Syntheses, crystal structures, and catalytic properties of dioxomolybdenum(VI) complexes with hydrazone ligands

Article abstract of DOI:10.1080/00958972.2012.731049

Two new dioxomolybdenum(VI) complexes, [MoO₂L ¹(CH₃OH)] (1) and [MoO₂L²(H ₂O)] (2), where L1 and L2 are dianionic form of N′-(2-hydroxy- 3-methoxybenzylidene)-4methoxybenzohydrazide and N′-

Full text of DOI:10.1080/00958972.2012.731049

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Syntheses, crystal structures,
and catalytic properties of
dioxomolybdenum(VI) complexes with
hydrazone ligands
Nan-Yan Jin a
a College of Chemical Engineering and Pharmacy, Jingchu
University of Technology, Jingmen, Hubei 448000, P.R. China
Accepted author version posted online: 18 Sep 2012.Published
online: 01 Oct 2012.
To cite this article: Nan-Yan Jin (2012) Syntheses, crystal structures, and catalytic properties of
dioxomolybdenum(VI) complexes with hydrazone ligands, Journal of Coordination Chemistry, 65:22,
4013-4022, DOI: 10.1080/00958972.2012.731049
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Journal of Coordination Chemistry
Vol. 65, No. 22, 20 November 2012, 4013–4022
Syntheses, crystal structures, and catalytic properties of
dioxomolybdenum(VI) complexes with hydrazone ligands
NAN-YAN JIN*
College of Chemical Engineering and Pharmacy, Jingchu University of Technology,
Jingmen, Hubei 448000, P.R. China
(Received 20 May 2012; in final form 13 August 2012)
Two new dioxomolybdenum(VI) complexes, [MoO2L1(CH3OH)] (1) and [MoO2L2(H2O)] (2),
where L1 and L2 are dianionic form of N0-(2-hydroxy-3-methoxybenzylidene)-4methoxybenzo-
hydrazide and N0-(2-hydroxy-3methoxybenzylidene)-2-hydroxybenzohydrazide, respectively,
have been synthesized and structurally characterized by spectroscopic methods and single-
crystal X-ray determination. The complexes are mononuclear molybdenum(VI) compounds.
Mo in each complex is octahedral. The difference in the substituent groups in the
benzohydrazides leads to coordination of different solvent molecules. Crystals of the complexes
are stabilized by hydrogen bonds. The complexes are effective catalysts for sulfoxidation.
Keywords: Hydrazone; Molybdenum complex; Crystal structure; Hydrogen-bonding;
Sulfoxidation
1. Introduction
The mechanism of molybdenum oxotransferase has received considerable interest. The
synthesis, characterization, and reactivity of a number of dioxomolybdenum complexes
with Schiff bases have been reported [1–4]. Some of the complexes possess oxygen atom
transfer properties as they oxidize thiols, hydrazone, polyketones, and tertiary
phosphines [5, 6]. Sulfoxidation activity has been reported for molybdenum salts,
composite metal oxides, molybdate doped porous carbon atoms, and molybdenum
containing molecular sieves [7–10]. The catalytic ability of dioxomolybdenum(VI)
complexes with hydrazone ligands toward oxidation of sulfides has received satisfactory
results [11, 12]. However, the number of documented dioxomolybdenum(VI) complexes
catalyzing the peroxidic oxidation of sulfides is still very limited. In this article, two new
dioxomolybdenum(VI) complexes, [MoO2L1(CH3OH)] (1) and [MoO2L2(H2O)] (2),
where L1 and L2 are the dianions of N0-(2-hydroxy-3methoxybenzylidene)-4-methox-
ybenzohydrazide (H2L1; scheme 1) and N0-(2-hydroxy-3-methoxybenzylidene)-2-hydro-
xybenzohydrazide (H2L2; scheme 1), respectively, have been prepared, structurally
characterized, and investigated for peroxidic oxidation of sulfides.
*Email: nanyanjin@163.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2012 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2012.731049

4014
N.-Y. Jin
OMe
HO
H
H
N
N
N
N
O
O
OH
OH
OMe
OMe
Scheme 1. The hydrazone ligands H2L1 (left) and H2L2 (right).
2. Experimental
2.1. Materials and methods
All chemicals and solvents of analytical reagent grade were used as received.
Microanalyses (C, H, and N) were performed using a Perkin-Elmer 2400 elemental
analyzer. 1H NMR spectra were recorded on a Bruker Avance 200 spectrometer.
Infrared (IR) spectra were carried out using the JASCO FT-IR model 420
spectrophotometer with KBr discs from 4000 to 200 cmꢀ1
.
2.2. Syntheses of H2L1 and H2L2
H2L1 and H2L2 were prepared by similar procedure. 3-Methoxysalicylaldehyde (0.152 g,
1 mmol) dissolved in methanol (50 mL) was added to a stirred methanol solution
(30 mL) of substituted benzohydrazide, namely 4-methoxybenzohydrazide (0.166 g,
1 mmol) for H2L1 and 2-hydroxybenzohydrazide (0.152 g, 1 mmol) for H2L2. Then the
reaction mixtures were refluxed for 2 h on a water bath, and the solvent was evaporated
to give colorless solid products. Yields: 92% for H2L1 and 95% for H2L2. Anal. Calcd
for C16H16N2O4 (H2L1; %): C, 64.0; H, 5.4; N, 9.3. Found (%): C, 63.8; H, 5.4; N, 9.4.
Anal. Calcd for C15H14N2O4 (H2L2; %): C, 62.9; H, 4.9; N, 9.8. Found (%): C, 62.8;
H, 5.0; N, 9.9.
2.3. Synthesis of [MoO2L1(CH3OH)] (1)
A hot methanol solution (10 mL) of MoO2(acac)2 (0.033 g, 0.1 mmol) was added to a
hot methanol solution (10 mL) of H2L1 (0.030 g, 0.1 mmol). The resulting brown
solution was stirred for 10 min at reflux, and then cooled to room temperature, to give
brown solution. Single crystals suitable for X-ray diffraction were formed by slow
evaporation of the methanol solution of the complex in air for a few days. Yield: 51%.
Anal. Calcd for C17H18MoN2O7 (%): C, 44.6; H, 4.0; N, 6.1. Found (%): C, 44.4;
H, 3.9; N, 6.2.
2.4. Synthesis of [MoO2L2(H2O)] (2)
A hot methanol solution (10 mL) of MoO2(acac)2 (0.033 g, 0.1 mmol) was added to a
hot methanol solution (10 mL) of H2L2 (0.029 g, 0.1 mmol). The resulting brown
solution was stirred for 10 min at reflux, and then cooled to room temperature, to give

Dioxomolybdenum(VI) with hydrazone
4015
brown solution. Single crystals suitable for X-ray diffraction were formed by slow
evaporation of the methanol solution of the complex in air for a few days. Yield: 63%.
Anal. Calcd for C15H14MoN2O7 (%): C, 41.9; H, 3.3; N, 6.5. Found (%): C, 41.8;
H, 3.4; N, 6.5.
2.5. X-ray structure determination
X-ray measurements were performed using a Bruker Smart 1000 CCD diffractometer
˚
with graphite monochromated Mo-Ka radiation (ꢀ ¼ 0.71073 A) using the !-scan
technique. Determination of the Laue class, orientation matrix, and cell dimensions
were performed according to the established procedures where Lorentz polarization and
absorption corrections were applied. Absorption corrections were applied by fitting a
pseudoellipsoid to the -scan data of selected strong reflections over a wide range of 2ꢁ
angles. The positions of almost all non-hydrogen atoms were located with direct
methods. Subsequent Fourier syntheses were used to locate the remaining non-
hydrogen atoms. All non-hydrogen atoms were refined anisotropically. The methanol
hydrogen atoms in 1 and the water hydrogen atoms in 2 were located from difference
Fourier maps and refined isotropically, with O–H, N–H, and Hꢁ ꢁ ꢁH distances
˚
restrained to 0.85(1), 0.90(1), and 1.37(2) A, respectively. Other hydrogen atoms were
placed in calculated positions and constrained to ride on their parent atoms. The
analysis was performed with SHELXS-97 and SHELXL-97 suite [13, 14]. 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.
2.6. Catalytic oxidation
The molybdenum complex (0.001 mol Lꢀ1) and phenyl methyl sulfide (0.1 mol Lꢀ1) were
dissolved at room temperature in a mixture of CH2Cl2 and CH3OH (6:4) together with
1,3,5-trimethoxybenzene (0.1 mol Lꢀ1) as an internal standard. The resulting solution
was cooled to 283 K and H2O2 (35% w/w) added dropwise (0.125 mol Lꢀ1). An aliquot
of the reaction solution (2.0 mL) was quenched with 5 mL of a stock solution of Na2SO3
(0.1 mol Lꢀ1) at 60 min and 2 h, and extracted with dichloromethane (three times 4 mL).
From the collected organic phases the solvent was removed under reduced pressure to
dryness and the residue was redissolved in deuterated chloroform (0.6 mL) and analyzed
by 1H NMR to determine the yield with reference to the internal standard
1,3,5-trimethoxybenzene.
3. Results and discussion
3.1. Syntheses
The hydrazones were readily prepared by condensation of 4-methoxysalicylaldehyde
with 4-methoxybenzohydrazide and 2-hydroxybenzohydrazide, respectively, in meth-
anol, with no side-products. The dioxomolybdenum(VI) complexes were synthesized by
reaction of MoO2(acac)2 with the hydrazones in methanol in a 1 : 1 mole proportion

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N.-Y. Jin
Table 1. Crystal data for 1 and 2.
Complex
1
2
Chemical formula
Formula weight
Crystal shape/color
Crystal size (mm3)
Temperature (K)
Wavelength (Mo-Ka) (A)
Crystal system
Space group
Unit cell dimensions (A, )
C
458.27
Block/brown
0.15 ꢃ 0.13 ꢃ 0.12
298(2)
0.71073
Monoclinic
P21/c
17H18MoN2O7
C15H14MoN2O7
430.22
Block/brown
0.27 ꢃ 0.27 ꢃ 0.26
298(2)
0.71073
Triclinic
P-1
˚
ꢂ
˚
a
b
c
ꢂ
ꢃ
ꢄ
13.104(1)
9.939(1)
14.970(2)
90
112.509(1)
90
1801.2(4), 4
0.771
0.8931
0.9131
1.690
3922/250
3356
1
1.083
0.0203
0.0257
0.0610
0.0331
0.0646
8.343(2)
10.570(3)
10.602(3)
75.193(2)
72.384(2)
67.541(2)
813.1(4), 2
0.848
0.8033
0.8096
1.757
3419/234
2559
3
˚
Volume (A ), Z
Absorption coefficient (Mo-Ka) (cmꢀ1
Temperature (min)
)
Temperature (max)
Calculated density (g cmꢀ3
Reflections/parameters
Unique reflections
Restraints
)
3
1.037
Goodness of fit on F2
Rint
0.0421
0.0478
0.1081
0.0702
0.1214
R1 [I ꢄ 2ꢅ(I)]
wR2 [I ꢄ 2ꢅ(I)]
R1 (all data)
wR2 (all data)
at reflux. The reaction (scheme 2) is accompanied by a color change of the solution
from colorless to yellow. The chemical formulae of the complexes have been confirmed
by elemental analyses, IR spectra, and single-crystal X-ray structure determinations.
Methanol, ethanol, and water readily coordinate to Mo, generating six-coordinate
octahedral geometry. The only difference between the two hydrazones is the substituent
of the benzohydrazides, which leads to coordination of different solvents.
3.2. Crystal structure description
The molecular structures of the complexes are shown in figures 1 and 2. The structures
of the complexes are very similar, except for the small difference in the hydrazone
ligands and the solvent. The coordination geometry around Mo can be described as
slightly distorted octahedral. In 1, the equatorial plane is defined by phenolic O, imino
N, and enolic O of the dianionic hydrazone ligand L1, and by one oxo, and the two axial
positions are occupied by methanol and the other oxo. In 2, the equatorial plane is
defined by the phenolic O, imino N, and enolic O of L2, and by one oxo, and the two
axial positions are occupied by water and the other oxo. The hydrazone ligands
coordinate to Mo meridional forming five- and six-membered chelate rings with bite
angles of 71.3(1)ꢂ and 81.1(1)ꢂ for 1 and 71.8(1)ꢂ and 81.2(1)ꢂ for 2. Dihedral angles

Dioxomolybdenum(VI) with hydrazone
4017
ꢂ
˚
Table 2. Selected bond lengths (A) and angles ( ) for 1 and 2.
1
Bond lengths
Mo1–O1
Mo1–O5
Mo1–O7
1.9156(15)
1.6972(15)
1.6897(17)
Mo1–O3
Mo1–O6
Mo1–N1
2.0092(14)
2.3861(16)
2.2347(16)
Bond angles
O7–Mo1–O5
O5–Mo1–O1
O5–Mo1–O3
O7–Mo1–N1
O1–Mo1–N1
O7–Mo1–O6
O1–Mo1–O6
N1–Mo1–O6
106.06(8)
O7–Mo1–O1
O7–Mo1–O3
O1–Mo1–O3
O5–Mo1–N1
O3–Mo1–N1
O5–Mo1–O6
O3–Mo1–O6
100.46(8)
102.35(7)
97.70(7)
95.52(7)
81.07(6)
171.51(7)
81.91(6)
76.72(6)
96.77(7)
148.66(6)
156.97(7)
71.33(6)
81.20(7)
77.62(6)
2
Bond lengths
Mo1–O1
Mo1–O5
Mo1–O7
1.920(3)
1.688(3)
1.683(4)
Mo1–O3
Mo1–O6
Mo1–N1
2.017(3)
2.292(3)
2.248(3)
Bond angles
O7–Mo1–O5
O5–Mo1–O1
O5–Mo1–O3
O7–Mo1–N1
O1–Mo1–N1
O7–Mo1–O6
O1–Mo1–O6
N1–Mo1–O6
105.9(2)
102.9(2)
98.1(2)
93.6(1)
81.2(1)
170.0(1)
81.3(1)
76.5(1)
O7–Mo1–O1
O7–Mo1–O3
O1–Mo1–O3
O5–Mo1–N1
O3–Mo1–N1
O5–Mo1–O6
O3–Mo1–O6
98.6(2)
96.0(2)
149.9(1)
159.1(2)
71.8(1)
83.8(2)
79.8(1)
ꢂ
˚
Table 3. Distances (A) and angles ( ) involving hydrogen-bonding of the complexes.
D–Hꢁ ꢁ ꢁA
d(D–H)
0.85(1)
d(Hꢁ ꢁ ꢁA)
d(Dꢁ ꢁ ꢁA)
ff (D–Hꢁ ꢁ ꢁA)
1
O6–H6Aꢁ ꢁ ꢁN2i
1.98(1)
2.808(2)
171(3)
2
O4–H4ꢁ ꢁ ꢁN2
O6–H6Aꢁ ꢁ ꢁO4i
O6–H6Bꢁ ꢁ ꢁO2ii
O6–H6Bꢁ ꢁ ꢁO1ii
0.82
1.85
2.573(4)
2.748(4)
2.811(5)
2.983(4)
146
178(8)
141(7)
146(7)
0.85(1)
0.85(1)
0.85(1)
1.90(5)
2.10(5)
2.24(4)
i
ii
Symmetry codes: ꢀx, 1ꢀy, 1ꢀz; 1ꢀx, 1ꢀy, ꢀz.
between the two substituted benzene rings of the hydrazone ligands are 0.4(3)ꢂ for 1 and
3.8(3)ꢂ for 2, indicating the ligands are nearly coplanar. Displacements of Mo from the
˚
˚
equatorial mean planes toward the apical oxo O7 are 0.344(2) A for 1 and 0.308(2) A for
2. The hydrazone ligands are coordinated as dianions, evident from the N2–C8 and
˚
O3–C8 bond lengths with values of 1.308(3) and 1.313(2) A for 1 and 1.306(5) and
˚
1.309(5) A for 2, indicating the presence of the enolate form of the amide. The Mo–O,
Mo–N, and Mo¼O bonds are within normal ranges and similar to those observed in
similar dioxomolybdenum(VI) complexes [15–19].

4018
N.-Y. Jin
OMe
OMe
OMe
H
N
N
N
N
N
N
MoO2(acac)2
O
O
OH
HO
O
OH
OH
Mo
O
OMe
OMe
OMe
OMe
O
H
(1)
HO
HO
H
N
N
N
N
N
N
MoO2(acac)2
O
O
OH
O
Mo
OH
OH
OH2
OMe
O
OMe
OMe
O
(2)
Scheme 2. Preparation of the complexes.
Figure 1. Molecular structure of 1 with 30% probability thermal ellipsoids.
Figure 2. Molecular structure of 2 with 30% probability thermal ellipsoids. Hydrogen bond is shown as a
dashed line.

Dioxomolybdenum(VI) with hydrazone
4019
Figure 3. Molecular packing of 1 viewed along the b-axis. Hydrogen bonds are shown as dashed lines.
In the crystal structure of 1, adjacent [MoO2L1(CH3OH)] complexes are linked by
two methanols through two intermolecular O6–H6Aꢁ ꢁ ꢁN2 hydrogen bonds to form a
˚
dimer (figure 3). The adjacent Moꢁ ꢁ ꢁMo distance in 1 is 7.12 A. In the crystal structure
of 2, adjacent [MoO2L2(H2O)] molecules are linked by two water molecules through
two intermolecular O6–H6Aꢁ ꢁ ꢁO4 hydrogen bonds to form a dimer. The dimers are
further linked through two intermolecular O6–H6Bꢁ ꢁ ꢁO1 hydrogen bonds and two
intermolecular O6–H6Bꢁ ꢁ ꢁO2 hydrogen bonds to form 1-D chains (figure 4).
˚
Adjacent Moꢁ ꢁ ꢁMo distances in 2 are 5.28 and 9.08 A. In addition, there are
also weak ꢆꢁ ꢁ ꢁꢆ stacking interactions among C1–C6 and C9–C14 benzene rings and
Mo1–O3–C8–N2–N1 chelate planes (table 4).
3.3. IR spectra
A weak band centered at 3434 cmꢀ1 for 1 can be assigned to ꢇOH of methanol and those
centered at 3453 and 3372 cmꢀ1 for 2 can be assigned to ꢇOH of water and the hydroxyl
group. The Mo¼O stretches occur as a pair of sharp strong bands at 937 and 909 cmꢀ1
for 1 and 945 and 915 cmꢀ1 for 2, assigned to the anti-symmetric and symmetric
stretches of dioxomolybdenum(VI). Bands due to ꢇC¼O and ꢇNH were absent in the

4020
N.-Y. Jin
Figure 4. Molecular packing of 2 viewed along the b-axis. Hydrogen bonds are shown as dashed lines.
Table 4. Parameters among planes.
Distance
between ring
˚
centroids (A)
Perpendicular
distance of
Cg(I) on Cg(J) (A)
Perpendicular
distance of Cg(J)
˚
on Cg(I) (A)
Dihedral
angle (ꢂ)
˚
Cg
1
Cg(1)–Cg(1)iii
Cg(1)–Cg(2)iv
Cg(2)–Cg(3)v
Cg(2)–Cg(3)iii
4.439
4.469
3.783
3.757
0.02
18.24
3.55
3.306
3.601
3.506
3.635
3.306
3.284
3.494
3.632
0.39
2
Cg(1)–Cg(1)vi
Cg(1)–Cg(2)vi
Cg(2)–Cg(3)i
Cg(2)–Cg(3)i
Cg(5)–Cg(6)vi
4.971
4.526
4.839
4.973
3.878
0.00
5.61
7.26
3.79
3.79
3.575
3.555
3.434
3.458
3.484
3.575
3.679
3.157
3.224
3.398
i
iii
iv
v
vi
Symmetry codes: ꢀx, 1 ꢀ y, 1 ꢀ z; ꢀx, 1 ꢀ y, 1 ꢀ z; ꢀx, 1/2 þ y, 1/2 ꢀ z; ꢀx, ꢀ1/2 þ y, 1/2 ꢀ z; 1 ꢀ x, 1 ꢀ y, 1 ꢀ z.
Cg(1), Cg(2), and Cg(3) are the centroids of Mo1–O3–C8–N2–N1, C1–C2–C3–C4–C5–C6, and C9–C10–C11–C12–C13–C14,
respectively.
complexes and new C–O stretches appear at 1262 cmꢀ1 for 1 and 1254 cmꢀ1 for 2,
suggesting keto-imine tautomerization of the hydrazone ligands during coordination.
Strong bands indicative of C¼N–N¼C in the complexes are shifted to 1609 cmꢀ1 for 1
and 1611 cmꢀ1 for 2. New weak peaks at 350–800 cmꢀ1 may be attributed to Mo–O and
Mo–N vibrations in the complexes. IR spectra of the two complexes are similar,

Dioxomolybdenum(VI) with hydrazone
4021
O
S
S
Complex
H2O2/TMB
Scheme 3. Oxidation with 1 and 2 as catalysts.
indicating the complexes have similar structures, as evidenced by the single-crystal
X-ray determination.
3.4. Catalytic sulfoxidation
Using the complexes for oxidation of sulfides under homogeneous conditions in
solution using methyl phenyl sulfide (thioanisol) as substrate are shown as in scheme 3.
Hydrogen peroxide was used as oxidant in a slight excess of 1.25 equivalents based on
the sulfide substrate. Reactions were run with 1 mol% of catalyst based on the substrate
at 10ꢂC. NMR was used to monitor formation of the sulfoxides with 1,3,5-
trimethoxybenzene as internal standard to determine the yields. The reaction was
started by addition of hydrogen peroxide. A control reaction under the same condition
without any complex present leads to less than 1% sulfide conversion within 4 h. In the
presence of the complexes conversions of sulfide to the corresponding sulfoxide of
about 77% for 1 and 82% for 2 within 60 min were observed. After 2 h conversions of
sulfide were complete. Under the given conditions no over oxidation to the sulfone
could be detected. The complexes are effective for the sulfoxidation, with 2 a little
superior to 1. Properties of the complexes in this work are a little better than those of
the previously reported similar dioxomolybdenum(VI) complexes [11, 12].
4. Conclusion
Two new dioxomolybdenum(VI) complexes with similar hydrazone ligands, N0-(2-
hydroxy-3methoxybenzylidene)-4-methoxybenzohydrazide and N0-(2-hydroxy-3-meth-
oxybenzylidene)-2hydroxybenzohydrazide, have been prepared and structurally char-
acterized. The dianionic hydrazone ligands coordinate to Mo through the phenolic O,
imino N, and enolic O. Mo has octahedral coordination. Methanol and water are co-
ligands during coordination. The complexes are efficient catalysts for the peroxidic
oxidation of sulfides.
Supplementary material
CCDC reference numbers 854792 for 1 and 854793 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 the Cambridge Crystallographic Data Center, 12 Union

4022
N.-Y. Jin
Road, Cambridge CB2 1EZ, UK; Fax: þ44 1 223 336 033; E-mail:
deposit@ccdc.cam.ac.uk
Acknowledgments
The authors greatly acknowledge Jingchu University of Technology for financial
support.
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