A partially water-soluble cationic Mn(III)-salphen complex for catalytic epoxidation

Article abstract of DOI:10.1139/v11-094

Synthesis and application of new cationic Mn(III)-salphen complexes for catalytic epoxidation of olefins to epoxides in a homogeneous reaction media have been reported. The experimental data showed that the cationic salphen is more active than its neutral form. The acceleration of the reaction rate is attributed to the phase-transfer capability of the built-in phenazinium salt of the Mn(III)-salphen catalyst.

Full text of DOI:10.1139/v11-094

1202
A partially water-soluble cationic Mn(III)–salphen
complex for catalytic epoxidation
Amir Abdolmaleki and Saeed Malek-Ahmadi
Abstract: Synthesis and application of new cationic Mn(III)–salphen complexes for catalytic epoxidation of olefins to epox-
ides in a homogeneous reaction media have been reported. The experimental data showed that the cationic salphen is more
active than its neutral form. The acceleration of the reaction rate is attributed to the phase-transfer capability of the built-in
phenazinium salt of the Mn(III)–salphen catalyst.
Key words: phase transfer, phenazinium salt, cationic Mn(III)–salphen complex, epoxidation.
Résumé : On a effectué la synthèse et examiné le domaine d’application d’un nouveau complexe cationique, le Mn(III)–
salphène, pour la transformation catalytique d’oléfines en époxydes, dans un milieu réactionnel homogène. Les données ex-
périmentales montent que le salphène cationique est plus actif que sa forme neutre. L’augmentation de la vitesse de réaction
est attribuée à la capacité de transfert de charge du sel de phénazinium inclus dans le catalyseur Mn(III)–salphène.
Mots‐clés : transfert de phase, sel de phénazinium, complexe cationique Mn(III)–salphène, époxydation.
[Traduit par la Rédaction]
mentioned catalytic system to stabilize the catalyst by ligation
and to increase the epoxidation reaction rate by drawing the
active oxidant into the organic phase.21,16 Also, the addition
of some phase-transfer catalysts could accelerate the epoxida-
tion reaction in the presence of a Mn(III)–salphen catalyst.22
Moreover, it was reported that the epoxidation activity could
be enhanced in the two-phase reaction medium using Mn
(III)–salphen catalysts with a built-in phase-transfer capabil-
ity, constructed by introducing tertiary amine unit(s) to the
salen ligand.23–25
Recently, Sun and co-workers found that the internal pyri-
dinium salt of the catalyst could play a built-in phase-transfer
role for the acceleration of the epoxidation in an aqueous / or-
ganic biphasic medium as 2 mol% of external pyridinium
salts and ammonium halides, with a tertiary amine unit that
was used.26,27
Introduction
Epoxides are versatile synthetic intermediates that can be
readily converted into a large variety of useful compounds
by means of a regioselective ring-opening reaction.1–3 There-
fore, the design of new catalysts for epoxidation of alkenes
constitutes an important strategy for the synthesis of pharma-
ceuticals and fine chemicals.4–6
In the field of homogeneous catalysis, salen and salophen
ligands4,7–11 have been investigated for many years and are
among the most widely developed ligands in the catalysis
toolbox of modern chemistry for a range of organic transfor-
mations. Many synthetic approaches toward metallosalen
complexes are known that start off with double Schiff base
precursors. These precursors commonly originate from a con-
densation process involving 1 equiv of a diamine and 2 equiv
of salicylaldehyde or a ketone analogue. The resultant metal-
losalen frameworks are thermally and kinetically stable enti-
ties controlled by the presence of a tetradentate ligation
mode of the N2O2 pocket that is characteristic of a salen li-
gand. This type of stabilization allows, for instance, the crea-
tion of vital catalytic intermediates generated by a change in
or accommodation of a higher oxidation state of the metal
ion coordinated by the salen ligand.12–15
In this work, the synthesis of a new phenazinium salphen
and its cationic complex with Mn(III) is reported. The cationic
complex Mn(III)–salphen was applied in the epoxidation of
olefins to epoxides.
Experimental
In this area, Jacobsen and co-workers16 developed a practi-
cal epoxidation procedure using a two-phase system, with an
aqueous phase containing H2O2 and an organic phase com-
posed of a solution of substrates.10,17–20 For practical applica-
tion, there has been a compelling interest in the development
of catalytic epoxidation systems that can increase reaction ac-
tivity in the aqueous H2O2 solution / organic biphasic system.
Usually a pyridine N-oxide derivative is added to the afore-
General remarks
All chemicals were purchased from Merck Chemical Co.
o-Phenylenediamine was sublimated for further purification.
Other solvents and chemicals were purified by common pro-
1
cedures. H NMR (500 MHz) spectra were obtained in deu-
terated dimethylsulfoxide (DMSO-d6) on a Bruker Avance
500 instrument (Bruker, Rheinstetten, Germany), UV–vis ab-
sorption spectra were obtained in DMSO (ca. ×10–5 mol/L)
Received 2 March 2011. Accepted 4 June 2011. Published at www.nrcresearchpress.com/cjc on 7 September 2011.
A. Abdolmaleki and S. Malek-Ahmadi. Department of Chemistry, Isfahan University of Technology, Isfahan-84156/83111, Iran.
Corresponding author: Amir Abdolmaleki (e-mail: abdolmaleki@cc.iut.ac.ir).
Can. J. Chem. 89: 1202–1206 (2011)
doi:10.1139/V11-094
Published by NRC Research Press

Abdolmaleki and Malek-Ahmadi
1203
1
Fig. 1. H NMR spectra of cationic salphen (4).
filtered, washed with ethanol. and was purified by column
chromatography. Yield (%): 82; mp 267 °C. FT-IR (KBr,
cm–1) n: 3421 (w), 2956 (s), 1636 (m), 1616 (m), 1527 (m),
1442 (s), 1398 (m), 1360 (m), 1292 (m), 1251 (m), 1144
(m), 983 (m), 847 (w), 741(m). 1H NMR (500 MHz,
DMSO-d6, ppm) d: 13.84 (s, 2H, HO–Ar), 8.43 (s, 2H,
CH=N), 8.12, 7.91, 7.64, 7.54 (m, 10H, aromatic H), 1.48
(s, 18H, CH3), 1.39 (s,18H, CH3). Anal. calcd for
C42H50N4O2 (%): C 78.47, H 7.84, N 8.72; found: C 78.37,
H 7.73, N 7.80.
Synthesis of Mn(III) salphen (3)
In a 50 mL round-bottom flask equipped with a magnetic
stirring bar, compound 1 (0.21 g, 1 mmol), 3,5-di-tert-butyl-
2-hydroxybenzaldehyde (0.468 g, 2 mmol), Mn(OAc)2·4 H2O
(0.245 g, 1 mmol) and an excess amount of LiCl (0.5 g) in
absolute ethanol were stirred under reflux for 12 h. For oxi-
dation of Mn(II) to Mn(III), air was bubbled into the solution
during the reaction. Then the reaction mixture was cooled to
room temperature and filtered. The brown solid was washed
with ethanol to remove all impurities. Yield (%): 78; mp >
300 °C. FT-IR (KBr n, cm–1): 2962 (m), 2921 (m), 1648
(m), 1610 (m), 1592 (s), 1492 (m), 1445 (m), 1220 (m), 770
(w), 643 (w), 616 (w), 488 (w). UV–vis (DMSO, nm) lmax
263, 346, 447.
:
Synthesis of cationic salphen (4)
In a 25 mL round-bottom flask, 15 mL of methyl iodide
was added to compound 2 (0.032 g, 0.05 mmol) and refluxed
for 7 days. The mixture was cooled to room temperature. The
unreacted methyl iodide was removed under reduced pres-
sure. The brown solid was obtained after the removal of
methyl iodide. Yield (%): 83; mp > 300 °C (decomp.). FT-
IR (KBr, cm–1) n: 3417 (w), 2958 (m), 2923 (m), 2853 (m),
1653 (m), 1624 (s), 1571 (m), 1525 (m), 1457 (m), 1384
(m), 1260 (m), 1199 (w), 1169 (w), 1097 (w), 1024 (m),
on a JASCO-570 UV–vis spectrometer, and IR spectra were
obtained as KBr pellets with a 680 plus-JASCO. Elemental
analysis was also performed by Malek-Ashtar University of
Technology, Tehran, Iran.
Synthesis of 2,3-diaminophenazine (1)
2,3-Diaminophenazine was prepared by using a modified
procedure reported in the literature.10 In a typical experiment,
6 mL of 0.08 mol/L aq FeCl3 solution was rapidly added into
30 mL of a 0.02 mol/L aqueous o-phenylenediamine solution
with vigorous stirring. A quick color change from purple-
black to reddish-brown was observed upon the addition of
FeCl3. After 5 h at ambient temperature, a large amount of
white solid was formed. The precipitate was washed three
times with deionized water, then separated by centrifugation
and sublimated to afford 1 as a white needlelike crystal.
Yield (%): 98; mp > 300 °C. FT-IR (KBr, cm–1) n: 3394 (s),
3313 (s), 3188 (w), 1692 (m), 1633 (m), 1531 (s), 1478 (s),
1383 (m), 1371 (m), 1240 (m), 1149 (m), 891 (w), 834 (m),
1
800 (w), 751 (m). H NMR (500 MHz, DMSO-d6, ppm) d:
13.85 (br, 2H, HO–Ar), 8.43 (s, 2H, CH=N), 8.26, 8.12,
7.93, 7.52 (m, 10H, aromatic H), 5.04 (br, 6H, CH3–N+),
1.49 (s, 18H, CH3), 1.40 (s, 18H, CH3) (Fig. 1). Anal. calcd
for C44H56I2N4O2 (%): C 57.02, H 6.09, N 6.05; found: C
56.99, H 6.11, N 5.97.
Synthesis of cationic Mn(III)–salphen (5)
In a 25 mL round-bottom flask, 15 mL of methyl iodide
was added to compound 3 (0.034 g, 0.02 mmol) and refluxed
for 7 days; after this period of time, the reaction mixture was
cooled to room temperature. After removal of the unreacted
methyl iodide under reduced pressure, a brown solid was ob-
tained. The residue was purified by chromatography on a
silica gel column (MeOH/CH2Cl2, 1:8) to give the desired
complex. Yield (%): 94; mp > 300 °C. FT-IR (KBr, cm–1) n:
2957 (m), 2925 (m), 2850 (m), 1650 (m), 1622 (s), 1570
(m), 1525 (m), 1460 (m), 1384 (m), 1261 (m), 1200 (w),
1171 (w), 1097 (w), 1024 (m), 799 (w), 751 (m). Anal. calcd
for C44H54ClI2MnN4O2 (%): C 52.06, H 5.36, N 5.52; found:
C 51.93, H 5.27, N 5.50.
1
772 (m), 750 (m). H NMR (500 MHz, DMSO-d6, ppm) d:
8.01 (s, 2H, aromatic H), 7.715 (s, 2H, aromatic H), 7.01 (s,
2H, aromatic H), 3.43 (br, 4H, –NH2). EI/MS m/z
(C12H10N4): M+, 210. Anal. calcd for C12H10N4 (%): C
68.56, H 4.79, N 26.65; found: C 68.42, H 4.71, N 26.59.
Synthesis of salphen (2)
Compound (1) (0.21 g, 1 mmol) and 3,5-di-tert-butyl-2-
hydroxybenzaldehyde (0.468 g, 2 mmol) were dissolved in
10 mL of absolute ethanol in a 25 mL flask equipped with a
magnetic stirring bar. The reaction mixture was stirred at
room temperature for 12 h. The dark brown precipitate was
Epoxidation of olefins in the presence of Mn(III)–salphen
(3) and cationic Mn(III)–salphen (5)
An aqueous solution of H2O2 (30%, 0.8 mmol) was added
Published by NRC Research Press

1204
Can. J. Chem. Vol. 89, 2011
Scheme 1. Reagents and conditions for catalyst syntheses.
OH
N
N
NH2
NH2
OHC
t-Bu
+
t-Bu
Diaminophenazine 1
N
N
N
N
N
N
N
O
N
O
Mn
t-Bu
t-Bu
Cl
t-Bu
OH HO
t-Bu t-Bu
Salphen 2
t-Bu
t-Bu
t-Bu
Mn(III)-salphen 3
CH3I/ reflux
CH3I/ reflux
I-
H3C
I-
I-
I-
H3C
N
N CH3
N
N
N CH3
N
O
N
N
Mn
t-Bu
O
t-Bu
Cl
t-Bu
OH HO
t-Bu t-Bu
t-Bu
t-Bu
t-Bu
Cationic Mn(III)-salphen 5
Cationic salphen 4
portionwise to a solution of olefins (0.4 mmol) and Mn(III)–
salphen complex (0.004 mmol) in CH2Cl2 (1 mL). The mix-
ture was stirred at 25 °C and the progress of the reaction was
monitored by gas chromatography. The aqueous and organic
phases were separated and the aqueous layer was extracted
twice with CH2Cl2 (2 × 3 mL). The combined organic layer
was washed with brine (2 × 3 mL) and dried over anhydrous
sodium sulfate.
signed to the out-of-plane bending vibrations of the C–H
bonds in the 1,2,4,5-tetra-substituted benzene ring of the
1
phenazine unit. The results obtained from the H NMR spec-
trum (d 8.01 (m, 2H), d 7.71 (m, 2H), d 7.01 (s, 2H), and d
3.43 (br, 4H, –NH2)) support the structure of compound 1.
The elemental analysis result is consistent with the proposed
chemical formula of 1.
Synthesis of salphen (2)
Salphen (2) was synthesized by using a stoichiometric
ratio of 3,5-di-tert-butyl-2-hydroxybenzaldehyde and 2,3-di-
aminophenazine (2:1) (Scheme 1) and characterized by FT-IR
and H NMR spectroscopies. The H NMR data (CH=N in d
8.43 ppm; d 13.84 ppm for OH; all aromatic hydrogens in d
8.12, 7.91, 7.64, and 7.51 ppm; and two kinds of aliphatic
hydrogen of the tert-butyl groups in d 1.48 and 1.39 ppm)
confirm the formation of compound 2. The FT-IR spectrum
of compound 2 shows vibrational peaks at 2929 and
2959 cm–1 (stretching of the methyl group of tert-butyl), at
Results and discussion
The procedure for catalyst preparation is depicted in the
Scheme 1.
1
1
2,3-Diaminophenazine (1) was synthesized by oxidation of
o-phenylenediamine. In the FT-IR spectrum, the two major
peaks of 2,3-diaminophenazine are observed between 3400
and 3150 cm–1 owing to N–H stretching vibrations of thea-
mino groups, which indicate the presence of NH2 groups in
the structural unit. The bands at 834 and 891 cm–1 are as-
Published by NRC Research Press

Abdolmaleki and Malek-Ahmadi
1205
Table 1. Epoxidation of olefins catalyzed by 1% mol Mn(III)–salphen (3) and cationic Mn(III)–salphen (5).
Mn(III)–salphen (3) Cationic Mn(III)–salphen (5)
Conversion
(%)
Reaction time
(min)
Conversion
(%)
Reaction time
(min)
Catalysts
20
73
45
86
20
75
45
91
20
20
69
67
45
45
84
79
n-C6H13
61
45
77
20
Epoxidation of olefins
Scheme 2. Epoxidation of olefins.
The epoxidation reaction of olefins (Scheme 2) was done
using a 0.01 molar ratio of each of the catalysts, Mn(III)–
salphen (3) and cationic Mn(III)–salphen (5). The conversion
was measured by GC–MS spectroscopy. The results show
that the cationic Mn(III)–salphen (5) had a higher catalytic
activity than the neutral Mn(III)–salphen (3) (Table 1).
The addition of excess CH3I to the 2,3-diaminophenazine
moiety and formation of the cationic Mn(III)–salphen (5) can
considerably facilitate the epoxidation reaction of olefins,
which is presumably because of the formation of the more
water-soluble phenazinium groups in the structure (5). The
neutral Mn(III)–salphen is slightly soluble in water and, con-
sequently, shows a low catalytic activity toward the epoxida-
tion of olefins.9
R2
R2
1% mol salphen (3 or 5)
R3
R3
R1
R1
O
CH2Cl2
H2O2 (30%)
rt
1636 cm–1 (stretching of the C=N bond), and at 3421 cm–1
for stretching of the OH group.
Preparation of Mn(III)–salphen (3)
Complex 3 was obtained according to the procedure shown
in Scheme 1. The complex was synthesized in the presence
of excess LiCl as a counterion. The Mn(II) was converted to
Mn(III) via in situ oxidation and the progress of the oxidation
reaction was monitored by UV–vis spectroscopy. The Mn
(II)–salphen shows an absorption band at l = 420 nm,
whereas a maximum was observed at 447 nm for Mn(III)–
salphen (3).
Conclusions
We explore the synthesis of new types of salphen com-
plexes with built-in inherent phase-transfer capability. The
complexes were synthesized in good yields and applied to-
ward two catalytic processes. The cationic Mn(III)–salphen
complex with two water-soluble phenazinium groups showed
an appropriate catalytic activity for the epoxidation of olefins.
This method could also be useful for the preparation of other
transition-metal catalysts for organic reactions.
Cationic salphen (4)
As shown in Scheme 1, the cationic salphen (4) was pre-
pared by reaction of salphen 2 with excess methyl iodide
1
under reflux. The H NMR spectrum of 4 supports the pro-
+
Acknowledgements
posed structure (Fig. 1). The protons of CH3 –N appear at
5.04 ppm, and this evidence is important because the methyl
iodide protons have no chemical shift higher than 3.5 ppm.
This research was sponsored by the Isfahan University of
Technology (IUT), Ishafan, Iran.
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