Syntheses, X-ray structures, and catalytic oxidations of dioxomolybdenum(VI) complexes with tridentate benzohydrazones

Article abstract of DOI:10.1080/00958972.2013.773587

Two new dioxomolybdenum(VI) complexes, [MoO₂L ¹(CH₃₀H)]CH₃₀H (1) and [MoO₂L ²(CH₃₀H)] (2), where L¹ and L² are dianionic forms of N'-(3-bromo-2-hydroxybenzylidene)-4-hydroxybenzohydrazide and N'-(2-hydroxybenzylidene)-2-hydroxy-3-methylbenzohydrazide, respectively, have been synthesized. The complexes have been characterized by elemental analyzes, IR spectra, molar conductance data, and single crystal X-ray determination. Mo in each complex is octahedral. The crystals of the complexes are stabilized by hydrogen bonds and weak π π stacking interactions. The complexes function as effective olefin epoxidation catalysts with hydrogen peroxide as oxidant and sodium hydrogen carbonate as a co-catalyst at room temperature.

Full text of DOI:10.1080/00958972.2013.773587

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Syntheses, X-ray structures,
and catalytic oxidations of
dioxomolybdenum(VI) complexes with
tridentate benzohydrazones
Xiao-Qiang He a
a College of Chemical Engineering and Pharmacy , Jingchu
University of Technology , Jingmen Hubei , P.R . China
Accepted author version posted online: 12 Feb 2013.Published
online: 20 Mar 2013.
To cite this article: Xiao-Qiang He (2013) Syntheses, X-ray structures, and catalytic oxidations
of dioxomolybdenum(VI) complexes with tridentate benzohydrazones, Journal of Coordination
Chemistry, 66:6, 966-976, DOI: 10.1080/00958972.2013.773587
To link to this article: http://dx.doi.org/10.1080/00958972.2013.773587
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Journal of Coordination Chemistry
Vol. 66, No. 6, 20 March 2013, 966–976
Syntheses, X-ray structures, and catalytic oxidations of
dioxomolybdenum(VI) complexes with tridentate
benzohydrazones
XIAO-QIANG HE*
College of Chemical Engineering and Pharmacy, Jingchu University of Technology,
Jingmen Hubei, PR China
(Received 13 September 2012; in final form 30 November 2012)
Two new dioxomolybdenum(VI) complexes, [MoO2L1(CH3OH)]·CH3OH (1) and [MoO2L2(CH3OH)]
(2), where L1 and L2 are dianionic forms of N'-(3-bromo-2-hydroxybenzylidene)-4-hydrox-
ybenzohydrazide and N'-(2-hydroxybenzylidene)-2-hydroxy-3-methylbenzohydrazide, respectively,
have been synthesized. The complexes have been characterized by elemental analyzes, IR spectra,
molar conductance data, and single crystal X-ray determination. Mo in each complex is octahedral.
The crystals of the complexes are stabilized by hydrogen bonds and weak π Á Á Á π stacking interac-
tions. The complexes function as effective olefin epoxidation catalysts with hydrogen peroxide as
oxidant and sodium hydrogen carbonate as a co-catalyst at room temperature.
Keywords: Benzohydrazone; Molybdenum; Crystal structure; Hydrogen bonding; Olefin
epoxidation
1. Introduction
Selective oxidation of hydrocarbons has attracted considerable attention in organic industry
[1–3]. Molybdenum complexes have been widely used, in particular for oxidations of
organic compounds [4–7]. Dioxomolybdenum complexes have been investigated as
oxidation catalysts for organic substrates, particularly for epoxidation of olefins [8–11]. Syn-
thesis, characterization, and reactivity studies of a number of dioxomolybdenum complexes
with Schiff bases have been reported [12–15]. Some of the complexes possess oxygen atom
transfer properties, oxidizing thiols, polyketones, and tertiary phosphines [16,17]. The
catalytic abilities of dioxomolybdenum complexes with benzohydrazones have also been
studied [8,18]. However, documented dioxomolybdenum complexes used as catalysts are
still limited. The author reports herein the syntheses, crystal structures, and catalytic
properties of two new dioxomolybdenum(VI) complexes, [MoO2L1(CH3OH)]·CH3OH (1)
and [MoO2L2(CH3OH)] (2), where L1 and L2 are dianions of N'-(3-bromo-2-hydroxybenzy-
lidene)-4-hydroxybenzohydrazide (H2L1; scheme 1) and N'-(2-hydroxybenzylidene)-2-hydroxy-
3-methylbenzohydrazide (H2L2), respectively.
*Email: hexq2012@163.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online Ó 2013 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2013.773587

2
X.-Q. He
OH
OH
O
H
N
H
N
N
N
O
OH
OH
Br
H2L1
H2L2
Scheme 1. The benzohydrazone ligands.
2. Experimental
2.1. Materials and methods
All chemicals and solvents of analytical reagent grade were used as received. Microanaly-
ses (C, H, N) were performed using a Perkin-Elmer 2400 elemental analyzer. Infrared
spectra were carried out using a JASCO FT-IR model 420 spectrophotometer with KBr
disk from 4000 to 200 cm–1. GC analyzes were carried out using a Shimadzu GC-2014C
gas chromatograph. Molar conductivities were carried out using a DDS-11A digital
conductivity meter.
2.2. Synthesis of the benzohydrazones
H2L1: 3-Bromosalicylaldehyde (0.20 g, 1 mmol) dissolved in methanol (50 mL) was added
to a stirred methanolic solution (30 mL) of 4-hydroxybenzohydrazide (0.15 g, 1 mmol).
The reaction mixture was refluxed for 2 h on a water bath and the solvent was evaporated
to give colorless crystalline products. Yields: 95%. Anal. Calcd for C14H11BrN2O3: C,
50.2; H, 3.3; N, 8.4%. Found: C, 50.0; H, 3.4; N, 8.4%.
H2L2: Salicylaldehyde (0.12 g, 1 mmol) dissolved in methanol (50 mL) was added to a
stirred methanolic solution (30 mL) of 2-hydroxy-3-methylbenzohydrazide (0.17 g,
1 mmol). The reaction mixture was refluxed for 2 h on a water bath and the solvent was
evaporated to give colorless crystalline products. Yields: 93%. Anal. Calcd for
C15H14N2O3: C, 66.7; H, 5.2; N, 10.4%. Found: C, 66.5; H, 5.2; N, 10.3%.
2.3. Synthesis of [MoO2L1(CH3OH)]·CH3OH (1)
A methanolic solution (10 mL) of MoO2(acac)2 (0.033 g, 0.1 mmol) was added to a
methanolic solution (10 mL) of H2L1 (0.034 g, 0.1 mmol). The resulting brown solution
was stirred for 10 min at reflux and then cooled to room temperature. Single crystals of 1
suitable for X-ray diffraction were formed by slow evaporation of the methanolic solution
of the complex in air for 5 days. Yield: 62%. Anal. Calcd for C16H17BrMoN2O7: C, 36.6;
H, 3.3; N, 5.3%. Found: C, 36.8; H, 3.2; N, 5.4.
2.4. Synthesis of [MoO2L2(CH3OH)] (2)
A methanolic solution (10 mL) of MoO2(acac)2 (0.033 g, 0.1 mmol) was added to a
methanolic solution (10 mL) of H2L2 (0.027 g, 0.1 mmol). The resulting brown solution
was stirred for 10 min at reflux and then cooled to room temperature. Single crystals of 2

Dioxomolybdenum(VI) complexes
3
suitable for X-ray diffraction were formed by slow evaporation of the methanolic solution
of the complex in air for 11 days. Yield: 51%. Anal. Calcd for C16H16MoN2O6: C, 44.9;
H, 3.8; N, 6.5%. Found: C, 44.8; H, 3.7; N, 6.7.
2.5. X-ray structure determination
X-ray measurements were performed using a Bruker Smart 1000 CCD diffractometer with
graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) using the ω-scan technique.
Determinations of the Laue class, orientation matrix, and cell dimensions were performed
according to the established procedures where Lorentz polarization and absorption correc-
tions were applied. Absorption corrections were applied by fitting a pseudoellipsoid to the
ψ-scan data of selected strong reflections over a wide range of 2h angles [19]. The
positions of almost all nonhydrogen atoms were located with direct methods. Subsequent
Fourier syntheses were used to locate the remaining nonhydrogen atoms. All nonhydrogen
atoms were refined anisotropically. The methanol H in 1 was located from a difference
Fourier map and refined isotropically, with OÀH distance restrained to 0.85(1) Å. The
remaining hydrogens were placed in calculated positions and constrained to ride on their
parent atoms. The analysis was performed with SHELXS-97 and SHELXL-97 programs
[20,21]. The crystallographic data for the complexes are summarized in table 1. Selected
bond lengths and angles are given in table 2. Hydrogen bonds are listed in table 3.
Table 1. Crystallographic data for 1 and 2.
Complex
1
2
Formula
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C
16H17BrMoN2O7
525.2
Monoclinic
P21/c
16.270(2)
8.691(1)
14.586(1)
90
116.093(3)
90
C16H16MoN2O6
428.2
Triclinic
P-1
7.560(2)
10.330(3)
11.210(3)
97.920(2)
98.050(2)
102.340(2)
834.0(4)
γ (°)
V (Å3)
1852.4(3)
4
Z
2
λ (Å)
0.71073
2.907
1.883
0.71073
0.822
1.705
298(2)
μ (MoKα) (mmÀ1
)
ρcalcd (g cmÀ3
T (K)
)
298(2)
F(000)
1040
432
Crystal size (mm)
h range (°)
h/k/l
0.17 Â 0.15 Â 0.15
2.79–26.49
À20, 20/À10, 18/À18, 18
0.6378
0.27 Â 0.23 Â 0.21
2.52–26.99
À9, 9/À13, 13/À12, 14
0.8085
T (min)
T (max)
0.6696
0.8463
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness of fit on F2
Rint
3819
3513
3819/1/251
1.097
0.0337
3522
2211
3522/1/232
1.044
0.0383
Final R indices [I > r2(I)]
R indices (all data)
0.0244, 0.0609
0.0274, 0.0625
0.618, À0.521
0.0448, 0.1052
0.0999, 0.1310
0.864, À0.739
Largest peak and hole (eÅÀ3
)

4
X.-Q. He
Table 2. Selected bond lengths (Å) and angles (°) for 1 and 2.
1
Bond lengths
Mo1–O1
Mo1–N1
Mo1–O5
1.9313(16)
2.2332(18)
1.6898(19)
Mo1–O2
Mo1–O4
Mo1–O6
2.0087(15)
2.3270(17)
1.7023(17)
Bond angles
O5–Mo1–O6
O6–Mo1–O1
O6–Mo1–O2
O5–Mo1–N1
O1–Mo1–N1
O5–Mo1–O4
O1–Mo1–O4
N1–Mo1–O4
105.56(9)
O5–Mo1–O1
O5–Mo1–O2
O1–Mo1–O2
O6–Mo1–N1
O2–Mo1–N1
O6–Mo1–O4
O2–Mo1–O4
99.09(9)
97.33(8)
149.49(7)
156.79(8)
71.59(6)
82.57(8)
79.82(7)
104.09(8)
95.98(7)
95.63(8)
81.33(6)
171.69(8)
80.26(7)
76.07(6)
2
Bond lengths
Mo1–O1
Mo1–N1
Mo1–O5
1.894(4)
2.214(4)
1.667(4)
Mo1–O2
Mo1–O4
Mo1–O6
2.000(3)
2.329(4)
1.670(4)
Bond angles
O5–Mo1–O6
O6–Mo1–O1
O6–Mo1–O2
O5–Mo1–N1
O1–Mo1–N1
O5–Mo1–O4
O1–Mo1–O4
N1–Mo1–O4
105.4(2)
O5–Mo1–O1
O5–Mo1–O2
O1–Mo1–O2
O6–Mo1–N1
O2–Mo1–N1
O6–Mo1–O4
O2–Mo1–O4
104.01(18)
96.51(16)
149.53(15)
95.15(17)
71.70(13)
170.22(15)
79.06(15)
99.52(18)
96.38(18)
157.46(17)
81.08(14)
83.80(16)
81.01(15)
75.24(13)
Table 3. Distances (Å) and angles (°) involving hydrogen bonding of 1 and 2.
DÀH Á Á Á A
d(DÀH)
d(HÁ Á ÁA)
d(DÁ Á ÁA)
\(DÀH Á Á ÁA)
1
O4ÀH4 Á Á Á N2i
O7ÀH7 Á Á Á O3ii
O3ÀH3 Á Á Á O7iii
2
0.85(1)
0.82
0.82
1.88(1)
2.14
1.92
2.711(2)
2.910(3)
2.729(3)
167(4)
157
170
O4ÀH4A Á Á Á O3iv
0.85(1)
0.82
1.94(2)
1.82
2.763(5)
2.550(5)
163(6)
147
O3ÀH3 Á Á Á N2
Symmetry codes: (i) 1 À x, 1/2 + y, 1/2 À z; (ii) 2 À x, À1/2 + y, 3/2 À z; (iii) x, 1 + y, z; (iv) Àx, Ày, 2 À z.
2.6. Catalytic epoxidation of olefins
To a solution of olefins (0.28 mmol), NaHCO3 (0.11 mmol), and catalyst (9.4 Â 10À4 mmol)
in MeCN (0.5 mL) was added H2O2 (1.1 mmol, 30% H2O solution) as oxidant. After the
reaction was over, for products analysis, the solution was subjected to multiple ether extrac-
tions, and the extract was concentrated to 0.5 cm3 by distillation in a rotary evaporator at
room temperature and then a sample (2 μL) was taken from the solution and analyzed by
GC. The retention times of the peaks were compared with those of commercial standards
and chlorobenzene was used as an internal standard for GC yield calculation.

Dioxomolybdenum(VI) complexes
5
3. Results and discussion
3.1. Synthesis
The two complexes were readily prepared by reaction of MoO2(acac)2 with the benzohyd-
razone ligands in methanol. The reaction process of the complexes (scheme 2) is accompa-
nied by a color change of the solution from colorless to brown. The crystals of the
complexes are stable in air at room temperature and soluble in methanol, ethanol, and
acetonitrile. The molar conductance values of 1 and 2 in methanol at 10–4 M are 8 and
11 ΩÀ1 cm2 MÀ1, indicating they are nonelectrolytes [22].
3.2. Crystal structure description
The molecular structures of the complexes are shown in figures 1 and 2. Complex 1 con-
tains a dioxomolybdenum complex and a methanol. The structures of the dioxomolybde-
num complexes are very similar to each other, except for the substituent groups of the
benzohydrazone ligands. Beyond the basic N'-(2-hydroxybenzylidene) benzohydrazide
skeleton, there are one bromo and one hydroxyl substituent in H2L1, and there are one
methyl and one hydroxyl substituent in H2L2. The hydroxyl is present in both complexes
but its position in the ring is not the same, which might influence molecular and crystal
structures. In each of the complexes, the coordination geometry around Mo can be
described as slightly distorted octahedron, with the equatorial plane defined by one pheno-
lic O, one imino N, and one enolic O of the dianionic benzohydrazone, and one oxo, and
with the two axial positions occupied by methanol and the other oxo. The benzohydrazone
ligands coordinate meridional to Mo forming five- and six-membered chelate rings with
bite angles of 71.59(6) and 81.33(6)° for 1 and 71.70(13) and 81.08(14)° for 2. The dihe-
dral angles between the two substituted benzene rings of the benzohydrazones are 19.7(3)°
for 1 and 2.2(3)° for 2, indicating the ligand in 1 is more twisted than in 2. Displacements
of Mo from the equatorial mean planes toward the axial oxo are 0.332(2) Å for 1 and
0.698(2) Å for 2. The benzohydrazones are coordinated in their dianionic forms, evident
from the N2–C8 and O2–C8 bond lengths with values of 1.302(3) and 1.318(3) Å for 1
and 1.285(6) and 1.295(5) Å for 2, indicating the presence of the enolate form of the
OH
OH
OH
H
N
N
N
N
N
N
MoO2(acac)2
O
O
OH
CH3OH
O
OH
OH
Mo
OMe
Br
Br
Br
O
H
O
(1)
OH
OH
O
OH
H
N
N
N
N
N
N
MoO2(acac)2
O
OH
O
Mo
OH
OH
OMe
H
O
O
(2)
Scheme 2. Preparation of the complexes.

6
X.-Q. He
Figure 1. An ORTEP diagram of 1 with atom labeling scheme and 30% probability thermal ellipsoids for all
nonhydrogen atoms.
Figure 2. An ORTEP diagram of 2 with atom labeling scheme and 30% probability thermal ellipsoids for all
nonhydrogen atoms. Intramolecular hydrogen bond is shown as a dashed line.
ligand amide. The Mo–O, Mo–N, and Mo=O bonds are within normal ranges and are sim-
ilar to those observed in similar dioxomolybdenum(VI) complexes [23–25].
In the molecular packing structure of 1, [MoO2L1] units are linked by methanols through
intermolecular O–H Á Á Á N and O–H Á Á Á O hydrogen bonds to form 1-D chains running
along the b axis (figure 3). In the molecular packing of 2, adjacent [MoO2L2] units are
linked by two methanols through two intermolecular O–H Á Á Á O hydrogen bonds to form a
dimer (figure 4). There are also weak π Á Á Á π stacking interactions among C1–C6 and
C9–C14 benzene rings, as specified in table 4 [26].

Dioxomolybdenum(VI) complexes
7
Figure 3. Packing diagram of 1-D chains of 1, viewed along the b axis. Hydrogen bonds are shown as dashed
lines.
Figure 4. Packing diagram of dimers of 2, viewed along the b axis. Hydrogen bonds are shown as dashed lines.
3.3. IR spectra
Weak and broad bands centered at 3435 cm–1 for 1 and 3421 cm–1 for 2 can be assigned to
the νOH vibrations of methanol and the hydroxyl groups of the benzohydrazone ligands.
The Mo=O stretching modes occur as a pair of sharp strong bands at 938 and 912 cm–1
for 1 and 939 and 917 cm–1 for 2, which are assigned to the antisymmetric and symmetric
stretching modes of the dioxomolybdenum(VI). The bands due to νC=O and νNH were

8
X.-Q. He
Table 4. Parameters among planes.
Distance between
ring centroids
Perpendicular
distance
Perpendicular
distance
Dihedral
Cg
(Å)
angle (°)
of Cg(I) on Cg(J) (Å)
of Cg(J) on Cg(I) (Å)
1
Cg(1)-Cg(2)v
2
3.935
12.41
3.571
3.498
Cg(1)-Cg(2)vi
Cg(1)-Cg
(2)vii
3.741
4.461
2.29
2.29
3.412
3.446
3.373
3.354
Symmetry codes: (v): 1 À x, À1/2 + y, 1/2 À z; (vi): 1 À x, 2 À y, Àz; (vii): 2 À x, 2 À y, Àz. Cg(1) and Cg(2) are
the centroids of C1–C2–C3–C4–C5–C6 and C9–C10–C11–C12–C13–C14, respectively.
Table 5. Catalytic oxidation of olefins catalyzed by 1 and 2a.
Conversion (%)b
(TON)c
Selectivity
(%)d
Substrate
Product
1
2
95 (263)
97 (271)
1
2
100
100
1
2
92 (281)
94 (290)
1
2
100
100
1
2
97 (288)
95 (283)
1
2
100
100
1
2
92 (269)
93 (272)
1
2
100
100
1
2
96 (277)
96 (285)
1
2
100
100
1
2
98 (292)
94 (287)
1
2
100
100
aThe molar ratios for catalyst: substrate:NaHCO3 : H2O2 are 1 : 298 : 117 : 1170. The reactions were performed in
(70 : 30) mixture of CH3OH/CH2Cl2 (1.2 mL) under air at room temperature. bThe GC conversion (%) are mea-
d
sured relative to the starting olefin after 74.5 min. cTON= (mmol of product)/mmol of catalyst. Selectivity to
epoxide = (epoxide%/(epoxide% + aldehyde%)) Â 100.

Dioxomolybdenum(VI) complexes
9
absent in the complexes and new C–O stretches appear at 1265 cm–1 for 1 and 1270 cm–1
for 2, suggesting keto-imine tautomerization of the benzohydrazone ligands during coordi-
nation. Strong bands indicative of C=N–N=C groups in the complexes shift to 1608 cm–1
for 1 and 1613 cm–1 for 2. New weak peaks at 300–800 cm–1 may be attributed to Mo–O
and Mo–N vibrations in the complexes. IR spectra of the two complexes are similar,
indicating the complexes are of similar structures, as evidenced by the single crystal X-ray
determination.
3.4. Catalytic properties
The complexes showed effective catalytic oxidation of various olefins to their correspond-
ing epoxides. The details of catalytic properties with respect to epoxidation of olefins with
1 and 2 as catalysts are given in table 5. Generally, excellent epoxide yields and selectivities
were observed for all aliphatic and aromatic substrates. The catalytic studies using dioxo-
molybdenum catalysts reveal that the efficiency of catalysts toward all substrates is similar
with maximum conversion, TON, and selectivities. The mechanism (scheme 3) for the
epoxidation of various olefins to epoxides using the two complexes as catalysts has been
proposed by comparison with the literature [27]. When H2O2 was used as a sole oxidant
the catalytic efficiency is not high, but when NaHCO3 was added as a co-catalyst the
efficiency of the system increases many fold. The key aspect of such a reaction is that
H2O2 and hydrogen carbonate react in an equilibrium process to produce peroxymonocar-
bonate, HCO4À, which is a more reactive nucleophile than H2O2 and speeds up the
epoxidation [28]. The basic principle of the catalytic reaction is the conversion of oxoper-
oxo complex to dioxo complex transferring oxo to the olefins and the conversion of dioxo
complex to the oxoperoxo complex reacting with HCO4À to regain catalytic activity.
Comparing the catalytic properties of the complexes in the present work with those of
other similar dioxomolybdenum(VI) complexes bearing benzohydrazone ligands [6,8]
O
O
O
_
Mo
O
HCO3
N
OMe
H
O
O
O
O
O
O
O
O
O
Mo
Mo
O
OH
_
N
N
O
OMe
H
OMe
O
H
O
_
HCO4
H2O
_
H2O2
HCO3
Scheme 3. Probable mechanism of catalytic oxidation of olefins to epoxides.

10
X.-Q. He
indicates that the substituent groups of the benzohydrazone ligands have little influence to
the catalytic processes.
4. Conclusions
Two new dioxomolydenum(VI) complexes with similar benzohydrazone ligands, N′-(3-bromo-
2-hydroxybenzylidene)-4-hydroxybenzohydrazide and N'-(2-hydroxybenzylidene)-2-hydroxy-
3-methylbenzohydrazide, have been prepared and structurally characterized using X-ray
structure analysis, IR spectra, molar conductance data, and elemental analyzes. The dianionic
benzohydrazones coordinate to Mo through the phenolic O, imino N, and ethanolic O. Mo is
coordinated by benzohydrazone and methanol, as well as two oxo groups, forming
octahedral coordination. The complexes showed effective catalytic properties in oxidation of
various olefins to their corresponding epoxides. The results are in accord to those reported
previously for molybdenum complexes catalyzing epoxidation of alkenes [28–31].
Supplementary material
CCDC reference numbers 901036 for 1 and 901037 for 2 contain the supplementary
crystallographic data for this article. These data can be obtained free of charge at http://
www.ccdc.cam.ac.uk or from Cambridge Crystallographic Data Center, 12 Union Road,
Cambridge CB2 1EZ, UK; Fax: +44 1223 336 033; E-mail: deposit@ccdc.cam.ac.uk.
Acknowledgements
The author greatly acknowledges Jingchu University of Technology for financial support.
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