Synthesis and application of new Schiff base Mn(III) complexes containing crown ether rings as catalysts for oxidation of cyclohexene and cyclooctene by Oxone

Article abstract of DOI:10.1080/10610278.2011.563854

Six catalysts MnClL¹-MnClL⁶, containing two crown ether rings, were synthesised and characterised by IR spectroscopy and CHN microanalysis. A combination of Oxone, as oxidant, and these catalysts was used for the oxidation of cyclohe

Full text of DOI:10.1080/10610278.2011.563854

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Synthesis and application of new Schiff base Mn(III)
complexes containing crown ether rings as catalysts for
oxidation of cyclohexene and cyclooctene by Oxone
Seyed Mohammad Seyedi a , Gholam Hossein Zohuri a & Reza Sandaroos a
a Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, Mashhad,
Iran
Version of record first published: 24 May 2011
To cite this article: Seyed Mohammad Seyedi, Gholam Hossein Zohuri & Reza Sandaroos (2011): Synthesis and application of
new Schiff base Mn(III) complexes containing crown ether rings as catalysts for oxidation of cyclohexene and cyclooctene by
Oxone, Supramolecular Chemistry, 23:7, 509-517
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Supramolecular Chemistry
Vol. 23, No. 7, July 2011, 509–517
Synthesis and application of new Schiff base Mn(III) complexes containing crown ether rings as
catalysts for oxidation of cyclohexene and cyclooctene by Oxone
Seyed Mohammad Seyedi*, Gholam Hossein Zohuri and Reza Sandaroos
*
Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran
(Received 3 October 2010; final version received 27 December 2010)
Six catalysts MnClL1–MnClL6, containing two crown ether rings, were synthesised and characterised by IR spectroscopy
and CHN microanalysis. A combination of Oxone, as oxidant, and these catalysts was used for the oxidation of cyclohexene
and cyclooctene. Among the prepared catalysts, MnClL3 and MnClL4 exhibited the best catalytic efficiency. Catalysts
MnClL1, MnClL2 and MnClL6 showed a moderate efficiency and MnClL1 showed the lowest efficiency. Comparison of
MmclL1–MnClL4 and MnClL6 containing crown ether rings with an identical mixture of uncrowned complex MnClL7
[manganese N,N0-bis(salicylidene)ethylenediamine chloride] and crown ether 2 (40-hydroxybenzo-15-crown-5), revealed a
more important role for the crown ether than increasing solubility of Oxone in the organic phase. The effect on reaction
times and chemical yields of temperature, pyridine as the axial base, and different alkali metal salts was also investigated.
Keywords: crown ether; Schiff base; Oxone
Introduction
inexpensive. Oxone has been widely studied as a routine
reagent for epoxidation reactions, oxidation of aldehydes to
carboxylic acids, and in many other oxidation processes
(24). Oxone is an efficient oxidising agent that has low
solubility in organic solvents, but is far more soluble in
aqueous media. The use of Oxone in an aqueous medium
for the epoxidation of alkenes leads to extensive hydrolysis
of the reaction products. Mixtures of water–methanol
(or ethanol) or water–ethanol–acetic acid overcome the
low solubility of Oxone in organic solvents, but ethanol is
also oxidised, and a large excess of Oxone is needed to
obtain good yields of epoxides (25–37).
Oxidation of alkenes to epoxides is very important in
organic synthesis and has been of immense interest in the
area of transition metal complex-mediated reactions over
the past decades (1–9). In this regard, metalloporphyrins
have been used extensively owing to their direct
relationship to enzymatic oxidation with cytochrome P-
450 (10–12). Parallel to the porphyrin chemistry, the same
catalytic reactions are mimicked by various transition
metal complexes, in particular, Schiff base complexes.
Such complexes are of interest because of their cheap and
easy synthesis and their chemical and thermal stability
(13–19). Application of Schiff base complexes containing
crown ether rings in aerobic oxidation of various organic
compounds is a topic of much current interest (20–22).
Complexation of a hard cation in the crown cavity of these
catalysts, close to the transition-metal centre, perturbs
oxygen-binding properties of the metal centre which results
in improved catalytic behaviour of some catalysts in
comparison with their uncrowned analogues (23).
Oxidation of alkenes by Oxone, in a biphasic mixture of
water, a ketone (usually acetone), dichloromethane and a
phase-transfer catalyst, has been reported several times
(29–33). The active oxidant in this system is a dioxirane
generated by the reaction of Oxone with the ketone.
Without the ketone no epoxidation occurs. In these kinds of
systems, control of pH was necessary to avoid Baeyer–
Villiger oxidation of the ketones. The maximum yield of
cyclohexene oxide (calculated based on initial cyclohex-
ene) was 78%, which was obtained at pH 7.5 and after 3 h
reaction time (29). Ford and co-workers obtained
cyclooctene oxide in 97% yield using Oxone as oxidant
and without an added catalyst after 5 h in an unbuffered
aqueous medium; control of pH was needed to obtain high
yields of more aqueous soluble epoxides such as
cyclohexene oxide and styrene oxide (38). The maximum
yield of cyclohexene oxide (based on initial cyclohexene)
Catalytic oxidation of alkenes has been carried out
using a variety of oxidants such as PhIO, NaOCl, H2O2,
alkyl hydroperoxides, percarboxylic acids, magnesium
monoperoxyphthalate, molecular oxygen and potassium
peroxymonosulphate (KHSO5). Potassium peroxymono-
sulphate (KHSO5), which is commercially named Oxone,
is a stable oxidising agent, which is easy to handle, is non-
toxic, generates non-polluting byproducts, and is relatively
*Corresponding authors. Emails: smseyedi@yahoo.com; r_sandaroos@yahoo.com
ISSN 1061-0278 print/ISSN 1029-0478 online
q 2011 Taylor & Francis
DOI: 10.1080/10610278.2011.563854
http://www.informaworld.com

510
S.M. Seyedi et al.
was 95% obtained at pH . 7.5 after 5 h reaction time.
Epoxidations of alkenes with Oxone catalysed by
manganese porphyrins (34, 35) and platinum complexes
(36) have also been reported, but are impractical since these
organometallic complexes decompose rapidly during the
epoxidation. Compared with manganese porphyrins,
manganese hemiporphyrazines showed better stability in
epoxidation reactions (39). The highest yield of cyclohex-
ene oxide reported using homogenous manganese hemi-
porphyrazines was 51%. It is believed that oxo manganese
(V) generated by the reaction of Oxone and hemiporphyr-
azines-manganese (III) is the active oxidant for the
oxidation of alkene.
rings, for epoxidation reaction in the presence of Oxone as
oxidant. We investigated the effect of the crown ether
rings, the structure of the ligands and the reaction
conditions on the efficiency of the prepared catalysts. Our
epoxidation system showed better performance compared
with other methods (38, 39).
Experimental
All chemicals and solvents were supplied by Merk
Chemical Co. (Darmstadt, Germany) and Fluka Co. and
were used as received. 1H NMR and IR spectra were
recorded on a Bruker BRX-100 AVANCE and a Shimadzu
spectrometer, respectively. 40-Aminobenzo-15-crown-5
(1) and MnClL6 named as manganese N,N0-bis(salicylide-
In this contribution, we report the synthesis, character-
isation and application of new Schiff base catalysts
MnClL1–MnClL6 (Figure 1), including two crown ether
Figure 1. Synthesis of complexes MnClL1–MnClL6 and ligands H2L1–H2L6.

Supramolecular Chemistry
511
ne)ethylenediamine chloride (EUK-8) were prepared
according to the literature (40, 41).
filtration and washed with cold ethanol. The crude product
was recrystallised from ethanol to give a pure sample.
DFT calculations were done with Gaussian 03 program
and the B3LYP/lan2Dz level of theory was used to
optimise the geometry of molecule (42).
H2L1
1H NMR (CDCl3, 100 MHz) d: 3.76 (m, 20H, CH2ZO,
NvCH2), 3.95 (m, 8H, CH2ZO), 4.17 (m, 8H, CH2ZO),
6.8–7.01 (m, 4H, ArH), 8.24 (s, 2H, CHvN), 13.10 (s,
2H, OH); IR (KBr, film) nmax: 3228, 2931, 2868, 1620,
1604, 1500, 1260, 1121, 1054, 932 cm21; Anal. Calcd for
C32H44N2O12: C, 59.25; H, 6.84; N, 4.32%. Found: C,
59.16; H, 6.81; N, 4.29%.
Synthesis of 40-hydroxybenzo-15-crown-5 (2)
To a warm solution of concentrated sulphuric acid (5 ml)
in water (40 ml), 40-aminobenzo-15-crown-5 (40.0 mmol,
11.4 g) was added slowly with vigorous stirring. The
mixture was cooled and 20 g of ice was added. A solution
of potassium nitrite (31.7 mmol, 3.2 g) in water (20 ml)
was added in small quantities, with stirring, to the initial
solution, until all the amine groups were diazotised, which
was determined by testing the solution for nitrous acid.
The mixture was heated at 408–508C on a water bath for
2 h. The mixture was saturated with potassium chloride
and extracted with ether (3 £ 100 ml). The solvent of the
combined extracts was removed by reduced pressure and
40-hydroxybenzo-15-crown-5 was collected in 68% yield
H2L2
1H NMR (CDCl3, 100 MHz) d: 1.60–1.68 (m, 2H,
CCH2C), 3.74 (m, 20H, CH2ZO, NvCH2), 3.92 (m, 8H,
CH2ZO), 4.19 (m, 8H, CH2ZO), 6.78–7.08 (m, 4H,
ArH), 8.30 (s, 2H, CHvN), 12.58 (s, 2H, OH); IR (KBr,
film) nmax: 3230, 2921, 2866, 1622, 1608, 1505, 1261,
1130, 1055, 930 cm21; Anal. Calcd for C33H46N2O12: C,
59.81; H, 7.00; N, 4.23%. Found: C, 59.70; H, 6.97; N,
4.20%.
1
(27 mmol, 7.8 g). H NMR (CDCl3, 100 MHz): d 3.77 (s,
8H, CH2ZO), 3.95 (m, 4H, CH2ZO), 4.19 (m, 4H,
CH2ZO), 6.61–7.02 (m, 3H, ArH), 12.28 (s, 1H, OH).
Anal. Calcd for C14H20O6: C, 59.14; H, 7.09%. Found: C,
59.01; H, 7.00%.
H2L3
1H NMR (CDCl3, 100 MHz) d: 1.62–1.90 (m, 4H,
CCH2C), 2.17–2.32 (t, 7H, NCH2, NCH3), 3.77 (m, 20H,
CH2ZO, NvCH2), 3.97 (m, 8H, CH2ZO), 4.15 (m, 8H,
CH2ZO), 6.82–7.08 (m, 4H, ArH), 8.29 (s, 2H, CHvN),
13.21 (s, 2H, OH); IR (KBr, film) nmax: 3224, 2931, 2861,
1618, 1601, 1500, 1261, 1129, 1048, 926 cm21; Anal.
Calcd for C37H55N3O12: C, 59.56; H, 7.55; N, 5.37%.
found: C, 59.51; H, 7.53; N, 5.34%.
Synthesis of 40-formyl-50-hydroxybenzo-15-crown-5 (3)
To a stirred ethylmagnesium bromide (3.0 M in Et2O,
27.0 mmol, 9 ml), 25.0 mmol of 40-hydroxybenzo-
15-crown-5 (2) (7.72 g) in 30 ml of THF, was added
dropwise over 15 min at 08C. The mixture was stirred for
2 h at room temperature. Dried toluene (40 ml) and a
mixture of triethylamine (35.0 mmol, 3.5 g) and paraf-
ormaldehyde (purity 94%, 60.0 mmol, 1.92 g) were
added and the mixture was stirred for further 2 h at
808C. After this time, the reaction mixture was cooled to
08C and then acidified to pH 3 by HCl (2 N). The
organic phase was separated, dried over MgSO4, and the
solvent was removed. The yield of the reaction was
about 75% (18.75 mmol, 5.89 g). 1H NMR (CDCl3,
100 MHz): d 3.74 (s, 8H, CH2ZO), 3.92 (m, 4H,
CH2ZO), 4.19 (m, 4H, CH2ZO), 6.82 (s, 1H, Ar), 7.32
(s, 1H, Ar), 10.0 (s, 1H, CHO), 12.8 (s, 1H, OH). Anal.
Calcd for C15H20O7: C, 57.69; H, 6.45%. Found: C,
57.64; H, 6.39%.
H2L4
1H NMR (CDCl3, 100 MHz) d: 3.74 (s, 16H, CH2ZO),
3.97 (m, 8H, CH2ZO), 4.15 (m, 8H, CH2ZO), 6.54–7.23
(m, 7H, ArH, PyH), 8.34 (s, 2H, CHvN), 13.28 (s, 2H,
OH); IR (KBr, film) nmax: 3223, 2930, 2867, 1621, 1602,
1505, 1251, 1124, 1051, 932 cm21; Anal. Calcd for
C35H43N3O12: C, 60.25; H, 6.21; N, 6.02%. Found: C,
60.18; H, 6.17; N, 6.00%.
H2L5
General procedure for ligands H2L1–H2L6
1H NMR (CDCl3, 100 MHz) d: 1.16–1.78 (m, 8H, CyH),
3.74 (m, 18H, CH2ZO, NvCH2), 3.92 (m, 8H, CH2ZO),
4.16 (m, 8H, CH2ZO), 6.9–7.06 (m, 4H, ArH), 8.29 (s,
Compound 3 (0.01 mol) was dissolved in ethanol (20 ml)
and mixed with the diamine (0.005 mol) and a trace
amount of formic acid. The mixture was stirred at 508C
under N2 for 6 h. The precipitate was collected by suction
2H, CHvN), 12.91 (s, 2H, OH); IR (KBr, film) nmax
3230, 2924, 2861, 1623, 1600, 1504, 1259, 1131, 1048,
:

512
S.M. Seyedi et al.
924 cm21; Anal. Calcd for C36H50N2O12: C, 61.52; H,
7.17; N, 3.99%. Found: C, 61.49; H, 7.14; N, 3.92%.
53.48; H, 5.26; N, 5.35%; found: C, 53.41; H, 5.21; N,
5.32%. Lm (S cm2 mol21): 116.71, Molar magnetic
susceptibility xM ¼ 1.07 £ 1021 J mol21 T22, magnetic
moment mm ¼ 4.81 £ 10223 J T21
.
H2L6
1H NMR (CDCl3, 100 MHz) d: 3.79 (s, 16H, CH2ZO),
3.92 (m, 8H, CH2ZO), 4.11 (m, 8H, CH2ZO), 6.69–7.41
(m, 8H, ArH), 8.33 (s, 2H, CHvN), 13.07 (s, 2H, OH); IR
(KBr, film) nmax: 3227, 2925, 2867, 1624, 1601, 1505,
MnClL5
IR (KBr, film) nmax: 2926, 2860, 1611, 1601, 1505, 1261,
1131, 1050, 922; Anal. Calc. for C36H48ClMnN2O12: C,
54.65; H, 6.12; N, 3.54%; found: C, 54.59; H, 6.09; N,
3.51%. Lm (S cm2 mol21): 115.98, Molar magnetic
susceptibility xM ¼ 0.98 £ 1021 J mol21 T22, magnetic
1252, 1122, 1050, 934 cm21
; Anal. Calcd for
C36H44N2O12: C, 62.06; H, 6.37; N, 4.02%. Found: C,
61.59; H, 6.34; N, 4.00%.
moment mm ¼ 4.70 £ 10223 J T21
.
General procedure for complexes MnClL1–MnClL6
MnClL6
A solution of ligand (1.0 mmol) and MnCl2·4H2O
(1.1 mmol) in EtOH (15 ml) was stirred for 2 h under a
N2 atmosphere at 708C, and the mixture was then cooled,
filtered and washed with EtOH to give the complexes. The
pure product was obtained after recrystallisation from
EtOH.
IR (KBr, film) nmax: 2927, 2865, 1613, 1602, 1507, 1250,
1122, 1052, 936 cm21; Anal. Calcd for C36H42ClMnN2-
O12: C, 55.07; H, 5.39; N, 3.57%. Found: C, 54.87; H,
5.34; N, 3.53%.
Catalyst characterisation
MnClL1
Catalysts MnClL1–MnClL6 have nearly similar IR spectra
compared with the IR spectra of the free ligands except for
the CvN stretching vibration which shifted slightly (11–
14 cm21) to lower frequency and increased in intensity
relative to the free ligand. Additionally, the characteristic
vibration of OH at <3225 cm21 disappeared but the
CZOZC stretching vibration remained without change.
These observations demonstrate that the manganese only
interacts with OH and CvN (43).
IR (KBr, film) nmax: 2930, 2865,1607,1602,1498, 1262,
1120, 1055, 932; Anal. Calcd for C32H42ClMnN2O12: C,
52.14; H, 5.74; N, 3.80%; found: C, 52.03; H, 5.69; N,
3.76%. Lm (S cm2 mol21): 117.12, Molar magnetic
susceptibility xM ¼ 1.08 £ 1021 J mol21 T22, magnetic
moment mm ¼ 4.80 £ 10223 J T21
.
MnClL2
The observed molar conductance of all complexes in
the DMF solution (1.0 £ 1023 mol l21) at 258C also
showed that they were electrolytes (44). The molar
Magnetic Susceptibility xM and magnetic moment mm of
all complexes indicated that manganese has four non-
paired electrons. Combining this phenomenon with the
results of the molar conductances and magnetic moments of
the complexes indicates that manganese in the complexes is
trivalent. The elemental analysis of the complexes
indicated that H2L1–H2L6 formed 1:1 (ligand/metal)
complexes.
IR (KBr, film) nmax: 2923, 2864, 1611, 1609, 1506, 1261,
1132, 1054, 931; Anal. Calcd for C33H44ClMnN2O12: C,
52.77; H, 5.90; N, 3.73%; found: C, 52.64; H, 5.87; N,
3.69%. Lm (S cm2 mol21): 115.56, Molar magnetic
susceptibility xM ¼ 1.06 £ 1021 J mol21 T22, magnetic
moment mm ¼ 4.86 £ 10223 J T21
.
MnClL3
IR (KBr, film) nmax: 2929, 2863, 1604, 1600, 1500, 1261,
1128, 1049, 928; Anal. Calcd for C37H53ClMnN3O12: C,
54.05; H, 6.50; N, 5.11%; found: C, 54.00; H, 6.49; N,
5.08%. Lm (S cm2 mol21): 117.02, Molar magnetic
susceptibility xM ¼ 1.10 £ 1021 J mol21 T22, magnetic
Procedure of epoxidation
In a typical experiment of two-phase epoxidation, a
required volume of aqueous solution of potassium
peroxomonosulphate (1.5 M) was added to a stirred
mixture of chloroform (5 ml), the required amount of
Schiff base complex and the alkene at room temperature.
The progress of reactions was monitored by TLC. At the
end of the reaction, the chloroform layer was separated and
the aqueous phase was extracted with chloroform
moment mm ¼ 4.73 £ 10223 J T21
.
MnClL4
IR (KBr, film) nmax: 2920, 2867, 1608, 1604, 1506, 1253,
1126, 1051, 930; Anal. Calcd for C35H41ClMnN3O12: C,

Supramolecular Chemistry
513
Figure 2. (a) Structure for MnClL1 obtained by DFT calculation. (b) Proposed structure for co-complexation of MnClL1 and KHSO5.
Some H and Cl atoms were omitted for clarity.
(3 £ 10 ml). The combined chloroform extracts were dried
(MgSO4), and after the removal of the solvent in vacuo,
the residue was purified by column chromatography (silica
gel, n-hexane-Et2O, 4:1).
The non-catalytic system (Table 1, run 1) afforded
only a trace amount of epoxide, which must be due to the
low solubility of KHSO5 in the organic phase. On the other
hand, catalytic systems containing MnClL1–MnClL4 or
MnClL6 decreased the reaction times significantly and
enhanced chemical yields (Table 1, runs 2–5 and 7).
Compared with MnClL1–MnClL4 or MnClL6, the mixture
of crown ether 2 and uncrowned complex MnClL7 did
not exhibit desirable efficiency (Table 1, run 8). This
observation is consistent with our hypothesis that
incorporation of crown ether and Schiff base complex is
Results and discussion
We initially synthesised MnClL1–MnClL6 because we
anticipated that the presence of crown rings bonded to
these complexes would not only increase the solubility of
the oxidant (KHSO5) in the organic phase, but also may
facilitate bringing the KHSO5 into the vicinity of the
manganese, leading to faster oxidation of manganese (III)
to oxo manganese (V) which is a better oxidant than
KHSO5 (Figure 2b). DFT studies for MnClL1 gave the
structure illustrated in Figure 2a. We envisioned that
MnClL1–MnClL6 would bind KHSO5 as sketched in
Figure 2b (geometry not DFT optimised).
Table 1. Oxidation of cyclohexene (1.0 mmol) by KHSO5
(2.0 ml, 1.5 M, 3.0 mmol), in the presence of MnClL1–MnClL6
(6.0 mmol) or an identical mixture of MnClL7 (6.0 mmol) and
crown (2) (6.0 mmol) in CHCl3 (5 ml) at room temperature and
normal pressure.
Accordingly, complexes MnClL1–MnClL6 as cata-
lysts and Oxone as oxidant were employed for the
oxidation of cyclohexene and cyclooctene in a biphasic
medium including water and CHCl3 at normal pressure
and room temperature. In order to elucidate the importance
of complex-crown incorporation in the efficiency of the
oxidation system, an identical mixture of uncrowned
complex MnClL7 and crown 2 (40-hydroxybenzo-15-
crown-5) was also used for the oxidation of cyclohexene
(Table 1).
Run
Catalyst
Yield (%)a
Time (h)
1
2
3
4
5
6
7
8
–
Trace
68
70
80
76
24
73
31
120
27.0
30.0
3.5
6.2
47.0
18
MnClL1
MnClL2
MnClL3
MnClL4
MnClL5
MnClL6
MnClL7 þ crown (2)
40.0
a Cyclohexene oxide yields are calculated on the initial amount of cyclohexene.

514
S.M. Seyedi et al.
a crucial requirement for MnClL1–MnClL4 and MnClL6
to exhibit high catalytic activities. As explained, crown
ether situates the oxidant adjacent to the metal through
trapping the potassium ion of the oxidant in its cavity and
so facilitates the oxidation of Mn (III) to MvO (V)
(Figure 2). The general catalytic efficiency order of the
catalysts is: MnClL3 . MnClL4 .. MnClL6 . MnClL1
. MnClL2 .. MnClL7 and crown ether 2 . MnClL5.
This trend might be due to the electron density of central
metal ion being enhanced by the electron-donating effect
of the ligands which facilitates the oxidation of Mn (III)
to MnvO (V). Accordingly, MnClL3 and MnClL4
including an additional electron donating N atom on
their ligands showed the highest catalytic performance.
Additionally, steric congestion around the coordinated
metal in MnClL5, originating from chair conformation of
cyclohexyl ring, prevents either the substrate or the
oxidant from approaching the central metal of MnClL5.
Therefore, the reaction catalysed by MnClL5 exhibited the
lowest chemical yield and the longest reaction time (Table
1, run 6). Furthermore, the N > N bridge group in MnClL6
is phenyl, which contributes to the formation of the p-
extended coordination structures. This contribution might
facilitate the oxidation and so afford better catalytic
performance of MnClL6 compared to MnClL1 and
MnClL2 (Table 1, run 7).
from the complexation of H2L3 with manganese ion
showed the highest activity. To find a reasonable
relationship between the catalytic performances and
ligand structures, the density of positive charge on the
central metal of catalysts MnClL1–MnClL6 was measured
using DFT studies (Table 2). This study revealed that
MnClL3 and MnClL4 possessed the lowest positive charge
on their central metal, which made them more susceptible
to be oxidised compared to their analogues.
It is worthy of mention that a colour change from brown
to orange occurred upon adding oxidant to the reaction
mixture when there was not any alkene, which might be
due to a variation of the oxidation level of manganese.
It is known that the addition of an axial base such as
pyridine is necessary to promote the catalytic activity of
manganese complexes since it increases electron donation
of the ligands in the complex and subsequently improves
the susceptibility of manganese (III) to be oxidised to
manganese (V) (39). Accordingly, the experiments
reported in Table 1 were repeated in the presence of
pyridine as an axial base (Table 3). The addition of pyridine
improved the chemical yields of all the reactions to some
extent. Furthermore, the reaction times of MnClL3 and
MnClL4 showed less sensitivity to the addition of pyridine
and MnClL5, which remained nearly constant, whereas
those of MnClL1, MnClL2 and MnClL6 were dramatically
reduced (Table 3, runs 1–6). It is concluded that the
additional N atom on the bridge groups of MnClL3 and
MnClL4 itself might play the role of axial base, therefore,
the presence of pyridine as an external axial base only
slightly improves corresponding reaction times in these
cases. We decided to examine the effect of pyridine
Among typical catalyst components, ligands play a
predominant role in the oxidation process. During electron
exchange between metal and substrate, ligand aids metal
to balance its electron density with releasing electrons,
when required to facilitate oxidation of the metal, and with
receiving electrons during the oxidation of the substrate.
Accordingly, to obtain a highly active catalyst, existence
of ligands with notable balance between their electron
donating and withdrawing properties, evidenced by the
calculation of energy gap between the highest occupied
molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) of them, is a predominant
requirement. Accordingly, the energy gap between HOMO
and LUMO of H2L1–H2L6 was studied using the DFT
calculation (Table 2). As can be seen, H2L6 and H2L4 own
the lowest energy gaps while MnClL3 which was obtained
Table 3. Effect of axial base (6 mmol) on the oxidation of
cyclohexene by KHSO5 in the presence of MnClL1–MnClL6.
Run Catalyst Axial base
Yield (%)a Time (h)
1
2
3
4
5
6
7
8
MnClL1 Pyridine
79
78
89
84
28
77
76
72
83
80
29
75
72
69
84
76
19
71
3.0
4.1
0.9
1.4
42
2.5
8
11
2
4
61
13
10
11.5
3.4
5
MnClL2 Pyridine
MnClL3 Pyridine
MnClL4 Pyridine
MnClL5 Pyridine
MnClL6 Pyridine
MnClL1 pyridine-N-oxide
MnClL2 pyridine-N-oxide
MnClL3 pyridine-N-oxide
MnClL4 pyridine-N-oxide
MnClL5 pyridine-N-oxide
MnClL6 pyridine-N-oxide
MnClL1 N-methylimidazole
MnClL2 N-methylimidazole
MnClL3 N-methylimidazole
MnClL4 N-methylimidazole
MnClL5 N-methylimidazole
MnClL6 N-methylimidazole
9
Table 2. Energy gap (eV) between HOMO and LUMO of
H2L1–H2L6; positive charge on the central metals of catalysts
obtained by DFT studies.
10
11
12
13
14
15
16
17
18
Catalyst
Positive charge
Ligand
Energy gap (eV)
MnClL1
MnClL2
MnClL3
MnClL4
MnClL5
MnClL6
1.2351
1.2364
1.0631
1.1002
1.2237
1.1124
H2L1
H2L2
H2L3
H2L4
H2L5
H2L6
3.9
4.0
4.0
3.1
4.3
2.9
59
10
Other conditions are the same as given in Table 1.
a Cyclohexene oxide yields are calculated on the initial amount of cyclohexene.

Supramolecular Chemistry
515
N-oxide as an axial base on the epoxidation of cyclohexene.
The reason of this attempt was originated by the possibility
that pyridine-N-oxide resistance to oxidation could
improve its ability for activating the manganese complex.
Unfortunately, the basicity of this oxygenated ligand
revealed not to be efficient enough to improve catalytic
activities compared to those obtained by pyridine (Table 3,
run 7–12). Similar unsatisfactory results were obtained
with N-methylimidazole (Table 3, run 13–18).
opening of epoxide. Additionally, an excess amount of
cyclohexene decreased the reaction time somewhat, but at
the expense of lower chemical yield which may be due to
the increase of side reactions (Table 4, run 9). Similar
results were obtained when the effect of reagent ratio on
the catalytic performance of MnClL1 was investigated
(Table 4, runs 10–18).
In order to investigate the influence of temperature on
the chemical yield of reactions that were performed over
MnClL1–MnClL4 and MnClL6, these catalysts were used
between temperatures of 15–608C for the oxidation of
cyclohexene (Figure 3). As can be seen, the highest
chemical yield of reactions performed over MnClL3 and
MnClL4 were obtained at 358C while the maximum yields
of reactions performed over MnClL1, MnClL2 and
MnClL6 were obtained at 258C. At temperatures between
35 and 608C, chemical yields obtained for MnClL3 and
MnClL4 as catalysts remained nearly constant; however,
chemical yields obtained by MnClL1, MnClL2 and
MnClL6 were adversely affected between 25 and 608C.
The higher thermal stability of MnClL3 and MnClL4 might
be due to the presence of more atoms contributing in the
coordination sphere of the metal.
Next, the effect of reagent ratio on the reactions
performed over MnClL1 was investigated (Table 4). When
using an excess oxidant with respect to pyridine, yields
were enhanced at the expense of longer reaction time
(Table 4, compare runs 1–3). It is believed that the
presence of excess amounts of oxidant leads to the loss of
catalytic activity as a consequence of free pyridine
exhaustion (39). Accordingly, excess amounts of pyridine
and oxidant with respect to catalyst and cyclohexene were
used in order to shorten reaction times and improve
chemical yields of these reactions (Table 4, runs 4–7). As
can be seen (Table 4, run 6), with a sixfold excess of
pyridine and a threefold excess of oxidant with respect to
catalyst and cyclohexene, the best result in terms of
chemical yield and reaction time was obtained. Chemical
yield and reaction time remained nearly constant after
further increase in the pyridine excess (Table 4, run 7).
With increasing amount of catalyst, chemical yield was
adversely affected (Table 4, run 8). It is probable that
MnClL1 could act as a Lewis acid, which facilitates ring
It is interesting that the addition of NaNO3 to the
catalytic system dramatically reduced the reaction times
and slightly improved the chemical yields while the
addition of Ba(NO3)2 and LiNO3 was almost ineffective
(Table 5). It is conceivable that there is a good match
between the cavity size of 15-crown-5 (d ¼ 0.18–
0.22 nm) (45) and the dimension of the alkali metal Naþ
(d ¼ 0.19 nm), resulting in a better extraction of HSO25 to
the aqueous medium. On the other hand, the dimension of
Liþ (d ¼ 0.136 nm) is too small and those of Baþ2 and Kþ
(d ¼ 0.266 nm) are too large to fit with the cavity size of
the crown.
Table 4. Effect of molar ratio of reagents on chemical yields
and times of reactions catalysed by MnClL1 and MnClL2 in 5 ml
chloroform.
Molar ratio of reagents
KHSO5 :Cat:Py:Cy
a
Run
Yield (%)b
Time (h)
To elucidate the merit of our epoxidation system, it
was compared with other systems (38, 39) in Table 6. Our
1c
1:0.006:1:1
3:0.006:1:1
6:0.006:1:1
3:0.006:2:1
3:0.006:4:1
3:0.006:6:1
3:0.006:8:1
3:0.012:6:1
3:0.002:6:2
1:0.006:1:1
3:0.006:1:1
6:0.006:1:1
3:0.006:2:1
3:0.006:4:1
3:0.006:6:1
3:0.006:8:1
3:0.012:6:1
3:0.002:6:2
68
74
75
66
72
79
78
48
41
62
70
69
62
72
78
79
37
34
24
25
25
11
8
2c
3c
4c
5c
6c
3
7c
3.2
2.6
2.5
27
23
24
14
9
4.1
4.5
3.4
2.5
8c
9c
10d
11d
12d
13d
14d
15d
16d
17d
18d
Figure 3. Effect of temperature on the yield of oxidation of
cyclohexene by KHSO5 in the presence of MnClL1–MnClL4 and
MnClL6. Reaction time is 30 min; other conditions are the same
as given in Table 3.
a A 1.5 M solution in water.
b Cyclohexene oxide yields were calculated on the initial amount of cyclohexene.
c MnClL1 was used as a catalyst.
d MnClL2 was used as a catalyst.

516
S.M. Seyedi et al.
Table 5. Effect of alkali metal salts (3.0 mmol) on the oxidation of cyclohexene (1 mmol) by KHSO5 in the presence of MnClL1–
MnClL4 and MnClL6.
Yield (%)a
NaNO3
Time (h)
NaNO3
Catalyst
–
Ba(NO3)2
LiNO3
–
Ba(NO3)2
LiNO3
MnClL1
MnClL2
MnClL3
MnClL4
MnClL6
79
78
89
84
78
79
79
87
85
80
88
90
95
94
90
80
77
90
80
77
3.0
4.1
0.9
1.4
2.5
3.1
4.2
0.8
1.3
2.6
1.7
2.0
0.3
0.5
1.2
2.8
4.0
0.8
1.4
2.4
Other conditions are the same as given in Table 3.
a Cyclohexene oxide yields were calculated on the initial amount of cyclohexene.
Table 6. Comparisons of epoxidations.
Alkene (mmol)
KHSO5 (mmol)
Yield (%)a
Time (h)
Reference
Cyclohexene (0.35 mmol)
Cyclohexene (1.6 mmol)
Cyclohexene (1 mmol)
Cyclohexene (1 mmol)
Cyclooctene (0.35 mmol)
Cyclooctene (1.6 mmol)
Cyclooctene (1 mmol)
Cyclooctene (1 mmol)
0.44
3.20
3.0
95
51
95
94
97
60
96
95
5
(38)b
0.75
0.3
0.5
5
16
0.2
0.4
(39)c
This workd
This worke
(38)b
3.0
0.60
3.20
3.0
(39)c
This workd
This worke
3.0
a Cyclohexene oxide and cyclooctene oxide yields were calculated on the initial amount of alkene.
b Reactions were performed using manganese hemiporphyrazine complex (8.0 mmol), in the presence of 2,4,6-trimethylpyridine (8.0 mmol), in 5 ml dichloroethane, at 258C.
c Reactions were performed at 238C in an aqueous media and without catalyst.
d MnClL3 was used as a catalyst.
e MnClL4 was used as a catalyst; conditions were the same as given in Table 5.
catalytic system exhibited either higher chemical yield or
shorter reaction time and, in some cases, exhibited both of
them when compared with literature precedents.
oxidant (Table 7). As can be seen, conjugated p-electron-
withdrawing groups with double bond of alkenes decrease
the reactivity for epoxidation (Table 7, run 8); additionally,
more reactivity of cis-alkenes in comparison with trans-
ones may be due to the cis-orientation of the substituents,
Eventually, the reaction was extended to the different
alkenes using MnClL3 as a catalyst and Oxone as an
Table 7. Epoxidation of different alkenesa.
Run
Substrate
Yieldb (%)
Time (h)
3.5
Run
Substrate
Yieldb (%)
Time (h)
0.6
1
74
5
96
2
3
52
94
4.0
1.6
6
7
69
21
2.0
15
NO2
NO2
4
92
1.0
8
0
16
a MnClL3 was used as a catalyst; conditions were the same as given in Table 5.
b Epoxide yields were calculated on the initial amount of cyclohexene.

Supramolecular Chemistry
517
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We elaborated the effect of a crown ether ring bonded to
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