Manganese(II) complexes of hydrazone based NNO-donor ligands and their catalytic activity in the oxidation of olefins

Article abstract of DOI:10.1080/00958972.2017.1281918

The reaction between tridentate NNO donor hydrazone ligands, (E)-2-cyano-N′-(phenyl(pyridin-2-yl)methylene)acetohydrazide (HL¹) and (E)-2-cyano-N′-(1-(pyridin-2-yl)ethylidene)acetohydrazide (HL²), with MnCl₂·4H₂O in methanol resulted in [Mn(HL¹)Cl₂(CH₃OH)] (1) and [Mn(HL²)Cl₂(CH₃OH)] (2). Molecular structures of the complexes were determined by single-crystal X-ray diffraction. All of the investigated compounds were further characterized by elemental analysis, FT-IR, TGA, and UV–Vis spectroscopy. These complexes were used as catalysts for olefin oxidation in the presence of tert-butylhydroperoxide (TBHP) as an oxidant. Under similar experimental conditions with equal manganese loading, the presence of [Mn(HL²)Cl₂(CH₃OH)] (2) resulted in higher conversion than [Mn(HL¹)Cl₂(CH₃OH)] (1).

Full text of DOI:10.1080/00958972.2017.1281918

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Manganese(II) complexes of hydrazone based
NNO-donor ligands and their catalytic activity in
the oxidation of olefins
Rahman Bikas, Massomeh Ghorbanloo, Roya Sasani, Ingo Pantenburg &
Gerd Meyer
To cite this article: Rahman Bikas, Massomeh Ghorbanloo, Roya Sasani, Ingo Pantenburg &
Gerd Meyer (2017): Manganese(II) complexes of hydrazone based NNO-donor ligands and
their catalytic activity in the oxidation of olefins, Journal of Coordination Chemistry, DOI:
10.1080/00958972.2017.1281918
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Date: 17 January 2017, At: 01:15

Publisher: Taylor & Francis
Journal: Journal of Coordination Chemistry
DOI: http://dx.doi.org/10.1080/00958972.2017.1281918
Manganese(II) complexes of hydrazone based NNO-donor ligands and their
catalytic activity in the oxidation of olefins
RAHMAN BIKAS*†, MASSOMEH GHORBANLOO†, ROYA SASANI†, INGO PANTENBURG‡ and
GERD MEYER‡§
†Department of Chemistry, Faculty of Science, University of Zanjan, 45371-38791 Zanjan, Iran
‡Department für Chemie, Institut für Anorganische Chemie, Universität zu Köln, Greinstraße 6,
50939 Köln, Germany
§Present address: Department of Chemistry, Iowa State University, Ames, IA 50011, USA
The reaction between tridentate NNO donor hydrazone ligands, (E)-2-cyano-N'-(phenyl(pyridin-
2-yl)methylene)acetohydrazide (HL1) and (E)-2-cyano-N'-(1-(pyridin-2-
yl)ethylidene)acetohydrazide (HL2), with MnCl2·4H2O in methanol resulted in
[Mn(HL1)Cl2(CH3OH)] (1) and [Mn(HL2)Cl2(CH3OH)] (2). Molecular structures of the
complexes were determined by single-crystal X-ray diffraction. All of the investigated
compounds were further characterized by elemental analysis, FT-IR, TGA and UV-Vis
spectroscopy. These complexes were used as catalysts for olefin oxidation in the presence of
tert-butylhydroperoxide (TBHP) as an oxidant. Under similar experimental conditions with equal
manganese loading, the presence of [Mn(HL2)Cl2(CH3OH)] (2) resulted in higher conversion
than [Mn(HL1)Cl2(CH3OH)] (1).
Keywords: Manganese(II) complex; Crystal structure; Spectroscopic studies; Oxidation; Catalyst
1. Introduction
The central role played by epoxides in all areas of synthetic organic chemistry [1] caused the
expansion of new sustainable and environmentally friendly processes for olefin oxidation [2]. In
this regard, researchers have focused on replacing methods based on rare and potentially toxic
metals such as chromium, ruthenium and osmium with metals of less toxicity such as iron,
*Corresponding author. Email: bikas_r@yahoo.com
1

titanium, manganese, tungsten, and molybdenum [3]. In the area of oxidative transformations,
one major aim is replacement of classical toxic waste-producing oxidants, with catalytic
procedures using an environmentally safe oxidant. In bulk chemistry, stoichiometric reagents
(such as K2CrO7, KMnO4 and derivatives) are regularly applied in these reactions. Nevertheless,
these reagents are expensive and produce a stoichiometric amount of (often toxic) inorganic salts
as waste [4]. Other suitable oxidants being considered include molecular oxygen, hydrogen
peroxide and tert-butyl hydroperoxide (TBHP) where there is either no by-product, the by-
product is environmentally benign, or the by-product can be easily recovered and recycled [5].
tert-Butanol, a useful high-octane compound, is formed as the by-product when tert-butyl
hydroperoxide is used as the oxidant [6].
Synthesis of coordination compounds based on hydrazone ligands have been reported
[7, 8]. Metal complexes of aroylhydrazone ligands have remained an attractive area of research
for inorganic chemists due to their versatile coordination chemistry and ability to generate
different molecular structures [9, 10]. Aroylhydrazone metal complexes have biological and
pharmacological applications [11, 12]. The biological activity associated with these compounds
is attributed to the presence of the –C(=O)–NH–N=CH– moiety [13]. Aroylhydrazone Schiff
base complexes of transition metals have shown high catalytic activities in chemical reactions
such as C–N bond formation using Chan-Lam coupling [14], trans amidation of carboxamides
with amines [15], epoxidation of olefins [16, 17] and polymerization of ethylene [18]. A series of
Mn(II) complexes containing substituted aroylhydrazone ligands have been used in catalytic
epoxidation of olefins with tert-butylhydroperoxide [19-21].
As a result of our ongoing investigations of the structural and catalytic properties of
transition metal complexes with hydrazone ligands, we report here the synthesis, characterization
and efficiency in hydrocarbon oxidation of two manganese complexes (scheme 1). These
complexes were obtained by coordinated single condensation products of 2-cyanoacetohydrazide
with 2-benzoylpyridine or 2-acetylpyridine and characterized by spectroscopic methods and
single crystal X-ray analysis.
2

Scheme 1. Schematic representation of the synthesis of ligands and complexes.
2. Experimental
2.1. Materials
All chemicals and solvents were purchased from Merck and used as received. FT-IR spectra
were recorded in KBr disks with a Bruker FT-IR spectrophotometer. Elemental analyses (carbon,
hydrogen and nitrogen) were obtained applying a Carlo ERBA Model EA 1108 analyzer. Atomic
absorption analysis was carried out using Varian Spectra AA 220 equipment. Thermogravimetric
analysis (TGA) was performed on a STA409PC Netzsch thermal analyzer in flowing N2 (60 mL
min-1) at a heating rate of 5 °C min-1. 1H and 13C NMR spectra of the ligands in DMSO-d6
solution were recorded on a Bruker 250 MHz spectrometer and chemical shifts are indicated in
ppm relative to tetramethylsilane (TMS). The reaction products of oxidation were determined
and analyzed using an HP Agilent 6890 gas chromatograph equipped with a HP-5 capillary
column (phenyl methyl siloxane 30 m × 320 μm × 0.25 μm) and gas chromatograph-mass
spectrometry (Hewlett-Packard 5973 Series MS-HP gas chromatograph with a mass-selective
detector).
2.2. Synthesis of (E)-2-cyano-N'-(phenyl(pyridin-2-yl)methylene)acetohydrazide (HL1)
HL1 was synthesized by refluxing a mixture of 2-cyanoacetohydrazide (10 mmol, 0.99 g) with
2-benzoylpyridine (10 mmol, 1.83 g) in 20 mL of methanol for 5 h [22]. The volume of the
solution was decreased to 5 mL by removing the solvent under reduced pressure as the solution
was cooled to room temperature. The obtained solids were separated and filtered off, washed
3

with cooled methanol and then dried in air. Yield 85.1% (2.25 g). Anal. Calcd. for C15H12N4O
(%): C, 68.17; H, 4.58; N, 21.20. Found: C, 68.13; H, 4.61; N, 21.27. Selected FT-R (KBr, cm-1):
3103 (m, NH), 2998 (w), 2918 (w, C-H aliphatic), 2260 (m, C≡N), 1686 (vs, C=O), 1665 (vs),
1598 (s, C=N), 1567 (s), 1472 (w), 1384 (w, C–O), 1282 (s), 1247 (vs), 1121 (m), 940 (m),
799 (s), 744 (s), 696 (vs), 661 (s), 452 (m), 412 (s). 1H NMR (250.13 MHz, DMSO-d6, 25 °C,
TMS, ppm): δ = 4.36 (2H, s, CH2-CN), 7.36-7.48 (3H, m), 7.89-8.05 (3H, m), 8.43 (1H, d, J =
8.00 Hz), 8.68 (1H, d, J = 4.50 Hz), 9.16 (1H, d, J = 7.50 Hz), 10.54 ppm (1H, s, NH). 13C NMR
(62.9 MHz, DMSO-d6, 25 °C, TMS, ppm): δ = 25.4 (-CH2CN), 121.0, 124.5, 128.7, 128.9,
129.2, 129.3, 129.6, 129.9, 137.1, 138.5, 149.0 and 165.9 ppm. UV-Vis (MeOH, λmax
(ε, M-1cm-1): 204 (30400), 292 nm (17200).
2.3. Synthesis of (E)-2-cyano-N'-(1-(pyridin-2-yl)ethylidene)acetohydrazide (HL2)
HL2 was synthesized by refluxing a mixture of 2-cyanoacetohydrazide (10 mmol, 0.99 g) with
2-acetylpyridine (10 mmol, 1.21 g) in 20 mL of methanol for 5 h with the same procedure used
for HL1. Yield 88.5% (1.79 g). Anal. Calcd. for C10H10N4O (%): C, 59.40; H, 4.98; N, 27.71.
Found: C, 59.37; H, 5.03; N, 27.80. Selected FT-IR (KBr, cm-1): 3208 (w), 3150 (w, NH),
2976 (w), 2934 (w, C-H aliphatic), 2264 (m, C≡N), 1690 (vs, C=O), 1578 (s, C=N), 1460 (s),
1393 (s, C–O), 1316 (s), 1249 (s), 1108 (m), 1047 (m), 928 (m), 852 (s), 786 (vs), 728 (vs),
702 (vs), 626 (s), 568 (s), 474 (w), 439 (s), 406 (s). 1H NMR (250.13 MHz, DMSO-d6, 25 °C,
TMS, ppm): δ = 2.29 (3H, s, CH3), 4.27 (2H, s, CH2-CN), 7.37 (1H, t, J = 7.25 Hz), 7.79 (1H, t,
J = 7.25 Hz), 8.05 (1H, d, J = 7.75 Hz), 8.55 (1H, d, J = 7.50 Hz), 11.19 ppm (1H, s, NH).
13C NMR (62.9 MHz, DMSO-d6, 25 °C, TMS, ppm): δ = 12.6 (-CH3), 25.4 (-CH2CN), 116.6,
120.7, 124.5, 137.0, 148.9, 150.2, 155.0 and 166.4 ppm. UV-Vis (MeOH, λmax (ε, M-1cm-1): 212
(13080), 289 nm (19780).
2.4. Synthesis of [Mn(HL1)Cl2(CH3OH)] (1)
HL1 (1.0 mmol, 0.26 g) and MnCl2·4H2O (2.0 mmol, 0.39 g) were dissolved in 20 mL of CH3OH
and the resulting solution refluxed for 8 h. The solution was left at room temperature for two
days and the obtained crystalline dark brown material was filtered off and dried in air. Yield:
(63%, 0.26 g). Anal. Calcd. for C16H16Cl2MnN4O2 (%): C, 45.52; H, 3.82; N, 13.27; Mn, 13.01.
Found: C, 45.59; H, 3.85; N, 13.32; Mn, 12.96. Selected FT-IR (KBr, cm-1): 3478 (m, br, OH),
4

3319 (m, br, OH), 3196 (m, NH), 2947 (w), 2911 (w, C-H aliphatic), 2262 (w, C≡N), 1667 (vs,
C=O), 1616 (s, C=N), 1525 (s), 1465 (m), 1392 (m, C–O), 1329 (s), 1262 (s), 1228 (s), 1193 (s),
1057 (m), 917 (w), 792 (s), 745 (s), 705 (vs), 665 (s), 574 (Mn-O), 436 (Mn-N).; UV-Vis
(MeOH): λmax, nm (ε, M-1cm-1): 205 (30240), 289 (17240), 350 nm (7320).
2.5. Synthesis of [Mn(HL2)Cl2(CH3OH)] (2)
Complex 2 was synthesized by reaction of HL2 (1.0 mmol, 0.20 g) and MnCl2·4H2O (2.0 mmol,
0.39 g) in methanol applying the same procedure as reported for 1. Dark brown crystals of 2
were obtained by slow solvent evaporation during three days. Yield: (71%, 0.26 g). Anal. Calcd.
for C11H14Cl2MnN4O2 (%): C, 36.69; H, 3.92; N, 15.56; Mn, 15.26. Found: C, 36.63; H, 3.88; N,
15.49; Mn, 15.32. Selected FT-IR (KBr, cm-1): 3364 (br, w, OH), 3162 (w, NH), 2941 (w), 2859
(w, C-H aliphatic), 2259 (m, C≡N), 1658 (vs, C=O), 1596 (m, C=N), 1538 (s), 1471 (m), 1387
(w, C–O), 1205 (s), 1001 (s), 808 (s), 566 (Mn-O), 429 (Mn-N). UV-Vis (MeOH): λmax, nm
(ε, M-1cm-1): 214 (12860), 276 (17340), 339 nm (5220).
2.6. X-ray diffraction data collection and refinement
A summary of the crystal data and refinement details for the compounds discussed in this paper
is given in table 1. Only special features of the analyses are mentioned here. Single crystals of 1
and 2 were selected directly from the mother liquors under a microscope and sealed in thin-
walled glass capillaries. Their quality was checked on a single-crystal X-ray diffractometer (Stoe
Image Plate Diffraction System, IPDS 2T) and complete intensity data sets were collected using
graphite-monochromated Mo-Kα radiation. All data sets were corrected for Lorentz, polarization
and absorption effects. The structures were solved by direct methods using SHELXS97 [23] and
SIR92 [24] for 1 and 2, respectively, completed with difference Fourier synthesis and refined
with full-matrix least-square procedures based on F2 using SHELXL97. All non-hydrogen atoms
were refined with anisotropic displacement parameters. The C-bonded hydrogens were placed in
idealized positions and constrained to ride on their parent atoms. The molecular structure plots
were prepared using Diamond [25].
CCDC 1504221 and 1504222 contain supplementary crystallographic data for 1 and 2,
respectively. These data can be obtained free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
5

2.7. General oxidation procedure
Oxidation reactions were carried out under air with acetonitrile as solvent and TBHP (80%
solution in water) as oxidant. In a typical experiment, a mixture of 2.4×10-3 mmol of 1, 2.0 mL
solvent and 1.0 mmol of cyclohexene was placed in a 25 mL round bottomed glass flask. The
mixture was heated to the desired temperature and after addition of TBHP, the solution was
placed in an oil bath. At appropriate intervals, aliquots were removed and analyzed immediately
by GC. The oxidation products were identified by comparison of their retention times with those
of authentic samples. Yields are based on cyclohexene.
3. Results and discussion
3.1. Preparation and characterization of 1 and 2
The reaction of 2-cyanoacetohydrazide with 2-benzoylpyridine or 2-acetylpyridine in methanol
gave the desired tridentate Schiff bases (HL1 or HL2) in high yield and purity. Elemental analysis
together with 1H NMR and 13C NMR spectra of HL1 and HL2 in DMSO-d6 confirmed the
proposed structure of the ligands (scheme 1). In the 1H NMR spectrum of these ligands the signal
of the NH-group appears at δ 10.54 (in HL1) and 11.9 (in HL2) which are substantially
broadened, indicating their involvement in hydrogen bonding interactions.
The infrared spectra of HL1 and HL2 (figures S1 and S2) display bands at ~3150, ~2260,
~1690 and ~1590 cm-1, which are assigned to υ(N-H), υ(C≡N), υ(C=O) and υ(C=N) of the free
Schiff base, respectively. Complexes 1 and 2 were synthesized by reaction of the corresponding
Schiff bases, HL1 and HL2, with MnCl2·4H2O in methanol. Dark brown crystals of 1 and 2 grew
by slow evaporation of methanol and were collected by filtration. The C=O bands of the free
ligands are shifted to lower frequencies in the FT-IR spectrum of 1 and 2 (figures S3 and S4),
which confirm coordination of HL1 and HL2 as neutral ligands in the keto form [26]. Moreover,
there is a band at 3150 cm-1 in the FT-IR spectrum of 1 and 2 which is due to the presence of an
amidic N-H bond and confirms coordination of ligands in the keto form. In the FT-IR spectra of
the complexes, the presence of Mn-O and Mn-N are indicated by weak bands at ~570 and
~430 cm-1, respectively [27]. The very broad band at 3400 cm-1 in the FT-IR spectrum of 1 and 2
is attributed to O-H of coordinated water or methanol molecules involved in hydrogen bonding
interactions [28].
6

The UV–Vis spectra of HL1 and HL2 and of 1 and 2 were obtained in methanolic
solutions and are shown in figure 1. The complexes exhibit intense bands at ~350 nm, attributed
to charge transfer (CT) transitions [29]. The intense higher-energy bands at ~270 nm can be
attributed to intra-ligand n→π* and π→π* transitions [30].
3.2. Thermal investigation
To examine thermal stabilities of complexes, thermogravimetric analysis (TGA) of 1 was made
at a heating rate of 5 °C/min from 30–800 °C. TGA curve of 1 (figure S5) shows that 1 is stable
to 115 °C, where the coordinated CH3OH is removed [observed (7.79%), calculated (7.58%)]
between 115–145 °C, then at 270 °C the Cl ligands are removed [observed (16.35%), calculated
(16.80%)]. Finally, hydrazone ligand is removed during third step up to 440 °C. The overall
mass loss to 800 °C is 80% of starting mass and indicates that the final product should be oxides
of manganese.
3.3. X-ray crystal structures of 1 and 2
In order to define the coordination sphere of Mn in 1 and 2, single-crystal X-ray diffraction
analyses were performed. The molecular structure of 1 and 2 are shown in figures 2 and 3,
respectively. Selected bond lengths and angles are given in table 2. The manganese(II) in both
complexes has distorted octahedral coordination with one oxygen and two nitrogen donors
provided by the Schiff base ligands, one oxygen, provided by coordinated water in 1 and
methanol in 2, and two chlorides. In both complexes, the two chlorides are located cis with
respect to each other with Cl1–Mn1–Cl2 angles of 97.81(3)° and 94.02(2)° in 1 and 2,
respectively. One Cl is axial and the other is equatorial. The Mn–Cl, Mn–N and Mn–O bond
lengths are in normal ranges as reported for Mn(II) complexes [31]. In 1 the C110=O110 and
C110–N19 bond lengths are 1.227(3) and 1.350(3) Å, respectively, and in 2 these bond lengths
are 1.227(2) and 1.350(2) Å, respectively. These bond lengths are close to those observed in
hydrazone ligands coordinated in the keto form [32]. Although the value of the C=O bond length
is considerably shorter than the C–O bond length in reported complexes containing the enol form
of hydrazone ligands, they are relatively longer than the C=O bond lengths of free hydrazones
[33]. Moreover, the C–NH– bond lengths in both 1 and 2 are considerably longer that the C=N
bond lengths in coordinated hydrazones in the enol form [34]. All these observations indicate
7

coordination of both HL1 and HL2 as neutral tridentate ligands in the keto form, which is also
confirmed by the IR spectrum. A coordinated water or methanol is present in the crystal
structures of 1 and 2, respectively. These molecules are engaged in O−H···Cl hydrogen bonding
interactions. Moreover, there are N−H···Cl and several directed C−H···Cl interactions which
further stabilize the crystal structures of both 1 (figure S6) and 2 (figure S7).
3.4. Catalytic activity
The oxidation of cyclohexene as a substrate with H2O2 (30%) and TBHP (80% solution in water)
was carried out in the presence of 1 as a catalyst precursor. In order to find the optimum
conditions for the oxidation reactions, oxidation of cyclohexene was studied under various
conditions. The oxidation of cyclohexene in the presence of H2O2 was not significant which can
be attributed to degradation of H2O2 in the presence of manganese [19]. Oxidation of
cyclohexene was significant when TBHP was used as oxidant, so, it was selected as oxidant in
the rest of the reactions. The conversion increased with increasing amounts of tert-
butylhydroperoxide from 0.5 mmol to 1 mmol, but further increase resulted in no notable
increase of conversion (table 3, entries 1-3).
It may be pointed out that the addition of the catalyst led to an increase in conversion of
cyclohexene. A relatively low conversion (36%) was obtained with 1.2×10-3 mmol catalyst, but a
higher conversion (71%) with 2.4×10-3 mmol catalyst. A still larger quantity resulted in lower
conversion, 67% with 4.8×10-3 mmol catalyst. This may be due to increased degradation of the
oxidant at a higher concentration of the catalyst [35]. In the blank reaction under similar
conditions with cyclohexene, only 5% of cyclohexene was oxidized after 24 h [36]. Thus, the
presence of 1 as a catalyst precursor is necessary for the oxidation reaction.
When the temperature was increased (from 40 to 80 °C), the conversion correspondingly
increased from 28% to 71%, as shown in table 3 (entries 2, 6 and 7). Solvent in the catalytic
oxidation of cyclohexene was examined in the current catalytic system (table 4). It was found
that the cyclohexene oxidation reaction had the highest reactivity and selectivity when the
reaction was performed in 1,2-dichloroethane, whereas methanol and ethanol were not the most
suitable solvents for [Mn(HL1)Cl2(CH3OH)]. The efficiency of the catalysts in different solvents
decreases in the order 1,2-dichloroethane > acetonitrile > methanol > ethanol > CCl4, indicating
8

a significant role of solvent for the oxidation reactions as investigated in this work (conversion
7–71%).
To establish the scope for the activity of [Mn(HL1)Cl2(CH3OH)], this study was further
extended to the catalytic oxidation of linear and cyclic olefins, namely styrene, α-methyl styrene,
1-octene, 1-decene, cyclooctene and indene, using [Mn(HL1)Cl2(CH3OH)] (table 5). Generally,
excellent epoxide selectivity was observed for the tested aliphatic and aromatic substrates
(table 5, entries 1-7). The present catalytic system is completely selective for epoxidation of
cyclooctene, 1-octene, 1-decene and indene, but is not selective for styrene and α-methyl styrene
because benzaldehyde and acetophenone were byproducts, respectively. The production of
benzaldehyde in the oxidation of styrene is due to over-oxidation of styrene oxide in the presence
of TBHP. The lower catalytic activity of α-methyl styrene compared to styrene seems to be due
to the steric hindrance of the methyl substituent of the former [37]. It should be noted that if
electronic effects due to the methyl substituent were important, then, α-methyl styrene as a more
electron-rich olefin would be expected to show higher reactivity than styrene.
In the following, to investigate substituent effects on catalytic activity, we have examined
the catalytic activity of [Mn(HL2)Cl2(CH3OH)] (2) under the same conditions as were optimized
for [Mn(HL1)Cl2(CH3OH)] (1) as the catalyst (table 6, entries 1-7). It is clear from the table that
the percentage conversion of olefins varies in the order 2 > 1. Both electron donation and steric
effect of the phenyl substituent rather than methyl in the hydrazone Schiff base ligand decrease
the catalytic activity of 1 relative to 2 [38]. Steric effect of the phenyl substituent rather than
methyl in the hydrazone Schiff base ligand decrease the catalytic activity of 1 relative to 2 [38].
Electron-donor substituents (such as methyl) seem to facilitate oxidation of M(II) to M(III)
[39, 40]. It should be noted that the oxidation of Mn(II) to Mn(III) has been proposed as an
important step in the Mn-hydrazone catalyzed oxidation of hydrocarbons with TBHP. In general,
electronic and steric effects of the ligand substituent are rather obvious since they are near the
Mn(II) center.
Although the details of catalytic mechanism are not studied in the present work, on the
basis of the electronic absorption spectroscopy studies (figure S8) and also by considering the
previous studies on catalytic oxidation by manganese complexes [41-43], it is predicted that the
key step in this catalytic process is the oxidation of substrates by a Mn-(oxido/peroxido)-
hydrazone Schiff base species formed after addition of TBHP to solution of complex.
9

4. Conclusion
Two new manganese(II) complexes were synthesized and characterized by various analyses and
their structures were determined by single crystal X-ray analysis. These complexes were used as
catalysts in oxidation of olefins in the presence of TBHP as oxidant. The catalytic potential of
the manganese(II) complexes in oxidation of olefins was studied, choosing oxidation of
cyclooctene, styrene, α-methyl styrene, indene, cyclohexene, 1-octene and 1-decene as model
reactions. These complexes catalyze the oxidative conversion of styrene to styrene oxide and its
successive products, and cyclohexene to cyclohexene oxide, 2-cyclohexene-1-ol, indene to
indene oxide and ketone, α-methyl styrene to α-methyl styrene epoxide and acetophenone, while
the present catalytic system was selective for epoxidation of cyclooctene, 1-octene and 1-decene.
All the catalytic processes have clean, safe and operationally simple characteristics.
Acknowledgements
The authors are grateful to the University of Zanjan and the Universitat zu Köln for their support
of this study.
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13

Figure 1. UV-Vis spectra of (a) HL1, (b) [Mn(HL1)Cl2(CH3OH)] (1), (c) HL2 and
(d) [Mn(HL2)Cl2(CH3OH)] (2).
d
14

Figure 2. Molecular structure of 1; thermal ellipsoids are drawn at 50% probability level.
15

Figure 3. Molecular structure of 2; thermal ellipsoids are drawn at 50% probability level
16

Table 1. Crystal data for [Mn(HL1)Cl2(H2O)] (1) and [Mn(HL2)Cl2(CH3OH)] (2).
1
2
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
Crystal size (mm)
a (Å)
C15H14Cl2MnN4O2
408.14
C11H14Cl2MnN4O2
360.10
293
293
Monoclinic
Cc
0.3 × 0.2 × 0.2
13.4270(7)
19.0994(8)
7.0430(4)
90
Monoclinic
P21/c
0.3 × 0.1 × 0.1
7.1368(3)
18.0670(7)
12.8545(5)
90
b (Å)
c (Å)
α (°)
108.044(4)
90
117.114(3)
90
β (°)
γ (°)
V (Å3)
1717.33(15)
4
1475.30(11)
4
Z
D
calc (g cm−3)
1.579
1.621
F(000)
828
732
μ (mm−1)
1.10
1.26
1.9-26.8
12777
2.1-26.8
17129
θ range (°)
Reflections collected
Unique reflections, R(int)
Observed reflections [I > 2σ(I)]
Goodness-of-fit (GOF)
Absorption correction
3617, 0.068
3564
3112, 0.057
2859
1.06
1.13
Numerical
0.5751, 0.8585
0.035, 0.092
0.01
Numerical
0.6098, 0.9047
0.029, 0.074
Tmin, Tmax
Final R1, wR2 [I > 2σ(I)]
Flackx
Largest diff. peak and hole (eÅ−3)
1.09, −0.39
0.49, −0.65
17

Table 2. Selected bond distances and angles in the molecular structures of 1 and 2.
Complex 1
Complex 2
Mn1—O005
Mn1—N18
Mn1—N11
Mn1—O110
Mn1—Cl1
Mn1—Cl2
O005—Mn1—N18
O005—Mn1—N11
N18—Mn1—N11
O005—Mn1—O110
N18—Mn1—O110
N11—Mn1—O110
O005—Mn1—Cl1
N11—Mn1—Cl1
N18—Mn1—Cl2
N11—Mn1—Cl2
O110—Mn1—Cl2
Cl1—Mn1—Cl2
2.226(2)
2.256(2)
2.272(2)
2.2797(19)
2.4048(8)
2.5015(7)
81.34(8)
90.50(8)
69.54(8)
80.35(8)
70.36(8)
139.75(7)
99.35(7)
100.26(6)
83.08(6)
93.64(6)
84.78(5)
97.81(3)
Mn1—N8
Mn1—O101
Mn1—N1
Mn1—O21
Mn1—Cl1
Mn1—Cl2
N8—Mn1—O101
N8—Mn1—N1
N8—Mn1—O21
O101—Mn1—O21
N1—Mn1—O21
N1—Mn1—Cl1
O21—Mn1—Cl1
N8—Mn1—Cl2
O101—Mn1—Cl2
N1—Mn1—Cl2
O21—Mn1—Cl2
Cl1—Mn1—Cl2
2.2387(12)
2.2495(11)
2.2711(13)
2.2717(12)
2.3757(4)
2.5312(5)
70.99(4)
70.36(5)
81.71(4)
80.11(4)
89.21(5)
101.49(3)
92.81(3)
92.53(3)
90.25(3)
96.62(3)
169.96(3)
94.018(15)
18

Table 3. Catalytic activity of [Mn(HL1)Cl2(CH3OH)] on cyclohexene oxidation.a
[TBHP]/[C6H10]
Molar ratio
Conversion
(%) b
Epoxide selectivity
(%) c
TOFe
(h-1)
Entry
TON d
1
2
3
0.5
1
1.5
1
1
1
23
71
74
36
67
28
53
73
71
70
75
70
84
78
96
19
296
308
300
139.6
104
221
59.2
61.6
60
28
21
4 f
5 g
6 h
7 i
1
44
a
Reaction conditions: catalyst ([Mn(HL1)Cl2(CH3OH)]) (2.4×10-3 mmol); reaction
temperature, 80 °C; cyclohexene, 1 mmol; 1,2-DCE (as solvent), 2 mL; reaction time, 5 h;
b Conversions are based on the starting substrate for homogeneous conditions; c The other
product is 2-cyclohexene1-ol; d Turnover number (TON) = (mmol of product)/(mmol of
catalyst); e Turnover frequency (TOF) = (mmol of oxide)/(mmol of catalyst × time); f catalyst
amount = 1.2×10-3 mmol; g catalyst amount = 4.8×10-3 mmol, h Temperature = 40 °C;
i Temperature = 60 °C
19

Table 4. Effect of various solvents on cyclohexene oxidation by [Mn(HL1)Cl2(CH3OH)].a
TOFd
Entry
Solvent
Conversion (%) b
TON c
(h-1)
1
2
3
4
5
EtOH
CCl4
1,2-DCE
CH3CN
MeOH
16
7
71
60
28
66
29
296
250
117
13.2
5.8
59.2
50
23.4
a
Reaction conditions: catalyst ([Mn(HL1)Cl2(CH3OH)]) (2.4×10-3 mmol); reaction
temperature, 80 °C; cyclohexene, 1 mmol; solvent, 2 mL; reaction time, 5 h;
TBHP, 1 mmol; b Conversions are based on the starting substrate; c Turnover
number (TON) = (mmol of product)/(mmol of catalyst); d Turnover frequency
(TOF) = (mmol of oxide)/(mmol of catalyst × time)
20

Table 5. Oxidation of alkenes with TBHP catalyzed by [Mn(HL1)Cl2(CH3OH)] (1).a
Conversion Epoxide selectivity
TOFg
Entry
Substrate
TONf
(%) b
(%) c
(h-1)
1
2
3
4
5
6
Styrene
α-Methyl styrene
Indene
Cyclooctene
1-Octene
1-Decene
67
56
26
43
17
9
40c
279
233
108
179
71
55.8
46.6
21.6
35.8
14.2
7.5
34d
>99
100
>99
>99
71e
37.5
296
7
Cyclohexene
71
59.2
a
Reaction conditions: catalyst (2.4×10-3 mmol), substrate (1.0 mmol), 1,2-dichloroethane
(2 mL), TBHP (1 mmol) and temperature 80 °C; b Conversions are based on the starting
substrate; c The other product is benzaldehyde; d The other product is acetophenone; e The
other product is 2-cyclohexene1-ol; f Turnover number (TON) = (mmol of product)/(mmol of
catalyst); g Turnover frequency (TOF) = (mmol of oxide)/(mmol of catalyst × time)
21

Table 6. Oxidation of alkenes with TBHP catalyzed by the [Mn(HL2)Cl2(CH3OH)] (2).a
Conversion
(%) b
Epoxide selectivity
(%) c
TOF g
(h-1)
Entry Substrate
TON f
1
2
3
4
5
6
7
Styrene
α-Methyl styrene
Indene
Cyclooctene
1-Octene
1-Decene
74
68
39
50
25
14
80e
50
308
283
162.5
208
104
58
61.6
56.6
32.5
41.6
20.8
11.6
66.6
65d
68
100
>99
>99
84
Cyclohexene
333
a
Reaction conditions: catalyst (2.4×10-3 mmol), substrate (1.0 mmol), 1,2-dichloroethane (2 mL),
TBHP (1 mmol) and temperature 80 °C; b Conversions are based on the starting substrate; c The other
product is benzaldehyde; d The other product is acetophenone; e The other product is
2-cyclohexene1-ol; f Turnover number (TON) = (mmol of product)/(mmol of catalyst); g Turnover
frequency (TOF) = (mmol of oxide)/(mmol of catalyst × time)
22

Graphical abstract
23