Selective oxidation of sulfides and olefins by a manganese (III) complex containing an N,O-type bidentate oxazine ligand

Article abstract of DOI:10.1080/00958972.2012.761339

A new manganese(III) complex [(N-O)₂Mn(OAc)] was synthesized using 2-(2'-hydroxyphenyl)-5,6-dihydro-1,3-oxazine (N-O) as a bidentate O, N donor. The complex has been characterized by elemental analysis, IR, UV-vis spectroscopy, and X-ray structure analysis. Oxidation of sulfides and epoxidation of olefins, respectively, to their corresponding sulfoxides and epoxides were conducted by this catalyst using urea hydrogen peroxide as oxidant at room temperature under air. The catalyst is efficient in oxidation reactions giving high yields and selectivities.

Full text of DOI:10.1080/00958972.2012.761339

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Selective oxidation of sulfides and
olefins by a manganese(III) complex
containing an N,O-type bidentate
oxazine ligand
Mojtaba Amini a , Mojtaba Bagherzadeh b , Zeinab Moradi-Shoeili b
, Davar M. Boghaei b , Arkady Ellern c & L. Keith Woo c
a Faculty of Science, Department of Chemistry, University of
Maragheh, Maragheh, Iran
b Chemistry Department, Sharif University of Technology, Tehran,
Iran
c Chemistry Department, Iowa State University, Ames, IA, USA
Published online: 29 Jan 2013.
To cite this article: Mojtaba Amini , Mojtaba Bagherzadeh , Zeinab Moradi-Shoeili , Davar M.
Boghaei , Arkady Ellern & L. Keith Woo (2013): Selective oxidation of sulfides and olefins by a
manganese(III) complex containing an N,O-type bidentate oxazine ligand, Journal of Coordination
Chemistry, 66:3, 464-472
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Journal of Coordination Chemistry
Vol. 66, No. 3, 10 February 2013, 464–472
Selective oxidation of sulfides and olefins by a manganese
(III) complex containing an N,O-type bidentate oxazine
ligand
MOJTABA AMINI†*, MOJTABA BAGHERZADEH‡*, ZEINAB MORADI-SHOEILI‡,
DAVAR M. BOGHAEI‡, ARKADY ELLERN§ and L. KEITH WOO§
†Faculty of Science, Department of Chemistry, University of Maragheh, Maragheh, Iran
‡Chemistry Department, Sharif University of Technology, Tehran, Iran
§Chemistry Department, Iowa State University, Ames, IA, USA
(Received 15 October 2012; in final form 30 November 2012)
A new manganese(III) complex [(N–O)2Mn(OAc)] was synthesized using 2-(2′-hydroxyphenyl)-
5,6-dihydro-1,3-oxazine (N–O) as a bidentate O, N donor. The complex has been characterized by
elemental analysis, IR, UV–vis spectroscopy, and X-ray structure analysis. Oxidation of sulfides
and epoxidation of olefins, respectively, to their corresponding sulfoxides and epoxides were
conducted by this catalyst using urea hydrogen peroxide as oxidant at room temperature under air.
The catalyst is efficient in oxidation reactions giving high yields and selectivities.
Keywords: Manganese(III) complex; Oxazine; Crystal structure; Olefin epoxidation; Sulfide oxidation
1. Introduction
Transition metal complex catalyzed oxidation of olefins and sulfides is of increasing signifi-
cance in synthetic chemistry offering effective synthesis of important compounds that have
industrial and medical applications [1–4]. Manganese complexes by virtue of their range of
stable and chemically accessible oxidation states have been subject of much research as
oxygen atom transfer reactions [5]; many manganese complexes oxidize hydrocarbons [6, 7].
Oxazoline-based and oxazine-based ligands are easily synthesized and form complexes
with almost all metal ions [8, 9]. Many complexes with these ligands show excellent cata-
lytic activity in various reactions and there have been many reports on applications in
homogeneous and heterogeneous catalysis [10–12]. The presence of N and O donor sites
in oxazolines and oxazines may modify significantly the steric and electronic properties of
the ligand. We reported highly efficient methods for oxidation of olefins and sulfides by
using two manganese(III)–oxazoline complexes [Mn(phox)2(CH3OH)2]ClO4 and Mn
(phox)3 (Hphox = 2-(2′-hydroxylphenyl)oxazoline) [11, 12]. There are no reports of manga-
nese complexes with oxazine-based ligands as catalysts for oxidation.
Herein, we describe the synthesis of a new Mn(III) complex [(N–O)2Mn(OAc)] ((N–O)
=2-(2′-hydroxyphenyl)-5,6-dihydro-1,3-oxazine) and use of this complex as a catalyst in
*Corresponding authors. Email: mamini@maragheh.ac.ir; bagherzadeh@sharif.edu
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online Ó 2013 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2012.761339

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N,O-Type bidentate oxazine
465
epoxidation of olefins and oxidation of sulfides in the presence of urea hydrogen peroxide
(UHP) as oxidant under air at room temperature (scheme 1). This is the first report on the
catalytic activity of an oxazine–Mn(III) complex in oxidation reactions.
[Mn(N-O)2(OAc)]/ImH/UHP
CH3OH/ CH2Cl2 , r.t.
R
R
O
O
S
O
O
S
[Mn(N-O)2(OAc)]/ImH/UHP
CH3OH/ CH2Cl2 , r.t.
R
R'
R
S
R'
R
R'
+
Scheme 1. Epoxidation of olefins and oxidation of sulfides in the presence of [Mn(N–O)2(OAc)].
2. Experiment
Chemicals and solvents were purchased from Fluka and Merck Chemical companies.
2-(2′-Hydroxyphenyl)-5,6-dihydro-1,3-oxazine (N–O) was synthesized according to a
published procedure [8].
Carbon, hydrogen, and nitrogen analyses were obtained from a Carlo ERBA Model EA
1108 analyzer. FT-IR spectrum was obtained by using a Unicam Matson 1000 FT-IR spec-
trophotometer using KBr disks at room temperature. UV–vis spectra were recorded by a
CARY 100 Bio VARIAN UV–vis spectrophotometer in methanol. Products of oxidation
reactions were determined and analyzed by an HP Agilent 6890 gas chromatograph
equipped with a HP-5 capillary column (phenyl methyl siloxane 30 m · 320 lm · 0.25 lm)
and a flame-ionization detector.
2.1. Synthesis of [(N–O)2Mn(OAc)]
To a solution of Mn(OAc)2·4H2O (0.173 g, 1.00 mmol) in 15 mL of methanol was added a
solution of 2-(2′-hydroxyphenyl)-5,6-dihydro-1,3-oxazine (N–O), (0.355 g, 2.00 mmol) in
10 mL of methanol. After 2 h stirring at room temperature, the solution was filtered and
evaporated under reduced pressure to give the complex. Recrystallization from acetonitrile
yielded the complex as brown crystals. Yield: 0.253 g, 48%. Anal. Calcd for
C22H23MnN2O6: C, 56.66; H, 4.97; N, 6.00. Found: C, 56.51; H, 4.83; N, 6.11. Selected
IR frequency (KBr disk, cmÀ1): 1608 (νC=N). Electronic absorption spectrum in MeOH
λmax (nm) (ɛ (MÀ1 cmÀ1)): 298 (1.8 Â 104), 243 (2.2 Â 104).
2.2. General procedure for epoxidation of olefins
To a solution of olefin (0.2 mmol), imidazole(ImH) (0.2 mmol), chlorobenzene (0.2 mmol)
as internal standard, and [(N–O)2Mn(OAc)] (0.01 mmol) in (1 : 1) mixture of CH3OH/
CH2Cl2 (1 mL) was added 0.6 mmol UHP as oxidant. The mixture was stirred at room tem-
perature and the reaction progress monitored by gas chromatography (GC). Assignments
of products were made by comparison with authentic samples.
2.3. General procedure for sulfide oxidation
To a solution of sulfide (0.2 mmol), imidazole(ImH) (0.2 mmol) as axial ligand, chloroben-
zene (0.2 mmol) as internal standard, and [(N–O)2Mn(OAc)] (0.01 mmol) in a 1 : 1 mixture

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466
M. Amini et al.
of CH3OH/CH2Cl2 (1 mL) was added 0.4 mmol UHP as oxidant. The mixture was stirred
at room temperature and the reaction progress monitored by GC. Assignments of products
were made by comparison with authentic samples.
3. Results and discussion
3.1. Complex characterization
The complex was obtained by reaction of 2 equiv. of ligand (N–O) and Mn(OAc)2·4H2O
(scheme 2). The fast color change from a colorless to a green solution during the synthesis
indicated that the manganese ion was oxidized by air in the presence of ligand [10].
CH3
O
O
Mn(OAc) .
4H2O
N
O
2
Mn
O
N
CH3OH
O
OH
N
O
O
Scheme 2. Synthesis of [(N–O)2Mn(OAc)].
Elemental analyses and spectral data confirm the assigned composition of the complex
[(N–O)2Mn(OAc)]. IR spectra of the complex clearly showed coordination of the ligands,
as exemplified by the shift of the C=N absorption [13]. The complex has a similar absorp-
tion pattern in the UV–vis spectra, showing ligand-to-metal charge transfer (LMCT) and
π → π⁄ transition with absorption maxima around 380 and 253 nm [10, 13].
The structure of complex was determined by X-ray crystallography and an ORTEP view
of the complex is shown in figure 1. Single crystals of the complex were obtained with
slow evaporation from Me–CN solution. Single-crystal X-ray structure refinement data are
shown in table 1. The Mn(III) complex crystallizes in the monoclinic crystal system, space
group P21/c, and consists of a Jahn–Teller distorted octahedral MnN2O4 chromophore. The
manganese is ligated by four donors [N(1), N(2), O(3) and O(4)] from two chelating
ligands (N–O) and two oxygens [O(1) and O(2)] of one acetate. The structure of the com-
plex is mononuclear, consisting of discrete monomeric units of [(N–O)2Mn(OAc)], in
which acetate is equatorial, with bond distances of 2.2834(16) and 2.0939(14) Å, respec-
tively, for the Mn1–O1 and Mn1–O2 bonds, while two donors of each oxazine occupy one
equatorial and one axial position. The rather short Mn1–O3 and Mn1–O4 distances of
1.864(11) and 1.868(11) Å compared to the equatorial coordination distances indicate com-
pressed Jahn–Teller effect. Axial compressed distortion in this complex is very unusual,
since most other Mn(III)-containing oxazoline moieties exhibit axial elongation [10, 13].
3.2. Catalytic reactivity
To compare catalytic reactivity of [(N–O)2Mn(OAc)] with [Mn(phox)2(CH3OH)2]ClO4 and
Mn(phox)3, we used the same optimization conditions from our previous work [11, 12].