Cd(ii) coordination compounds as heterogeneous catalysts for microwave-assisted peroxidative oxidation of toluene and 1-phenylethanol

Article abstract of DOI:10.1039/d0nj01408g

The aroylhydrazone Schiff bases 2-hydroxy(2-hydroxybenzylidene)benzohydrazide (H₂L¹) and 2-amino(2-hydroxybenzylidene)benzohydrazide (H₂L²) have been used to synthesize the new Cd(ii) coordination polymer [Cd(μ-

Full text of DOI:10.1039/d0nj01408g

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Cd(II) coordination compounds as heterogeneous
catalysts for microwave-assisted peroxidative
oxidation of toluene and 1-phenylethanol†
Cite this:DOI: 10.1039/d0nj01408g
a
ab
Manas Sutradhar, *a Tannistha Roy Barman,
Elisabete C. B. A. Alegria,
Hugo M. Lapa, ab M. Fatima C. Guedes da Silva a and Armando J. L. Pombeiro
*
a
´
The aroylhydrazone Schiff bases 2-hydroxy(2-hydroxybenzylidene)benzohydrazide (H2L1) and 2-amino(2-
hydroxybenzylidene)benzohydrazide (H2L2) have been used to synthesize the new Cd(II) coordination
polymer [Cd(m-1kONO0:2kO:3kO00-L1)(DMF)]n (1) and the dinuclear complex [Cd(1kONO0:2kO-HL2)(kOO0-
Ac)]2ꢀ2DMF (2). Both 1 and 2 have been characterized by elemental analysis, FT-IR and single crystal X-ray
diffraction analysis. The catalytic performances of 1 and 2 were evaluated towards the microwave (MW)
assisted peroxidative oxidation (with tert-butylhydroperoxide, TBHP) of toluene and 1-phenylethanol under
heterogeneous conditions. The MW-assisted peroxidative oxidation of toluene by a Cd(II) catalytic system is
reported for the first time. At 50 1C, the 2/TBHP/MW catalytic system shows good activity for toluene
oxidation with a total (benzaldehyde + benzyl alcohol) product yield of 49% in 1 h, in the absence of any
additives. In the case of neat solvent-free peroxidative oxidation of 1-phenylethanol the 1/TBHP/MW catalytic
system resulted in 24% total yield of acetophenone under optimized conditions. After the catalytic reactions,
both 1 and 2 were successfully recovered and reused.
Received 21st March 2020,
Accepted 5th May 2020
DOI: 10.1039/d0nj01408g
rsc.li/njc
of cadmium-based homogeneous28–30 and heterogeneous31–33
Introduction
catalysts, semiconductors, etc.34–36
Coordination polymers (CPs) are crystalline multifunctional
Herein, we report the syntheses of Cd(II) compounds, a
materials widely used in gas storage and separation,1,2 fluorescence coordination polymer [Cd(m-1kONO0:2kO:3kO00-L1)(DMF)]n (1)
and electrocatalytic sensors,3,4 molecular recognition,5,6 drug and a dinuclear complex [Cd(1kONO0:2kO-HL2)(kOO0-Ac)]2ꢀ2DMF
delivery,7 synthesis of magnetic materials8,9 and catalysis.10,11 (2), using two different aroylhydrazones. They have been screened
Aroylhydrazones, as ligands, can stabilize metal complexes in as heterogeneous catalysts towards the peroxidative oxidation of
various coordination geometries and nuclearities,12–22 their metal toluene and 1-phenylethanol as model substrates of aromatic
complexes can have a recognized potential to act as catalysts alkanes and secondary alcohols, respectively, and using tert-butyl
for oxidation reactions and some of them exhibit interesting hydroperoxide (TBHP) as the oxidizing agent, under microwave
magnetic properties.11,19–22
(MW) irradiation. Benzaldehyde is obtained as the main product
In this work, we have explored the heterogeneous catalytic from the oxidation of toluene whereas 1-phenylethanol produces
activity of a new Cd(II) CP. Though cadmium compounds are acetophenone. These carbonyl compounds are useful starting
known to be carcinogenic,23 they have been used as versatile materials in the industry of dyes, perfumes and pharm-
materials in different areas, particularly in the preparation of aceuticals37–40 and building blocks for many organic compounds
Metal–Organic Frameworks (MOFs) and CPs, semiconducting and fine chemicals.41–46
sulphides and metal nanoparticles.24–27 There are different types
The establishment of sustainable conditions in catalytic
oxygenation processes is a continuing demand and herein we
explore the advantages of using microwave (MW) irradiation
which provides a simpler and energy saving method compared
to conventional heating.47–55 The rate of the catalytic reaction
under MW irradiation is also compared to the conventional
heating. Peroxidative oxidation catalysed by Cd(II) complexes
has been scaly reported56,57 and MW-assisted peroxidative oxi-
dation of alkyl benzene (toluene) by a heterogeneous Cd(II)
system is now reported for the first time.
a
´
´
Centro de Quımica Estrutural, Instituto Superior Tecnico, Universidade de Lisboa,
Av. Rovisco Pais, 1049-001 Lisboa, Portugal. E-mail: manas@tecnico.ulisboa.pt,
pombeiro@tecnico.ulisboa.pt
b
´
´
Area Departamental de Engenharia Quımica, Instituto Superior de Engenharia de
´
´
Lisboa, Instituto Politecnico de Lisboa, R. Conselheiro Emıdio Navarro, 1,
1959-007 Lisboa, Portugal
† Electronic supplementary information (ESI) available. CCDC 1991861 (1) and
1991862 (2). For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/d0nj01408g
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Results and discussion
The Cd(II) coordination polymer [Cd(m-1kONO0:2kO:3kO00-L1)-
(DMF)]n (1) and the dinuclear complex [Cd(1kONO0:2kO-HL2)-
(kOO0-Ac)]2ꢀ2DMF (2) have been synthesized under solvothermal
conditions (in DMF-MeOH) using two different aroylhydrazone
Schiff bases, namely 2-hydroxy-(2-hydroxybenzylidene)benzohydr-
azide (H2L1) and 2-amino(2-hydroxybenzylidene)benzohydrazide
(H2L2),58–60 and Cd(OAc)2ꢀ2H2O (Scheme 1). In both cases, the
ligand exhibits the keto form, but in 1 it behaves as a di-negative
chelator (L1)2ꢁ whereas in 2 it acts as a uni-negative chelator
(HL2)ꢁ. In both complexes, phenolate oxygens at the ortho-position
form phenoxido bridges. The phenolic –OH group present in the
hydrazide moiety of H2L1 deprotonates during the reaction condi-
tions and coordinates to another Cd(II) ion which helps to form 2D
polymeric network of 1. In the case of H2L2, the presence of ꢁNH2
group in the hydrazide moiety does not participate in coordination
and thus 2 exists as dinuclear complex. Characterizations of 1
and 2 have been carried out by elemental analysis, IR spectro-
scopy and X-ray diffraction (single crystal) analysis. Besides the
similar characteristic stretching signals of the ligands, the
bands at 1602 and 1514 cmꢁ1 in the IR spectrum of 2 corre-
spond to the asymmetric and symmetric –COO stretching
frequency of the acetate ligand.61
Fig. 1 Ellipsoid plot of 1 (drawn at 30% probability level) with atom
numbering scheme. Symmetry operations to generate equivalent atoms:
(i) 1 ꢁ x, 1/2 + y, 1.5 ꢁ z; (ii) x, 1/2 ꢁ y, ꢁ1/2 + z; (iii) 1 ꢁ x, ꢁ1/2 + y, 1.5 ꢁ z;
(iv) x, 1/2 ꢁ y, 1/2 + z. Selected bond distances (Å) and angles (1):
Cd1–O1 2.172(2), Cd1–O2iii 2.327(2), Cd1–O3iii 2.271(2), Cd–O3iv 2.314(3),
Cd1–O4 2.384(3), Cd1–N2iv 2.320(3), N1–N2 1.375(4), C1–O1 1.321(4),
C7–O2 1.255(4), C14–O3 1.322(4); O1–Cd1ꢁN2iv 163.08(10), O2iiiꢁCd1–O3iii
143.67(9), O3ivꢁCd1–O4 164.09(11).
ligand is in the dianionic keto form and symmetry expansion
reveals a 2D polymeric structure with a metal-to-ligand ratio of
1 : 1, where every (L1)2ꢁ moiety bridge chelate the metals in the
following way (Fig. 2): one of the cations via the Oketo-, the
N
imine- and the Ophenolate-atoms, this latter atom also binding a
X-ray structures
second cation, and the other Ophenolate coordinating to a third
X-ray quality crystals of 1 and 2 were obtained under solvothermal cadmium. The Cd(II) cations have distorted octahedral NO5
synthesis from a mixture of DMF/methanol.
geometries and are located in the least-square planes con-
The asymmetric unit of 1 contains one ligand (L1)2ꢁ, one structed with the basal coordinating atoms from two symmetry
DMF molecule and one Cd(II) ion (Fig. 1). The aroylhydrazone related L1 ligands; the apical positions are engaged with the
Scheme 1 Syntheses of 1 and 2.
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Fig. 2 Ball and stick representation of the 2D polymeric structure of 1
(arbitrary view).
O-atom of a third L1 moiety and the ODMF. The (L1)2ꢁ assemblies
are not planar (the least-square planes of the phenyl groups
make an angle of 28.851) and are stacked in the polymeric
structure. No classical H-bond interactions were found in 1,
except the intramolecular contacts involving the Himine atoms
and the Ophenolate
.
Fig. 4 A view (down the crystallographic a axis) of the infinite 2D network
derived from H-bond interactions (represented in light-blue dashed lines).
The asymmetric unit of 2 consists of a (HL2)ꢁ ligand ONO0
coordinated to a cadmium center and one chelating acetate
anion; a crystallization DMF solvent molecule is also present.
Symmetry expansion leads to the formation of a dinuclear
species with an inversion center in the middle of a {Cd2O2}
core (Fig. 3). As found in 1, the aroylhydrazone ligands are in
planar (the least-square planes of the phenyl groups make an
angle of 20.121). Contrasting with 1, the structure of 2 reveals
H-bond interactions involving the donor Nimine and the acceptor
Oacetate [dDꢀ ꢀ ꢀA 2.839(2) Å, +DꢁHꢀ ꢀ ꢀA 163(3)1], and the donor
Namine and the acceptor ODMF [dDꢀ ꢀ ꢀA 2.952(4) Å, +DꢁHꢀ ꢀ ꢀA
169(2)1] therefore generating a 2D infinite network (Fig. 4).
the keto form and chelate by means of the Ophenolate-, the Oketo
-
and the Nimine-atoms; the former type of atoms act as bridging
bidentate donors to two metal cations. Despite the 6-membered
NO5 environments, every Cd(II) center exhibits a geometry that
resembles a square pyramid in which the chelating acetate, as a
whole, occupy the apical site. Relative to the least-square plane
formed by the basal binding atoms, the metal is displaced
0.804 Å towards the acetate. The (HL2)ꢁ ligand is also not
Catalytic studies
To examine the catalytic performance of Cd(II) complexes 1 and
2 under microwave irradiation, we have selected toluene and
1-phenylethanol as model substrates for the peroxidative oxidation
of alkanes and alcohols, respectively. All the reactions were
carried out under mild conditions, using t-BuOOH as oxidant
(Scheme 2).
Fig. 3 Ellipsoid plot of 2 (drawn at 30% probability level) with atom
numbering scheme. DMF molecule of crystallization is omitted for clarity.
Symmetry operation to generate equivalent atoms: (i) 1 ꢁ x, 1 ꢁ y, 1 ꢁ z.
Selected bond distances (Å) and angles (1): Cd1–O1 2.1946(14), Cd1–O1i
2.2824(13), Cd1–O2 2.3345(14), Cd1–O3 2.3903(16), Cd1–O4 2.3086(17),
Cd1–N1 2.2993(16), N1–N2 1.385(2), C1–O1 1.328(2), C8–O2 1.254(2),
C14–N3 1.368(3); O1–Cd1ꢁO2 135.50(6), O1–Cd1ꢁN1 133.71(6).
Scheme 2 Microwave-assisted peroxidative oxidation of toluene (a) and
1-phenylethanol (b) by 1 and 2.
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Microwave-assisted peroxidative oxidation of toluene by the
Cd(II) compounds
The microwave-assisted peroxidative oxidation of toluene
(Scheme 2a and Table 3) was undertaken under low MW
irradiation (5 W) in acetonitrile at 50 1C for 1 h (typically) and
using t-BuOOH (aq. 70%, 2 eq.) as oxidant, in the presence of
[Cd(m-1kONO0:2kO:3kO00-L1)(DMF)]n (1) or [Cd(1kONO0:2kO-HL2)-
(kOO0-Ac)]2ꢀ2DMF (2). Both complexes are active as catalysts,
being 2 the most promising one. Under the above-mentioned
reaction conditions, toluene is oxidized to yield predominantly
benzaldehyde, along with some amount of benzyl alcohol. For
longer reaction periods, residual amounts of benzyl acetate and
benzoic acid among other oxidation products were detected
(GC-MS). The simple cadmium acetate Cd(OAc)2ꢀ2H2O was also
tested for comparative purposes and, under typical reaction
conditions (1 h, 50 1C, 5 W) less than 1% total yield (entry 15,
Table 1) was reached, thus showing the importance of the N
and O donor ligands in the metal coordination sphere in
order to increase the catalytic performance. Despite this, the
ligands HL1 and HL2 do not exhibit any activity by themselves
(entries 16 and 17, Table 1), demonstrating also the key role
played by the metal coordination entity.
Fig. 5 Yields of the major products of oxidation of toluene (benzaldehyde
and benzyl alcohol) catalyzed by 2 along time (0.083–3 h). Reaction
conditions: [toluene] = 1.67 M; [2] = 3.3 ꢂ 10ꢁ3 M; [TBHP] = 3.3 M, in
NCMe, under MW irradiation (5 W).
An examination of the reaction kinetic profile of the toluene
oxidation in the presence of 2 during 3 hours at 50 1C shows
that the reaction proceeds gradually to yield increasing
amounts of the major reaction products (benzaldehyde and
benzyl alcohol) (Fig. 5) up to a maximum total yield of ca. 49%.
The selectivity towards benzaldehyde is always above 90%
(Fig. 6) even during the initial reaction period.
A highly favorable effect of MW irradiation is observed in
this study, even with the very low power of 5 W. Hence, e.g. for
Fig. 6 Selectivity of the major products of oxidation of toluene (benzal-
dehyde and benzyl alcohol) catalyzed by 2 along time (0.083–3 h).
Reaction conditions: [toluene] = 1.67 M; [2] = 3.3 ꢂ 10ꢁ3 M; [TBHP] =
3.3 M, in NCMe, under MW irradiation (5 W) during 0–180 min.
Table 1 Yields (benzaldehyde and benzyl alcohol) in the microwave-
assisted oxidation of toluene by TBHP at 50 1C with 1 or 2 as catalysta
Yieldb (%)
Co-catalyst
(mmol)
Benzyl
the 2/TBHP/MW system, 68% of total product yield (benzalde-
hyde + benzyl alcohol) was obtained after 24 h reaction using
conventional heating and stirring at 50 1C in acetonitrile with
TBHP (70% aqueous solution) as oxidant, 15% using H2O2
(30% aqueous solution) or less than 2% when using atmo-
spheric oxygen instead, while under a low microwave power (5 W)
already 39.5% or 49% of total product yield was achieved after only
1 or 3 h reaction, respectively, at 50 1C (entries 8 or 9, respectively,
Table 1). A very interesting achievement is the considerable yield of
ca. 22% obtained after 1 h reaction under MW irradiation and in
the absence of any added solvent (entry 11, Table 1).
The temperature of 50 1C was found to be the optimum one
for this catalytic system. Attempt to perform the peroxidative
oxidation of toluene in the presence of 2 at a lower temperature
of 30 1C resulted in an important yield drop. The temperature
rises from 50 to 80 1C resulted in a decrease in the total yield
from 39.5 to 31%, respectively (entries 8 and 10, Table 1),
suggesting the formation in higher amount of the unwanted
benzoic acid (oxidation of benzyl alcohol to benzaldehyde and
sequential oxidation to benzoic acid).
Entry Catalyst
Benzaldehyde alcohol Total TONc
1
1
2
—
—
—
—
13.7
18.6
11.5
5.2
3.9
6.5
0.2
0.3
—
0.4
—
3.3
4.9
0.7
0.2
0.1
—
17.6 88
25.1 126
11.7 59
5.5 28
1.9 10
2.9 15
2d
3e
4f
5
6
7
8
TEMPO (30) 1.9
HNO3 (7) 2.5
HNO3 (35) 0.5
—
—
—
—
TEMPO (30) 3.1
HNO3 (7) 2.2
HNO3 (35) 0.5
0.5
3
36.2
39.5 198
49.1 246
31.0 155
21.8 109
3.2 16
2.2 11
9d
10e
11f
12
13
14
15
16
17
44.2
30.3
21.6
—
—
—
—
0.5
0.6
—
3
3
—
—
Cd(OAc)2ꢀ2H2O —
0.6
—
—
HL1
HL2
—
—
—
a
Reaction conditions: toluene (1.67 M), catalyst precursor 1 or 2 (3.3 ꢂ
10ꢁ3 M), TBHP (70% aq. 3.3 M), NCMe (3 mL), 50 1C, 1 h, MW (5 W).
b
Moles of products (benzaldehyde or benzyl alcohol)/100 mol of
c
toluene, determined by GC. Turnover number (TON) = number of
moles of products per mol of Cd(II) compound. 3 h reaction. e Reaction
d
f
performed at 80 1C, 10 W. Solvent-free conditions.
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In the present system, especially in the presence of 2, higher (ca. 39% total yield and high selectivity) of the mononuclear
yields of oxidized products at moderate temperature (50 1C) aroylhyrazone-Cu(II) complex [Cu(kNOO0-HL)Cl(CH3OH)] and
were achieved after 1 h (39.5% total yield, entry 8, Table 1), higher than that of the 1D aroylhyrazone-Cu(II) coordination
compared with those reported e.g. for nanoparticles of cadmium polymer [Cu(1kNOO0,2kO0,3kO00-L)]n studied by us as catalysts
peroxide CdO2 for which a total yield of 30% was obtained after for the oxidation of toluene under similar reaction conditions.17
1 h at 180 1C.62 To check the heterogenous character of the This shows that cadmium complexes may become important
catalysts we have performed powder XRD of the compounds players in a game that is being dominated by more studied
(1 and 2) before and after catalytic reactions (Fig. S1, ESI†). transition metal centers, such as those typically used in oxidation
Similar powder patter as well as same elemental analyses reactions of alkanes, namely of copper or iron.
suggest that the compounds remain unchanged after catalytic
Cd(II) + t-BuOOH - Cd(I) + t-BuOOꢃ + H+
Cd(I) + t-BuOOH - Cd(II) + t-BuOꢃ + HOꢁ
t-BuOꢃ + RH - t-BuOH + Rꢃ
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
reactions. The reusability of the most active catalyst 2 was
also explored. Compound 2 was isolated by centrifugation after
catalyzing the microwave-assisted oxidation of toluene by TBHP
for 1 h at 50 1C (Table 1, entry 8) and reused in 4 more cycles. It
was found that for the reaction performed for 1 h, there was a
reduction of ca. 11% yield in the 2nd cycle compared to the first
cycle. Between the 2nd and 3rd cycles a 7% yield reduction was
detected, and between 3rd, 4th and 5th cycles there was no
significant change (Fig. 7).
The mechanism of this reaction should initially involve the
formation of oxygen-based radicals t-BuOOꢃ and t-BuOꢃ (eqn (1)
and (2)) upon oxidation or reduction of the oxidant t-BuOOH by
CdII or CdI metal centers, respectively.17,63,64
Rꢃ + O2 - ROOꢃ
ROOꢃ + RH - ROOH + Rꢃ
ROOH + Cd(I) - Cd(II) + ROꢃ + HOꢁ
ROꢃ + RH - ROH + Rꢃ
ROOꢃ + t-BuOOꢃ - RHO + t-BuOH + O2
The t-BuOOꢃ radical can undergo dismutation to the t-BuOꢃ
radical and O2, and the t-BuOꢃ radical can abstract an H atom
from toluene (RH) to form the benzyl radical Rꢃ (eqn (3)) which
upon reaction with O2 forms the benzyl peroxyl radical ROOꢃ
(eqn (4)) which upon dismutation leads to ROH (benzyl alcohol)
and benzaldehyde (plus O2). Furthermore, H-abstraction from
toluene (RH) by the ROOꢃ radical originates ROOH (plus Rꢃ)
(eqn (5)) which upon reduction by a CdI center, forms the
benzyloxyl ROꢃ radical (plus hydroxide) (eqn (6)) that leads
to benzyl alcohol (plus Rꢃ) by H-abstraction from toluene
(eqn (7)).63
The selectivity for benzaldehyde (our main product) can be
favoured by the terminal reaction of ROOꢃ + t-BuOOꢃ to yield
the aldehyde (plus t-BuOH and O2) (eqn (8)) or by further metal
promoted oxidation of benzyl alcohol with t-BuOOH.63
The dinuclear Cd(II) complex [Cd(1kONO0:2kO-HL2)(kOO0-
Ac)]2ꢀ2DMF (2) exhibits a catalytic activity similar to that
Microwave-assisted solvent-free peroxidative oxidation of
1-phenylethanol by the Cd(II) compounds
The oxidations 1-phenylethanol (Scheme 2, reaction b) using
t-BuOOH (aq. 70%, 2 eq.) as the oxidizing agent, were carried
out following a previously reported procedure.55,65 Under typical
conditions, the catalyst 1 or 2 (10 mmol) was placed in the
reaction tube as well as 1-phenylethanol (2.5 mmol) and
t-BuOOH (5 mmol, aq. 70%), in the absence of any added
solvent. The reactions were performed for 1 hour at 80 or
120 1C and under low-power (5 or 15 W) microwave irradiation.
Acetophenone is the only oxidized product detected by GC-MS
chromatography in the abovementioned conditions. The results
in terms of yield and of turnover number (TON, moles of
product/mol of catalyst) are summarized in Table 2.
Table 2 Selected results for the MW-assisted oxidation of 1-phenylethanol
by TBHP with 1 or 2 as catalysta
Temperature Additive
Yieldb
(%)
Entry Catalyst
(1C)
(mmol)
TONc
1
2
3
4
5
6
7
8
9
1
80
120
80
80
120
80
120
80
80
—
—
12
24
30
60
38
23
33
18
25
10
8
TEMPO (30) 15
—
—
TEMPO (30)
TEMPO (30) 10
Hpca (50)
—
2
9
13
7
5
3
Cd(OAc)2ꢀ2H2O
a
Reaction conditions (unless stated otherwise): 2.5 mmol of substrate,
10 mmol (0.4 mol% vs. substrate), 5 mmol of t-BuOOH (aq. 70%), 80 or
b
120 1C, 1 h, microwave irradiation (5–15 W). Molar yield (%) based on
Fig. 7 Recycling studies for 2. Reaction conditions: toluene (1.67 M),
catalyst precursor 2 (3.3 ꢂ 10ꢁ3 M), TBHP (70% aq. 3.3 M), NCMe (3 mL),
50 1C, 1 h, MW (5 W).
1-phenylethanol, i.e. moles of acetophenone per 100 mol of 1-phenyl-
c
ethanol determined by GC. Turnover number (TON) = number of
moles of acetophenone per mol of Cd(II) compound.
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Under the above conditions (120 1C and 1 h reaction time) metal source, namely Cd(OAc)2ꢀ2H2O, was used for the synthesis
and in the presence of 1 (catalyst/substrate molar ratio of 0.4%) of complexes.
a substantial conversion of 1-phenylethanol into acetophenone
was already reached (24% yield, entry 2, Table 2).
C, H, and N elemental analyses were carried out by the
´
Microanalytical Service of the Instituto Superior Tecnico. Infra-
The presence of 2,2,6,6-tetramethylpiperydil-1-oxyl (TEMPO) red spectra (4000–400 cmꢁ1) were recorded on a Bruker Vertex
radical has not a considerable significant effect on the aceto- 70 instrument in KBr pellets; wavenumbers are in cmꢁ1. The
phenone yield in both catalytic systems. While in the presence 1H NMR spectra were recorded at room temperature on a
of 1 a slight increase on the acetophenone yield from 12 to 15% Bruker Avance II + 400.13 MHz (UltraShieldTM Magnet, Rhein-
(in the absence and in the presence of TEMPO, respectively, stetten, Germany) spectrometer. Tetramethylsilane was used as
entries 1 and 3, Table 2) is observed, in the system catalyzed by the internal reference and the chemical shifts are reported in
2 even a small decline is observed (from 9 to 7% in the absence ppm. The catalytic tests performed under microwave (MW)
and in the presence of TEMPO, respectively, entries 4 and 6, irradiation were undertaken in a focused Anton Paar Monowave
Table 2).
300 microwave incorporating a rotational system and an IR
The effect of an acidic additive was tested by using the temperature detector, using a 10 mL capacity reaction tube with a
heteroaromatic 2-pyrazynecarboxylic acid (Hpca) in the 2/TBHP/ 13 mm internal diameter. Gas chromatographic (GC) measure-
MW system and also an inhibiting effect was observed, e.g., ments were carried in a FISONS Instruments GC 8000 series gas
a yield drop from 9 to 5% was detected in the absence and in chromatograph with a capillary DB-WAX column (30 m ꢂ 0.32 mm)
the presence [n(acid)/n(catalyst 2) = 5], respectively, when the and a FID detector under the following conditions: program 120 1C
reaction was performed at 80 1C during 1 h.
for 1 min, 10 1C minꢁ1, 200 1C for 1 min, injector at 240 1C and
The temperature has an accelerating effect on the conversion helium as the carrier gas. Gas Chromatography-Mass Spectrometry
of 1-phenylethanol and at 120 1C yields of 24% and 13% were (GC-MS) analyses were performed using a PerkinElmer Clarus
obtained after 1 h reaction in the presence of 1 or 2, respectively 600 1C (Shelton, CT, USA) instrument (He as the carrier gas). The
(Table 2, entries 2 and 5), which are considerably higher than ionization voltage was 70 eV. Gas chromatography was conducted in
those observed at 80 1C (12 and 9%, respectively) (Table 2, the temperature-programming mode, using a SGE BPX5 column
entries 1 and 4).
(30 m ꢂ 0.25 mm ꢂ 0.25 m). All reaction products achieved from
The replacement of 1 or 2 by the precursor salt, cadmium(II) the catalytic oxidation reactions were identified by their retention
acetate dihydrate resulted in a strong decrease of activity (compare times, confirmed with those of commercially available samples. The
e.g., entries 1 and 9, Table 2) indicating again the important role of mass spectra of reaction products were compared to fragmentation
the Schiff base ligand in the catalytic performance of the Cd(II) patterns obtained from the NIST spectral library stored in the
compounds for the oxidation reaction.
computer software of the mass spectrometer. Powder X-ray diffrac-
The formation of acetophenone catalyzed by 1 and 2 does tion (PXRD) was conducted in a D8 Advance Bruker AXS (Bragg
not occurred when Ph2NH or CBrCl3 (well known66 oxygen- or Brentano geometry) theta-2theta diffractometer, with copper radia-
carbon-radical traps, respectively), are added to the reaction tion (Cu Ka, l = 1.5406 Å) and a secondary monochromator,
mixture, suggesting the involvement of radical species in the operated at 40 kV and 40 mA. Flat plate configuration was used,
oxidation mechanism. Therefore, we believe that the mechanism and the typical data collection range was between 51 and 401.
t
may involve (see above) the metal-assisted generation of BuOOꢃ
and t-BuOꢃ radicals upon oxidation or reduction of t-BuOOH,
Syntheses of the pro-ligands H2L1 and H2L2
respectively, which further behave as hydrogen atom abstractors The aroylhydrazone Schiff base pro-ligand 2-hydroxy(2-hydroxy-
from the alcohol64,67 with possible involvement of the Schiff-based benzylidene)benzohydrazide (H2L1) and 2-amino(2-hydroxy-
ligand in proton transfer steps. Compared with other Cd(II) Schiff- benzylidene)benzohydrazide (H2L2) (Scheme 1) were prepared
based heterogeneous catalysts,68 the catalytic activity of the present by a reported method58–60 upon condensation of the 2-hydroxy-
Cd compounds 1 and 2 proved to be comparable (slightly higher) benzohydrazide or 2-aminobenzohydrazide with 2-hydroxybenzal-
for the alcohol oxidation reaction,68 the most promising catalyst 1 dehyde.
being recovered and reused for at least 3 consecutive cycles. For
the reaction performed at 120 1C for 1 h, it was verified that there
was no significant change in the yield between the first and second
Synthesis of the Cd(II) compounds
[Cd(l-1jONO0:2jO:3jO00-L1)(DMF)]n (1). A mixture of 2 mL
cycles, but it decreased by ca. 15% from the second to the third DMF solution of H2L1 (5 mg, 0.02 mmol) and 5 mL methanolic
round. The loss of activity could indicate the occurrence of catalyst solution of Cd(OAc)2ꢀ2H2O (5 mg, 0.02 mmol) was sealed in a
partial deactivation.
capped glass vessel and heated to 80 1C for 48 h and, after
subsequent gradual cooling to room temperature, good quality
crystals were isolated by filtration, washed 3 times with methanol,
and then dried open in air. This compound is insoluble in
common organic solvents and water. Yield: 58% (based on Cd).
Experimental
General materials and procedures
The synthetic part of this study was performed under atmo- Anal. calc. for 1 (C17H17CdN3O4): C, 46.43; H, 3.90; N, 9.56.
spheric air. Commercially available reagents and solvents were Found: C, 46.37; H, 3.86; N, 9.53. IR (KBr; cmꢁ1): 3476 n(OH)
used as received, without further purification or drying. The 3157 n(NH), 1663 n(CQO), 1189 n(N–N).
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[Cd(1jONO0:2jO-HL2)(jOO0-Ac)]2ꢀ2DMF (2). This compound in NCMe (3 mL) were as follows: toluene (1.67 M), catalyst
was synthesized using a similar procedure as mentioned in the precursor 1 or 2 (3.3 ꢂ 10ꢁ3 M), TBHP (70% aq. 3.3 M), 50 1C,
case of 2 but H2L2 was used instead of H2L1.
1–3 h, MW (5 W). After the reaction, 90 mL of cycloheptanone
Anal. calc. for 2 (C38H44Cd2N8O10): C, 45.75; H, 4.45; N, (internal standard) and 10 mL of diethyl ether (for extraction)
11.23. Found: C, 45.72; H, 4.41; N, 11.20. IR (KBr; cmꢁ1): 2986 were added and after stirring, a sample was analysed by GC. For
n(NH), 1684 n(CQO), 1176 n(N–N).
the solvent-free oxidation of 1-phenylethanol, the following
conditions were applied: 2.5 mmol of the alcohol, 10 mmol
(0.4 mol% vs. substrate) of [Cd(m-1kONO0:2kO:3kO00-L1)(DMF)]n
(1) or [Cd(1kONO0:2kO-HL2)(kOO0-Ac)]2ꢀ2DMF (2), 5 mmol of
t-BuOOH (aq. 70%). The sealed tube was placed in the micro-
wave reactor and the system was left under stirring and under
irradiation (5 or 15 W), at 80 or 120 1C for 1 h. After reaction,
150 mL of benzaldehyde (internal standard) and 2.5 mL of NCMe
(for extraction) were added and the mixture stirred. Similarly, a
sample from the upper organic layer was analysed by GC and
the product yield determined using the internal standard
method. Blank experiments (without metal catalyst) were also
performed to confirm the catalytic activities of Cd(II) complexes,
however no oxidation products were detected.
X-ray measurements
Good quality single crystals suitable for X-ray diffraction of 1
and 2 were immersed in cryo-oil, mounted on Nylon loops and
measured at 297 K. Intensity data were collected using a Bruker
APEX-II PHOTON 100 diffractometer with graphite mono chromated
Mo Ka (l 0.71073) radiation. Data were collected using phi and
omega scans of 0.51 per frame and full sphere of data were obtained.
Cell parameters were retrieved using Bruker SMART69 software
and refined using Bruker SAINT69 on all the observed reflections.
Absorption corrections were applied using SADABS.70 Structures
were solved by direct methods by using SHELX-9771 and refined
with SHELXL-2014.72 Calculations were performed using WinGX
version 2014.1.73 All non-hydrogen atoms were refined anisotro-
pically. The H-atoms bonded to carbon were included in the
model at geometrically calculated positions and refined using a
riding model, using Uiso defined as 1.2Ueq or 1.5Ueq of the parent
carbon atoms for phenyl or methyl groups, respectively. The
N-bound hydrogen atoms were located in the difference Fourier
synthesis and refined. Least square refinements with anisotropic
thermal motion parameters for all the non-hydrogen atoms and
isotropic for the atoms were employed. Crystallographic data and
processing parameters are presented in Table 3.
Conclusions
The Cd(II) compounds [Cd(m-1kONO0:2kO:3kO00-L1)(DMF)]n (1)
and [Cd(1kONO0:2kO-HL2)(kOO0-Ac)]2ꢀ2DMF (2) have been
applied as heterogeneous catalysts in the oxidation of toluene
and 1-phenylethanol. Peroxidative oxidations catalysed by Cd(II)
complexes are not yet well explored and in this study we have
reported for the first time the MW-assisted peroxidative oxidation
of an alkyl benzene (toluene) by an heterogeneous Cd(II) system.
The 2/TBHP/MW system is more active in the toluene oxidation at
moderate temperature (50 1C), achieving a total (benzaldehyde +
benzyl alcohol) product yield of 49% after 1 h, in absence of any
additives, whereas 68% of total product yield was obtained after
24 h reaction using conventional heating. The 1/TBHP/MW catalytic
system shows a better catalytic activity in the solvent-free per-
oxidative oxidation of 1-phenylethanol (24% yield of acetophenone).
After the catalytic reactions, both 1 (for the 1-phenylethanol
oxidation) and 2 (for the toluene oxidation) were successfully
recovered and reused.
Catalytic studies
The microwave-assisted (MW) peroxidative oxidations of toluene
and 1-phenylethanol were performed in an appropriate Pyrex
tube (10 mL capacity reaction tube with a 13 mm internal
diameter) which was placed in an Anton Paar Monowave 300
reactor fitted with a rotational system and an IR temperature
detector. For toluene oxidation, the concentrations of the reactants
Table 3 Crystal data and structure refinement details for complexes 1 and 2
This study shows a promising catalytic activity of Cd(II)
complexes in peroxidative oxidation reactions, namely those for
which Cu(II) and Fe(II or III) complexes provide typical catalysts,
and the obtained results encourage further extension to other
oxidation reactions and/or to different Cd(II) complexes with
benzohydrazide ligands.
1
2
Empirical formula
Formula weight
Crystal system
Space group
C17H17CdN3O4
439.73
Monoclinic
P21/c
11.0012(10)
13.1453(11)
12.5273(12)
101.791(4)
1773.4(3)
4
C38H44Cd2N8O10
997.61
Monoclinic
P21/c
12.732(2)
12.938(2)
13.914(2)
113.651(6)
2099.7(6)
2
a/Å
b/Å
c/Å
b/1
Conflicts of interest
V (Å3)
Z
There are no conflicts to declare.
Dcalc (g cmꢁ3
F000
)
1.647
880
1.257
1.578
1008
1.077
m (Mo Ka) (mmꢁ1
)
Acknowledgements
Rfls. collected/unique/observed 28 675/3628/3171 33 215/4309/3725
Rint
0.0309
0.0293, 0.0728
1.164
0.0277
0.0208, 0.0494
1.120
b
Final R1a, wR2 (I Z 2s)
˜
ˆ
This work has been supported by the Fundaçao para a Ciencia e
Goodness-of-fit on F2
Tecnologia (FCT) 2020–2023 multiannual funding to Centro de
1
P
P
P
P
R = ||Fo| ꢁ |Fc||/ |Fo|. wR(F2) = [ w(|Fo|2 ꢁ |Fc|2)2/ w|Fo|4]2.
Quımica Estrutural (project UIDB/00100/2020). Authors are also
a
b
´
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21 M. Sutradhar, T. Roy Barman, E. C. B. A. Alegria, M. F. C.
Guedes da Silva, H.-Z. Kou, C.-M. Liu and A. J. L. Pombeiro,
Dalton Trans., 2019, 48, 12839–12849.
grateful to the FCT project PTDC/QUI-QIN/29778/2017 for
financial support. M. S. acknowledges the FCT and IST for a
working contract ‘‘DL/57/2017’’ (Contract no. IST-ID/102/2018).
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