Synthesis, characterization, and X-ray structure of n′-(2-hydroxy- 5-methylbenzylidene)-4-trifluoromethylbenzohydrazide and its dioxidomolybdenum(vi) complex with catalytic property

Article abstract of DOI:10.1080/24701556.2016.1241270

A new hydrazone compound N′-(2-hydroxy-5-methylbenzylidene)-4-trifluoromethylbenzohydrazide (H₂L) was prepared. Based on H₂L, a new dioxidomolybdenum(VI) complex [cis-MoO₂L(MeOH)], was obtained. Single crystals of H_2<

Full text of DOI:10.1080/24701556.2016.1241270

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Synthesis,
Characterization
and
X-Ray
Structure
and
of
N’-(2-Hydroxy-5-methylbenzylidene)-4-trifluoromethylbenzohydrazide
Dioxidomolybdenum(VI) Complex with Catalytic Property
Peng Liang, and Zhe Hong*
Its
School of Chemical Engineering, Eastern Liaoning University, Dandong 118003, P. R. China
*Corresponding author. E-mail: hongzhe57@126.com
Abstract
A
new
hydrazone
compound
N’-(2-hydroxy-5-methylbenzylidene)-4-trifluoromethylbenzohydrazide (H2L) was prepared.
Based on H2L, a new dioxidomolybdenum(VI) complex [cis-MoO2L(MeOH)], was obtained.
Single crystals of H2L and the complex were obtained by slow evaporation of methanolic
solution containing the compounds, and were characterized by elemental analysis, infrared
spectroscopy, and single crystal X-ray diffraction techniques. The geometry around the
molybdenum center is distorted octahedral in which the tridentate benzohydrazone ligand with
two anionic oxygens and one neutral imine nitrogen occupies meridional position. The
octahedral geometry of the cis-dioxomolybdenum center is additionally completed by a
coordinated methanol solvent molecule. The complex was tested for the homogeneous oxidation
of various olefins.
Keywords
molybdenum dioxido complex, synthesis, X-ray structure, catalytic property
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INTRODUCTION
Hydrazones prepared by the condensation of salicylaldehydes with hydrazides are an
important class of versatile ligands.[1-3] Transition metal complexes derived from such ligands
have been widely investigated due to their interesting properties, such as magnetic exchange,[4]
biological activities,[1, 5, 6] as well as catalytic materials.[7, 8] Molybdenum is an important metal
that is capable of forming various complexes with organic ligands,[8-10] and usually show
essential catalytic properties.[11-19] Epoxides are very important and versatile intermediates for
manufacturing a range of important commercial products such as pharmaceuticals and polymers.
In addition, it can be easily transformed to a variety of functional groups.[20] Many transition
metal complexes including molybdenum complexes have been reported for oxidation of olefins.[8,
11, 21-23]
As a continuation of the work, in this report, a new hydrazone compound
N’-(2-hydroxy-5-methylbenzylidene)-4-trifluoromethylbenzohydrazide
dioxidomolybdenum(VI) complex [cis-MoO2L(MeOH)] are reported.
EXPERIMENTAL
(H2L)
and
its
Materials and Methods
5-Methylsalicylaldehyde, 4-trifluoromethylbenzohydrazide, and MoO2(acac)2 of AR grade
were purchased from Aldrich and used as received. CHN elemental analyses were performed
with a Perkin-Elmer 240C elemental analyzer. I.r. spectra (KBr disks) were recorded on a
Perkin-Elmer 257 spectrophotometer. The oxidation products were analyzed with a gas
chromatograph (Shimadzu, GC-14B) equipped with a SAB-5 capillary column (phenyl methyl
1
siloxane 30 mm × 320 mm × 0.25 mm) and a flame ionization detector. H NMR spectrum was
obtained in DMSO-d6 solution with a Bruker 300 MHz spectrometer.
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Preparation of H2L
5-Methylsalicylaldehyde (0.68 g, 5 mmol) and 4-trifluoromethylbenzohydrazide (0.10 g, 5
mmol) were dissolved in 30 mL methanol. The mixture was heated to reflux for 1 h, and cooled
to room temperature. Single crystals suitable for X-ray diffraction were obtained by slow
1
evaporation of the solvent. Yield: 77%; H NMR (DMSO-d6, 300 MHz): δ = 2.34 (s, 3H, CH3),
6.93 (d, 1H, ArH), 7.07 (d, 1H, ArH), 7.49 (s, 1H, ArH), 7.78 (d, 2H), 7.99 (d, 2H), 8.65 (s, 1H,
-CH=N-), 11.27 (s, 1H, NH). IR (KBr pellet): ν(OH) 3366; ν(NH) 3226; ν(C=N) 1657 cm-1. Anal.
calcd. for C17H17F3N2O3: C, 57.63; H, 4.84; N, 7.91. Found: C, 57.51; H, 4.92; N, 7.98%.
Preparation of the Complex
H2L (0.16 g, 5 mmol) and MoO2(acac)2 (1.63 g, 5 mmol) were mixed in methanol (20 mL). the
mixture was stirred at room temperature for 30 min to give a yellow solution. Single crystals
1
suitable for X-ray diffraction were obtained by slow evaporation of the solvent. Yield: 45%; H
NMR (DMSO-d6, 300 MHz): δ = 2.32 (s, 3H, PhCH3), 3.13 (s, 3H, CH3), 6.91 (d, 1H, ArH),
7.13 (d, 1H, ArH), 7.45-7.72 (m, 3H, ArH), 8.13 (d, 2H, ArH), 8.45 (s, 1H, -CH=N-). IR (KBr
pellet): ν(OH) 3460; ν(C=N) 1628 cm-1; ν(MoO2) 951, 860 cm-1. Anal. calcd. for
C17H15F3MoN2O5: C, 42.52; H, 3.15; N, 5.83. Found: C, 42.40; H, 3.32; N, 5.71%.
Crystal Structure Determination
Crystals suitable for X-ray measurement were grown from methanol by slow evaporation
method. The single crystals of H2L and the complex were chosen and glued to thin glass fibers
by epoxy glue in air for data collection. The diffraction data were collected on a Bruker Apex2
CCD with Mo Kα radiation (λ = 0.71073 Å) at 298(2) K using the ω scan method. An empirical
absorption correction was applied. The structures were solved by direct methods and difference
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Fourier synthesis. Crystal data collection, parameters, and refinement statistics for the
compounds are listed in Table 1. All of the non-H atoms were refined anisotropically. The
methanol H atoms were located from electronic density maps and refined with O···H distances
restrained to 0.85(1) Å. All other H atoms of the compounds were included in calculated
positions, and assigned isotropic thermal parameters which were set to ride on the parent atoms.
The CF3 group of H2L is disordered over two sites, with occupancies of 0.602(3) and 0.398(3).
All calculations were performed using the SHELXTL-97.[24]
Catalytic Oxidation Procedure
Catalytic experiments were carried out in a 50 mL glass reaction flask fitted with a water
condenser. In a typical procedure, 0.032 mmol dioxidomolybdenum(VI) complex was dissolved
in 10 mL CH2Cl2. Then 10 mmol alkene was added to the reaction mixture and 30 mmol TBHP
was added. The reaction mixture was refluxed for 1 h. The reaction products were monitored at
periodic time intervals using gas chromatography. The oxidation products were identified by
comparison with authentic samples (retention times in GC).
RESULTS AND DISCUSSION
The hydrazone ligand used for the preparation of the dioxidomolybdenum complex was
derived from condensation of 5-methylsalicylaldehyde and 4-trifluoromethylbenzohydrazide.
The complex was prepared by simple ligand exchange reaction of the hydrazone ligand with
MoO2(acac)2 in methanol (Scheme 1). Single crystals of the compounds were successfully grown
by slow evaporation of methanolic solution at room temperature. The molar conductance of the
complex is 22 Ω-1 cm2 mol-1, indicating the non-electrolytic nature.
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IR Spectra
The compounds were characterized by IR spectroscopy. The free hydrazone showed stretching
bands attributed to C=O, C=N, C–OH and NH at 1657, 1636, 1153 and 1232, and 3317 cm-1,
respectively. The sharp band at 3226 cm-1 for H2L is absent in the spectrum of the complex,
indicating the coordination of the hydrazone ligand through enolate form. The molybdenum
complex showed two prominent bands at 951 and 860 cm-1 attributed to cis-dioxomolybdenum
(MoO2) group.[14] The bands due to νC=O and νNH are absent in the complex, but a new C–O
stretch appeared at 1272 cm-1. This suggest occurrence of keto-imine tautomerization of the
ligand during complexation. The C=N stretching band at 1657 cm-1 in the free hydrazone shifted
to 1628 cm-1 upon coordination to Mo atom. A broad and weak band centered at 3460 cm-1 in the
spectrum of the complex is also appeared, which may be attributed to the coordinate methanol
O–H group.
Crystal Structure Description of H2L and the Complex
The molecular structure of H2L is shown in Fig. 1 and the bond parameters are given in Table
2. There is a hydrazone molecule and a methanol molecule in the compound. The bond distances
and angles are normal.[25] The dihedral angle between the two benzene rings is 138.8(3)°. The
molecule of the compound displays an E geometry about the C=N bond. The hydrazone
molecules are linked by methanol molecules through hydrogen bonds (Table 3) into chains along
a axis (Fig. 2).
The molecular structure of the complex is shown in Fig. 3 and the bond parameters are given
in Table 2. The mononuclear complex has a distorted octahedral geometry with one nitrogen
atom and five oxygen atoms. Three atoms, O(1), N(1) and O(2) (from the tridentate ligand), and
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one terminal oxo atom, O(4), occupy the meridional plane. The Mo(1) atom lies 0.709(2) Å out
from this plane in the direction of the other terminal oxygen atom O(4). Both O(4) and O(3), the
latter from the coordinated methanol molecule, occupy the axial positions forming an
O(4)–Mo(1)–O(3) angle of 170.76(8)° which indicate the distorted octahedral nature of the
donor environment around the Mo(VI) center. The MoO2 moiety has a cis configuration. The
Mo(1)–O(3) bond trans to the terminal oxygen atom O(4) is significantly longer compared to the
length of other molybdenum oxygen bonds, indicating that the methanol molecule is weakly
coordinated to the metal center and can behave as a substrate-binding site. The coordinate bond
parameters are consistent with those reported in the literature.[13, 14] In the crystal, the molecules
are linked via intermolecular hydrogen bonds (Table 3), forming dimers, as shown in Fig. 4.
Catalytic Results
The catalytic property of the complex was investigated in the epoxidation of cyclooctene, as a
model substrate, and tert-butyl hydroperoxide as the oxygen donor. In the absence of catalyst, the
reactions did not proceed even under reflux. The results are presented in Table 4. Oxidation of
4-fluorostyrene, 4-chlorostyrene, 4-methylstyrene, and 4-methoxystyrene gave the corresponding
epoxide as the sole product, while in the oxidation of styrene some benzaldehyde was were
detected as a by-product. From the results, it can be seen that the electron donating group in the
salicylidene ring can decrease the activity of the catalyst, which results from the decreasing
Lewis acidity of the molybdenum center and therefore the deduction of TBHP activation.[11] In
the proposed catalytic cycle (Scheme 2), at first step, TBHP will be activated by coordination to
the molybdenum center and formation of hepta-coordinated molybdenum intermediate I. It
seems that heterolytic cleavage of the TBHP occurs. Therefore catalysts with higher lewis acidity
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character increase the efficiency of the coordinated peroxo group. Then olefin as a nucleophile
will attack to the electrophile oxygen atom of the coordinated TBHP. A comparison of the results
obtained for this work with those reported in the literature which used TBHP as oxidizing agent
reveals that reaction time is lower than other systems and higher conversion for epoxide
formation will be achieved. In addition, the catalytic oxidation property of the complex is
comparable to those reported in the literature.[26,27]
ACKNOWLEDGEMENTS
We acknowledge eastern Liaoning University for funding this study.
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TABLE 1 Crystallographic and experimental data for H2L and the complex
H2L
C17H17F3N2O3
354.33
298(2)
0.71073
Block/colorless
0.15  0.13  0.13
Monoclinic
P21/c
6.7399(5)
35.0990(12)
7.6570(7)
90
108.517(2)
90
1717.6(2)
4
The complex
C17H15F3MoN2O5
480.25
298(2)
0.71073
Block/yellow
0.31  0.29  0.28
Triclinic
Empirical Formula
Formula weight
Temperature/K
Wavelength (MoK) /Å
Crystal shape/colour
Crystal size/mm
Crystal system
Space group
a /Å
P-1
8.0778(5)
9.4758(6)
12.4185(8)
90.1490(10)
92.9880(10)
91.0060(10)
949.11(10)
2
b /Å
c /Å
α /°
 /°
 /°
V /Å3
Z
Calculated density/g cm-3
Absorption
coefficient/mm-1
1.370
0.116
1.680
0.750
Theta range for data 3.04-25.50
collection
2.53-25.50
Index ranges/h, k, l
Tmin
Tmax
Reflections/parameters
Unique reflections
Observed reflections [I 
2(I)]
-8, 8; -42, 34; -8, 9
-9, 9; -9, 11; -15, 14
0.8007
0.8174
4974/258
3479
3251
0.9828
0.9851
8349/261
3021
2350
Restraints
61
1
F(000)
736
1.039
0.0851, 0.1689
0.1065, 0.1831
480
1.080
0.0251, 0.0676
0.0275, 0.0699
0.448, -0.450
Goodness of fit on F2
R1, wR2 [I  2(I)]a
R1, wR2 (all data)a
Largest diff. peak and 0.215, -0.224
hole/e Å-3
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aR1 = Fo – Fc/Fo, wR2 = [w(Fo2 – Fc2)2/w(Fo2)2]1/2.
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TABLE 2 Selected bond lengths (Å) and bond angles (°) for the compounds
H2L
N(1)–N(2)
C(8)–O(2)
The complex
C(7)–N(1)
N(2)–C(8)
1.283(4)
1.342(4)
1.387(4)
1.232(4)
Mo(1)–O1
Mo(1)–O(3)
Mo(1)–O(5)
1.9170(18)
2.3618(17)
1.6930(17)
Mo(1)–O(2)
Mo(1)–O(4)
Mo(1)–N(1)
O(4)–Mo(1)–O(1)
O(4)–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(3)
O(2)–Mo(1)–O(3)
2.0183(17)
1.6928(19)
2.2495(18)
98.68(10)
96.89(9)
149.49(7)
156.78(8)
71.28(7)
O(4)–Mo(1)–O(5) 106.43(10)
O(5)–Mo(1)–O(1) 102.23(8)
O(5)–Mo(1)–O(2) 98.26(8)
O(4)–Mo(1)–N(1) 95.57(8)
O(1)–Mo(1)–N(1) 81.21(7)
O(4)–Mo(1)–O(3) 170.76(8)
O(1)–Mo(1)–O(3) 81.99(7)
N(1)–Mo(1)–O(3) 75.38(6)
82.32(7)
78.60(7)
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TABLE 3 Hydrogen bond distances (Å) and bond angles (°) for the compounds
H2L
O(1)–H(1)∙∙∙N(1)
O(1)–H(1)∙∙∙O(3)i
O(3)–H(3)∙∙∙O(2)ii
N(2)–H(2)∙∙∙O(3)
0.82
2.04
2.728(3)
3.051(4)
2.679(3)
2.849(4)
141(3)
120(3)
162(3)
177(5)
0.82
2.55
0.82
1.89
0.90(1)
1.95(1)
The complex
1.96(1)
O(3)–H(3A)∙∙∙N(2)iii 0.84(1)
2.796(2)
170(4)
Symmetry codes for i: –1 + x, y, z; ii: 1 + x, y, z; iii: 1 – x, 1 – y, – z.
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b
TABLE 4 Epoxidation of olefins using TBHP catalyzed by the complex in CH2Cl2
Alkene
Conversion Product
(%)c
Selectivity Time
to epoxides (h)
(%)
TOF
(h-1)
O
41
100
1
97.0
82.5
F
F
O
38
100
1
Cl
Cl
O
23
1
1
41.7
28.2
100
100
O
15
MeO
MeO
O
26
1
72.1
100
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Fig. 1. ORTEP diagram (30% thermal ellipsoid) of H2L with atom labeling scheme. Only the
major component of the CF3 group is shown. Hydrogen bonds are drawn as dashed lines.
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Fig. 2. Intermolecular hydrogen bonds (dotted lines) linked structure of H2L.
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Fig. 3. ORTEP diagram (30% thermal ellipsoid) of the complex with atom labeling scheme.
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Fig. 4. Intermolecular hydrogen bonds (dotted lines) linked structure of the complex.
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CF3
CF3
H
N
N
N
N
MoO2(acac)2
+
O
O
O
O
Mo
OH
OMe
H
O
Scheme 1. The synthesis procedure of the complex
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CF3
CF3
N
N
N
N
TBHP
O
O
O
O
O
O
Mo
O
Mo
OMe
OMe
H
H
Me3C O
O
I
O
H
TBOH
CF3
CF3
N
N
O
N
N
O
O
O
O
O
Mo
Mo
OMe
OMe
H
O
H
Me3C O
Me3C
O
O
O
O
H
H
Scheme 2. The catalytic procedure of the oxidation.
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