Synthesis, crystal structures, and catalytic oxidation of olefins of dioxomolybdenum(VI) complexes with aroylhydrazone ligands

Article abstract of DOI:10.1134/S1070328417030058

Two Mo(VI) aroylhydrazone complexes, cis-[MoO₂(L¹)(CH₃OH)] (I) and cis-[MoO₂(L²)(CH₃OH)] (II), derived from 2-bromo-N'-(3,5-dibromo-2-hydroxybenzylidene)benzohydrazide (H₂L¹

Full text of DOI:10.1134/S1070328417030058

ISSN 1070-3284, Russian Journal of Coordination Chemistry, 2017, Vol. 43, No. 3, pp. 189–195. © Pleiades Publishing, Ltd., 2017.
Synthesis, Crystal Structures, and Catalytic Oxidation of Olefins
of Dioxomolybdenum(VI) Complexes with Aroylhydrazone Ligands1
D. L. Penga, b
aKey Laboratory of Surface and Interface Science of Henan Province, School of Material and Chemical Engineering,
Zhengzhou University of Light Industry, Zhengzhou, 450001 P.R. China
bHenan Collaborative Innovation Center of Environmental Pollution Control and Ecological Restoration,
Zhengzhou University of Light Industry, Zhengzhou, 450001 P.R. China
e-mail: pengdonglai2015@sina.com
Received January 3, 2016
Abstract⎯Two Mo(VI) aroylhydrazone complexes, cis-[MoO2(L1)(CH3OH)] (I) and cis-
[MoO2(L2)(CH3OH)] (II), derived from 2-bromo-N'-(3,5-dibromo-2-hydroxybenzylidene)benzohydrazide
(H2L1) and 2-bromo-N'-(2-hydroxy-4-methoxybenzylidene)benzohydrazide (H2L2), respectively, are
reported. The complexes were characterized by elemental analyses, infrared and electronic spectroscopy, and
single crystal structure analysis (CIF files CCDC nos. 1443679 (I) and 1443678 (II)). The Mo atoms are
coordinated by two cis terminal oxygen, ONO from the aroylhydrazone ligand, and methanol oxygen. Com-
plex I crystallized as monoclinic space group P21/c with unit cell dimensions a = 8.075(2), b = 13.905(1), c =
16.448(1) Å, β = 91.282(2)°, V = 1846.5(4) Å3, Z = 4, R1 = 0.0859, wR2 = 0.2066. Complex II crystallized as
triclinic space group P , with unit cell dimensions a = 8.0824(6), b = 10.5919(8), c = 10.7697(8), α =
1
96.432(2)°, β = 97.438(2)°, γ = 103.119(2)°, V = 880.8(1) Å3, Z = 2, R1 = 0.0271, wR2 = 0.0571. The complexes
were tested as catalyst for the oxidation of olefins and showed effective activity.
Keywords: cis-dioxomolybdenum(VI) complexes, aroylhydrazone ligands, catalysis, oxidation, olefins
DOI: 10.1134/S1070328417030058
INTRODUCTION
tures and catalytic activity of Mo(VI) hydrazone com-
plexes, cis-[MoO2(L1)(CH3OH)] (I) and cis-
[MoO2(L2)(CH3OH)] (II), derived from 2- bromo-N'-
(3,5-dibromo-2-hydroxybenzylidene)benzohydrazide
(H2L1) and 2-bromo-N'-(2-hydroxy-4-methoxyben-
zylidene)benzohydrazide (H2L2), respectively, are
reported.
Molybdenum is an essential metal that is capable of
forming metal complexes with various ligands [1–5].
Molybdenum is not only much less toxic than many
other metals of industrial importance but it is also an
essential constituent of certain enzymes that catalyze
reduction of molecular nitrogen and nitrate in plants
and oxidation (hydroxylation) of xanthine and other
purines and aldehydes in animals [6]. Epoxides are very
important chemicals for manufacturing a range of
important commercial products such as pharmaceuti-
cals and polymers. Many transition metal complexes
have been reported for oxidation of olefins [7–13].
Molybdenum complexes are known for considerable
use in organic chemistry, in particular for the various
oxidations of organic compounds [2, 4, 14]. Oxido-per-
oxido molybdenum complexes have been intensively
investigated as oxidation catalysts for variety of organic
substrates, particularly for sulfoxidation and epoxida-
tion of olefins [15–18]. In order to further study the cat-
alytic potentiality of the oxido–peroxido complexes of
molybdenum, in this paper, the synthesis, crystal struc-
EXPERIMENTAL
Materials and methods. All the reagents and sol-
vents used in the synthesis were procured commer-
cially and used without subsequent purification. The
starting material, bis(acetylacetonato) dioxomolybde-
num(VI), [MoO2(Acac)2] was prepared as described
in the literature [19]. Microanalyses (C, H, N) were
performed using a Perkin-Elmer 2400 elemental ana-
lyzer. Infrared spectra were carried out using the
JASCO FT-IR model 420 spectrophotometer with
KBr disk in the region 4000–400 cm–1. Electronic
absorption spectra measurement in acetonitrile was
1
1
The article is published in the original.
measured using a Labda 900 spectrometer. H NMR
189

190
PENG
Table 1. Crystallographic data and refinement parameters for complexes I, II
Value
Parameter
I
II
Formula weight
Crystal size, mm
Temperature, K
Crystal system
Space group
634.93
0.17 × 0.15 × 0.13
298(2)
507.15
0.30 × 0.27 × 0.27
298(2)
Monoclinic
P21/c
Triclinic
P
1
a, Å
8.075(2)
13.905(1)
16.448(1)
90
8.0824(6)
10.5919(8)
10.7697(8)
96.432(2)
97.438(2)
103.119(2)
880.8(1)
b, Å
c, Å
α, deg
β, deg
γ, deg
91.282(2)
90
V, Å3
1846.5(4)
Z
4
2
calcd, g cm–3
2.284
1.912
ρ
μ(MoKα), mm–1
7.231
3.049
F(000)
1208
12439
2942
500
7602
3213
Number of measured reflections
Number of observed reflections
with I > 2σ(I)
Unique reflections
Parameters
2349
239
2770
240
Number of restraints
R1, wR2 (I > 2σ(I))*
R1, wR2 (all data)*
13
1
0.0859, 0.2066
0.1078, 0.2200
1.055
0.0271, 0.0571
0.0361, 0.0608
1.067
Goodness of fit of F 2
2
2
2
2 1/2
2
* R = ∑||F | – |F ||/∑|F |, wR = [∑w(
–
) /∑w( ) ]
Fc Fo
.
Fo
1
o
c
o
2
IR (KBr; νmax, cm–1): 3451 br.w, ν(OH), 1612 m
ν(C=N), 949 s and 853 s ν(Mo=O). Absorption spec-
trum in acetonitrile (λmax, nm (ε, L mol–1 cm–1)): 300
(11300), 325 (12770), 410 (2123).
spectra were measured in DMSO-d6 on Bruker
300 MHz spectrometer.
Synthesis of I. 3,5-Dibromo-2-hydroxybenzalde-
hyde (0.280 g, 1.0 mmol) and 2-bromobenzohydra-
zide (0.215 g, 1.0 mmol) were dissolved and mixed in
20 mL methanol. Then, MoO2(Acac)2 (0.328 g,
1.0 mmol) dissolved in 20 mL methanol was poured
into the above mixture. The final mixture was stirred at
room temperature for 30 min to give yellow solution.
After leaving the solution for a few days at room tem-
perature, fine orange crystals were formed. The prod-
uct was filtered and washed with methanol. The yield
was 73%.
For C15H11N2O5Br3Mo
anal. calcd., %:
Found, %:
C, 28.38;
C, 28.12;
H, 1.75;
H, 1.86;
N, 4.41.
N, 4.53.
Synthesis of II. 2-Hydroxy-4-methoxybenzalde-
hyde (0.152 g, 1.0 mmol) and 2-bromobenzohydrazide
(0.215 g, 1.0 mmol) were dissolved and mixed in
20 mL methanol. Then, MoO2(Acac)2 (0.328 g,
1.0 mmol) dissolved in 20 mL methanol was poured
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SYNTHESIS, CRYSTAL STRUCTURES
191
into the above mixture. The final mixture was stirred at O–H distances restrained to 0.85(1) Å. The remaining
room temperature for 30 min, to give yellow solution. H atoms were calculated at idealized positions and
After leaving the solution for a few days at room tem- refined with the riding models. Crystallographic data
perature, fine orange crystals were formed. The prod- for the compounds are summarized in Table 1.
uct was filtered and washed with methanol. The yield Selected bond lengths and angles are listed in Table 2.
was 73%.
Supplementary material for structures has been
IR (KBr; νmax, cm–1): 3467 br.w ν(OH), 1608 m
deposited with the Cambridge Crystallographic Data
ν(C=N), 950 s and 859 s ν(Mo=O). Absorption spec-
trum in acetonitrile (λmax, nm (ε, L mol–1 cm–1)): 297
(12178), 320 (13040), 400 (2050).
Centre (CCDC nos. 1443679 (I) and 1443678 (II);
deposit@ccdc.cam.ac.uk
cam.ac.uk).
or
http://www.ccdc.
Catalytic epoxidation of olefins. In a typical cata-
lytic experiment, TBHP (1 mmol) was added to a
solution of olefin (1 mmol), chlorobenzene
(1 mmol) as an internal standard and the complexes
(2 × 10–4 mmol) in 1,2-dichloroethane (0.5 mL). The
mixture was stirred at 80°C under air and the course of
the reaction was monitored using gas chromatography
(Agilent Technologies Instruments 6890N, equipped
with a capillary column (19019J-413 HP-5, 5% Phenyl
Methyl Siloxane, Capillary 60.0 m × 250 μm ×
1.00 μm) and a flame ionization detector). Assign-
ments of products were made by comparison with
authentic samples. All the reactions were run two
times.
For C16H15N2O6BrMo
anal. calcd., %:
Found, %:
C, 37.89;
C, 37.70;
H, 2.98;
H, 3.07;
N, 5.52.
N, 5.45.
X-ray structure determination. X-ray data for the
compounds were collected on a Bruker SMART
APEXII diffractometer equipped with graphite-
monochromated MoKα radiation (λ = 0.71073 Å). A
preliminary orientation matrix and cell parameters
were determined from three sets of ω scans at different
starting angles. Data frames were obtained at scan
intervals of 0.5° with an exposure time of 10 s frame–1.
The reflection data were corrected for Lorentz and
polarization factors. Absorption corrections were car-
ried out using SADABS [20]. The structures were
solved by direct methods and refined by full-matrix
least-squares analysis using anisotropic thermal
RESULTS AND DISCUSSION
The aroylhydrazone ligands were reacted with
parameters for non-H atoms with the SHELXTL pro- bis(acetylacetonato)dioxomolybdenum(VI) in metha-
gram [21]. The water H atoms were located from dif- nol solution, generating fine crystalline complexes, as
ference Fourier maps and refined isotropically, with shown in scheme:
H
N
N
Br
Br
N
N
+ MoO2(Acac)2
O
Br
O
Br
OH
O
Mo
O
OMe
O
Br
Br
H
(I)
H
N
N
N
N
+ MoO2(Acac)2
O
Br
O
OMe
Br
MeO
OH
MeO
O
Mo
O
O
H
(II)
Scheme.
Crystals of the complexes are stabilized in air at
room temperature, and soluble in methanol, ethanol, are shown in Fig. 1. Structures of the dioxomolybde-
and acetonitrile. num complex molecules of I and II are very similar to
The molecular structures of the complexes I and II
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192
PENG
Table 2. Selected bond lengths (Å) and angles (deg) for complexes I and II
Bond
d, Å
Bond
d, Å
I
Mo(1)–O(1)
1.928(8)
1.683(10)
2.299(9)
Mo(1)–O(2)
2.033(9)
1.730(10)
2.232(10)
Mo(1)–O(3)
Mo(1)–O(5)
Mo(1)–O(4)
Mo(1)–N(1)
II
Mo(1)–O(1)
Mo(1)–O(4)
Mo(1)–O(6)
1.929(2)
2.342(2)
1.693(2)
Mo(1)–O(2)
Mo(1)–O(5)
Mo(1)–N(1)
2.009(2)
1.698(2)
2.237(2)
Angle
ω, deg
Angle
ω, deg
I
O(3)Mo(1)O(4)
O(4)Mo(1)O(1)
O(4)Mo(1)O(2)
O(3)Mo(1)N(1)
O(1)Mo(1)N(1)
O(3)Mo(1)O(5)
O(1)Mo(1)O(5)
N(1)Mo(1)O(5)
106.1(5)
103.3(4)
98.7(4)
94.6(4)
80.9(4)
170.2(4)
84.3(4)
76.0(3)
O(3)Mo(1)O(1)
O(3)Mo(1)O(2)
O(1)Mo(1)O(2)
O(4)Mo(1)N(1)
O(2)Mo(1)N(1)
O(4)Mo(1)O(5)
O(2)Mo(1)O(5)
97.1(5)
95.5(5)
150.5(4)
158.0(4)
71.5(3)
82.9(4)
79.1(4)
II
O(6)Mo(1)O(5)
O(5)Mo(1)O(1)
O(5)Mo(1)O(2)
O(6)Mo(1)N(1)
O(1)Mo(1)N(1)
O(6)Mo(1)O(4)
O(1)Mo(1)O(4)
N(1)Mo(1)O(4)
105.99(11)
102.31(10)
98.74(9)
96.66(10)
81.03(8)
O(6)Mo(1)O(1)
O(6)Mo(1)O(2)
O(1)Mo(1)O(2)
O(5)Mo(1)N(1)
O(2)Mo(1)N(1)
O(5)Mo(1)O(4)
O(2)Mo(1)O(4)
98.74(10)
96.72(10)
149.24(8)
156.17(10)
70.85(8)
81.81(9)
171.63(9)
82.24(9)
75.23(8)
78.81(8)
each other, except for the substituent groups of the with bite angles of 71.5(3)° and 80.9(4)° for I and
aroylhydrazone ligands. In each of the complexes, the 70.85(8)° and 81.03(8)° for II. The dihedral angles
coordination geometry around the Mo atom can be between the two phenyl rings of the aroylhydrazone
described as slightly distorted octahedron, with the ligands are 11.8(3)° for I and 7.3(5)° for II. The dis-
equatorial plane defined by one phenolic O, one placements of the Mo atoms from the equatorial mean
imino N, and one enolic O atoms of the dianionic planes toward the axial oxo atoms are 0.302(2) Å for I
hydrazone ligand, and one oxo O atom, and with the and 0.335(3) Å for II. The aroylhydrazone ligands are
two axial positions occupied by one O atom of a meth- coordinated in their dianionic forms, which are evi-
anol ligand and the other oxo O atom. The aroylhydra- dent from the N–C and O–C bond lengths, indicating
zone ligands coordinate to Mo atoms in meridional the presence of the enolate form of the ligand amide
fashion forming five- and sixmembered chelate rings groups. The Mo–O, Mo–N, and Mo=O bonds are
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SYNTHESIS, CRYSTAL STRUCTURES
(a)
193
Br(2)
C(5)
C(13)
C(14)
C(6)
C(7)
N(2)
C(12)
C(11)
C(4)
C(1)
N(1)
C(9)
O(3)
C(8)
O(2)
O(5)
C(3)
C(2)
O(1)
C(10)
Mo(1)
Br(1)
Br(3)
O(4)
C(15)
(b)
C(14)
C(13)
C(5)
C(6)
C(7)
C(1)
N(2)
N(1)
C(9)
C(12)
C(4)
O(6)
C(8)
C(10)
C(15)
C(11)
O(3)
C(3)
C(2)
O(1)
O(2)
Mo(1)
O(5)
O(4)
Br(1)
C(16)
Fig. 1. Molecular structure of I (a) and II (b) with atom labeling scheme and 30% probability thermal ellipsoids for all non-hydro-
gen atoms.
within normal ranges and are similar to those observed the dioxomolybdenum(VI) moieties. The bands due to
in similar dioxomolybdenum(VI) complexes [22, 23]. the ν(C=O) and ν(NH) of the hydrazones were absent
in the complexes, and new C–O stretches appear at
1155 cm–1 for the complexes. This suggests occurrence
of keto-imine tautomerization of the aroylhydrazone
ligands during coordination. The strong bands indica-
tive of the C=N–N=C groups in the complexes are
shifted to 1612 cm–1 for I and 1608 cm–1 for II. The
new weak peaks observed in the range 400–800 cm–1
may be attributed to the Mo–O and Mo–N vibrations
in the complexes. The IR spectra of the complexes are
similar to each other, indicating the complexes
are similar structures, as evidenced by the single crys-
tal X-ray determination.
In the crystal of I, the complex molecules are
linked by methanol ligands through intermolecular
O–H···N hydrogen bonds to form 1D chains along the
x axis. The chains are further linked through weak
Br···O interactions, to form 2D sheets parallel to the xy
plane (Fig. 2a). In the crystal of II, adjacent complex
molecules are linked by two methanol molecules
through two intermolecular O–H···N hydrogen bonds
to form a dimer (Fig. 2b).
The weak and broad bands in the region 3400–
3500 cm–1 for the complexes are assigned to the
ν(OH) vibrations. The Mo=O stretching modes occur
as a pair of sharp strong bands at about 950 and
Considering the structures of two complexes are
855 cm–1 for the complexes, which are assigned to the similar, the present work used complex I as catalyst to
anti-symmetric and symmetric stretching modes of explore suitable experimental conditions. The cata-
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194
PENG
lytic property of complex I was first studied in the
(a)
homogeneous epoxidation of cyclohexene, used as a
model substrate, and tert-butyl hydrogen peroxide
(TBHP) as the oxygen donor. The influence of several
solvents was evaluated (Table 3). It is clear that toluene
and 1,2-dichloroethane produce the highest yields and
the catalytic activity is the best when 1,2-dichlo-
roethane was used as the solvent. At 80°C with 1,2-
dichloroethane, a turnover number (TON) of 5210
with the complex can be achieved. So, 1,2-dichlo-
roethane can be used as solvent in the catalyst experi-
ment.
z
0
y
x
The catalytic epoxidation of some olefins using the
complexes as catalyst is summarized in Table 4. In
general, both complexes show similar catalytic proper-
ties. It is clear that the epoxide yields and selectivity
are good for cyclohexene (entries 1 and 2). However,
the catalytic property decreased for styrene, p-chloro-
styrene, p-bromostyrene, and p-methylstyrene, and
substituted styrene. The electronic effects may play
key roles in the catalytic process. The catalytic prop-
erty of p-fluorostyrene, p-chlorostyrene and p-bro-
mostyrene is better than styrene (entries 3–10). The
complexes were highly selective for the epoxide for
cyclohexene, but lower for various styrenes with benz-
aldehyde and acetophenone derivatives as side prod-
ucts.
(b)
z
y
0
Thus two new structurally similar Mo(VI) aroylhy-
drazone complexes have been synthesized and charac-
terized by X-ray structure analysis and spectroscopy
methods. The aroylhydrazone ligands coordinate to
the Mo atoms through the phenolic O, imino N and
ethanolic O atoms. The Mo atoms in the complexes
are coordinated by aroylhydrazone and methanol
ligands, as well as two oxo groups, forming octahedral
coordination. The complexes were employed as cata-
lyst for the epoxidation of various olefins with tert-
butylhydroperoxide in dichloroethane. The complexes
can catalyze cyclohexene to its epoxide with TBHP as
the oxidant, with high yield and selectivity.
x
Fig. 2. Molacular packing diagrams of I viewed along the
z axis (a) and II viewed along the x axis (b). Hydrogen
bonds are shown as dashed lines.
Table 3. The solvent effects on the catalytic epoxidation of cyclohexene with TBHP using complex I as the catalyst*
Entry
Solvent
Conversion
Time
TON**
1
2
3
4
5
6
MeOH
EtOH
83
92
1 h
3550
4130
4220
4600
4870
5210
1 h
MeCN
93
1 h
Cyclohexane
Toluene
90
1 h
98
1 h
1,2-Dichloroethane
100
30 min
* The molar ratio for the complex : cyclohexene : TBHP is 1 : 5000 : 5000. The reactions were carried out at 80°C.
** TON = (mmol of product)/mmol of catalyst.
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SYNTHESIS, CRYSTAL STRUCTURES
195
Table 4. Catalytic oxidation results*
Entry
Substrate
Product
Compound Conversion, % Selectivity, %
TON
1
2
I
100
100
100
100
4600
3800
O
II
O
3
4
5
I
II
I
58
55
67
43
38
46
1058
987
O
1220
F
Cl
Br
6
7
II
I
71
73
77
59
62
50
61
53
44
39
1195
1107
1083
930
F
Cl
Br
O
8
II
I
O
9
10
II
915
* The molar ratio for the catalyst : olefin : TBHP is 1 : 5000 : 5000. The reactions were carried out at 80°C for 1 h.
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