3)
2.234(3)
1.7913(16)
1.988(2)
2.069(2)
Fe(1)ꢀN(1)
Fe(1)ꢀN(2)
Fe(1)ꢀN(3)
Fe(1)ꢀO(5)
Fe(1)ꢀO(2)
Fe(1)ꢀO(3)
Fe(2)ꢀN(4)
Fe(2)ꢀN(5)
Fe(2)ꢀN(6)
Fe(2)ꢀO(5)
Fe(2)ꢀO(1)
Fe(2)ꢀO(4)
2.145(3)
2.192(3)
2.216(3)
1.796(2)
2.069(2)
2.037(2)
2.159(3)
2.181(3)
2.225(3)
1.792(2)
2.021(2)
2.068(2)
Fe(1)ꢀN(1)
Fe(1)ꢀN(2)
Fe(1)ꢀN(3)
Fe(1)ꢀO(1)
Fe(1)ꢀO(2)
Fe(1)ꢀO(5)
Fe(2)ꢀN(4)
Fe(2)ꢀN(5)
Fe(2)ꢀN(6)
Fe(2)ꢀO(1)
Fe(2)ꢀO(3)
Fe(2)ꢀO(4)
2.148(4)
2.219(4)
2.203(4)
1.788(3)
2.039(3)
2.001(3)
2.158(5)
2.198(5)
2.192(4)
1.794(3)
2.029(3)
2.057(3)
Bond angles/ °
O(1)ꢀFe(1)ꢀO(2)
O(1)ꢀFe(1)ꢀO(3)
O(2)ꢀFe(1)ꢀO(3)
O(1)ꢀFe(1)ꢀN(2)
O(2)ꢀFe(1)ꢀN(2)
O(3)ꢀFe(1)ꢀN(2)
O(1)ꢀFe(1)ꢀN(1)
O(2)ꢀFe(1)ꢀN(1)
O(3)ꢀFe(1)ꢀN(1)
N(2)ꢀFe(1)ꢀN(1)
O(1)ꢀFe(1)ꢀN(3)
O(2)ꢀFe(1)ꢀN(3)
O(3)ꢀFe(1)ꢀN(3)
N(2)ꢀFe(1)ꢀN(3)
98.98(8)
O(5)ꢀFe(1)ꢀO(3) 101.31(9)
O(5)ꢀFe(1)ꢀO(2) 99.95(9)
O(3)ꢀFe(1)ꢀO(2) 86.61(10)
O(5)ꢀFe(1)ꢀN(1) 96.52(10)
O(3)ꢀFe(1)ꢀN(1) 161.72(9)
O(2)ꢀFe(1)ꢀN(1) 86.34(10)
O(1)ꢀFe(1)ꢀO(5)
O(1)ꢀFe(1)ꢀO(2)
O(2)ꢀFe(1)ꢀO(5)
O(1)ꢀFe(1)ꢀN(1)
O(5)ꢀFe(1)ꢀN(1)
O(2)ꢀFe(1)ꢀN(1)
100.51(14)
98.14(13)
90.30(14)
98.47(15)
160.62(16)
83.15(14)
97.60(16)
88.13(16)
164.20(16)
93.23(16)
172.18(16)
86.59(17)
84.97(15)
74.70(17)
95.08(8)
97.36(10)
98.43(10)
161.45(10)
87.41(11)
173.69(8)
87.26(9)
O(5)ꢀFe(1)ꢀN(2) 170.54(11) O(1)ꢀFe(1)ꢀN(3)
O(3)ꢀFe(1)ꢀN(2) 87.26(10)
O(2)ꢀFe(1)ꢀN(2) 84.41(10)
N(1)ꢀFe(1)ꢀN(2) 75.28(11)
O(5)ꢀFe(1)ꢀN(3) 96.35(10)
O(3)ꢀFe(1)ꢀN(3) 87.45(10)
O(5)ꢀFe(1)ꢀN(3)
O(2)ꢀFe(1)ꢀN(3)
N(1)ꢀFe(1)ꢀN(3)
O(1)ꢀFe(1)ꢀN(2)
O(5)ꢀFe(1)ꢀN(2)
83.18(9)
75.47(10)
89.99(10)
92.81(11)
167.78(9)
80.84(11)
O(2)ꢀFe(1)ꢀN(3) 163.46(10) O(2)ꢀFe(1)ꢀN(2)
N(1)ꢀFe(1)ꢀN(3) 94.65(10) N(1)ꢀFe(1)ꢀN(2)

Dalton Transactions
Page 28 of 37
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DOI: 10.1039/C6DT01059H
N(1)ꢀFe(1)ꢀN(3)
Fe(1)ꢀO(1)ꢀFe(1)
90.58(10)
N(2)ꢀFe(1)ꢀN(3) 79.90(11)
O(5)ꢀFe(2)ꢀO(1) 102.55(9)
O(5)ꢀFe(2)ꢀO(4) 100.03(9)
O(1)ꢀFe(2)ꢀO(4) 85.32(9)
O(5)ꢀFe(2)ꢀN(4) 97.66(10)
N(3)ꢀFe(1)ꢀN(2)
79.25(17)
99.78(14)
98.03(14)
89.09(13)
99.1(2)
120.27(16)
O(1)ꢀFe(2)ꢀO(3)
O(1)ꢀFe(2)ꢀO(4)
O(3)ꢀFe(2)ꢀO(4)
O(1)ꢀFe(2)ꢀN(4)
O(1)ꢀFe(2)ꢀN(4) 159.49(10) O(3)ꢀFe(2)ꢀN(4)
O(4)ꢀFe(2)ꢀN(4) 87.75(9) O(4)ꢀFe(2)ꢀN(4)
O(5)ꢀFe(2)ꢀN(5) 170.61(10) O(1)ꢀFe(2)ꢀN(6)
160.0(2)
81.70(16)
96.78(16)
89.17(16)
165.17(16)
95.21(18)
172.34(17)
87.02(16)
85.51(16)
74.6(2)
O(1)ꢀFe(2)ꢀN(5) 85.04(10)
O(4)ꢀFe(2)ꢀN(5) 85.97(9)
N(4)ꢀFe(2)ꢀN(5) 75.22(10)
O(5)ꢀFe(2)ꢀN(6) 94.68(10)
O(1)ꢀFe(2)ꢀN(6) 85.48(9)
O(3)ꢀFe(2)ꢀN(6)
O(4)ꢀFe(2)ꢀN(6)
N(4)ꢀFe(2)ꢀN(6)
O(1)ꢀFe(2)ꢀN(5)
O(3)ꢀFe(2)ꢀN(5)
O(4)ꢀFe(2)ꢀN(6) 164.04(10) O(4)ꢀFe(2)ꢀN(5)
N(4)ꢀFe(2)ꢀN(6) 96.44(10)
N(5)ꢀFe(2)ꢀN(6) 80.27(10)
N(4)ꢀFe(2)ꢀN(5)
N(6)ꢀFe(2)ꢀN(5)
79.69(18)
119.32(16)
Fe(1)ꢀO(5)ꢀFe(2) 116.96(11) Fe(1)ꢀO(1)ꢀFe(2)

Page 29 of 37
Dalton Transactions
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DOI: 10.1039/C6DT01059H
Table 3. UVꢀVisible spectral data and electrochemical data of the diiron(III) complexes in
DCM:ACN mixture at 25 °C
Complex
λmax/nm
Ep,c (CV)
(V)
ꢀ0.713
E1/2 (DPV)
(V)
ꢀ0.665
Redox Process
(ε/M–1 cm–1)
[Fe2(O)(OOCH)2(L)]2+ 1 468 (1640)
FeIIIFeIII →FeIIFeIII
511 (1060)
713 (215)
[Fe2(O)(OAc)2(L)]2+
[Fe2(O)(Me3Ac)2(L)]2+
[Fe2(O)(OBz)2(L)]2+
[Fe2(O)(Ph2Ac)2(L)]2+
[Fe2(O)(Ph3Ac)2(L)]2+
2
3
4
5
6
466 (1413)
506(1153)
713 (230)
464 (1236)
507 (929)
713 (134)
466 (1445)
507 (1108)
720 (171)
467 (1090)
509 (940)
713 (150)
466 (1028)
510 (832)
685 (144)
ꢀ0.825
ꢀ0.841
ꢀ0.743
ꢀ0.695
ꢀ0.633
ꢀ0.757
ꢀ0.770
ꢀ0.679
ꢀ0.630
ꢀ0.595
FeIIIFeIII →FeIIFeIII
FeIIIFeIII →FeIIFeIII
FeIIIFeIII →FeIIFeIII
FeIIIFeIII →FeIIFeIII
FeIIIFeIII →FeIIFeIII
Potential measured vs. Ag/AgNO3 (0.001 M, 0.1 M TBAP); add 0.544 V to convert to NHE.