Catalytic potentials of homodioxo-bimetallic dihydrazone complexes of uranium and molybdenum in a homogeneous oxidation of alkenes

Article abstract of DOI:10.1007/s00706-015-1477-9

The catalytic potentials of dioxomolybdenum(VI) and dioxouranium(VI) homobimetallic bis-ONO tridentate (2-hydroxy-1-benzylidene)malonyl-, succinyl-, and terephthalo-dihydrazone complexes were studied in homogeneous oxidation processes of various aliphatic and cyclic alkenes using aqueous H₂O₂ or TBHP (tert-butyl hydroperoxide) as a terminal oxidant. The catalytic potentiality is quantitative and highly selective to afford the corresponding oxide product with Mo^VIO₂ complexes which is 5 times more than that with U^VIO₂ complexes using aqueous H₂O₂ or TBHP. Effect of various solvents and temperatures was investigated in the oxidation of 1,2-cyclooctene catalyzed by Mo^VIO₂ complexes using aqueous H₂O₂ results that the most favored solvent is acetonitrile at an optimal temperature is 70 °C. The mechanistic pathway was tentatively described and discussed

Full text of DOI:10.1007/s00706-015-1477-9

Monatsh Chem
DOI 10.1007/s00706-015-1477-9
ORIGINAL PAPER
Catalytic potentials of homodioxo-bimetallic dihydrazone
complexes of uranium and molybdenum in a homogeneous
oxidation of alkenes
Mohamed Shaker S. Adam1,2
Received: 23 January 2015 / Accepted: 12 April 2015
Ó Springer-Verlag Wien 2015
Abstract The catalytic potentials of dioxomolybde-
num(VI) and dioxouranium(VI) homobimetallic bis-ONO
tridentate (2-hydroxy-1-benzylidene)malonyl-, succinyl-,
and terephthalo-dihydrazone complexes were studied in
homogeneous oxidation processes of various aliphatic and
cyclic alkenes using aqueous H2O2 or TBHP (tert-butyl
hydroperoxide) as a terminal oxidant. The catalytic poten-
tiality is quantitative and highly selective to afford the
corresponding oxide product with MoVIO2 complexes
which is 5 times more than that with UVIO2 complexes
using aqueous H2O2 or TBHP. Effect of various solvents
and temperatures was investigated in the oxidation of 1,2-
cyclooctene catalyzed by MoVIO2 complexes using aqueous
H2O2 results that the most favored solvent is acetonitrile at
an optimal temperature is 70 °C. The mechanistic pathway
was tentatively described and discussed.
Graphical abstract
Keywords Dihydrazones Á Dioxomolybdenum(VI) Á
Dioxouranium(VI) Á Catalytic oxidation Á Alkenes
Introduction
The most important particular application of the catalytic
oxidation of alkenes and arenes is the production of in-
dustrial chemicals and fine chemicals [1–3]. High attraction
on the high-valent molybdenum(VI) complexes is consid-
erable interest in research due to their catalytic activity as
internal catalyst [4] for the oxidation of alkenes and arenes.
Many transition metal complexes have been reported for
oxidation of alkenes [5–10], while dioxomolybdenum
Schiff base complexes [4, 11–13] have been of great in-
terest due to their ease and low cost in formation, as well as
their sterical and electronical structural features. The cat-
alytic oxidation of alkenes by Mo(VI) complexes has been
demonstrated by Mimoun et al. [14]. He suggested that the
Mo(VI) complexes form with the oxo reagent intermedi-
ates. Those intermediates are responsible for the oxygen
transfer in the catalytic processes. Mimoun presented a
mechanism, in which the olefin binds to the metal, inserting
into the Mo-oxo bond. Subsequent reductive elimination of
Electronic supplementary material The online version of this
article (doi:10.1007/s00706-015-1477-9) contains supplementary
material, which is available to authorized users.
& Mohamed Shaker S. Adam
shakeradam61@yahoo.com;
madam@kfu.edu.sa
1
Department of Chemistry, College of Science, King Faisal
University, P.O. Box 380, Al Hufuf 31982, Al Hassa,
Saudi Arabia
2
Department of Chemistry, Faculty of Science, Sohag
University, Sohag 82534, Egypt
123

M. S. S. Adam
the resulting metallacycle leads to a metal oxide and the
epoxide product. Sharpless et al. [15] suggested an alter-
native mechanism involving direct transfer of oxygen from
the coordinated peroxide to the alkenes.
Results and discussion
Synthesis and characterization
Consequently, the catalytic activity of MoO2(VI) cata-
lysts is relevance to the active sites of the majority of
molybdo-enzymes [16, 17]. The most valuable types for
the active site of several Mo complexes are dioxo com-
plexes of MoO2(VI) with polydentate nitrogen, sulfur, and
oxygen ligands [18–21]. Such active sites are linked to
coordinative unsaturation in the MoO2(VI) complexes
catalysts which means coordinatively unsaturated centers.
The active sites in MoO2(VI) complexes may be occupied
by labile ligands, i.e. the solvent molecules. This behavior
is the major role of a catalytic redox site [22, 23].
MoO2(VI) complexes have already been shown to be ef-
fective catalyst precursors uptake [24–26]. The
coordination chemistry of uranium in the form of the ura-
nyl cation species has been largely received considerable
attention recently because of their interest coordination
chemical behavior, reactivity, and interesting optical,
magnetic and catalytic applications, e.g. photocatalysis
[27]. The uranyl catalysts not yet find many particular
applications mainly in the catalytic activity toward alkenes’
epoxidation with various oxidants of high-valent UO2(VI)
complexes [28, 29].
Synthesis of ligands bis-(2-hydroxy-1-benzylidene)-
malonyldihydrazone (L1), bis-(2-hydroxy-1-benzylidene)-
succinyldihydrazone (L2), and bis-(2-hydroxy-1-benzyli-
dene)terephthalodihydrazone (L3) is a common method
and reported elsewhere [32–35, 38]. Complexation of L1–
L3 with either bis(acetylacetonato)dioxomolybdenum or
uranyl acetate dehydrate in methanol (1:2) is reported re-
1
cently by us [26] (Scheme 1). 1–6 are characterized by H
NMR, 13C NMR, and UV–Vis spectra, thermogravimetric
analyses (TGA), and elemental analyses (EA). Moreover,
they are characterized by IR spectra which are recorded in
Table 2.
All complexes 1–6 show high air stability in solid phase
with decomposition above 300 °C. Ligands in the com-
plexes behaved in a bis-dianionic tridentate manner bonded
2?
2?
to the cis-MoO2 and UO2 ions through the phenolic
oxygen, the amide oxygen, and Schiff base nitrogen atoms
as proved by IR and NMR spectra and previously by X-ray
analysis of complex 1 [32]. The X-ray analysis of complex
2?
1 [32] afforded cis-MoO2 complexes.
In the solid phase, cis-MoO2 complexes 1, 2, and 3 are
2?
yellow to orange while the UO2 complexes 4, 5, and 6
Acyl dihydrazones and their metal complexes have wide
applicability, e.g., analytical, medicinal chemistry and
biotechnology [30, 31]. Due to their facile keto–enol tau-
tomerization, they have high availability of several
potential donor sites can act as multidentate ligands and
coordinate with metals of low and high oxidation states
[32–35]. At present, significant contributions to literature
knowledge regarding the catalytic potentials of homo-
bimetallic complexes have been less explored in the alkene
oxidations, especially MoO2(VI) pattern [36] in combina-
tion with multidentate ligands, i.e. dihydrazones,
containing bulky fragments in their molecular skeleton.
Moghadam et al. [37] reported the catalytic activity of new
binuclear molybdenum bis-oxazoline complex in the
oxidation of alkenes and sulfides by tert-butyl hydrogen
peroxide. In the view of those considerations, we devel-
oped in this work the complexation of commonly reported
multidenate dihydrazone ligands (2-hydroxy-1-benzyli-
dene)malonyl-, succinyl-, and terephthalo-dihydrazone
L1–L3 with dioxomolybdenum(VI) and dioxouranium(VI)
ions awarding homodioxo-bimetallic complexes 1–6
(Scheme 1). The catalytic potentials of those complexes as
homogenous catalysts in the oxidation of some various
alkenes using either aqueous H2O2 or tert-butyl hydroper-
oxide (TBHP) as external oxidants were presented and
studied and investigated in various conditions.
are red to brownish red (Table 1). The current complexes
are partially soluble in common organic solvents (ace-
tonitrile, acetone, and methanol) but highly dissolved in the
coordinated solvents, such as DMSO and DMF.
1H NMR spectra of L1–L3 and complexes 1–6 gave
conclusive evidence for the complexation mode and the
2?
2?
coordinated atoms of L1–L3 to MoO2 and UO2 ions.
The NH and OH resonating signals in L1 (11.85, 11.47,
11.40, 11.07, and 11.05 ppm, resulted from the diketo-,
keto–enol, and dienol forms of L1) disappeared completely
in the corresponding complexes 1 and 4. The same be-
havior was observed in the complexation of L2 and L3
2?
2?
with MoO2 and UO2 ions. The resonating signals of
the NH and OH observed at 11.71, 11.70, 11.28, 11.21,
11.17, and 10.13 ppm (due to the tautomeric behavior) in
L2 disappeared in 2 and 5. In L3, the NH and OH proton
resonances (11.19, 11.22, and 12.24 ppm) disappeared in 3
and 6 after complexation. These regarding involved that
L1–L3 coordinated to the metal ions in the enol form
through the phenolic OH group and the dienol OH group.
There is a downfield shift of the HC=N resonating proton
from 8.28 and/or 8.42 ppm in L1 to 8.80 and 9.19 ppm in 1
and 4, respectively, from 8.28 and/or 8.35 ppm in L2 to
8.77 and 9.14 ppm in 2 and 5, respectively, and from 8.67
and/or 8.69 ppm in L3 to 9.00 and 9.32 ppm in 3 and 6,
respectively. This could be due to the coordination of the N
123

Catalytic potentials of homodioxo-bimetallic dihydrazone…
Scheme 1
Complex
X
M
n
(metal ion) (crystalline molecules)
MoVI
MoVI
MoVI
1
1
1
-CH2-
1
2
-C2H4-
3
UVI
UVI
UVI
1
0
0
-CH2-
4
5
-C2H4-
6
123

M. S. S. Adam
Table 1 Characteristic electronic spectra for the current complexes ([complex] = 1.0 9 10-4 mol dm-3) at 298 K
Complex
MW/g mol-1
Electronic spectra (in MeOH)
max/mol-1 cm-1
Color
k
max/nm
e
Assign.
1
[(MoO2)2L1(H2O)2]ÁH2O
[(MoO2)2L2(H2O)2]ÁH2O
[(MoO2)2L3(H2O)2]ÁH2O
646.22
391
338
251
381
335
254
404
346
258
392
290
395
290
399
294
33,150
12,820
17,970
31,900
18,780
17,940
34,470
12,540
17,970
29,100
16,650
25,730
18,210
29,330
14,960
LMCT
n–p*
Yellow
p–p*
2
3
660.25
708.29
LMCT
n–p*
Orange
Yellow
p–p*
LMCT
n–p*
Orange
p–p*
4
5
6
[(UO2)2 L1(H2O)2]Á3H2O
[(UO2)2 L2(H2O)2]Á2H2O
[(UO2)2 L3(H2O)2]Á2H2O
966.43
962.22
1010.48
LMCT
p–p*
Brownish
Red
LMCT
p–p*
Brownish
Red
LMCT
p–p*
Red
Table 2 Structural significant IR spectral data (KBr, m/cm-1) of the studied ligands L1–L3 and their corresponding complexes 1–6
ꢀ
Group
H2O
L1
1
4
L2
2
5
L3
3
6
3734 w
3734 m br
3434 s br
3736 w
3438 br
3630 w br
3401 s br
3680 w br
3416 s br
3751 w br
3451 s br
3440 s br
OH
NH
3406 m br
3264 s br
3208 s br
3058 br
2976 br
1672 s
3436 m br
3207 s br
3381 m br
3208 s br
3218 s br
3226 s br
3213 s br
CH aromatic
CH aliphatic
C=O
3056 m br
3058 br
2928 br
1663 s
1555 s
1486 m
1406 s
1273 m
1034 w
3050 m br
2925 w br
3035 br
3067 m br
2927 w br
1643 s
1551 m
1487 m
1365 s
1284 m
1034 w
C=N
1566 s
1617 s
1556 m
1448 w
1337 w
1260 w
908 m
813 w
1610 s
1545 s
1445 w
1390 m
1298 m
905 m
760 w
599 w
483 w
411 w
1615 s
1558 m
1445 m
1329 m
1265 m
868 m
760 w
1603 s
1608 s
1551 w
1443 w
1333 m
1290 m
921 m
745 w
1712 w
1599 m
1449 w
1388 m
1298 s
897 m
758 w
597 w
522 w
403 w
1482 m
1374 m
1263 s
NCO amide
C–O
1448 w
1395 m
1303 m
902 m
762 m
599 w
519 w
409 w
N–N
1033 w
M=O
M–O phenolic
M–O carbonyl
M–N hydrazone
575 w
585 w
618 w
461 w
467 w
463 w
w week band, m medium band, s strong band, br broad band
atom of the HC=N group (Schiff base group) to the central
metal ions. Comparison of the 13C NMR spectra between
the free ligands L1–L3 and their corresponding complexes
1–6 indicates that the L1–L3 coordinated to the central
metal ion in the dienol form. The signal associated with the
C=O group (182, 172, and 165 ppm in L1, L2, and L3,
respectively) disappeared and new signals appeared after
complexation at 167 and 170 ppm in 1 and 4, respectively,
at 173 and 177 ppm in 2 and 5, respectively, and at 167 and
170 ppm in 3 and 6, respectively, due to the formation of
the C=N group in the dienol from (Scheme 1).
Complexes 1–6 and their corresponding ligands were
characterized by IR spectra and are recorded in Table 2. IR
spectral data supported that the L1–L3 formed complexes
123

Catalytic potentials of homodioxo-bimetallic dihydrazone…
2? 2?
with either MoO2 or UO2 within dienol form. Strong
vibrational bands at 1672, 1663, and 1663 cm-1 assigned
to m(CO) of the uncoordinated dihydrazones, L1, L2, and L3
respectively, disappeared in 1–6. In L1, the assigned band
Thermogravimetric analyses results of complexes 1–6
were evaluated using nitrogen as the gas carrier. The
complexes were heated at rate of 10 °C per min from 30 to
600 °C. The thermogram of the studied complexes shows
that the complexes decomposed into two successive steps.
This attributed to the loss of water molecules in the crystal,
coordinated water molecules in the complexes for the first
and second steps, respectively (Supplementary Material).
TGA data confirmed the tentative molecular structure of
the recent complexes 1–6.
of m(C=N) appeared as couple of bands at 1566–1482 cm-1
,
was shifted to 1617–1556 and 1601–1545 cm-1 after co-
2?
2?
ordination to MoO2
and UO2
ions in 1 and 4,
respectively. Also, in L2 and L3, the same behavior was
observed that m(C=N) shifted from 1555–1486 to
1551–1487 cm-1
,
respectively, to 1615–1558 and
1603 cm-1 in 2 and 5, respectively, and to 1608–1551 and
1712–1559 cm-1 in 3 and 6, respectively (Table 2) [28].
According to the tautomeric behavior of L1–L3 [32–35],
the m(OH) bands at 3406, 3436, and 3381 cm-1 disappeared
after complexation with appearance of new bands at the
same region as doubled signals (3734–3340, 3736–3438,
3680–3416, 3734–3434, 3630–3401, and 3751–3451 cm-1
in 1, 2, 3, 4, 5, and 6, respectively) due to the presence of
coordinated water molecules in the complexes [28].
In the thermogram of the complexes, the first en-
dothermic weight loss of 1 was observed between 100 and
120 °C with mass loss value (Dmrel = 2.19 %) ap-
proximately
agreeing
with
the
expected
one
(Dmrel = 2.78 %) and ascribed to the loss of one water
molecule. This suggests that the water molecule was part of
the lattice structure in complex 1. Similar behavior for
complexes 2 and 3, the first endothermic weight loss was
observed in the same temperature range with mass loss
values (Dmrel = 2.11 and 2.04 %) approximately agrees
with the expected one (Dmrel = 2.72 and 2.54 %) for 2 and
3, respectively, suggesting that both 2 and 3 have one water
molecule in crystal lattice. Also in complex 4, the loss of
the crystalline water molecule detected in the same area
with experimental mass loss value (Dmrel = 1.57 %)
agrees with the calculated mass loss (Dmrel = 1.86 %).
Complexes 5 and 6 did not show any weight loss at the
range from 180 to 230 °C (Supplementary Materials). This
means that the crystal lattice of complexes 5 and 6 prob-
ably does not have any water molecules (Scheme 1). In
complexes 1–3, the two coordinated water molecules were
lost at 180–230 °C with an observed mass loss value
(Dmrel = 5.33, 5.24, and 5.01 %) which agrees with the
expected value (Dmrel = 5.73, 5.57, and 5.21 % for 1, 2,
and 3, respectively).
L1–L3 show assigned bands in the UV region only,
while their corresponding complexes show remarkable
colors in the solution which gives characteristic bands in
UV and visible regions (Table 1). Electronic spectra of the
complexes have been recorded in methanol at
&1.0 9 10-4 mol dm-3. The ligands show bands assigned
to intraligand p ? p* and n ? p* transitions [32–35]. The
red shift of the UV bands of L1–L3 [28] with com-
plexation to 290–296 and 345–356 nm, respectively,
proved the complexation of the current ligands to the
2?
2?
central metal MoO2 and UO2 ions. In addition to the
intraligand bands in the complexes (290–296 nm), all
complexes show new bands at 381–404 nm, assigned to
ligand-to-metal charge transfer (LMCT) [28].
Unfortunately, attempts to characterize the complex
structure by single crystallography were unsuccessful,
despite the crystallographic data of 1 were presented by
Zhang et al. [32, 33]. In 1 as cited by Zhang et al. [32,
33], the molybdenum atoms have a distorted octahedral
coordination with the donor atoms from the Schiff base
tetra-anion and one oxo group occupying the sites of the
equatorial planes affording cis-MoO2-units. The other
oxo group occupies the apical sites. The remaining apical
sites are occupied by water molecules, instead of
methanol molecule as remarked elsewhere [32, 33]. The
nitrogen atoms of the Schiff base ligands are trans to one
of the double bonded oxo groups, as observed previously
[39].
The decomposition losses (Dmrel = 7.27, 7.05, and
6.91 %) occurred at the range 180–230 °C agree with the
theoretical Dmrel = 7.59, 7.48, and 7.12 %, respectively, to
complexes 4, 5, and 6. This is due to the loss of the four
coordinated water molecules to the two central UO22? ions.
The TGA results support that the two homo-uranyl ions in
each complex molecule of 4, 5, and 6 have seven coordi-
nation number to the bis-dihydrazone ligand and water
molecules (Scheme 1).
All complexes 1–6 decomposed without melting above
300 °C.
Recently, similar dihomo diuranyl dihydrazone complex
of hydroxylpyridinyl oxalohydazoide has been reported
with X-ray crystallographic analyses [40], in which the
solvent molecule (DMSO) completed the seven coordina-
tion number with a distorted pentagonal bipyramidal
geometrical structure.
Catalytic activity
The catalytic performance of cis-MoO2- and UO2 com-
plexes 1–6 was investigated in the oxidation of some
aliphatic and cyclic alkenes with aqueous H2O2 and tert-
butyl hydroperoxide (TBHP) as oxygen donors. A series of
123

M. S. S. Adam
Table 3 Oxidation yield (1,2-epoxycyclooctane) of 1,2-cyclooctene catalyzed by 1–6 using aqueous H2O2 in acetonitrile
Entry
Complex
Conversion/%a
Selectivity product yield/%b
Reactant residue/%
TONc
TOFd
H2O2
TBHP
H2O2
TBHP
H2O2
TBHP
H2O2
TBHP
H2O2
TBHP
1
2
3
4
5
6
a
1
2
3
4
5
6
95
97
96
48
53
49
97
99
98
53
56
54
89
95
93
20
25
17
93
96
94
23
25
19
5
3
4
2
89
95
93
20
25
17
93
96
94
23
25
19
11.12
11.87
11.62
2.50
46.50
48.0
4
2
47.0
52
47
51
47
44
46
11.50
12.50
9.50
3.12
2.12
The reaction of 1,2-cyclooctene (1.0 mmol) and complexes 1–6 (0.01 mmol) with aqueous H2O2 (4.16 mmol) in 10 cm3 acetonitrile at 70 °C
for 8 h or TBHP (2.0 mmol) at 70 °C for 2 h
b
The yield percentage of the target oxide product (1,2-epoxycyclooctene) based on GC results
c
TON (turnover number) = ratio of moles of product (here is the obtained oxide) to the moles of catalyst
The corresponding TOF (turnover frequency) (TON/h) values are shown in parentheses [mol (mol catalyst)-1 h-1
d
]
blank experiments revealed that the presence of both cat-
alyst and oxidant is essential for an effective catalytic
oxidation cis-1,2-cyclooctene (Table 3). To find the suit-
able reaction conditions, the effect of various reaction
parameters that may affect the chemo- and stereoselective
conversion was studied. Type and concentration of cata-
lysts, nature and concentration of oxidants, solvent, and
temperature are the factors that have been evaluated to
optimize the catalytic processes.
The uranyl units 4, 5, and 6 attain low conversion and
control selectivity in the same conditions (20, 25, and 17 %
with H2O2 and 23, 25, and 19 % with TBHP using 4, 5, or
6, respectively) with remaining high amount of the reactant
in the reaction media and some other unknown side prod-
ucts, as observed by GC (Table 3, entries 4–6). UO2
complexes 4–6 afforded low catalytic potential compared
to cis-MoO2 complexes 1–3. Consequently, the catalytic
potentiality of 1,2-cyclooctene oxidation is quantitative
and highly selective to the corresponding oxide product
with cis-MoO2 complexes 1–3 which is 5 times more than
that with UVIO2 complexes 4–6 using either aqueous H2O2
or TBHP as terminal oxidant in the same conditions. UO2
complexes are found to serve many catalytic processes as
active catalysts, e.g. thermal oxidation of CO, NO, and
volatile organic compounds [41, 42]. Nevertheless, UO2
complex catalysts are yet to find many practical applica-
tions, mainly because of their inherent natural radioactivity
[29, 43]. From TGA data, the two particular coordinated
water molecules complete the coordination number seven
of the UO2 complexes, c.f. Scheme 1, compared to similar
reported UO2 complexes [40]. According to the less crys-
tallographic data of 4–6, the most common geometry of the
ligand around the two uranyl atoms is close to pentagonal
bipyramidal with the axial O=U=O moiety, as reported
before [40, 44, 45]. Probably, the three donor atoms of the
ligand N, O, and O with the two coordinated water mole-
cules may be in the equatorial position. This tentative
geometrical structure of 4–6 may support the reason of
their low catalytic potential toward oxidation of alkenes
with either aqueous H2O2 or TBHP as oxygen source.
Catalyst complexes effect
To evaluate the catalytic activity of complexes 1–6,
oxidation of cis-1,2-cyclooctene as a model substrate in
acetonitrile catalyzed 1–6 (0.01 mmol) has been investi-
gated using either aqueous H2O2 (4.16 mmol) at 70 °C for
8 h or TBHP (2 mmol) at 70 °C for 2 h as the oxygen
source. The oxide product (1,2-epoxycyclooctene) thus
obtained was analyzed by GC. Control experiments
showed that no epoxide was formed in a measurable extent
in the absence of any catalyst. All experimental results
obtained were collected in Table 3. The controlled cat-
alytic processes using 1–6 were entirely chemo- and
stereoselective for oxidation. 1,2-Epoxycyclooctene as the
only product was obtained in highest scale but in some
cases it was found some other side products in various
conditions as recorded by the turnover numbers and turn-
over frequencies (Table 3), especially with catalysts 4, 5,
and 6.
cis-MoO2 complexes 1–3 afforded high catalytic con-
trol and chemoselectivity for the conversion of 1,2-
cyclooctene using either aqueous H2O2 or TBHP as a
terminal oxidant. Table 3 presents excellent conversion
percentage (89, 95, and 93 % with H2O2 and 93, 96, and
94 % with TBHP using 1, 2, or 3, respectively (Table 3,
entries 1–3).
Oxidant effect
The catalytic potential toward oxidation of 1,2-cy-
clooctene with either aqueous H2O2 or TBHP as oxygen
123

Catalytic potentials of homodioxo-bimetallic dihydrazone…
Table 4 Oxidation yield
product of some alkenes
catalyzed by 1, 2, or 3 using
aqueous H2O2
Entry
Alkene
Producta
Conversion product yield /%b,
selectivity /%
Time /h
1
2
3
O
1
2
95 (89)
97 (93)
96 (93)
8
8
O
90 (86)
92 (89)
92 (89)
3
4
85 (91)
79 (42)
87 (90)
81 (44)
89 (92)
83 (45)
8
9
O
O
O
5
6
7
73 (68)
62 (56)
66 (55)
77 (71)
64 (57)
64 (54)
80 (75)
67 (61)
65 (57)
10
10.5
10
O
O
HO
HO
a
Reaction of alkene (1.0 mmol) with aqueous H2O2 (4.16 mmol), catalyzed by complexes 1, 3,
or 5 (0.01 mmol) in 10 cm3 acetonitrile at 70 °C
b
The yield based on GC results and conversion percentage (selectivity percentage)
The catalyst/substrate molar ratios
source catalyzed by complexes 1–3 is presented in
Table 3. The obtained results from Table 3 assumed that
the catalytic processes with TBHP are highly successful
than those with aqueous H2O2.
The effect of the catalyst was discovered by insertion of
different molar ratios of the complex catalysts 1–3 to 1,2-
cyclooctene in the oxidation process (0.01, 0.02, or 0.04:1,
respectively) using aqueous H2O2 in acetonitrile at 70 °C
for 8 h. The results are reported in Table 6 and presented
in Fig. 1a. The increase of the catalyst molar ratio to two
double amounts improved the rate of 1,2-cyclooctene
oxidation affording good yield of conversion after 4–5 h
of the running processes (Table 6, entries 4 and 5; 77, 84,
and 62 % with 0.01, 0.02, and 0.04 mmol of 1, respec-
tively). Similar behavior took place for catalysts 2 and 3.
After 4 h of the 1,2-cyclooctene oxidation with catalyst 2,
the control conversion reached to the highest values using
0.02 and 0.04 mmol afford 90 and 72 %, respectively
(entries 3 and 4). With catalyst 3, the conversion afforded
85 and 75 % of the yield product after 4 and 3 h using
0.02 and 0.04 mmol of 3, respectively (entries 3 and 4).
But by running the time with the two double amounts of
catalysts (0.02 and 0.04 mmol, entries 5 and 6), the con-
trol chemo- and stereoselectivity reduced to afford lower
yield with enhancing of other side unknown products
(controlled by GC). After long time (ca. 10–12 h), the 1,2-
cyclooctene oxidation using 0.02 or 0.04 mmol of catalyst
1, 2, or 3, showed that the control conversion of 1,2-
Aqueous H2O2 as terminal green oxidant is less ef-
fective in alkenes oxidation compared to the organic
peroxides, i.e. TBHP, as reported in the literatures [46–
49]. Particularly, oxidation processes of some aliphatic
and cyclic alkenes catalyzed by either 1, 2, or 3 and
oxidized by aqueous H2O2 afforded high potential and
chemoselectivity compared to those with TBHP
(Tables 3, 4, 5). Oxidation processes with aqueous H2O2
consumed longer time (8–10.5 h) compared to TBHP
(2–3.5 h) in the same conditions (molar ratio of sub-
strate:catalyst = 100:1). The excess required amount of
aqueous H2O2 (4.16 mmol, Table 4) compared to that of
TBHP (2.0 mmol, Table 5) was resulted either from its
decomposition in the presence of the cis-MoO2 complex
[50] or the presence of high amount of water [28]. The
high amount of water in aqueous H2O2 (30 % mol per-
centage of H2O2) may play a major for the catalytic
potentiality, which reduced the reactivity of aqueous
H2O2 as an oxygen source compared to TBHP with low
amount of water. Furthermore, the organic nature of
TBHP in the organic solvent, e.g. acetonitrile, may im-
prove its reactivity compared to aqueous H2O2 [49].
123

M. S. S. Adam
Table 5 Oxidation yield product of some alkenes catalyzed by 1, 2, or 3 with TBHP
Entry
Alkene
Producta
Conversion product yield /%b,
selectivity /%
Time, h
1
2
3
O
1
2
97 (93)
99 (96)
98 (94)
2
2
O
96 (94)
97 (96)
99 (97)
3
4
93 (88)
68 (83)
96 (92)
65 (87)
96 (93)
71 (88)
2
O
O
2.5
O
5
6
7
77 (67)
74 (63)
69 (59)
82 (75)
73 (65)
67 (58)
82 (73)
77 (64)
71 (59)
3
O
2.5
3.5
O
HO
HO
a
Reaction of alkene (1.0 mmol) with TBHP (2 mmol), complexes 1, 3, or 5 (0.01 mmol) in 10 cm3 acetonitrile at 70 °C
The yield based on GC results and conversion percentage (selectivity percentage)
b
Table 6 The time dependence of the product percentage of 1,2-cyclooctene oxidation catalyzed by 1, 2, or 3 with different amounts of oxidant
(molar ratios) aqueous H2O2
Entry
Time/h
Oxidation product of 1,2-epoxycyclooctene yield/% (selectivity)a,b
1
2
3
0.01
0.02
0.04
0.01
0.02
0.04
0.01
0.02
0.04
1
1
18
41
57
68
77
80
85
89
89
89
19.4
21
44
61
70
84
82
79
78
77
74
20.5
27
46
63
62
58
52
32
58
74
81
88
89
92
95
95
95
36
65
86
90
82
80
80
79
78
76
38
67
72
70
61
22
43
69
77
85
88
90
93
93
93
25
50
72
85
26
53
75
72
70
2
2
3
3
4
4
5
5
6
6
89
7
7
55
54
67
65
8
8
47
86
83
9
10
12
10
Slopec
44
50
62
20.8
a
Reaction of 1,2-cyclooctene (1.0 mmol) with aqueous H2O2 (4.16 mmol), complexes 1, 3, or 5 in 10 cm3 acetonitrile at 70 °C to give the
maximum yield of the oxide product
b
The yield based on GC results and selectivity percentage
c
The slope is a deduced value for the first 3 h of the rate of oxidation conversion of 1,2-cyclooctene reaction
cyclooctane reduced to the lowest amounts, especially
with 0.04 mmol of 1, 2, or 3 to afford 44, 50, and 62 % of
the control product, respectively (Table 6, entries 9 and
10). On the other hand, using the low mount of the cat-
alyst (0.01 mmol of 1, 2, or 3) in the 1,2-cyclooctene
oxidation with aqueous H2O2 in acetonitrile after 8 h at
70 °C is highly control chemo- and stereoselective to the
target oxide product (Table 6, entry 8) awarding 89, 95,
and 93 % yield of the oxide product with 1, 2, or 3, re-
spectively, with unremarkable amount of the reactant in
the reaction media, as recorded by GC (ca. 2–5 %), cf.
Table 3.
123

Catalytic potentials of homodioxo-bimetallic dihydrazone…
100
Solvent effect
0.01 mmol (catalyst 1)
0.02 mmol
0.04 mmol
A
The catalytic potential of some transition metal complex
catalysts has been remarkably impacted by the nature of the
solvent on the alkene oxidation which was studied recently
with different oxidants [49, 52–54]. The influence of dif-
ferent solvents, i.e. acetonitrile, methanol, acetone,
tetrahydrofuran (THF), dichloromethane, and chloroform,
on the oxidation of 1,2-cyclooctene catalyzed by com-
plexes 1, 2, or 3 with either aqueous H2O2 or TBHP as the
oxygen donor at 70 °C was investigated.
80
60
70
40
60
B
50
40
30
20
20
The results in Table 7 illustrate the conversion control
and chemoselectivity of 1,2-cyclooctene in the presence of
either aqueous H2O2 or TBHP with a catalytic amount of cis-
MoO2 complexes 1, 2, or 3 (0.01 mmol) in different
solvents. The trend of the observed solvent effect was
ordered as: acetonitrile[ chloroform [dichloromethane[
methanol[ acetone[ THF. The highest conversion per-
centages were obtained in acetonitrile (89, 95, and 93 %
with aqueous H2O2 after 8 h and 93, 96, and 94 % with
TBHP after 2 h, catalyzed by 1, 2, and 3, respectively).
It seems that in aprotic solvents, i.e. acetonitrile, and in
lowest dielectric constants, i.e. chloroform and dichlor-
omethane, a high oxidation conversion is observed using
catalyst complexes 1, 2, or 3. These results were consistent
with those previous results published by Rayati et al. [52]
in the investigated catalytic activities of vanadium(IV)
Schiff base complexes in the 1,2-cyclooctene epoxidation
reaction with TBHP and also by Grivani et al. [51].
In methanol, the conversion control percentages with
either aqueous H2O2 or TBHP were moderate compared to
the results obtained in acetonitrile (Table 7, 49, 39 and
48 % with aqueous H2O2 and 43, 45, and 51 % with
TBHP, catalyzed by 1, 2, and 3, respectively). As seen
from Scheme 2 (II), the presence of unoccupied coordi-
nation sites on the metal center (Mo) of the catalyst is
crucial for its catalytic performance [1]. Therefore, with
aqueous H2O2 or TBHP, methanol is of smaller size
10
0
0
1
2
3
Time /h
0
0
2
4
6
8
10
12
14
Time/h
Fig. 1 a The time dependence of the conversion percentage of 1,2-
cyclooctene catalyzed by in various ratios (0.01, 0.02, or
0.04 mmol) with aqueous H2O2 as a terminal oxidant. b The time
dependence of the conversion percentage of 1,2-cyclooctene cat-
alyzed by 1 in various ratios (0.01, 0.02, or 0.04 mmol) with aqueous
H2O2 as a terminal oxidant for the first 3 h
1
Although the increase of the catalyst complexes of the
2?
MoO2 unit amounts (from 0.01 to 0.02 and 0.04 mmol)
enhanced the reaction rate and the catalysts’ reactivity, it
did not improve the control chemo- and stereoselectivity of
conversion. It increased the further 1,2-cyclooctene
oxidation to other side unknown products (observed by
GC) (Fig. 1a), as observed previously [51, 52]. The slope
of the conversion rate of 1,2-cyclooctene oxidation with
aqueous H2O2 catalyzed by 1 for the first 3 h could be
deduced from Fig. 1b and recorded in Table 6. The ob-
served rate constant values indicate that the rate of
oxidation within highest amount of catalyst complex 1
(0.04 mmol) is faster than that within the lower amounts
(0.01 and 0.02 mmol) of the catalyst. This may be at-
tributed to the catalytic potential of the studied complex
catalysts.
Table 7 Oxidation yield product (1,2-epoxycyclooctane) of 1,2-cyclooctene catalyzed by 1, 2, or 3 with aqueous H2O2 or TBHP in
different solvents
Solventa
Acetonitrile
H2Ob2 TBHPc
Methanol
H2O2
Acetone
H2O2
THF
CH2Cl2
H2O2
CHCl3
H2O2
TBHP
TBHP
H2O2
TBHP
TBHP
TBHP
Oxidation product yield/%
1
2
3
89
95
93
93
96
94
49
39
48
43
45
51
15
18
13
37
45
42
26
17
27
27
22
27
9
9
4
45
46
53
16
13
23
56
57
59
a
The reaction of 1,2-cyclooctene (1.0 mmol) with aqueous H2O2 (4.16 mmol) or with TBHP (2.0 mmol) catalyzed by 1, 3, and 5 (0.01 mmol)
in 10 cm3 solvent at 70 °C
b
The percentage of the oxide product (selectivity) obtained by GC using aqueous H2O2 as terminal oxidant after 8 h
c
The percentage of the oxide product (selectivity) obtained by GC using TBHP as terminal oxidant after 2 h
123

M. S. S. Adam
the organic nature of TBHP may elucidate its high
oxidizing reactivity in chloroform and dichloromethane
catalyzed by 1, 2, or 3 to award good selective conversion
of 1,2-cyclooctene (45, 46, and 53 % in dichloromethane
and 56, 57, and 59 % in chloroform, catalyzed by 1, 2, and
3, respectively).
Scheme 2
O
Mo
O
O
O
N
O
alkene
O
H
R
O
III
O
O
Mo
O
The consumed long time for the 1,2-cyclooctene
oxidation (8 h at 70 °C) with aqueous H2O2 in acetonitrile
within 1, 2, or 3 as a catalyst compared to that with TBHP
(2 h at 70 °C) in the same conditions is probably explained
by the high amount of water in the reaction media with
aqueous H2O2. The high amount of water perhaps deacti-
vates the catalytic potentials of the catalyst complexes by a
competition through coordination to the unsaturated active
site to the central metal ion [55]. Additionally, the organic
nature of TBHP may improve the oxidizing reactivity in
the organic solvent (acetonitrile) compared to aqueous
H2O2. In other words, the need to insure the homogeneous
conditions of the substrates, 1,2-cyclooctene is usually
immiscible within H2O2/H2O in acetonitrile phase [55]
which proceeded long time and at high temperature
(Table 3). The coordinated water molecules as labile li-
gands should also be taken into account, which may also
play a major role for inhibition of the complexes’ catalytic
potentials (Scheme 2, I). cis-MoO2 complexes 1–3 contain
coordinatively unsaturated centers that play a major role of
a catalytic redox site [4]. In general, the proposed
mechanisms for the oxidation of alkenes with cis-MoO2
complexes involve coordination of the oxidant to the metal
center [56, 57] within substitution of the solvent molecule
by the oxidant in the catalytic processes [58]. This could be
explained by the redshift of the characteristic electronic
absorbance band from 381 to 403 nm of complex 2 after
the addition of aqueous H2O2 to the reaction media (Fig. 2)
of the oxidation of 1,2-cyclooctene. The substitution of the
coordinated water molecule by H2O2 molecule in the
oxygenation process caused little shift of the characteristic
absorption band in the catalyst complex 2.
O
O
O
Mo
N
R
O
N
O
H
O
O
R
R = H, t-Bu
O
H
II
IV
O
ROH
O
O
Mo
N
O
O
H2O2 or TBHP
O
R
H
I
First step (R = H)
alkene oxide
with recycling (R = H (H2O2), t-Bu (TBHP))
compared to acetonitrile, and its coordination to the metal
center is expected to be more facile than with acetonitrile.
Accordingly, the lower catalytic activity of the title cata-
lysts 1, 2, and 3 in methanol, compared to that in
acetonitrile, it seems to be due to the greater competition
between the peroxide and the later for occupying the co-
ordination sites to Mo ion (Scheme 2). These results might
be confirmed also by the effect of strong coordinatively
solvents, i.e. acetone and THF. The conversion percentage
of 1,2-cyclooctene using either aqueous H2O2 or TBHP
reduced significantly in THF (26, 17, and 27 % with
aqueous H2O2 and 27, 22, and 27 % with TBHP, catalyzed
by 1, 2, and 3, respectively) (Table 7). Consequently, the
higher chemo- and stereoselective conversion of 1,2-cy-
clooctene in acetonitrile relative to methanol and THF is
due to the high coordination ability of those solvents to the
central metal ion.
The solubility of cis-MoO2 complexes 1, 2, or 3 [28]
may also play an observable role in their catalytic poten-
tials. 1, 2, and 3 are sparingly soluble in dichloromethane
and chloroform in which the catalytic ability of the current
complex catalysts deactivated and exhibited very low yield
of the oxidation process heterogeneously, especially with
aqueous H2O2 (Table 7). In addition, H2O2 has a very low
miscibility in low dielectric constants, i.e. dichloromethane
and chloroform, but is highly miscible in the other polar
solvents with highest dielectric constants, i.e. acetonitrile
and methanol, as observed by Monfared et al. [54]. This
may explain the low chemo- and stereoselective conversion
of 1,2-cyclooctene in dichloromethane and chloroform
with aqueous H2O2 as an oxidant (9, 9, and 4 % in
dichloromethane and 16, 13, and 23 % in chloroform,
catalyzed by 1, 2, and 3, respectively; Table 7). Moreover,
Temperature effect
Attempts were made to investigate the effect of tem-
perature on the rate and control selectivity of the
conversion protocol of 1,2-cyclooctene. The reaction car-
ried out in various temperatures at 40, 50, 60, and 70° C
catalyzed by 1. The results are collected and presented in
Fig. 3. Figure 3 shows the time-dependent curves of the
oxidation reactions employing 1 as catalysts and aqueous
H2O2 as oxygen donor. The observed kinetic profiles pre-
sent that the catalytic activity of 1 is performed with
increase of the reaction temperature. At low temperatures,
40 and 50 °C, the catalytic potential of 1 was very slow and
gave low chemo- and stereoselective conversion after long
123

Catalytic potentials of homodioxo-bimetallic dihydrazone…
This will be discussed in the mechanistic aspects
(Scheme 2, II).
The mechanistic aspects
Various alkenes were subjected to the oxidation protocol
under the influence of 1, 2, and 3 catalysts, to establish the
catalytic applicability of the current complex catalysts.
Tables 4 and 5 summarized the oxidation product yield of
the some different alkenes by chemo- and stereoselectivity
using either aqueous H2O2 or TBHP in acetonitrile cat-
alyzed by 1, 2, or 3 (entries 1–7) at 70 °C.
Inspections of the results in Tables 4 and 5 indicate
several useful features of this catalytic process. The least
reactive aliphatic terminal and cyclic alkenes were oxi-
dized in desired times in good/excellent yields and
excellent selectivities (Table 4, entries 1–3 with aqueous
H2O2 and Table 5, entries 1–3 with TBHP) in which the
chemo- and stereoselectivity of the procedure was notable
and measured by GC. The complete retention of con-
figuration in the oxidation of styrene was obtained in
excellent stereoselectivity (Tables 4 and 5, entry 4),
whereas the stereoselective conversion of the chain
aliphatic alkenes was good/excellent as shown in Tables 4
and 5 (entries 4–6). It took longer time with aqueous H2O2
(ca. 10 h) compared with TBHP (ca. 2.5 h).
Fig. 2 a Electronic spectral scan of complex 2 before and after
addition of aqueous H2O2 to the reaction media in the presence of 1,2-
cyclooctene in acetonitrile at 70 °C. b The repeated electronic
spectral scan of complex 2 with delay time 1 h
100
90
80
70
60
50
40 °C
40
30
20
10
0
50 °C
60 °C
70 °C
Due to the structural features of cis-MoO2 unit with bis-
tridentate Schiff base ligands [4, 13], cis-MoO2 complexes
1, 2, and 3 have an open coordination site for the oxidant
activation occupied by water molecule (Scheme 2, I) which
is useful for catalytic oxidation reactions, replacing of the
coordinated water molecule by the oxidant molecule, i.e.
H2O2 or TBHP (Scheme 2, II). This behavior has been
widely reported [4, 49, 50] and proved particularly by the
electronic spectral scan of 2 in the reaction media before
and after addition of aqueous H2O2 (Fig. 2a). The elec-
tronic spectral scan of 2 in acetonitrile solution before and
after adding aqueous H2O2 at 70 °C indicates that the
catalyst is stable under the catalysis conditions (Fig. 2a).
The above observations are in stark contrast with previous
reports, where 1,2-cyclooctene oxidation by MoVI com-
plexes was described as efficient only for TBHP as an
oxidant in non-aqueous media, whereas the presence of
water in excess or the use of aqueous H2O2 as an oxidant
was reported to give rise to catalyst deactivation [13, 46,
47]. The most probability for the stability of Mo ions in the
reaction media has elucidated by monitoring the repeated
electronic spectral scans of complex in the oxidation pro-
cess (Fig. 2b). The intensity of the characteristic absorption
bands remains approximately unchanged during the reac-
tion time (381 and 403 nm) [13].
0
2
4
6
8
10
12
Time/h
Fig. 3 Kinetic profiles of 1,2-cyclooctene oxidation with aqueous
H2O2 in acetonitrile in presence of 1 at different temperatures for 12 h
time (16 % after 10 h and 62 % after 12 h, respectively).
At high temperatures, 60 and 70 °C, the catalytic process
afforded the highest conversions (87 % after 12 h and
89 % after 8 h, respectively), especially at 70 °C which
gave an optimal chemo- and stereoselective and controlled
yield product of 1,2-epoxycyclooctane-catalyzed complex
1 with aqueous H2O2 with shorter time (8 h).
The solubility of the studied complexes may play a
major role for their catalytic activity. Complex 1 is not
highly active at low temperature probably due to the low
solubility of such dihydrazone complexes in acetonitrile
converting the catalytic process to be heterogeneous [32–
35, 38]. The ability to open a coordination site for the
oxidant molecule, by replacing the coordinative solvent
molecule (water) with the oxidant molecule may also have
responsibility for their catalytic activity, which is not easy
to carry out at low temperatures to form activated species.
The initial decrease in the intensity upon addition of the
oxidant may be attributed to the formation of active species
123

M. S. S. Adam
in the oxidation reaction (Scheme 2, II) [13, 58, 59]. Ac-
cording to the designed tentative mechanism, the second
stage of the process is the interaction between the alkene
molecule and the activated H2O2 or TBHP molecule in the
coordination sphere of the molybdenum complex
(Scheme 2, III) [49, 50]. The Lewis acidity of the Mo
center increases the oxidizing power of the oxo group and
the alkene is subsequently oxygenated by nucleophilic at-
tack on an electrophilic oxygen atom of the coordinated
H2O2 or TBHP (Scheme 2, III) [53]. The reaction
mechanism involves an activation state formed between the
complex, oxidant, and alkene (Scheme 2, III) [51, 52],
affording an activated structural intermediate of the com-
plex, solvent (water from H2O2 or tert-butanol from TBHP)
and alkene oxide (Scheme 2, IV) [4, 13]. In the catalytic
process, in higher conversions, water (from H2O2) or tert-
butanol (t-BuOH, from TBHP) probably competes with
H2O2 or TBHP (Scheme 2, recycling of the catalytic pro-
cess, I) for coordination to the molybdenum center,
forming a less reactive species that leads to the decreasing
reaction rate with either H2O2 or TBHP, respectively [51].
The impact of the electronic and structural features of
the dihydrazone ligands 1–3 on the stability and catalytic
potential of the Mo catalyst was investigated. Different
electronically and structurally Mo complexes were sub-
jected to the oxidation of some various alkenes by either
aqueous H2O2 or TBHP (Tables 4, 5). The order of cat-
alytic activity was found to be 3 [ 2 [ 1 according to
turnover frequency of the catalysts per hour (Scheme 1).
As expected, an electron withdrawing ability of the phenyl
ring of the terephthalo group in 3 increases the effective-
ness of the catalyst, resulting from the elevated Lewis
acidity of the molybdenum center and therefore the acti-
vation of aqueous H2O2 or TBHP (Scheme 2, III) [59]. The
remarkable reductions of the catalytic activity of 1 and 2
compared to 3 may be due to the electron-donating ability
of methylene and ethylene groups in 1 and 2, respectively,
which is resulted from the low Lewis acidity of the
molybdenum center toward aqueous-H2O2 or TBHP.
The performing results for the activity and stability of
these easily prepared current complexes during the oxida-
tion reactions (Scheme 2, I), leading to high/excellent
conversion at reasonably low reaction times with TBHP,
along with excellent chemo- and stereoselectivity conver-
sion are strong advantages of the present cis-MoO2
complexes as oxidation catalysts [4].
solvents were used as received from commercial sup-
pliers (Sigma-Aldrich and Acros). The bis-tridentate
2-(o-hydroxybenzylidene)dihydrazone ligands L1–L3
were obtained from the common condensation of mal-
onyl [32, 33], succinyl [34, 35], and terephthalo [38]
dihydrazide with salicylaldehyde [28] (Supplementary
Material). Complexes 1–6 are characterized by IR, 1H
NMR, 13C NMR, and UV–Vis spectra, thermogravi-
metric analyses (TGA), and elemental analyses (EA)
[26] (Tables 1, 2).
NMR spectra were measured in the Central Lab,
Chemistry Department, Faculty of Science, Sohag
University at 25 °C on a multinuclear FT-NMR spec-
trometer Bruker ARX400 at 400.1 (1H) and 100.6 (13C)
1
MHz. The H and 13C chemical shifts d are given in ppm.
Coupling constants refer to JHH in 1H and JCC in 13C NMR
unless denoted otherwise. Splitting pattern for larger and
smaller coupling constants is given in the same order as the
J values. Mass spectra were recorded in Central Lab, As-
siut University, Jeol JMS600 spectrometer superconducting
magnet with EI?. Elemental analyses were carried out
with a CHNS-932 analyzer from LECO using standard
conditions in Microanalytical Center in Cairo University.
The electronic spectra of the current ligands and complexes
1–6 were measured using 10 mm silica cells in the ther-
mostatted cell holder of a Jasco UV–Vis spectrophotometer
(model V-530). The thermostatted cell holder was supplied
by an ultrathermostat water circulator (HAAKE Model F3-
k). Thermogravimetric analysis (TGA) was carried out in
dynamic nitrogen atmosphere (20 cm3 min-1) with heating
rate 10 °C using Shimadzu TGA-50H thermal analyzer.
Melting points were obtained by a Thermo Scientific 9100
apparatus.
General procedure for the synthesis of complexes 1–6
The metal compound (bis(acetylacetonato)dioxomolybde-
num(VI) or uranyl acetate) dissolved in 10 cm3 methanol
was added vigorously to a methanolic solution (15 cm3) of
L1–L3 (as suspended solution). The reaction mixture was
refluxed under stirring for few hours. The metal complexes
were precipitated after cooling then filtrated, washed many
times with methanol and diethyl ether, and dried in
vacuum. The metal complex was recrystallized in hot so-
lution of methanol–acetonitrile (1:1).
Bis(2-hydroxy-1-benzylidene)malonyldihydrazone
dimolybdenylÁmonohydrate (1)
Experimental
Prepared from 0.10 g L1 (0.28 mmol) and 0.19 g MoO2
(acac)2 (0.58 mmol); reflux was for 2 h. Orange solid;
m.p.: [300 °C; yield 0.12 g (63 %). The NMR results are
in accordance with the reported complex by Zhang et al.
[32].
All reactions were carried out with magnetic stirring and
held at the chosen temperature by immersion in a
thermostated oil bath. H2O2, TBHP, all alkenes, and
123

Catalytic potentials of homodioxo-bimetallic dihydrazone…
Bis(2-hydroxy-1-benzylidene)succinyldihydrazone
dimolybdenylÁmonohydrate (2)
8.14 (d, J = 8.0 Hz, 2H, ArH), 8.51 (d, J = 8.0 Hz, 2H,
ArH), 8.55 (s, 2H), 9.32 (d, J = 4.4 Hz, 2H, CH=N) ppm;
13C NMR (100 MHz, DMSO-d6): d = 115.9 (CH), 116.0
(CH), 119.3 (Cq), 122.8 (CH), 123.0 (CH), 127.5 (CH),
127.7 (CH), 128.6 (CH), 130.1 (Cq), 132.6 (CH), 132.8
(CH), 133.2 (CH), 133.3 (CH), 136.8 (Cq), 139.9 (Cq),
157.8 (CH), 158.6 (CH), 165.4 (Cq), 167.5 (CH), 167.6
(CH), 170.0 (Cq), 170.9 (Cq) ppm.
Prepared from 0.15 g L2 (0.42 mmol) and 0.27 g MoO2(-
acac)2 (0.84 mmol); reflux was for 2 h. Orange solid; m.p.:
[300 °C; yield 0.18 g (66 %). The NMR results are in
accordance with the reported complex by Ahmed et al.
[34].
Bis(2-hydroxy-1-benzylidene)terephthalodihydrazone
dimolybdenylÁmonohydrate (3, [(MoO2)2L3(H2O)2]ÁH2O)
Prepared from 0.15 g L3 (0.372 mmol) and 0.24 g MoO2
(acac)2 (0.745 mmol); reflux was for 2 h. Orange solid;
Catalytic procedure
1,2-Cyclooctene (0.13 cm3, 1.0 mmol) or other alkene
(1.0 mmol) was added to a solution of 1–6 contacted to
air (0.01, 0.02, or 0.04 mmol) in 10 cm3 of acetonitrile
(or other solvents) at 40, 50, 60, or 70 °C in an oil bath
under magnetic stirring. The reaction was initiated by
charging with either 0.5 cm3 aqueous H2O2 (30 %,
4.16 mmol) or 0.2 cm3 TBHP (70 % in water, 2.0 m-
mol) at the typical temperature. The reaction was
monitored by gas chromatographic analyses, using
computerized standard calibration curve. The oxidation
products were identified by comparing their retention
times with those of authentic samples. Control reactions
were carried out by withdrawing samples (ca. 2 cm3) of
the reaction mixture and treatment of the reaction mix-
ture with MnO2 to quench the excess aqueous H2O2 or
TBHP and with anhydrous sodium sulfate, under the
same conditions in the catalytic runs. The resulting
slurry was filtered on Celite, and the filtrate was injected
in the GC. This allowed independent measurements for
each sample. The conversion of 1,2-cyclooctene to 1,2-
epoxycyclooctene was calculated according to comput-
erized standard calibration curves.
1
m.p.: [300 °C; yield 0.10 g (62 %). H NMR (400 MHz,
DMSO-d6): d = 3.89 (s, H2O), 6.97 (d, J = 8.2 Hz, 2H,
ArH), 7.10 (t, J = 7.3 Hz, 2H, ArH), 7.56 (t, J = 7.5 Hz,
2H, ArH), 7.76 (d, J = 7.1 Hz, 2H, ArH), 8.07–8.14 (m,
4H, ArH), 9.00 (s, 2H, CH=N) ppm; 13C NMR (100 MHz,
DMSO-d6): d = 118.0 (CH), 119.5 (Cq), 121.1 (CH), 127.6
(CH), 127.7 (CH), 129.0 (CH), 131.6 (Cq), 132.5 (Cq),
133.6 (Cq), 133.9 (CH), 134.6 (CH), 156.0 (CH), 156.3
(CH), 158.8 (CH), 165.0 (Cq), 167.1 (Cq) ppm.
Bis(2-hydroxy-1-benzylidene)malonyldihydrazone
diuranylÁmonohydrate (4, [(UO2)2 L1(H2O)4]ÁH2O)
Prepared from 0.10 g L1 (0.28 mmol) and 0.24 g UO2
(OAc)2Á2H2O (0.58 mmol); reflux was for 1 h. Brownish
1
red solid; m.p.: [300 °C; yield 0.22 g (80 %). H NMR
(400 MHz, DMSO-d6): d = 3.35 (s, H2O), 3.87 (s, CH2),
6.70 (d, J = 4.4 Hz, 2H, ArH), 6.93 (s, 2H, ArH), 7.46 (d,
J = 4.7 Hz, 2H, ArH), 7.53 (s, 2H, ArH), 9.19 (s, 2H,
CH=N) ppm; 13C NMR (100 MHz, DMSO-d6): d = 40.9
(CH2), 115.7 (CH), 119.2 (Cq), 122.8 (CH), 132.3 (CH),
133.0 (CH), 156.5 (CH), 167.4 (Cq), 174.4 (Cq) ppm.
Bis(2-hydroxy-1-benzylidene)succinyldihydrazone diuranyl
(5, [(UO2)2 L2(H2O)4])
Gas chromatography is computerized Agelient 5890A
1909 J-413: 325 °C equipped with a flame ionization de-
tector and a HP-5 capillary column (phenyl methyl
siloxane 30 m 9 320 lm 9 0.25 lm). All the reactions
were run at least in duplicate.
Prepared from 0.10 g L2 (0.28 mmol) and 0.23 g UO2
(OAc)2Á2H2O (0.56 mmol); reflux was for 1 h. Brownish
1
red solid; m.p.: [300 °C; yield 0.19 g (71 %). H NMR
(400 MHz, DMSO-d6): d = 3.15 (s, 4H, CH2), 3.37 (s,
H2O), 6.68 (t, J = 7.3 Hz, 2H, ArH), 6.94 (d, J = 8.1 Hz,
2H, ArH), 7.46 (t, J = 7.7 Hz, 2H, ArH), 7.51 (d,
J = 7.3 Hz, 2H, ArH), 9.14 (s, 2H, CH=N) ppm; 13C
NMR (100 MHz, DMSO-d6): d = 31.2 (CH2), 115.8 (CH),
119.1 (CH), 122.9 (Cq), 132.3 (CH), 132.9 (CH), 156.4
(CH), 167.3 (Cq), 177.1 (Cq) ppm.
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