Non-heme iron(III) complex with tridentate ligand: Synthesis, structures and catalytic oxidations of alkanes

Article abstract of DOI:10.1016/j.catcom.2017.02.012

We report synthesis and characterization of a mononuclear iron(III) complex [Fe(LH₂)Cl₂]⁺, 1 having tridentate ligand LH₂ and their application in oxidation of alkanes. Complex, 1 was characterized by various analytical, spectroscopic, electrochemical, crystallographic and theoretical studies. The molecular structure determination showed the presence of distorted square pyramidal FeN₃Cl₂ coordination sphere. Selective oxidation of benzyl alcohol, fluorene, adamantane, 1-phenyl ethanol and n-octane in presence of catalytic amount [Fe^III(LH₂)Cl₂]⁺ with H₂O₂ under mild conditions give products in good to excellent yields through Fe^IV[dbnd]O intermediate. Several studies were done to support the reactive intermediate. Generation of Fe^IV[dbnd]O intermediate was also supported by UV–visible titration of [Fe^III(LH₂)Cl₂]⁺ and H₂O₂ at low temperature and their visual colour change from red (500 nm, ? = 1050 M^{? 1} cm^{? 1}) to green (770 nm, ? = 153 M^{? 1} cm^{? 1}).

Full text of DOI:10.1016/j.catcom.2017.02.012

Accepted Manuscript
Non-heme iron(III) complex with tridentate ligand: Synthesis,
structures and catalytic oxidations of alkanes
Nidhi Tyagi, Ovender Singh, Kaushik Ghosh
PII:
DOI:
Reference:
S1566-7367(17)30063-8
doi: 10.1016/j.catcom.2017.02.012
CATCOM 4940
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2 December 2016
8 February 2017
14 February 2017
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ACCEPTED MANUSCRIPT
Non-Heme Iron(III) Complex with Tridentate Ligand: Synthesis,
Structures and Catalytic Oxidations of Alkanes
Nidhi Tyagi, Ovender Singh, Kaushik Ghosh*
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India
Email Address of Corresponding Author: ghoshfcy@iitr.ernet.in
Fax : +91-1332-273560
1

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Abstract
We report synthesis and characterization of a mononuclear iron(III) complex
[Fe(LH2)Cl2]+, 1 having tridentate ligand LH2 and their application in oxidation of alkanes.
Complex, 1 was characterized by various analytical, spectroscopic, electrochemical,
crystallographic and theoretical studies. The molecular structure determination showed the
presence of distorted square pyramidal FeN3Cl2 coordination sphere. Selective oxidation of
benzyl alcohol, fluorine, adamantine, 1-phenyl ethanol and n-octane in presence of catalytic
amount [FeIII(LH2)Cl2]+ with H2O2 under mild conditions give products in good to excellent
yields through FeIV=O intermediate. Several studies were done to support the reactive
intermediate. Generation of FeIV=O intermediate was also supported by UV-visible titration
of [FeIII(LH2)Cl2]+ and H2O2 at low temperature and their visual colour change from red (500
-1
-1
nm, ɛ = 1050 M-1cm ) to green (770 nm, ɛ = 153 M-1cm ).
Key Words: High-spin Fe(III) complex, Distorted square pyramidal Fe complex, Oxidation
of alkanes, FeIV=O intermediate, Single crystal analysis.
2

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1.
Introduction
Transition metal-catalyzed reactions have played vital roles in the synthesis of natural
and unnatural compounds via late transition metals, such as palladium, ruthenium, rhodium,
gold and platinum etc. [1]. However, the relatively high price and considerable toxicity of
these metals limited their applications. Therefore research has been focused on more
efficient, convenient, cost-effective and environmentally benign metals. Among the first row
transition metals, iron, which is second-most abundant metal in the earthʼs crust, possesses
above advantages and unique features which showed its magical catalytic capability to
facilitate many catalytic and bioactivities. Various iron-catalyzed organic transformations
have been realized, including nucleophilic additions, substitutions, reductions, oxidations,
hydrogenations, cycloadditions, isomerizations, rearrangements as well as polymerizations
[1-4].
Selective oxidation of alkanes under mild conditions is important in view of synthetic
and industrial aspects and because of the lack of the reactivity of alkanes (due to their high C-
H bond energy), catalytic oxidation of alkanes under mild conditions are quite difficult [5].
Therefore, these reactions can be catalyzed by naturally occurring metalloenzymes which
include cytochrome P-450, methane monooxygenase and Rieske dioxygenase as well as a
variety of oxidant such as, molecular oxygen (O2), t-butyl hydroperoxide, hydrogen peroxide,
m-chloroperbenzoic acid etc. [6,7,8]. Literature described large number of biomimetic model
have been established to mimic the reactivity of these enzymes [9,10]. For eg. Groves et al.
first reported biomimetic oxidation of hydrocarbons with iodosylbenzene using [Fe(TPP)Cl]
catalyst [11]. In 2003, Que and co-worker described, the first X-ray crystal structure of a
mononuclear non-heme iron(IV)-oxo complex [FeIV(O)(tmc)(MeCN)][OTf]2 has been
prepared by reacting [FeII(tmc)(OTf)2] with PhIO in acetonitrile at -40 °C [12]. Kim and co-
worker reported mononuclear non-heme iron complex of tetradentate carboxamido ligand,
[FeIII(Me2bpb)Cl(H2O)] for alcohol oxidation in presence of m-chloroperbenzoic acid and
mechanism of this conversion involve a high-valent iron-oxo species, which was supported
18
by KIE (kH/kD) and H2 O exchange experiments, as well as the use of peroxyphenylacetic
acid (PPAA) as an oxidant [13]. Similarly, in Gif-system, Fe(III) catalysts and t-butyl
hydroperoxide were used for alcohols transformation into their corresponding ketones with
good yields [14].
Oxidation reaction using hydrogen peroxide catalyzed by ferrous ions (Fenton’s
reagent) has also been carefully investigated [15]. Several reports on Fe(III) porphyrin
complexes with H2O2 has been extensively studied through O–O activation and oxygen atom
transfer mechanism [16-18]. Bandyopadhyay and co-workers reported heterogeneous
iron(III)-cyclam (intercalated
onto
smectite
montmorillonite
K-10) for alkane
hydroxylation using benign H2O2 at room temperature with a very high selectivity for
alcohols in comparison to [FeIII(cyclam)Cl2]Cl/H2O2 under homogeneous condition through
high-valent iron–oxo intermediates [19]. All the above literature revealed the important role
3

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of structure of supporting chelating ligand plays in influencing the formation of high-valent
iron-oxo species.
With the view of above literature data, herein, we have design, synthesize and
characterized a novel tridentate ligand LH2 (shown in Scheme 1), their iron complex
[Fe(LH2)Cl2]+, 1 and their catalytic activity. Complex, [FeIII(LH2)Cl2]+ was characterized by
various analytical, spectroscopic and electrochemical studies. Molecular structure of
[Fe(LH2)Cl2]+, 1, was also determined by X-ray crystallography. Efficient and selective
oxidation of benzyl alcohol, fluorine, adamantine, 1-phenylethanol and n-octane in presence
of catalytic amount [FeIII(LH2)Cl2]+ with H2O2 (due to its environmental impact, as water is
the only by product) under mild conditions and affording products in good to excellent yields.
We found that chelating ligand (LH2) support for the formation of high-valent iron-oxo
species to give good catalytic oxidation products. Generation of intermediate was supported
by UV-visible titration of [FeIII(LH2)Cl2]+ and H2O2 at low temperature.
2.
Results and Discussion
White solid of a new tridentate ligand, LH2 has been synthesized by stirring of 2-(1-
phenylhydrazinyl)pyridine (1 equivalent) and pyridine-2,6-dicarbonyl dichloride at 0oC in
dichloromethane in the presence of the base triethylamine (1.25 equivalent) followed by
overnight refluxing. White product was further purified by silica column chromatography
(DCM:MeOH). Ligand, LH2 reacted with FeCl3 in 1:1 equivalent ligand to metal ratio
yielded red solid of [Fe(LH2)Cl2]Cl, 1 in good yield. The synthetic procedure of ligand, LH2
and their Fe(III) complex [Fe(LH2)Cl2]Cl, 1 were described in Scheme 1.
---------------
Scheme 1
----------------
The structures of the ligand, LH2 as well as complex [Fe(LH2)Cl2]+, 1 have been
characterized by various spectral and analytical techniques (Fig. S1-S7, ESI†). Coordination
of amine N to iron metal centre was confirmed by shifting of νC=O from 1690 cm−1 in free
ligand to 1630 cm−1 in metal complex through IR spectral studies. The IR spectrum of 1 was
dominated by vibrations of the pyridyl rings as evidenced by the absorptions 1474 and 1588
-1
cm [20]. The ESI-MS spectrum of 1 showed a peak at m/z = 555 for [L2-+Fe]+. Molar
-1
conductivity (DMF, at ca. 10−3 M) was found to be 58.0 Ω-1cm2mol , indicating uni-
univalent (1:1) electrolyte behaviour [21]. Complex 1 showed a magnetic moment of 5.89
BM in the solid state at 296 K, suggesting a high-spin (S = 5/2) electronic configuration of
iron(III) centre [22]. The electrochemical properties of complex 1 exhibited a cathodic peak
at -422 mV vs Ag/AgCl, which is assignable to the Fe(III)/Fe(II) couple (Fig. S7). The UV-
visible spectrum of 1 was recorded in dichloromethane, the peak near 320 and 240 nm was
assigned as chloro-to-Fe(III) charge transfer transition and π-π*, n-π* transitions of ligand,
-1
respectively [23]. Complex 1 also showed a broad band near 500 nm (ε = 1050 M-1cm ) due
to ligand-to-metal charge transfer transition (LMCT) [24].
The ORTEP diagram for cation of mononuclear iron(III) complex [Fe(LH2)Cl2]+, 1 is
shown in Fig. 1. Matrix parameters, selected bond distances and bond angles are described in
4

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Table S1-S2. The coordination environment around the iron atom is described as distorted
square pyramidal with the trigonality index τ of 0.31 [τ = (β-α)/60, where β = N2–Fe1–N3 =
144.68◦ and α = N5–N2–Fe1 = 125.96◦; for perfect square pyramidal and trigonal
bipyramidal geometries the τ values are zero and unity, respectively, according to the
Addison/Reedijk geometric criterion] [22,25]. Ligand, LH2 bonded to iron(III) at the three
corners and a chloride ion of the basal plane of the square pyramid and other chloride ion
occupies the apical site in the molecular structure of 1. The free phenyl and pyridine rings
attached to amine nitrogen in 1 looks like as butterfly wings. The iron (III) centre is 0.512 Å
above the basal plane generated by N1, N2, N3 and Cl2 and lies in the direction of the apical
atom Cl(1). The Fe–NPy distance is 2.099(4) Å, whereas, Fe-Nam distances, Fe-N2, Fe-N3 are
2.052(4) and 2.067(4) Å, respectively, which is consistent with the reported data [26,27]. The
equatorial Fe1–Cl2, Fe–Cl1 bond distance are 2.2190(15) and 2.2207(17) Å, respectively
which is close to the literature values [28]. The intermolecular packing interaction along b
axis shown hydrogen bonding with counter ion Cl- and all Npy of complex 1 with distance
~3.1 Å (free pyridine) and ~3.2 Å (pyridine bonded to iron centre) as well as a weak π–π
stacking between adjacent pyridyl and phenyl rings with centroid-centroid distance of ~4.04
Å. Due to these intermolecular interactions (H-bonding and weak π–π stacking) of complex 1
leads to the formation of zigzag structure (Fig. S8) as π–π interactions plays an important role
in controlling the packing or assembly of compounds. In general, multi-dentate ligands with
pyridine groups or nitrogen heterocycles conspicuously contribute as building blocks in the
design of metal–ligand networks.
---------------
Fig. 1
---------------
In
addition to
the spectroscopic and
crystallographic evidences,
the
complex 1 was unambiguously supported through theoretical analysis. In order to describe
the electronic properties of the transitions involved in the absorption processes of complex 1,
we optimized the geometry of 1 (Fig. S9(A)) followed by the analysis with the aid of time-
dependent density functional theory (TD-DFT) (B3LYP/LANL2DZ) [29-31]. Transitions
with oscillator strength (f =0.0002) at around 490 nm mainly corresponds to H-
11(β)→L+1(β), and H-4(β)→L+3(β) transitions with orbital contribution of 33% and 24%,
respectively (Fig. S6, Table S3). Single occupied frontier molecular orbitals (SOMO)
diagram of complex 1 and significant contribution of iron and ligand in SOMO and shown in
Fig. 2. The percentage contribution of complex 1 in both alpha and beta orbitals is tabulated
in Table S4. Mulliken spin-density plot (Fig. S9(B)) of 1 reveals +3.73 spin on the metal,
ligand holds +1.27 spin over entire ligand along with chloride atoms and justify the oxidation
state of iron as trivalent (atomic spin density is tabulated in Table S5 in Supporting
Information). The geometrical parameters as bond lengths and bond angles in gas phase was
also calculated using Gaussian 09 package and their comparison with crystallographic data
were shown in Table S2.27
---------------
5

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Fig. 2
---------------
To test the catalytic activities of this robust catalyst, oxidation of alkanes were
o
examined at 40 C and we found good results with acetonitrile as solvent and H2O2 as oxidant
under air. Control experiments revealed that in the absence of complex 1, only less than 1%
of the oxidized product was detected and the efficiency of alkane oxidation with H2O2
catalysed by only FeCl3 is also very low (less than 1%). Oxidation of alcohol has its chemical
and biological importance and selective oxidation of alcoholic part to its corresponding
aldehyde or ketone, without over oxidation to acid group is important. In this study, benzyl
alcohol selectively converted to benzaldehyde (Scheme S1(a), Table 1) using 1 mol%
catalyst, [Fe(LH2)Cl2]+ and 150 eq. of H2O2 in good yields (40-95%, TOF = 5-2 h-1) at 40oC,
when reaction was monitored 8-48 h, excitingly with 20 mol% catalyst ca. 100% yield was
obtained in 4h under similar condition. The results of TON and TOF (h-1) are summarized in
Table S6. Under the similar condition this reaction works well with sterically hindered alkane
such as fluorene or 2-adamantanol (Scheme S1(b),(c), Table S6). However, fluorene
oxidation to fluoren-9-ol (Scheme S1(b)) show less % conversion around 25 % (TOF= 0.52
h-1) and no significant enhancement in % conversion was observed even after 48 h of reaction
(Table 1). Interestingly, when 20 mol% catalysts used ca. 60% yield was obtained in 4h
under similar condition. We have also studied the oxidation of adamantane and the products
from the oxidation of adamantane are shown in Scheme S1(c). Adamantane has six
equivalent secondary carbon atoms (C2), each one with two hydrogen atoms, and four
equivalent tertiary carbon atoms (C3), each one with one hydrogen atom. The oxidation at C2
gives adamantan-2-ol (A2) and adamantanone (K), while the oxidation at C3 gives
adamantan-1-ol (A1). We calculated the normalized reactivity ratio (C3:C2) as the proportion
of the products on C3 positions (adamantan-1-ol, A1) obtained in relation to those on C2
positions (adamantan-2-ol (A2) and adamantanone (K)) obtained. For this purpose, we
divided the area obtained by GC corresponding to a product by the number of its isomers
(equivalent positions), that is, 4 for adamantan-1-ol, 12 for adamantan-2-ol and 6 for
adamantanone (Table S6). With use of 1 mol% catalyst and 150 eq. of H2O2 at 40oC showed,
A1 (34-43%), A2+K (59-66%) when reaction was monitored for 8-48 h, interestingly this
conversion was increased drastically A1 (83%) and A2+K (92%) with 20 mol% catalyst in
4h. Under similar conditions, 1-phenylethanol showed good conversion of 93% with 20
mol% catalyst in 4h (Scheme S1(d). We have also monitored the conversion of n-octane to 2-
, 3-, 4- octanol (68%), and 2-octanone (22%) with 20 mol% catalyst, 150 eq. of H2O2 in 4h
(Scheme S1(e).
-------------
Table 1
--------------
The possible mechanism for transfer oxidation reaction using the complex 1 is shown in
Scheme 2. The loss of a chloride ligands followed by the addition of peroxide molecule
generates the active [FeIII(LH2)Cl(OOH)]+ species which was supported by ESI-MS
6

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spectrum. The ESI-MS mass spectrum of complex 1 in acetonitrile in presence of H2O2
showed a mass peak at around m/z = 626.34, corresponding to [[Fe(LH2)Cl(OOH)]+ (Fig.
S10). The [FeIII(LH2)Cl(OOH)]+ species upon hemolysis generate•OH radical and formed a
FeIV=O intermediate, as shown in step A. Generation of FeIV=O intermediate can be further
supported by UV-visible titration of [FeIII(LH2)Cl2]+ and H2O2 at low temperature. The band
-1
at around 500 nm (ɛ = 1050 M-1cm ) in complex 1 decrease after addition of H2O2 (1.5
-1
equiv.) and a band around 770 nm (ɛ = 153 M-1cm ) appear with a fast color change of red to
o
green at 0 C (shown in Fig. 3). Martinho et al. reported a green colored [(TPEN)FeIV=O]2+
species obtained by treating the [(TPEN)Fe]2+ (in acetonitrile) precursor with 1-2 equivalent
-1
m-CPBA at 0 °C and characterized by absorption band at around 730 nm (ɛ ≈ 380 M-1cm )
[32]. Similarly, green species [(N4Py)FeIV=O] is also observed around 700 nm (ε > 170 M-
-1
1cm ) in absorption spectra [33]. To confirm the oxidizing species, we used Shul’pin method,
where a comparison was made after and before addition of PPh3 to the catalytic reaction
mixture and we found no change in the GC spectra, suggesting that the main oxidizing
species is not a hydroxyl radical, probably a ferryl ion, (FeIV=O), similarly FeCl3 shown
oxidation proceed (partially) with the formation of ferryl ion, (FeIV=O) (Fig. S11(a),(b)) [34-
35]. Additional, we have done the catalytic reaction in presence and absence of air and found
no change in A/K ratio, suggesting non-involvement of O2 that propagates a radical chain
autoxidation process [36]. Furthermore, optimized HOMO energies of [Fe(LH2)Cl2],
[Fe(LH2)(OOH)Cl] and [Fe(LH2)(O)Cl] intermediate were found to be -8.8, -5.3 and -6.4 eV,
respectively, which also supported FeIV=O could be a probable intermediate (Fig. 4).
This high valent oxo-species further upon addition of substrate molecule generate a
•
[Fe(LH2)Cl(OH)]+ and R• species (step B). Further upon subsequent addition of OH radical
leads to alcohol formation in step C and water elimination in step D followed by
[Fe(LH2)Cl(OH2)]+ species (step D). According to this mechanism the substrate will
coordinate to the high valent oxo-species (as shown in step B) and results in the formation of
substrate complex to give the desired product.
It has been found in literature that FeIV=O species have been prepared generally by
treating an iron (III/II) precursor bearing an amine/pyridine ligand with a single oxygen-atom
donor, such as ClO-, PhIO, peroxides, or even with dioxygen [12,37-39]. Shul’pin and
coworker mentioned that the number of iron-bound chloride ions (in FeCl3) changes the
•
process selectivity by replacing OH to FeIV=O, as oxidizing species [35]. Hence, with
important role of structure of supporting chelating ligand (LH2) for the formation of high-
valent iron-oxo species we are able to get good catalytic oxidation products.
----------------
Fig. 3
----------------
Fig. 4
----------------
Scheme 2
7

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----------------
3.
Conclusion
In summary, we report the synthesis, characterization and catalytic oxidation activity of
a novel mononuclear iron(III) complex [Fe(LH2)Cl2]+, 1. Complex, 1 was characterized by
various analytical, spectroscopic and electrochemical studies. The molecular structure of
complex [FeIII(LH2)Cl2]+, 1 was determined by single crystal X-ray diffraction and revealed
the presence of distorted square pyramidal FeN3Cl2 coordination sphere. Furthermore,
theoretical studies completely support the experimental findings such as electronic spectra
and oxidation state of metal centre. This robust complex [Fe(LH2)Cl2]+, 1 was tested for
catalytic oxidation of selective alkanes eg. benzyl alcohol, fluorine, adamantine, 1-phenyl
ethanol and n-octane in presence of catalytic amount 20 mol% of [FeIII(LH2)Cl2]+ with H2O2
in good to excellent yield under mild conditions. Proposed mechanism suggested the
generation of FeIV=O intermediate which was supported by UV-visible titration of
[FeIII(LH2)Cl2]+ and H2O2 at low temperature with a visual colour change. Efforts to develop
asymmetric catalyzed oxidation of various alkanes of higher technological interest by
introducing the chiral moiety to ligands as well as their detailed mechanistic aspects are under
progress.
Acknowledgements
K.G. is thankful to CSIR Project of India for financial assistance
(no.01/(2720/13/EMR-II). Nidhi Tyagi is thankful CSIR and Ovender Singh is thankful to
MHRD for financial assistance. We are thankful to IISER, Thiruvananthapuram for single
crystal X-ray facility.
Supplementary data
Supplementary data to this article (Experimental section, Fig. S1-S9 and Tables S1-S7
showing the characterization of the compound and properties under different conditions,
CCDC for the complex 1 is 1520051) can be found online.
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38. V. Balland, M. F. Charlot, F. Banse, J. J. Girerd, T. A. Mattioli, E. Bill, , J. F. Bartoli,
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Scheme, Figures and Table Caption List
Scheme 1. Synthetic procedure for ligand LH2 and complex 1, [Fe(LH2)Cl2]+.
Scheme 2. Proposed mechanism for catalytic oxidation reaction by [Fe(LH2)Cl2]+, 1 with
H2O2 as oxidant.
Fig. 1. ORTEP view of the complex [Fe(LH2)Cl2]Cl, 1 with ellipsoids at 50% probability
level. Selected bond lengths are as follows, Fe1-N2: 2.052(4), Fe1-N3: 2.067(4), Fe1-N1:
2.099(4), Fe1-Cl2: 2.2190(15) and Fe1-Cl1: 2.2207(17). (Counter ion and hydrogen atoms
have been omitted for clarity)
Fig. 2. Single occupied frontier molecular orbitals diagram of complex 1, showing significant
contribution of iron and ligand in SOMO.
Fig. 3. UV-Visible spectra of complex 1 (black) alone and in presence of H2O2 (green).
Fig. 4. Optimized HOMO energies of [Fe(LH2)Cl2], [Fe(LH2)(OOH)Cl] and [Fe(LH2)(O)Cl]
intermediate.
Table 1. Catalytic oxidation of benzyl alcohol, fluorene in presence of catalytic amount of
complex [Fe(LH2)Cl2]+, 1 with H2O2 in 8, 24 and 48 h.
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Scheme 1
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Scheme 2
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Fig. 1
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Fig. 2
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Fig. 3
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Fig. 4
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Table 1
S.
Substrate
Product
TON
TOF (h-1)
No.
8 h
Benzyl alcohol Benzaldehyde 40
Fluorene fluoren-9-ol 15
24 h 48 h
8 h
5
2
24 h 48 h
1
2
60
20
95
25
3
1
2
0.5
aReaction is carried out with 1 mol% catalyst in CH3CN at 40 (℃). The yields were
measured by GC 8, 24 and 48 h after the beginning of the reaction. TON defined as [TON=
{(% conversion ) × (mM of substrate)/(mM of catalyst)×100}, TOF= Actual TON/time].
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Highlights




Iron(III) complexof tridentateligand was synthesized and characterized.
Molecularstructure of mononuclearhigh-spin Fe(III)complex was determined.
The complex catalysed oxidation of alkane undermild condition.
Role of high-valentFeIV=O species was speculated duringcatalyticactivity.
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