Synthesis, structure, and catalytic property of a mononuclear dioxomolybdenum(VI) complex containing MoON core

Article abstract of DOI:10.1134/S1070328414040101

Reaction of molybdenyl(IV) oxide bis(2,4-pentanedionate) with N'-(3-ethoxy-2-hydroxybenzylidene)-2-methoxybenzohydrazide in methanol affords a mononuclear dioxomolybdenum(VI) complex containing MoO₅N basic core. The complex has been characterized by various physicochemical techniques (IR and elemental analysis), and single crystal X-ray diffraction. X-ray crystal structure determination reveals that the complex crystallizes as monoclinic space group P2₁/c, with unit cell dimensions a = 9.251(1), b = 11.910(2), c = 17.636(3) A, β = 103.220(2)°, V = 1891.7(5) A³, Z = 4, R ₁ = 0.0693, wR ₂ = 0.1691. The Mo atom in the complex is octahedrally coordinated, with the tridentate ONO ligand occupying the meridional sites. Thermal stability analysis was performed. The complex shows high catalytic property for the oxidation of various olefins.

Full text of DOI:10.1134/S1070328414040101

ISSN 1070ꢀ3284, Russian Journal of Coordination Chemistry, 2014, Vol. 40, No. 5, pp. 268–272. © Pleiades Publishing, Ltd., 2014.
Synthesis, Structure, and Catalytic Property of a Mononuclear
Dioxomolybdenum(VI) Complex Containing MoON Core1
F. M. Wang
Key Laboratory of Coordination Chemistry and Functional Materials in Universities of Shandong, Department of Chemistry,
Dezhou University, Dezhou, Shandong, 253023 P.R. China
eꢀmail: wfm99999@126.com
Received August 12, 2013
Abstract—Reaction of molybdenyl(IV) oxide bis(2,4ꢀpentanedionate) with N'ꢀ(3ꢀethoxyꢀ2ꢀhydroxybenꢀ
zylidene)ꢀ2ꢀmethoxybenzohydrazide in methanol affords a mononuclear dioxomolybdenum(VI) complex
containing MoO5N basic core. The complex has been characterized by various physicochemical techniques
(IR and elemental analysis), and single crystal Xꢀray diffraction. Xꢀray crystal structure determination reveals
that the complex crystallizes as monoclinic space group
P
21/
c
, with unit cell dimensions
a = 9.251(1), b =
11.910(2), = 17.636(3) β = 103.220(2)
c
Å,
°,
V
= 1891.7(5) Å3
,
Z
= 4, 1 = 0.0693, wR2 = 0.1691. The Mo atom
R
in the complex is octahedrally coordinated, with the tridentate ONO ligand occupying the meridional sites.
Thermal stability analysis was performed. The complex shows high catalytic property for the oxidation of various
olefins.
DOI: 10.1134/S1070328414040101
1
INTRODUCTION
PerkinElmer 2400 analyzer. IR spectra (as KBr pellet)
were recorded using a PerkinElmer RXI FTꢀIR specꢀ
trophotometer. Solution electrical conductivity was
measured with a DDSꢀ11A conductivity meter. Therꢀ
mal analysis of the complex was carried out by
Molybdenum is an essential transition metal that of
great importance in life sciences. Enzymes containing
molybdenum at their active sites catalyze a wide range
of reactions in carbon, sulfur, and nitrogen metaboꢀ
lism [1–3]. In particular the molybdenum mediated
oxo transfer in enzymatic systems has attracted much
attention in reactivity and coordination chemistry of
cisꢀdioxomolybdenum(VI) complexes [4–6]. In reꢀ
cent years, a number of molybdenum complexes have
been used as catalysts for the epoxidation and hydroxꢀ
ylation of olefines [7–9], oxidation of alcohols [10],
and oxoꢀtransfer reactions [11]. Hydrazone ligands are
widely used to prepare complexes with various metal
salts. Molybdenum complexes with such ligands are alꢀ
so reported, but the number is still less when compared
to other metals. In the present work, we report the synꢀ
thesis, structure, and catalytic property of a new dioxoꢀ
molybdenum(VI) complex, [MoO2L(OH2)], derived
heating at a rate of 10°C per minute on a PerkinElmer
TGAꢀ4000 thermo balance. The oxidation products
were analyzed with a gas chromatograph (Shimadzu,
GCꢀ14B) equipped with a SABꢀ5 capillary column
(phenyl methyl siloxane 30
flame ionization detector.
×
320
×
0.25 mm) and a
Synthesis of the complex. The ligand H2L (0.31 g,
1.0 mmol) was dissolved in 30 mL methanol and
[MoO2(Acac)2] (0.33 g, 1.0 mmol) was added to the
solution and the mixture was refluxed for 1 h at room
temperature. The color of the solution changed from
colorless to yellow. The solution was filtered and alꢀ
lowed to evaporate slowly at room temperature. After
a week, yellow single crystals, suitable for Xꢀray difꢀ
fraction analysis, were obtained by filtration. The yield
was 55%.
from a new hydrazone ligand
xybenzylidene)ꢀ2ꢀmethoxybenzohydrazide (H2L).
N'ꢀ(3ꢀethoxyꢀ2ꢀhydroꢀ
For C17H18N2O7Mo
EXPERIMENTAL
anal. calcd., %: C, 44.55;
H, 3.96;
H, 4.10;
N, 6.11.
N, 6.27.
Materials and physical measurements. Reagent
grade solvents were used as received. All other chemiꢀ
cals were of AR grade, obtained from commercial
sources and used as received. The ligand H2L was preꢀ
pared by using a reported procedure [12]. Elemental
analyses (C, H, and N) were done with a
Found, %:
C, 44.32;
IR (KBr;
ν
, cm–1): 3443 and 3341
(C–N), 1262r
(MoO2), 500–800 (Mo–O/N).
ν
(O–H), 1609
ν
(C7=N1), 1670
ν
ν
(C–O), 941 and
893
ν
ν
Xꢀray crystallography. A suitable single crystal of
the complex was mounted on a thin glass fiber without
1
The article is published in the original.
268

SYNTHESIS, STRUCTURE, AND CATALYTIC PROPERTY
269
Table 1. Crystal data and structure refinement for the complex
protection. Cell dimensions were determined at
298(2) K from the setting angles of a Bruker SMART
1K CCD diffractometer using a graphite monochroꢀ
Parameter
Formula weight
Value
mated Mo
K
(
λ
= 0.71073
Å
) radiation source. Data
α
458.3
collection was completed using the
ω
/2 scan techꢀ
θ
Crystal system
Monoclinic
niques. Structure of the complex was solved by direct
method, developed by successive difference Fourier
synthesis and refined on F2 by a fullꢀmatrix leastꢀ
squares procedure using SHELXLꢀ97 [13]. The posiꢀ
tions of all nonꢀhydrogen atoms were refined with
anisotropic displacement factors. The water hydrogen
atoms were located from a difference map, and with
Space group
P21/c
Unit cell dimensions:
a
, Å
, Å
9.215(1)
11.910(2)
17.636(3)
103.220(2)
1891.7(5)
4
b
c
, Å
β
, deg
O–H and H···H distances of 0.85(1) and 1.37(2) Å, reꢀ
Volume, Å3
spectively. The remaining hydrogen atoms were geoꢀ
metrically calculated and isotropically fixed at posiꢀ
tions recalculated after each cycle of refinement (C–H
Z
ρcalcd, g cm–3
Absorption coefficient, mm–1
1.609
0.93–0.97
Å
with the isotropic thermal parameters of
iso(C) and 1.5 iso(Cmethyl)). Absorption
0.734
Uiso(H) = 1.2
U
U
F
(000)
928
corrections based on multiꢀscan using SADABS were
applied [14]. Data reduction was accomplished using
the SAINT plus [15].
Crystal size, mm
Limiting indices
0.18
×
0.18
×
0.16
–10
–9
–17
≤
≤
≤
h
k
l
≤
9,
14,
20
Pertinent cell parameters, data collection condiꢀ
tions, and refinement details are provided in Table 1.
The important bond lengths and angles are provided in
Table 2. Supplementary material for the complex has
been deposited with the Cambridge Crystallographic
Data Centre (no. 9506932; deposit@ccdc.cam.ac.uk
or http://www.ccdc.cam.ac.uk).
≤
≤
Reflections collected/unique
4433/2332
Observed reflections, (
I
> 2
σ
(
I
))
1975
Parameters
Restraints
252
3
Goodnessꢀofꢀfit on F 2
Final indices, ( > 2
indices (all data)
Largest difference in peak and
1.307
Catalytic oxidation procedure. Catalytic experiꢀ
ment was carried out in a 50 mL glass reaction flask fitꢀ
ted with a water condenser. 0.032 mmol dioxoꢀmolybꢀ
denum(VI) complexes were dissolved in 10 mL
1,2ꢀdichloroethane. Then 10 mmol alkene was added
to the reaction mixture and 30 mmol tertꢀbutyl hydroꢀ
peroxide (TBHP) was added. The reaction
mixture was refluxed for 1 h. The reaction products
were monitored at periodic time intervals using gas
chromatography. The oxidation products were identiꢀ
fied by comparison with authentic samples.
R
I
σ(I))
R
1 = 0.0693, Rw2 = 0.1691
1 = 0.0796, Rw2 = 0.1735
1.031 and –0.517
R
R
hole,
e
Å–3
solution of the complex in acetonitrile at 25°C is
–1
21.5
lytic behavior in the solution [16].
Ω
cm2 mol–1. The data indicates nonꢀelectroꢀ
An ORTEP diagram of the complex is illustrated in
Fig. 1. In the complex a distorted octahedral geometry
is observed for the molybdenum atom, with an [ONO]
RESULTS AND DISCUSSION
The synthesis of the hydrazone ligand requires a donor set from the hydrazone ligand, a complementꢀ
two step reaction. In the first step the 2ꢀmethoxybenꢀ ing water O donor, and the two oxo groups of the
zohydrazide was synthesized by reaction of the methyl MoO2 moiety. For the latter Mo–O bond lengths of
2ꢀmethoxybenzoate with 1.3 equivalent of hydrazine
hydrate under reflux and continuous stirring for 6 h.
Subsequently the hydrazone ligand was obtained via
condensation reaction of 2ꢀmethoxybenzohydrazide
with 3ꢀethoxysalicylaldehyde in methanol solution at
room temperature. Stoichiometric reaction of the hyꢀ
drazone ligand with MoO2(Acac)2 as molybdenum
1.700(7) and 1.726(7)
Å and OMoO bond angle of
105.7(4) are found, which are in excellent agreement
°
with the corresponding values observed in similar diꢀ
oxomolybdenum complexes [17, 18]. The equatorial
plane at the molybdenum atom is given by the tridenꢀ
tate hydrazone ligand (O(1), N(1), and O(2)) and one
of the oxo groups (O(7)). The ligand coordinates in a
source in refluxing methanol yields the corresponding meridional fashion forming fiveꢀ and sixꢀmembered
cisꢀdioxomolybdenum(VI) complex. The reaction chelate rings at the MoO2 moiety with bite angles of
progress is accompanied by an immediate color change
of the solution from colorless to yellow. The complex is
soluble in methanol, ethanol, acetonitrile, DMF, and
71.6(3)
ligand is coordinated in its dianionic form. This is evꢀ
ident from the N(2)–C(8) and O(2)–C(8) bond
, reꢀ
°
and 82.1(3)°, respectively. The hydrazone
DMSO. The molar conductance of a ~0.10 mmol dm–3 lengths with values of 1.315(13) and 1.324(12)
Å
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 40
No. 5
2014

270
WANG
Table 2. Selected bond lengths (Å) and bond angles (deg)
for the complex
mean plane towards the axial oxygen atom O(6) by
0.324(1) . The coordinate bond lengths are comparaꢀ
Å
ble with those observed in similar dioxomolybdeꢀ
num(VI) complexes [17–19]. The crystal packing of
the complex is depicted in Fig. 2. The complex moleꢀ
cules are linked by water molecules through intermoꢀ
lecular O–H⋅⋅⋅O hydrogen bonds (Table 3), to form
Bond
d
, Å
Bond
d, Å
Mo(1)–O(1)
Mo(1)–N(1)
Mo(1)–O(6)
1.915(7) Mo(1)–O(2)
2.215(9) Mo(1)–O(5)
1.700(7) Mo(1)–O(7)
1.998(6)
2.340(7)
1.726(7)
chains running down the x axis.
Angles
ω
, deg
Angles
ω
, deg
The ligand showed stretching bands attributed to
C=O, C=N, C–OH, and NH at 1658, 1636, 1155
and 1227, and 3317 cm–1, respectively. In addition, a
strong band found at 1621 cm–1 is attributed to
C=N–N=C group. This IR evidence has been regisꢀ
tered earlier for the similar class of ligands that behave
as tridentate dibasic ligands upon enolization [20].
The dioxomolybdenum complex showed two promiꢀ
nent bands at 941 and 893 cm–1 attributed to cisꢀdioxoꢀ
molybdenum (MoO2) group. The bands due to
O(1)Mo(1)O(2) 150.9(3)
O(2)Mo(1)N(1) 71.6(3)
O(1)Mo(1)N(1) 82.1(3)
O(6)Mo(1)O(5) 170.8(3)
O(1)Mo(1)O(5) 82.3(3) O(2)Mo(1)O(5) 79.3(3)
O(6)Mo(1)O(1) 99.2(4)
O(6)Mo(1)O(7) 105.7(4)
O(7)Mo(1)O(1) 103.1(3)
O(7)Mo(1)N(1) 156.3(3)
N(1)Mo(1)O(5) 75.2(3)
O(6)Mo(1)O(2) 95.7(3)
O(6)Mo(1)N(1) 96.0(3)
O(7)Mo(1)O(2) 96.6(3)
O(7)Mo(1)O(5) 82.6(3)
ν
(C=O) and (NH) were absent in the complex, but a
ν
new C–O stretch appeared at 1262 cm–1. This suggests
occurrence of ketoꢀimine tautomerization of the
ligand during complexation. The C=N–N=C frameꢀ
work seen at 1621 cm–1 in the ligand shifted to
1609 cm–1 upon coordination to Mo atom. Bands
have also appeared in the range 3200–3600 cm–1 that
may be attributed to coordinated water. Three new
peaks found at 777, 586, and 519 cm–1 may be attribꢀ
uted to Mo–O and Mo–N linkages in the complex.
Table 3. Geometric parameters of hydrogen bonds of the
complex*
Distance, Å
⋅⋅⋅ D⋅⋅⋅A
Angle
D–H⋅⋅⋅A,
deg
Contact D–H⋅⋅⋅
A
D–H
H
A
O(5)–H(5
O(5)–H(5
O(5)–H(5
A
)
⋅⋅⋅O(7)i 0.85(1) 2.45(12) 2.758(10) 102(9)
⋅⋅⋅O(4)ii 0.85(1) 2.39(10) 2.967(11) 126(11)
⋅⋅⋅O(2)ii 0.90(1) 2.11(5) 2.903(11) 155(10)
The doubly
(C=N) found at 1609 and 1670 cm–1 in
ν
B
)
the complex may be due to different C(7)–N(1) and
C(8)–N(2) bond distances.
B
)
i
ii
* Symmetry codes: 2 – x, 2 – y, –z; 1 – x, 2 – y, –z.
Thermo analysis of the complex was carried out in
the temperature range 50–1000
10
C min–1. The TG and DTA curves of the complex
are shown in Fig. 3. The complex is stable up to 187 C.
Above this temperature, the TG curve show three steps
of weigh losses. The first step of decomposition in the
temperature range 187–237 C involved the removal
of the coordinated water molecule and the ethoxy and
methoxy groups of the ligand. The second step of deꢀ
composition in the temperature range 238–603 C inꢀ
°C at a heating rate of
°
spectively, which are indicative for the presence of the
enolate form of the ligand amide group. The two axial
positions are occupied by the second oxo group O(6)
of the MoO2 unit and an oxygen atom of a water molꢀ
ecule (O(5)). The rather long Mo(1)–O(5) bond and
consequently weak bonding is due to the trans effect of
the oxo group O(6). This is accompanied by a signifiꢀ
°
°
°
cant displacement of the Mo atom from the equatorial volved the removal of the remaining moiety of the
C(5)
C(6)
C(4)
C(15)
C(7)
O(4)
C(1)
N(2)
C(10)
O(6)
C(2)
C(3)
O(1)
N(1)
C(9)
C(11)
C(16)
C(8)
O(3)
Mo(1)
C(17)
O(2)
C(12)
C(13)
C(14)
O(5)
O(7)
Fig. 1. An ORTEP drawing at 30% thermal ellipsoid of the crystal structure of the complex, with the atomic numbering scheme.
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 40 No. 5 2014

SYNTHESIS, STRUCTURE, AND CATALYTIC PROPERTY
271
5
1.0
0.8
0.6
0
–5
x
–10
–15
–20
–25
z
0
0.4
0.2
0
200
400 600
Temperature,
800
1000
y
°
C
Fig. 3. TG–DTA curves of the complex.
The catalytic results are listed in Table 4. Generally
speaking, excellent epoxide yields and selectivity were
observed for both aliphatic and aromatic substrates.
Oxidation of aromatic substrates gave the correspondꢀ
ing epoxides in 100% yields, while in the oxidation of
aliphatic substrates, the conversions are less than
100%. This is in accordance with those reported in the
literature [21]. The possible catalytic procedure is is
illustrated below:
Fig. 2. The molecular packing of the complex, viewed
along the axis. H atoms, except those related to hydrogen
bonding, are omitted for clarity. Hydrogen bonds are indiꢀ
cated by dashed lines.
x
ligand. With further elevation in temperature until
880°C, the MoO2 residue was volatilized.
N
N
O
O
OMe
Mo
O
OH2
O
OEt
(H3C)3C
O
O
OH
OH
(H3C)3C
N
N
N
N
H O
HO
O
O
OMe
O
OMe
Mo
O
Mo
O
O
OH2
OH2
O
OEt
OEt
O
O
O
(H3C)3C
(H3C)3C
To start with, TBHP was activated by coordination
to the Mo atom and formation of heptaꢀcoordinated
molybdenum intermediate. Then a molecule of an
olefin as a nucleophile attacks the electrophilic oxygen
atom of the coordinated TBHP. As a result, the correꢀ
sponding epoxides were formed, and TBHP was reꢀ
duced to tertꢀbutyl alcohol.
Thus, a new mononuclear dioxomolybdenum(VI)
complex containing MoO5N basic core derived
from N'ꢀ(3ꢀethoxyꢀ2ꢀhydroxybenzylidene)ꢀ2ꢀmethoxyꢀ
benzohydrazide in methanol has been prepared and
characterized. The Mo atom is sixꢀcoordinated in octaꢀ
hedral geometry. The molecular structure of the complex
is stabilized by hydrogen bonds. Thermal analysis was alꢀ
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 40
No. 5
2014

272
WANG
Table 4. Catalytic oxidation results*
Substrate
Product
O
Conversion, % (TON)**
95 (251)
Selectivity, %
100
Cl
Cl
97 (272)
100
100
O
100 (275)
O
O
100 (295)
100 (266)
100
100
O
O
100 (287)
100
Cl
Cl
* The molar ratio of catalyst : substrate : TBHP is 1 : 300 : 1000. The reactions were performed in mixture of CH OH–CH Cl (
1.5 mL).
V : V = 1 : 1;
3
2
2
** The GC conversion (%) was measured relative to the starting substrate after 60 min; TON = (mmol of product)/(mmol of catalyst).
9. Amini, M., Haghdoost, M.M., and Bagherzadeh, M.,
Coord. Chem. Rev., 2013, vol. 257, nos. 7–8, p. 1093.
so performed to understand the stability of the complex.
The complex shows high catalytic property for the oxidaꢀ
tion of various olefins.
10. BeltowskaꢀBrzezinska, M., Wesierski, T., Luczak, T.,
et al., Electrochim. Acta, 2012, vol. 63, no. 1, p. 89.
11. Ma, X.L. and Schulzke, C., Inorg. Chim. Acta, 2013,
vol. 395, p. 218.
ACKNOWLEDGMENTS
12. Zhang, M., Xian, D.ꢀM., Li, H.ꢀH., et al., Aust. J.
Chem., 2012, vol. 65, no. 4, p. 343.
13. Sheldrick, G.M., Acta Crystallogr., A, 2008, vol. 64, no. 2,
The Department of Chemistry of Dezhou Univerꢀ
sity is acknowledged for financial support.
p. 112.
14. Sheldrick, G.M., SADABS
,
Program for Empirical Abꢀ
ttingen
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