Catalytic behavior of an iron(III) complex containing an N,O-type bidentate oxazoline ligand for selective oxidation of sulfides

Article abstract of DOI:10.1007/s11243-015-0001-8

The reaction of Fe(acac)₃ and 2-(2′-hydroxyphenyl)oxazoline (Hphox) as a bidentate O,N donor ligand afforded a six-coordinated iron(III) complex [Fe(phox)₂(acac)] with distorted octahedral configuration. The complex was isolated as an air-stable crystalline solid and characterized by elemental analysis, FTIR, solution electrical conductivity, and by single-crystal X-ray structure analysis. The structure, electronic properties, and vibrational normal modes of the complex were investigated by DFT. The use of this complex as a catalyst for the oxidation of sulfides to their corresponding sulfoxides using urea hydrogen peroxide as the primary oxidant was investigated. The catalyst shows very efficient reactivity, giving high yields and selectivities at room temperature under air.

Full text of DOI:10.1007/s11243-015-0001-8

Transition Met Chem
DOI 10.1007/s11243-015-0001-8
Catalytic behavior of an iron(III) complex containing an N,O-type
bidentate oxazoline ligand for selective oxidation of sulfides
Mojtaba Amini1 Mostafa Khaksar2 Ali Arab3 Reza Masoomi Jahandizi4
•
•
•
•
Mojtaba Bagherzadeh2 Davar M. Boghaei2 Arkady Ellern5 L. Keith Woo5
•
•
•
Received: 12 September 2015 / Accepted: 5 November 2015
Ó Springer International Publishing Switzerland 2015
Abstract The reaction of Fe(acac)3 and 2-(20-hydrox-
yphenyl)oxazoline (Hphox) as a bidentate O,N donor
Introduction
ligand afforded
a
six-coordinated iron(III) complex
Selective oxidation of sulfides to sulfoxides is an essential
step in many biological and industrial processes. The
resulting sulfoxides are essential intermediates for a num-
ber of applications. A number of methods have been
developed for the conversion of sulfides into sulfoxides [1–
4]. Iron compounds are not only low cost and environ-
mentally friendly, but also used by nature in a variety of
metalloenzymes and especially in oxidation reactions.
Thus, in the last two decades, considerable effort has been
invested in the search for synthetic iron catalysts with
various ligands to promote oxidation reactions [5–8].
Compounds containing an oxazoline ring have become
one of the most successful, versatile, and commonly used
classes of ligands for catalysis of various reactions, due to
their accessibility from readily available amino alcohols in
short and high yielding syntheses, as well as their modular
nature and applicability in a wide range of metal-catalyzed
transformations [9–11]. Many complexes of such ligands
show excellent catalytic activity, and there have been many
recent reports on their applications in homogeneous and
heterogeneous catalysis [12–14].
[Fe(phox)2(acac)] with distorted octahedral configuration.
The complex was isolated as an air-stable crystalline solid
and characterized by elemental analysis, FTIR, solution
electrical conductivity, and by single-crystal X-ray struc-
ture analysis. The structure, electronic properties, and
vibrational normal modes of the complex were investigated
by DFT. The use of this complex as a catalyst for the
oxidation of sulfides to their corresponding sulfoxides
using urea hydrogen peroxide as the primary oxidant was
investigated. The catalyst shows very efficient reactivity,
giving high yields and selectivities at room temperature
under air.
& Mojtaba Amini
mamini@maragheh.ac.ir
Density functional theory (DFT) has been widely used
in the prediction of structure, electronic properties, and
spectroscopic properties of metal complexes [15–17].
Electron transfer process in iron complexes with cyanide,
pyrazine, 4,40-bipyridine, and azide ligands has been
studied by the B3LYP hybrid density functional method
[15]. Jaworska et al. [16] have investigated the electronic
structures and UV–Vis spectra of iron complexes with
EDTA and NO ligands using the B3LYP functional.
Conradie et al. [17] have employed DFT calculations using
different functionals in order to study the properties of
isomers of tris(beta-diketonato)3 iron complexes, and their
1
Department of Chemistry, Faculty of Science, University of
Maragheh, P.O. Box. 55181-83111, Maragheh, Iran
2
Chemistry Department, Sharif University of Technology,
P.O. Box 11155-3615, Tehran, Iran
3
Department of Chemistry, Semnan University,
P.O. Box. 35131-19111, Semnan, Iran
4
Department of Biology, Faculty of Science, University of
Maragheh, Maragheh, Iran
5
Chemistry Department, Iowa State University, Ames,
IA 50011-3111, USA
123

Transition Met Chem
O
S
O
S
General procedure for sulfide oxidation
[Fe(phox)2(acac)], UHP
CH2Cl2/ CH3OH, r.t.
+
R
R'
R
R'
R
S
R'
The following standard procedure was used for sulfide oxida-
tion experiments. To a solution of sulfide (0.2 mmol),
chlorobenzene (0.2 mmol) as internal standard and
[Fe(phox)2(acac)] (0.01 mmol) in a 1:1 mixture of CH3OH/
CH2Cl2 (1 mL), was added UHP (0.4 mmol). The mixture was
stirred at room temperature, and the reaction progress was
monitored by GC. Products were identified by comparison with
authentic samples.
O
Scheme 1 Oxidation of sulfides by the [Fe(phox)2(acac)]/UHP
catalytic system
results showed that the B3LYP functional provides the best
description of both the spin state and geometry of such
complexes.
Continuing our studies on the oxidation of sulfides using
five-coordinated iron(III) complexes with ligands based on
oxazoline, oxazine, and thiazoline rings [18–20], herein we
describe the synthesis of a new six-coordinated Fe(III) complex
[Fe(phox)2(acac)] (Hphox = 2-(20-hydroxyphenyl)oxazoline
and acac = acetylacetonate). The complex has been charac-
terized by X-ray crystallography as well as DFT calculations
and examined as a catalyst in the oxidation of sulfides in the
presence of urea hydrogen peroxide (UHP) as an oxidant under
air at room temperature (Scheme 1).
X-ray crystallography
The crystal evaluation and data collectionwere performed on
the Bruker SAINT Software package using a narrow-frame
algorithm. A red block-like crystal of C23H23FeN2O6,
approximate
dimensions
0.090 mm 9 0.170 mm 9
0.700 mm, was used for the X-ray crystallographic analysis.
The frames were integrated with the Bruker SAINT software
package using a narrow-frame algorithm. The integration of
the data using a monoclinic unit cell yielded a total of 23118
˚
reflections to a maximum h angle of 25.25° (0.83 A resolu-
Experimental section
General procedures
tion), of which 3941 were independent (average redundancy
5.866, completeness = 99.5 %, Rint = 9.15 %) and 2560
(64.96 %) were [2r(F2). The final cell constants of
a = 11.809(2) A, b = 14.117(3) A, c = 13.887(3) A,
3
b = 108.789(3)°, and volume = 2191.7(7) A are based
˚
˚
˚
Chemicals and solvents were purchased from Fluka and
Merck. 2-(20-Hydroxyphenyl)oxazoline (Hphox) was syn-
thesized according to a published procedure [21].
Elemental analyses (C, H, N) were obtained with a Carlo
ERBA Model EA 1108 analyzer. FTIR spectra were
obtained with a Unicam Matson 1000 FTIR spectropho-
tometer using KBr disks at room temperature. Molar con-
ductances were determined in methanol (ca. 10-3 M) at
room temperature using a Toa CM 405 conductivity meter.
The products of oxidation reactions were determined and
analyzed with an HP Agilent 6890 gas chromatograph
equipped with a HP-5 capillary column and flame ioniza-
tion detector.
˚
upon therefinement of the XYZ-centroidsof 2674 reflections
above 20 r(I) with 4.647° \ 2h \ 43.13°. Data were cor-
rected for absorption effects using the multi-scan method
(SADABS). The ratio of minimum to maximum apparent
transmission was 0.851. The calculated minimum and
maximum transmission coefficients (based on crystal size)
were 0.6290 and 0.9370. The final anisotropic full-matrix
least-squares refinement on F2 with 291 variables converged
at R1 = 4.55 %, for the observed data and wR2 = 12.05 %
for all data. The goodness-of-fit was 1.007. The largest peak
in the final difference electron density synthesis was
-
3
-
3
˚
0.503 e /A , and the largest hole was -0.437 e /A with an
˚
-
3
˚
RMS deviation of 0.066 e /A . On the basis of the final
model, the calculated density was 1.452 g/cm3 and F(000),
996 e-.
Synthesis of the complex
To a solution of Fe(acac)3 (0.353 g, 1.00 mmol) in ethanol
(15 mL) was added a solution of 2-(20-hydroxyphenyl)
oxazoline and (0.326 g, 2.00 mmol) of the ethanol
(10 mL). After stirring at room temperature for 4 h, the
solution was filtered and evaporated under reduced pres-
sure to give a precipitate. Recrystallization from acetoni-
trile yielded the complex as dark brown crystals. Yield:
0.361 g, 75 %. Anal. Calcd for C23H23FeN2O6: C, 57.6;
H, 4.8; N, 5.8. Found: C, 57.5; H, 4.8; N, 5.9 %. Selected
IR frequency (KBr disk, cm-1): 1611 (mC=N); KM (X-1
cm2 mol-1): 11.5.
Computational methods
The structure of the complex was geometry-optimized
using the B3LYP hybrid density functional and
6-31?G* basis set. A frequency calculation was per-
formed on the optimized structure in order to confirm that
it was a local minimum on the potential energy surface.
All calculations were performed using the Gaussian03
program [22].
123

Transition Met Chem
Results and discussion
Complex characterization
The complex was obtained by the reaction of two equiva-
lents of 2-(20-hydroxyphenyl) oxazoline with one equiva-
lent of Fe(acac)3 (Scheme 2). The fast color change from
cherry red to a dark brown solution during the synthesis
indicated coordination of ligand.
The new iron(III) complex is dark brown, stable in air
and light and soluble in methanol, ethanol, acetonitrile,
methylene chloride, DMF, and DMSO. The elemental
analyses and spectroscopic data for the complex agree very
well with its formulation as [Fe(phox)2(acac)].
In order to study the binding mode of the oxazoline
ligand, the IR spectrum of free Hphox was compared with
the spectrum of the complex. The shift of the band assigned
to C=N from 1624 (free ligand) to 1611 cm-1 (complex)
indicates coordination of the azomethine nitrogen to iron
[23, 24].
Fig. 1 ORTEP diagram of the complex [Fe(phox)2(acac)] with
thermal ellipsoids drawn at a 50 % probability limit
˚
and Fe1–O4 distances of 1.934(2) and 1.951(2) A compared
to the equatorial coordination distances are indications of
tetragonal compression [26].
The molar conductivity of the complex in methanol was
11.5 X-1 cm2 mol-1, indicating a non-electrolytic nature
[25].
DFT results
The structure of the complex has been determined by
X-ray crystallography, and an ORTEP view of the complex
with atom numbering scheme is shown in Fig. 1. A suit-
able single crystal of the complex was obtained by slow
evaporation from MeCN solution. A summary of the X-ray
structure refinement is shown in Table 1, and a crystal
packing diagram of the complex is shown in Fig. 2.
The complex crystallizes in the monoclinic crystal sys-
tem and space group P 1 21/n 1 and has a tetragonally
compressed octahedral FeN2O4 chromophore. The iron
atom is ligated by four donor atoms [N(1), N(2), O(2) and
O(4)] from two chelating phox ligands plus two oxygen
atoms [O(5) and O(6)] of one acac ligand. The complex is
mononuclear, consisting of discrete monomeric units of
[Fe(phox)2(acac)], in which the acetate ligand is in an all-
equatorial position, with bond distances of 2.014(3) and
The geometry-optimized structure of the complex obtained
from DFT calculations has a distorted octahedral configu-
ration, in accordance with the X-ray crystallographic
analysis. Table 2 presents some selected calculated and
experimental bond lengths and bond angles for the com-
plex. The calculated bond lengths are almost all slightly
shorter than the corresponding experimental values. The
error is in the 2–7 % range for Fe–O(N) bond lengths and
\1 % for other bond lengths. Both shorter and longer
calculated bond lengths, compared to experimental values,
have been reported previously [27–29]. The differences
have been attributed to environmental effects as well as the
choice of DFT method [27–29].
The calculated natural bond orbital (NBO) atomic
charge on Fe1 is ?0.717, while those on O1, O2, O3, O4,
O5, O6, N1, and N2 are -0.529, -0.592, -0.529, -0.592,
-0.573, -0.573, -0.443, and -0.443, respectively. The
charge on the iron atom is thus significantly lower than the
formal charge of ?3 indicating charge transfer from the
ligands to the metal center. Figure 3 shows the HOMO-1
(-5.06 eV), HOMO (-5.59 eV), LUMO (-1.38 eV), and
LUMO?1 (-1.28 eV) of the complex. It is clear that while
the HOMO-1 and HOMO orbitals show smaller contribu-
tions on Fe, the LUMO and LUMO?1 orbitals show
greater contributions on and around the Fe atom. The
calculated HOMO–LUMO energy gap is 4.21 eV, indi-
cating good stability and chemical hardness of the
complex.
˚
2.030(3) A for Fe1–O5 and Fe1–O6, respectively, while the
two donor atoms of each oxazoline ligand occupy one
equatorial and one axial position. The rather short Fe1–O2
O
O
N
O
Fe(acac)3
Fe
O
N
2
O
C2H5OH
OH
N
O
O
Scheme 2 Synthesis of [Fe(phox)2(acac)]
123

Transition Met Chem
Table 1 Crystal data and
structure refinement for
complex [Fe(phox)2(acac)]
[Fe(phox)2(acac)]
Empirical formula
Formula weight
Temperature
C23H23FeN2O6
479.28
173 (2) K
˚
0.71073 A
Wavelength
Crystal system
Space group
Monoclinic
P 1 21/n 1
˚
a = 11.809(2) A
Unit cell dimensions
a = 90°
b = 14.117 (3) A
˚
b = 108.789 (3)°
˚
c = 13.887 (3) A
c = 90°
2191.7 (7) A
3
˚
Volume
Z
4
Density (calculated)
Absorption coefficient
F(000)
1.452 g/cm3
0.731 mm-1
996
Crystal size
0.090 9 0.170 9 0.700 mm
1.98 to 25.25°
-14 B h B 14, -16 B k B 16, -16 B l B 16
Theta range for data collection
Index ranges
Reflections collected
Independent reflections
Completeness to theta = 28.70°
Absorption correction
Maximum and minimum transmission
Refinement method
Data/restraints/parameters
Goodness of fit on F2
Final R indices [I [ 2R(I)]
R indices (all data)
Largest difference peak and hole
23118
3941 [R(int) = 0.0915]
99.5 %
Multi-scan
0.9370 and 0.6290
Full-matrix least-squares on F2
3941/0/291
1.007
R1 = 0.0455, wR2 = 0.1018
R1 = 0.0900, wR2 = 0.1205
-3
˚
0.503 and -0.437eA
Rint = R|Fo2-Fo2(mean)|/R[Fo2]
R1 = R||Fo|-|Fc||/R|Fo|
GOOF = S = {R[w(Fo2-Fc2)2]/(n-p)}1/2
wR2 = {R[w(Fo2-Fc2)2]/R[w(Fo2)2]}1/2
w = 1/[r(Fo2) ? (aP)2 ? bP] where P is [2Fc2 ? Max(Fo2, 0)]/3
A frequency calculation on the complex at the B3LYP/
6-31G* level of theory confirmed that the optimized
structure is a minimum on the potential energy surface.
Some of the experimental and corresponding calculated
vibrational frequencies along with the probable assign-
ments are reported in Table 3. It should be added that the
calculated vibrational frequencies have been scaled by a
factor of 0.9613 in order to correct for theoretical errors
such as electron correlation and basis set deficiencies [30].
Good agreement between experimental and calculated
vibrational frequencies is observed. According to the
computational results, vibrations in the region of
3093–3054 cm-1 are related to the aromatic C–H stretch-
ing of benzene groups in agreement with the results
reported in the literature [31]. Vibrations in the region of
3035–2936 cm-1 are related to the C–H stretching modes
of the oxazoline and acac rings. The calculated C=N
stretching frequency of the complex is 1610 cm-1, in very
good agreement with the experimental value of
1611 cm-1. The aromatic C–H in-plane and out-of-plane
bending modes of benzene and its derivatives have been
reported in the region of 1300–1000 and 1000–675 cm-1
,
respectively [32]. Our calculated frequencies for the aro-
matic C–H in-plane bending and C–H out-of-plane bending
123

Transition Met Chem
Fig. 2 Crystal packing diagram of the complex [Fe(phox)2(acac)]
Fig. 3 Calculated HOMO-1, HOMO, LUMO, and LUMO?1 for
the complex
Table 2 Observed (X-ray) and calculated (DFT) geometrical
parameters for the complex
˚
DFT (A) X-ray (A) Angle
˚
Bond
DFT (°) X-ray (°)
Initially, oxidation of MPS was examined without cat-
alyst and with either Fe(acac)3 or [Fe(phox)2(acac)]. As
reported in Table 4, the maximum activity was observed
with [Fe(phox)2(acac)], whereas oxidation of MPS without
catalyst gave almost no conversion. The conversion of
MPS increased with increasing amount of catalyst from 0
to 0.01 mmol. When the amount of catalyst was further
increased to 0.015 mmol, the selectivity for sulfoxide
product was reduced from 96 to 89 %. Next, dichlor-
omethane, chloroform, acetonitrile, acetone, methanol, and
1:1 mixtures of CH2Cl2/CH3OH and CH2Cl2/CH3CN were
investigated as solvents. Among these choices, the 1:1
mixture of CH3OH/CH2Cl2 was found to be the best for
this protocol (Table 4). The amount of UHP also had a
significant effect on both the conversion and methylphenyl
sulfoxide selectivity (Table 4, entries 12–16). When the
amount of UHP was increased from 0.1 to 0.4 mmol, the
conversion of MPS increased drastically from 22 to 73 %.
With a further increase in UHP to 0.5 mmol, the selectivity
for methylphenyl sulfoxide decreased from 96 to 89 %,
while the conversion of MPS increased from 73 to 80 %. In
other words, the selectivity for sulfoxide would appear to
be better for reactions with 2 equivalents of the oxidant
compared to higher amounts.
Fe1–O2 1.897
Fe1–O4 1.897
Fe1–O5 1.945
Fe1–O6 1.945
Fe1–N1 1.962
Fe1–N2 1.962
N1–C1 1.473
N1–C3 1.300
N2–C10 1.473
N2–C12 1.300
O2–C9 1.306
O4–C18 1.306
O5–C19 1.275
O6–C22 1.275
1.934(2)
1.951(2)
2.014(3)
2.030(3)
2.097(3)
2.114(3)
1.473(5)
1.293(4)
1.478(5)
1.289(4)
1.313(4)
1.318(4)
1.277(5)
1.284(5)
O2–Fe1–O4 175.46
171.06(10)
168.92(11)
168.03(11)
85.89(11)
85.90(11)
100.94(11)
88.55(11)
98.82(11)
86.17(10)
85.21(11)
89.31(11)
88.37(10)
99.70(11)
89.70(11)
84.44(11)
N1–Fe1–O6 177.59
N2–Fe1–O5 177.59
O5–Fe1–O6
O6–Fe1–N2
N2–Fe1–N1
N1–Fe1–O5
O2–Fe1–O5
O2–Fe1–O6
O2–Fe1–N1
O2–Fe1–N2
O4–Fe1–O5
O4–Fe1–O6
O4–Fe1–N1
O4–Fe1–N2
93.07
87.14
92.75
87.14
90.34
86.52
91.08
92.06
85.52
90.36
92.07
91.08
modes are 1144 and 745 cm-1, respectively, in good
agreement with the reported values.
Catalytic reactivity
In order to compare the present catalytic system with
recently reported protocols, we compared the results
obtained for MPS oxidations in the presence of other five-
coordinated iron(III) complexes. As shown in Table 5,
[Fe(phox)2(acac)] gave lower oxidation yield but higher
In order to evaluate the catalytic activity of this complex
for the oxidation of sulfides, the reaction conditions were
optimized with respect to the oxidation of methylphenyl
sulfide (MPS) with respect to solvent, quantity of catalyst,
and the amount of UHP.
123

Transition Met Chem
Table 3 Experimental and calculated vibrational frequencies of the complex
Assignment
Experimental
frequency (cm-1
Calculated frequency
)
Unscaled (cm-1
)
Scaled (cm-1
)
a(C–H)
3150 (m)
1611 (s)
3218–3177
3157–3054
1675
3093–3054
3035–2936
1610
benzene rings
a(C–H)phox, acac
a(C=N)
aasymm(C–C)benzene rings
asymm(C–C)benzene rings
aasymm(C–C)acac
1646
1582
1581
1520
1570
1509
d(C–H)phox
asymm(C–C)benzene rings ? b(C–H)benzene rings
d(C–H)acac
1551–1533
1512
1491–1474
1453
a
1508–1502
1490
1450–1444
1432
a
a
a
a
a
asymm(C–C)benzene rings ? b(C–H)benzene rings
asymm(C=O)acac
1457
1401
asymm(C=O)oxazoline
1368 (s)
1439
1383
asymm(C–C)benzene rings ? a(C=O)
? bending of (C–H)phox
? bending of (C–H)phox
1396
1342
benzene rings
asymm(C–C)benzene rings ? a(C=O)
1389
1335
benzene rings
b(C–H)benzene rings
v(C–H)phox
1190
1144
1140
1096
c(C–H)benzene rings
775
745
a
a
a
a
asymm(Fe–O
)
584 (w)
584 (w)
646
621
benzene rings
symm(Fe–O
)
631
606
benzene rings
symm(Fe–Oacac
symm(Fe–N)
)
451
434
399
384
a, stretching; b, in-plane bending; c, out-of-plane bending; d, scissoring; v, rocking; band intensities are classified as strong (s), medium (m) or weak (w)
selectivity than the other complexes. Although the mech-
anism of these oxidation reactions is not yet known, it is
probably related to the availability of a site for coordina-
tion of the substrate or oxo-intermediate in five-coordi-
nated iron(III) complexes. Hence, the six-coordinated
iron(III) complex [Fe(phox)2(acac)] must gain a free site
before the reaction can proceed.
LMCT [phenolate (pp) ? iron(III) (dr*)] [33, 34] of
[Fe(phox)2(acac)] disappeared gradually, and the solution
turned from brown to pale yellow. The marked decrease in
the absorption intensity at 230 nm after 15 min (Fig. 4)
indicates the destruction of the catalyst, resulting in loss of
reactivity. In the presence of greater amounts of the oxidant
the decomposition of the catalyst increased, for example
when a large excess of UHP (1 mmol) was added to the
solution, the absorption band disappeared immediately and
the brown solution became almost colorless.
The present complex [Fe(phox)2(acac)] proved to be
capable of catalyzing the oxidation of a range of struc-
turally diverse sulfides (Table 6). Arylalkyl (Table 6,
entries 1,2), arylbenzyl (Table 6, entry 3), dibenzyl
(Table 6, entry 4), diaryl (Table 6, entry 5) and dialkyl
(Table 6, entries 6–8) sulfides underwent clean and selec-
tive oxidation to the corresponding sulfoxide under air,
with impressive selectivities. Very good conversions of
substrates were obtained for all cases. Aromatic sulfides
were found to undergo oxidation more easily than aliphatic
substrates. In the case of dialkyl sulfides (Table 6, entries
6–8), no over-oxidation to the sulfone was observed.
In general, the reactivity of the catalytic system com-
pletely ceased after 15 min. Upon addition of UHP to a
solution of the iron complex for the oxidation of methyl-
phenyl sulfide, the absorption band at 230 nm assigned to
Conclusions
The iron(III) complex [Fe(phox)2(acac)] has been synthe-
sized and characterized by physicochemical methods and
by X-ray crystallography. The complex proved to be an
effective catalyst for the oxidation of sulfides to their
corresponding sulfoxides. However, the lack of a free site
may explain the lower oxidation yields of the present
complex compared to others reported previously. Never-
theless, easy preparation, mild reaction conditions, high
yields of the products, short reaction times, and high
123

Transition Met Chem
Table 4 The effect of various conditions in the oxidation of methylphenyl sulfide by [Fe(phox)2(acac)]/UHP
Entry
Catalyst
Amount of
catalyst (mmol)
Amount of
UHP (mmol)
Solvent (1 mL)
Conversion
(%) (TON)a
Selectivity to
sulfoxide (%)b
1
–
–
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0
CH2Cl2:CH3OH
CH2Cl2:CH3OH
CH2Cl2:CH3OH
CH2Cl2:CH3OH
CH2Cl2:CH3OH
CH2Cl2
Trace
–
2
Fe(acac)3
0.01
0.01
0.005
0.015
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
54 (10.8)
73 (14.6)
48 (9.6)
81 (16.2)
15 (3.0)
13 (2.6)
51 (10.2)
44 (8.8)
39 (7.8)
55 (11.0)
Trace
98
96
99
89
100
100
95
99
98
94
–
3
Fe(phox)2(acac)
Fe(phox)2(acac)
Fe(phox)2(acac)
Fe(phox)2(acac)
Fe(phox)2(acac)
Fe(phox)2(acac)
Fe(phox)2(acac)
Fe(phox)2(acac)
Fe(phox)2(acac)
Fe(phox)2(acac)
Fe(phox)2(acac)
Fe(phox)2(acac)
Fe(phox)2(acac)
Fe(phox)2(acac)
4
5
6
7
CHCl3
8
CH3CN
9
CH3OH
10
11
12
13
14
15
16
a
CH3COCH3
CH2Cl2:CH3CN
CH2Cl2:CH3OH
CH2Cl2:CH3OH
CH2Cl2:CH3OH
CH2Cl2:CH3OH
CH2Cl2:CH3OH
0.1
0.2
0.3
0.5
22 (4.4)
35 (7.0)
51 (10.2)
80 (16.0)
100
100
97
89
TON = (mmol of sulfoxide ? mmol of sulfone)/mmol of catalyst
b
Selectivity to sulfoxide = (sulfoxide %/(sulfoxide % ? sulfone %)) 9 100
Table 5 Recently reported
catalytic systems for oxidation
of sulfides by Fe(III)
complexesa
Entry
1
Catalyst
Conversion (%)
73
Selectivity (%)
96
References
Present work
O
O
Fe
O
N
O
N
O
O
2
3
4
83
85
91
92
86
86
[18]
[19]
[20]
O
O
S
Cl
Fe
N
O
O
N
O
Cl
Fe
N
O
O
N
O
Cl
Fe
N
O
O
N
S
a
Reaction conditions: 1 mol % of catalyst, 0.4 mmol UHP, 1 mL of CH2Cl2/CH3OH/rt/15 min
123

Transition Met Chem
Table 6 Oxidation of sulfides catalyzed by [Fe(phox)2(acac)]/UHPa
Entry Substrate
Conversion (%)b (TON)c Selectivity (%)d
References
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1
2
3
4
5
6
7
8
a
PhSMe
73 (14.6)
70 (14.0)
74 (14.8)
96
96
PhSEt
PhSCH2Ph
95
PhCH2SCH2Ph 81 (16.2)
91
Ph2S
72 (14.4)
41 (8.2)
35 (7.0)
33 (6.6)
98
Et2S
100
100
100
Bu2S
(C8H17)2S
The molar ratios for [Fe(phox)2(acac)]/substrate/oxidant were
1:20:40. The reactions were performed in a (1:1) mixture of CH2Cl2/
CH3OH (1 mL) under air at room temperature
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The GC yields (%) are reported relative to the starting sulfide
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c
TON = (mmol of sulfoxide ? mmol of sulfone)/mmol of catalyst
d
Selectivity to sulfoxide = (sulfoxide %/(sulfoxide % ? sulfone %)) 9
100
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123