Synthesis, X-ray studies, DFT calculations and catalytic activity of a novel iron complex containing an N, O-type bidentate oxazine ligand

Article abstract of DOI:10.1016/j.poly.2013.05.049

The reactions of FeCl₃·3H₂O and 2-(2′-hydroxyphenyl)-5,6-dihydro-1,3-oxazine (N-O) as a bidentate O, N donor ligand afforded a five-coordinated iron(III) complex, [Fe(N-O) ₂Cl], with a distorted trigonal bipyramidal configuration. The complex [Fe(N-O)₂Cl] was isolated as an air-stable crystalline solid and was fully characterized, including by single-crystal X-ray structure analysis. The complex [Fe(N-O)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 under air. To get an insight into the electronic structure of the complex, the experimental studies have been accompanied by density functional theory (DFT) computations.

Full text of DOI:10.1016/j.poly.2013.05.049

Polyhedron 61 (2013) 94–98
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, X-ray studies, DFT calculations and catalytic activity of a novel
iron complex containing an N,O-type bidentate oxazine ligand
b
b,
c
Mojtaba Amini a, , Mohammad Mehdi Haghdoost , Mojtaba Bagherzadeh , Arkady Ellern ,
⇑
⇑
L. Keith Woo c
a Department of Chemistry, Faculty of Science, University of Maragheh, P.O. Box. 55181-83111, Maragheh, Iran
b Chemistry Department, Sharif University of Technology, P.O. Box 11155-3615, Tehran, Iran
c Chemistry Department, Iowa State University, Ames, IA 50011-3111, USA
a r t i c l e i n f o
a b s t r a c t
The reactions of FeCl3ꢀ3H2O and 2-(20-hydroxyphenyl)-5,6-dihydro-1,3-oxazine (N–O) as a bidentate O,N
donor ligand afforded a five-coordinated iron(III) complex, [Fe(N-O)2Cl], with a distorted trigonal bipyra-
midal configuration. The complex [Fe(N-O)2Cl] was isolated as an air-stable crystalline solid and was fully
characterized, including by single-crystal X-ray structure analysis. The complex [Fe(N-O)2Cl] shows very
efficient reactivity in the oxidation of sulfides to their corresponding sulfoxides using urea hydrogen per-
oxide (UHP) as the oxidant at room temperature under air. To get an insight into the electronic structure
of the complex, the experimental studies have been accompanied by density functional theory (DFT)
computations.
Article history:
Received 31 January 2013
Accepted 19 May 2013
Available online 4 June 2013
Keywords:
Iron(III) complex
Oxazine
Crystal structure
Sulfide oxidation
DFT
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
We recently reported a highly efficient method for the oxidation
of sulfides by using a Fe(III) complex with salicylidene benzoyl
The selective oxidation of sulfides to sulfoxides remains a chal-
lenge and is of interest because of the importance of sulfoxides and
sulfones as synthetic intermediates towards the synthesis of vari-
ous agrochemical and pharmaceutical products, and biologically
active molecules [1–4]. Although a variety of catalytic systems
have been used to achieve this conversion [5–8], most are associ-
ated with drawbacks, such as long reaction time, harsh reaction
conditions, expensive and toxic catalysts, and low selectivity.
Iron complexes have recently attracted considerable interest as
catalysts for the subject of much research in the area of oxygen
atom transfer reactions [9–15]. Due to the non-toxicity and low
cost of iron, the development of new iron-based catalysts for oxi-
dation of sulfides is a subject that has attracted intensive research
recently [16–17].
hydrazine as a ligand [17]. In order to study the effect of the ligand
and complex structure on the catalytic reactivity, herein we de-
scribe the synthesis of a new Fe(III) complex, [Fe(N-O)2Cl] ((N–
O) = 2-(20-hydroxyphenyl)-5,6-dihydro-1,3-oxazine), and the use
of this complex as a catalyst in the oxidation of sulfides in the pres-
ence of urea hydrogen peroxide (UHP) as an oxidant under air at
room temperature (Scheme 1). This is the first report on the cata-
lytic activity of an oxazine–Fe(III) complex in oxidation reactions.
2. Experimental
2.1. General procedure
The use of oxazine and oxazoline type ligands has been largely
developed in recent years for a broad range of catalytic reactions
based on a variety of metals [18–20]. Many complexes with these
ligands show excellent catalytic activity in various reactions and
therefore over the past few years there have been many reports
on their applications in homogeneous and heterogeneous catalysis
[21,22].
Chemicals and solvents were purchased from the Fluka and
Merck Chemical companies. The ligand 2-(20-hydroxyphenyl)-5,6-
dihydro-1,3-oxazine (N–O) was synthesized according to a pub-
lished procedure [22].
The elemental analysis (carbon, hydrogen and nitrogen) was
obtained from a Carlo ERBA Model EA 1108 analyzer. The FT-IR
spectrum was obtained using a Unicam Matson 1000 FT-IR
spectrophotometer with KBr disks at room temperature. The MS
spectrum was recorded on a Finnigan MAT95 spectrometer. The
products of the oxidation reactions were determined and analyzed
by an HP Agilent 6890 gas chromatograph equipped with an HP-5
⇑
Corresponding authors. Tel.: +98 421 2278900; fax: +98 421 2276066.
E-mail addresses: mamini@maragheh.ac.ir (M. Amini), bagherzadeh@sharif.edu
(M. Bagherzadeh).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.05.049

M. Amini et al. / Polyhedron 61 (2013) 94–98
95
Scheme 1. Synthesis and catalytic activity of the [Fe(N-O)2Cl] complex.
Fig. 1. ESI-MS spectrum of [Fe(N-O)2Cl].
capillary column and a flame-ionization detector. The crystal eval-
uation and data collection were performed on a Bruker APEX dif-
fractometer (Mo Ka radiation, k = 0.71073 Å).
functional of Spartan 08 (Wavefunction, Irvine, CA). The calcula-
tions were performed using the basis set 6-31(G⁄) with LACVP⁄
pseudopotentials [23,24]. The electronic structures were also cal-
culated at the same level. The starting atomic coordinates of the
complex were taken from the X-ray structure. No constraint was
applied in the calculations and all atoms were free to optimize.
2.2. Synthesis of the complex [Fe(N-O)2Cl]
To a solution of FeCl3ꢀ6H2O (0.270 g, 1.00 mmol) in 15 mL of
absolute ethanol was added a solution of the ligand, 2-(20-hydroxy-
phenyl)-5,6-dihydro-1,3-oxazine (N–O), (0.355 g, 2.00 mmol) in
10 mL of ethanol. After 2 h stirring at room temperature, the solu-
tion was filtered and evaporated under reduced pressure to give
the complex. Recrystallization from acetonitrile yielded the com-
plex as dark brown crystals. Yield: 0.253 g, 48%. Anal. Calcd for
2.4. General procedure for sulfide oxidation
To
a solution of the sulfide (0.2 mmol), chlorobenzene
(0.2 mmol) as an internal standard and [Fe(N-O)2Cl] (0.01 mmol)
in a 1:1 mixture of CH3OH/CH2Cl2 (1 mL) was added 0.4 mmol
UHP as an oxidant. The mixture was stirred at room temperature
and the reaction progress was monitored by GC. Assignments of
the products were made by a comparison with authentic samples.
C
20H20ClFeN2O4: C, 54.14; H, 4.54; N, 6.31. Found: C, 54.38; H,
4.63; N, 6.49%. Selected IR frequency (KBr disk, cmꢁ1): 1635 (
mC@N).
ESI-MS (C20H20ClFeN2O4) (m/z): 408.2 [MꢁCl]+.
3. Results and discussion
2.3. Computational details
3.1. Complex characterization
The gas phase geometries of the iron complex were optimized
without any symmetry restrictions with spin-unrestricted density
functional theory (DFT) calculations using the hybrid (U)B3LYP
The complex was obtained by the reaction of 2 equivalents of
the ligand (N–O) with FeCl3ꢀ6H2O. The fast color change from a

96
M. Amini et al. / Polyhedron 61 (2013) 94–98
Table 1
Crystal data and structure refinement for the complex [Fe(N-O)2Cl].
[Fe(N–O)2Cl]
C20H20ClFeN2O4
443.68
173(2)
0.71073
Monoclinic
P121/c1
Empirical formula
Formula weight
Temperature, K
Wavelength, Å
Crystal system
Space group
Unit cell dimensions
a (Å)
9.8342(15)
b (Å)
8.2556(13)
c (Å)
24.539(4)
a
(°)
90
b (°)
99.335(2)
c
(°)
90
1965.9(5)
V (Å3)
Z
4
Dcalc (Mg/m3)
Absorption coefficient (mmꢁ1
F(000)
1.499
0.932
916
)
Crystal size (mm)
h (°)
Index ranges
0.11 ꢂ 0.11 ꢂ 0.26
1.68–28.85
ꢁ13 6 h 6 13, ꢁ11 6 k 6 11,
ꢁ33 6 l 6 33
20023
Fig. 2. ORTEP diagram of the complex [Fe(N-O)2Cl] with thermal ellipsoids drawn
Reflections collected
at the 50% probability limit.
Independent reflections (Rint
Completeness to theta = 28.70° (%)
Absorption correction
Maximum and minimum
transmission
Refinement method
)
5122 (0.0602)
99.3
multi-scan
0.9044 and 0.7937
pale yellow to a dark brown solution during the synthesis indi-
cated the coordination of the ligand. The elemental analyses and
spectral data confirm the assigned composition of the complex,
[Fe(N-O)2Cl]. The IR spectrum of the complex clearly showed the
coordination of the ligands, as exemplified by the shift of the
absorption band assigned to the C@N bonds from 1645 (free li-
gand) to 1635 cmꢁ1 (Fe complex) [21,22]. The ESI mass spectrum
of the complex showed a molecular ion peak at m/z 408.2
([MꢁCl]+), which clearly reveals the loss of the chlorine atom
(Fig. 1). The absence of the molecular ion peak [Fe(N-O)2Cl]+ sug-
gests that the coordinated chloro ligand is readily dissociated from
the complex. The fragment ion peak at m/z 232.1 was attributed to
the loss of one phenyl oxazine ligand from the molecular ion peak.
The structure of complex has also been determined by X-ray
crystallography and an ORTEP view of the complex, with the atom-
ic numbering scheme, is shown in Fig. 2. The summary of the single
crystal X-ray structure refinement is shown in Table 1. The com-
plex crystallizes in the monoclinic crystal system with space group
P121/c1. In the crystal structure of [Fe(N-O)2Cl], weak intermolec-
ular C–Hꢀ ꢀ ꢀCl interactions connect the molecules throughout the
lattice (Fig. 3).
The structure shows that the two ligands are coordinated to
iron in the expected bidentate fashion through O and N atoms.
The coordination geometry of iron may thus be described as trigo-
nal bipyramidal, with near C2 symmetry. The oxygen donor and
chloride ions occupy three equatorial sites and the nitrogen donors
occupy the two axial positions. The bond angle between the axial
nitrogen atoms and the central iron atom, N1–Fel–N2, is
169.77(7)° and the angles within the equatorial plane range be-
tween 113.73(5)° and 126.51(7)°. The Fe–Cl bond distance of
2.2581(6) Å is typical of Fe(III)–Cl distances in mononuclear high
spin iron(III) complexes.
full-matrix least-squares on F2
5122/0/253
1.029
R1 = 0.0385, wR2 = 0.0857
R1 = 0.0592, wR2 = 0.0947
0.583 and ꢁ0.460
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
Final R indices [I > 2
r
(I)]
R indices (all data)
Largest difference in peak and hole
(e Åꢁ3
)
P
P
2
Rint
=
=
|Fo ꢁ Fo2(mean)|/ [Fo2].
P
P
R1
||Fo| ꢁ |Fc||/ |Fo|.
P
GOOF = S = { [w(Fo ꢁ Fc2)2]/(n ꢁ p)}1/2
.
2
P
P
wR2 = { [w(Fo ꢁ Fc2)2]/ [w(Fo2)2]}1/2
.
2
w = 1/[
r
(Fo2) + (aP)2 + bP] where P is [2Fc2 + Max(Fo2, 0)]/3.
bond lengths are equal to +0.002 and +0.008 Å. The Fe–N bond
lengths are also sufficiently reproduced (the deviations of the Fe–
N distances are equal to ꢁ0.034 and ꢁ0.042 Å). The highest error
in the bond lengths (0.06 Å) was obtained for the Fe–Cl bond.
The difference in the geometrical parameters in the crystal and
gas phase may be caused by the influence of the crystal packing.
The electron distribution was investigated by means of Natural
Population Analysis (NPA) (Table 3). The calculated charge on the
iron atom in the [Fe(N-O)2Cl] complex is equal to +1.932 and it is
3.2. DFT optimized calculations
(U)B3LYP/DFT optimized calculations were carried out to get an
insight into the electronic structure. The optimized geometric
parameters were compared with the experimental solid state
structure and the agreement was found to be very good, suggesting
that the DFT/B3LYP method and the basis sets used in the
calculations are reasonable (Table 2). The deviations of the Fe–O
Fig. 3. Weak intermolecular C–Hꢀ ꢀ ꢀCl interactions creating a 1D polymeric array
along the a axis for the complex [Fe(N-O)2Cl].

M. Amini et al. / Polyhedron 61 (2013) 94–98
97
Table 2
The experimental and optimized bond lengths (Å) and angles (°).
Selected bonds
Experimental
Optimized
Selected angels
Experimental
Optimized
Fe1–N1
Fe1–N2
Fe1–O2
Fe1–O3
Fe1–Cl1
N1–C4
2.108(2)
2.100(2)
1.872(2)
1.869(2)
2.2581(6)
1.294(3)
1.292(3)
2.142
2.142
1.874
1.877
2.318
1.297
1.297
N1–Fe1–Cl1
N2–Fe1–Cl1
O2–Fe1–Cl1
O3–Fe1–Cl1
N1–F1–O2
N1–Fe1–O3
N2–Fe1–O2
N2–Fe1–O3
N1–Fe–N2
94.28(5)
95.85(5)
113.73(5)
119.73(5)
88.24(7)
86.5(7)
88.99(7)
87.39(7)
169.77(7)
126.51(7)
93.72
91.62
118.36
119.84
86.99
89.12
91.71
87.00
174.47
121.80
N2–C17
O2–Fe1–O3
Table 3
thesis and industrial production. In order to compare the
catalytic reactivity of the complex with some Fe(III) complexes,
we used the same optimization conditions as for our previous
work [17].
Atomic charges from the natural atomic orbital popula-
tion analysis.
Atom
Charge
Fe1
Cl1
N1
N2
O2
O3
C4
1.932
When the oxidation of methylphenylsulfide was performed in
the absence of catalyst, no reaction occurred, whereas the [Fe(N-
O)2Cl]/UHP oxidizing system was able to oxidize methylphenyl
sulfide to the corresponding sulfoxide at room temperature and
in short reaction times (15 min, Table 4, entry 1). A series of var-
ious types of structurally diverse sulfides were subjected to the
oxidation reaction using the [Fe(N-O)2Cl] complex as the catalyst
and UHP as the oxidant. Arylalkyl (Table 4, entries 1, 2), arylben-
zyl (Table 4, entry 3), dibenzyl (Table 4, entry 4), diaryl (Table 4,
entry 5) and dialkyl (Table 4, entries 6–8) sulfides underwent
clean and selective oxidation to the corresponding sulfoxides un-
der air, with impressive selectivity (86–99%). Significantly, very
good conversions of the substrates, depending on the nature of
the sulfide, in the range 63–91% (TON = 11.4–18.2) were obtained
in all cases. It was observed that aromatic sulfides undergo oxida-
tion reactions more easily than aliphatic substrates. The highest
and the lowest conversions were obtained for ethylphenyl sulfide
(91%) and dioctyl sulfide (57%), respectively (Table 4, entries 2
and 8).
ꢁ0.712
ꢁ0.695
ꢁ0.695
ꢁ0.832
ꢁ0.827
0.641
C17
0.641
obviously lower than the formal charge of +3. Moreover, the natu-
ral atomic orbital populations of the dxy, dxz, dyz, dz2 and dx2 Fe
ꢁy2
orbitals are 1.171, 1.214, 1.260, 1.656 and 1.216. These results sug-
gest charge donation from the donor atoms of the ligands to the
metal center. All the nitrogen and oxygen atoms in the complex
bear negative charges, while the carbon atoms and the iron center
bear positive charges. As expected, the coordination of the N atom
to the Fe ion induces a significant positive charge on the carbon
atom of the imino group. Because of these charge redistributions,
the dipole moment of the molecule is 1.88 Debye.
The highest occupied-lowest unoccupied molecular orbital
(HOMO–LUMO) energy separation is useful for estimating the ki-
netic stability of molecules. A large HOMO–LUMO gap can be asso-
ciated with high kinetic stability and low chemical reactivity [25].
The orbital energy level analysis for the [Fe(N-O)2Cl] complex at
the B3LYP level shows that the HOMO–LUMO gap is 4.58 eV, which
In comparison with other iron complexes [13–15], the easy
preparation, mild reaction conditions, the use of UHP as a green
oxidant, high yields of the products, short reaction time, almost
no further oxidation, high selectivity and low cost make
[Fe(N-O)2Cl]/UHP a useful catalytic system for the oxidation of
sulfides.
suggests that this complex is stable. The
of the [Fe(N–O)2Cl] complex are delocalized on
a
-spin LUMO to LUMO+3
p-anti-bonding
orbitals of the phenyl ring and the coordinated N and O atoms of
the oxazine ligands. The d-type orbitals of the metal are found
among the unoccupied b-spin MOs (LUMO to LUMO+4) (Fig. 4).
The HOMO to HOMOꢁ4 molecular orbitals are located mainly on
the oxazine and Cl ligands, with only a minor population localized
on the metal center.
4. Conclusion
The novel five-coordinated Fe(III) complex [Fe(N-O)2Cl] has
been synthesized and characterized by physico-chemical methods.
X-ray diffraction studies revealed the coordination of the 2-(20-
hydroxyphenyl)-5,6-dihydro-1,3-oxazine ligand to the metal
center in a chelating way via N- and O-donor atoms. Solid state
studies further confirm the trigonal bipyramidal geometry around
the Fe atom. In this study we have demonstrated the effectiveness
of this complex as a catalyst for the oxidation of sulfides to their
corresponding sulfoxides. In addition, the electronic structure of
3.3. Catalytic activity
The selective oxidation of sulfides to the corresponding sulf-
oxides is a fundamental transformation, both in laboratory syn-
Fig. 4. Contour plots of some selected MOs (b-spin) of the [Fe(N-O)2Cl] complex.

98
M. Amini et al. / Polyhedron 61 (2013) 94–98
Table 4
Oxidation of sulfides catalyzed by [Fe(N-O)2Cl]/UHP.a
Entry
1
Substrate
Conversion (%)b (TON)c
85 (17)
Selectivity (%)d
86
2
3
91 (18.2)
87 (17.4)
86
88
4
5
84 (16.8)
79 (15.8)
87
90
6
7
8
68 (13.6)
63 (12.6)
57 (11.4)
97
99
99
a
The molar ratios for [Fe(N-O)2Cl]:substrate:oxidant are 1:20:40. The reactions were performed in a 1:1 mixture of CH2Cl2/CH3OH
(1 mL) under air at room temperature within 15 min.
b
The GC yields (%) are measured relative to the starting sulfide.
TON = (mmol of sulfoxide + mmol of sulfone)/mmol of catalyst.
Selectivity to sulfoxide = (sulfoxide%/(sulfoxide% + sulfone%)) ꢂ 100.
c
d
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M. A., M. B. and L. K.W. thank the Research Council of University
of Maragheh, Sharif University of Technology and the National Sci-
ence Foundation for funding of this work.
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Appendix A. Supplementary data
The CIF file of the crystal structure for the complex [Fe(N-O)2Cl]
has been deposited with the CCDC, No. 920792. This data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic Data Cen-
tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336
033; or e-mail: deposit@ccdc.cam.ac.uk.
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