Two dioxomolybdenum(VI) complexes with hydrazone ligands: synthesis, characterization, crystal structures and catalytic properties

Article abstract of DOI:10.1007/s11243-016-0086-8

A pair of Mo(VI) complexes, [MoO₂L¹(MeOH)] (1) and [MoO₂L²(MeOH)] (2), where L¹ and L² are the dianions of 2-amino-N’-(3-bromo-5-chloro-2-hydroxybenzylidene)benzohydrazide (H₂L

Full text of DOI:10.1007/s11243-016-0086-8

Transit Met Chem
DOI 10.1007/s11243-016-0086-8
Two dioxomolybdenum(VI) complexes with hydrazone ligands:
synthesis, characterization, crystal structures and catalytic
properties
Dong-Lai Peng1,2
Received: 20 June 2016 / Accepted: 1 August 2016
Ó Springer International Publishing Switzerland 2016
Abstract A pair of Mo(VI) complexes, [MoO2L1(MeOH)]
(1) and [MoO2L2(MeOH)] (2), where L1 and L2 are the
dianions of 2-amino-N’-(3-bromo-5-chloro-2-hydroxyben-
zylidene)benzohydrazide (H2L1) and 2-amino-N’-(3,5-di-
bromo-2-hydroxybenzylidene)benzohydrazide (H2L2),
respectively, have been prepared and characterized by
physico-chemical methods and single-crystal X-ray
diffraction. The Mo atom in each complex is coordinated
by the phenolate oxygen, imino nitrogen and enolate
oxygen of the hydrazone ligand, together with a methanol
ligand and two oxo groups, giving a distorted octahedral
geometry. The complexes proved to be effective catalysts
for the oxidation of various olefins.
of molecular nitrogen and nitrate and the oxidation (hy-
droxylation) of xanthine and other purines and aldehydes
[6]. Epoxides are very important chemicals for the manu-
facture of a range of important commercial products such
as pharmaceuticals and polymers. Many transition metal
complexes have been investigated as catalysts for the
oxidation of olefins [7–13]. Molybdenum complexes find
considerable use in organic chemistry, in particular for the
oxidation of various organic compounds [1, 2, 4]. Oxido–
peroxido molybdenum complexes have been intensively
investigated as oxidation catalysts for variety of organic
substrates, particularly for sulfoxidation and epoxidation of
olefins [14–17]. In order to further explore the catalytic
potential of oxido–peroxido complexes of molybdenum, in
this paper, two new molybdenum complexes, [MoO2L1
(MeOH)] (1) and [MoO2L2(MeOH)] (2), where L1 and L2
are the dianions of 2-amino-N’-(3-bromo-5-chloro-2-hy-
droxybenzylidene)benzohydrazide (H2L1) and 2-amino-N’-
(3,5-dibromo-2-hydroxybenzylidene)benzohydrazide (H2L2),
respectively, are presented.
Introduction
In recent years, considerable attention has been focused on
molybdenum 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 the reduction
Experimental
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-016-0086-8) contains supplementary
material, which is available to authorized users.
Materials and methods
& Dong-Lai Peng
pengdonglai2015@sina.com
All the reagents and solvents used in the syntheses were
procured commercially and used without purification.
Microanalyses (C, H, N) were obtained with a PerkinElmer
2400 elemental analyzer. Infrared spectra were recorded on
a JASCO FT-IR model 420 spectrophotometer with KBr
disks in the region of 4000–400 cm-1. Electronic absorp-
tion spectra were recorded in acetonitrile using a Lambda
900 spectrometer. 1H NMR spectra were measured in
DMSO-d6 on a Bruker 300 MHz spectrometer.
1
Key Laboratory of Surface and Interface Science of Henan
Province, School of Material and Chemical Engineering,
Zhengzhou University of Light Industry, Zhengzhou 450001,
People’s Republic of China
2
Henan Collaborative Innovation Center of Environmental
Pollution Control and Ecological Restoration, Zhengzhou
University of Light Industry, Zhengzhou 450001,
People’s Republic of China
123

Transit Met Chem
Table 1 Crystallographic data
and refinement parameters for
the complexes
Complex
1
2
Empirical formula
Formula weight
C15H13BrClMoN3O5
526.58
C15H13Br2MoN3O5
571.04
Crystal size (mm)
0.23 9 0.23 9 0.20
298(2)
0.17 9 0.15 9 0.15
298(2)
Temperature (°C)
˚
Wavelength (A)
0.71073
0.71073
Monoclinic
P21/n
Crystal system
Space group
Monoclinic
P21/n
˚
a (A)
13.7629(17)
9.7735(12)
14.1856(18)
110.509(2)
1787.2(4)
4
13.8832(8)
9.8296(4)
14.1980(8)
110.719(2)
1812.24(16)
4
˚
b (A)
˚
c (A)
b (°)
3
˚
V (A )
Z
Dcalc (g cm-3
)
1.957
2.093
l (Mo Ka) (mm-1
)
3.151
5.165
F(000)
1032
1104
Number of measured reflections
Number of observations (I [ 2r(I))
Unique reflections
15,567
9329
3165
3371
2743
2827
Parameters
245
245
Number of restraints
R1, wR2 (I [ 2r(I))a
R1, wR2 (all data)a
4
4
0.0269, 0.0633
0.0353, 0.0670
0.0280, 0.0619
0.0391, 0.0666
1.033
Goodness of fit of F2
1.041
P
P
P
P
a
R1 = ||Fo| - |Fc||/ |Fo|, wR2 = [ w(Fo2 - Fc2)2/ w(Fo2)2]1/2
Synthesis of complex 1
(0.328 g, 1.0 mmol) in methanol (20 mL) was then added
in one portion. The mixture was stirred at room tempera-
ture for 30 min, giving a yellow solution. Upon keeping
this solution for 7 days at room temperature, fine orange
crystals were formed. The product was filtered off and
washed with methanol. Yield 0.405 g (71 %). IR (KBr)
(mmax/cm-1): 3446 (br, w, mOH), 3125 (w, mNH), 1636 (m,
3-Bromo-5-chloro-2-hydroxybenzaldehyde
(0.235 g,
1.0 mmol) and 2-aminobenzohydrazide (0.151 g,
1.0 mmol) were dissolved in methanol (20 mL), and a
solution of MoO2(acac)2 (0.328 g, 1.0 mmol) in
methanol (20 mL) was then added in one portion. The
resulting mixture was stirred at room temperature for
30 min, giving a yellow solution. Upon keeping this for
5 days at room temperature, fine orange crystals were
formed. The product was filtered off and washed with
methanol. Yield 0.326 g (62 %). IR (KBr) (mmax/cm-1):
3450 (br, w, mOH), 3167 (w, mNH), 1632 (m, mC=N), 955 s
and 850 s (mMo=O). Absorption spectrum in acetonitrile
[kmax (nm), e (L mol-1 cm-1)]: 297 (12,520), 330
(10,130), 415 (3728). Anal.: Calcd. for C15H13-
BrClMoN3O5: C, 34.21; H, 2.49; N, 7.98 %. Found: C,
34.08; H, 2.41; N, 8.15 %.
m
C=N), 955 s and 850 s (mMo=O). Absorption spectrum in
acetonitrile [kmax (nm), e (L mol-1 cm-1)]: 300 (13,170),
325 (11,210), 427 (4530). Anal.: Calcd. for C15H13Br2-
MoN3O5: C, 31.55; H, 2.29; N, 7.36 %. Found: C, 31.39;
H, 2.38; N, 7.27 %.
X-Ray crystallography
X-ray data for both complexes were collected on a Bruker
SMART APEXII diffractometer equipped with graphite-
˚
monochromated Mo Ka radiation (k = 0.71073 A). A
preliminary orientation matrix and cell parameters were
determined from three sets of x 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 made using SADABS [18].
Synthesis of complex 2
3,5-Dibromo-2-hydroxybenzaldehyde (0.280 g, 1.0 mmol)
and 2-aminobenzohydrazide (0.151 g, 1.0 mmol) were
dissolved in methanol (20 mL). A solution of MoO2(acac)2
123

Transit Met Chem
The structures were solved by direct methods and refined
by full-matrix least-squares analysis using anisotropic
thermal parameters for non-H atoms with the SHELXTL
program [19]. The methanol and amino hydrogen atoms
were located from difference Fourier maps and refined
isotropically, with O–H, N–H and HÁÁÁH distances
internal standard and the required complex (2 9 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 Tech-
nologies Instruments 6890 N, equipped with a capillary
column (19,019 J-413 HP-5, 5 % phenyl methyl siloxane,
Capillary 60.0 m 9 250 lm 9 1.00 lm) and a flame ion-
ization detector). Products were identified by comparison
with authentic samples. All the reactions were run in
duplicate (Table 2).
˚
restrained to 0.85(1), 0.90(1) and 1.45(2) A, respectively.
The remaining H atoms were calculated at idealized posi-
tions and refined with riding models. Crystallographic data
for the complexes are summarized in Table 1.
Catalytic experiments
Results and discussion
Chemistry
In a typical catalytic experiment, TBHP (tert-butyl
hydroperoxide, 1 mmol) was added to a solution of the
required olefin (1 mmol), chlorobenzene (1 mmol) as
The hydrazones were reacted with MoO2(acac)2 in
methanol, followed by slow evaporation of the resulting
solutions, to generate fine orange crystalline complexes, as
shown in Scheme 1. Crystals of the complexes are stable in
air at room temperature and soluble in methanol, ethanol
and acetonitrile.
˚
Table 2 Selected bond lengths (A) and angles (°) for the complexes
1
Mo1-O1
1.937(2)
1.702(2)
Mo1-O2
1.993(2)
2.289(2)
Mo1-O3
Mo1-O4
Mo1-O5
1.685(2)
Mo1-N1
2.250(3)
O5-Mo1-O3
O3-Mo1-O1
O3-Mo1-O2
O5-Mo1-N1
O1-Mo1-N1
O5-Mo1-O4
O1-Mo1-O4
N1-Mo1-O4
2
105.00(12)
104.98(10)
97.07(9)
O5-Mo1-O1
O5-Mo1-O2
O1-Mo1-O2
O3-Mo1-N1
O2-Mo1-N1
O3-Mo1-O4
O2-Mo1-O4
98.91(11)
98.76(10)
146.96(9)
161.29(10)
71.54(8)
Structure descriptions of the complexes
The molecular structures of the complexes are shown in
Figs. 1 and 2, respectively. The two structures are very
similar. In both cases, the coordination geometry around
the Mo atom can be described as a slightly distorted
octahedron. The equatorial plane is defined by one phenolic
O, one imino N and one enolic O atom from the dianionic
hydrazone ligand, plus one oxo atom, while the axial
positions are occupied by one O atom of a methanol ligand
and the other oxo atom. The hydrazone ligands coordinate
in meridional fashion, forming five- and six-membered
chelate rings with bite angles of 71.54(8)° and 80.26(9)°
for 1 and 71.50(10) and 80.37(10)° for 2. The dihedral
angles between the two phenyl rings of the hydrazone
ligands are 12.2(3)° for 1 and 12.1(4)° for 2. The dis-
placements of the Mo atoms from the equatorial mean
91.64(11)
80.26(9)
171.14(11)
79.64(9)
83.77(10)
78.62(9)
79.50(9)
Mo1-O1
1.939(2)
2.288(3)
Mo1-O2
1.994(2)
1.703(2)
Mo1-O3
Mo1-O4
Mo1-O5
1.686(3)
Mo1-N1
2.250(3)
O5-Mo1-O4
O4-Mo1-O1
O4-Mo1-O2
O5-Mo1-N1
O1-Mo1-N1
O5-Mo1-O3
O1-Mo1-O3
N1-Mo1-O3
104.92(14)
105.17(11)
96.90(10)
91.85(12)
80.37(10)
170.97(12)
79.80(10)
79.12(10)
O5-Mo1-O1
O5-Mo1-O2
O1-Mo1-O2
O4-Mo1-N1
O2-Mo1-N1
O4-Mo1-O3
O2-Mo1-O3
98.69(12)
98.57(11)
147.29(10)
161.06(12)
71.50(10)
84.02(11)
78.81(10)
˚
planes toward the axial oxo atoms are 0.313(2) A for 1 and
˚
0.310(2) A for 2. The hydrazone ligands are coordinated in
their dianionic forms, which is evident from the N–C and
O–C bond lengths, which indicate the presence of the
H
N
X
N
X
N
N
MoO2(acac)2
+
O
NH2
O
NH2
OH
O
Mo
O
OMe
Br
Br
O
H
Scheme 1 Synthetic procedure for the complexes. X = Cl (complex 1) or Br (complex 2)
123

Transit Met Chem
enolate form of the ligand amide groups. The Mo–
O(methanol) bonds trans to the terminal oxygen atoms are
˚
significantly longer (2.29 A) compared to the other
molybdenum oxygen bonds; the Mo–O(enolate) distances
˚
are about 1.99 A and Mo–O(phenolate) distances about
˚
1.94 A. The Mo–O(oxo) bond lengths are similar to those
2?
found in other MoO2 cores [20, 21]. The bonds between
Mo and the azomethine nitrogen (N1) in the complexes are
˚
relatively long, at 2.25 A. This can be attributed to the
trans effect of the oxo groups trans to the Mo–N1 bonds
˚
[20]. The C8–O2 bond lengths in the complexes are 1.32 A,
which are closer to a single bond rather than a C=O double
bond. Similarly, shortening of the C8–N2 lengths in the
˚
complexes of about 1.30 A, instead of the more normal
Fig. 1 Molecular structure of 1 with atom labeling scheme and 30 %
probability thermal ellipsoids for all non-hydrogen atoms. Hydrogen
bond is shown as a dashed line
˚
1.38 A, together with elongation of the N1–N2 lengths to
˚
about 1.40 A provide further evidence for electron delo-
calization within the ligands [22, 23]. Each ligand forms
both five- and six-membered chelate rings with the Mo
center.
In the crystal structures of both 1 and 2, the complex
molecules are linked by their methanol ligands through
hydrogen bonds (Table 3) to form 1D chains. These chains
are further linked through hydrogen bonds involving the
NH2 groups, giving 2D sheets (Figs. 1S and 2S).
Infrared and UV–Vis spectra
In the IR spectra of both complexes, weak and broad bands
in the region of 3400–3500 cm-1 are assigned to mOH
vibrations (Figs. 3S and 4S). Weak and sharp bands at
3167 cm-1 for 1 and 3125 cm-1 for 2 are assigned to the
Fig. 2 Molecular structure of 2 with atom labeling scheme and 30 %
probability thermal ellipsoids for all non-hydrogen atoms. Hydrogen
bond is shown as a dashed line
m
NH vibrations. The Mo=O stretching modes occur as a pair
of sharp, strong bands at 955 and 850 cm-1 for both
complexes, which are assigned to the anti-symmetric and
symmetric stretching modes, respectively. This is in
accordance with observations on similar complexes
[20, 24]. The bands due to the mC=O and mNH of the free
hydrazones were absent in the spectra of the complexes,
˚
Table 3 Hydrogen bond distances (A) and bond angles (°) for the
complexes
whereas new C–O stretches were observed at 1160 cm-1
.
D–H_A
d(D–H) d(H_A) d(D_A) Angle (D–H_A)
1
Table 4 Solvent effects on the catalytic epoxidation of cyclohexene
with TBHP using complex I as the catalysta
O4–H4ÁÁÁN3i
0.85(1) 1.96(2)
N3–H3BÁÁÁO3ii 0.90(1) 2.17(2)
2.784(4) 165(5)
3.039(4) 162(4)
3.112(4) 140(3)
2.727(4) 125(3)
Entry
Solvent
Conversion
Time
TONb
N3–H3AÁÁÁO5iii 0.90(1) 2.37(3)
1
2
3
4
5
6
a
MeOH
87
89
1 h
3645
3860
4170
4325
4610
5530
N3–H3AÁÁÁN2
O3–H3ÁÁÁN3iv
0.90(1) 2.11(3)
0.85(1) 1.97(2)
EtOH
1 h
2
MeCN
92
1 h
2.782(4) 162(5)
3.080(4) 163(4)
3.106(4) 141(3)
2.738(4) 124(3)
Cyclohexane
Toluene
88
1 h
N3–H3AÁÁÁO4v 0.90(1) 2.21(2)
99
1 h
N3–H3BÁÁÁO5vi 0.90(1) 2.36(3)
1,2-Dichloroethane
100
30 min
N3–H3BÁÁÁN2
0.90(1) 2.13(3)
The molar ratio for the complex:cyclohexene:TBHP is
Symmetry codes: (1) 1 - x, 2 - y, -z; (2) x, 1 ? y, z; (3) 3/2 - x,
1/2 ? y, 1/2 - z; (4) -x, 1 - y, -z; (v) x, -1 ? y, z; (5) 1/2 - x, –
1/2 ? y, 1/2 - z
1:5000:5000. The reactions were carried out at 80 °C
b
TON = (mmol of product)/mmol of catalyst
123

Transit Met Chem
Table 5 Catalytic oxidation resultsa
Entry Substrate
1
Product
Compound Conversion (%) Selectivity (%)
TON
5530
100
100
46
100
100
53
1
2
1
O
2
3
5450
2723
O
4
5
2
1
2
1
2
51
73
69
78
80
47
50
55
46
48
2612
1330
1415
1202
1153
O
O
O
F
F
6
7
Cl
Cl
8
9
1
2
65
72
51
44
890
876
Br
Br
10
a
The molar ratio for the catalyst:olefin:TBHP is 1:5000:5000. The reactions were carried out at 80 °C for 1 h
This suggests keto-imine tautomerization of the hydrazone
ligands upon coordination. Strong bands indicative of the
C=N groups are observed at 1632 cm-1 for complex 1 and
1636 cm-1 for complex 2, which are similar to those
observed in similar complexes [20, 24, 25]. Weak features
in the range 400–800 cm-1 may be attributed to the Mo–O
and Mo–N vibrations in the complexes.
the spectra of the two complexes may be due to the dif-
ferent electronic properties of the ligands [20].
Catalytic epoxidations
Molybdenum complexes find considerable use in organic
chemistry, in particular for the oxidation of various organic
compounds [28, 29]. Oxido–peroxido molybdenum com-
plexes have been intensively investigated as oxidation
catalysts for a variety of organic substrates, particularly for
sulfoxidation and epoxidation of olefins, and can show high
efficiency and selectivity [30, 31]. Since the two complexes
presented here are very similar, we investigated only
complex 1 with respect to optimization of the experimental
The electronic spectra of the complexes (Figs. 5S and
6S) recorded in acetonitrile solution both display two
strong absorption bands centered at about 420 nm and
300 nm. These peaks are assigned to charge transfer tran-
sitions of the type N(pp)–Mo(dp) LMCT and O(pp)–
Mo(dp) LMCT, respectively [26, 27], as the ligand-based
orbitals are either N or O donor types. Small variation in
123

Transit Met Chem
conditions for catalysis. Thus, complex 1 was first studied
in the homogeneous epoxidation of cyclohexene as a model
substrate, with tert-butyl hydrogen peroxide (TBHP) as the
oxygen donor. The effects of various solvents were eval-
uated (Table 3). Toluene and 1,2-dichloroethane produced
the highest yields, and the catalytic activity was best when
1,2-dichloroethane was used as the solvent. At 80 °C with
1,2-dichloroethane, a turnover number (TON) of 5530 was
achieved for this reaction. Hence, 1,2-dichloroethane was
used as solvent for subsequent experiments.
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Acknowledgments The author acknowledges the Zhengzhou
University of Light Industry for supporting this work.
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