Dioxomolybdenum(VI) complexes of hydrazone phenolate ligands - syntheses and activities in catalytic oxidation reactions

Article abstract of DOI:10.1016/j.jics.2021.100006

The new cis-dioxomolybdenum (VI) complexes [MoO₂(L²)(H₂O)] (2) and [MoO₂(L³)(H₂O)] (3) containing the tridentate hydrazone-based ligands (H₂L² = N'-(3,5-di-tert-butyl-2-hydroxybenzylidene)-4-methylbenzohydrazide and H₂L³ = N'-(2-hydroxybenzylidene)-2-(hydroxyimino)propanehydrazide) have been synthesized and characterized via IR, ¹H and ¹³C NMR spectroscopy, mass spectrometry, and single crystal X-ray diffraction analysis. The catalytic activities of complexes 2 and 3, and the analogous known complex [MoO₂(L¹)(H₂O)] (1) (H₂L¹ = N'-(2-hydroxybenzylidene)-4-methylbenzohydrazide) have been evaluated for various oxidation reactions, viz. oxygen atom transfer from dimethyl sulfoxide to triphenylphosphine, sulfoxidation of methyl-p-tolylsulfide or epoxidation of different alkenes using tert-butyl hydroperoxide as terminal oxidant. The catalytic activities were found to be comparable for all three complexes, but complexes 1 and 3 showed better catalytic performances than complex 2, which contains a more sterically demanding ligand than the other two complexes.

Full text of DOI:10.1016/j.jics.2021.100006

Journal of the Indian Chemical Society 98 (2021) 100006
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Dioxomolybdenum(VI) complexes of hydrazone phenolate ligands -
syntheses and activities in catalytic oxidation reactions
,
b
c
d
e
Md Kamal Hossain a , Maxym O. Plutenko , Jorg A. Schachner , Matti Haukka ,
€
d
c
a,*
€
Nadia C. Mosch-Zanetti , Igor O. Fritsky , Ebbe Nordlander
a Chemical Physics, Department of Chemistry, Lund University, P.O. Box 124, SE-22100, Lund, Sweden
b Department of Chemistry, Jahangirnagar University, Savar, Dhaka, 1342, Bangladesh
c Department of Chemistry, National Taras Shevchenko University, Volodymyrska Street 64, 01601, Kyiv, Ukraine
d Institute of Chemistry, University of Graz, Schubertstraße 1, 8010, Graz, Austria
e
€
€
€
€
Department of Chemistry, P.O. Box 35, University of Jyvaskyla, FI-40014, Jyvaskyla, Finland
A R T I C L E I N F O
A B S T R A C T
The new cis-dioxomolybdenum (VI) complexes [MoO2(L2)(H2O)] (2) and [MoO2(L3)(H2O)] (3) containing the
tridentate hydrazone-based ligands (H2L2 ¼ N'-(3,5-di-tert-butyl-2-hydroxybenzylidene)-4-methylbenzohydrazide
and H2L3 ¼ N'-(2-hydroxybenzylidene)-2-(hydroxyimino)propanehydrazide) have been synthesized and charac-
terized via IR, 1H and 13C NMR spectroscopy, mass spectrometry, and single crystal X-ray diffraction analysis. The
catalytic activities of complexes 2 and 3, and the analogous known complex [MoO2(L1)(H2O)] (1) (H2L1 ¼ N'-(2-
hydroxybenzylidene)-4-methylbenzohydrazide) have been evaluated for various oxidation reactions, viz. oxygen
atom transfer from dimethyl sulfoxide to triphenylphosphine, sulfoxidation of methyl-p-tolylsulfide or epoxida-
tion of different alkenes using tert-butyl hydroperoxide as terminal oxidant. The catalytic activities were found to
be comparable for all three complexes, but complexes 1 and 3 showed better catalytic performances than complex
2, which contains a more sterically demanding ligand than the other two complexes.
Keywords:
Dioxomolybdenum(VI) complexes
Hydrazone
Schiff base
Oxidation
Epoxidation
Sulfoxidation
1. Introduction
of ligands consists of tri- and tetradentate phenol-containing N,O-donor
ligands. Several molybdenum (VI) dioxo complexes with tridentate Schiff
Oxygen atom transfer (OAT) processes attract considerable interest
because of their importance in nature and their potential applications in
industrial oxidation reactions, e.g. sulfoxidation processes [1–3]. In
biological systems a large number of such reactions are catalysed by
molybdopterin-dependent enzymes with active sites containing a mo-
lybdenum cofactor (Moco) [4–7]. In order to emulate and understand
such biological reactions, a plethora of molybdenum (VI) and molybde-
num (IV) complexes have been developed as structural and functional
model complexes for molybdenum-containing cofactors [8–11]. Most
such modelling studies involve the use of sulfur-containing ligands, as the
various mononuclear molybdenum cofactors always contain the ligand
molybdopterin, where a dithiolene unit of the pterin ligand chelates the
molybdenum ion [12–15].
base ligands, including salan ligands, catalyse oxo transfer between
DMSO and different phosphines as well as the oxidation of benzoin to
benzil [21–24].
Hydrazone-based ligands have attracted attention because of their
versatile coordination properties and abilities to coordinate in both
deprotonated and non-deprotonated forms [25,26] and their potential
biological activities [27,28]. The results obtained with phenol-containing
thiosemicarbazone ligands are particularly relevant [29]. A number of
hydrazone complexes containing the cis-Mo(VI)O2 entity have been
prepared, and in some cases they have been investigated as catalysts for
oxygen atom transfer processes [30–32].
Here we describe the synthesis and characterization of octahedral cis-
Mo(VI)O2 complexes based on three tridentate phenol-containing
hydrazone ligands: N'-(2-hydroxybenzylidene)-4-methylbenzohydrazide
(H2L1), N'-(3,5-di-tert-butyl-2-hydroxybenzylidene)-4-methylbenzohy-
drazide (H2L2) and N'-(2-hydroxybenzylidene)-2-(hydroxyimino)pro-
panehydrazide (H2L3) (Fig. 1 and Scheme 1). The three ligands include a
A number of molybdenum complexes that are based on sulfur-free
ligands and reveal OAT/oxidation activities have been investigated
[16–21]. Such ligands include, for example, tris(pyrazol)borates [16,17],
β-ketiminates [18,19] and the salan-type ligands [20,21]. The latter class
* Corresponding author.
E-mail address: Ebbe.Nordlander@chemphys.lu.se (E. Nordlander).
https://doi.org/10.1016/j.jics.2021.100006
Received 27 November 2020; Accepted 8 January 2021
0019-4522/© 2021 Indian Chemical Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/
licenses/by-nc-nd/4.0/).

M.K. Hossain et al.
Journal of the Indian Chemical Society 98 (2021) 100006
amide (RCONH) and aromatic OH groups, respectively. The disappear-
ance of both peaks in the 1H NMR spectrum of 3 indicates that both
groups are deprotonated and coordinated to the molybdenum ion while
the oxime NOH proton resonance is retained in the complex and is shifted
to 12 ppm, which suggests that the oxime group remains protonated in
the complex. In the 13C NMR spectra of both 2 and 3, the chemical shifts
of the carbonyl, phenolate and methine carbon atoms are shifted to lower
field in agreement with coordination of the ligands in the described
manner.
The infrared spectra of complexes 2 and 3 showed shifts to lower
Fig. 1. Hydrazone ligands employed in this study.
–
wave numbers for the ν(C
stretch of the amide group (1613 and
–
O)
1604 cmꢀ1 for 2 and 3, respectively) in relation to the spectra of the
corresponding free ligands, in agreement with coordination of the amide
oxygen to the metal ion. The IR spectra also showed two strong bands
around 916–920 and 944-954 cmꢀ1 that are assigned as asymmetric and
symmetric M ¼ O stretches, respectively.
potential O,N,O0-donor set formed by the amide and phenol oxygens, and
the azomethine nitrogen atoms. The ligand H2L2 contains sterically bulky
tert-butyl substituents in the aromatic ring (Fig. 1), while the ligand H2L3
is electron-depleted in comparison with H2L1 and H2L2 because of the
presence of the oxime electron acceptor group. The potential of the
molybdenum (VI) oxo complexes to effect catalytic oxygen atom transfer
from dimethyl sulfoxide (DMSO) to a phosphine, sulfoxidation of methyl-
p-tolylsulfide, and olefin epoxidation reactions are reported. The Mo(VI)
O2 complex of H2L1 has been previously reported by Lei et al. [25] but its
catalytic activities have not been studied.
Solutions of complexes 2 in acetonitrile and 3 in methanol were
studied by ESI mass spectrometry. The mass spectrum of 2 exhibits a
molecular ion [MoO2(L2) (H2O)þMeCN]þ peak at m/z ¼ 551 and a
ligand molecular ion [H2L2þNa]þ at m/z ¼ 389. For complex 3, a pattern
corresponding to mononuclear [MoO2(L3)]þH]þ (m/z ¼ 349.97) was
found. Furthermore, the formation of the associates of the mononuclear
species with alkaline metal ions {[MoO2(L3)]þNaþ}þ (m/z ¼ 371.95)
and {[MoO2(L3)]þKþ}þ (m/z ¼ 387.93) were also detected as predomi-
nant species. In addition, the ESI-MS spectrum of 3 revealed the presence
of a pattern corresponding to the dinuclear species [2{MoO2(L3)}þNa]þ
(m/z ¼ 716; Fig. S1, Supplementary Material).
2. Results and discussion
All three complexes were synthesized following the procedure pub-
lished by Lei et al. for the synthesis of [MoO2(L1)(MeOH)] (1) [33]. The
reaction of equimolar amounts of [MoO2(acac)2] with H2L2 or H2L3 in
(wet) methanol or methanol-water solutions afforded [MoO2(L2)(H2O)]
(2) and [MoO2(L3)(H2O)] (3) (Scheme 1). Crystallization from the re-
action mixtures at room temperature by slow evaporation of the solvent
led to the formation of orange crystals in 69–85% yields. In these com-
pounds, methanol (1) or water (2, 3) is coordinated in the vacant coor-
dination site. It should be noted that the methanol solvate analogue of 2,
[MoO2(L2)(MeOH)], has been published recently [34].
3. Crystal and molecular structures of 2 and 3
The solid-state structures of [MoO2(L2)(H2O)] (2) and
[MoO2(L3)(H2O)]⋅2H2O (3⋅2H2O) were determined by single crystal X-
ray diffraction. Relevant crystallographic data for 2 and 3 are summa-
rized in Table 1 and selected bond lengths and angles are given in
Table 2. The molecular structures of 2 and 3 are shown in Fig. 2. The
complexes are chiral with stereogenic centers at the molybdenum ions,
but both enantiomers are present in the centrosymmetric monoclinic and
triclinic structures.
The solid state structures confirm that the fully deprotonated hydra-
zone ligands coordinate by their phenolate O, azomethine N and enolic O
atoms (O(1), N(2), O(2)) in a planar tridentate O,N,О0 fashion, imposing
what is effectively a mer coordination geometry with the remaining three
positions of the distorted octahedron occupied by the two oxido ligands
and a coordinated water molecule. The Mo–O bond lengths fall in the
range 1.6972(8)-1.7189(15) Å, which are typical for Mo(VI)¼O double
bonds, and the O(3)-Mo(1)-O(4) angles are also typical for cis-Mo(VI)O2
entities (105.71 (7)ꢁ and 105.42 (4)ꢁ for 2 and 3, respectively). The O(3)
oxido ligand is located trans to the azomethine nitrogen, N(2), with
The complexes were characterized by IR, 1H and 13C{1H} NMR
spectroscopy, ESI mass spectrometry and single crystal X-ray diffraction
analysis of 2 and 3. The complexes are stable at room temperature; they
are well soluble in DMSO and DMF, and moderately soluble in methanol,
chloroform and acetonitrile. The discussion below is restricted to the data
for complexes 2 and 3, as all relevant characterization data for 1 -
including its crystal structure - have been published previously [33].
The hydrazone ligands are coordinated in their doubly deprotonated
form to the metal center, i.e. via their phenolate substituent and the ox-
ygen of the deprotonated amide group so that they become tridentate
O,N,O0-donors. This is confirmed by the 1H NMR and 13C NMR spectra of
the free ligands and their corresponding complexes. In the 1H NMR
spectrum of H2L3, the signals at δ 11.39 and 11.62 are attributed to the
Scheme 1. Syntheses of complexes 1–3.
2

M.K. Hossain et al.
Journal of the Indian Chemical Society 98 (2021) 100006
Table 1
Summary of crystallographic data for [MoO2(L2)(H2O)] (2) and
[MoO2(L3)(H2O)]⋅2H2O (3⋅2H2O).
2
3.2H2O
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
C23H30MoN2O5
510.43
123 (2)
1.54184
Monoclinic
P21/n
9.44913 (13)
21.6798 (3)
11.33150 (16)
90
100.6173 (14)
90
2281.57 (5)
4
1.486
C10H15MoN3O8
401.19
170 (2)
0.71073
Triclinic
P-1
7.24920 (15)
9.55895 (17)
10.9038 (2)
94.2393 (16)
108.402 (2)
90.9357 (16)
714.35 (3)
2
b (Å)
c (Å)
Fig. 2. Mercury plots of the molecular structures of (a) [MoO2(L2) (H2O)] (2)
and (b) [MoO2(L3) (H2O)] (3) showing the atom numbering scheme. Hydrogen
atoms (except for the aqua ligands) and solvent molecules have been omitted for
the sake of clarity. Thermal ellipsoids are drawn at the 50% probability level.
α
(ꢁ)
β (ꢁ)
γ (ꢁ)
Volume (Å3)
Z
nitrogen being close to those expected for an octahedron - 92.49 (7)ꢁ and
90.43 (3)ꢁ for 2 and 3, respectively. The Mo-Owater and Mo–N distances
Mo(1)–O(5) (2.3257 (14) Å for 2 and 2.2685 (7) Å for 3) and Mo(1)–N(2)
(2.2442 (16) Å for 2 and 2.2419 (7) Å for 3) are elongated due the trans
influence of the oxido ligands. The five-membered chelate rings Mo(1)-
O(1)-C(1)-N(1)-N(2) are almost planar with N–N, N–C and C–O bond
lengths that are typical for corresponding rings in other structurally
characterized Mo(VI)O2 hydrazone complexes [29,33,35–37].
Density (calculated) (Mg mꢀ3
)
1.865
0.964
μ
(Mo K
α
) (mmꢀ1
)
5.006
F (000)
Crystal size (mm)
θ range (ꢁ)
1056
404
0.20 ꢂ 0.03 x 0.03
4.08 to 77.00
ꢀ9 ꢃ h ꢄ 11
ꢀ27 ꢃ k ꢄ 27
ꢀ13 ꢃ l ꢄ 14
19797
0.38 ꢂ 0.23 x 0.05
3.03 to 38.48
ꢀ12 ꢃ h ꢄ 12
ꢀ16 ꢃ k ꢄ 16
ꢀ19 ꢃ l ꢄ 19
28688
Limiting indices
Reflections collected
Independent reflections (Rint
Completeness to
)
4777 (0.0315)
99.1%
7939 (0.0194)
98.6%
Intermolecular hydrogen bond interactions are found in both crystal
structures. The hydrogen bonding parameters for 2 and 3 are collected in
Table 3. In the crystal structure of 2, there is a strong intermolecular
hydrogen bonding interaction between the coordinated water molecule
and the oxido ligand O(3) with the O(5)-H(5B)⋯O (3)i (i ¼ -xþ1,-yþ1,-
zþ1) distance being 2.765 (2) Å, and between the water molecule and the
nitrogen of the azomethine-hydrazone fragment - O(5)-H(5A)⋅⋅⋅N(1)ii (ii
¼ -xþ2,-yþ1,-zþ1) ¼ 2.825 (2) Å (Fig. 3).
theta ¼ 26.000ꢁ
Absorption correction
Semi-empirical from
equivalents
0.8701 and 0.4305
Full-matrix least-
squares on F2
4777/0/288
Analytical
Max. and min. transmission
Refinement method
0.9500 and 0.7125
Full-matrix least-
squares on F2
7939/0/208
Data/restraints/parameters
Goodness of fit on F2
1.039
1.140
In the crystal packing arrangement of 3, the [MoO2(L3)(H2O)]
molecule is connected with the neighboring identical molecule through
two O(6)–H(6)⋅⋅⋅O(3) hydrogen bonds. In addition, these two molecules
are connected through two co-crystallized water molecules with O(7)-
H(7B)⋯O(2) and O(7)-H(7A)⋅⋅⋅N(3) intermolecular hydrogen bonds.
These pairs are also connected through O(5)-H(5A)⋅⋅⋅N(2) hydrogen
bonds forming a supramolecular 3-dimensional network along the c axis.
The other hydrogen bonds, O(5)-H(5B)⋯O(8) and O(8)–H(8)⋅⋅⋅O(7) of
the coordinated water, connect these chains, as well (Fig. 4).
Final R indices [I > 2
σ
(I)]
R1 ¼ 0.0263,
R1 ¼ 0.0191,
wR2 ¼ 0.0544
R1 ¼ 0.0210,
wR2 ¼ 0.0558
0.625 and ꢀ0.607
wR2 ¼ 0.0697
R1 ¼ 0.0298,
R indices (all data)
wR2 ¼ 0.0719
0.401 and ꢀ0.786
Largest difference in peak and
hole (e Åꢀ3
)
Table 2
Selected bond lengths (Å) and bond angles (ꢁ) for [MoO2(L2)(H2O)] (2) and
[MoO2(L3)(H2O)] (3).
4. Catalysis studies
2
3
4.1. Catalytic oxygen atom transfer
Mo (1)–O (1)
Mo (1)–O (2)
Mo (1)–O (3)
Mo (1)–O (4)
Mo (1)–O (5)
Mo (1)–N (2)
2.0184 (14)
1.9212 (14)
1.7189 (15)
1.6920 (15)
2.3257 (14)
2.2442 (16)
2.0397 (7) (Mo (1)–O (2))
1.9164 (7) (Mo (1)–O (1))
1.7156 (7)
1.6972 (8)
2.2685 (7)
The reactivities of complexes 1–3 in catalytic oxygen atom transfer
reactions were studied by using triphenylphosphine as an oxygen
acceptor substrate. The reactions were performed in deuterated dimethyl
sulfoxide (DMSO‑d6) that functioned as the ultimate oxygen donor
(Scheme 2) and were monitored by 31P NMR spectroscopy.
The studies on complexes 1–3 were carried out with low catalyst
loadings (1 mol %) at 65 ꢁC but the conversions were found to be very
low for all catalysts after 24 h as well as after 48 h. After increasing the
catalyst loadings to 5 mol%, complexes 1–3 gave conversions of approx.
65%, 20% and 85% after 20 h, and 90%, 40% and >99% after 40 h,
respectively. The results are summarized in Table 4.
The catalytic activities for phosphine oxidation are thus clearly
3 > 1>2, with 3 showing good activity and 1 acceptable activity. How-
ever, complex 2 showed poor activity. We attribute the poor catalytic
efficiency of that complex to the steric hindrance of the tert-butyl sub-
stituents of the ligand, in particular the tert-butyl substituent in 6-position
on the salicylaldimine ring.
2.2419 (7)
O (1)-Mo (1)-O (2)
O (3)- Mo (1)–O (4)
O (4)- Mo (1)–O (2)
O (3)- Mo (1)–O (2)
O (4)- Mo (1)–O (1)
O (3)- Mo (1)–O (1)
O (4)- Mo (1)–O (5)
O (3)- Mo (1)–O (5)
O (4)- Mo (1)–N (2)
O (3)- Mo (1)–N (2)
O (2)- Mo (1)–N (2)
O (1)- Mo (1)–N (2)
148.32 (6)
105.71 (7)
99.14 (7)
101.66 (7)
96.41 (7)
100.41 (6)
169.67 (6)
83.61 (6)
92.49 (7)
161.01 (6)
80.25 (6)
71.63 (6)
150.33 (3)
105.42 (4)
98.89 (4)
106.55 (3)
94.69 (4)
94.93 (3)
169.33 (3)
84.88 (3)
90.43 (3)
159.99 (3)
82.37 (3)
71.23 (3)
similar O(3)-Mo(1)-N(2) angles of 161.01 (6)ꢁ for 2 and 159.99 (3)ꢁ for
3. The other oxido group, O (4), sits trans to the coordinated water sol-
vent molecule with O(4)-Mo(1)-O(5) angles of 169.67 (6)ꢁ for 2 and
169.33 (3)ꢁ for 3, with the cis O(4)-Mo(1)-N(2) angles to the azomethine
3

M.K. Hossain et al.
Journal of the Indian Chemical Society 98 (2021) 100006
Table 3
Table 4
Yields (%) of OPPh3 with DMSO to PPh3 after 20/40 ha.
Hydrogen-bond parameters (Å,ꢁ) for the crystal structures of [MoO2(L2) (H2O)]
(2) and [MoO2(L3) (H2O)]⋅2H2O (3⋅2H2O).
DMSO
1
2
3
D-H⋅⋅⋅A
D-H (Å)
H⋅⋅⋅A (Å)
D⋅⋅⋅A (Å)
D-H⋅⋅⋅A (ꢁ)
20 h
40 h
65
90
20
40
85
>99
2
O(5)-H(5B)...O(3)i
O(5)-H(5A) … N(1)ii
3⋅2H2O
0.88
0.88
1.90
1.99
2.765 (2)
2.825 (2)
165.4
157.2
a
Conditions: 5 mol % catalyst at 65 ꢁC.
O(6)–H(6) … O(3)#1iii
O(5)-H(5A) … N(2)iv
O(7)-H(7A) … N(3)iii
0.84
0.87
0.85
2.00
2.05
2.03
2.8126 (10)
2.8825 (10)
2.8675 (11)
161.9
159.8
170.4
reactions were run in CD3CN solutions at 25 ꢁC at 1 mol% catalysts
loadings, and the yields of sulfoxide were monitored via 1H NMR mea-
surements at given intervals with the solvent residual signal as internal
standard. Control experiments in the absence of catalyst gave no reaction.
The catalytic reactions started immediately without any noticeable in-
duction times. No colour change of the solutions were observed over a
period of 24 h, suggesting that the catalysts remained intact. A high
selectivity for sulfoxide formation was detected, with the corresponding
sulfone being formed as a minor product (ca. 2–5%). Although both the
catalysts and sulfoxide product are chiral (vide supra), the potential
enantioselectivites of the reactions were not evaluated as racemates of
the molybdenum catalysts were used at all times. The results are sum-
marized in Table 5.
Symmetry codes: (i) -xþ1, -yþ1, -zþ1; (ii) -xþ2, -yþ1, -zþ1; (iii) -xþ1, -yþ1, -z;
(iv) -xþ1, -yþ1, -zþ1.
All three complexes were found to catalyse sulfide oxidation with
moderate yields. The catalytic efficiencies, as measured in product yields,
are in the order 1 > 3>2. Again, we attribute the relatively low reactivity
of 2 primarily to steric reasons.
4.3. Catalytic epoxidation
Fig. 3. A Mercury plot of the molecular packing of 2 viewed along the c-axis.
Hydrogen bonds are shown as dashed lines.
Complexes 1–3 were tested as catalysts in the epoxidation of five
olefinic substrates, viz. cis-cyclooctene, 1-octene, styrene, limonene and
α
-terpineol (Fig. 5). In the case of the two latter substrates, racemic
mixtures of the R- and S-enantiomers were used.
The catalytic epoxidation reactions were conducted with 1 mol%
catalyst loading in CHCl3 at 50 ꢁC with 3 equiv. of tert-butyl hydroper-
oxide (TBHP) oxidant with respect to alkene substrate. The progress of
the catalytic reactions was monitored by withdrawing aliquots of the
reaction mixtures during the reactions and subjecting them to GC-MS
analysis. The results are shown in Table 6.
Complex 2 showed the overall poorest catalytic activity, only dis-
playing mediocre activity in the epoxidation of cis-cyclooctene. For 1-
octene no conversion of substrate could be observed, whereas for the
substrates styrene, limonene and
α-terpineol no selectivity for the
respective epoxides were observed in experiments with complex 2.
Complexes 1 and 3 showed a slightly better catalytic reaction profile for
the challenging substrates 1-octene and limonene. Similar to 2, both
complexes 1 and 3 did not show any selectivity in the epoxidation of
styrene; only benzaldehyde was observed as the major product, the result
of over-oxidation. In the epoxidation of 1-octene, complexes 1 and 3 gave
very poor activities and selectivities, with 2-octanone being the major
side-product. Interestingly, for limonene rather good activities were
observed, with a high selectivity towards the epoxidation of the endo-
cyclic double bond of limonene. For complex 3, traces of limonene di-
oxide (~3%), i.e. epoxidation of both the endocyclic and exocyclic
double bonds, could be detected. In contrast to limonene, the epoxidation
Fig. 4. A Mercury plot of the molecular packing of 3 viewed along the a-axis.
Hydrogen bonds are shown as dashed lines.
of
quantities of the desired epoxide. It appears that the hydroxy group of
-terpineol interferes with catalytic activity. Attempts to replace TBHP
α-terpineol yields a mix of several oxidation products and only trace
α
with the more eco-friendly oxidant H2O2 did not result in conversion of
any substrate when experiments were carried out under the same con-
ditions as with TBHP. This is in contrast to the observations by Peng who
found the analogue [MoO2(L2) (MeOH)] to be a good epoxidation cata-
lyst for cyclooctene and styrene in an MeOH/CH2Cl2 mixture when H2O2
was used as an oxidant in the presence of base (NaHCO3) [34].
Scheme 2. Dioxidomolybdenum (VI) catalysed OAT reaction from DMSO
to PPh3.
4.2. Catalytic sulfoxidation
The oxidation of methyl-p-tolylsulfide by tert-butylhydroperoxide
using complexes 1–3 as catalysts was investigated (Scheme 3). The
4

M.K. Hossain et al.
Journal of the Indian Chemical Society 98 (2021) 100006
Table 6
Conversion (selectivity) of epoxide for complexes 1–3.
[%]
1
2
3
cyclooctene, S1
1-octene, S2
styrene, S3
83 (77)
10 (79)
unsel.
53 (96)
unsel.
33 (94)
no conv.
unsel.
unsel.
unsel.
>95 (81)[a]
59 (88)
unsel.
58 (96)
unsel.
Scheme 3. Dioxidomolybdenum (VI) catalysed oxidation of methyl-p-
tolylsulfide.
limonene, S4
α
-terpineol, S5
General conditions: 1 mol% catalyst, 0.5 ml CHCl3, 50 ꢁC, 3 equiv. TBHP, con-
version (selectivity) of epoxide after 24 h; [a] max. conversion of epoxide reached
after 4 h.
Table 5
Sulfoxidation of methyl-p-tolylsulfide using TBHP.
1
2
3
T
Yield [%]
75
Yield [%]
55
Yield [%]
67
(dd), triplet (t) and multiplet (m or unresolved), coupling constants are
given in Hz. Samples for infrared spectroscopy were recorded on Bruker
Vertex 70 or PerkinElmer Spectrum BX spectrometers with resonances
given in wave number (cmꢀ1) and intensities (br ¼ broad, vs ¼ very
strong, s ¼ strong, m ¼ medium, w ¼ weak). Mass spectrometry was
performed with a Waters ZQ 4000 or Bruker Apex Ultra FT-ICR spec-
trometer. Results are denoted as cationic mass peaks; unit is the mass/
charge ratio.
TBHP
24 h
Reaction conditions: Yield of the product measured by 1H NMR spectroscopy.
1 mol % of catalyst (1–3), 3.0 equivalents of tert-BuOOH at 25 ꢁC.
5. Summary and conclusions
In summary, two new cis-Mo(VI)O2 complexes, viz. [MoO2(L2)(H2O)]
(2) and [MoO2(L3)(H2O)] (3), where L2 and L3 are tridentate hydrazone-
based O,N,O0-donors, have been synthesized and characterized. These
two complexes, and the known complex [MoO2(L1)(H2O)] (1) have been
investigated as catalysts for oxygen atom transfer and oxidation re-
actions. Complexes 1–3 are active catalysts for oxygen atom transfer from
DMSO to triphenylphosphine and oxidation of methyl-p-tolylsulfide
using by tert-butyl hydroperoxide. Catalytic epoxidation proved to be
more difficult, with relative divergent results and poor to non-existent
conversions for some of the substrates investigated. Complex 3 proved
to be the best overall oxidation catalyst, with complex 2 proving to be
consistently the worst catalyst for the reactions that have been studied.
The reactivity of 3 may be influenced by the presence of the electron-
withdrawing oxime functionality in the hydrazone ligand; this substitu-
ent should render the oxido groups of 3 less basic but it should make a
presumptive Mo(IV) mono-oxo intermediate formed in the oxo atom
transfer reaction more prone to abstract an oxygen atom from the sub-
strate (DMSO). In oxidation reactions employing 3 as a catalyst, the
oxime functionality may assist in guiding tert-butylhydroperoxide to the
complex to form the type of molybdenum peroxido complex that has
been proposed to be the active oxidant in epoxidation catalysis [38–40].
We ascribe the low reactivity of 2 to be due primarily to steric hindrance
imposed by the ligand. Further reactivity studies and computational
modelling will be required to fully assess the catalytic properties of the
complexes.
6.2. Crystallography
The crystals of 2 and 3 were immersed in cryo-oil, mounted in a loop,
and measured at a temperature of 123–170 K. The X-ray diffraction data
were collected on a Rigaku Oxford Diffraction Supernova diffractometer
using Cu Kα (2) or Mo Kα (3) or radiation. The CrysAlisPro [41] software
package was used for cell refinements and data reductions. A multi-scan
(2) or analytical (3) absorption correction (SADABS [42] (2), CrysAlisPro
[41] (3)) was applied to the intensities before structure solution. The
structures were solved by charge flipping method using the SUPERFLIP
[43] software. Structural refinements were carried out using SHELXL
[44] software with SHELXLE [45] graphical user interface. The hydrogen
atoms were positioned geometrically and constrained to ride on their
parent atoms with C–H ¼ 0.95–0.98, O–H ¼ 0.84–0.875 Å and
Uiso ¼ 1.2–1.5 Ueq (parent atom). Crystallographic data for the structures
in this paper have been deposited with the Cambridge Crystallographic
Data Centre as supplementary publication numbers. The following CCDC
deposition numbers were assigned: 2045993 (2), 2045994 (3). These
data can be obtained free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
6.3. Catalytic oxygen atom transfer reaction
The oxygen atom transfer reactions to PPh3 (0.25 mmol) catalysed by
complexes 1–3 (0.0125 mmol) were run in DMSO‑d6 (0.5 mL) at 65 ꢁC.
The reaction progress was monitored by 31P NMR using a 15-min inter-
val. The set of experiments involved PPh3 reveals that a singlet at ꢀ6 ppm
was converted to OPPh3 at approx. 27 ppm.
6. Experimental section
6.1. Materials and physical measurements
Commercial grade chemicals were used without further purification
in the syntheses and HPLC grade solvents were used as purchased. All
syntheses and manipulations were performed under ambient laboratory
atmosphere. The 1H and 13C NMR spectra were recorded using a Varian
Inova 500 MHz spectrometer or Bruker AC-400 400 Mhz spectrometer in
deuterated chloroform or dimethylsulfoxide (CDCl3 or DMSO‑d6) solu-
tions at room temperature and referenced to the residual signal of the
solvent. Peaks are reported as singlet (s), doublet (d), doublet of doublets
6.4. Epoxidation experiments
Epoxidation experiments were conducted using a Heidolph Parallel
Synthesizer. In a typical experiment, 2–3 mg of catalyst (1 mol %) was
dissolved in 0.5 mL of CHCl3 and mixed with 1 equiv. of substrate. Then
mesitylene was added as internal standard, and the reaction mixtures
were heated to 50 ꢁC, whereupon the oxidant (3 equiv.) was added in one
Fig. 5. Alkene substrates used for epoxidation experiments. In the cases of limonene and α-terpineol, racemic mixtures of the R- and S-isomers were used.
5

M.K. Hossain et al.
Journal of the Indian Chemical Society 98 (2021) 100006
portion. Aliquots for GCꢀMS (20
μ
L) were withdrawn with a calibrated
[MoO2(L3)(H2O)]⋅2H2O (3): A mixture of H2L3 (0.221 g, 1.0 mmol)
and [MoO2(acac)2] (0.328 g, 1.0 mmol) in 5 mL of methanol was heated
to 60 ꢁC for 15 min. After cooling to room temperature, a portion of
10 mL of water was added and the resultant solution was stirred for
1 min. A yellow crystalline precipitate of compound 3 formed during
10–15 min. The resultant product was filtered off, washed with acetone
and dried in air. Yield 0.28 g (69%). Yellow single crystals suitable for X-
ray analysis were grown by slow evaporation of a methanol-water (1/1,
w/w) solution. 1H-NМR (400 MHz, DMSO‑d6, 25 ꢁC): 2.10 (3H, s, CH3),
6.89 (1H, d, J ¼ 8.4 Hz), 7.02 (1H, t, J ¼ 7.2 Hz), 7.48 (1H, t, J ¼ 7.2 Hz),
Socorex Acura 825, 10–100 μL variable volume pipet at given time in-
tervals, quenched with MnO2, and diluted with HPLCꢀgrade ethyl ace-
tate. The reaction products were analyzed by GCꢀMS (Agilent
Technologies 7890 GC System), and the epoxide produced from each
reaction mixture was quantified versus mesitylene as the internal
standard.
6.5. Typical procedure for sulfoxidation
7.64 (1H, d, J ¼ 7.2 Hz), 8.78 (1H, s, CH), 12.00 (1H, s, oxime-OH). 13
C
Reactions were carried out at room temperature in deuterated
acetonitrile solutions using 1:3 molar ratios of substrate/tBuOOH (0.2 M:
–
–
–
NMR (126 MHz, DMSO‑d ) δ 167.03 (C O, enolate), 159.52 (C NOH),
–
6
157.05, 147.75, 135.23, 134.47, 121.54, 120.01, 118.61 (Ar–C), 11.29
0.6 M) and 10 μL of 1,2-dichloroethane added as an internal standard in a
(CH3). Selected FT-IR (cmꢀ1) 936s (Mo O), 912s (Mo O), 1604
5 mm NMR tube. The reactions were monitored continuously by 1H NMR
spectroscopy using a 15-min interval for up to 24 h. The relative in-
tensities of substrate and product resonances were estimated on the basis
of the integrated intensities of spectra. Concentrations of the sulfide
methyl singlet at 2.45 ppm and the sulfoxide methyl singlet at 2.71 ppm
were measured with respect to the internal standard, 1,2-dichloroethane
(3.73 ppm).
–
–
–
–
(C¼OAmide), 1020 (N-Ooxime). ESI-MS: m/z ¼ 349.97 ({[MoO2(L3)]þ
Hþ}þ), 371.95 ({[MoO2(L3)]þNaþ}þ), 387.93 ({[MoO2(L3)]þKþ}þ),
716.92 ({2 [MoO2(L3)]þNaþ}þ), 1063.88 ({3 [MoO2(L3)]þNaþ}þ).
Notes
The authors declare no competing financial interest.
Declaration of competing interest
6.6. Preparation of ligands
Ligands H2L1 and H2L2 were prepared according to the procedure of
Lei and Fu [33].
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
6.7. Preparation of H2L3
N'-(2-hydroxybenzylidene)-2-(hydroxyimino)propanehydrazide
Acknowledgements
(H2L3) was synthesized according to a slight modification of a published
procedure [46].
A solution of 2-(hydroxyimino)propanehydrazide
This research has been carried out within the framework of COST
Action CM1003 Biological oxidation reactions - mechanisms and design of
new catalysts. M.K.H. thanks the European Commission for an Erasmus
Mundus predoctoral fellowship. E.N and I.F. thank the Swedish Institute
for a joint collaborative grant from the Visby program.
(1.17 g, 10 mmol), prepared as described elsewhere [47], in methanol
(30 mL) was treated with salicylaldehyde (1.22 g, 10 mmol). The
resulting mixture was heated under reflux for 1.5 h. On cooling to room
temperature, a solid yellowish precipitate was formed. The solid was
filtered off, washed with methanol and dried in air. Some additional
amount of ligand coud be obtained from the filtrate by partial evapora-
tion of the solvent under vacuum and filtration off the formed precipitate.
Yield 2.04 g (92%). Found: C, 54.18; H, 5.10; N, 19.04. Calc. for
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://do
i.org/10.1016/j.jics.2021.100006.
C
10H11N3O3: C, 54.29; H, 5.01; N, 19.00.1H-NМR (400 MHz, DMSO‑d6) δ
11.74 (1H, s, oxime-OH), 11.62 (1H, s, phenol-OH), 11.39 (1H, s, amide,
NH), 8.57 (1H, s, CH), 7.31 (1H, t, J ¼ 8.0 Hz), 7.23 (1H, t, J ¼ 7.4 Hz),
6.85 (2H, t, J ¼ 8.8 Hz), 2.00 (3H, s, CH3). 13C NMR (126 MHz, DMSO‑d6)
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