Synthesis, X-ray studies, and catalytic efficacy of a novel iron complex containing an N,O-type bidentate thiazoline ligand

Article abstract of DOI:10.1002/zaac.201300302

The reactions of FeCl₃·6H₂O and 2-(2′-hydroxyphenyl)-2-thiazoline as a bidentate O-N donor thiazoline ligand (thoz) afford a five-coordinate Fe^III complex [Fe(thoz) ₂Cl] with a distorted square pyramidal configuration. Complex [Fe(thoz)₂Cl] was isolated as air-stable crystalline solids and fully characterized, including by single-crystal X-ray structure analysis. Complex [Fe(thoz)₂Cl] shows very efficient reactivity in the oxidation of sulfides to their corresponding sulfoxides using urea hydrogen peroxide (UHP) as the oxidant at room temperature in air. Copyright

Full text of DOI:10.1002/zaac.201300302

ARTICLE
DOI: 10.1002/zaac.201300302
Synthesis, X-ray Studies, and Catalytic Efficacy of a Novel Iron Complex
Containing an N,O-Type Bidentate Thiazoline Ligand
Mojtaba Amini,*[a] Mina Bigdeli,[a] Sepideh Delsouz-Hafshejani,[a] Arkady Ellern,[b] and
L. Keith Woo[b]
Keywords: Thiazoline; Iron; Crystal structure; Sulfide oxidation
Abstract. The reactions of FeCl3·6H2O and 2-(2Ј-hydroxyphenyl)-2-
crystal X-ray structure analysis. Complex [Fe(thoz)2Cl] shows very
thiazoline as a bidentate O-N donor thiazoline ligand (thoz) afford efficient reactivity in the oxidation of sulfides to their corresponding
a five-coordinate FeIII complex [Fe(thoz)2Cl] with a distorted square sulfoxides using urea hydrogen peroxide (UHP) as the oxidant at room
pyramidal configuration. Complex [Fe(thoz)2Cl] was isolated as air-
stable crystalline solids and fully characterized, including by single-
temperature in air.
well as exceptional catalytic activity and in recent years, sig-
nificant advances in the oxidation of sulfides has been ac-
complished by applying iron catalysts.[17–20]
Introduction
Model compounds play a significant role in establishing the
structure of enzymic intermediates and contribute to a better
understanding of their catalytic properties and behavior.[1,2]
Iron is the most prominent transition metal in biological per-
spective, as many enzymes containing ferrous or ferric ion cat-
alyze various bio-transformations. Therefore a growing interest
in the coordination chemistry of iron is based on the recogni-
tion of its importance from a biological, pharmacological, and
catalytic perspective.[3,4]
Multidonor ligands containing nitrogen and sulfur atoms,
and their versatile chelating ability with transition metal ions,
have attracted considerable interest in a wide range of asym-
metric catalytic reactions.[5–9] Among these ligands, 2-(2Ј-hy-
droxyphenyl)-2-thiazoline (Hthoz), found in the microbial in
the iron chelator (S)-(–)-desferrithiocin. Several metal com-
plexes bearing 2-(2Ј-hydroxyphenyl)-2-thiazoline have been
reported in the literature.[10,11]
We recently reported highly efficient method for the oxi-
dation of sulfides by using of a FeIII complex with 2-(2Ј-hy-
droxyphenyl)-5,6-dihydro-1,3-oxazine as oxazine ligand.[21] In
order to study the effect of the ligand on the catalytic reactiv-
ity, herein we describe the synthesis of a new FeIII complex
[Fe(thoz)2Cl] [Hthoz = 2-(2Ј-hydroxyphenyl)-2-thiazoline] and
the use of this complex as a catalyst in the oxidation of sulfides
in the presence of urea hydrogen peroxide (UHP) as an oxidant
in an air atmosphere at room temperature (Scheme 1). This is
the first report on the catalytic activity of a thiazoline-FeIII
complex in oxidation reactions.
Scheme 1. The oxidation of sulfides by the [Fe(thoz)2Cl]/UHP cata-
The oxidation of sulfides to sulfoxides is of significant im-
portance in organic chemistry, both for fundamental research
and for a wide range of applications.[12–14] Organic sulfoxides
are useful synthetic intermediates for the construction of vari-
ous chemically and biologically active molecules since then, a
number of methods have been developed for the conversion of
sulfides into sulfoxides.[15,16] In particular, iron would be an
ideal candidate to be an environmentally benign catalyst be-
cause of its easy availability, low toxicity, and low price as
lytic system.
Results and Discussion
Complex Characterization
The complex was obtained by the reaction of 2 equiv. of
the ligand 2-(2Ј-hydroxyphenyl)-2-thiazoline and FeCl3·6H2O
(Scheme 2). The fast color change from a pale yellow to a dark
* Prof. Dr. M. Amini
Fax: +98-421-2276066
E-Mail: mamini@maragheh.ac.ir
[a] Department of Chemistry, Faculty of Science
University of Maragheh
P.O. Box. 55181–83111, Maragheh, Iran
[b] Chemistry Department
Iowa State University
Iowa 50011–3111, USA
Scheme 2. Synthesis of the complex [Fe(thoz)2Cl].
Z. Anorg. Allg. Chem. 2014, 640, (2), 385–389
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385

M. Amini et al.
ARTICLE
Figure 1. ORTEP diagram of the complex [Fe(thoz)2Cl] with thermal ellipsoids are drawn at a 50% probability limit.
brown solution during the synthesis indicated the coordination
of ligand.
Table 1. Crystal data and structure refinement for complex
[Fe(thoz)2Cl].
The elemental analyses and spectroscopic data confirm the
assigned composition of the complex [Fe(thoz)2Cl]. The IR
spectra of the complex clearly showed the coordination of the
ligands, as exemplified by the shift of the absorption band as-
signed to C=N bonds from 1624 (free ligand) to 1611 cm–1
(iron complex).[21–23]
Fe(thoz)2Cl
Empirical formula
Formula weight
Temperature /K
Wavelength /Å
Crystal system
Space group
C18H16ClFeN2O2S2
447.75
173(2)
0.71073
triclinic
¯
P1
Also the structure of the complex was determined by X-ray
crystallography and an ORTEP view of the complex with atom
numbering scheme is shown in Figure 1. The summary of sin-
gle-crystal X-ray structure refinement is shown in Table 1. The
five-coordinate iron(III) complex crystallize in the triclinic
Unit cell dimensions
a = 8.2324(12) Å
α = 90.055(2)°
b = 9.6609(14) Å
β = 98.540(2)°
c = 12.2752(17) Å
γ = 107.110(2)°
921.7(2)
¯
crystal system and space group P1. In the crystal structure of
Volume /Å3
[Fe(thoz)2Cl] the weak intermolecular C–H···Cl interactions
connect the molecules throughout the lattice (Figure 2).
The structure shows that the two ligands are coordinated
to iron in the expected bidentate fashion through oxygen and
nitrogen atoms. This complex consists of the distorted square
pyramidal FeIII unit with two nitrogen atoms and two oxygen
atoms of two deprotonated ligand, 2-(2Ј-hydroxyphenyl)-2-
thiazoline, occupying the basal plane [Fe–N distances
2.0935(13) Å and 2.1028(13); Fe–O distances 1.8736(11) and
1.8870(12) Å] and the chloride ion in the apical position with
the significantly longer bond length of 2.2576(5) Å.
Z
2
Density (calculated) /Mg·cm–3
Absorption coefficient /mm–1
F(000)
1.613
1.205
458
Crystal size /mm
Theta range for data collection /° 1.68 to 28.81
Index ranges
0.08ϫ0.26ϫ0.26
–11 Յ h Յ 11
–13 Յ k Յ 13
–16 Յ l Յ 16
9963
4756 [R(int) = 0.0174]
98.6%
multi-scan
0.9098 and 0.7447
Full-matrix least-squares on F2
4756 / 0 / 235
Reflections collected
Independent reflections
Completeness to θ = 28.70°
Absorption correction
Max. and min. transmission
Refinement method
Data / restraints / parameters
Catalytic Activity
Goodness-of-fit on F2
1.041
a)
Final R indices [I Ͼ 2σ(I)]
R1 = 0.0265, wR2 = 0.0676
R1 = 0.0308, wR2 = 0.0699
In order to evaluate the catalytic activities of complex
[Fe(thoz)2Cl] for the oxidation of sulfides, the reactions were
optimized according to the oxidation of methylphenyl sulfide
through the investigation of the influence factors of the oxi-
dation, such as the solvent, the amount of the catalyst and the
amount of the UHP.
a)
R indices (all data)
Largest diff. peak and hole /e·Å–3 0.456 and –0.251
2
2
2
a) Rint = Σ|Fo – Fo (mean)| / Σ[Fo ]. R1 = Σ||Fo| – |Fc|| / Σ|Fo|. GOOF
= S = {Σ[w(Fo – Fc ) ] / (n – p)}1/2. wR2 = {Σ[w(Fo – Fc ) ] /
2
2 2
2
2 2
Σ[w(Fo ) ]}1/2. w = 1 / [σ(Fo ) + (aP)2 + bP] where P is [2Fc
+
2 2
2
2
2
Max(Fo , 0)] / 3.
386
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© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2014, 385–389

Iron Complex Containing an N,O-Type Bidentate Thiazoline Ligand
amount of catalyst is increased to 0.015 mmol, the selectivity
of the methylphenylsulfide oxidation is reduced from 86 to
71% (Table 2, entry 9). The reaction without catalyst is found
to proceed only very slightly. Hence, the amount of catalyst
can enhance the reaction rate for the selective oxidation of
sulfides. The amount of UHP could also significantly affect
the conversion and methylphenylsulfoxide selectivity (Table 2,
entries 10–13). When the amount of UHP was increased from
0.1 to 0.4 mmol, the conversion of methylphenylsulfide was
drastically increased from 26 to 91%. With a further increase
of UHP to 0.5 mmol, the selectivity to methylphenylsulfoxide
decreases from 86 to 65%, where the conversion of methyl-
phenylsulfide was also increased from 91 to 99%. In other
words selectivity to sulfoxide would appear to be better for
reactions with 2 equiv. of the oxidant compared to this with
2.5 equiv. of UHP. Also, when UHP was replaced with H2O2,
the activity towards the oxidation of methylphenylsulfide was
decreased from 91 to 68%.
After optimization, a series of various types of structurally
diverse sulfides was subjected to the oxidation reaction using
the complex [Fe(thoz)2Cl] as catalyst and UHP as oxidant.
Arylalkyl (Table 3, entries 1,2), arylbenzyl (Table 3, entry 3),
dibenzyl (Table 3, entry 4), diaryl (Table 3, entry 5), and di-
alkyl (Table 3, entries 6–8) sulfides underwent clean and selec-
tive oxidation to the corresponding sulfoxide in air in impres-
sive selectivity (81–100%). Significantly, the very good con-
versions of substrates, depending on the nature of the sulfide
in the range of 49–97% (TON = 11.4–18.2), were obtained
for all cases. It was observed that aromatic sulfides undergo
oxidation reactions more easily than aliphatic substrates.
Moreover in the case of dialkyl sulfides (Table 3, entries 6–8),
no over-oxidation to sulfone was observed. The highest and
the lowest conversions were obtained for dibenzyl sulfide
(97%) and dioctyl sulfide (49%), respectively (Table 3, entries
4,8).
Figure 2. Depicting weak intermolecular C–H···Cl interactions bonded
1D polymeric array along a axis for complex [Fe(thoz)2Cl].
For finding the best solvent for the sulfide oxidation, dichlo-
romethane, chloroform, acetonitrile, acetone, methanol and a
1:1 mixture of CH3OH/CH2Cl2 were employed as solvents.
Among the solvents examined, the 1:1 mixture of CH3OH/
CH2Cl2 was found to be the best for this protocol (Table 2).
The effect of amount of catalyst on the conversion and the
selectivity of the methylphenylsulfide oxidation at room tem-
perature for 15 min in CH2Cl2/H2O was also studied. The con-
version of methylphenylsulfide monotonously increases with
In order to show the merit and efficiency of the present cata-
lytic system in comparison with recently reported protocols,
the addition of catalyst of 0 to 0.01 mmol (Table 2). When the we compared the results of the methylphenyl sulfide oxidation
Table 2. The effect of various conditions in the oxidation of methylphenylsulfide by [Fe(thoz)2Cl]/UHP.
Entry
Amount of catalyst /mmol Amount of UHP /mmol
Solvent /1 mL
Conversion /% (TON)a)
Selectivity to
sulfoxide /%
b)
1
2
3
4
5
6
7
8
0.005
0.005
0.005
0.005
0.005
0.005
0
0.01
0. 015
0.01
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.1
0.2
0.3
0.5
0.4
CH2Cl2
CHCl3
CH3CN
CH3OH
5(2)
100
100
100
99
100
97
–
86
71
100
98
11(4.4)
22(8.8)
47(18.8)
35(15)
71(28.4)
trace
91(18.2)
93(12.4)
26(5.2)
43(8.6)
68(13.6)
99(19.8)
68 (13.6)
CH3COCH3
CH2Cl2:CH3OH
CH2Cl2:CH3OH
CH2Cl2:CH3OH
CH2Cl2:CH3OH
CH2Cl2:CH3OH
CH2Cl2:CH3OH
CH2Cl2:CH3OH
CH2Cl2:CH3OH
CH2Cl2:CH3OH
9
10
11
12
13
14
0.01
0.01
0.01
0.01
94
65
96
c)
a) TON = (mmol of sulfoxide + mmol of sulfone)/mmol of catalyst. b) Selectivity to sulfoxide = [sulfoxide% / (sulfoxide% + sulfone%)]ϫ100.
c) H2O2 as oxidant.
Z. Anorg. Allg. Chem. 2014, 385–389
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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387

M. Amini et al.
ARTICLE
a)
Table 3. Oxidation of sulfides catalyzed by [Fe(thoz)2Cl]/UHP
.
in the presence of iron catalysts. As shown in Table 4, our
catalytic system is superior to some of the previously reported
catalysts in terms of reaction conditions, conversion, and selec-
tivity. In contrast to similar, previously reported systems, the
catalytic system presented in this paper does not suffer from
the harsh reaction conditions, such as using large amounts of
hazardous solvents (Table 4, entries 2 and 4), high reaction
temperature (Table 4, entry 3), long reaction time (Table 4, en-
tries 3–5) and using an additive (Table 4, entry 4). As illus-
trated in Table 4, both FeIII complexes [Fe(thoz)2Cl] and
[Fe(N–O)2Cl] (Table 4, entry 6) are almost equally effective
sulfide oxidation catalysts.
Conclusions
The novel five-coordinate FeIII complex [Fe(thoz)2Cl] was
synthesized and characterized by physico-chemical methods.
X-ray diffraction studies revealed the coordination of the bi-
dentate thiazoline ligand 2-(2Ј-hydroxyphenyl)-2-thiazoline to
the iron in a chelate way by oxygen and nitrogen atoms. The
solid state studies further exemplified the distorted square py-
a) The molar ratios for [Fe(thoz)2Cl]:substrate:oxidant are 1:20:40.
The reactions were performed in a 1:1 mixture of CH2Cl2/CH3OH
(1 mL) in air at room temperature within 15 min. b) The GC yields ramidal configuration around the iron atom. In this study we
/% are measured relative to the starting sulphide. c) TON = (mmol of
have demonstrated the effectiveness of this complex as a cata-
sulfoxide + mmol of sulfone)/mmol of catalyst. d) Selectivity to sulf-
lyst for the oxidation of sulfides to their corresponding
oxide = [sulfoxide%/(sulfoxide% + sulfone%)]ϫ100.
Table 4. Recently reported catalytic systems for the oxidation of sulfides by iron catalysts.
a) Selectivity to sulfoxide = [sulfoxide%/(sulfoxide% + sulfone%)]ϫ100.
388 www.zaac.wiley-vch.de
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2014, 385–389

Iron Complex Containing an N,O-Type Bidentate Thiazoline Ligand
sulfoxides. Easy preparation, mild reaction condition, the use
of UHP as green oxidant, high yields of the products, short
Acknowledgements
reaction time, low over-oxidation to sulfones (0–18%), high M.A. and L.K.W. thank the Research Council of the University of
Maragheh and the National Science Foundation, respectively, for fund-
ing of this work.
selectivity, and low cost make [Fe(thoz)2Cl]/UHP as a useful
catalytic system for the oxidation of sulfides.
Experimental Section
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Crystallographic data (excluding structure factors) for the structure in
this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies
of the data can be obtained free of charge on quoting the depository
number CCDC-946247 for [Fe(thoz)2Cl] (Fax: +44-1223-336-033;
E-Mail: deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk).
Received: June 24, 2013
Published Online: October 2, 2013
Z. Anorg. Allg. Chem. 2014, 385–389
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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