Ligand coordination sphere effect of Schiff base cis-dioxomolybdenum(VI) complexes in selective catalytic oxidation of alcohols

Article abstract of DOI:10.1002/kin.21530

Several cis-dioxomolybdenum(VI) complexes with Schiff bases-derived ligands were synthesized and fully characterized. The catalytic performances of these complexes were tested in the alcohol oxidation under solvent-free condition using H₂O₂ as oxidant giving high results. The influence of the oxygen, sulfur, and nitrogen atom within the coordination sphere around the molybdenum center was studied (S?>?N?>?O). From this study, we suggest that there exists a relationship between the electronegativity of the atom and the catalytic performance in alcohol oxidation.

Full text of DOI:10.1002/kin.21530

Received: 27 January 2021
DOI: 10.1002/kin.21530
Revised: 24 June 2021
Accepted: 18 July 2021
R E S E A RC H A RT I C L E
Ligand coordination sphere effect of Schiff base
cis-dioxomolybdenum(VI) complexes in selective catalytic
oxidation of alcohols
Haiwen Liu
Tong Li
Zihan Zhuo
Zuodong Mao
Yan Zhang
Weili Wang
Hang Wei
Wenxin Zhang
School of Chemistry and Material Science,
Ludong University, Yantai, China
Abstract
Several cis-dioxomolybdenum(VI) complexes with Schiff bases-derived ligands
were synthesized and fully characterized. The catalytic performances of these
complexes were tested in the alcohol oxidation under solvent-free condition
using H2O2 as oxidant giving high results. The influence of the oxygen, sulfur,
and nitrogen atom within the coordination sphere around the molybdenum cen-
ter was studied (S > N > O). From this study, we suggest that there exists a rela-
tionship between the electronegativity of the atom and the catalytic performance
in alcohol oxidation.
Correspondence
WeiliWang, School ofChemistryand
MaterialScience, LudongUniversity,
186HongqiMiddle Road, Yantai264025,
China.
Email:wangweilishou@163.com
HaiwenLiu, Zihan Zhuo and YanZhang
contributedequally to this work.
Fundinginformation
K E Y WO R D S
Research Foundation forAdvancedTal-
entsofLudong University, Grant/Award
Number: LB2017003; University Stu-
dentInnovation andentrepreneurship
trainingProgram of ShandongProvince,
Grant/AwardNumbers:S201910451123,
S201910451158
alcohol, catalysis, molybdenum complex, oxidation, Schiff base, substituent effect
1
INTRODUCTION
For this reason, in pursue of a green chemistry
approach,13–15 great changes have been introduced to
Aldehydes, ketones, and carboxylic acids are essen-
tial industry products and intermediates.1 Among them,
aldehydes,2–5 carboxylic acids,6–8 amides,9,10 and other
derivatives are commonly found in literature as starting
materials or building blocks in several synthetic method-
ologies. These derivatives are commonly prepared by
oxidation of the corresponding alcohols as reported in
literature11; in this regard, organic solvents and nongreen
oxidants (CrO3, NaClO, KMnO4, etc.) are frequently used12
in the conventional oxidation protocols; however, these
compounds are toxic and nonselective, including the fact
that they are commonly added stoichiometrically which,
together with other disadvantages made them undesired in
the production of fine chemicals, especially when a green
chemistry approach is highly desired.
develop a greener and more environmentally friendly oxi-
dation protocols with the use of oxidants such as TBHP,16,17
H2O2,18 dioxygen,19 and so on. On another hand, sol-
vent free or aqueous solvents are preferred prevailing
against the traditional solvent conditions, resulting in
safer, environmentally friendlier, easy-to-handle, and gen-
erally improved techniques. In this context, the develop-
ment of catalytic systems to fulfill the requirements of
greener protocols is highly desirable and is a part of cur-
rent research in applied catalysis.20
In this regard, Schiff bases have been extensively used
for many applications, for instance, adsorbents for toxic
metal ion removal21–23 and ligands to develop metal com-
plexes catalysts.24,25 Coordinated with different metals,
such as Mn(III),26 MoO3/Cu,27 Co(II) and Zn(II) Metal
Int J Chem Kinet. 2021;1–9.
wileyonlinelibrary.com/journal/kin
© 2021 Wiley Periodicals LLC
1

2
LIU et al.
S C H E M E 1 Synthesis of
cis-dioxomolybdenum Schiff base complexes
X = O, ONO
X = S, ONS
CHO
OH
EtOH
N
H2N
X = COO, ONOCO
X = N-Ph, ONN
XH
OH
XH
MoO2(acac)2
Reflux
N
Mo
O
EtOH
Heat
O
O
N
Mo
O
X
X
H
O
Mo
O
O
O
O
X
O
N
Organic Frameworks (MOFs),28 Schiff bases complexes
have shown good outstanding activity as catalysts for alco-
hols oxidation.
ever, to the best of our knowledge, there are few reports
concerning the catalytic activity of dioxomolybdenum
(VI) complexes towards alcohol oxidation; hence, the
electronic environment provided by the ligands have not
been evaluated yet. To continue our work on molybdenum
catalysis, herein, we report the study of the influence
of donor atoms in different tridentate ligands over the
catalytic activity of cis-dioxomolybdenum(VI) complexes
([MoO2L]2, Scheme 1) towards alcohol oxidation. The
ligands (H2L, with O, S, N) are recorded for short as follow,
H2SAP for salicylideneaminophenol (ONO), H2SATP
for salicylideneaminothiophenol (ONS), H2SAN for
N-(salicylidene)-2-aminobenzoic acid (ONO-CO), and
H2SADA for N-(salicylidene)-N’-phenyl-o-pheylene-
diamine (ONN).
Recently, several Schiff base molybdenum(VI) com-
plexes have been developed and used as catalysts for
various oxygen atom transfer reactions, such as olefins
epoxidation,29–31 sulfides oxidation,32 and alcohols
oxidation.33 Regarding alcohol oxidations, various
Schiff base molybdenum complexes, such as magnetic
complex,34 Mo−V bimetallic complex,35 and μ-O{MoO2}2
core complex,36 have been evaluated in alcohol oxidation
with TBHP or H2O2 as oxidants yielding the correspond-
ing aldehydes or ketones. Nevertheless, dioxomolybde-
num(VI) complexes37,38 and peroxo-molybdenum(VI)
complexes39,40 have been used for olefin epoxidation
but not been fully exploited as catalysts in alcohols
oxidation.
Solvent-free conditions are one aspect of green chem-
istry, which are reported for oxidation of primary alcohols
to aldehydes.41,42 Our research group have studied
several [MoO2L]n (L = ONO, ONS ligands) type cis-
dioxomolybdenum(VI) Schiff base complexes as catalysts
in solvent-free olefin epoxidation, observing high catalytic
activity.24,43–45 We have to point out that the solvent-free
condition here means organic solvent-free condition with
no further added organic solvent. The introduction of an
electron withdrawing group (─NO2) and/or an electron
donor group (─Net2) substituent into ligand have been
also studied; hence, the electronic density distribution
through the ligand backbone and its influence in the
metallic center and consequently, in the catalytic cycle
have been evaluated.43 The coordination spheres (atom
substitution) catalytic activity were studied in olefin epox-
idation (S > O).24 DFT theoretical calculations also have
validated the catalytic mechanism providing some insight
about the process, thus showing a trend that the energy
span is in accordance with the experimental results. How-
2
EXPERIMENTAL
2.1
Materials and methods
All reactions were carried out in air. Water was dis-
tilled before use. Organic solvents (ethanol, methanol,
diethylether. and acetrontrile) and organic compounds
(H2O2 30% in water), benzylic alcohol (99%), benzaldehyde
(99%), salicylaldehyde (99%), o-anilines (2-aminophenol
(99%), 2-aminothiophenol (98%), anthranilic acid (99%),
and N-phenyl-o-pheylenediamine (99%)) were used from
commercial without any further purification. The thermo-
gravimetric analyses were performed on a thermal ana-
lyzer. The sample was placed into a nickel/platinum alloy
crucible and heated at 50◦C min–1 in a reconstituted air
flow from 15◦C to 700◦C. An empty crucible was used as a
reference. HRMS were performed on Thermo Scientific Q
Exactive. Infrared spectra were recorded at room temper-
ature with Mattson Genesis II FTIR or a Bruker spectrom-
eter. NMR spectra were recorded on Bruker spectrometer.

LIU et al.
3
Catalytic reactions were followed by gas chromatography
on a Gas Chromatograph equipped with FID detector
with a HF-5 column (30 m × 0.32 mm × 0.5 μm). The
GC parameters were quantified with authentic samples
of the reactants and products. The conversion of alcohols
and the formation of aldehydes were calculated from
calibration curves (r2 = 0.9995) relatively to an internal
standard.
2.3.1
[MoO2(ONN)(EtOH)]
Red powder, yield: 87%.
1H NMR (400 MHz, DMSO-d6, (ppm)): δ = 9.21 (s, 1H,
CH=N), 7.78 (dd, 2H, J = 8.5, 3.1 Hz, Ar─H), 7.52 (s,
1H, Ar─H), 7.38 (t, 2H, J = 7.6 Hz, Ar─H), 7.24–7.11 (m,
3H, Ar─H), 7.01 (q, 2H, J = 7.9 Hz, Ar─H), 6.88 (d, 1H,
J = 8.3 Hz, Ar─H), 6.80 (t, 1H, J = 7.7 Hz, Ar─H), 6.16 (d,
1H, J = 8.2 Hz, Ar─H), 4.34 (t, 1H, J = 5.1 Hz, ─OH), 3.51–
3.40 (m, 2H, ─CH2), 1.07 (t, 3H, J = 6.9 Hz, ─CH3).
13C NMR (126 MHz, DMSO) δ 162.86, 157.55, 155.94,
152.55, 136.16, 135.90, 135.71, 129.68, 129.40, 126.68, 125.22,
122.66, 120.28, 119.92, 119.37, 116.71, 115.20, 56.50 (─CH2─),
19.04 (─CH3).
2.2
General procedure for synthesis of
the ligands
In a 100 ml Erlenmeyer flask, salicylaldehyde (0.2442 g,
2 mmol) was dissolved in 2 ml of ethanol and the corre-
sponding o-aniline (2 mmol) was added. The mixture was
refluxed under magnetic stirring for 4 h. The resulting pre-
cipitate was filtered and washed with ethanol to obtain col-
ored powders.
2.3.2
[MoO2(ONN)]2
Black powder, IR (ATR, cm–1) 1611(C=N), 947 (Mo=O), 810
(Mo─O─Mo).
1H NMR (500 MHz, DMSO-d6, (ppm)): δ = 9.20 (s, 1H,
2.2.1
H2SADA (ONN)
CH=N), 7.80–7.73 (m, 2H, Ar─H), 7.50 (s, 1H, Ar─H), 7.37
(t, J = 7.6 Hz, 2H, Ar─H), 7.19 (d, J = 7.8 Hz, 2H, Ar─H),
7.14 (t, J = 7.4 Hz, 1H, Ar─H), 7.00 (dt, J = 10.3, 7.6 Hz, 2H,
Ar─H), 6.87 (d, J = 8.3 Hz, 1H, Ar─H), 6.78 (t, J = 7.6 Hz,
1H, Ar─H), 6.15 (d, J = 8.2 Hz, 1H, Ar─H).
Yellow needles, yield: 94%. IR (ATR, cm–1) 3402 (NH), 1608
(C═N).
1H NMR (500 MHz, DMSO-d6, (ppm)): δ = 12.77 (s, 1H,
OH), 8.88 (s, 1H, CH═N), 7.69 (s, 1H, NH-Ph2), 7.68 (d,
1H, Ar─H), 7.40 (td, J = 8.4, 7.9, 1.8 Hz, 1H, Ar─H), 7.32
(td, J = 7.4, 6.9, 1.4 Hz, 2H, Ar─H), 7.20 (t, J = 7.7 Hz,
3H, Ar─H), 7.02 (dd, J = 13.6, 7.5 Hz, 3H, Ar─H), 6.96 (t,
J = 7.5 Hz, 1H, Ar─H), 6.91 (d, J = 8.2 Hz, 1H, Ar─H), 6.81
(t, J = 7.3 Hz, 1H, Ar─H).
13C NMR (126 MHz, DMSO) δ = 162.86, 157.56, 155.94,
152.56, 136.16, 135.90, 135.72, 129.69, 129.40, 126.68, 125.22,
122.66, 120.28, 119.92, 119.38, 116.72, 115.20.
HRMS (ESI) calcd. for C19H15N2O3Mo, [M++1] 417.0137,
found 417.0132 [M++1].
TGA as [MoO2(ONN)]2: 34.7% MoO3 residue (requires
34.6%). Anal. Calc. For C19H14MoN2O3 (Mr = 414.27): C,
55.09; H, 3.41; N, 6.76. Found: C, 55.11; H, 3.42; N, 6.75.
13C NMR (126 MHz, DMSO-d6) δ 162.98, 160.56, 144.39,
140.19, 137.66, 133.47, 132.76, 129.50, 127.91, 122.19, 120.42,
120.15, 119.97, 119.43, 119.09, 117.51, 116.97.
HRMS (ESI) calcd. for C19H16N2O, [M-1] 287.1184, found
287.1189 [M-1]
2.4
Catalytic oxidation of alcohols with
H2O2 under solvent-free condition
2.3
General procedure for synthesis of
The oxidation of alcohols catalyzed by [MoO2(L)]2 was car-
ried according to the below general procedures. The opti-
mization experiment was performed as follow: the selected
alcohol (10 mmol), complex (0.025 mmol), acetophenone
(0.1201 g, 1 mmol, internal standard), and H2O2 30% aque-
ous (2.1 ml, 20 mmol) were added in a 50 ml two necked
flask. The reaction mixture was stirred at 70◦C for 4 h. The
reaction was monitored by GC-FID, the column temper-
ature was initially held at 80◦C for 2 min, and then the
temperature was raised to 220◦C at a rate of 10◦C min–1,
from 220 to 280◦C at a rate of 20◦C per minute and finally
held for 15 min. The total run time was 34 min. Nitrogen gas
was used as the carrier gas. The injector temperature was
molybdenum complexes
In a 100 ml Erlenmeyer flask, the corresponding ligand
(1.2 mmol) was dissolved in 2 ml of ethanol and then
[MoO2(acac)2] (0.3914 g, 1.2 mmol) was added. The mix-
ture was refluxed under magnetic stirring for 4 h lead-
ing to complexes as red powder (orange to red powder or
crystal). The isolated precipitate was dried under reduced
pressure at room temperature for 24 h as monomer
[MoO2(ONN)(EtOH)] (see NMR), and heated at 80◦C to
obtain brown or black dimeric compounds [MoO2(ONN)]2
(proved by IR, NMR, and TGA).

4
LIU et al.
TA B L E 1 Relevant IR, TGA and NMR data for ligand and Mo complexes
Characterizations
Compound
IR (cm–1)
TGAa
1H NMR
C═N
Mo═O
–
Mo─O─Mo
–
Δm Exp
Δm Theo
(CH═N)
H2SAP (ONO)
[MoO2(ONO)]2
[MoO2(ONS)]2
H2SADA (ONN)
[MoO2(ONN)]2
1633
1608
1602
1608
1611
8.99
9.25
9.06
8.89
9.21
937
920
–
790
781
–
58.5
58.8
57.6
59.5
947
810
34.7
34.6
aΔm is the mass losses of the molybdenum complexes by TG analyses.
maintained at 300◦C and the injection volume was 0.5 μl
in the splitless mode with acetophenone as an internal
standard.
[Mo]
OH
O
H O , 70 °C
2
2
S C H E M E 2 Oxidation of benzyl alcohol
3
RESULTS AND DISCUSSIONS
Characterizations
3.2
Oxidation of alcohols catalyzed by
3.1
cis-dioxomolybdenum(VI) complexes
The H2SAP,24 H2SATP,24 and H2SAN46 compounds were
synthesized according to the literature. [MoO2(acac)2] was
synthesized as previously described47 and used freshly pre-
pared. Complex [MoO2(SAP)]2 was prepared as previous
described.24 The complexes were fully characterized by
IR, NMR, TG analysis (See Table 1) or elemental analysis.
For NMR, all the synthesized ligands have typical imine
(CH═N) resonance signals at around 9.0 ppm, which after
coordination with {MoO2}2+ center due to the consequent
deprotection of OH. Both, the imine (CH═N) and the aro-
matic signals in NMR displayed different chemical shifts
due to the different patterns of substitution within the lig-
ands. After coordination, besides the downfield shift of
imine group, it is important to notice the absence of the
two signals of the hydroxyl groups that are no longer vis-
ible upon coordination. Concerning FT-IR, a characteris-
tic band was assigned to the C═N stretching vibration at
around 1603–1613 cm–1. It is noteworthy to point out two
signals at 920–947, which are typical from the monomer,
whereas only one strong and narrow band around 930–
950 cm–1 is visible for the dimeric structure, plus a complex
pattern around 780–840 cm–1 as a result of the Mo2(μ-O)2
moiety.17 As discussed in the previous articles, the differ-
ence between dimer and polymer is observable in IR by
the value of the Mo─O─Mo vibration. When it is a dimer,
this band is narrow and around 800. In case of polymer,
the value is lower and the signal broader. For instance,
the complex [MoO2(ONN)(EtOH)] yields the correspond-
ing monomer from ethanol solution (see NMR), while,
at higher temperature, the decoordination of the solvent
yields the dimeric structure as shown by FT-IR, which is
consistent with literature.
The
catalytic
ability
of
the
prepared
cis-
dioxomolybdenum(VI) complexes was tested in alcohols
oxidation. Benzyl alcohol was taken as a model substrate
(Scheme 2), and using 30% H2O2 aqueous as oxidant for the
reactions with no additional organic solvent seems better
than TBHP. Though TBHP has similar activity with H2O2,
with water as the byproduct, H2O2 is cleaner than the other
one. In this condition, the molecular oxygen as oxidant at
atmosphere does not work for this oxidation reaction.
Blank experiments were carried out and gave the
expected results. Without catalyst, none of aldehydes or
acids were observed, showing the catalytic effectiveness
of the complexes. Once the catalyst is added, the ben-
zaldehyde yield enhances continuously. As 0.25 mol% of
the catalyst is added and the reaction is carried out for
4 h, the benzaldehyde yield can reach up to about 68%
(TON = 57) with high selectivity (99%; Table 3), and the
benzaldehyde was the sole product as showed by GC anal-
ysis. The addition of too much catalyst no longer effects on
the reaction after the amount of catalyst is increased more
than 0.25 mol%. The catalytic oxidation produces a trace
amount of benzoic acid only after 24 h of reaction. In addi-
tion, at room temperature, no benzoic acid was observed,
indicating that the catalytic oxidation of alcohol strongly
depends on the reaction temperature (Table 2). At low tem-
perature, the reaction occurs slowly, while it turns to faster
and reaches up to 69% in 4 h. For the organic solvent, the
catalytic activity is lower than none organic media. There-
fore, organic condition is used. Therefore, the optimized
catalytic experiments were performed using two equiva-
lents of H2O2 per benzyl alcohol with a 0.25% Mo/alcohol
ratio at 70◦C.

LIU et al.
5
TA B L E 2 Solvent-free oxidation of benzyl alcohol with [MoO2(ONO)]2 as catalysta
Entry
1
2
3
4
5
6
7
T (◦C)
25
40
50
60
70
70
70
70
70
70
Cat. (%)
0.5
0.5
0.5
0.5
0.5
0
0.05
0.1
0.25
1
0.25
0.25
0.25
0.25
0.25
Oxidant
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
TBHP
H2O2
H2O2
H2O2
O2
Solvent
Conversion (%)
–
–
–
–
–
trace
5.3
11.5
43
69
Trace
42
55
68
71
72
–
–
–
–
8
9
10
11
12
13
14
15
70
70
70
70
–
CH3CN
CHCl3
Toluene
–
62
33
35
0
70
aConditions: Mo/benzyl alcohol/oxidant = X/100/200; t = 4 h.
TA B L E 3 Solvent-free oxid◦ation of benzyl alcohol with different molybdenum complexesa
Catalyst
T ( C) %Cat. Oxidant Sol.
Time % C onv. % Select.b TON TOFc (h–1) References
[MoO2(ONO)]2
[MoO2(ONS)]2
[MoO2(ONOCO)]2
[MoO2(ONN)]2
SB-Mn (III)
70
70
70
70
rt
0.25
0.25
0.25
0.25
10
–
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
TBHP
O2
–
–
–
–
–
–
–
4 h
4 h
4 h
4 h
5 h
5 h
68
94
85
99
98
99
98
57
271
92
108
229
95
This work
This work
This work
This work
26
27
28
33
34
37
48
92
92
225
164
MoO3/Cu nanoparticles 80
100
89
63
85
40.2
92
100
87
–
–
–
–
–
3D Zn/Co-MOF
CPS-[MoO2(ALGL)2]
Fe2O3-SB-Mo
90
90
80
90
25
–
–
2
1
–
100
–
148
–
–
Toluene 10 h
H2O2
H2O2
TBHP
EtOH
–
3 h
24 h
2 h
Polynuclear SB-Mo
Bidentate SB-Mo
–
–
2
98
45
22.5
aSB = Schiff base.
bAldehyde selectivity.
cTOF is calculated on the time interval with maximum slope for the conversion plot.
The kinetic studies for all complexes were showed in
Figure 1. All our complexes in the alcohol oxidation share
the same trend, such as, they have a quick reaction speed
at the beginning, then it turns to slow and slow with the
consumption of starting materials. All the conversions are
more than 68% at 70◦C, suggesting the dependence upon
the temperature (Table 3). Good to excellent catalytic activ-
ities were observed compared with the literature, but with
lower temperature and catalyst loading, which showing
the importance of this process.
influences on their catalytic ability. It is generally accepted
that the Lewis acidity of dioxidomolybdenum(VI) com-
plexes are one of the most important characteristics that
determine catalytic performance in oxidation. As shown
in Table 2, the influence of the coordination spheres
(O, S, N) on the coordinated cites of tridentate ligand
is observed. Among these complexes, [MoO2(ONO)]2 is
already active (conversion: 68%, selectivity: 99%), but is the
lowest one. The substitution of O with S or N brings better
catalytic conversion, shows higher catalytic activity than
the complex [MoO2(ONO)]2 (the conversions > 85%, up to
94% for [MoO2(ONS)]2). This may be due to the fact that
the O is more electronegative and then reduce the Lewis
It is known to all, the modification of the ligand
framework affects the electrochemical performances of
the molybdenum complexes.49 This will certainly have

6
LIU et al.
F I G U R E 1 Kenitic study of conversion
in alcohol oxidation with
dioxomolybdenum(VI) complexes
([MoO2(ONO)]2 (◆),[MoO2(ONS)]2 (■),
[MoO2(ONOCO)]2 (×), and [MoO2(ONN)]2
(▲))
acidity of the metal center that makes the metal center
less active to contact with oxidant for catalysis. That is to
form a shielding effect on the metal center. For S or N,
electronegative is weaker than the O, allows the metal
to be easier contacted with oxidant, hence the activity
is stronger (Figure 1). The catalytic conversions are in
accordance with the electronegative of the substitute in
the order as follow: (S > N > O).
O
H O
2
O
O
H
O
O
Mo
O
H
O
N
H
O
O
O
O
H
O
O
H
O
Mo
O
O
Mo
O
O
O
To investigate the scope of this methodology, the oxi-
dation of a variety of primary and secondary alcohols
under the optimized reaction conditions was studied with
[MoO2(ONS)]2 as catalyst (Table 4). Several of substituted
benzyl alcohols with either electron-donating or with-
drawing groups were also converted to their related alde-
hydes in good to excellent yields (entries 1–6) by this cata-
lyst. Furthermore, good yield in our system was observed
for heteroaromatic alcohols, which are difficult to oxi-
dize in most transition-metal catalytic conditions (entries
7 and 8, Table 3). Besides, hindered benzyl alcohol was
also oxidized to aldehydes in high yields (entries 9 and
10). Aliphatic primary alcohols also yield the correspond-
ing aldehydes (entries 11–15), while secondary alcohols
were oxidized to the corresponding ketones in high yields
(entries 9, 10, and 15).
N
N
H
O
Mo
O
H
O
O
O
H O
2
O
H
N
O
H
O
S C H E M E 3 Probable mechanism of catalytic cycle for benzyl
alcohol oxidation catalyzed by dioxomolybdenum(VI) complexes
strate. The first step of the catalytic cycle involves a
H2O2 proton exchange in the presence of one of the cis
Mo═O and the HOO─ coordinated to the Lewis-acidic
metal center, giving the peroxo intermediate. Then, the
intermediate oxidizes alcohol and loses one molecule
H2O, giving an intermediate that releases aldehyde and
H2O and finally regenerates the dioxomolybdenum(VI)
complex.
Concerning a plausible mechanism (See Scheme 3), it is
known that the dimeric complexes of the type [MoO2L]2
dissociate to give monomers while in solution, particu-
larly in the presence of σ donor solvents45 in this case,
the hydrogen peroxide is a σ donor capable of disrupt the
dimeric structure therefore activating the oxygen atom.
The mechanism for oxidation of alcohols by oxidometal
complexes in the presence of H2O2 is not clear yet but
based on both, existing literature48,50 and previous results
of our group, we propose that the alcohol oxidation starts
with the formation of peroxido-molybdenum Schiff base
intermediate as the active species that oxidize the sub-
4
CONCLUSIONS
Few molybdenum Schiff base complexes were synthesized
from salicylaldehyde and o-substituted anilines, thus vary-
ing the coordination sphere of the metallic center (O, S, N).
These molybdenum Schiff base complexes were used as

LIU et al.
7
TA B L E 4 Oxidation of alcohols with H2O2 catalyzed by the [MoO2(ONS)]2 complex under solvent-free conditionsa
Entry
Substrate
Productb
Conv. (%)
Selectivityc (%)
TON
1
94
98
314
2
3
4
5
90
92
98
97
95
95
95
96
302
306
334
326
6
7
83
96
94
93
248
317
8
9
90
97
91
88
292
332
10
87
89
326
11
85
83
93
87
288
161
OH
O
12
OH
O
O
13
14
90, 90.6d
87
97
90
197
187
OH
O
OH
O
OH
15
92
89
212
aConditions: Mo/alcohol/H2O2 (aq.) = 0.25/100/200; T = 70◦C; t = 4 h.
bThe products were identified by comparison of physical and spectroscopic properties with authentic compounds.
cAldehyde selectivity.
dCalculated by NMR.
catalysts for alcohol oxidation, the reaction was performed
in aqueous H2O2 solution without other organic solvent,
the molybdenum complexes [MoO2L]2 have good catalytic
abilities to perform the desired reaction. Experiments have
shown that the molybdenum Schiff base complexes have
good conversion and selectivity for the oxidation of
alcohol. The ligand coordinate sphere surrounded the
molybdenum (atom substitution) effects on this catalysis
reaction were studied. It shows that substitutions of
coordinate spheres (O, S, N) from ligands have slight
differences in catalytic activities due to their electrochem-
ical performances derived from different electronegative.

8
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AC K N OW L E D G M E N T S
This work was financially supported by the Research Foun-
dation for Advanced Talents of Ludong University (No.
LB2017003) and the University Student Innovation and
entrepreneurship training Program of Shandong Province
(No. S201910451123; No. S201910451158).
C O N F L I C T O F I N T E R E S T
The authors declare no conflict of interest.
DATA AVA I L A B I L I T Y S TAT E M E N T
All data included in this study are available upon request
by contact with the corresponding author.
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S U P P O RT I N G I N FO R M AT I O N
Additional supporting information may be found online
in the Supporting Information section at the end of the
article.
How to cite this article: Liu H, Zhuo Z, Zhang Y,
et al. Ligand coordination sphere effect of Schiff
base cis-dioxomolybdenum(VI) complexes in
selective catalytic oxidation of alcohols. Int J Chem
Kinet. 2021;1–9. https://doi.org/10.1002/kin.21530
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