New mononuclear dioxidomolybdenum(VI) complexes with hydrazone ligands: Synthesis, crystal structures and catalytic performance

Article abstract of DOI:10.1016/j.ica.2018.11.020

Three new dioxomolybdenum (VI) complexes, [MoO₂L¹(MeOH)] (1), [MoO₂L²(MeOH)] (2) and [MoO₂L³(H₂O)]·EtOH (3), where L¹, L² and L³ are the dianionic form of N′-(2-hydroxybenzylidene)-3-methylbenzohydrazide, 4-bromo-N′-(2-hydroxybenzylidene)benzohydrazide and 2-bromo-N′-(2-hydroxy benzylidene)benzohydrazide, respectively, have been prepared and characterized by IR, UV–Vis and NMR spectra, as well as single crystal X-ray diffraction. The Mo atoms in the complexes are in octahedral coordination, with the ONO donor set of the hydrazone ligands, O atoms of the solvents, and two oxo groups. Crystals of the complexes are stabilized by hydrogen bonds of types O–H?N and O–H?O. Moreover, the catalytic properties of complexes 1–3 are compared in cyclohexane oxidation. It was found that sebacic acid and FeCl₃ could promote the catalytic activities of the complexes.

Full text of DOI:10.1016/j.ica.2018.11.020

Accepted Manuscript
Research paper
New mononuclear dioxidomolybdenum(VI) complexes with hydrazone li-
gands: synthesis, crystal structures and catalytic performance
Cong Wang, Na Xing, Wenjing Feng, Sihan Guo, Mingyang Liu, Yue Xu,
Zhonglu You
PII:
S0020-1693(18)31273-8
https://doi.org/10.1016/j.ica.2018.11.020
ICA 18635
DOI:
Reference:
Inorganica Chimica Acta
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15 November 2018
16 November 2018
Please cite this article as: C. Wang, N. Xing, W. Feng, S. Guo, M. Liu, Y. Xu, Z. You, New mononuclear
dioxidomolybdenum(VI) complexes with hydrazone ligands: synthesis, crystal structures and catalytic performance,
Inorganica Chimica Acta (2018), doi: https://doi.org/10.1016/j.ica.2018.11.020
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New mononuclear dioxidomolybdenum(VI) complexes with
hydrazone ligands: synthesis, crystal structures and catalytic
performance
Cong Wang, Na Xing*, Wenjing Feng, Sihan Guo, Mingyang Liu, Yue Xu and
Zhonglu You*
College of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian
116029, P. R. China
Corresponding author. E-mail address: scarllet_na@163.com (N. Xing)
youzhonglu@163.com (Z. You)
Abstract
Three new dioxomolybdenum (VI) complexes, [MoO2L1(MeOH)] (1),
[MoO2L2(MeOH)] (2) and [MoO2L3(H2O)]∙EtOH (3), where L1, L2 and L3 are the
dianionic
form
of
N'-(2-hydroxybenzylidene)-3-methylbenzohydrazide,
4-bromo-N'-(2-hydroxybenzylidene)benzohydrazide and 2-bromo-N'-(2-hydroxy
benzylidene)benzohydrazide, respectively, have been prepared and characterized by
IR, UV-Vis and NMR spectra, as well as single crystal X-ray diffraction. The Mo
atoms in the complexes are in octahedral coordination, with the ONO donor set of the
hydrazone ligands, O atoms of the solvents, and two oxo groups. Crystals of the
complexes are stabilized by hydrogen bonds of types O–H∙∙∙N and O–H∙∙∙O.
Moreover, the catalytic properties of complexes 1–3 are compared in cyclohexane
oxidation. It was found that sebacic acid and FeCl3 could promote the catalytic
activities of the complexes.
Keywords: Hydrazone; Molybdenum complex; Crystal structure; Catalytic oxidation;
Cyclohexane
1. Introdution
The selective oxidation of hydrocarbons is an important research object in the
world [1-5]. Catalytic oxidation of cyclohexane (Cy) is a representative one, for its
products of cyclohexanol (CyOH), cyclohexanone (CyO), adipic acid, glutaric acid
and so on, are all important industrial products [6]. Soluble cobalt salts and complexes
1

are often used as industrial catalysts for this reaction. However, it usually takes higher
temperature and pressure to overcome the inertness of C-H bond in the hydrocarbons
[7-13]. Therefore, research has been done to improve the conditions of Cy reaction in
industrial production [14]. For example, Sankar et al. reported that they used
CoAlPO-11 as catalysis to Cy oxidation in air. The conversion of Cy could reach up to
o
16.1% at 403 K (130 C) under 1.5 MPa [15]. Huang et al. also gave a similar
o
condition that was realized on zinc oxide supported iron porphyrin at 423 K (150 C)
under 0.7 MPa [16]. Song et al. observed that the Al-Cu-Co catalyst was applied to Cy
oxidation, and the selectivity of CyO reached 58.7% at 413 K (140 oC) under 0.5 MPa
[17]. However, it is still a goal for catalytic oxidation of Cy at lower temperature
below the boiling point of Cy (80.7 oC) to receive more reaction selective control and
relieve the hazard [18].
In recent years, the main research has been focusing on metal salts beside Co, such
as iron, copper, manganese and their complexes as catalysts [6, 19-21]. Wang et al.
revealed that the total selectivity of the products (adipic acid+CyOH+CyO) reached to
79.7% in Cy oxidation with utilizing manganese porphyrins as catalyst and PhCOOH
as co-catalyst [6]. Lindhorst et al. showed that the highest TON value of products was
317.3 when molecular iron complexes were used as catalysts and H2O2 as oxidant in
alkane oxidation [21]. Li et al. reported that the yield of the product could reach to
46.2% with a porphyrinic MOF, PCN-222(Fe)-F7 as catalyst in the catalytic oxidation
of Cy [22]. Among them, molybdenum complexes are particularly studied, for the
molybdenum enzymes have been found to have excellent catalytic properties. They
can regulate many physiological processes, such as hormone metabolism, carbon
metabolism, nitrogen metabolism and active oxygen metabolism in plants [23-26].
Therefore, the catalytic activity of molybdenum complexes is worth being studied,
especially these synthesized with molybdenum enzyme as template. The recent
research has focused on the synthesis of Mo complexes with molybdenum oxidase,
2+
such as nitrate reductase, sulfite oxidase and xanthine oxidase that all contain MoO2
group [27]. Biswal et al. showed that the dioxidomolybdenum (VI) complex
[MoO2L]2•THF with an ONO donor ligand L (H2L= 2-aminobenzoylhydrazone of
2

benzoylacetone) had a very good catalytic activity for the styrene oxidation (TOF=
2+
4900 h-1) [28]. Bagherzadeh et al. displayed that the complex containing MoO2 and
salicylidene 2-picoloyl hydrazone ligand had an excellent performance (the selectivity
of epoxides are all 66 % above) in the epoxidation of seven olefins [29]. Gomes et al.
reported that the selectivity of the acetalisation of benzaldehyde with ethanol at 55 °C
was 100% at 10 min when complex [MoO2Cl2(L)] with L=
4,4’-di-tert-butyl-2,2’-bipyridine was used as a catalyst [30]. To sum up, it indicates
that the complexes with MoO22+ as active site would have potential catalytic activity.
On the other hand, Schiff base complexes play important roles in many reactions
due to their low cost, thermodynamic property stability, and high catalytic
performance [31-33]. Hydrazone, a special kind of Schiff base, has more stable
structure than common Schiff base in some aspects. Therefore, the synthesis of
oxomolybdenum complexes with hydrazon and their catalytic activities in the
oxidation of Cy are attracting much attention at present [34].
In this paper, three new dioxomolybdenum (VI) complexes, [MoO2L1(MeOH)] (1),
[MoO2L2(MeOH)] (2) and [MoO2L3(H2O)]∙EtOH (3), where L1, L2 and L3 are the
dianionic
form
of
N'-(2-hydroxybenzylidene)-3-methylbenzohydrazide,
4-bromo-N'-(2-hydroxybenzylidene)benzohydrazide and 2-bromo-N'-(2-hydroxy
benzylidene)benzohydrazide, respectively, have been prepared and structurally
characterized. Their catalytic properties are also studied. By adjusting the temperature
of the reaction, altering the solvent of the reaction, changing the amount of the
reaction catalyst and the acid auxiliary, the optimum reaction conditions are found.
Then, the effects of adding mixed acids on the reaction are explored. At last, the
activity of the three complexes is compared.
2. Experimental
2.1. Materials and methods
Salicylaldehyde,
3-methylbenzohydrazide,
4-bromobenzohydrazide,
and
2-bromobenzohydrazide were purchased from Sigma-Aldrich company. All other
chemicals and solvents with AR grade were obtained from Xiya Reagent Co. Ltd.
Elemental analyses were performed on a Perkin-Elmer 240C elemental analyzer. IR
spectra were recorded on a Jasco FT/IR-4000 spectrometer as KBr pellets in the
3

4000–400 cm–1 region. UV-Vis spectra were recorded on a Perkin-Elmer Lambda 35
spectrometer. NMR spectra were recorded on a Bruker 500 MHz instrument. Single
crystal structures were determined by a Bruker D8 Venture CCD area diffractormeter.
The 30% aqueous solution of H2O2 as a primary oxidant is used in Cy reactions. The
Cy, CyOH and CyO are analyzed by GC-9790 series gas chromatograph that equips
a FID detector and a capillary column. The carrier gas is nitrogen.
2.2. Synthesis of the complexes
Synthetic procedure
Salicylaldehyde (1.21 g, 0.01 mol) was diluted by 30 mL methanol (complex 1 and
2) or ethanol (complex 3), to which was added 3-methylbenzohydrazide (complex 1,
1.50 g, 0.01 mol), 4-bromobenzohydrazide (complex 2, 2.15 g, 0.01 mol) or
2-bromobenzohydrazide (complex 3, 2.15 g, 0.01 mol). The mixtures were magnetic
stirred for 30 min at room temperature to give colorless solution. Then, methanol or
ethanol solution (30 mL) of MoO2(acac)2 (3.30 g, 0.01 mol) was added to the above
solution. The final mixtures were further stirred for 30 min at room temperature, to
give orange solution. Single crystals of the complexes were obtained by slow
evaporation of the solution in air for a period of a week.
Spectroscopic data
The complex 1: The yield is 0.27 g (67%). IR data (KBr, cm-1): 3447w, 1605s,
1553m, 1516m, 1477w, 1443m, 1385w, 1346m, 1269m, 1225w, 1124w, 1020w, 949m,
904s, 860m, 814w, 760m, 723w, 685w, 609m, 523w. UV–Vis data [methanol, λ/nm]:
205, 259, 298, 389. Anal. Calcd for C16H16MoN2O5: C, 46.6; H, 3.9; N, 6.8. Found: C,
46.7; H, 4.0; N, 6.7%.
The complex 2: The yield is 0.35 g (73%). IR data (KBr, cm-1): 3449w, 1610s,
1558m, 1528w, 1445m, 1393m, 1358m, 1269w, 1163s, 1074s, 951s, 860s, 750w,
677w, 548s, 523s. UV–Vis data [methanol, λ/nm]: 205, 268, 312, 393. Anal. Calcd for
C15H13BrMoN2O5: C, 45.4; H, 3.3; N, 7.0. Found: C, 45.2; H, 3.3; N, 7.1%.
The complex 3: The yield is 0.25 g (49%). IR data (KBr, cm-1): 3252w, 1607s,
1558m, 1516m, 1472w, 1441m, 1383w, 1355m, 1273m, 1159s, 1074s, 949s, 903m,
860s, 761w, 644w, 548s, 521s. UV–Vis data [methanol, λ/nm]: 205, 282, 292, 314,
4

380. Anal. Calcd for C16H17BrMoN2O6: C, 37.7; H, 3.4; N, 5.5. Found: C, 37.9; H, 3.3;
N, 5.6%.
2.3. X-ray structure determination
Diffraction intensities for the complexes were collected at 298(2) K using a Bruker
D8 Venture diffractometer with MoKα radiation (λ = 0.71073 Å). The collected data
were reduced with SAINT [35], and multi-scan absorption correction was performed
using SADABS [36]. Structures of the complexes were solved by direct methods and
refined against F2 by full-matrix least-squares method using SHELXTL [37]. All of
the non-hydrogen atoms were refined anisotropically. The hydrogen atoms of the
methanol and water ligands were loctaed from difference Fourier maps and refined
isotropically, with OH and H···H distances restrained to 0.85(1) and 1.37(2) Å,
respectively. The remaining hydrogen atoms were placed in calculated positions and
constrained to ride on their parent atoms. Crystallographic data for the complexes are
summarized in Table 1. Selected bond lengths and angles are given in Table 2.
Table 1. Crystal data for complexes.
1
C16H16MoN2O5
412.2
2
C15H13BrMoN2O5
477.1
3
C16H17BrMoN2O6
509.2
Formula
FW
Crystal shape/colour
Crystal system
Space group
a (Å)
block/orange
Triclinic
P-1
block/orange
Monoclinic
P21/c
block/orange
Triclinic
P-1
7.844(1)
9.922(2)
11.164(2)
85.289(2)
73.854(2)
76.412(2)
811.2(2)
2
12.218(1)
9.4756(9)
14.867(2)
90
8.007(2)
11.066(2)
11.644(2)
70.317(3)
85.220(3)
71.357(3)
920.1(3)
2
b (Å)
c (Å)
α (º)
104.234(2)
90
 (º)
 (º)
V (Å3)
1668.4(3)
4
Z
0.71073
298(2)
0.71073
298(2)
0.71073
298(2)
 (MoK) (Å)
T (K)
 (MoK) (cm–1)
0.837
3.208
2.919
Reflections
6023
9617
6824
5

Unique reflections
Observed reflections
[I  2(I)]
4034
2920
3109
2590
3416
4564
Parameters
221
1
221
1
243
3
Restraints
Goodness of fit on
F2
1.059
1.048
1.043
0.0557, 0.1139
0.0888, 0.1340
0.0346, 0.0939
0.0447, 0.0999
0.0434, 0.1120
0.0661, 0.1256
R1, wR2 [I  2(I)]
R1, wR2 (all data)
Table 2. Selected bond lengths (Å) and angles (o) for the complexes.
1
Bond lengths
Mo1−O1
1.918(4)
1.695(4)
2.384(4)
Mo1−O2
Mo1−O4
Mo1−N1
2.004(3)
1.694(4)
2.246(4)
Mo1−O3
Mo1−O5
Bond angles
O4−Mo1−O3
O3−Mo1−O1
O3−Mo1−O2
O4−Mo1−N1
O1−Mo1−N1
O4−Mo1−O5
O1−Mo1−O5
N1−Mo1−O5
107.2(2)
98.34(19)
97.32(18)
155.59(17)
81.49(15)
81.78(16)
81.88(16)
74.92(13)
O4−Mo1−O1
O4−Mo1−O2
O1−Mo1−O2
O3−Mo1−N1
O2−Mo1−N1
O3−Mo1−O5
O2−Mo1−O5
102.54(17)
97.29(16)
149.73(15)
95.78(18)
71.27(14)
170.60(16)
78.56(14)
2
Bond lengths
Mo1−O1
1.929(2)
2.342(3)
1.692(2)
Mo1−O2
Mo1−O4
Mo1−N1
2.010(2)
1.697(3)
2.238(3)
Mo1−O3
Mo1−O5
Bond angles
6

O5−Mo1−O4
O4−Mo1−O1
O4−Mo1−O2
O5−Mo1−N1
O1−Mo1−N1
O5−Mo1−O3
O1−Mo1−O3
N1−Mo1−O3
105.81(14)
99.76(13)
97.11(13)
159.04(12)
80.64(9)
O5−Mo1−O1
O5−Mo1−O2
O1−Mo1−O2
O4−Mo1−N1
O2−Mo1−N1
O4−Mo1−O3
O2−Mo1−O3
103.41(12)
97.53(11)
148.25(10)
93.54(12)
71.62(10)
170.55(11)
77.75(10)
82.88(11)
81.44(10)
77.36(9)
3
Bond lengths
Mo1−O1
1.921(3)
1.702(3)
2.269(3)
Mo1−O2
Mo1−O4
Mo1−N1
2.035(3)
1.692(3)
2.233(3)
Mo1−O3
Mo1−O5
Bond angles
N1−Mo1−O5
O1−Mo1−O2
O2−Mo1−N1
O3−Mo1−N1
O3−Mo1−O2
O4−Mo1−N1
O4−Mo1−O2
O4−Mo1−O5
76.55(11)
150.55(13)
71.25(12)
95.31(14)
95.24(15)
157.87(14)
97.66(14)
82.86(13)
O1−Mo1−N1
O1−Mo1−O5
O2−Mo1−O5
O3−Mo1−O1
O3−Mo1−O5
O4−Mo1−O1
O4−Mo1−O3
81.31(12)
82.81(13)
80.64(13)
97.79(16)
171.68(14)
104.27(14)
104.94(16)
Table 3. Distances (Å) and angles (°) involving hydrogen bonding of the complexes
D−H∙∙∙A
d(D−H)
0.85(1)
0.85(1)
d(H∙∙∙A)
1.94(2)
1.94(1)
d(D∙∙∙A)
2.770(5)
2.778(4)
Angle(D−H∙∙∙A)
167(7)
1
O5−H5∙∙∙N2i
2
O3−H3∙∙∙N2ii
173(6)
3
7

O6−H6A∙∙∙O3iii
O5−H5A∙∙∙O6
O5−H5B∙∙∙N2iv
0.82
2.01
2.798(5)
2.611(5)
2.814(5)
163(7)
166(6)
167(5)
0.85(1)
0.85(1)
1.78(2)
1.98(2)
Symmetry codes: i) 1 – x, 1 – y, – z; ii) – x, – y, – z; iii) 1 + x, y, z; iv) 1 – x, 2 – y, 2 – z.
2.4. Catalytic oxidation reaction
The Cy oxidation reactions are carried out in a glass flask with condenser pipes
placed in a water bath with magnetic stirring. In a typical experiment, 0.0003 g of the
catalysts (complex 1-3) are dissolved in 3.00 mL of solvent. Then the required amount
of Cy (n(Cy): n(catalyst) =15000:1) and HNO3 (n(HNO ): n(catalyst) =2000:1) are added
3
according to this order. At last, a certain amount of 30% H2O2 (n(H O ): n(catalyst)
2
2
=9000:1) is added, that was stirred for 5 h at 80 oC.
For the products analysis, 0.03 g of methylbenzene (internal standard) and 1.5 mL
of diethyl ether (to extract the substrate and the organic products from the reaction
mixture) were added to 1.5 ml sample to detect the moles of CyOH and to calculate
the conversion of Cy (a). 0.03 g of methylbenzene, 1.5 mL of diethyl ether and 0.3 g
PPh3 were added to 1.5 ml sample to detect the moles of CyO (b). The obtained
mixtures were stirred for 10 min and then a sample (0.5 μL) was taken from the
organic phase and analyzed by the internal standard method in the GC. The mole of
CyOOH is calculated according to the equation of [CyOH(b) - CyOH(a) + (CyO(a)
-
CyO(b))] and the TON values are the moles of products (CyOH / CyOOH or CyO) /
mole of catalyst [38].
3. Results and discussion
3.1. Synthesis
The three complexes were synthesized by the reaction of MoO2(acac)2 with the
respective hydrazone compound in methanol for 1 and 2, and in ethanol for 3. For
complexes 1 and 2, the methanol solvent coordinated to the Mo atoms, while for
complex 3, the water solvent coordinated to the Mo atom.
8

z
z
H
N
N
Y
N
MoO2(acac)2
+
Y
N
O
O
X
O
X
O
Mo
OH
L'
O
1: X = Z = H, Y = Me, L’ = MeOH; 2: X = Y = H, Z = Br, L’ = MeOH; 3: X = Br, Y = Z = H, L’ = H2O
3.2. Crystal structure description of the complexes
The molecular structures of the complexes are shown in Fig. 1-3. In each complex,
the Mo atom is coordinated with the phenolate O, imino N, and enalate O atoms of the
hydrazone ligand, and one oxo group, forming an octahedral coordination. These
define the equatorial plane, and with one oxo group and one solvent O atom
(methanol for 1 and 2, water for 3) occupying the axial positions. The hydrazone
ligands coordinate to the Mo atoms in meridional fashion forming five- and
six-membered chelate rings with bite angles of 71.27(14)° and 81.49(15)° for 1,
71.62(10)° and 80.64(9)° for 2, and 71.25(12)° and 81.31(12)° for 3. The hydrazone
ligands are coordinated in enolate form, which are evident from the N2–C8 and
O2–C8 bond lengths with values of 1.29-1.32 Å for the complexes. The Mo1–O1 and
Mo1–N1 bond lengths in the complexes are comparable to each other, ranging from
1.918(4) to 1.929(2) Å and 2.246(4) to 2.238(3) Å, respectively. However, the
Mo1-O2 bond lengths in 1 and 2 are much shorter than 3, which might be caused by
the effect of the substituent groups of the phenyl rings. The coordinate bond angles of
the complexes are ranging from 71.2 to 107.2º for the cis angles and from 148.2 to
171.7º for the trans angles, which are comparable to those observed in similar
molybdenum(VI) complexes [39-43].
In the crystal structure of 1 and 2, adjacent two complex molecules are linked by
two methanol molecules through intermolecular O–H∙∙∙N hydrogen bonds to form a
dimer (Fig. 4 and 5). In the crystal structure of 3, adjacent two complex molecules are
linked by two water molecules through intermolecular O–H∙∙∙N hydrogen bonds to
form a dimer. The dimers are further linked by ethanol molecules through O–H∙∙∙O
hydrogen bonds, to form chains along the a axis (Fig. 6).
9

Fig. 1. Molecular structure of 1 with 30% probability thermal ellipsoids.
Fig. 2. Molecular structure of 2 with 30% probability thermal ellipsoids.
10

Fig. 3. Molecular structure of 3 with 30% probability thermal ellipsoids.
Fig. 4. Molecular packing of 1, viewed along the b axis. Hydrogen bonds are shown as dashed lines.
11

Fig. 5. Molecular packing of 2, viewed along the b axis. Hydrogen bonds are shown as dashed lines.
Fig. 6. Molecular packing of 3, viewed along the b axis. Hydrogen bonds are shown as dashed lines.
3.3. IR spectra
The infrared spectra of the complexes are shown in Fig. S1-3. The weak and broad
vibration of O-H groups appears in the range of 3252–3449 cm-1. The strong
12

vibrations observed at about 1607 cm-1 for the free hydrazones are assigned to the
C=N groups in the spectra of complexes 1–3 [44-45]. As a result of the
υasym(O=Mo=O) and υsym(O=Mo=O) modes, the complexes display two sharp bands
in the 900-950 cm-1 region which are assigned to symmetric and anti-symmetric
stretching vibrations of the cis-[MoO2]2+ moiety [46-47]. The bands in the low wave
400-700 cm−1 may be assigned to the Mo-O and Mo-N bonds.
3.4. UV-vis spectra
The UV-Vis spectra of the complexes are shown in Fig. S4-6. All the complexes
display intense absorption bands in the 380–393 nm region with εmax = 2121–4638
M-1cm-1 that are marked as L(pπ) - Mo(dπ) LMCT transitions involving the
promotions of electrons from the filled HOMO of the ligand to the empty LUMO of
molybdenum [48-49]. The bands at 298 nm, 312nm and 314 nm (εmax = 7193–13370
M-1cm-1) are attributable to the n→π* transitions of C=N group. These peaks are
designated as transitions based on their extinction co-efficient values between
electronic energy levels [46]. In addition, the peaks centered at 259 nm, 268 nm, 282
nm and 291 nm (εmax = 9207–12858 M-1cm-1) are attributed to the transition of the
π→π* in the hydrazone ligands of the complexes.
3.5 1H and 13C NMR spectra
1
The H NMR spectra of the three complexes determined in d6– DMSO are shown
in Fig. 7-9 and their typical chemical shifts are presented in Table S1. In all
complexes, the mono hydrogen on –CH=N– has a significant peak which is attributed
to δ = 8.95–8.94 ppm. The Ar–H atoms in all the hydrazone ligands are assigned to δ
= 7.93–6.95 ppm. A singlet at δ = 2.39 (3H) for 1, a singlet at δ = 3.18 (3H) for 1, 2
and two singlets at δ = 3.44 ppm (2H), δ = 1.06 ppm (3H) for 3 are ascribed to the
PhCH3, Mo···HOCH3 and Mo···HOCH2CH3 respectively. The absence of a signal
from –OH confirms an enolization prior to complexation, followed by a deprotonation
during the complexation.
Table 4 provides 13C NMR spectral data of all compounds and Fig. S7-9
reproduces their spectra. In all complexes, the signals at δ = 169–168 ppm are
assignable to carbon bearing enolic oxygen. The absorption band at δ = 159 ppm is
13

attributed to carbons bearing azomethine nitrogen, and the carbons bearing phenolic
oxygen are observed at δ = 155–157 ppm. The carbons on the aromatic ring are
assigned to δ = 138–118 ppm. The carbon on coordinated methanol in complex 1 and
2 are assigned to δ = 48.6 ppm. The carbon on coordinated ethanol in complex 3 are
assigned to δ = 56.0 ppm and δ = 18.5 ppm. It was found that fewer numbers of
aromatic carbon signals were observed in complex 2 due to the presence of symmetric
axis, and the signal of terminal –CH3 in complex 3 could be located at δ = 20.9 ppm
[50].
Fig. 7 1H NMR spectrum of complex 1
Fig. 8 1H NMR spectrum of complex 2
14

Fig. 9 1H NMR spectrum of complex 3
Table 4. 13C NMR spectral data of complex 1–3 .
138.06, 134.87,
134.22, 132.58,
129.88, 128.66,
128.35, 125.15,
121.53, 120.26,
118.52
Complex
168.79 155.85 159.36
20.89 48.57
\
1
135.04, 134.33,
131.88, 129.86,
Complex
167.91 156.32 159.38 129.20, 125.71,
\
48.57
\
2
121.64, 120.20,
118.54
135.15, 134.44,
133.80, 132.34,
Complex
132.18, 131.16,
169.48 157.00 159.49
56.00,
18.52
\
\
127.70, 121.56,
3
120.69, 120.04,
118.60
15

3.6 Thermal analysis
The thermal analyses of complexes 1, 2 and 3 were carried out in order to study the
thermal stability in air atmosphere (Figures S10-12). For complex 1, the first weight
o
o
loss of 6.82% at the temperature range from 88.9 C to 280 C is due to the loss of
CH3OH (calc. 7.76%). The second step can be caused by the weight loss of the ligand
o
o
L1, 60.9% with the temperature range from 280 C to 581 C (calc. 61.1%). For
o
complex 2, the first weight loss of 6.12% at the temperature ranges from 83.8 C to
337 oC is due to the loss of CH3OH (calc. 6.71%). The second step can be assigned to
the weight loss of ligand L2, 64.3% from 337 oC to 574 oC (calc. 66.4%). For complex
o
o
3, the first weight loss of 4.25% at the temperature range from 97.6 C to 118 C is
due to the loss of H2O (calc. 3.53%). The second step can be attributed to the weight
o
o
loss of ligand L3, 63.2% from 236 C to 567 C (calc. 62.3%). At this stage, the
complexes turned to MoO3. Then, with the increasing of the temperature, MoO3
gradually sublimated [51].
3.7. Catalytic activity
100
80
60
40
20
0
100
80
60
40
20
0
Cy
CyOH
CyOOH
0
2
4
6
8
10
Time (h)
Fig. 10. The effect of time on Cy oxidation. Reaction conditions: Cy (0.7924 g, 9.434 mmol), complex 2 as
catalyst (0.0003 g, 0.0006 mmol), H2O2 (0.6416 g, 5.661 mmol), HNO3 (0.07924 g, 1.258 mmol), CH3CN (3 mL),
40 oC.
The influence of reaction time on Cy oxidation is investigated by keeping the
amounts of Cy (0.7924 g), complex 2 as catalyst (0.0003 g), H2O2 (0.6416 g) and
16

o
HNO3 (0.07924 g) in CH3CN (3 mL) at 40 C, as shown in Fig. 10. The Cy
conversion curve is continuously rising from 50 to 90% with the reaction time from
0.5 to 9 h, which is different from that of CyOH. The TON curve of CyOH is rising at
first from 0 to 5 h, and reached the maximum value of 74.4. However, it decreased
after that, which may be due to the further oxidation of CyOH to other by-products.
What can be sure is that the CyOH or Cy cannot be oxidated further to CyO under this
situation, for trace CyO (TON< 10) can be detected in all of our catalytic experiments,
which is too small to be of significance for this discussion. On the other hand, the
TON value curve of CyOOH has a similar shape with CyOH, and reaches the highest
value of 18.7 at 6 h.
100
80
60
40
20
80
60
40
20
0
CyOH
Cy
CyOOH
0
2250:1
4500:1
9000:1
13500:1
18000:1
n
:ncatalyst
H2O2
Fig. 11. The effect of the amount of H2O2 on Cy oxidation. Reaction conditions: Cy (0.7924 g, 9.434 mmol),
complex 2 as catalyst (0.0003 g, 0.0006 mmol), HNO3 (0.07924 g, 1.258 mmol), CH3CN (3 mL), 40 oC, 5 h.
The effect of H2O2 on Cy oxidation over complex 2 is explored by keeping the
amounts of Cy (0.7924 g), complex 2 (0.0003 g) and HNO3 (0.07924 g) in CH3CN (3
o
mL) at 40 C for 5 h. As shown in Fig. 11, the curve of CyOH with a peak of 74.4
TON value at 9000:1 ratio and decreases after that. As for CyOOH, The TON value is
only 13.0 at that ratio, which indicates that most of the CyOOH is converted into
CyOH, since the TON value of CyOH is the highest. Although CyOOH has the
highest TON value (70.2) at 13500:1 ratio, the TON of CyOH drops sharply to 30.4.
This may be due to the hindrance of the water in the decomposition of CyOOH into
17

the target product. What also can be seen is that when there is no H2O2 in the system,
a certain amount of products are still generated (CyOH TON= 8.83). It indicates that
there exists another weak oxidant, HNO3. However, the Cy conversion without H2O2
is much lower than that with H2O2, suggesting that H2O2 is the main oxidant in the
reaction.
80
70
60
50
40
30
20
10
0
100
80
60
40
20
0
CyOH
CyOOH
Cy
PE
Dichloromethane
Ethanol
Methanol
Acetonitrile
Solvent
Fig. 12. The effect of the solvents on Cy oxidation. Reaction conditions: Cy (0.7924 g, 9.434 mmol), complex 2 as
catalyst (0.0003 g, 0.0006 mmol), H2O2 (0.6416 g, 5.661 mmol), HNO3 (0.07924 g, 1.258 mmol), Solvent (3 mL),
40 oC, 5 h.
A total of five different solvents (3 mL) CH3CN, MeOH, EtOH, CH2Cl2, and
petroleum ether (PE) are evaluated in Cy oxidation, while complex 2 as catalyst
(0.0003 g) at 40 oC and running for 5 h, in which the amount of Cy (0.7924 g), HNO3
(0.07924 g), H2O2 (0.6416 g) were kept fixed, as shown in Fig. 12. It can be seen that
the reaction in PE almost does not occur with the conversion rate of Cy being only
17.9 %. Though the reaction in CH2Cl2 yields a certain amount of CyOOH (TON=
18.7), the TON value of CyOH is only 32.2, indicating that CyOOH does not
conductively transform to CyOH in CH2Cl2. Compared to EtOH (TON= 38.6), the
reaction of MeOH yields more CyOH (TON= 56.5). The highest TON(CyOH), 74.4, is
obtained in CH3CN, and the CyOOH can also be detected. In summary, the order of
TON(CyOH) is PE< CH2Cl2< EtOH< MeOH< CH3CN, which is in good accordance
with their dielectric constants (ε/ε0): PE (~2)< CH2Cl2 (9.1)< EtOH (26.6)< MeOH
18

(32.7)< CH3CN (37.5) [52].
100
95
90
85
80
75
70
90
80
70
60
50
40
30
20
10
0
CyOH
Cy
CyOOH
65
0
1000:1
2000:1
5000:1
9000:1
n
:ncatalyst
HNO3
Fig. 13. The effect of the amount of HNO3 on Cy oxidation. Reaction conditions: Cy (0.7924 g, 9.434 mmol),
complex 2 as catalyst (0.0003 g, 0.0006 mmol), H2O2 (0.6416 g, 5.661 mmol), CH3CN (3 mL), 40 oC, 5 h.
The effect of HNO3 molar ratio on the catalytic activity of complex 2 for Cy
oxidation is investigated, when the amount of Cy (0.7924 g), complex 2 (0.0003 g),
H2O2 (0.6416 g), CH3CN (3 mL) was set, at 40 oC and running for 5 h (shown in Fig.
13). It can be seen that with the increasing of HNO3 mole ratio, the TON value of
CyOH increases gradually, reaches the maximum value of 74.4, and decreases after
that, which may be due to the deep oxidation of Cy to the by-product caused by a
large amount of HNO3. The TON value curve of CyOOH displays a similar shape as
that of CyOH, with the highest TON 13.0 at the radio of 2000:1 too. In particular,
when there is no HNO3, CyOH can still be found with the TON value is 18, indicating
that H2O2 alone can oxidate Cy to CyOH directly.
19

120
100
80
20
15
10
5
CyOH
Cy
60
40
CyOOH
0
20
0
2250:1
4500:1
9000:1
13500:1
18000:1
n
:ncatalyst
H2O2
Fig. 14. The effect of the amount of H2O2 without HNO3 on Cy oxidation. Reaction conditions: Cy (0.7924 g,
9.434 mmol), complex 2 as catalyst (0.0003 g, 0.0006 mmol), CH3CN (3 mL), 40 oC, 5 h.
To discuss further the oxidation performance of H2O2 without acid, the amount of
H2O2 is changed under the conditions of Cy (0.7924 g), complex 2 as catalyst (0.0003
g), CH3CN (3 mL), 40 oC and 5 h, as shown in Fig. 14. It is intriguing that an obvious
amount of CyOH with the largest TON value of 18.2 can be detected without acid but
H2O2. However, almost no CyOOH can be detected without HNO3 considering the
trend of TON(CyOOH), which is quite different to the situation with HNO3. It seems that
the addition of HNO3 is the key to determine whether CyOOH can be produced.
Therefore, it is speculated that when H2O2 is added alone, Cy is likely to be directly
oxidated to CyOH. Otherwise, the reaction will go through the process of converting
Cy to CyOOH first, and then to CyOH with HNO3.
20

100
80
60
40
20
0
250
200
150
100
50
Cy
CyOH
CyOOH
0
10
20
30
40
50
60
70
80
90
100
Temperature (oC)
Fig. 15. The effect of the temperature on Cy oxidation. Reaction conditions: Cy (0.7924 g, 9.434 mmol), complex
2 as catalyst (0.0003 g, 0.0006 mmol), H2O2 (0.6416 g, 5.661 mmol), HNO3 (0.07924 g, 1.258 mmol), CH3CN (3
mL), 5 h.
The effect of reaction temperature for the Cy oxidation is detected under the
conditions of Cy (0.7924 g), complex 2 (0.0003 g), H2O2 (0.6416 g), HNO3 (0.07924
g), CH3CN (3 mL) and 5 h catalyzed by complex 2, and the results are shown in Fig.
15. As can be seen, the Cy conversion rate is almost unchanged, but begins to decline
o
after 50 C. However, more and more CyOH are generated (TON= 224) with the
temperature rising to 80 oC. The reaction also produces much CyOOH (TON= 138) at
o
80 C, which means that high temperature enhances the production of main products
by promoting the production of intermediate products. Since the yield of the main
o
product has not changed at 90 C and a very small amount of CyO (TON= 9.04) has
o
been generated, 80 C is chosen as the best temperature considering the energy
consumption.
21

250
200
150
100
50
100
80
60
40
20
0
CyOH
CyOOH
Cy
0
phosphoric acid
HCl
lactic acid
sulphuric acid
nitric acid
The type of the acids
Fig. 16. The effect of the type of the acid on Cy oxidation. Reaction conditions: Cy (0.7924 g, 9.434 mmol),
complex 2 as catalyst (0.0003 g, 0.0006 mmol), H2O2 (0.6416 g, 5.661 mmol), acid (1.258 mmol), CH3CN (3 mL),
80 oC, 5 h.
As a common view, acid additives can enhance the catalyst’s performance in
oxidation. Tang et al. found that HCl was a good acid additive (TON= 76.1) when
XPMo12-nVnO40 was used as catalyst in MeCN and N2 atmosphere [53]. Wu et al.
detected that the TON value of CyO could reach to 83 when lactic acid was acting as
acid additive promoted by ammonium paramolybdate, comparing to that in HNO3 was
only 41 [54]. Sutradhar et al. studied the performance of pyrazine carboxylic acid in
the reaction of Cy oxidation in H2SO4 and found that the TON values of CyOH and
CyO were higher than those in the traditional acid auxiliary TFA [55].
Herein, H3PO4, HCl, lactic acid, H2SO4 and HNO3 are chosen as acid additives
under the condition of Cy (0.7924 g), complex 2 as catalyst (0.0003 g), H2O2 (0.6416
o
g), CH3CN (3 mL), 80 C and 5 h, as shown in Fig. 16. It is found that our catalyst
has the lowest activity in H3PO4, and is better in lactic acid and HCl. It produces the
most CyOH and CyOOH in HNO3, with the TON values equal to 224 and 138,
respectively. Though the TON of CyOH is slightly lower in H2SO4, there is no
intermediate product of CyOOH produced at all. In addition, a very small amount of
CyO is detected in both HNO3 (TON= 3.46) and H2SO4 (TON= 0.450).
22

120
100
80
60
40
20
0
100
80
60
40
20
0
CyOH
CyOOH
Cy
C
T
A
O
N
S
F
Acids mixing with HNO3
Fig. 17. The effect of acid mixture on Cy oxidation.Oxidation conditions: Cy (0.7924 g, 9.434 mmol), complex 2
as catalyst (0.0003 g, 0.0006 mmol), H2O2 (0.6416 g, 5.661 mmol), N= HNO3 (0.0396 g, 0.6289 mmol), the molar
ratio of acid to HNO3 is 1:1 [C (citric acid), T (tartaric acid), A (AlCl3), O (oxalate), S (Sebacic acid), F (FeCl3)],
CH3CN (3 mL), 80 oC, 5 h.
Moreover, other acids also have the ability to promote reactions. For instance,
Pokutsa et al. showed that when oxalic acid was acid auxiliaries, the selectivity of
CyOH and CyO is 22% and 44% and the total TON value of products is 944 [56]. Yiu
et al. reported that when FeCl3 was treated as acid auxiliary, the highest yield was at
91% when [nBu4N][Os(N)Cl4] was the catalyst [57]. Ho et al. observed that when
AlCl3 was the acid promoter, Cy was oxidized to chlorocyclohexane in CH3COOH—
CH2Cl2 with a yield of 24% with the barium ferrate as catalyst [58]. Du et al. also
reported citric acid was used as acid additives, the conversion of Cy reached 93.3%
and the selectivity of CyO reached 33.3% [59]. At the same time, we speculate that
acid containing hydroxyl as an acid promoter may have a significant role in
promoting the reaction due to the performance of lactic acid in the previous work [54],
so tartaric acid may also have the possibility to enhance the reaction. It can be
expected that the ability to enhance the reaction will be further enhanced by mixing
the acid with HNO3. Herein, the effects of citric acid, tartaric acid, oxalate, sebacic
acid, FeCl3 and AlCl3 on Cy oxidation is discussed under the condition of Cy (0.7924
g), complex 2 as catalyst (0.0003 g), H2O2 (0.6416 g), HNO3 (0.0396 g), CH3CN (3
23

o
mL), 80 C, 5 h and the molar ratio of acid to HNO3 is 1:1, as shown in Fig. 17. It is
observed that the Cy conversion is not significantly affected by different acids, when
the maximum value being 69.5% with sebacic acid was obtained. It indicates that the
addition of acids have no special effect on the Cy conversion. Unfortunately, the TON
values of CyOH and CyOOH decrease when citric acid, tartaric acid, AlCl3, or oxalic
acid is added to the system, indicating that these acids can not be acted as promoters
with HNO3 together. However, it increases when the acids are sebacic acid and FeCl3,
indicating that those two acids are potential good additives with HNO3, though more
study need to be done to clear out the reason. Moreover, FeCl3 may have the potential
to act as a catalyst alone considering it is a metal chloride and a type of Lewis acid.
Therefore, we carry out a reaction with FeCl3 under the same conditions without
complex, and found that the Cy conversion is 55.1%, the TON value of CyOH 47.3
and no CyOOH is formation. Comparing with the catalytic performance of complex 2
+ FeCl3 together (Cy conversion= 61.8 %, CyOH TON= 99.3 and CyOOH TON=
33.4), we believe that FeCl3 is acting as auxiliaries rather than catalyst in the reaction.
250
200
150
100
50
100
80
60
40
20
0
CyOH
CyOOH
Cy
0
complex 1
complex 2
complex 3
The type of catalyst
Fig. 18. The effect of the type of catalyst on Cy oxidation. Reaction conditions: Cy (0.7924 g, 9.434 mmol),
complex 1: [MoO2L1(MeOH)], complex 2: [MoO2L2(MeOH)] and complex 3: [MoO2L3(H2O)]∙EtOH as catalysts
(0.0003 g), H2O2 (0.6416 g, 5.661 mmol), HNO3 (0.07924 g, 1.258 mmol), CH3CN (3 mL), 80 oC, 5 h.
Finally, the catalytic properties of our three different Mo complexes in the reaction
of Cy (0.7924 g), H2O2 (0.6416 g), catalyst (0.0003 g), HNO3 (0.07924 g), CH3CN (3
mL), 80 oC and 5 h are compared, as shown in Fig. 18. It is observed that the catalytic
24

performance of complex 2 is the best: the Cy conversion (Conv.%= 52.2) and the
TON value of its products (CyOH TON= 224, CyOOH TON= 138) are all the highest
one in the three complexes. As we know, the stronger the electron withdrawing effect
the ligand has, the easier it is to be close to the electrons of the center metal, which
may have high catalytic performance by exposing more active metal center. As for the
three complexes, the electron withdrawing effect of the bromine atoms in the
complexes 2 and 3 are much stronger than that of methyl in complex 1. Therefore, the
catalytic activities is 2, 3> 1. On the other hand, if the bromine atoms at ligand
terminals are adjacent to the coordination group, a certain aggregation of electrons at
the coordination groups will be formed, which is not conducive to the exposure of the
active sites to the central metal. That is why the catalytic activity of complex 2 is
stronger than that of complex 3.
Table 5. The comparison of [MoO2L2(MeOH)] (complex 2) with other catalysts for Cy oxidation.
nCatalyst
(mmol)
0.0006
0.0020
1.6*10-5
TON(Cy
Entry
Catalyst
Time (h) Temp. (oC)
Reference
OH+CyO)
227
28
[MoO2L2(MeOH)]
[MoO2(L1)]
5
9
80
70
This work
[60]
1
2[a]
3[b]
16
R.T.
[61]
36
[(TpMoO2)2(μ-O)]·H2O
[Cu6(H2tea)6Fe(CN)6]n(NO3)2n•6nH
2O
[CuCoIIICoII (MeDea)3Cl3(CH3OH)
0.55(H2O)0.45](H2O)0.45
[FeCl2{ŋ3-HC(pz)3}] supported at
CNT-Oxi-Na
4[c]
5[d]
6[e]
6
5
6
25
20
0.0010
0.0100
0.0099
[62]
[63]
[64]
110
15
2
R.T.
78
7[f]
8[g]
9[h]
3
24
65
R.T.
/
0.0500
0.0012
0.0023
[65]
[66]
[67]
13.6
43
3.9
[Fe(salhd)Cl]
[Fe(NCCN)(MeCN)(CNtBu)](PF6)2
[FeII(Cl3TPAFeX2)(OTf)2]
4.5
[a]H2L1= 2,3-dihydroxybenzylidene-2-hydroxybenzohydrazide.
[b]Tp= tris (pyrazolyl) borate. [c]H3tea= triethanolamine.
[d]MeDea= N-methyldiethanolamine. [e]pz= pyrazol-1-yl.
[f][Fe(salhd)Cl]= chloro-[N,N’-bis(salicylaldehyde)-cyclohexanodiminate] iron(III).
[g]NCCN= a tetradentate bis (pyridyl-N-heterocyclic carbene) ligand.
[h]ClnTPAFeX2= a series of a-chlorinated tripods in the tris (2-pyridylmethyl) amine series ClnTPA (n= 1–3) and
fully characterized the corresponding FeX2 complexes (X= Cl, CF3SO3).
The catalytic activity of complex 2 in Cy oxidation is also compared with other
catalysts reported in the literature in Table 5. The total amount of CyOH+CyO
produced by complex 2 is the highest one.
25

4. Conclusion
Three new complexes have been successfully synthesized by the reaction of
MO(acac)2 with hydrazone or salicylaldehyde for the first time. All complexes are
characterized by IR, UV and thermostability. The central metals are six-coordinate in
the complexes and in an octahedral environment detected by single-crystal X-ray
diffraction analyses. Their catalytic activities are studied in Cy oxidation using H2O2
as oxidant. The optimization of parameters, such as the concentration of oxidant, the
type of solvent, the amount of acids, reaction temperature and the type of acids have
been found. Interesting, Sebacic acid and FeCl3 show positive effect on the oxidation
with HNO3. The sequencing of the catalytic ability is complex 2 > 3 > 1, due to the
ability of ligands to release the active sites of central metal.
Supplementary material
CCDC reference numbers 1586416 for 1, 1586417 for 2, and 1586418 for 3 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;
Email: deposit@ccdc.cam.ac.uk.
Acknowledgments
This work was financially supported by the support plan of high-level personnel
innovation of Dalian [no. 2015R070], the Basic research project of Key Laboratory of
Liaoning Province [no. LZ2015048], the Education Office of Liaoning Province
[Project No. LF201783611] and the Innovative Entrepreneurial Training Program of
Liaoning Normal University [Project No. cx201802017].
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