Pd-nanoparticles stabilized by pyridine-functionalized poly(ethylene glycol) as catalyst for the aerobic oxidation of α,β-unsaturated alcohols in water

Article abstract of DOI:10.1002/pola.26645

The synthesis and coordination chemistry of 4-pyridinemethylene end-capped MeO-PEG (L^Py) to Pd(II) was investigated. The PEG-based macroligand L^Py was straightforwardly synthesized by pyridine end-functionalization of PEG monomethyl ether and employed to stabilize Pd(II) in a neutral (1a) and bis-cationic (2a) macrocomplex, characterized by a LPy to Pd molar ratio of 2 and 4, respectively. The homogeneous distribution of the Pd-NPs in 2b exerted a significant stabilizing effect on the poly(ether) chain of L^Py against its oxidative thermal degradation. From a screening of the Pd(II)- and Pd-NP-based catalysts in the aerobic oxidation of unsaturated alcohols in water emerged 2b as the most stable catalyst, showing TOF-values up to 200^-h and high chemoselectivity even for prolonged reaction times.

Full text of DOI:10.1002/pola.26645

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Pd-Nanoparticles Stabilized by Pyridine-Functionalized Poly(ethylene
glycol) as Catalyst for the Aerobic Oxidation of
a,b-Unsaturated Alcohols in Water
Guido Giachi,1 Werner Oberhauser,2 Marco Frediani,1 Elisa Passaglia,3
Laura Capozzoli,4 Luca Rosi1
1
ꢀ
Dipartimento di Chimica, Universita di Firenze, via della Lastruccia 13, 50019 Sesto Fiorentino, Firenze, Italy
2Istituto di Chimica dei Composti Organometallici (ICCOM-CNR), Area di Ricerca CNR di Firenze, via Madonna del Piano 10,
50019 Sesto Fiorentino, Italy
3ICCOM-CNR, UOS Pisa, Area della Ricerca, via Moruzzi 1, 56124 Pisa, Italy
4Centro di Microscopie Elettroniche (Ce.M.E.), Area di Ricerca CNR di Firenze, via Madonna del Piano 10,
50019 Sesto Fiorentino, Italy
Correspondence to: W. Oberhauser (E-mail: werner.oberhauser@iccom.cnr.it) or M. Frediani (E-mail: marco.frediani@unifi.it)
Received 7 January 2013; accepted 17 February 2013; published online 13 March 2013
DOI: 10.1002/pola.26645
KEYWORDS: aerobic alcohol oxidation; catalysis; Pd-nanoparticle; polyether
structure of PEG are suitable to stabilize metal cations of dif-
ferent atom size and metal NPs.26,27
INTRODUCTION The selective oxidation of alcohols to carbonyl
compounds or acids is a reaction of long standing academic
and industrial research interest.1,2 Because of the increasing
demand for environmentally compatible oxidation protocols,
which avoid the generation of a large amount of heavy-metal
waste and usage of organic solvents as reaction medium, a
great deal of research activity is been focusing on alternative
“greener” synthetic protocols using supercritical carbon diox-
ide3–7 or water8 as reaction medium and oxygen as the final
hydrogen acceptor.8 Efficient homogeneous catalysts applied in
water-phase aerobic alcohol oxidation reactions are Ru9- and
Pd-based10–13 with acetate,9 water soluble phenanthrolines11,12
or biquinolines13 as stabilizing ligands. Conversely, well estab-
lished heterogeneous catalysts employed for the same catalytic
process are based on metal nanoparticles (NPs) which are ei-
ther incarcerated in organic cross linked polymers,14,15 anch-
ored onto carbon,16 alumina17 and hydroxyapatite18 or
stabilized by end-functionalized poly(ethylene glycol) (PEG).19–22
The latter polymer, which is cheap, biocompatible and water-
soluble, decreases the polarity of water by reducing
hydrogen bond interactions among water molecules.23 As a
consequence, PEG is a co-solvent increasing the solubility of
organic compounds in water which is mandatory for an effi-
cient catalytic substrate conversion. Furthermore, the easy
end-functionalization of PEG with metal coordinating units
brings about the formation of water-soluble macroligands
which stabilize well-defined molecular metal complexes or
metal NPs.24,25 Moreover, oxygen atoms of the poly(ether)
Herein, we present the synthesis and coordination chemistry
of 4-pyridinemethylene end-capped MeO-PEG (LPy) to Pd(II).
The obtained Pd(II) macrocomplexes gave, upon reduction
with dihydrogen pressure LPy-stabilized Pd-NPs. The Pd(II)-
and Pd(0)-based catalysts were applied to convert a,b-unsat-
urated alcohols into the corresponding carbonyl compounds
in water under aerobic oxidative reaction conditions.
RESULTS AND DISCUSSION
The macroligands of the formula MeO-(CH2CH2O)116CH2CH2OR
with R 5 4-pyridinemethylene (LPy) and benzyl (LBn) were
straightforwardly obtained by a reported mesylation reaction
of MeO-(CH2CH2O)116CH2CH2OH (LOH),28 followed by the reac-
tion of the isolated mesylate with NaH in dry DMF in the pres-
ence of the corresponding alcohol (i.e., 4-hydroxymethyl
pyridine and benzylalcohol) [Scheme 1(a)]. LPy and LBn were
obtained as white solids in 86 and 71% yield, respectively.
The successful end-functionalization of LOH was proved by
1H NMR spectroscopy. As a result, the 1H NMR spectra of
LPy and LBn, acquired in CD2Cl2, showed for the benzylic
hydrogen atoms a singlet at 4.59 and 4.55 ppm, respectively,
which is slightly shifted to lower frequency compared to the
analogous 1H NMR signal of PyCH2OH (4.73 ppm) and
PhCH2OH (4.66 ppm, J 5 6.7 Hz). Moreover, 1H NMR spectra
of LPy and LBn, acquired in DMSO-d6, confirmed the absence
Additional Supporting Information may be found in the online version of this article.
C
V
2013 Wiley Periodicals, Inc.
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SCHEME 1 Syntheses of PEG-based macroligands, Pd(II)-macrocomplexes and Pd-NPs.
of a triplet centered at 4.56 ppm (J 5 5.2 Hz), which was
assigned to the terminal hydroxyl group of LOH 29
The num-
that of LPy (7.29 ppm), while the o-pyridine hydrogen atoms
of 2a showed a chemical shift at 9.64 ppm, which is diagnos-
tic for their interaction with the oxygen atoms of p-toluene-
sulfonate, as proved for related Pd(II) compounds.31,32 UV–
vis spectra, acquired in water, showed for 1a an adsorption
shoulder centered at 225 nm due to the Pd-pyridine interac-
tion, which is lacking in the spectrum of LPy [Fig. 1(a)], while
in 2a the UV-vis contribution originating from MLCT is
masked by the absorption peak of the tosylate anion at 228
nm, as confirmed by the comparison of the UV–vis spectra of
2a and (HLPy)(OTs) [Fig. 1(b)].
.
ber-averaged molar mass (Mn) of LPy and LBn was deter-
mined by 1H NMR spectroscopy (Mn 5 5200 g/mol) [i.e., by
integration of the singlets assigned to (CH2CH2O)n (3.60
ppm) and to the terminal CH3O unit (3.36 ppm)]. In addi-
tion, (GPC)-analyses carried out on the isolated macroligands
in THF showed a Mn of 7750 g/mol and a polydispersity
index (PDI) of 1.05. The molar mass values determined by
1H NMR integration and GPC-analysis are significantly differ-
ent. This experimental fact is related to the use of polysty-
rene to calibrate GPC columns.30
The macrocomplexes 1a and 2a were reduced with dihydro-
gen (15 bar) in solution (i.e., CH2Cl2) at room temperature,
yielding Pd-NPs stabilized by (HLPy)(OAc) (1b) or by LPy
and (HLPy)(OTs) present in a 1:1 molar ratio in 2b. Both lat-
ter pyridinium salts are obtained upon the protonation reac-
tion of the corresponding free acid (i.e., HOAc and p-TsOH,
generated by the reduction of Pd(OAc)2 and Pd(OTs)2) with
LPy. In contrast, Pd-NPs in 2b0 and 3b were obtained by in
situ reductions of 1a in the presence of 2LPy and of
Pd(OAc)2 in the presence of LBn, respectively [Scheme 1(c)].
The isolated macroligand LPy was used to coordinate
Pd(OAc)2 (OAc 5 acetate) in CH2Cl2, yielding the neutral
Pd(II) macrocomplex of the formula trans-[Pd(OAc)2(LPy)2]
(1a) [Scheme 1(b)]. The addition of two molequivalents of
LPy to a CH2Cl2 solution of 1a in the presence of p-toluene-
sulfonic acid (p-TsOH) gave the bis-cationic Pd(II) macrocom-
plex of the formula [Pd(LPy)4](OTs)2 (2a). Both latter
macrocomplexes were isolated in ca. 80% yield, as yellowish
(1a) and beige (2a) powders. The occurred coordination of
1
LPy to Pd(II) in 1a and 2a has been proved by H NMR and
1
UV–vis spectroscopy. As a result, the H NMR spectrum of 1a
UV–vis spectra of 1b and 2b, acquired in water, showed a
significantly lower UV-absorption in the region from 200 to
245 nm, due to the absence of the MLCT contribution,
showed a doublet at 7.38 ppm (i.e., m-H atoms of the pyri-
dine unit) which is shifted to higher frequency compared to
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FIGURE 1 UV–vis spectra acquired in H2O: (a) LPy, 1a and 1b; (b) LPy, (HLPy)(OTs), 2a and 2b.
instead a featureless broad plasmon resonance, typically
found for spherical Pd-NPs, was shown.33 In this context, it
is interesting to note that 1b showed a significantly more
intense plasmon resonance in the range of 280 to 360 nm
compared to 2b, which is due to the higher Pd(0) content in
1b (1.1 wt %) than in 2b (0.51 wt %).
which contains the benzyl-functionalized macroligand LBn
.
Since 2b and 3b were the two extreme cases concerning the
tendency of Pd-NP aggregation, we tested the Pd-NP-stabiliz-
ing ability of the corresponding macroligands. To this pur-
pose, we carried out centrifugation experiments with water
solutions of 2b and 3b under identical experimental condi-
tions. As a result, only from the water solution of 3b sepa-
rated a black solid, which was identified by means of X-ray
powder diffraction (XRPD) analysis as polymer free Pd-NPs
with a mean diameter of 6.0 nm (i.e., determined by the
Debye-Scherrer method34 using the Pd(111) Bragg reflex of
the XRPD pattern of Fig. 3, trace b). (TEM)-analyses com-
bined with the result of the centrifugation experiments cor-
roborate the importance of LPy to control the aggregation
process of NPs and hence their homogeneous distribution
within the polymer matrix.
The Pd-NPs size and their degree of aggregation has been
revealed by (TEM)-analysis. Representative (TEM)-images of
1b and 2b, shown in Figure 2, clearly proved for 2b the
occurrence of spherical and well-separated Pd-NPs with an
average size of 6.33 6 1.40 nm [Fig. 2(b)], whereas 1b
showed non-defined worm-like NP aggregates [Fig. 2(a)].
The degree of Pd-NP aggregation found in 2b0 [Fig. 2(c)] was
comparable to that of 1b [Fig. 2(a)], while the highest degree
of particle aggregation was encountered in 3b [Fig. 2(d)]
FIGURE 2 (TEM)-images of: (a) 1b, (b) 2b, (c) 2b0 and (d) 3b.
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FIGURE 3 XRPD spectra of: (a) 3b and (b) Pd-NPs isolated
from 3b after the centrifugation experiment.
FIGURE 4 TGA of LOH, LPy, LBn, 1b, 2b and 3b carried out in air
To compare the oxidative thermal degradation behavior of
LPy and LBn with that of 1b, 2b and 3b in the solid state,
thermogravimetric analyses (TGA) were carried out in a tem-
perature range from 30 to 700 ꢀC and in the presence of
an air flux.35 The results of these analyses are shown in
Figure 4, which presents the weight loss of all compounds in
an interval from 5% (onset) to 50%.36
atmosphere.
polymer ether chains against oxidative thermal degradation
increases in the order 3b < 1b < 2b as does the distribution
of the Pd-NPs within the polymer matrix (Fig. 4).
The stability of LPy in solution against oxidative degradation
has been tested by applying real oxidative catalytic conditions
(i.e. water solution of LPy was heated at 60 C in the presence
From an analysis of the TGA data emerged that: (a) the ther-
mal behavior of LPy and LBn is comparable to that of com-
mercial LOH, which is in accordance with a random scission
of the poly(ether) chain and not with an unzipping process
of the polymer chain which starts from the polymer chain
ends;37 (b) the presence of Pd-NPs in the polymer matrix
exerts a stabilizing effect on the macroligands against oxida-
tive thermal degradation (Fig. 4), which is superior to that
observed for Fe-NPs present in poly(ether)-based poly-
mers;38 (c) the degree of Pd-NP aggregation, which deter-
mines the contact surface area between the polymer phase
and the NPs, is crucial for the thermal stability of the PEG-
based macroligands. As a consequence, the stability of the
ꢀ
of K2CO3 and 30 bar of air for 14 h) and by analyzing after-
wards the isolated polymers by GPC.36 As a result, LPy proved
not to degrade significantly under the chosen experimental
conditions (i.e., LPy: Mn 5 7750 g/mol, PDI: 1.05; after treat-
ment under oxidative experimental conditions: Mn 5 8300 g/
mol, PDI: 1.08), since the variation of Mn and of the PDI are
within the error range of a typical (GPC)-analysis.
The Pd(II) macrocomplexes 1a and 2a and the macroligand-
stabilized Pd-NPs 1b, 2b and 3b were applied to catalyze
the aerobic oxidation (i.e., 30 bar of air) of selected a,b-un-
saturated alcohols [i.e., cinnamyl alcohol (A), trans22-
SCHEME 2 Syntheses of PEG-based macroligands, Pd(II)-macrocomplexes and Pd-NPs.
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TABLE 1 Aerobic Oxidation of a,b-Unsaturtaed Alcohols in Water Catalyzed by 1a, 1b, 2a, 2b, 2b’, 3b and 4a
Conv (%)/TOFb
Sel (%)c
4 h
Entries
Catalyst
Substrate
2 h
4 h
14 h
2 h
14 h
Pd(II)d
1
2e
1a
1a
2a
2a
4
A
A
A
A
A
57/29
83/41
52/26
87/43
71/36
78/20
78/20
85/21
98
99
96
99
85
99
95
85
3
4e
5
Pd(0)f
6
1b
2b
2b
2b
2b
2b
2b
2b
2b
2b’
3b
3b
A
A
A
A
A
A
B
C
D
A
A
A
30/200
29/193
30/200
27/180
76/145
99/189
98/187
88/177
93
90
77
98
91
89
75
98
7
8g
9e
10h
11h,i
12
13
14
15
16
17e
98/127
97126
95
94
27/180
4/27
88/168
14/27
100
100
99
100
100
98
29/193
40/267
30/200
21/140
90/171
79/150
70/133
61/116
91
93
94
88
99
99
a
f
Catalytic conditions: T (60 ꢀC), water (5.0 mL), p(air) (30 bar).
TOF expressed in mmol product 3 (mmol precatalyst 3 h)21
Chemoselectivity referred to the a,b-unsaturated carbonyl compound.
Pd amount (0.01 mmol), substrate (1.00 mmol).
Addition of K2CO3 (0.25 mmol).
Pd amount (0.003 mmol), substrate (8.00 mmol).
p(air) (1 bar).
b
c
g
h
i
.
Substrate (0.778 mmol).
d
e
Fourth catalytic cycle.
methylcinnamyl alcohol (B), geraniol (C) and 2-cyclohexen-1-
ol (D)] in water, giving the corresponding a,b-unsaturated
carbonyl compound as major product (Scheme 2). Aromatic
and aliphatic alcohols showed under identical catalytic con-
ditions a low conversion to the corresponding carbonyl com-
pound of around 10–20% after a reaction time of 14 h.
accordance with the formation of Pd-black in the course of
the catalytic reaction (Table 1, entries 1–5).
The Pd-NPs-based catalysts showed a cinnamyl alcohol con-
version which is comparable for a reaction time of 4 h (Table
1, entries 6, 7, 15, and 16) and in the range of activity found
for a Pt/Bi-catalyst supported on alumina (i.e., TOF 144
h21), which is one of the most active and chemoselective
heterogeneous catalyst for the oxidation of the latter sub-
strate in water in the presence of a detergent.18,39
The performance of the catalysts was screened by using cin-
namyl alcohol as substrate. The amount of catalyst used for
the oxidative reactions was based on its Pd content which
was determined by ICP-OES.
By carrying out analogous catalytic reactions lasting 14 h,
catalysts suffered from deactivation, except 2b, which
showed the highest longevity, regardless of the substrate
employed (Table 1, entries 7, 12, 13, and 14). (TEM)-images,
acquired on recovered 2b after catalytic reactions lasting 4 h
[Fig. 5(a)] and 14 h [Fig. 5(b)], showed a size distribution
for the Pd-NPs which is comparable to that of as-synthesized
2b (i.e., 6.33 6 1.40 nm (as-synthesized), 6.11 6 0.93 nm (af-
ter 4 h catalysis) and 5.65 6 1.38 nm (after 14 h catalysis)),
corroborating hence the catalyst longevity of 2b.
As far as the catalytic activity is concerned, the Pd-NPs-based
catalysts showed an almost ten times higher activity com-
pared to the Pd(II)-based macrocomplexes. As a conse-
quence, a substrate to Pd molar ratio of 2600 was applied in
1b, 2b, 2b0 and 3b-catalyzed reactions, while 1a and 2a
gave acceptable substrate conversions only with a substrate
to Pd molar ratio of 100 (Table 1, entries 6,7, 15 and 16 vs.
entries 1 and 3). The performance of the latter Pd(II)-cata-
lysts in terms of activity is comparable to that of Pd(OAc)2-
(bathoneocuproine-disulfonate-disodium) (4), which is
known to be one of the most efficient Pd(II) catalysts for the
aerobic alcohol oxidation in water.12 The activity of all
Pd(II)-based catalysts decreased with time, which is in
The rather low conversion of geraniol (substrate C) by 2b is
due to the weak interaction of the aliphatic carbon backbone
with the surface of the Pd-NPs, which is a prerequisite for
the oxidative addition of the alcohol functionality (i.e.,
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FIGURE 5 (TEM)-images of 2b: (a) after 4 h, (b) after 14 h of catalysis and (c) after 4 h catalysis in the presence of K2CO3.
formation of the Pd-alcoholate as the key step in the alcohol
oxidation reaction) at a low coordinated Pd atom located at
the edge or corner of the Pd-NPs.40
minor products is the result of a competition reaction between
either substrate or product and oxygen to accept dihydrogen
from Pd-NPs. The higher the adsorption probability of sub-
strate and product on the Pd-NP surface, the lower the chemo-
selectivity for the unsaturated carbonyl compound.39,41
As far as the chemoselectivity of the catalytic oxidation reac-
tions is concerned, substrates B and C were converted into the
corresponding a,b-unsaturated carbonyl compound with a che-
moselectivity of 100%. Conversely, the oxidation of substrates
A and D gave the corresponding a,b-unsaturated carbonyl com-
pound as major product (Scheme 2). The formation of the cor-
responding saturated alcohols and carbonyl compounds as
As a consequence, catalytic reactions carried out in the pres-
ence of only 1bar of air gave a very low chemoslectivity (i.e.,
77%) for the a,b-unsaturated carbonyl compound, while the
substrate conversion remained comparable to that obtained in
the presence of 30 bar of air (Table 1, entry 8 vs. 7). This latter
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experimental result is in accordance with the role of oxygen in
these catalytic reactions as dihydrogen acceptor and not as
scavenger of strongly Pd-NP-adsorbing compounds, such as
CO, which may be formed by decarbonylation reactions.41
and dissolved in anhydrous DMF (80.0 mL) with vigorous
stirring at 60 ꢀC under a N2 atmosphere. After a reaction
time of 4 h, the orange reaction mixture was cooled to room
temperature and filtered through a sand path and then
through a paper filter to obtain a clear orange solution. After
cooling the latter solution to 0 ꢀC, cold 2-propanol (100.0
mL) was added, obtaining a pale-yellow precipitate which
was separated from solution by filtration on a Buchner filter
and successively washed with cold 2-propanol (50.0 mL)
and n-hexane (20.0 mL). The white solid obtained was dried
under vacuum to constant weight. Yield: 7.96 g (86%).
Mn 5 5200 g/mol. Mn, determined by 1H NMR spectroscopy,
refers to the polymer chain.
The chemoselectivity of the oxidation reactions with substrate
A was positively altered in the presence of a Broensted base
such as K2CO3, regardless of the catalyst employed (Table 1).
As a result, the associated catalysts’ activity increased with 1a
and 2a, and decreased with 2b and 3b compared to catalytic
reactions carried out in the absence of base. These catalytic
results may be rationalized with the role of K2CO3 as an
external base, which stabilizes the Pd(II) catalyst in case of
1a and 2a. The negative effect of K2CO3 on the catalytic activ-
ity of 2b (Table 1, entry 9 vs. 7) and 3b (Table 1, entry 17 vs.
16) might be ascribed to the coverage of the Pd-NP surface
with (OH)-groups which hamper the access of substrate and
product to the reactive metal atoms. As a consequence, the
chemoselectivity of the oxidation reaction is increased on the
expense of catalytic activity.41 It is important to mention at
this point, that the mean size of the Pd-NPs of 2b did not
change significantly during catalysis carried out in the pres-
ence of K2CO3 for 4 h (i.e., 6.266 1.51 nm) as shown by the
corresponding (TEM)-image [Fig. 5(c)].
Synthesis of (HLPy)(OTs)
In a round-bottom flask LPy (500.0 mg, 0.100 mmol) and p-
TsOH monohydrate (37.0 mg, 0.200 mmol) were dissolved
in water (5.0 mL) and stirred at room temperature for 3 h.
Then 2-propanol (20.0 mL) and cold Et2O (40.0 mL) were
added to the clear colorless reaction mixtures, causing the
precipitation of the polymer product as a colorless viscous
liquid, which solidified after treating it with cold Et2O (20.0
mL). The obtained solid was dried by vacuum at room tem-
perature giving a waxy product. Yield: 451.0 mg (88%).
Synthesis of LBn
Recycling experiments were carried out with 2b in the pres-
ence of cinnamyl alcohol at a substrate to Pd molar ratio of
260, showing a constant substrate conversion (i.e., 98%) and
chemoselectivity (i.e., 95%) for four consecutive catalytic
runs (Table 1, entries 10 and 11). Importantly, the organic
phase containing product, not converted substrate and
diethyl ether (i.e., extraction medium) exhibited a Pd content
of ca. 25 ppb (i.e., determined by ICP-OES) for each catalytic
cycle.
Mesylated LOH (1.14 g, 0.220 mmol), benzyl alcohol (45.5 lL,
0.440 mmol) and NaH (11.0 mg, 0.440 mmol) were placed in a
round-bottom flask and dissolved in anhydrous DMF (10.0 mL)
ꢀ
under vigorous stirring at 60 C under a N2 atmosphere. After
a reaction time of 4 h, the pale-yellow reaction mixture was
cooled to room temperature and filtered successively through a
sand path and filter paper to give a clear colorless solution. Af-
ter cooling the latter solution to 0 ꢀC, cold 2-propanol (50.0
mL) was added and the obtained white precipitate was sepa-
rated from solution by filtration with a Buchner filter and
washed with cold 2-propanol (20.0 mL) and n-hexane (10.0
mL). The obtained white waxy solid was dried by vacuum at
room temperature to constant weight. Yield: 771.0mg (71%).
Mn (1H NMR)5 5200 g/mol, refers to the polymer chain.
EXPERIMENTAL
Materials
LOH (Mn 5 5,200 g/mol), 4-hydroxymethyl pyridine, benzyl
alcohol, n-trioctyl amine; Pd(OAc)2, NaH, mesyl chloride,
Na2CO3 and p-TsOH monohydrate were purchased from
Sigma-Aldrich. Compound 4 was prepared as described by a
literature procedure.12 4-Hydroxymethyl pyridine and NaH
were utilized and stored under dry N2 atmosphere. LOH was
Synthesis of 1a
LPy (4.00 g, 0.750 mmol) and Pd(OAc)2 (87.0 mg, 0.390
mmol) were dissolved in CH2Cl2 (40.0 mL) and the resulting
turbid solution was stirred in air for 24 h. Afterwards, the
obtained solution was filtered through a paper filter and to
the clear yellow solution was added n-hexane (50.0 mL),
which caused the precipitation of a brown viscous oil. The
supernatant was decanted off and the obtained brown oil
was washed twice with n-hexane (30.0 mL) to give a brown
waxy product which was dried by vacuum to constant
weight yielding a beige solid: Yield: 3.33 g (82%). Pd content
(ICP-OES): 0.96 wt %.
ꢀ
dried under vacuum at 70 C for 1 h before use. CH2Cl2, sta-
bilized with amylene, was purchased from Panreac; DMF (an-
hydrous) and THF (HPLC-grade and BHT-stabilized) were
purchased from VWR international; 2-propanol (purum) was
purchased from Fluka Chemika; n-hexane (puriss.) and Et2O
(puriss. p.a. and stabilized with BHT) were purchased from
Sigma-Aldrich. All solvents were used without further purifi-
cation and anhydrous DMF was handled under a dry N2 flux.
Synthesis
Synthesis of LPy
Synthesis of 2a
Mesylated LOH (9.10 g, 1.750 mmol), which was synthesized
by following a reported procedure,29 4-hydroxymethyl pyri-
dine (384.0 mg, 3.500 mmol) and NaH (84.0 mg, 3.500
mmol) were weighted in a round-bottom flask (250.0 mL)
1a (910.5 mg, 0.084 mmol) and LPy (891.7 mg, 0.168 mmol)
were dissolved in CH2Cl2 (20.0 mL) under stirring by means
of a magnetic stirring bar. To the obtained clear amber col-
ored solution was added p-TsOH monohydrate (32.0 mg,
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0.168 mmol). After a reaction time of 24 h the solution was
successively filtered throughꢀa HPLC cartridge (0.2 lm PTFE
membrane) and cooled to 0 C. Then cold 2-propanol (100.0
mL) was added to the latter solution and an off-white slurry
mmol) and water (5.0 mL) at room temperature. The auto-
clave was then sealed, heated to 60 ꢀC by means of an oil
bath and charged with air (30 bar). After a reaction time of
2 h the autoclave was cooled to room temperature, the air
pressure released, and the suspension transferred into a sep-
aration funnel. The water phase was extracted with diethyl
ether (3 3 8.0 mL). Afterwards, the recovered water phase
was placed again in the autoclave and cinnamyl alcohol
(100.0 lL, 0.778 mmol) added. The second, third, and fourth
catalytic cycle were carried out as described above.
ꢀ
was formed which was kept at 0 C for 1 h. Afterwards, the
obtained solid was successively filtered, washed with cold 2-
propanol (20.0 mL) and n-hexane (20.0 mL) and dried by
vacuum at room temperature. Yield: 1.47 g (81%). Pd con-
tent (ICP-OES): 0.47 wt %.
Synthesis of 1b, 2b, 2b0, and 3b
Centrifugation Experiments with Water Solutions of 2b
and 3b
1a and 2a (400.0 mg) were dissolved in deareated CH2Cl2
(30.0 mL) at room temperature. Afterwards the clear solu-
tions were transferred under nitrogen into a stainless steel
autoclave (80.0 mL). The autoclave was charged with dihy-
drogen (15 bar) at room temperature and stirring started.
After a reaction time of 1 h, the autoclave was successively
depressurized, the black solutions transferred into a round
bottom flask and concentrated to dryness, giving 1b and 2b
as black solids. Pd content (ICP-OES): 1.1 wt % (1b) and
0.51 wt % (2b).
Water solutions of 2b and 3b with a concentration of
10 mg/mL were subjected to centrifugation at 6000 rpm for
6 h. During this time period only in the case of 3b a black
powder separated from the corresponding water solution.
The black solid was successively separated from solution,
washed with deionized water, dried by vacuum at room tem-
perature and analyzed by XRPD. Even after doubling the cen-
trifugation time (i.e., 12 h at 6000 rpm) no precipitate
separated from water solution of 2b.
The in situ syntheses of 2b0and 3b were carried out as
follows:
Instruments and Characterization
UV–vis spectra were recorded on a Varian-Cary 4000 UV–vis
spectrophotometer using a quartz cuvette with an optical
path length of 10 mm. The measurements were carried out
on deionized water solutions of the polymer compounds
using a concentration of 0.3 mg/mL.
2b0: 1a (400.0 mg, 0.037 mmol) was dissolved in deareated
CH2Cl2 (30.0 mL) at room temperature. Afterwards LPy
(392.8 mg, 0.074 mmol) was added to the latter solution,
which was transferred into a stainless steel autoclave (80.0
mL) under nitrogen and pressurized with dihydrogen (15
bar). The synthesis proceeded as described above for 1b and
2b. Pd content (ICP-OES): 1.2 wt %.
(TEM)-analyses were carried out on a TEM PHILIPS CM 12,
equipped with an OLYMPUS Megaview G2 camera and using
an accelerating voltage of 80 keV. Samples for (TEM)-analy-
ses were prepared by depositing a drop of the water solu-
tion of the corresponding sample on a holey film of a 300
mesh Cu grid, followed by the evaporation of the solvent.
3b: LBn (400.0 mg, 0.075 mmol) and Pd(OAc)2 (4.3 mg,
0.019 mmol) were dissolved in deareated CH2Cl2 (30.0 mL)
at room temperature. The obtained yellow solution was
stirred for half an hour and then transferred into a stainless
steel autoclave (80 mL), which was pressurized with dihy-
drogen (15 bar). The synthesis proceeded as describe above
for 1b and 2b. Pd content (ICP-OES): 1.1 wt %.
TGA were performed under an air flux of 200 mL/min by
using a SEIKO TG/DTA 7200 instrument calibrated within as
reference sample. Measurements were carried out in the
tempeꢀrature interval from 30 to 700 ꢀC with a heating rate
of 10 C/min.
Aerobic Alcohol Oxidation Reactions Catalyzed by
1a, 2a, 1b, 2b, 2b0, and 3b
A stainless steel autoclave (80.0 mL) was successively charged
with water (5.0 mL), the desired amount of catalyst, substrate
and where used with K2CO3. Then, the autoclave was sealed
XRPD spectra were acquired on a X’Pert PRO (PANalytical)
powder diffractometer using Cu Ka radiation (k 5 1.541874
Å) and a parabolic MPD mirror for Cu radiation. Spectra
were acquired in a 2H range from 10 to 60ꢀ with a step size
of 0.1050ꢀ.
ꢀ
and heated to 60 C by means of an oil bath. Once the latter
reaction temperature was reached the autoclave was charged
with air (30 bar) and magnetic stirring was started. After the
desired reaction time, the autoclave was cooled to room tem-
perature by means of an ice-water bath and the air pressure
released. The crude reaction mixture was transferred into a
separation funnel and the water suspension was extracted
with diethyl ether (3 3 8.0 mL). The combined organic phases
were subjected to (GC)- and (GC-MS)-analyses.
(GC)-analyses were performed with a Shimadzu 2010 gas
chromatograph equipped with a flame ionization detector
and a 30 m (0.25 mm i.d., 0.25 lm film thickness) VF-
WAXms capillary column. (GC-MS)-analyses were performed
with a Shimadzu QP 5000 apparatus, equipped with a 30 m
(0.32 mm i.d., 0.50 lm film thickness) CP-WAX 52 CB WCOT-
fused silica column.
Recycling Experiments with 2b
A stainless steel autoclave (80.0 mL) was charged with 2b
(65.0 mg, 0.003 mmol), cinnamyl alcohol (100.0 mL, 0.778
(ICP-OES)-analyses were carried out on an ICP apparatus of
the type Perkin Elmer Optima 2000 OES DV.
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15 H. Miyamura, R. Matsubara, S. Kobayashi, Chem. Commun.
2008, 44, 2031–2033.
CONCLUSIONS
The PEG-based macroligand LPy was straightforwardly syn-
thesized by pyridine end-functionalization of PEG mono-
methyl ether and employed to stabilize Pd(II) in a neutral
(1a) and bis-cationic (2a) macrocomplex, characterized by a
LPy to Pd molar ratio of 2 and 4, respectively. Unlike 1a, 2a
revealed to be a suitable molecular precursor for the genera-
tion of spherical and well-dispersed Pd-NPs (2b) with a
mean diameter of 6.33 6 1.40 nm. The homogeneous distri-
bution of the Pd-NPs in 2b exerted a significant stabilizing
effect on the poly(ether) chain of LPy against its oxidative
thermal degradation. From a screening of the Pd(II)- and
Pd-NP-based catalysts in the aerobic oxidation of a,b-
unsaturated alcohols in water emerged 2b as the most stable
catalyst, showing TOF-values up to 2002h and high chemose-
lectivity even for prolonged reaction times. Consequently, 2b
proved to be an efficient recyclable catalyst releasing a low
amount of Pd (ca. 25 ppb per catalytic cycle) into the or-
ganic phase.
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