Synthesis, x-ray structural characterization, and catalytic property of dioxomolybdenum(VI) Complexes with N-(3-Bromo-2-hydroxybenzylidene)-2- hydroxybenzohydrazide and N-(5-Chloro-2-hydroxybenzylidene)-4- nitrobenzohydrazide

Article abstract of DOI:10.1080/15533174.2013.778883

Two new dioxomolybdenum(VI) complexes [MoO2(Bhbz) (CH3OH)] (1) and [MoO2(Cnbz)(CH3OH)] (2) with the benzohydrazone ligands H2Bhbz and H2Cnbz derived from 5-chlorosalicylaldehyde with 4-nitrobenzohydrazide and 3-bromosalicylaldehyde with 2-hydroxybenzohydrazide, respectively, have been synthesized and structurally characterized by physicochemical methods and single crystal X-ray determination. The crystal of (1) crystallizes in triclinic space group P-1, with a = 8.0014(9), b = 9.6294(10), c = 11.4204(12) A, = 89.193(2), β = 87.833(2), γ = 89.110(2)°, V = 879.11(16) A³, Z = 2, R 1 = 0.0464, wR 2 = 0.1185, S = 1.051. The crystal of (2) crystallizes in monoclinic space group P21/c, with a = 7.4108(3), b = 18.7901(7), c = 12.6293(5) A, β = 104.4850(10)°, V = 1702.72(12) A³, Z = 4, R 1 = 0.0236, wR 2 = 0.0576, S = 1.061. X-ray analysis indicates that the structures of both complexes are similar to each other. The molybdenum atom in each complex is in an octahedral coordination environment, constructed by two oxo groups and NO2 donor set of the ligand, and one methanol O atom. The complexes have effective catalytic property for the oxidation of several olefins.

Full text of DOI:10.1080/15533174.2013.778883

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Synthesis, X-Ray Structural Characterization,
and Catalytic Property of Dioxomolybdenum(VI)
Complexes With N’-(3-Bromo-2-hydroxybenzylidene)-2-
hydroxybenzohydrazide and N’-(5-Chloro-2-
hydroxybenzylidene)-4- nitrobenzohydrazide
Yan Lei a b , Qiwen Yang a c , Gangcai Chen a d & Qingling Yang d
a School of City Construction and Environmental Engineering , Chongqing University ,
Chongqing , P. R. China
b School of Chemistry and Environmental Engineering , Chongqing Three Gorges University ,
Chongqing , P. R. China
c China Huaxi Engineering Design & Construction Co. , ltd, Sichuan , P. R. China
d Chongqing Institute of Environmental Science Research , Chongqing , P. R. China
Accepted author version posted online: 23 Sep 2013.Published online: 21 Nov 2013.
To cite this article: Yan Lei , Qiwen Yang , Gangcai Chen & Qingling Yang (2014) Synthesis, X-Ray Structural
Characterization, and Catalytic Property of Dioxomolybdenum(VI) Complexes With N’-(3-Bromo-2-hydroxybenzylidene)-2-
hydroxybenzohydrazide and N’-(5-Chloro-2-hydroxybenzylidene)-4- nitrobenzohydrazide, Synthesis and Reactivity in
Inorganic, Metal-Organic, and Nano-Metal Chemistry, 44:4, 590-597, DOI: 10.1080/15533174.2013.778883
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 44:590–597, 2014
Copyright ꢀ Taylor & Francis Group, LLC
C
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2013.778883
Synthesis, X-Ray Structural Characterization, and Catalytic
Property of Dioxomolybdenum(VI) Complexes With N’-(3-
Bromo-2-hydroxybenzylidene)-2-hydroxybenzohydrazide
and N’-(5-Chloro-2-hydroxybenzylidene)-4-
nitrobenzohydrazide
Yan Lei,1,2 Qiwen Yang,1,3 Gangcai Chen,1,4 and Qingling Yang4
1School of City Construction and Environmental Engineering, Chongqing University, Chongqing,
P. R. China
2School of Chemistry and Environmental Engineering, Chongqing Three Gorges University, Chongqing,
P. R. China
3China Huaxi Engineering Design & Construction Co., ltd, Sichuan, P. R. China
4Chongqing Institute of Environmental Science Research, Chongqing, P. R. China
INTRODUCTION
Two new dioxomolybdenum(VI) complexes [MoO2(Bhbz)
(CH3OH)] (1) and [MoO2(Cnbz)(CH3OH)] (2) with the ben-
zohydrazone ligands H2Bhbz and H2Cnbz derived from
5-chlorosalicylaldehyde with 4-nitrobenzohydrazide and 3-
bromosalicylaldehyde with 2-hydroxybenzohydrazide, respec-
tively, have been synthesized and structurally characterized by
The coordination chemistry of molybdenum(VI) has
attracted considerable attention due to its biological im-
portance[1–3] and their application as catalysts in various
oxidations reactions.[4–6] Recent reports indicated that the
molybdenum(VI) complexes with hydrazone ligands possess
physicochemical methods and single crystal X-ray determination. oxygen atom transfer properties as they were found to oxidize
thiols, hydrazine, polyketones, and tertiary phosphines.[7,8]
The crystal of (1) crystallizes in triclinic space group P-1, with a
= 8.0014(9), b = 9.6294(10), c = 11.4204(12) Å, α = 89.193(2),
Moreover, molybdate as a functional mimic for vanadium-
haloperoxidase enzymes shows a higher catalytic activity as
β = 87.833(2), γ = 89.110(2)◦, V = 879.11(16) Å3, Z = 2, R1 =
0.0464, wR2 = 0.1185, S = 1.051. The crystal of (2) crystallizes in
monoclinic space group P21/c, with a = 7.4108(3), b = 18.7901(7),
c = 12.6293(5) Å, β = 104.4850(10)◦, V = 1702.72(12) Å3, Z =
4, R1 = 0.0236, wR2 = 0.0576, S = 1.061. X-ray analysis indi-
cates that the structures of both complexes are similar to each
other. The molybdenum atom in each complex is in an octahe-
dral coordination environment, constructed by two oxo groups and
NO2 donor set of the ligand, and one methanol O atom. The com-
plexes have effective catalytic property for the oxidation of several
olefins.
compared to vanadate.[9] However, in comparison with the
oxovanadium complexes, the number of crystal structures of
molybdenum complexes with hydrazone ligands especially
benzohydrazone ligands is rather few. Herein we report the
synthesis, X-ray crystal structures, and catalytic oxidation
property of two new dioxomolybdenum(VI) complexes,
[MoO2(Bhbz)(CH3OH)] (1) and [MoO2(Cnbz)(CH3OH)] (2),
with similar tridentate benzohydrazone ligands N’-(3-bromo-2-
hydroxybenzylidene)-2-hydroxybenzohydrazide (H2Bhbz) and
N’-(5-chloro-2-hydroxybenzylidene)-4-nitrobenzohydrazide
(H2Cnbz), respectively.
Keywords Benzohydrazone ligand, synthesis, catalytic oxidation,
crystal structure, molybdenum complex
EXPERIMENTAL
Materials
Received 29 January 2013; accepted 19 February 2013.
This work was supported by the Fundamental Research Funds for
the Central Universities (Project No. 0969801), and Key Laboratory of
Water Environment Evolution and Pollution Control in Three Gorges
Reservior (Chongqing Three Georges University).
Address correspondence to Yan Lei, School of Chemistry and
Environmental Engineering, Chongqing Three Gorges University,
Chongqing 404000, P. R. China. E-mail: leiyan222@aliyun.com
Ammonium molybdate, acetylacetone, 5-chlorosalicylalde
hyde, 3-bromosalicylaldehyde, 4-nitrobenzohydrazide, and
2-hydroxybenzohydrazide were purchased from Aldrich.
[MoO2(acac)2] was prepared and purified as described previ-
ously.[10] All other reagents were used as received without fur-
ther purification.
590

DIOXOMOLYBDENUM(VI) COMPLEXES
591
TABLE 1
TABLE 2
Crystallographic data for the single crystals of (1) and (2)
Selected bond distances (Å) and bond angles (◦) for (1) and (2)
(1)
(2)
(1)
Bond distance
Mo1–O1
Mo1–N1
Mo1–O6
Bond angle
Empirical
formula
Formula weight
Temperature
(K)
C15H12ClMoN3O7 C15H13BrMoN2O6
1.923(3)
2.255(3)
1.698(3)
Mo1–O2
Mo1–O5
Mo1–O7
2.021(3)
2.349(3)
1.691(3)
477.7
493.1
298(2)
298(2)
O7–Mo1–O6 106.30(18)
O6–Mo1–O1 100.95(16)
O6–Mo1–O2 99.85(16)
O7–Mo1–N1 95.33(16)
O1–Mo1–N1 80.69(13)
O7–Mo1–O5 170.58(15)
O1–Mo1–O5 82.96(14)
N1–Mo1–O5 75.87(12)
O7–Mo1–O1 99.10(18)
O7–Mo1–O2 96.73(16)
O1–Mo1–O2 149.05(14)
O6–Mo1–N1 157.64(15)
O2–Mo1–N1 71.48(12)
O6–Mo1–O5 82.17(15)
O2–Mo1–O5 77.45(13)
Crystal system
Space group
a (Å)
Triclinic
P–1
Monoclinic
P21/c
7.4108(3)
18.7901(7)
12.6293(5)
90
104.4850(10)
90
1702.72(12)
4
8.0014(9)
9.6294(10)
11.4204(12)
89.193(2)
87.833(2)
89.110(2)
879.11(16)
2
b (Å)
c (Å)
α (◦)
β (◦)
γ (◦)
(2)
V (Å3)
Bond distance
Z
Mo1–O1
Mo1–N1
Mo1–O5
1.9141(15)
2.2417(17)
1.6900(19)
Mo1–O2
Mo1–O4
Mo1–O6
2.0088(16)
2.3603(18)
1.6967(16)
F(000)
476
3227/1/248
968
3520/1/231
Data/restraints/
parameters
Goodness-of-fit
on F2
Final R indices
[I > 2σ(I)]
Bond angle
1.051
1.061
O5–Mo1–O6 105.64(9)
O6–Mo1–O1 102.43(8)
O6–Mo1–O2 97.61(8)
O5–Mo1–N1 96.24(8)
O1–Mo1–N1 81.07(6)
O5–Mo1–O4 170.62(8)
O1–Mo1–O4 81.46(8)
N1–Mo1–O4 74.79(6)
O5–Mo1–O1 100.11(9)
O5–Mo1–O2 95.26(9)
O1–Mo1–O2 150.30(7)
O6–Mo1–N1 156.70(8)
O2–Mo1–N1 72.07(6)
O6–Mo1–O4 82.89(8)
O2–Mo1–O4 79.51(7)
R1 = 0.0464
R1 = 0.0236
wR2 = 0.1185
wR2 = 0.0576
Physical Measurements
Infrared spectra (4000-400 cm−1) were recorded as KBr discs
with a FTS-40 BioRad FT-IR spectrophotometer. Electronic
spectra were recorded on a Shimadzu UV 3101 spectrophotome-
ter. Microanalyses (C, H, N) of the ligands and the complexes
were carried out on a Carlo-Erba 1106 elemental analyzer. So-
lution electrical conductivity was measured at 298K using a
DDS-11 conductivity meter. GC analyses were performed on a
Shimadzu GC-2010 gas chromatograph.
experimental details for the structure analysis are summarized
in Table 1, and the selected bond lengths and angles are listed
in Table 2.
Preparation of H2Bhbz
Hot methanol solutions of 5-chlorosalicylaldehyde and 4-
nitrobenzohydrazide (1:1, v/v) were stirred under reflux for 1 h,
and cooled to room temperature. The yellow precipitate was
then filtered, washed with methanol and dried in vacuo. Yield:
97%. Anal. Calcd. for C14H10ClN3O4 (%): C, 52.6; H, 3.2; N,
13.1. Found (%): C, 52.4; H, 3.1; N, 13.3.
X-Ray Crystallography
Crystallographic data of complexes (1) and (2) were collected
on a Bruker SMART CCD area diffractometer with graphite
monochromated Mo-Kα radiation (λ = 0.71073 Å) at 298(2)
K. Absorption corrections were applied by using the multi-
scan program.[11] The structures were solved by direct meth-
ods and successive Fourier difference syntheses (SHELXS-97),
and anisotropic thermal parameters for all nonhydrogen atoms
were refined by full-matrix least-squares procedure against
F2 (SHELXL-97).[12] All non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were set in calculated posi-
tions and refined by a riding mode, with a common thermal
parameter. H atoms for the methanol molecules were located
in difference Fourier map and refined isotropically, with O H
Preparation of H2Cnbz
The synthesis of H2Cnbz was similar to that of H2Bhbz, but
with 3-bromosalicylaldehyde and 2-hydroxybenzohydrazide.
Yield: 95%. Anal. Calcd. for C14H11BrN2O3 (%): C, 50.2; H,
3.3; N, 8.4. Found (%): C, 50.1; H, 3.4; N, 8.3.
Preparation of [MoO2(Bhbz)(CH3OH)] (1)
To a stirred solution of H2Bhbz (0.32 g, 1 mmol) in 50 mL
distances restrained to 0.85(1) Å. The crystallographic data and methanol was added MoO2(acac)2 (0.33 g, 1 mmol). The

592
Y. LEI ET AL.
X'
Y'
O
Y'
H
N
Y
CHO
OH
Y
NH2
N
N
+
H
O
OH
X'
X
X
SCH. 1. The preparation of the benzohydrazone ligands. H2Bhbz: X = Yꢀ = H, Y = Cl, Xꢀ = NO2; H2Cnbz: X = Br, Y = Xꢀ = H, Yꢀ = OH.
X'
Y'
X'
Y'
H
N
O
Y
N
Y
N
N
MoO2(acac)2
+
O
O
O
Mo
OH
X
O
X
MeOH
SCH. 2. The preparation of the complexes. H2Bhbz: X = Yꢀ = H, Y = Cl, Xꢀ = NO2; H2Cnbz: X = Br, Y = Xꢀ = H, Yꢀ = OH.
resulting mixture was refluxed for 1 h. The orange reaction RESULTS AND DISCUSSION
solution was filtered and the solvent removed under reduced
pressure, yielding red solid of the complex. Yield: 51%. Red
Synthesis
The two benzohydrazone compounds are readily prepared
single crystals suitable for X-ray diffraction were obtained by
recrystallization of the solid from methanol. Anal. Calcd. for
C15H12ClMoN3O7 (%): C, 37.7; H, 2.5; N, 8.8. Found (%): C,
37.5; H, 2.7; N, 8.7.
by condensation reaction of benzohydrazides with salicylalde-
hydes in methanol (see Scheme 1). The stoichiometric reactions
of the benzohydrazone ligands with MoO2(acac)2 as molybde-
num source in refluxing methanol yielded the corresponding
dioxomolybdenum(VI) complexes (see Scheme 2). The reac-
tion progress is accompanied by an immediate color change
Preparation of [MoO2(Cnbz)(CH3OH)] (2)
The synthesis and crystallization of complex (2) was similar of the solution from yellow to orange. We have attempted to
to that of (1), but with H2Cnbz (0.34 g, 1 mmol) was used instead prepare and grow diffraction quality crystals of the complexes
of H2Bhbz. Yield: 67%. Anal. Calcd. for C15H13BrMoN2O6 from various solvents; yet, only methanol is suitable. The mo-
(%): C, 36.5; H, 2.7; N, 5.7. Found (%): C, 36.6; H, 2.6.7; lar conductance values of the complexes measured in absolute
N, 5.7.
methanol at the concentration of 10–3 M is 21 and 25 ꢀ–1 cm2
FIG. 1. ORTEP diagram of complex (1) (30% thermal ellipsoid).

DIOXOMOLYBDENUM(VI) COMPLEXES
593
FIG. 2. ORTEP diagram of complex (2) (30% thermal ellipsoid).
FIG. 3. The dimeric structure of complex (1) linked by the O H···N hydrogen bonds.

594
Y. LEI ET AL.
SCH. 3. Proposed catalytic oxidation mechanism.
mol−1, indicating the nonelectrolytic nature of the complexes in
methanol.[13]
ilar to each other. The benzohydrazone ligand in each complex is
approximately planar, with dihedral angles between the benzene
rings of 7.8(3)◦ for (1) and 6.5(3)◦ for (2). The benzohydrazone
ligands coordinate in dianionic fashion forming five- and six-
membered chelate rings with Mo atoms. This is evident from the
N2–C8 and O2–C8 bond lengths with values of about 1.30 and
Description of the Structures
The perspective views of the complexes (1) and (2) are shown
in Figures 1 and 2, respectively. The two complexes are very sim-
FIG. 4. The chain structure of complex (2) linked by the O—H···O hydrogen bonds and Br···O interactions.

DIOXOMOLYBDENUM(VI) COMPLEXES
595
TABLE 3
Catalytic oxidation resultsa
Substrate
Product
Conversion (%)b
Selectivity (%)
(1)
(2)
85
82
100
100
(1)
(2)
77
80
100
100
(1)
(2)
73
75
100
100
(1)
(2)
100
100
100
100
(1)
(2)
100
100
100
100
(1)
(2)
100
100
100
100
aThe molar ratio of catalyst:substrate:TBHP is 1:300:1000. The reactions were performed in mixture of CH3OH/CH2Cl2 (V:V = 7:3; 1.5 mL).
bThe GC conversion (%) was measured relative to the starting substrate after 1 h.
lated to the Mo atoms. The N1–Mo1–O2 bond angles are much
less than the ideal values of 90◦, as a result of the strain created
by the five-membered chelate rings Mo1–N1–N2–C8–O2.
In the crystal structures of complex (1), molecules are
linked via intermolecular O5—H5···N2i (O5—H5 = 0.85(1) Å,
H5···N2i = 1.97(1) Å, O5···N2i = 2.818(5) Å, O5—H5···N2i
= 175(7)◦; symmetry code for i: 1 – x, 1 – y, – z) hydrogen
bonds to form dimers, as shown in Figure 3. In the crystal struc-
tures of complex (2), molecules are linked via intermolecular
O4—H4A···O3ii (O4—H4A = 0.85(1) Å, H4A···O3ii = 1.95(1)
Å, O4···O3ii = 2.785(2) Å, O4—H4A···O3ii = 174(4)◦; symme-
try code for ii: – x, 1 – y, 1 – z) hydrogen bonds to form dimers.
The dimers are further linked through weak Br···O (3.156(2) Å)
interactions to form 1D chains, as shown in Figure 4.
1.31 Å, respectively, which are indicative for the presence of
the enolate form of the ligand amide groups. The coordination
around the Mo atom in each complex is distorted octahedral.
The benzohydrazone ligand coordinates through one phenolate
O, one imine N, and one ethanolic O atoms to the MoO2 moiety,
and the sixth weaker coordination comes from the O atom of the
methanol molecule. The equatorial donor atoms, O1, O2, O6,
and N1, form a high degree of planarity, with mean deviations
from the least-squares planes of 0.032(3) Å for (1) and 0.043(3)
Å for (2). The Mo atoms are displaced by 0.334(2) Å for (1)
and 0.323(3) Å for (2) toward the axial oxo O atoms from the
least-squares planes defined by the equatorial donor atoms. The
Mo O and Mo N bonds in both structures are comparable
to each other. The Mo O bond lengths are within previously
reported ranges.[4,7,14] The rather long Mo Omethanol bonds and
consequently weak bonding is due to the trans influence of the
oxo groups. The distortion of the octahedral coordination of
both structures can also be observed from the bond angles re-
Spectral Characterization
The weak bands centered at about 3350 cm–1 in the IR spectra
of both complexes may be attributed to the νOH vibrations of the

596
Y. LEI ET AL.
coordinated methanol molecules. The benzohydrazone ligands adopt keto-imine tautomerization and behave as tridentate dian-
show stretching bands attributed to C O, C N, C OH, ionic ligands. The complexes show effective catalytic property
and NH at about 1660–1675, 1635–1645, 1220–1230, and in the oxidation of olefins to their corresponding epoxides.
3250–3280 cm–1, respectively.[15] The Mo O stretching modes
in both complexes occur as a single and strong bands at 930 cm–1,
assigned to the stretching vibrations of the dioxomolybde-
Supplementary data are available from the Cambridge
num(VI) moieties.[16] The bands due to the νC O and νNH were
Crystallographic Data Center (CCDC), 12 Union Road,
absent in the complexes, and new C O stretches appear at
SUPPLEMENTARY MATERIALS
Cambridge CB2 1EZ, UK (fax: +44-1223-336-033; e-mail:
deposit@ccdc.cam.ac.uk; or via www.ccdc.cam.ac.uk/conts/
retrieving.html) on request, quoting the deposition numbers:
CCDC 921306 and 921307.
about 1250–1265 cm–1. This suggests occurrence of keto-imine
tautomerization of the benzohydrazone ligands during coordi-
nation. The strong bands indicative of the typical azomethine
groups in the two complexes are observed at 1609 cm–1 for (1)
and 1613 cm–1 for (2).[17] The new peaks observed in the low
wavenumbers can be attributed to the vibrations of the Mo–O
and Mo–N bonds. The comparison of the IR spectra of the free
ligands and the dioxomolybdenum(VI) complexes confirms the
enolate coordination mode of the benzohydrazone ligands.
The electronic spectra for the complexes recorded in ab-
solute methanol displayed absorption maxima in the range
390–410, 300–320, and 260–285 nm. The bands at 300–320
and 260–285 nm can be attributed to the internal ligand transi-
tions. The absorption maxima at 390–410 nm could be assigned
to the ligand-to-metal Mo(dπ) ← O(π) charge transfer (LMCT)
transition. This is in the range usually observed for MoO2 com-
plexes.[18]
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Catalytic Oxidation Results
The catalytic experiment was carried out according to
the literature method by using tert-butyl hydrogen peroxide
(TBHP) as the oxidant.[19] The results are summarized in Ta-
ble 3. Oxidation of cyclohexene, vinylbenzene, and 1-methyl-4-
vinylbenzene gave the corresponding epoxides in 100% yields,
while in the oxidation of pent-1-ene, hex-1-ene, and hept-1-ene,
the yields are less than 100%. Thus, it is obvious that the termi-
nal double bonds are less reactive than the conjugated double
bonds. This is in agreement with those reported in the litera-
ture.[19] The selectivity of the complexes for the oxidation of
all substrates is 100%. Comparison of the catalytic properties
of the complexes (1) and (2) shows that there is no obvious
difference between them. The proposed catalytic mechanism is
depicted by Scheme 3. At the first step, TBHP was activated by
coordination to the Mo atom and formation of heptacoordinated
molybdenum intermediate. Then olefin as a nucleophile attacked
to the electrophile oxygen atom of the coordinated TBHP. Fi-
nally, the epoxides were formed, and TBHP was reduced as
tert-butyl alcohol.
10. Chen, G.J.J.; McDonald, J.W.; Newton, W.E. Synthesis of Mo(IV) and
Mo(V) complexes using oxo abstraction by phosphines: mechanistic impli-
cations. Inorg. Chem. 1976, 15, 2612–2615.
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Crystallogr. A 1995, 51, 33–38.
12. Sheldrick, G.M. SHELXTL97, Program for refining crystal structure refine-
ment, University of Go¨ttingen: Germany, 1997.
13. Geary, W.J. Use of conductivity measurements in organic solvents for
characterization of coordination compounds. Coord. Chem. Rev. 1971, 7,
81–122.
14. Dinda, R.; Ghosh, S.; Falvello, L.R.; Toma´s, M.; Mak, T.C.W. Synthe-
sis, structure, and reactivity of some new dipyridyl and diamine-bridged
dinuclear oxomolybdenum(VI) complexes. Polyhedron 2006, 25, 2375–
2382.
CONCLUSIONS
Two new dioxomolybdenum(VI) complexes with similar tri-
dentate benzohydrazone ligands have been synthesized and
structurally characterized. In view of the microanalyses data,
evidence from spectral measurements and from single-crystal
X-ray determination, confirms that the benzohydrazone ligands
15. Vrdoljak, V.; Prugovecki, B.; Matkovic-Calogovic, D.; Pisk, J.; Dreos, R.;
Siega, P. Supramolecular hexagon and chain coordination polymer contain-
2+
ing the MoO2 core: Structural transformation in the solid state. Cryst.
Growth Des. 2011, 11, 1244–1252.

DIOXOMOLYBDENUM(VI) COMPLEXES
597
16. Dinda, R.; Sengupta, P.; Ghosh, S.; Mayer-Figge, H.; Sheldrick, W.S. A
family of mononuclear molybdenum-(VI), and –(IV) oxo complexes with
a tridentate (ONO) ligand. J. Chem. Soc. Dalton Trans. 2002, 4434–4439.
17. Ngan, N.K.; Lo, K.M.; Wong, C.S.R. Synthesis, structure studies and
18. Gupta, S.; Barik, A.K.; Pal, S.; Hazra, A.; Roy, S.; Butcher, R.J.; Kar, S.K.
Oxomolybdenum(VI) and (IV) complexes of pyrazole derived ONO donor
ligands – synthesis, crystal structure studies and spectroelectrochemical
correlation. Polyhedron 2007, 26, 133–141.
electrochemistry of molybdenum(VI) Schiff base complexes in the 19. Rayati, S.; Rafiee, N.; Wojtczak, A. cis-Dioxo-molybdenum(VI) Schiff
presence of different donor solvent molecules. Polyhedron 2011, 30,
base complexes: Synthesis, crystal structure and catalytic performance for
2922–2932.
homogeneous oxidation of olefins. Inorg. Chim. Acta. 2012, 386, 27–35.