Palladium complexes with 3-phenylpropylamine ligands: Synthesis, structures, theoretical studies and application in the aerobic oxidation of alcohols as heterogeneous catalysts

Article abstract of DOI:10.1039/c5ra17249g

The reaction of 3-phenylpropylamine with Pd(OAc)₂ by heating in toluene resulted in the nearly square-planar complex trans-[Pd(C₆H₅(CH₂)₃NH₂)₂(OAc)₂] (1). Complex 1 reacted with NaCl in methanol to obtain the corresponding product trans-[Pd(C₆H₅(CH₂)₃NH₂)₂Cl₂] (2). Treatment of 2 with triphenylphosphine in dichloromethane afforded trans-[Pd(C₆H₅(CH₂)₃NH₂)₂(PPh₃)₂]2Cl^- (3). All the palladium(ii) complexes (1-3) were fully characterized by IR and NMR spectroscopy. In addition, the crystal structures of 1 and 2 were determined by single-crystal X-ray diffraction analysis. In these structures, the acetate and chloride ligands are in trans geometry. Density functional theory (DFT) calculations gave bond lengths and angles that were noted as experimental values. Palladium nanoparticles that were derived from complexes (1-3) were supported on cucurbit[6]uril (CB[6]) and identified by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), inductively coupled plasma analysis (ICP) and high-resolution X-ray powder spectroscopy (HR-XPS). CB[6]-supported palladium nanoparticles (NPs) were used as heterogeneous catalysts for the aerobic oxidation of alcohols to the corresponding aldehydes or ketones without over-oxidation. CB[6]-Pd NPs (3) (prepared from complex 3) show better catalytic activity than CB[6]-Pd NPs (1), (2), as a higher yield was observed with them in a relatively short time. Factors such as the amount of catalyst, solvent, temperature and reaction time were all systematically investigated to determine their effects on the yield of catalytic alcohol oxidation reactions. This catalytic system displayed high activity and selectivity toward alcohols in mild conditions. The catalyst was reused five times without any significant loss of catalytic activity.

Full text of DOI:10.1039/c5ra17249g

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DOI: 10.1039/C5RA17249G
Journal Name
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Palladium complexes with 3ꢀphenylpropylamine
ligand: synthesis, structures, theoretical studies and
application in the aerobic oxidation of alcohols as
heterogeneous catalysts
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
a,
a
b
b
Kazem Karami, * Nasrin Haghighat Naeini, Vaclav Eigner, Michal Dusek, Janusz
c
d
a
Lipkowski, Pablo Hervés, and Hossein Tavakol
DOI: 10.1039/x0xx00000x
www.rsc.org/
Reaction of 3ꢀphenylpropylamine with Pd(OAc)
by heating in toluene, resulted in nearly square planar
2
complex transꢀ[Pd(C H (CH ) NH ) (OAc) ] (1). Complex 1 reacted with NaCl in methanol to give
6
5
2 3
2 2
2
corresponding product transꢀ[Pd(C H (CH ) NH ) (Cl) ] (2). Treatment of 2 with triphenylphosphine in
6
5
2 3
2 2
2
‾
dichloromethane afforded transꢀ[Pd(C H (CH ) NH ) (PPh ) ]2Cl (3). All of the palladium (II)
6
5
2 3
2 2
3 2
complexes (1–3) were fully characterized by IR and NMR spectroscopies. In addition, the crystal
structures of 1, 2 were determined by singleꢀcrystal Xꢀray diffraction analysis. In these structures, the
acetate and chloride ligands are in trans geometry. Density functional theory (DFT) calculations gave
bond lengths and angles in keeping with the experimental values. Palladium nanoparticles derived from
complexes (1–3), were supported on cucurbit[6]uril (CB[6]) and identified by Xꢀray powder diffraction
(XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), inductively
coupled plasma analyzer (ICP) and high resolution Xꢀray powder spectroscopy (HRꢀXPS). CB[6]ꢀ
supported palladium nanoparticles (NP) were used as heterogeneous catalysts for the aerobic oxidation
of alcohols to corresponding aldehydes or ketones without over oxidation. CB[6]ꢀPd NPs (3) (prepared
from complex 3) shows better catalytic activity than CB[6]ꢀPd NPs (1), (2), as higher yield was observed
with it in a relatively short time. The factors such as catalyst amount, solvent, temperature and reaction
time were all systematically investigated to elucidate their effects on the yield of catalytic alcohol
oxidation reactions. This catalytic system showed high activity and selectivity toward alcohols in mild
conditions. The catalyst was reused five times without any significant loss in the catalytic activity.
that use molecular oxygen as the oxidant is of considerable interest
Introduction
[6ꢀ8]. Molecular oxygen is an inexpensive and abundant oxidant for
Selective oxidation of alcohols to corresponding carbonyl
compounds is an important transformation in the chemical industry
the oxidation of alcohols.
The most important organic reactions have been performed with
homogeneous palladium catalysts, that these catalysts possess many
merits such as high turnover number, high activity and selectivity
and excellent yields, but from economic and environmental
standpoint, heterogeneous catalysts are highly desirable [9ꢀ14]. For
this purpose, researchers have immobilized palladium complexes
and nanoparticles on various supports such as polymers [15], silica
[
1,2]. The traditional oxidizing reagents, which have usually
required stoichiometric amounts of inorganic oxidants such as
chromate or permanganate, are expensive and have serious toxicity
issues associated with them and are not acceptable from the green
chemistry perspective [3,4]. Homogeneous transition metalꢀbased
catalysts such as palladium (II) complexes are known that can
promote this reaction without the use of toxic oxidants [5].
Hydrogen peroxide is considered as one of the least toxic oxidants as
it gives water as the only byꢀproduct. It also conforms with the
environmental concerns, where development of catalytic systems
[16], SBAꢀ15/16 [17], MCMꢀ41 [18], alumina [19], zeolite [20],
carbon [21], clay [22], PEG [23] and TiO2 [24], to create
heterogeneous catalysts, because the heterogeneous systems are easy
to handle and recover [25–33].
a
Department of Chemistry, Isfahan University of Technology, Isfahan 84156/83111, CB[6] is a family of macrocyclic molecules that can act as both
Iran.
support and capping agent (Fig. 1). CB[6] have been synthesized by
Eꢀmail: karami@cc.iut.ac.ir;Tel: +98 3113912351; fax: +98 3113912350
b
condensation of formaldehyde with glycoluril [34–38] and it has two
identical portals. At each entrance to the cavity, six polar carbonyl
groups are located and provide two negative fringes of bonding to
the surface of metals or nanomaterials. In spite of the polar fringed
portals, the CB[6] is nonꢀpolar and thus nearly insoluble in all
common organic solvents and water. In addition, CB[6] is the rigid
cyclic structure that has high thermal and chemical stability against
many oxidizing agents [39].
Institute of Physics, Na Slovance 2, 182 21, Praha 8, Czech Republic.
c
Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01ꢀ224
Warsaw, Poland.
d
Department of chemistry, University of Vigo, 36310, Vigo, Spain.
†
Electronic supplementary information (ESI) available: NMR and FTꢀIR spectra of all
complexes (1ꢀ3); single crystal data of 1 and 2 (CCDC 1060354 and 1060510), HRꢀXPS
N1s spectra of fresh CB[6]ꢀPd NPs (3) and CB[6]ꢀPd NPs (3) after fifth runs.
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NMR spectrum of complex 1 shows aliphatic and aromatic region
and the carbon signals of CO and CH appear at 180.18 and 23.51
3
ppm respectively.
O
The IR spectrum of complex 2 shows two peaks corresponding to
−1
NH2 at 3223 and 3270 cm while acetato ligand peaks are not
N
N
N
N
CH2
1
present. The H NMR spectrum of complex 2 displays four sets of
1
3
1
H
H
signals for the methylene protons and amine protons. The C{ H}
NMR spectrum of complex 2 shows aliphatic and aromatic region
CH2
and the carbon signals of CO and CH
are disappeared.
3
The IR spectrum of complex 3 shows two peaks corresponding to
−
1
1
O
NH
at 3395 and 3451 cm . The H NMR spectrum of complex 3
2
6
displays four sets of signals appear for the methylene protons and
amine protons. The P{ H} NMR of complex 3 shows a peak
corresponding to PPh at 23.26 ppm. Also this spectrum displays an
31
1
Fig. 1 Illustration of Cucurbit[6]uril.
3
additional peak with lower intensity at 17.86 ppm that belongs to
Here we report synthesis and characterization of palladium
complexes derived from 3ꢀphenyl prophyl amine. Theoretical studies
were also carried out in order to investigate why we have obtained
PPh in Pd(PPh ) [42]. Molar conductance value for the complex 3
3
3 4
ꢀ3
3
ꢀ1
2
ꢀ1
in acetone (1×10 mol/cm ) is 110 ꢁ cm mol which indicates its
:2 electrolytic nature.
1
complex
2 instead of orthopalladate (our expectation). The
The single crystal structures of 1 and 2 were established with Xꢀray
diffraction. However, attempts to crystallize complex 3 were
unsuccessful. The single crystal Xꢀray diffraction study of complex
optimized molecular parameters were close to the real structure that
obtained by Xꢀray analysis. In addition the LUMOꢀHOMO energy
gap and partial atomic charges of complex 2 (real product) and
complex 2ʹ (desired product) were obtained.
These complexes (1, 2 and 3) were used for preparation of
heterogeneous Pd catalysts, which were deposited on CB[6] using a
simple method. The supported Pd nanoparticles showed high activity
and selectivity in the aerobic oxidation of alcohols under mild
conditions. Finally, the catalyst was highly reusable at least five
times.
1
, 2 confirmed the details of the proposed new structures.
Fig. 2 and 3 show ORTEP plot of complexes 1 and 2 with selected
bond lengths and angles given in the figure caption. The crystal data
and parameters concerning the data collection and structure solution
and refinement are given in Table S1 of ESI†. Tables S2 and S3 in
ESI† contain selected bond lengths and angles for complexes 1 and
2
, respectively. H atoms in the figures as well as in the tables are
omitted for clarity. The palladium metal exhibits a distorted squareꢀ
planar geometry in both the complexes and two acetates and two
chlorides are trans to each other.
As indicated by the ligands at the Pd(II) center of complex 1, the
angles subtended are varying from 85.0(5) to 95.4(5) and from
Results and Discussion
Scheme 1 shows the complexes prepared in this work and the
labelling assigned to the methylene protons.
1
79.3(8) to 179.4(5). The summation of the bond angles around the
palladium is 360.0. The Pd–N bond lengths, 2.057(12) and 2.059(14)
Å of 1 are close to each other and greater than the value, 2.008(7)
and 2.021(6) Å reported for palladium complex 2c [43]. Also the
Pd–O bond distances (2.019(9) and 2.019(10) Å) of 1 have been
found to be somewhat longer than the values 1.953(10) and 1.985(6)
Å reported for palladium complex 2c [43].
3
1
OAc
NH
2
(i)
NH2 Pd NH2
2
OAc
(1)
(
ii)
PPh
Pd NH
3
Cl
(
iii)
NH
Pd NH
NH
2
2
Cl-
2
2
2
PPh
3
Cl
(3)
(2)
Scheme 1 (i) Pd(OAc) in toluene (60 °C, 24 h); (ii) excess NaCl in
2
methanol (room temperature, 12 h); (iii) 2PPh in dichloromethane
3
(room temperature, 6 h).
Synthesis and structure of palladium complexes
The reaction of 3ꢀphenylpropylamine Ph(CH ) NH with Pd(OAc)
2
3
2
2
in a 1:1 molar ratio was carried out in toluene at 60 °C to produce
palladium complex transꢀ[Pd(C H (CH ) NH ) (OAc) ].
Treatment of complex 1 with an excess of NaCl in methanol
afforded the corresponding product transꢀ
Pd(C H (CH ) NH ) (Cl) ] (2). Complex 2 reacted with PPh in a
1
6
5
2 3
2 2
2
[
6
5
2 3
2 2
2
3
1
:2 molar ratio to give complex 3 transꢀ[Pd(C H (CH ) NH )
6 5 2 3 2 2
ꢀ
(PPh ) ]2Cl [40]. The mononuclear palladium complexes 1, 2 and 3
3 2
were characterized by IR and NMR spectroscopy. These spectra for
complexes 1, 2 and 3 are given in the ESI (Fig. S1–S9†).
Fig. 2 ORTEP diagram of transꢀ[Pd(C H (CH ) NH ) (OAc) ] (1)
6
5
2 3
2 2
2
with 50% probability ellipsoids. All the hydrogen atoms are omitted
for clarity. Selected bond lengths (Å): Pd(1)–O(1), 2.019(9); Pd(1)–
O(3), 2.019(10); Pd(1)–N(1), 2.057(12); Pd(1)–N(2), 2.059(14).
Selected bond angles (°):O(1)ꢀPd(1)ꢀN(1), 85.3(5); O(3)ꢀPd(1)ꢀN(1),
The IR spectrum of complex 1 shows characteristic bands due to the
ꢀ
1
acetato ligand at 1573, 1388 cm [41] and two peaks corresponding
−1
1
13
1
31
1
to NH at 3218, 3118 cm . H NMR, C{ H} NMR and P{ H}
2
NMR spectra of complexes 1, 2 and 3 were recorded in CDCl . The
3
1
9
4.3(5); O(3)ꢀPd(1)ꢀN(2), 85.0(5); O(1)ꢀPd(1)ꢀN(2), 95.4(5); O(1)ꢀ
H NMR spectrum of complex 1 displays five sets of signals for the
methylene protons, acetatoꢀmethyl and amine protons. The C{ H}
13
1
Pd(1)ꢀO(3), 179.4(5); N(1)ꢀPd(1)ꢀN(2), 179.3(8).
2
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Synthesis and characterization of CB[6]ꢀPd NPs
Our goal for the synthesis of CB[6]ꢀPd NPs was to use simple
methods. To achieve this goal, CB[6] was prepared in water, and Pd
nanoparticles were dispersed on CB[6] in green solvents such as
water and ethanol (Scheme 2). In these reactions palladium
complexes 1ꢀ3 were used as precursor for the catalysts preparation.
Previous studies show that CB[6] has two identical portals lined by
carbonyl groups that provide two negative fringes capable of binding
to the surface of metals or other nanostructures [45,46]. The
Palladium ions can bind to carbonyl groups at the portal of CB[6]
before reducing agent (NaBH ) is added to the reaction mixture
4
[45,46]. When reducing agent is added to the reaction mixture, high
percentage of Pd(II) were transformed to Pd(0). This synthesis
procedure is based on the physical adsorption of Pd(0) on the surface
of CB[6] support, by the electronic interaction between the electrons
from Pd(0) and the vacant orbitals of C=O. However, electrostatic
interaction between the surface atoms of Pd and CB[6] can
effectively prevent the Pd NPs from agglomeration.
Fig. 3 ORTEP diagram of transꢀ[Pd(C H (CH ) NH ) (Cl) ] (2)
6
5
2 3
2 2
2
with 50% probability ellipsoids. All the hydrogen atoms are omitted
for clarity. Selected bond lengths (Å): Pd(1)ꢀN(1), 2.0361(15);
i
i
Pd(1)ꢀN(1 ), 2.0361(15); Pd(1)ꢀCl(1), 2.3031(5); Pd(1)ꢀCl(1 ),
.3031(5). Selected bond angles (°): N(1)ꢀPd(1)ꢀCl(1), 88.12(4);
2
i
i
i
i
N(1)ꢀPd(1)ꢀCl(1 ), 91.88(4); N(1 )ꢀPd(1)ꢀCl(1 ), 88.12(4); N(1 )ꢀ
Pd(1)ꢀCl(1), 91.88(4); Cl(1)ꢀPd(1)ꢀCl(1 ), 180.0(5); N(1)ꢀPd(1)ꢀ
N(1 ), 180.0(5).
i
O
i
CH
H SO , H O
2
2
O
O
H
N
N
N
N
H
2
H O
HCl
2
(NH
+
2
)
2
CO
4
2
H
H
N
N
N CH2
H
2
+ H O
As indicated by the ligands at the Pd(II) center of the complex 2, the
160 0
90 0
C
C H O
C
2
2
2
H
H
H
angles are 88.12(4), 91.88(4), 88.12(4) and 91.88(4) for N(1)ꢀPd(1)ꢀ
HCl
i
i
i
i
N CH
2
Cl(1), N(1)ꢀPd(1)ꢀCl(1 ), N(1 )ꢀPd(1)ꢀCl(1 ) and N(1 )ꢀPd(1)ꢀCl(1)
respectively. The summation of the bond angles around the
palladium is 360.0. In complex 2, the Pd1ꢀN1 (2.0361(15) Å) bond
O
O
6
CB[6]
i
Pd
Pd
length is same as Pd1ꢀN1 (2.0361(15) Å) distance. The Pd-Cl bond
Pd complex (1-3)
NaBH
lengths of 2 (2.3031(5) Å) are in the normal range and consistent
with the values reported for the Palladium(II) complexes [44].
The N–H⋯O interactions in the crystal of 1 result in the formation
of a three dimensional structure shown in Fig. 4, where as N–H⋯Cl
interactions shown in Fig. 5 result in the formation of three
dimensional network in the case of 2.
4
CB[6]
EtOH:H O (1:1)
2
[35,47]
Scheme 2 Preparation of CB[6]ꢀPd NPs 1ꢀ3.
The XRD pattern of the fresh CB[6]ꢀPd NPs (3) exhibits a broad
reflection peak belonging to the CB[6] at about 23° [48] and also
shows the peaks at 40°, 46°, 68° and 82° corresponding to (111),
(200), (220) and (311) crystallographic planes of Pd(0) nanoparticles
with the faceꢀcentred cubic Pd lattice (Fig. 6a). This is in good
agreement with the results achieved from literature (Fig. 6b) [49].
Fig. 4 Intermolecular N–H⋯O interaction in complex 1.
Fig. 6 Xꢀray powder diffraction pattern of (a) CB[6]ꢀPd NPs (3) (b)
Pd(0) [46].
Fig. 7 shows the HRꢀXPS spectra of the CB[6]ꢀPd NPs (3). The Pd
3
d spectra show the characteristic Pd 3d5/2 and 3d3/2 doublet peaks.
The expected positions for the Pd(0) and Pd(II) species respectively
at around 335.3 [50,52] and 337.6 eV [50,51] are shown.
Fig. 5 Intermolecular N–H⋯Cl interaction in complex 2.
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The Pd(II) peaks were fitted with a Gaussian Lorentzian peak shape, between CB[6]ꢀPd NPs (3) (Fig 8c) and CB[6] (Fig. 8b) were
while the Pd(0) peaks were fitted with an asymmetric shape. The observed, proving that there is no direct bonding between CB[6] and
fitting indicates that the Pd in fresh CB[6]ꢀPd NPs (3) is a mix of metal atoms [54]. In addition the absorption bands at 693 and 518
ꢀ
1
Pd(II) and Pd(0) states (Fig. 7a). After the fifth runs, Pd(0) is seen in cm showed the existence of PPh in CB[6]ꢀPd NPs (3) in a lower
3
a higher proportion, indicating that after catalytic cycles, the Pd(II) proportion (Fig. 8c), compared to palladium complex 3 (Fig. 8a).
cations were transformed to Pd(0).
The HRꢀXPS N 1s spectra of the fresh CB[6]ꢀPd NPs (3), and
CB[6]ꢀPd NPs (3) after fifth runs show a single peak at around 400.0
eV attributed to CꢀN bonds [49] coming from the CB[6] and ligand.
(Fig. S10†). After the fifth runs, CꢀN bonds is seen in a lower
proportion, indicating that after catalytic cycles, the Pd(II) were
transformed to Pd(0) and ligand removed from the complex 3.
Fig. 8 FTIR spectra of (a) palladium complex 3 (b) CB[6] (c) CB[6]ꢀ
Pd NPs.
The FEꢀSEM image of the corresponding CB[6]ꢀPd NPs (3)
confirms the uniform distribution of Pd NPs onto CB[6] (Fig. 9).
Fig. 7 The XPS patterns of (a) High resolution Pd 3d of fresh CB[6]ꢀ
Pd NPs (3), and (b) CB[6]ꢀPd NPs (3) after fifth runs.
Fig. 9 FEꢀSEM images corresponding to CB[6]ꢀPd NPs (3).
Fig. 8 shows the FTIR spectra of palladium complex 3 (a) CB[6] (b)
and CB[6]ꢀPd NPs (c). From the Fig. 8a it is clear that two bands at
around 2924 and 2849 cm could be attributed to the asymmetric
ꢀ
1
The corresponding TEM images reveal that the Pd NPs were formed
and well dispersed through the support surface (Fig. 10). The
average particle size of Pd(0) in the fresh CB[6]ꢀPd NPs based on
more than 243 particle sizes resulted 3.11 nm (Fig. 10a). The TEM
images after fifth runs are also shown in Fig. 10b. The average
particle size of Pd(0) in CB[6]ꢀPd NPs after use is 2.62 nm. Also the
TEM images show that aggregation of Pd NPs was not observed
even after fifth runs.
and symmetric stretching of CH groups of 3ꢀphenylpropylamine,
2
ꢀ
1
respectively. The peak at 3048 cm exhibited the stretching
vibration of CH in aromatic ring in palladium complex 3. Three
–
1
strong bands observed near 744, 693 and 518 cm in the complex
were attributable to the coordinated PPh ligand [53]. FTIR spectrum
3
ꢀ
1
of CB[6] (Fig. 8b) shows the peak at 1726 cm . This peak was
assigned to carbonyl stretching vibration at which no changes
4
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Table 1
a
Optimizing of the catalyst for oxidation reaction of benzyl alcohol .
b
Entry
catalyst
Conversion (%)
1
2
3
4
5
1
2
3
3
36
39
60
52
5
c
PPh
3
a
Reaction conditions: alcohol (1 mmol), K
2 3
CO (1 mmol), toluene (6
ml), catalyst (0.05 mol% Pd), 90 °C, 23 h, in air.
b
Determined by GC using biphenyl as an internal standard.
Catalyst (0.025 mol% Pd).
c
Although all catalysts were able to furnish the corresponding
benzaldehyde in good yield within the indicated time, catalyst 3
exhibited higher catalytic activity than that of catalysts 1, 2 (Table 1,
entries 1, 2 and 3). This is due to the fact that by changing the
ligands on the transition metal atom, the catalyst properties can be
altered and bulky electronꢀrich phosphines tend to increase the
reaction performance and they also act as catalysts themselves [55ꢀ
5
7]. For further investigation, a blank experiment with only PPh3
was performed ant it was found that PPh could not act as catalyst by
3
its own (Table 1, entry 5). In addition, two different amounts of
CB[6]ꢀPd NPs (3) (0.025, 0.05 mol% of Pd) were used, keeping with
all other reaction parameters fixed: namely temperature (90 ᵒC),
toluene (6 mmol), K CO , and reaction time (23 h) in the air. Indeed,
2
3
the amount of catalyst had a significant effect on the oxidation of
benzyl alcohol (Table 1, entries 3 and 4). The results are shown in
Table 1, indicating 52 and 60 % conversion corresponding to 0.025
and 0.05 mol% of Pd catalyst, respectively. Lower conversion of
benzyl alcohol into benzaldehyde with 0.025 mol% of Pd catalyst
may be due to fewer catalytic sites. Therefore, 0.05 mol% of the Pd
catalyst was taken to be optimal.
Our further investigation concerned the oxidation of benzyl alcohol
using CB[6]ꢀPd NPs (3), toluene and atmospheric air as the source of
molecular oxygen in the presence of various bases (Table 2).
It is reasonable that the base deprotonates the alcohol and starts the
catalytic reaction, therefore the alcohol oxidation over Palladium
based catalyst needs a base as a promoter generally [58]. Hence,
different bases were used to promote the benzyl alcohol oxidation
and K CO was found to be the most effective (Table 2, entry 7).
2
3
In addition, various solvents were investigated in the presence of
K CO and it was found that solvents such as acetonitrile (CH CN),
2
3
3
N,Nꢀdimethylformamide (DMF), acetone, MeOH and EtOH were
not effective (Table 2, entries 15ꢀ19). The conversion of benzyl
Fig. 10 TEM images of (a) fresh CB[6]ꢀPd NPs (3) and (b) CB[6]ꢀ
Pd NPs (3) after fifth runs: histogram illustrating the Pd nanoparticle
size distribution.
alcohol was low when H O:EtOH (1:1, 1:2, 1:4) was used as the
2
solvent (Table 2, entry 12ꢀ14) and toluene was the best choice for
this catalytic reaction (Table 2, entry 7). Under air, the reaction time
needed to be prolonged to 23 h at 90 °C. To reduce the reaction time,
catalytic oxidation of benzyl alcohol was studied by performing the
reaction of benzyl alcohol under air bubbling .In order to determine
the effect of air bubbling on the oxidation reaction of benzyl alcohol,
three different temperatures (rt, 40 °C and 90 °C) were investigated.
The results show that 75, 85, and 77% conversion were found
corresponding to rt, 40 °C and 90 °C, respectively (Table 2, entries
1, 4 and 6). At elevated temperature (90 °C), >95% benzyl alcohol
conversion was found with 77% selectivity of benzaldehyde and
18% selectivity for benzoic acid which is due to over oxidation of
banzaldehyde to benzoic acid at elevated temperature (Table 2, entry
6). Also, in the presence of CB[6] as catalyst, the reaction did
not occur (Table 2, entry 20).
Aerobic oxidation of alcohols by CB[6]ꢀPd NPs
Selective oxidation of alcohols to corresponding carbonyl
compounds is one of the most important transformations in the
synthesis of fine chemicals. Several excellent catalysts for the
oxidation of alcohols to corresponding aldehydes or ketones have
been disclosed, but most of them required heating and toxic
oxidants which violate the requirements of the “green chemistry”.
We first examined the ability of CB[6]ꢀPd NPs (Cat 1–3) to actually
catalyze the aerobic oxidation of benzyl alcohol. Initially, we
focused our attention on the aerobic oxidation of benzyl alcohol
under identical reaction conditions (90 ᵒC, toluene as solvent,
K CO as base, in air) in order to compare catalyst 3 with catalyst 1
2
3
and 2 (Table 1).
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For further investigation, Pd(OAc) and PdCl were also supported corresponding aldehyde or ketone is high for all the alcohol
2
2
on CB[6] and in comparison with the present catalyst (CB[6]ꢀPd oxidation reactions. High efficiency was obtained for
NPs (3)) showed lower conversion (Table 2, entry 21, 22).
2ꢀphenylethanol oxidation, where 100% conversion was observed at
h (Table 3, entry 2). CB[6]ꢀPd NPs (3) shows higher catalytic
2
activity for substituted aromatic alcohols containing electronꢀ
Table 2
donating groups (e.g., –OCH ) than those containing electron
Optimizing of the reaction condition for oxidation reaction of benzyl
alcohol, using CB[6]ꢀPd NPs (3).
3
withdrawing group (such as –NO ) (Table 3, entry 7 and 8). Also
2
this catalytic system was inactive toward linear alcohols (Table 3,
entry 9 and 10). The results show that CB[6]ꢀPd NPs (3) was highly
active and selective for the oxidation of alcohols that have at least a
benzylic ring.
Entry
Solvent
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Pd
mol %)
0.05
Base
Temp
. (°C)
40
Time
(h)
4
Conv.
a
(
(%)
85
Table 3
b
1
2
3
4
5
6
7
8
9
K
2
2
CO
CO
3
3
Aerobic oxidation of alcohols using CB[6]ꢀPd NPs (3).
−
K
90
rt
23
10
6
10
b
0.05
0.05
0.025
0.05
0.05
0.05
0.05
0.05
0.05
0.05
−
75
b
a
b
K
2
K
2
K
2
K
2
CO
CO
CO
CO
3
3
3
rt
68
Entry
Substrate
Time (h) Conv. (Yield) (%)
b
1
2
3
4
5
6
7
8
9
Benzyl alcohol
4
2
85(81)
100(94)
98(91)
96(90)
77(75)
80(78)
84(81)
90(83)
Trace
90
90
90
90
90
90
90
90
6
70
2ꢀPhenylethanol
b
c
6
77 (18)
1ꢀPhenylethanol
4
3
23
23
23
23
23
23
60
42
41
13
28
31
4ꢀHydroxybenzyl alcohol
2ꢀChlorobenzyl alcohol
2ꢀHydroxybenzyl alcohol
4ꢀNitrobenzyl alcohol
4ꢀMethoxybenzyl alcohol
3−Ηexanol
4
4
K
3
PO
4
2
.3H O
4
NaOH
KOH
4
1
1
1
0
1
2
4
Na
2
CO
3
10
10
1
0
2−methylꢀ1ꢀpropanol
Trace
EtOH:H
4:1)
2
2
2
O
O
O
K
2
2
CO
CO
3
a
Reaction conditions: alcohol (1 mmol), K
2
CO
3
(1 mmol), toluene (6
(
ml), catalyst (0.05 mol% Pd), 40 °C, under air bubbling.
1
3
EtOH:H
2:1)
0.05
K
3
90
23
43
a
Determined by GC using biphenyl as an internal standard.
Isolated yields.
(
b
1
1
4
5
EtOH:H
1:1)
0.05
0.05
K
2
2
CO
CO
3
3
90
90
23
23
55
4
(
The catalytic activity of complex 3 was also studied in the aerobic
oxidation of a variety of alcohols using the optimized heterogeneous
reaction conditions. As presented in Table 4, in these homogeneous
systems, different alcohols react to give the corresponding products.
In comparison with homogeneous system, oxidation of alcohols in
the presence of CB[6]ꢀPd NPs (3) (heterogeneous system) could be
considered as efficient and versatile catalytic reactions for the
Acetone
K
1
1
1
1
6
7
8
9
DMF
0.05
0.05
0.05
0.05
0.05
0.05
K
K
K
K
2
2
2
2
CO
CO
CO
CO
3
3
3
3
90
90
90
90
40
40
40
23
23
23
23
4
8
8
4
3
MeOH
EtOH
3
CH CN
aerobic oxidation of a variety of alcohols
.
c
Toluene
Toluene
Toluene
K2CO3
b,d
2
0
1
2
−
Table 4
d
e
K2CO3
b,e
2
2
4
68
Aerobic oxidation of alcohols using complex 3.
K2CO3
b,f
0.05
4
54
a
Reaction conditions: benzyl alcohol (1 mmol), base (1 mmol),
a
solvent (6 ml), catalyst (0.025, 0.05 mol% Pd), in air.
Entry
1
Substrate
Benzyl alcohol
Time (h)
Conv.
88
a
Determined by GC using biphenyl as an internal standard.
Under air bubbling.
Benzoic acid conversion.
The reaction was carried out in the presence of CB[6].
4
b
c
3
4
5
7
8
9
1ꢀPhenylethanol
4ꢀHydroxybenzyl alcohol
2ꢀChlorobenzyl alcohol
4ꢀNitrobenzyl alcohol
4ꢀMethoxybenzyl alcohol
3−Ηexanol
4
4
100
93
d
e
The reaction was carried out in the presence of Pd(OAc)
2
supported on
CB[6].
4
76
f
The reaction was carried out in the presence of PdCl
CB[6].
2
supported on
4
88
4
93
The scope of the catalytic system was then subsequently extended to
the aerobic oxidation of a variety of alcohols under optimized
conditions (Table 3). Several aromatic alcohols were employed to
study the generality of CB[6]ꢀPd NPs (3) for alcohol oxidation, and
the results are listed in Table 3, entry 1–8. The selectivity toward the
10
Trace
(1 mmol), toluene (6 ml),
a
Reaction conditions: alcohol (1 mmol), K
complex 3 (0.05 mol%), 40 °C, under air bubbling.
Determined by GC using biphenyl as an internal standard.
2
CO
3
a
6
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with low catalyst loading in short reaction time. In comparison with
previous works, this catalytic system provides higher turnover
frequency (TOF). In addition, the preparation process of the catalyst
CB[6]ꢀPd NPs (3) is very simple and environmentally friendly.
Heterogeneity and recyclability
Further experiments were performed to confirm the recyclability of
the solid catalyst. The consecutive oxidations of benzyl alcohol
under air bubbling were also carried out at 40 °C using toluene as
solvent. The results are summarized in Fig. 11. At palladium
loading of 0.05 mol%, CB[6]ꢀPd NPs (3) afforded 85% conversion
under air bubbling over 4 h. After the first reaction cycle, CB[6]ꢀ
Pd NPs (3) catalyst was recovered by centrifugation and was
washed with ethanol and water three times. The catalyst was dried
overnight at 100 ᵒC and for the second reaction cycle, the fresh
benzyl alcohol was added and other conditions were the same as
the first reaction cycle. A 87% conversion was obtained over 4 h.
From the third to 5th reaction, there was no significant change
observed in selectivity, and only little decrease in conversion was
observed which showed that the catalyst was stable and can be
regenerated for repeated use.
Theoretical approaches
In order to investigate the structure of the prepared molecule 2 in the
gas phase, theoretical calculations were performed. In the previous
sections we described the synthetic methods for preparation of
palladium complexes. According to our previous works [42(b)], with
these reaction conditions, we expected to obtain orthopallated
complex, but Xꢀray crystal structure confirmed trans geometry
instead. Therefore, the optimizations of two structures (the real (2)
and the expected (2ʹ) products) were performed to find a reason why
the trans product results.
The optimized parameters of 2 are in good agreement with the
experimental one determined by Xꢀray analysis. The most important
calculated parameters for the optimized structure of 2 are listed in
Table 6. The comparison between calculated parameters (2*) with its
Xꢀray structure parameters (2) confirms that this structure is close to
the real structure.
Table 6
Selected bond lengths (Å) and bond angles (°) for compound 2
(
calculated (2*) and Xꢀray structure (2)).
Bond lengths
2
2*
Pd1ꢀN1
Pd1ꢀN1
Pd1ꢀCl1
2.0361(15)
2.0361(15)
2.3031(5)
2.3031(5)
2.058
2.057
2.466
2.466
i
i
Pd1ꢀCl1
Bond angles
Fig. 11 A recyclability test for catalyst in the aerobic oxidation of
alcohols.
N1ꢀPd1ꢀCl1
88.12(4)
91.88(4)
91.88(4)
88.12(4)
180.0(5)
180.0(5)
89.9
90.1
90.1
89.9
180.0
180.0
ICPꢀOES analysis showed that palladium content of the fresh CB[6]ꢀ
Pd NPs (3) was 11.9 wt%. The palladium content after 5 consecutive
reaction cycles was almost the same as that of fresh catalyst, and
palladium could not be detected in the filtrate after the reaction. Also
the hot filtration test confirming no continued reaction after the
catalyst was removed. These results confirm the heterogeneous
i
N1 ꢀPd1ꢀCl1
i
N1ꢀPd1ꢀCl1
i
i
N1 ꢀPd1ꢀCl1
i
Cl1ꢀPd1ꢀCl1
i
N1ꢀPd1ꢀN1
character of the catalytically active species that limits pollution of According to the Xꢀray structure, complex 2 has a trans geometry
the environment by the catalyst.
and calculation of the Gibbs free energies of both complexes showed
that the trans structure (2) could be obtained more easily than the
ortopalladate. The calculated ꢂG values for the reactions which
Comparison with the other studies
The catalytic performance of the catalyst CB[6]ꢀPd NPs (3) for the produce 2 and 2ʹ are respectively ꢀ263.72 and ꢀ132.35 kcal/mol and
oxidation of alcohols was compared with some of the previous ꢂH values are ꢀ305.40 and 151.25 kcal/mol in the gas phase.
reported results using different types of the supports (Table 5) [15, Therefore, the Gibbs free energy confirms the trans compound must
5
9ꢀ63]. As shown in Table 5, comparing with other supported be the major product.
palladium catalyst, CB[6]ꢀPd NPs (3) exhibited comparable yields
Table 5
Comparison with reported results for Suzuki reaction on supported Pd catalysts
a
b
ꢀ1
b
Entry
Catalysts
Reaction conditions
Yields/%
TOF (h )
References
solvent/base/temp./time/oxidant
o
1
2
3
4
5
6
7
Pd@SBAꢀ15 (0.4)
PdNPs/PS (0.5)
PdHAPꢀ0 (0.6)
Pd/MILꢀ101 (1.5)
MSꢀPdGꢀG (1)
toluene/K
2
CO
3
/80 C/3.5 h/O
2
83
98
99
99
99
29
81
59
13
[59]
[15]
o
toluene/K
2
CO
3
/85 C/15 h/ꢀ
trifluorotoluene/ꢀ/90°C/1 h/O
2
165
44
[60]
toluene/ꢀ/80 °C/1.5 h/O
2
[61]
o
toluene/K
2
CO
3
/110 C/10 h/O
2
10
[62]
Pd(TOP)/MBꢀH
2
O
2
solvent free/ꢀ/80 °C/24 h/ꢀ
36
[63]
o
CB[6]ꢀPd NPs (0.05)
toluene/K
2
CO
3
/40 C/4 h/air bubbling
405
This work
a
Data in parentheses indicate mol % of Pd used.
The turnover frequency: mmol aldehyde product/(mmol catalyst × h reaction time).
b
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We also employed the population analyses for 2 and 2ʹ to extract the source (Magnesium Kα line of 1253.6 eV energy and a power
energies of frontier molecular orbitals (FMOs). Graphical applied of 250 W), placed perpendicular to the analyzer axis and
presentations of LUMO and HOMO and their related energy for both calibrated using the 3d5/2 line of Ag with a full width at half
structures are shown in Fig. 12. Their energy gaps of 2 and 2ʹ are maximum (FWHM) of 1.1 eV. An electron flood gun was used
respectively 0.0610 and 0.1647 eV. By comparison between the during measurements to neutralize charging effects. The selected
energies of frontier orbitals, the LUMOꢀHOMO energy gaps in 2 is resolution for the spectra of the different elements was 15 eV of Pass
less than that in 2ʹ, which shows this structure is more reactive. For a Energy and 0.15 eV/step. All Measurements were made in an ultra
ꢀ
8
more exact determination of the partial atomic charges, NBO high vacuum (UHV) chamber at pressure below 8·10 mbar. In the
calculations were used. The results of these calculations for complex fittings GaussianꢀLorentzian and assymetric functions were used
2
shows that the partial charges of Cl atoms in the complex 2 are (after a Shirley background correction).
about ꢀ0.5. Also the charge of Pd in this complex is 0 that shows Conversions were monitored using an Agilent Technologies 6890N
good interaction. gas chromatograph equipped with a flame ionization detector (FID)
+
The second order perturbation energies (E2) obtained from the NBO and an HBꢀ50 column (length = 30 m, inner diameter 320 ꢃm, and
calculations show that the Pdꢀligand bonds are stronger in 2 film thickness = 0.25 ꢃm). Products were identified by comparison
compared with 2ʹ. The E2 interaction energies between Cl and Pd is with authentic samples.
1
72 kcal/mol and between N and Pd is 154 kcal/mol for complex 2,
while the same values for complex 2ʹ are respectively 142 and 2 (Pdꢀ Crystallography
C has a real covalent bond and its energy should be added to the E2 Crystals suitable for Xꢀray molecular structure determination of 1
energy) kcal/mol. All these calculations confirm production of the and 2 were obtained by the diffusion method for a solution of 1 and
complex 2.
2 in dichloromethane/nꢀhexane (1:3 v/v) and chloroform/nꢀhexane
1:3 v/v) respectively. Xꢀray diffraction experiment was done at 120
(
K with the use of Agilent Gemini single crystal diffractometer (Cu
Kα radiation). The structure was solved using Superflip software
[64] and further refined with Jana2006 [65]. MCE software [66] was
used for Fourier maps visualization. The structure was refined by
full matrix least squares on F squared value. The atoms of palladium
complex were refined anisotropically, the positions of hydrogen
atoms were kept in expected geometry with Uiso set to 1.2 of Ueq of
the parent atom. The atoms O , O and O were refined isotropically
1
2
3
and they represented disordered solvent. The centers of these atoms
do not represent actual atomic positions.
Computational methods
DFT method was applied to optimize the structures and calculate
molecular and spectral parameters of compounds in the gas phase.
Gaussian 09 program package [67] was employed for optimizing the
structures and calculation of molecular properties. To perform DFT
calculations, Becke’s three parameter exchange functional [68] was
used in combination with the LeeꢀYangꢀParr correlation functional
(
B3LYP) with LANL2DZ basis set [69]. Molecules were used
Fig. 12 Frontier molecular orbital diagrams of complexes 2 and 2΄.
without any symmetry restriction and C1 symmetry
was assumed for all molecules. NBO analyses [70] were carried out
as implemented in the Gaussian program package using
B3LYP/LANL2DZ level of theory.
Experimental
General
All reactants were purchased from Merck Chemical Company and
Aldrich and used as received. Solvents were used without further
purification or drying.
Synthesis of transꢀ[Pd(C H (CH ) NH ) (OAc) ] (1)
Pd(OAc)2 (0.0448 g, 0.2 mmol) was added to a solution of
6
5
2 3
2 2
2
3
ꢀphenylprophylamine (28 µL, 0.2 mmol) in toluene (10 mL) and
Fourier transform infrared (FTIR) spectra were measured in KBr the resulting mixture was heated under reflux for 24 h. The solvent
pellets with a Jasco FT/IR 680 plus instrument. NMR spectra were was then evaporated and the resulting yellow solid residue was
1
obtained on a Bruker spectrometer at 400.13 MHz ( H). Elemental dissolved in nꢀhexane (10 mL) and CH Cl (2 mL). A pale yellow
2
2
analysis was performed on LECO, CHNSꢀ932 apparatus. Molar solid immediately precipitated. The mixture was stirred for 2 h at
ꢀ
3
conductance of complex 3 was measured in acetone at 1×10
M
room temperature and the resulting suspension was filtered. The pale
using an Elmetron CCꢀ505 conductivity meter.
yellow solid thus obtained and air dried to give (1).
Scanning electron microscopy (SEM) studies were recorded at 15 Complex 1: yellow solid, Yield: (0.06533 g, 66%); m.p. 135 °C. IR:
ꢀ
1
1
kV using a HITACHI Sꢀ4160 instrument (Japan). Transmission
νmax/cm 1573 and 1388 (CO), 3218 and 3118 (NH ). H NMR (400
2
electron micrographs (TEM) were obtained with a JEOL JEM 1010 MHz, CDCl , 25 °C, TMS): δ (ppm): 1.85 (s, 3H, MeCO ), 2.04 (m,
3
2
3
microscope operating at an acceleration voltage of 100 kV. 2H, H ), 2.53 (m, 2H, H ), 2.60 (t, J = 7.6 Hz, 2H, H ), 3.77 (m,
Palladium content of the catalyst was measured by Inductively 2H, NH ), 7.17ꢀ7.30 (m, 5H, C H ). C{ H} NMR (CDCl , 25 °C,
2
1
HH
3
1
3
1
2
6
5
3
Coupled
300DVspectrometer). Xꢀray powder diffraction (XRD) pattern was 126.15ꢀ140.85 (Caromatic), 180.18 (CO). Anal. calcd for
measured using a Scintag Xꢀray diffractometer with 1.54 Å (Cu Kα)
32N O Pd: C,53.39; H, 6.52; N, 5.66. Found: C, 53.26; H, 6.65;
Plasma
(ICPꢀOES)
analyzer
(Perkin–Elmer TMS): δ (ppm): 23.51 (Me), 32.50 (C ), 32.90 (C ), 43.23 (C ),
2 3 1
7
C22H
2
4
Xꢀray radiation source. HRꢀXPS experiments were performed in a N, 5.73.
SPECS Sage HR 100 spectrometer with a nonꢀmonochromatic Xꢀray
8
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DOI: 10.1039/C5RA17249G
ART LE
IC
Synthesis of transꢀ[Pd(C H (CH ) NH ) (Cl) ] (2)
Benzyl alcohol (1 mmol), base (1 mmol), solvent (6 ml) and catalyst
6
5
2 3
2 2
2
To a suspension of 2 in methanol was added excess NaCl and the (0.05 mol% Pd) were combined in a dry flask and the mixture was
resulting mixture stirred for 12 h at room temperature. The solvent stirred at given temperature in air or under air bubbling for a given
was then evaporated and the resulting yellow solid residue was time. After the reaction, the liquid was filtered and analyzed with
dissolved in nꢀhexane (10 mL) and CH Cl (2 mL). The mixture was GC to determine the conversion and selectivity.
2
2
stirred for 2 h at room temperature and the resulting suspension was
filtered. The pale yellow solid thus obtained and air dried to give (2). Recycle procedure for CB[6]ꢀPd NPs
Complex 2: yellow solid, Yield: (0.04656 g, 52%); m.p. 170 °C. IR: Benzyl alcohol was used to test the recycle ability of CB[6]ꢀPd NPs
ꢀ
1
1
ν
max/cm 3223 and 3270 (NH ). H NMR (400 MHz, CDCl , 25 °C, (3) in the aerobic oxidation of alcohols. After the first run was
2
3
TMS): δ (ppm): 2.03 (m, 2H, H ), 2.67 (m, 2H, H ), 2.69 (m, 2H, complete, the CB[6]ꢀPd NPs (3) catalyst was separated by
2
1
1
3
1
H ), 2.78 (m, 2H, NH ), 7.17ꢀ7.31 (m, 5H, C H ). C{ H} NMR centrifugation and was washed several times with ethanol and water
3
2
6
5
(
CDCl , 25 °C, TMS): δ (ppm): 32.85 (C ), 33.10 (C ), 44.87 (C ), to remove adsorbed organic substrates and salt. The catalyst was
3
2
3
1
1
26.22ꢀ140.65 (Caromatic). Anal. calcd for C18H26Cl N Pd: C, 48.29; dried overnight at 70 °C prior to being reused. Then the catalyst was
2 2
H, 5.85; N, 6.26. Found: C, 47.79; H, 5.92; N, 6.16.
used for the 2nd run without additional activation and the same
process was repeated for the next run.
ꢀ
Synthesis of transꢀ[Pd(C H (CH ) NH ) (PPh ) ]2Cl (3)
6
5
2 3
2 2
3 2
Conclusions
To a suspension of palladium complex 2 (0.275 g, 0.5 mmol) in
In conclusion, we prepared three palladium complexes derived from
primary benzylamine, we developed a simple method for their
dispersion in a form of palladium nanoparticles on the surface of
CB[6], and we tested application of these nanoparticles for the
aerobic oxidation of alcohols in toluene using air as the source of
molecular oxygen. The most promising catalytic effects exhibited
nanoparticles of the complex 3, CB[6]ꢀPd NPs (3), which were
dichloromethane (15 mL) was added PPh (0.262 g, 1 mmol). The
resulting solution was stirred for 6 h and then filtered through a plug
3
of MgSO . The filtrate was concentrated to ca. 2 mL, and nꢀhexane
4
(10 mL) was added to the precipitate 3 as a pale yellow solid, which
was collected and airꢀdried.
Complex 3: yellow solid, Yield: (0.05047 g, 56%); m.p. 171 °C
ꢀ
1
2
ꢀ1
ꢀ1
(
dec.). ∆M (
ꢁ
cm mol
)110. IR: νmax/cm 3395 and 3451 (NH2). evaluated as an efficient catalyst for the aerobic oxidation of
/
1
H NMR (400 MHz, CDCl , 25 °C, TMS): δ (ppm): 1.28 (m, 2H, alcohols. Importantly, all reactions were performed at 40 °C and the
3
3
H ), 2.09 (m, 2H, H ), 2.75 (t, J = 7.2 Hz, 2H, H ), 3.06 (m, 2H, corresponding products were obtained in good or excellent yields.
2
1
HH
3
3
1
1
NH ), 7.28ꢀ7.75 (m, C H ). P{ H} NMR (CDCl , 25 °C): δ (ppm): The small size of the Pd particles explains the remarkably high
2
6
5
3
2
3.26 (s, 1P, PPh (complex 3)), 17.85 (s, 1P, PPh (Pd(PPh ) ).
activity of this particular catalyst. TEM and ICP of regenerated
catalyst showed that it is stable under the present reaction conditions.
3
3
3 4
Preparation of Glycoluril and CB[6]
Glycoluril and CB[6] were prepared according to previous reports
71,72]. For preparation of glycoluril, to a solution of urea (60 g, 1
mol) in water (100 mL) was added a 40% aqueous solution of
glyoxal (50 g, 0.345 mol) and concentrated HCl (8.6 mL). The
resulting solution was heated at 90 °C until a heavy precipitate was
formed. Then the reaction mixture was allowed to cool to room
temperature and filtered. The precipitate was washed with copious
amounts of water (200 mL) followed by acetone to remove residual
water and dried under vacuum.
Acknowledgment
[
This work was supported by the Department of Chemistry, Isfahan
University of Technology and University of Vigo. Also we
gratefully acknowledge the funding support received for this project
from the Iranian Nano Technology Initiative Council.
Crystallography was supported by the Czech Science Foundation
project 15ꢀ12653S.
Notes and references
For synthesis of CB[6], a stirred mixture of glycoluril (1.5 g, 10.6 [1] R. A. Sheldon and J. K. Kochi, MetalꢀCatalyzed Oxidations of Organic
mmol), 37% aqueous formaldehyde solution (2.4 mL), concentrated Compounds, Academic Press, New York, 1981, ch. 7, p. 204.
sulfuric acid (1.43 mL) and water (10 mL) was heated several hours [2] M. Hudlicky, Oxidations in Organic Chemistry, ACS Monograph 186,
American Chemical Society, Washington, DC, 1990.
at 160 °C. The resulting reaction mixture was cooled to room
[
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General procedure for the aerobic oxidation of alcohols
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 9

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This journal is © The Royal Society of Chemistry 2012

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View Article Online
DOI: 10.1039/C5RA17249G
Table of contents:
Palladium
complexes
with
3-
phenylpropylamine ligand were synthesized
and characterized. Palladium nanoparticles
were supported on cucurbit[6]uril and used
as heterogeneous catalysts for the aerobic
oxidation of alcohols to corresponding
compounds.