A dioxidomolybdenum(VI) complex of tridentate ONO aroylhydrazone ligand: Synthesis, crystal structure and catalytic activity

Article abstract of DOI:10.1080/15533174.2016.1149730

A new dioxidomolybdenum complex with the formula [MoO₂L(MeOH)], derived from the tridentate aroylhydrazone 2-amino-N?-(3,5-dichloro-2-hydroxybenzylidene)benzohydrazide (H₂L), has been prepared and characterized by elemental analysis, FTIR, 1H NMR, and single-crystal structural X-ray diffraction. The complex has a distorted octahedral structure in which the aroylhydrazone ligand behaves as a binegative donor one, leaving the amino-N uncoordinated toward the metal center. Single-crystal structure analysis reveals that the Mo(VI) center is coordinated by the donor atoms of the binegative aroylhydrazone ligand, two oxido groups and one oxygen atom of a methanol ligand. Crystal of the complex is stabilized by hydrogen bonds. The complex functions as an effective olefin epoxidation catalyst with hydrogen peroxide as terminal oxidant and sodium hydrogen carbonate as a cocatalyst.

Full text of DOI:10.1080/15533174.2016.1149730

Synthesis and Reactivity in Inorganic, Metal-Organic, and
Nano-Metal Chemistry
ISSN: 1553-3174 (Print) 1553-3182 (Online) Journal homepage: http://www.tandfonline.com/loi/lsrt20
A Dioxidomolybdenum(VI) Complex of Tridentate
ONO Aroylhydrazone Ligand: Synthesis, Crystal
Structure and Catalytic Activity
Min Liang & Dong-Hui Zou
To cite this article: Min Liang & Dong-Hui Zou (2016): A Dioxidomolybdenum(VI) Complex of
Tridentate ONO Aroylhydrazone Ligand: Synthesis, Crystal Structure and Catalytic Activity,
Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, DOI:
10.1080/15533174.2016.1149730
To link to this article: http://dx.doi.org/10.1080/15533174.2016.1149730
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Date: 23 June 2016, At: 06:36

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A Dioxidomolybdenum(VI) Complex of Tridentate ONO Aroylhydrazone Ligand:
Synthesis, Crystal Structure and Catalytic Activity
MIN LIANGa,*, DONG-HUI ZOUb
aSchool of Chemistry and Chemical Engineering, Qiqihar University, Qiqihar, P. R. China.
bCollege of Food and Bio-Engineering, Qiqihar University, Qiqihar, P. R. China.
Correspondence author. E-mail: liangmin09@163.com
Abstract
A new dioxidomolybdenum complex with the formula [MoO2L(MeOH)], derived from the
tridentate aroylhydrazone 2-amino-N’-(3,5-dichloro-2-hydroxybenzylidene)benzohydrazide
(H2L), has been prepared and characterized by elemental analysis, FTIR, 1H NMR, and single
crystal structural X-ray diffraction. The complex has a distorted octahedral structure in which
the aroylhydrazone ligand behaves as a binegative donor one, leaving the amino-N
uncoordinated towards the metal center. Single crystal structure analysis reveals that the Mo(VI)
center is coordinated by the donor atoms of the binegative aroylhydrazone ligand, two oxido
groups and one oxygen atom of a methanol ligand. Crystal of the complex is stabilized by
hydrogen bonds. The complex functions as an effective olefin epoxidation catalyst with
hydrogen peroxide as terminal oxidant and sodium hydrogen carbonate as a co-catalyst.
Keywords
Aroylhydrazone, dioxidomolybdenum complex, catalytic activity, crystal structure
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Introduction
In recent years, considerable attention has focused on the coordination chemistry of molybdenum
because of its biochemical significance[1--3] as well as involvement of Mo(VI) compounds in
some industrial catalytic processes.[4--6] In spite of the important role of molybdenum in
biological processes, its high potential as catalyst has made it even more attractive. The
mechanism of molybdenum oxotransferase has been extensively investigated for a long time.
Molybdenum complexes are known for considerable use in organic chemistry, in particular for
the various oxidations of organic compounds. Dioxomolybdenum complexes have been
intensively investigated as oxidation catalysts for variety of organic substrates, particularly for
sulfoxidation and epoxidation of olefins.[7--10] In the present work, a new
dioxidomolybdenum(VI) complex with the formula [MoO2L(MeOH)], derived from the
tridentate aroylhydrazone 2-amino-N’-(3,5-dichloro-2-hydroxybenzylidene)benzohydrazide
(H2L; Scheme 1), was studied.
Experimental
Materials and Methods
All chemicals and solvents used were of analytical reagent grade and were used as received.
MoO2(acac)2 was prepared as described in the literature.[11] Micro analyses for C, H, N were
carried out using a Perkin Elmer 2400 CHNS/O elemental analyzer. FTIR spectra were recorded
on a FTIR 8400-Shimadzu as KBr discs in the range of 400--4000 cm1. 1H NMR spectra were
recorded at 25 ºC on the Bruker AVANCE 400 MHz spectrometer. X-ray diffraction data were
collected using a Bruker Smart Apex II diffractometer.
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Synthesis of H2L
A methanolic solution (10 mL) containing 1 mmol of 3,5-dichloro-2-hydroxybenzaldehyde (0.19
g) was added dropwise to a methanolic solution of 2-aminobenzohydrazide (1 mmol, 0.15 g)
with constant stirring. The mixture was refluxed for 30 min, and the resulting precipitate was
filtered off, washed with cold methanol and dried in desiccator over silica gel. Yield: 0.25 g,
77%. Anal. Calc. for C14H11Cl2N3O2: C, 51.87; H, 3.42; N, 12.96. Found: C, 51.75; H, 3.53; N,
13.04%. FTIR (KBr), cm1: ν(NH) 3211, ν(OH) 3372, ν(CHar) 2833--3067, ν(C = O) 1653, ν(C =
N) 1612, ν(N--N) 1147, ν(C--O) 1292. 1H NMR (400 MHz, DMSO-d6, ppm): δ = 12.27 (s, 1H;
NH), 11.12 (s, 2H, NH2), 10.85 (s, 1H; OH), 8.69 (s, 1H; CH = N), 6.80--7.70 (m, 6H, ArH).
Synthesis of the Dioxidomolybdenum Complex
A methanolic solution (10 mL) of MoO2(acac)2 (0.1 mmol, 0.033 g) was added to a solution of
H2L (0.1 mmol, 0.032 g) in methanol (10 mL) and the resulting orange mixture was refluxed for
30 min. After cooling, the solution was filtered and left to stand overnight. Orange single crystals
suitable for crystallography separated after a week and were dried in a vacuum desiccator over
silica gel. Yield: 0.033 g, 69%. Anal. Calc. for C15H13Cl2MoN3O5: C, 37.37; H, 2.72; N, 8.72.
Found: C, 37.13; H, 2.82; N, 8.61%. IR (KBr) cm1: ν(OH) 3351, ν(CHar) 2810--2990, ν(C = N)
1605, ν(C = Cring) 1453, ν(N--N) 1131, ν(C--O) 1254, νasym(cis-MoO2) 937, νsym(cis-MoO2) 912.
1H NMR (400 MHz, DMSO-d6, ppm): δ = 11.73 (s, 2H, NH2), 10.16 (s, 1H, OH), 8.81 (s, 1H;
CH = N), 6.70--7.90 (m, 6H; ArH), 3.41 (s, 3H; CH3).
X-Ray Structure Determination
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The crystal of the complex was measured on a Bruker SMART Apex II CCD diffractometer
using Mo Kα radiation (λ = 0.71073 Å) and a graphite monochromator at 25 ºC. Unit cell and
reflection data were obtained by standard methods[12,13] and are summarized in Table 1. The
structure was solved, refined, and prepared for publication using the SHEXTL package (structure
solution refinements and molecular graphics).[14] The structure was solved using direct methods
and refined on F2 using full matrix least squares techniques with anisotropic displacement factors
for all non-hydrogen atoms. The amino and methanol H atoms were located from a difference
Fourier map and refined isotropically, with N--H, O--H, and H···H distances restrained to
0.90(1), 0.85(1), and 1.43(2) Å, respectively. Positions of the remaining hydrogen atoms were
calculated from the geometry of the molecular skeleton and their thermal displacement
parameters were refined isotropically on a groupwise basis. Selected bond lengths and angles are
reported in Table 2. H-bonding distances and angles are shown in Table 3.
Catalytic Epoxidation of Olefins
To a solution of olefins (0.28 mmol), NaHCO3 (0.11 mmol) and catalyst (9.4×10−4 mmol) in
MeCN (0.5 mL) was added H2O2 (1.1 mmol, 30% H2O solution) as oxidant. After the reaction
was over, for the products analysis, the solution was subjected to multiple ether extraction, and
the extract was also concentrated down to 0.5 mL 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.
Results and Discussion
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Synthesis and Characterization
The reaction of MoO2(acac)2 with the tridentate aroylhydrazine ligand (H2L) in methanol led to
the formation of cis-MoO2 complex in a good yield. The ligand contributes in its enolic form that
the schematic diagram for synthesis of the complex is shown in Scheme 2. The complex is stable
at room temperature and soluble in DMSO, DMF, ethanol, acetonitrile and less soluble in other
common solvents like dichloromethane, chloroform, and insoluble in benzene, n-hexane and
CCl4.
1H NMR Spectra
1H NMR data of the aroylhydrazone ligand when compared with those of the complex reveals
that the ligand serves as tridentate binegative ONO donor and amino group remains free. The
amino residue of the ligand shows signal for N--H at 11.12 ppm, which is found unperturbed in
the complex. The disappearance of the signal at 12.27 ppm in the free ligand upon complexation
suggests that during complexation the [--CO--NH--] group undergoes tautomerization to
[--C(OH) = N--] followed by deprotonation of the --OH group. The azomethine C--H signal in
the complex is shifted up-field from its original position in the free ligand upon coordination of
the --CH = N-- group, on account of reduction of electron density at the azomethine C--H. The
aromatic protons also show some deviation in the complex as compared to the free ligand since
in the complex they are in direct conjugation to the coordinated O and N of the hydrazone
ligands. Appearance of characteristic signals for O--H at 10.16 ppm and CH3 at 3.41 ppm
indicates that the sixth coordination position is occupied by a methanol molecule.
IR Spectra
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IR spectrum of the free aroylhydrazone ligand shows multiple bands at 3211 cm1 for ν(N--H) of
amino group, 1653 cm1 for ν(C = O) and 3372 cm1 for ν(O--H).[15] The ν(C = O) band is absent
in the spectrum of the complex as the ligand binds in binegative mode losing protons from the
carbohydrazide group. The strong peak at 1605 cm1 can be assigned to ν(C = N).[16] The
complex exhibits two characteristic bands at 912 cm1 and 937 cm1 for symmetric and
antisymmetric stretching of cis MoO2 core.[17--19]
Structure Description
The perspective view of the complex together with the atom numbering scheme are shown in
Fig. 1. The coordination geometry around the molybdenum atom in the complex reveals a
distorted octahedral environment with an NO5 chromophore. The ligand molecule behaves as
binegative tridentate one binding through the phenolate oxygen, the enolate oxygen and the
imine nitrogen and occupies three positions in the meridional plane. One oxo group, O(4), is
located trans to the imine nitrogen in the same plane. The other oxo group, O(3), is located in the
axial plane with the methanol molecule. The molybdenum is found to be deviated from the mean
equatorial plane of O(1), O(2), O(4) and N(1) by 0.308(2) Å. The Mo--Omethanol bond length is
longer than the remaining coordinate bonds. This shows that the solvent molecule is loosely
attached to the Mo center. The C(8)--O(2) bond length is closer to single bond length rather than
C--O double bond length. However, the shorter length compared to C--O single bond may be
attributed to extended electron delocalization in the ligand. Similarly shortening of C(8)--N(2)
length together with the elongation of N(1)--N(2) length also supports the electron cloud
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delocalization in the ligand system. The ligand molecule forms a five-membered and a
six-membered ring with the Mo center.
In the molecular packing structure of the complex, molecules are linked through
intermolecular O--H∙∙∙N, N--H∙∙∙O and N--H∙∙∙N hydrogen bonds to form 2D sheets (Fig. 2).
Catalytic Property
The complex showed effective catalytic property in the oxidation of various olefins to their
corresponding epoxides. The details of catalytic properties with respect to epoxidation of olefins
with the complex as catalyst are given in Table 4. Excellent epoxide yields and selectivity were
observed for all aliphatic and aromatic substrates. The results of catalytic studies using
dioxomolybdenum catalysts reveal that the efficiency of catalyst toward all the substrates is
similar with maximum conversion, TON, and selectivity. The mechanism for the epoxidation of
various olefins to epoxides using the two complexes as catalysts has been proposed by
comparison with the literature report (Scheme 3).[20] 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 peroxymonocarbonate, HCO4 , which is a
more reactive nucleophile than H2O2 and speeds up the epoxidation reaction.[21] The basic
principle of the catalytic reaction is the conversion of oxoperoxo-complex to dioxo-complex
transferring oxo species to the olfins and the conversion of dioxo complex to the oxoperoxo
−
complex reacting with HCO4 to regain the catalytic activity. The catalytic property of the
complex is similar to those with similar molybdenum(VI) complexes.[22,23]
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Conclusion
A new dioxidomolydenum(VI) complex with aroylhydrazone ligand
2-amino-N'-(3,5-dichloro-2-hydroxybenzylidene)benzohydrazide has been prepared and
structurally characterized using X-ray structure analysis, FTIR and 1H NMR spectra. The
complex has octahedral geometry with positions around the central atom being occupied with
donor atoms of the aroylhydrazone ligand, oxido groups and O donor atom of coordinated
methanol solvent. The complex showed effective catalytic property in the oxidation of various
olefins to their corresponding epoxides.
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Table 1. Crystal data and structure refinement for the complex
Parameter
Molecular formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal size (mm)
Crystal system
Space group
a (Å)
Value
C15H13Cl2MoN3O5
482.12
298(2)
0.71073
0.18 × 0.17 × 0.17
Monoclinic
P21/n
13.6724(7)
9.7792(5)
14.0066(6)
110.468(1)
1754.5(2)
4
b (Å)
c (Å)
β (º)
V (Å3)
Z
Dcalc (g/cm3)
1.825
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Absorption coefficient (mm1)
F(000)
1.085
960
Theta range for data collection (º)
Index ranges
2.60-27.55
-17 ≤ h ≤ 17
-12 ≤ k ≤ 12
-14 ≤ l ≤ 18
17930
Reflections collected
Independent reflection (Rint)
Reflections observed (I > 2σ(I))
Data completeness
4028 (0.0361)
3177
99.6%
Maximum and minimum transmission
Data/restraints/parameters
Goodness-of-fit on F2
0.8287, 0.8371
4028/4/245
1.060
Final R indices (I > 2σ(I))
R indices (all data)
0.0353, 0.0669
0.0549, 0.0724
0.541, -0.370
Highest peak and deepest hole (e Å3)
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Table 2. Selected bond lengths (Å) and angles (º) for the complex
Bond lengths (Å)
Mo(1)-O(1)
1.936(2)
2.252(2)
1.703(2)
Mo(1)-O(2)
Mo(1)-O(3)
Mo(1)-O(5)
1.990(2)
1.686(2)
2.283(2)
Mo(1)-N(1)
Mo(1)-O(4)
Bond angles (º)
O(3)-Mo(1)-O(4)
O(4)-Mo(1)-O(1)
O(4)-Mo(1)-O(2)
O(3)-Mo(1)-N(1)
O(1)-Mo(1)-N(1)
O(3)-Mo(1)-O(5)
O(1)-Mo(1)-O(5)
N(1)-Mo(1)-O(5)
104.71(11)
104.50(9)
97.44(8)
O(3)-Mo(1)-O(1)
O(3)-Mo(1)-O(2)
O(1)-Mo(1)-O(2)
O(4)-Mo(1)-N(1)
O(2)-Mo(1)-N(1)
O(4)-Mo(1)-O(5)
O(2)-Mo(1)-O(5)
98.89(10)
98.54(9)
147.36(8)
161.51(9)
71.51(7)
83.99(9)
78.89(8)
91.83(10)
80.55(8)
171.22(10)
79.70(8)
79.39(8)
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Table 3. Distances (Å) and angles (º) involving hydrogen bonding
D−H∙∙∙A
d(D−H)
d(H∙∙∙A)
1.96(1)
2.18(2)
2.32(2)
2.15(3)
d(D∙∙∙A)
2.787(3)
3.044(3)
3.079(3)
2.726(3)
Angle(D−H∙∙∙A)
168(4)
O(5)−H(5)∙∙∙N(3)i
0.85(1)
N(3)−H(3A)∙∙∙O(4)ii 0.90(1)
N(3)−H(3B)∙∙∙O(3)iii 0.90(1)
N(3)−H(3B)∙∙∙N(2) 0.90(1)
165(4)
143(3)
121(3)
Symmetry codes: i) 1 -- x, -- y, -- z; ii) x, --1 + y, z; iii) 3/2 -- x, --1/2 + y, 1/2 -- z.
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Table 4. The catalytic oxidation resultsa
Substrate
Product
Conversion
Selectivity (%)d
(%)b (TON)c
97 (270)
92 (293)
100
O
O
O
100
85 (261)
87 (257)
100
100
O
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 measured relative to the starting olefin after 74.5 min.
cTON = (mmol of product)/mmol of catalyst.
d Selectivity to epoxide = (epoxide%/(epoxide% + aldehyde%)) × 100.
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Figure 1. An ORTEP diagram of the complex with atom labeling scheme and 30% probability
thermal ellipsoids for all non-hydrogen atoms.
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Figure 2. Packing diagram of the complex, viewed along the b axis. Hydrogen bonds are shown
as dashed lines.
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Scheme 1. H2L
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Scheme 2. Synthesis procedure of the complex
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O
O
O
Mo
O
N
_
OMe
H
HCO3
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. Proposed mechanism of catalytic oxidation process
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