Electronic Effects of Aromatic Rings on the Catalytic Activity of Dioxidomolybdenum(VI)–Hydrazone Complexes

Article abstract of DOI:10.1002/ejic.201601359

Nine dioxidomolybdenum(VI) complexes were synthesized through the reactions of MoO₃with tridentate hydrazone Schiff base ligands obtained from the reactions of aromatic acid hydrazides (3-hydroxy-2-naphthoic acid hydrazide, 4-pyridinecarboxylic

Full text of DOI:10.1002/ejic.201601359

A Journal of
Accepted Article
Title: Electronic effects of aromatic rings on the catalytic activity of
ꢀdioxidomolybdenum(VI)-hydrazone complexes
Authors: Rahman Bikas, Vito Lippolis, Nader Noshiranzadeh, Hossein
Farzaneh-Bonabꢀ, Alexander J. ꢀ Blake, Milosz Siczekꢀ,
Hassan Hosseini-Monfaredꢀ, and Tadeusz Lis
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201601359
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10.1002/ejic.201601359
European Journal of Inorganic Chemistry
FULL PAPER
Electronic effects of aromatic rings on the catalytic activity of
dioxidomolybdenum(VI)-hydrazone complexes
Rahman Bikas,[a,*] Vito Lippolis,[b,*] Nader Noshiranzadeh,[a] Hossein Farzaneh-Bonab,[a] Alexander J.
Blake,[c] Milosz Siczek,[d] Hassan Hosseini-Monfared,[a] and Tadeusz Lis[d]
Abstract:
Nine
dioxidomolybdenum(VI)
complexes
were
an important research area in organic synthesis and
bioinorganic modeling of oxygen transfer metalloenzymes.3 Over
the past decades, several transition metals have been widely
used as homogenous or heterogeneous catalysts in various
oxidation systems.4 Epoxidation of olefins and selective
oxidation of sulfides to sulfoxides are two kinds of oxidation
reactions which have attracted considerable attention during
recent decades.5 This interest is mainly due to their diverse
application in various fields. Sulfoxides are useful building blocks
as chiral auxiliaries in organic synthesis6 and possess a wide
range of biological activities, e.g. antimicrobial properties,7
inhibition of the biosynthesis of uric acid and gastric acid
secretion8 or the regulation of cholesterol catabolism.9 Epoxides
are well-known as interesting and versatile intermediates or
precursors for the synthesis of pharmaceuticals, agrochemicals,
and many other compounds.10 Treating alkenes with peroxide-
containing reagents that donate a single oxygen atom in the
presence of transition metal catalysts is a famous reaction for
generating epoxides under mild conditions.11 Using H2O2 as a
source of an O atom is very attractive in this method because
aqueous H2O2 is inexpensive, environmentally benign and easy
to handle.12 The reaction of H2O2 with the metal generates the
active metal catalyst with a peroxy ligand (MOOR), which then
transfers an O center to the substrate.13
synthesized by the reaction of MoO3 with tridentate hydrazone Schiff
base ligands obtained from the reaction of aromatic acid hydrazides
(3-hydroxy-2-naphthoic acid hydrazide, 4-pyridine carboxylic acid
hydrazide or 2-furane carboxylic acid hydrazide) and ortho-hydroxy
aldehyde derivatives (5-iodo-2-hydroxybenzaldehyde, 2-hydroxy-1-
naphthaldehyde or 2-hydroxy-3-methoxybenzaldehyde). All ligands
and complexes were characterized by elemental analysis and
spectroscopic methods. The structures of seven complexes were
further elucidated by single-crystal X-ray diffraction analysis which
indicated a distorted octahedral geometry at the metal centre.
Spectroscopic and X-ray analyses indicated that the ligands are
coordinated to the molybdenum(VI) ion as dinegative ligands due to
deprotonation of phenolic OH and amidic NH groups upon
complexation. These complexes were used as catalyst in the
oxidation of cyclooctene and thioanisole in the presence of hydrogen
peroxide as environmental friendly oxidant. In order to achieve the
highest catalytic activity, the effects of important parameters such as
solvent, temperature and the molar ratio of oxidant to substrate were
optimized. The results indicate that electron-withdrawing
substituents on the ligands increase the catalytic activity of
dioxidomolybdenum(VI)-hydrazone complexes.
Increasing catalytic activity and selectivity is one of the
challenges in chemical science and it has attracted considerable
attention in recent decades. In catalytic reactions it is obvious
that the catalytic activity and product selectivity depend on
several parameters.14 Reagents and experimental conditions for
catalytic reactions must be carefully chosen in order to maximize
conversion and minimize wastes. The structure and composition
of catalyst, the nature of oxidant and the temperature are some
of the well-known parameters which have powerful influences on
the activity of catalytic systems.15 In some cases, catalysts can
be modified with a range of additives or promoters that enhance
rates or selectivity.16 Some studies have shown that specific
steric and electronic properties of the ligand are also essential
parameters which may provide an opportunity to further enhance
the catalytic activity or selectivity.17
Among metal-based catalysts, Mo-containing compounds
are known to be highly active catalysts for oxidation reactions in
the presence of peroxides and some of them are currently used
in several industrial processes. Mo(VI)-oxido complexes
synthesized from the reaction of molybdenum(VI) ions and
organic ligands are one of the most efficient class of
(pre)catalysts for oxidation reactions which have been
investigated during recent decades.18 Interestingly, such
Introduction
Oxidation of organic compounds is an important chemical
reaction found both in nature and in the chemical industry.1 In
the latter, oxidation reactions are essential for the production of
oxygenated species which allow access to a large variety of
important organic compounds.2 Since most oxidation reactions
are very slow in the absence of catalysts, the catalytic oxidation
of organic substrates by transition metal complexes has become
[a] Department of Chemistry, Faculty of Sciences, University of
Zanjan 45195-313, Zanjan, Iran.
[b] Dipartimento di Scienze Chimiche e Geologiche, Università
degli Studi di Cagliari S.S. 554 Bivio per Sestu 09042 Monserrato,
Italy.
[c] School of Chemistry, The University of Nottingham, The
University Park, NG7 2RD Nottingham, UK.
[d] Faculty of Chemistry, University of Wroclaw, Joliot-Curie 14,
Wroclaw 50-383, Poland.
Corresponding authors: bikas_r@yahoo.com, lippolis@unica.it
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10.1002/ejic.201601359
European Journal of Inorganic Chemistry
FULL PAPER
complexes have structural similarities with
a class of Results and Discussion
mononuclear molybdoenzymes capable of transferring oxygen
atoms and can be use as suitable models for mimicking
biological systems.19 Although several studies have been
reported on the catalytic properties of Mo(VI) complexes,
information regarding the effects of various organic ligands on
the catalytic activity of such catalytic systems is relatively rare.
Hydrazone ligands obtained from the reaction of aromatic acid
hydrazides, R-C(=O)–NH–NH2, with ortho-hydroxybenzaldehyde
derivatives are special class of N- and O-donor Schiff base
ligands which have a high tendency to form transition metal
complexes.20 These ligands can stabilize transition metal ions in
high oxidation states and their complexes have high stability
under the conditions found in catalytic oxidation reactions.21
Although this type of ligand is usually tridentate (ONO),
depending on the nature of R-group connected to the –C(=O)–
NH–N= moiety they can act as ditopic ligands to form
multinuclear transition metal complexes and coordination
polymers.22 They can form Mo(VI) complexes with general
formula of Mo(VI)O2L (L = hydrazone ligand) which are known to
be efficient (pre)catalysts for various oxidation reactions.23
Synthesis of the ligands and Mo(VI)-complexes
The reaction of aromatic (2-hydroxynaphthalen, 4-pyridine or 2-
furane) carboxylic acid hydrazides with aromatic o-
hydroxyaldehyde derivatives in methanol gave the desired
tridentate Schiff base ligands (H2Lna-c, n = 1, 2, 3, see Scheme 1)
in excellent yields and purity. Dioxidomolybdenum(VI)
complexes of these hydrazone Schiff base ligands were
synthesized by the reaction of the appropriate ligand with
equimolar amount of MoO3 in methanol (see Experimental
Section and Electronic Supporting Information).† 1H and 13C
NMR spectral data of the ligands in DMSO-d6 confirmed the
proposed structure of the ligands. The signal at δ 12.08–12.68
ppm in the spectra of H2Lna-c (n = 1, 2, 3) is assigned to the
common NH-group, concomitant with the observation of a rapid
loss of these signals when D2O is added to the solution. The
signals in the range δ 10.69-12.24 in the spectra of H2Lna-c (n = 1,
2, 3) are also lost upon addition of D2O to the solution. Hence,
this signal is assigned to the aromatic –OH group. The
resonances observed in the range δ 8.3–8.7 ppm are attributed
to the azomethine group (–CH=N–) in the spectra of H2Lna-c (n =
1, 2, 3). The chemical shifts for the complexes are comparable
and very close to each other. On complexation the absence of
amidic N–H and phenolic O–H 1H NMR signal indicates the
coordination of ligands as dianionic ligands in the enol form. In
complexes 1, 2 and 3 the peak at about 11 ppm is due to the
By
considering
the
catalytic
applications
of
dioxidomolybdenum(VI) complexes in oxidation reactions and
our studies of the catalytic properties of hydrazone complexes,
we aimed to prepare Mo(VI)-hydrazone complexes and use
them in the oxidation of olefins and sulfides. In the present work
we report the synthesis, characterization, crystal structures and
catalytic activity of nine dioxidomolybdenum(VI) complexes
obtained from the reaction of MoO3 with the hydrazone Schiff
base ligands showed in Scheme 1. In this project we have
studied the effects of aromatic rings connected to the –C(=O)–
NH–N= group on the catalytic activity of Mo(VI)-hydrazone
complexes.
1
naphtholic O–H group. In the H NMR spectrum of complexes
(except complexes 4 and 5) the peak at about 3.7 ppm is due to
the presence of methanol in the structure of complexes. The
peak of OH group of methanol appears at about 4.1 ppm. A
comparison of the IR spectra of the complexes with free ligands
provides evidence for the coordination of the ligands in the
complexes. A list of the important vibrational frequencies (IR
spectra) of the free ligands and their dioxidomolybdenum
complexes, which are useful for determining the mode of
coordination of the ligands, are given in the in the Electronic
Supporting Information (ESI).† All hydrazone Schiff base ligands
[H2Lna-c (n = 1, 2, 3)] exhibit a broad band between 3170–3270
cm−1 due to the amidic –NH-vibrations. In addition a broad band
centered at 3400–3460 cm-1 in H2Lna-c is due to the aromatic O–
H groups, probably involved in hydrogen bonding interactions. In
the IR spectra of H2Lna-c a very strong band appears around
1645–1680 cm−1 which is due to C=O-vibration.24 On
complexation the absence of N-H and C=O bands and red shifts
in azomethine (–C=N–) band of the ligands show coordination of
H2Lna-c in the enol form. The infrared spectra of complexes
display IR absorption band around 1600 cm-1 which can be
assigned to the C=N stretching frequency of the coordinated
hydrazone ligand whereas for the free ligands the same band is
observed around 1610 cm-1.25 Two strong bands at 920–940
cm−1 and 960–985 cm−1 are observed as new peaks assigned to
ν(Mo=O), which are not present in the spectra of the free ligands.
Description of crystal structures
Scheme 1. a) Hydrazone Schiff-base ligands and b) Mo(VI)-
In order to define the coordination sphere conclusively, single-
crystal X-ray diffraction study was undertaken on high quality
complexes synthesized in the present work.
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10.1002/ejic.201601359
European Journal of Inorganic Chemistry
FULL PAPER
crystals of 1-6 and 9 (Scheme 1, see Experimental Section).
Selected bond lengths and angles are collected in Table S1 (in
the Electronic Supplementary Information, ESI).†
The molecular structure with the atom numbering scheme of
complex 1 is shown in Fig. 1a (see Figs. S1-S4 for complexes 2,
3, 5, 9, respectively). These complexes are neutral mononuclear
complexes of dioxidomolybdenum(VI) and in general have
similar coordination geometries at the metal centre.
for 1-3, 5 and 9, respectively. The molybdenum to oxygen bond
lengths follows the order Mo–oxido oxygen < Mo–phenolate
oxygen < Mo–enolate oxygen < Mo–methanol oxygen. The
amidic hydrogen of the ligands has been eliminated during the
complexation. The C8–O4 and C8–N2 bond lengths are close to
those reported for hydrazone based ligands coordinating to a
metal centre in the enolic form.27 This finding indicates that
ligands H2L1a-c, H2L2b, and H2L3c act as tridentate dinegative
ligands in the reported Mo complexes. In complexes 1-3, 5, and
9 two molecules of complex are connected together by
intermolecular O(methanol)–H···O (in 1-3) or O(methanol)–H···N (in 5
and 9) hydrogen bonds, thus creating a pseudo-dimer as
depicted in Fig. 1b (for 1) and Figs. S1-S4 (for complexes 2, 3, 5,
9, respectively). This intermolecular hydrogen bond stabilizes
the crystal structure of the mentioned complexes. There is an
intramolecular O(naphthol)–H···N hydrogen bond in the crystal
structure of 1-3 with the N atom of amidic group acting as
hydrogen bond acceptor, while in complex 5 an intramolecular
π-π stacking interaction between the pyridyl moieties of the
hydrazone ligands contributes to stabilize the dimers.
Parameters of hydrogen bonding geometry are given in Table
S2.
The molecular structure with the atom numbering scheme of
complex 4 is shown in Fig. 2a (see Fig. S5 for 6). These
compounds are neutral, one-dimensional coordination polymers
of dioxidomolybdenum(VI) and show a similar geometry. The
repetitive unit of these coordination polymers features two
crystallographically independent molecules of complex
[MoO2(L)] (L = (L2a)2- (4), (L2c)2- (6)) connected head-to-tail by the
pyridyl moiety of the hydrazone ligand via an Mo–N coordination
bond (see Figs. 2 and S5). In 4 and 6, each Mo(VI) atom is six-
coordinated by O1enol, N1imine, and O1phenol atoms from dinegative
hydrazone ligands (L2a)2- and (L2c)2-, respectively. The two oxido
ligands O1 and O2 and the N4-py from the second hydrazone
ligand complete a distorted octahedral coordination sphere. The
O,N,O donor atoms of the Schiff base ligands together with
oxygen atom of the oxido ligand O2 form the basal plane. One
axial position is occupied by the oxygen atom from oxido ligand
O1, while the other is occupied by the pyridyl nitrogen atom from
the hydrazone ligand of a second complex unit. As a result, the
hydrazone ligands H2L2a and H2L2c in their dianionic forms act as
ditopic ligands by coordinating one Mo(VI) core through the
O,N,O-donor set, and the second Mo(VI) core by N atom from
the pyridyl moiety. The Mo–N4-Py bond length is longer than the
other bond distances due to the trans influence of the oxido
ligand. The short Mo–Ooxido distances (≈1.69 Å) confirm the
presence of a molybdenum–oxygen double bond. The C8–O4
and C8–N2 bond lengths are close to those reported for
hydrazone based ligands coordinating to a metal centre in the
enolic form.27 There are several C–H···O, C–H···π and π···π
interactions in the crystal packing of both compounds 4 and 6
which stabilize the crystal structure and convert one-dimensional
polymeric chains to two-dimensional polymeric sheet (Figs. S6,
S7). It is noteworthy that polymeric complexes such as 4 and 6
particularly with isonicotinoyl hydrazide containing ligands where
the nitrogen atom of the pyridine residue coordinates to a
second metal centre are particularly rare.21a,c
Fig. 1. a) The structure of complex [MoO2(L1a)(CH3OH)] (1) with
the atom numbering scheme. Displacement ellipsoids are drawn
at the 30% probability level; b) Dimeric unit formed by
intermolecular hydrogen bonding in complex 1. Hydrogen bonds
are shown as green dashed lines (see Tables S1 and S2 for
selected bond distances and angles).
The molybdenum atom in these complexes has distorted
octahedral geometry in an NO5 donor environment with the
nitrogen and two oxygen atoms provided by the Schiff base
ligand, two oxygen atoms from oxido ligands and one oxygen
atom from a coordinated methanol molecule. The O,N,O donor
atoms from the Schiff base ligands together with the oxido ligand,
O2, form the basal plane of the pseudo-octahedral coordination
environment, while the oxygen atom of methanol molecule and
second oxido ligand O1 are located in the axial positions. The
oxido ligands, O1, O2, are mutually cis. The short Mo–Ooxido
distance (≈1.69 Å) indicates the presence of a molybdenum–
oxygen double bond which is commonly found in cis-[MoO2]2+
complexes.26 The Mo–Omethanol bond length is longer than the
other Mo–O bond distances, which can be attributed to the trans
influence of the oxido group. The equatorial MoNO3 unit is far
from planar. The deviation of the molybdenum atom from the
mean plane defined by the independent hydrazone Schiff base
nitrogen atom N1 and oxygen atoms O3, O4, and the oxygen
atom O2 is 0.337(6), 0.305(8), 0.351(3), 0.320(1) and 0.331(4) Å
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10.1002/ejic.201601359
European Journal of Inorganic Chemistry
FULL PAPER
Table 1 Comparison of the catalytic activities of complexes 1-9
in the oxidation of cyclooctene with hydrogen peroxide in
different conditions.
Entry Cat.
H2O2 Solvent T(oC)
Yield(%) TON*
mmol
1
2
3
4
5
6
7
8
-
3
-
1
2
2.5
3
4
CH3CN 60
CH3CN 60
CH3CN 60
CH3CN 60
CH3CN 60
CH3CN 60
CH3CN 60
CH3CN 25
CH3CN 40
CH3CN 70
CH3CN 80
0
0
-
-
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
1
3
4
5
6
7
8
9
45
77
83
83
80
23
51
89
88
8
225
385
415
415
400
115
255
445
440
40
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
DMF
70
70
70
70
DMSO
CH3Cl
CH2Cl2
6
30
47
41
69
72
64
94
90
96
92
93
89
84
88
235
205
345
360
320
470
450
480
460
415
445
420
440
CH3OH 70
EtOH
THF
70
70
CH3CN 70
CH3CN 70
CH3CN 70
CH3CN 70
CH3CN 70
CH3CN 70
CH3CN 70
CH3CN 70
Fig. 2. a) The structure of compound [MoO2(L2a)]n (4) with the
atom numbering scheme. Displacement ellipsoids are drawn at
the 30% probability level; one-dimensional polymeric chain of
compound 4 shown in b) ball and stick mode and c) space-filling
mode (see Tables S1 and S2 for selected bond distances and
angles).
Reaction conditions: catalyst, 2 μmol; cyclooctene, 1 mmol;
solvent, 3 mL; time, 4 h. * TON = (mmol of epoxide)/(mmol of
catalyst).
Control reactions indicated that the catalytic behavior of
complexes 4 and 6 which have polymeric structures is quite
similar to that of mononuclear complexes and there is no
significant difference in their catalytic behaviors. The catalytic
oxidation of cyclooctene by H2O2 in the presence of complexes
1-9 indicates that cyclooctene converts to the corresponding
epoxide with 100% selectivity at 60 °C. The effect of oxidant
concentration on the oxidation reactions by 2 at 60 °C is
illustrated in Fig. 3 and Table 1 (entries 3-7).
With other conditions such as solvent, temperature and the ratio
of substrate to catalyst kept constant, different oxidant/substrate
molar ratios (1:1, 2:1, 2.5:1, 3:1 and 4:1) were investigated. As
shown in Table 1, the conversion was increased with increasing
the hydrogen peroxide:cyclooctene molar ratios from 1:1 to
2.5:1; further increasing of the amount of H2O2 did not improve
the conversion. At H2O2:cyclooctene molar ratio of 1:1, 2:1 and
2.5:1, after 4 h the level of conversion reached 45%, 77% and
Catalytic studies
The catalytic oxidation of cyclooctene as a representative
substrate for olefins and methyl phenyl sulfide (thioanisole) as a
representative substrate for sulfides with hydrogen peroxide was
studied in the presence of Mo(VI) complexes. Aqueous solution
of hydrogen peroxide (30 wt%) was selected as cheap and
environmental benign oxidant in these reactions. The results of
control experiments revealed that the presence of both catalyst
and oxidant (H2O2) are essential for the oxidation reactions.
Preliminary tests indicated that the reactivity of thioanisole is
very high in comparison to cyclooctene, so the reactions for
thioanisole were carried out under different conditions from
those for cyclooctene. Results of the studies for cyclooctene and
thioanisole are summarized in Tables 1 and 2, respectively.
Some reaction conditions which can be changed to achieve the
maximum oxidation of substrates, such as the oxidant
concentration (moles of oxidant per moles of substrate),
temperature and solvent of the reaction were investigated. Since
the active core of complexes have the same geometry, complex
2 was used to establish suitable reaction conditions.
83%,
respectively.
Increasing
the
hydrogen
peroxide:cyclooctene ratio to 3:1 did not result in any further
increase in conversion. The degree of conversion decreased
with a ratio of 4:1. According to these plots, a 2.5:1 ratio of
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10.1002/ejic.201601359
European Journal of Inorganic Chemistry
FULL PAPER
H2O2:cyclooctene was selected as the optimal value to achieve
the highest yields of epoxide.
various temperatures including room temperature (25 oC), 40, 60,
70 and 80 °C (see Fig. 4). The results of these studies show that
the catalytic activity of complex 2 increases with increasing
temperature up to 70 °C but a further increase to 80 °C does not
improve the conversion.
The catalytic activity of 1-9 in the epoxidation of cyclooctene was
compared under the optimized conditions for catalyst
2
(H2O2:substrate molar ratio 2.5:1, acetonitrile, reaction
=
temperature 70 °C) and showed little variation in their catalytic
activity. All complexes are able to convert cyclooctene to
cycloctene oxide with 100% yield and selectivity after 5 hours.
However, by focusing on the results of catalytic reactions after 4
hours it is clearly seen that the complexes with electron-
withdrawing groups show higher catalytic activity. This finding
indicates that the substituents on ligands can affect the activity
of these complexes. The comparison of the catalytic activity of
complexes with same salicylidine moiety but different group
connected to hydrazone group, {(1, 4, 7), (2, 5, 8) or (3, 6, 9)},
shows that the catalytic activity of complexes with pyridine ring
(4-6) is a little higher than complexes with 2-naphtholic (1-3) or
furan (7-9) rings.
Fig. 3. Effect of H2O2 concentration on the oxidation of
cyclooctene by 2. Reaction conditions: catalyst, complex 2, 2
μmol; CH3CN, 3 mL; cyclooctene, 1 mmol; temperature, 60±2 °C.
Cyclooctene conversions (%) are reported with error bars.
Due to considerable effects of solvents in the homogeneous
catalytic systems, the influence of different solvents (including
acetonitrile, DMF, DMSO, methanol, ethanol, dichloromethane,
chloroform and THF) was studied in the catalytic oxidation of
cyclooctene in the presence of complex 2 and H2O2. Table 1
illustrates the influence of the solvent on the catalytic oxidation
of cyclooctene. Acetonitrile was found to be the most suitable
solvent for the catalytic system being studied, with the highest
conversion of cyclooctene, 83%, obtained after 4 hours in
acetonitrile at 60 °C. In DMF and DMSO the reaction was very
slow and the maximum conversion of cyclootene in these
solvents was below 10% after 4 hours; this can be attributed to
the higher donor number and coordinating ability to the metal
core of these solvents. Similar observation has been seen in
methanol and ethanol whose their coordinating ability is higher
than acetonitrile but is lower than DMF and DMSO.
Dichloromethane and chloroform have low donor properties but
the activity of complex 2 in these solvents also is lower than in
acetonitrile. This finding can be attributed to the fact that the
polarities of these solvents are low and also they are immiscible
with hydrogen peroxide. In addition the solubilities of
dioxidomolybdenum complexes in these solvents are low in
comparison with other solvents. In general, it is seen that the
solvents with moderate donation ability and high polarities
seemed to favour the oxidation reaction being studied. Solvents
with high coordinating properties and solvents with low polarities
are therefore not suitable solvents for this reaction.
Fig. 4. Effect of the reaction temperature on the oxidation of
cyclooctene by 2. Reaction conditions: catalyst, complex 2, 2
μmol; CH3CN, 3 mL; cyclooctene, 1 mmol; H2O2, 2.5 mmol.
Cyclooctene conversions (%) are reported with error bars.
This finding indicates that electron-withdrawing group connected
to the −C=(O)−NH−N=C moiety increases the catalytic activity of
dioxidomolybdenum-hydrazone
complexes.
Moreover,
comparing the catalytic reactivity of complexes with the same
ring connected to the hydrazone moiety but with various
salicylidine part confirms this finding. Complexes with the
electron-donating −OCH3 substituent have relatively lower
catalytic activity than complexes with an electron withdrawing −I
group. This finding supports the conclusion that a nucleophilic
attack by hydrogen peroxide on the dioxidomolybdenum center
forms the catalytically active species. Conversely electron-
withdrawing groups increase the formation speed of such
intermediates. Thus, it is predict that the dioxidomolybdenum-
Temperature can be considered as one of the other most
important factors in the activity of various catalytic systems.
Many biological and industrial catalytic systems are sensitive to
temperature and show higher activity in particular temperature
ranges. Thus, the oxidation of cyclooctene was also studied at
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10.1002/ejic.201601359
European Journal of Inorganic Chemistry
FULL PAPER
hydrazone complexes with stronger electron-withdrawing groups
will show even higher levels of catalytic activity.
oxidation of thioanisole in the presence of complex 2, the
oxidation of thioanisole was studied in the presence of other
oxidomolybdenum(VI) complexes (1, 3-9). The results are
collected in Table 2 and indicate these complexes have quite
similar catalytic activity in the oxidation of thioanisole and can be
considered as selective catalysts for the oxidation of sulfide to
sulfoxide at room temperature.
Since sulfoxides are useful building blocks in organic synthesis,6
selective oxidation of sulfides to sulfoxides is an other important
transformations in organic chemistry. Oxidation of sulfides
occurs under milder conditions than olefins and a wide range of
transition metal complexes can catalyze this reaction in the
presence of a suitable oxidant. However, the oxidation of
sulfides is very low in the absence of catalyst.28 In order to study
the catalytic properties of complexes 1-9 in the oxidation of
sulfides, the oxidation of thioanisole was studied under various
conditions in the presence of complex 2. The results (Table 2)
show that thioanisole is converted to the corresponding sulfoxide
with 100% selectivity by these complexes at room temperature
(25 oC).
Although the details of catalytic mechanism are not studied in
the present work, by considering the previous studies on similar
systems18,29 and also on the basis of the electronic absorption
spectroscopy studies of complex 2 (see Fig. S8), it is predicted
that the key step in this catalytic process is the oxidation of
substrates by oxidoperoxidomolybdenum-hydrazone species.
Electronic absorption spectroscopy indicates that the solution of
complex 2 in methanol is sensitive towards addition of H2O2. By
increasing H2O2 to the methanolic solution of complex 2, the
intensity of absorption bands at about 220, 290 and 330 nm
increases. Moreover, the shoulder at about 400 nm disappears
by addition of H2O2. These changes can be attributed to the
formation of oxidoperoxidomolybdenum species.18b,29,30 This is in
agreement with the hypothesis that electron-withdrawing
substituents on the hydrazone ligands might effect the formation
of such species and consequently the overall catalytic activity. In
comparison with the previously reported articles based on
catalytic oxidation of olefins and sulfides by Mo(VI)
complexes18,29 the catalytic activities of complexes reported in
this study are in the normal range expected for Mo(VI)
complexes.
Table 2 Comparison of the catalytic activities of complexes 1-9
in the oxidation of thioanisole with hydrogen peroxide in different
conditions.
Entry
Cat.
H2O2
T (oC)
Yield(%); Sulfoxide selectivity
mmol
10
20
30
40
min
min
min
min
1
-
3
0
1
2
2.5
3
4
2
2
2
2
2
2
2
2
2
2
2
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
40
60
70
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
0; 0
2; 100
0; 0
3; 100
5; 100
2
2
2
2
2
2
2
2
2
2
1
3
4
5
6
7
8
9
0; 0
0; 0
0; 0
3
26; 100
39; 100
45; 100
46; 95
43; 84
51; 80
58; 75
63; 64
41; 100
40; 100
45; 100
42; 100
39; 100
37; 100
31; 100
36; 100
38; 100
69; 100
65; 91
61; 86
66; 79
78; 74
90; 61
100; 58
78; 100
71; 100
82; 100
73; 100
77; 100
52; 100
54; 100
58; 100
52; 100
100; 100
89; 83
68; 100
4
-
5
100; 77
6
87; 78
100; 71
7
100; 60
100; 57
100; 45
100; 40
100; 100
100; 100
100; 100
100; 100
100; 100
86; 100
82; 100
87; 100
-
Conclusions
8
-
9
-
In this work nine dioxidomolybdenum(VI) complexes of tridentate
Schiff base ligands were synthesized and characterized by
spectroscopic methods and single crystal X-ray diffraction.
Spectroscopic and X-ray analyses indicated that the ligands are
coordinated to the metal core as dinegative ligands by
deporotonation through phenolic OH and amidic NH groups.
These complexes were used as catalysts in the oxidation of
cyclooctene and thioanisole in the presence of hydrogen
peroxide as environmental benign oxidant. The effects of
parameters such as solvent, temperature and the molar ratio of
oxidant to substrate were studied. These complexes are
effective and selective catalysts for oxidation of cyclooctene and
thioanisole and our results indicate that electron-withdrawing
substituents on the ligands increase the catalytic activity of
dioxidomolybdenum(VI) hydrazone complexes as they can
facilitate the nucleophilic attack of hydrogen peroxide to the
metal centre during the reaction.
10
11
12
13
14
15
16
17
18
-
-
-
-
-
-
100; 100
100; 100
100; 100
Reaction conditions: catalyst, 2 μmol; thioanisole, 1 mmol; acetonitrile 3
mL, r.t. = 25 oC.
Higher temperature increased the conversion of sulfide in a
shorter time of reaction. Nevertheless, the selectivity of sulfoxide
decreased when the reaction was carried out at higher
temperatures (40, 60 and 70 °C) (Table 2). At high temperatures
methylsulfonylbenzene (sulfone) was obtained as a byproduct
by further oxidation of methylsulfinylbenzene (sulfoxide). As
shown in Table 2, at room temperature the conversion of
thioanisole increases considerably as the molar ratio of H2O2 to
thioanisole is raised from 1.0 to 2, but a further increase
decreases the selectivity towards sulfoxide. This shows that the
higher amount of H2O2 facilitates the over-oxidation of sulfoxide
to sulfone. The effect of solvent in the oxidation of thioanisole
was very similar to the solvent effect in the oxidation of
cyclooctene. By considering the results obtained from the
Experimental Section
Materials and instrumentations
All chemicals were purchased from Aldrich and used as received.
Solvents of the highest grade commercially available (Merck)
were used without further purification. IR spectra were recorded
as KBr discs with a Bruker FT-IR spectrophotometer. UV-vis
solution spectra were recorded using a thermo-spectronic Helios
Alpha spectrometer. The elemental analyses (CHN) were
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10.1002/ejic.201601359
European Journal of Inorganic Chemistry
FULL PAPER
obtained using a Carlo ERBA Model EA 1108 analyzer. Atomic
absorption analyses were carried out using Varian Spectra AA
220 equipment. 1H and 13C NMR spectra of the ligands and
complexes in DMSO-d6 solution were recorded on a Bruker
spectrometer and chemical shifts were indicated in ppm relative
to tetramethylsilane (TMS). The products of oxidation reactions
were determined and analyzed using an HP Agilent 6890 gas
chromatograph equipped with a HP-5 capillary column (phenyl
methyl siloxane 30 m × 320 m × 0.25 m) and gas
chromatograph-mass spectrometry using an Hewlett-Packard
5973 Series MS-HP gas chromatograph with a mass-selective
detector.
solved by direct methods with SHELXS33 and refined by a full-
matrix least-squares technique on F2 using SHELXL-9733 with
anisotropic displacement parameters for all ordered non-H
atoms. The H atoms were found in the difference Fourier maps,
but in the final refinement cycles they were repositioned in their
calculated positions and refined using a riding model, with C–H =
0.95 Å (for aromatic CH), 0.98 Å (for CH3), and with Uiso(H) =
1.2Ueq(C) (for CH groups) and 1.5Ueq(C (for CH3 groups). The O-
bonded hydrogen atoms were located from difference maps and
treated with Uiso(H) = 1.5Ueq(O). The structure plots were
prepared with DIAMOND.34
CCDC 1501045-1501050 and 1501051 contain the
supplementary crystallographic data for complexes 1–6 and 9,
respectively. These data can be obtained free of charge from
Synthesis of the ligands. All ligands were prepared in a similar
manner by refluxing an equimolar mixture of the appropriate
aromatic acid hydrazide (3-hydroxy-2-naphthoic acid hydrazide,
4-pyridine carboxylic acid hydrazide or 2-furane carboxylic acid
hydrazide) and the ortho-hydroxy aldehyde derivative (5-iodo-2-
hydroxybenzaldehyde, 2-hydroxy-1-naphthaldehyde or 2-
hydroxy-3-methoxybenzaldehyde) in methanol (20 mL) for 3-6 h.
The solvent volume was then reduced to 5 mL by evaporation
on a steam bath and cooled to room temperature. The solids
obtained were separated and filtered off, washed with cooled
methanol (5 mL) and then dried in air. Completeness of the
reactions was checked by TLC on silica gel plates. Analytical
characterization of the ligands is reported in the ESI.†
The
Cambridge
Crystallographic
Data
Centre
via
www.ccdc.cam.ac.uk/data_request/cif, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ,
UK;
fax:
(+44)
1223-336-033;
or
e-mail:
deposit@ccdc.cam.ac.uk.
Catalytic studies. Liquid phase catalytic oxidations were carried
out under air (atmospheric pressure) in a 25 mL round bottom
flask equipped with a magnetic stirrer. In a typical experiment,
H2O2 (30 wt% aqueous solution) was added to a flask containing
the complex (2 × 10-3 mmol) and substrate (1 mmol) in a solvent
(3 mL). The course of the reaction was monitored using a gas
chromatograph equipped with a capillary column and a flame
ionization detector. The oxidation products were identified by
comparing their retention times with those of authentic reference
samples or by 1H NMR and GC–MS analyses. Control reactions
were carried out in the absence of catalyst or oxidant, under the
same conditions as the catalytic runs. Each experiment was
repeated three times and the reported conversions (%) with
error bars (standard deviations, Figs. 3 and 4) are the average
of the results obtained in the repeated experiments.
Synthesis of the complexes. The complexes were synthesized
by the reaction of 1.0 mmol of the appropriate ligand [H2Lna-c (n =
1, 2, 3)] and 1.0 mmol (0.144 g) of MoO3 using thermal gradient
method in a branched tube.31 General method: The appropriate
amount of materials was placed into the main arm of a branched
tube. Methanol was carefully added to fill the arms, the tube was
sealed and the reagents containing arm was immersed in an oil
bath at 60 C while the other arm was kept at ambient
temperature. The complexes were produced in the hot arm
during a week and slowly collected into the cooler arm. The
complexes were collected at cooler arm as high quality single
crystal (in case of complexes 1, 2, 3, 4, 5, 6 and 9) or pure low
quality crystals (in 7 and 8). After a week, the resulting
complexes were separated, washed with cooled absolute
ethanol and dried at room temperature. Analytical
characterization of the complexes is reported in the ESI.†
Acknowledgments
The authors are grateful to the University of Zanjan and the
Università degli Studi di Cagliari for financial support of this
study. We also thank EPSRC (UK) for funding.
X-ray crystallography. A summary of the crystal data and
refinement details for the compounds discussed in this paper are
given in Table S3. Only special features of the analyses are
mentioned here. Single crystal data collection was performed on
a Bruker SMART APEX CCD area detector for 2, 4, 5 and 6 and
on KUMA-KM4, Xcalibur R (RUBY detector) and Xcalibur PX
(ONYX detector) diffractometers for 1, 3 and 9, respectively
which were equipped with an Oxford Cryosystems open-flow
nitrogen cryostat, using  scans and a graphite-monochromated
Mo K ( = 0.71073 Å) radiation. The data were corrected for
Lorentz-polarization effects as well as for absorption. Data
collection, cell refinement, data reduction, analysis and
absorption correction were carried out using either Bruker
SMART32a or CryssAlisPro software.32b The structures were
Appendix
Electronic ESI (ESI) available: Additional information as noted in
the text including synthetic details for the preparation of ligands
H2Lna-c (n = 1, 2, 3) and complexes 1-9, Selected bond lengths
(Å) and angles (deg) in the crystal structure of complexes 1-6,
and 9 (Table S1), parameters of hydrogen bonding geometry
( Table S2), crystal data for complexes 1-6, and 9 (Table S3),
molecular structures of complexes 2, 3, 5 and 9, view of the
crystal structure for complexes 2-6, 9 (Figs. S1-S7) See DOI: .
Keywords: Crystal structure
• Catalytic studies • Mo(VI)
complexes • Epoxidation • Hydrazone ligands •
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10.1002/ejic.201601359
European Journal of Inorganic Chemistry
FULL PAPER
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Entry for the Table of Contents
Layout 1:
FULL PAPER
Text for Table of Contents
Author(s), Corresponding Author(s)*
Page No. – Page No.
Title
Layout 2:
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Rahman Bikas,[a,*] Vito Lippolis,[b,*] Nader
Noshiranzadeh,[a] Hossein Farzaneh-
Bonab,[a] Alexander J. Blake,[c] Milosz
Siczek,[d] Hassan Hosseini-Monfared,[a] and
Tadeusz Lis[d]
Page No. – Page No.
Electronic effects of aromatic rings
on the catalytic activity of
dioxidomolybdenum(VI)-hydrazone
complexes
Text for Table of Contents
The catalytic activity of dioxidomolybdenum(VI) hydrazone complexes in the
oxidation of cyclooctene and thioanisole in the presence of H2O2 is significantly
affected by the nature of the aromatic substituents in the hydrazine ligands.
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