Facile Light-Mediated Preparation of Small Polymer-Coated Palladium-Nanoparticles and Their Application as Catalysts for Alkyne Semi-Hydrogenation

Article abstract of DOI:10.1002/chem.201603297

A facile light-mediated preparation of small palladium nanoparticles (PdNPs) with a diameter of 1.3 nm and low dispersity by using low-priced and readily prepared photoactive polymers is presented. These polymers act as reagents for the photochemical reduction of Pd ions and they are also stabilizers for the PdNPs generated in situ. The PdNP–polymer hybrid materials prepared by this reliable approach are efficient hydrogenation catalysts that show high activity and Z-selectivity in the semi-hydrogenation of alkynes. These PdNP–catalyst hybrid materials can be readily recycled and reused up to five times.

Full text of DOI:10.1002/chem.201603297

DOI: 10.1002/chem.201603297
Full Paper
&
Heterogeneous Catalysis
Facile Light-Mediated Preparation of Small Polymer-Coated
Palladium-Nanoparticles and Their Application as Catalysts for
Alkyne Semi-Hydrogenation
Florian Mꢀsing,[a] Xi Wang,[a] Harald Nꢁsse,[b] Jꢁrgen Klingauf,[b] and Armido Studer*[a]
Abstract: A facile light-mediated preparation of small palla-
dium nanoparticles (PdNPs) with a diameter of 1.3 nm and
low dispersity by using low-priced and readily prepared pho-
toactive polymers is presented. These polymers act as re-
agents for the photochemical reduction of Pd ions and they
are also stabilizers for the PdNPs generated in situ. The
PdNP–polymer hybrid materials prepared by this reliable ap-
proach are efficient hydrogenation catalysts that show high
activity and Z-selectivity in the semi-hydrogenation of al-
kynes. These PdNP–catalyst hybrid materials can be readily
recycled and reused up to five times.
Introduction
lent selectivity). However, in many cases a complex multistep
preparation of the PdNPs including the use of non-commercial
chemicals and harsh reaction conditions is required, which
may hamper their future industrial applications.[4] In contrast,
Lipshutz et al. recently reported a simple PdNP/nanomicelle
system for semi-hydrogenation of alkynes to Z-alkenes in
water.[2j] The PdNP/nanomicelle catalyst charged with H2 was
elegantly prepared in situ by addition of NaBH4 and Pd(OAc)2
to an aqueous solution of nanomicelles made from the com-
mercially available surfactant TPGS-750-M. The Lipshutz cata-
lyst provided high yields of Z-alkenes and could be successfully
recycled several times. However, two equivalents of the addi-
tive LiCl are necessary to obtain good Z-selectivities.[2j]
Metal nanoparticles have found wide applications in catalysis
since they show high activity and high chemoselectivity in vari-
ous reactions.[1] Moreover, their ready recyclability allows for
the development of environmentally friendly processes.[1] For
instance, Pd, Fe, Ni, or Au nanocatalysts have been successfully
applied to the selective semi-hydrogenation of terminal and in-
ternal alkynes, using for example H2, ammonium formate, or
amine–borane complexes as reductants.[2] Different inorganic
(for example, CeO2, TiO2, Cu2O, SiO2) or organic materials (ionic
liquids, polymers, surfactants) have been used as supports for
these transition-metal nanocatalysts.[2] Importantly, these novel
hybrids may replace the well-established Lindlar catalyst in
future. It is well known that the Lindlar catalyst suffers from
a number of drawbacks: first, toxic Pb(OAc)2 and large
amounts of quinoline are required as catalyst poison to sup-
press over-hydrogenation of the targeted alkenes to the corre-
sponding alkanes. Second, reactions run with the Lindlar cata-
lyst often show problems with respect to Z/E-isomerization,
over-hydrogenation, irreproducibility, and a limited substrate
scope.[2i,j,3] Pd nanocatalysts, as compared to Fe-, Ni-, or Au-
based materials, generally reveal higher activity in the semi-hy-
drogenation of alkynes under very mild reaction conditions
(low catalyst loading, low temperature, low H2 pressure, excel-
The use of light is a powerful tool to drive organic reactions,
because light is readily available, cheap, and easy to apply in
industrial processes.[5] In a series of papers, Scaiano et al. dis-
closed the potential of the Norrish Type I photoreaction for
nanoparticle preparation by using the commercially available
photoinitiator Irgacure-2959.[6] Along these lines, we recently
introduced a method to prepare polymer coated AuNPs by
using photoactive homo- and copolymers for radical-mediated
photochemical AuNP synthesis.[7] These well-defined photoac-
tive styrene type homo- and copolymers bear photocleavable
a-hydroxyalkyl ketone (HAK) substituents that can be selective-
ly cleaved by the Norrish Type I photoreaction under irradia-
tion with UV light. Highly reductive ketyl radicals are generated
in the photoreaction that are able to reduce gold salts to
Au0.[7] However, the photoactive styrene type homo- and co-
polymers were prepared by using controlled radical polymeri-
zation, and the monomers used in these initial studies had to
be laboriously prepared in several steps.[7] Both the rather ex-
pensive controlled polymerization techniques and the multi-
step preparation of styrene monomers increase the price of
the photoactive polymers, which may prevent the application
of this method for preparation of polymer-coated NPs in indus-
try.
[a] F. Mꢀsing, Dr. X. Wang, Prof. Dr. A. Studer
Institute of Organic Chemistry
Westfꢀlische Wilhelms-Universitꢀt
Corrensstrasse 40, 48149 Mꢁnster (Germany)
E-mail: studer@uni-muenster.de
[b] H. Nꢁsse, Prof. Dr. J. Klingauf
Institute of Medical Physics and Biophysics
Westfꢀlische Wilhelms-Universitꢀt
Robert-Koch-Strasse 31, 48149 Mꢁnster (Germany)
Supporting information for this article is available on the WWW under:
http://dx.doi.org/10.1002/chem.201603297
Chem. Eur. J. 2016, 22, 1 – 6
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Therefore, we decided to focus on new low-cost processes
for the preparation of polymer-coated metal nanoparticles and
on their application in catalysis. Herein we present the ready
preparation of an acrylate copolymer bearing photoactive
HAK-groups as ester substituents and its successful application
in photochemical preparation of remarkably small PdNPs
(1.3 nm) (Scheme 1). The low-cost polymer plays a dual role in
NP formation: It first acts as an electron source for the photo-
reduction of Pd ions and then as a stabilizer for the PdNPs
generated in situ. We will show that these polymer-coated
PdNPs are efficient and Z-selective alkyne semi-hydrogenation
catalysts.
no)ethyl acrylate] (poly(HMPA-co-DAEA), 2) (HMPA/DAEA-ratio
in the copolymer ca. 1:2; Mn =4 900 gmolÀ1; PDI=2.1) where
the thioether entity is replaced by a dimethylamino functionali-
ty and the homopolymer poly(HMPA), 3 (Mn =8 800 gmolÀ1
PDI=2.0) were prepared for comparison.
;
We next investigated the photomediated PdNP synthesis
using copolymer 1 under an argon atmosphere in DMF, and ir-
radiation was performed at l=254 nm in a photoreactor (see
the Supporting Information). Irradiation of a mixture of 1 in
the presence of Pd(OAc)2 (ratio of photoactive entities (HMPA)/
Pd(OAc)2 =10:1) for 10 min at room temperature led to the for-
mation of a dark-brown solution indicating the successful syn-
thesis of PdNPs. TEM measurements revealed the formation of
small and monodisperse PdNPs (Pd@1*) with a diameter of
1.3Æ0.2 nm. Figure 1a shows the TEM image obtained for the
well-dispersed PdNPs.
Figure 1. TEM-images of a) Pd@1*, b) Pd@3*, and c) Pd@1* catalyst after 6
cycles.
DLS measurements of the Pd–polymer hybrid (8.4Æ2.3 nm)
indicate that the PdNPs are covered by a thin circa 3.6 nm
thick polymer layer (see the Supporting Information). The oxi-
dation state of the PdNPs was investigated by XPS. Only the
two signals with characteristic binding energies for Pd0 were
detected showing full photochemical reduction of the Pd salt
to Pd0 (see the Supporting Information).[10] PdNPs were readily
isolated by simple evaporation of the solvent under vacuum
and were stored under argon atmosphere at room tempera-
ture without aggregation or decomposition. The brown PdNP
powder obtained after solvent evaporation was also subjected
to thermogravimetric analysis revealing a Pd loading of
20 wt% in the Pd–polymer hybrid material (see the Supporting
Information). 1H NMR analysis of the Pd–polymer hybrids
showed that about 50% of the HAK groups were photochemi-
cally converted within 10 min of UV irradiation. The ketone
was converted into a dimethylamide moiety by photochemical
CÀC bond cleavage and subsequent amidation with HNMe2
derived from DMF (Scheme 2).[7,11] Upon increasing the irradia-
tion time to 30 min, the HAK entities were fully converted into
the corresponding carboxylic amides to afford the fully modi-
fied copolymer 1*. The PdNP diameter did not change upon
prolonging the irradiation time (see the Supporting Informa-
tion); however, DLS analysis indicated a slightly decrease of
the PdNP hybrid diameter (5.7Æ1.9 nm). The PdNPs derived
from the copolymer 2 (Pd@2*, 1.6Æ0.3 nm) and from homo-
polymer poly(HMPA) 3 (Pd@3*, 2.5Æ0.6 nm; Figure 1b) were
prepared in analogy to Pd@1* (see the Supporting Informa-
tion).
Scheme 1. a) Novel copolymers for the light-mediated preparation of small
PdNPs; b) application of PdNPs as catalysts for the alkyne semi-hydrogena-
tion.
Results and Discussion
Synthesis and characterization of Pd nanoparticles
We decided to prepare the statistical acrylate copolymer
1 bearing photoactive HAK entities and thioether units, be-
cause thioethers are known to interact with the surface of
PdNPs, thereby stabilizing them towards aggregation.[8] More-
over, the thioether moiety may act as a Pd catalyst poison,
leading to modulation of its reactivity.[8] The acrylate mono-
mers 2-(4-(2-hydroxy-2-methylpropanoyl)phenoxy)ethyl acry-
late (HMPA) and 2-(methylthio)ethyl acrylate (MTEA) were read-
ily obtained in one step by esterification of acrylic acid or
acryloyl chloride with two different commercially available al-
cohols.[9] The photoactive 2-hydroxy-1-(4-(2-hydroxyethoxy)-
phenyl)-2-methylpropan-1-one (I-2959) is a cheap and well-es-
tablished commercial photoinitiator and 2-(methylthio)ethan-1-
ol can be also purchased at low cost. AIBN-initiated copolymer-
ization of a 1:2 HMPA/MTEA mixture at 608C for 6 h in THF
gave the statistical copolymer poly[2-(4-(2-hydroxy-2-methyl-
propanoyl)phenoxy)ethyl acrylate-co-2-(methylthio)ethyl acry-
late] (poly(HMPA-co-MTEA), 1) in 81% yield of isolated product
(HMPA/MTEA ratio in the copolymer=1:2; Mn =16 300 gmolÀ1
PDI=2.0). Along with 1, the copolymer poly[2-(4-(2-hydroxy-2-
methylpropanoyl)phenoxy)ethyl acrylate-co-2-(dimethylami-
;
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Table 1. Reaction optimization and catalyst recycling.
Entry PdNP
Cycle t [h] Conv. 7a [%][a] 8a [%][a] (Z/E)[a] 9a [%][a]
1[b]
2[c]
3
Pd@1*
Pd@1*
Pd@1*
Pd@1*
Pd@1*
Pd@1*
Pd@2*
Pd@3*
Pd@1*
Pd@1*
Pd@1*
Pd@1*
Pd@1*
–
–
–
–
–
–
–
–
2
3
4
5
6
5
3
2.5
2.5
4
4.5
1.5
1.5
2.5
2.5
2.5
2.5
5
69
68 (95:5)
97 (96:4)
99 (97:3)
99 (97:3)
97 (97:3)
99 (97:3)
67 (91:9)
63 (91:9)
96 (97:3)
98 (97:3)
96 (93:7)
98 (97:3)
83 (97:3)
<1
2
<1
<1
2
<1
17
25
<1
1
>99
>99
>99
>99
>99
>99
>99
>96
>99
>99
>99
84
4[d]
5
6[e]
7
8
9
10
11
12
13
Scheme 2. Proposed mechanism for the photochemical generation of Pd-
nanoparticles Pd@1* using copolymer 1 and light in DMF.
3
1
<1
The following mechanism for PdNP formation is suggested.
During UV irradiation, the photoactive HAK-groups in 1 under-
go Norrish Type I CÀC bond homolysis to generate reductive
ketyl radicals 4 along with polybenzoyl radicals 5.[12] The ketyl
radicals 4 reduce Pd ions under formation of acetone as a by-
product.[7] The concomitantly generated polybenzoyl radicals 5
are trapped by dimethylamine,[13] which is derived from DMF.
The dimethylamine acyl radical adduct 6 also acts as a one-
electron reductant to give the dimethylamide functionality
upon oxidation.[14] The photochemically modified copolymer
1* eventually stabilizes the PdNPs generated in situ (Pd@1*).
Importantly, isolated Pd@1* showed high solubility and stabili-
ty in various organic solvents with different polarity, such as
acetone, CHCl3, DCM, DMF, DMSO, EtOAc, and THF, providing
flexibility for applications in catalysis.
[a] Determined by GC-FID using an internal standard; signals identified by
GC-MS. [b] Reaction in DCM. [c] Reaction in THF. [d] Pd@1* prepared by
30 min irradiation. [e] HMPA/MTEA-ratio in the copolymer=1:5; Mn =
27700 gmolÀ1; PDI=2.0.
duced the activity of the resulting Pd@1* catalyst (Table 1,
entry 6); however, the Z/E selectivity remained unaltered. This
observation can be explained by a reduced diffusion rate of
the substrate 7a through the larger protecting polymer layer
to the active PdNP site of the catalyst. The amino-functional-
ized hybrid catalyst Pd@2* was more active than Pd@1* and
complete conversion was achieved within 1.5 h (Table 1,
entry 7). However, over-hydrogenation product 9a was formed
in 17% yield along with the targeted alkene 8a (67%), and
semi-hydrogenation occurred in lower selectivity (91:9). A simi-
lar result was achieved with homopolymer-derived nanoparti-
cles (Pd@3*) (Table 1, entry 8), clearly revealing the importance
of the thioether moieties in these polymer coated PdNPs for
modulating their activity and selectivity.
Catalytic semi-hydrogenation of internal and terminal al-
kynes
As a first test reaction, we studied the semi-hydrogenation of
ethyl 3-phenylpropiolate (7a) to give Z/E-alkenes 8a with
Pd@1* as a catalyst at 408C under an H2 atmosphere (1 atm) in
different solvents (Table 1). In DCM, ethyl 3-phenylacrylate 8a
was formed in 68% yield with high Z-selectivity (Z/E=95:5)
after 5 h (Table 1, entry 1). Reaction was faster in THF. Quantita-
tive conversion was achieved in 3 h and 8a was formed in
97% yield with high Z-selectivity (96:4) (Table 1, entry 2). How-
ever, we identified 2% of the over-hydrogenation product 9a.
Result was improved upon switching to DMF (Table 1,
entry 3). Alkene 8a was formed in excellent yield (99%) and Z-
selectivity (97:3) in 2.5 h. The over-hydrogenated 9a was only
observed in traces. Extending the reaction time to 4 h did not
alter the Z/E ratio, showing that product isomerization does
not occur under the applied conditions, and that the 97:3 ratio
is the kinetic product selectivity (Table 1, entry 5). However, we
started to observe 9a in 2% yield. Notably, Pd@1* prepared
under 10 min or 30 min irradiation provided the same result in
the semi-hydrogenation of 7a (Table 1, compare entries 3 and
4). Increasing the amount of thioether moieties in copolymer
1 (ratio of HMPA/MTEA=1:5; Mn =27 700 gmolÀ1; PDI=2.0) re-
Under optimized conditions, recycling experiments using
the Pd@1* catalyst were performed. The PdNP hybrid material
was readily recovered after each run by the following proce-
dure: DMF and the reduction products were separated by
simple distillation from the catalyst. The Pd@1* catalyst re-
mained in the reaction vessel as a brown solid which was then
applied in the next cycle by simply charging the vessel with
alkyne 7a, DMF, and H2. Using this procedure, the catalyst was
recycled five times to give the Z-alkene 8a in good to excel-
lent yields (Table 1, entries 9–13). However, in the sixth run the
activity of the catalyst decreased significantly (Table 1,
entry 13). We analyzed the catalyst after 6 cycles by TEM (3.5Æ
1.0 nm) and DLS (10.7Æ7.7 nm), revealing a significant increase
of the Pd diameter from 1.3 to 3.5 nm (Figure 1a,c). The ripen-
ing of Pd@1* to larger nanoparticles may have an influence on
the decrease of its catalytic activity after the fifth cycle.[15]
Pd@1* catalyst recovery can also be achieved by simple pre-
cipitation by treatment of the reaction mixture with pentane
while the organic products stay in the liquid phase (see the
Supporting Information). Using this catalyst recycling proce-
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dure, Pd@1* was successfully reused over four cycles, provid-
ing the desired alkene 8a in excellent yields (96–99%) and Z-
selectivity (97:3; 2.5–4 h reaction time; see the Supporting In-
formation). However, in the fifth run the catalytic activity signif-
icantly decreased and reaction time had to be increased to
obtain complete conversion. Alkene 8a was obtained in 95%
yield with high Z-selectivity (96:4) after 9 h. To study possible
reasons for catalyst ripening and for the decreased catalytic ac-
tivity after the fourth run, the hybrid system was studied with
respect to Pd leaching. To this end, the solution obtained after
catalyst precipitation was analyzed regarding the amount of
leached Pd by total reflection X-ray fluorescence (TXRF).[16] In
the first three catalytic cycles, the amount of leached Pd was
found to be below the detection limit of 8 ppm. To determine
the catalytic active species in our Pd@1* catalyst system we
also conducted a poisoning experiment. The addition of Hg0
to the reaction mixture led to complete inhibition of the semi-
hydrogenation of alkyne 7a, indicating that a heterogeneous
Pd species is responsible for the semi-hydrogenation of alkyne
7a to alkene 8a.[17] The reaction solution obtained after Pd@1*
catalyst precipitation, which according to the TXRF analysis
contains less than 8 ppm Pd, was found to be inactive in the
semi-hydrogenation of alkyne 7a. This result indicates that the
Pd@1* hybrid is the active catalyst and that leached Pd is inac-
tive in this reaction.
Next, the substrate scope was investigated with alkynes 7b–
t (Scheme 3). Reactions were monitored by GC, and hydroge-
nation time was adapted according to the reactivity of the
given alkyne. All substrates 7b–t were smoothly converted
into the corresponding alkenes 8b–t in high conversions in
1 to 9 h. Arylalkyl alkynes turned out to be good substrates,
and for the arylpropyl alkynes the aryl group was systematical-
ly varied. The 3-pyridyl congener 7b reacted in excellent yield
(>98%) and Z-selectivity (97:3) to give 8b. The biphenyl deriv-
ative 7c was also tolerated to give the corresponding alkene
8c in good yield (>95%) and Z-selectivity (93:7). For aryl al-
kynes bearing an electron withdrawing substituent such as the
formyl, benzoyl, chloro, methoxy, trifluoromethoxy, and nitro
group at the aryl moiety, yields (91% up to>97%) and Z-selec-
tivities (93:7 up to 96:4) were high (see 8d–i). However, small
amounts (<1% up to 4%) of the over-hydrogenation products
9d–i were identified. Notably, the formyl, benzoyl, and also the
nitro functionality, which are readily converted using Pd on
charcoal as a catalyst, remained untouched.
An influence of steric shielding on the semi-hydrogenation
of internal alkynes 7j–7n was studied. All alkynes turned out
to be good substrates as documented by the semi-hydrogena-
tion of arylcyclopropyl alkyne 7j, and its cyclopentyl, cylohexyl,
phenyl, and isopropyl congeners 7k–n. The Z/E-selectivity re-
mained high (96:4 or 97:3), but reaction time had to be in-
creased up to 9 h for sterically more hindered systems in order
to get a quantitative conversion.
Scheme 3. Semi-hydrogenation of alkynes 7b-t. Conversion of 7b-t, yield
and Z/E-ratio of 8b-t and yield of 9b-t determined by GC-FID using an inter-
nal standard; signals identified by GC-MS. Semi-hydrogenation of terminal
alkynes performed with 1 mol% Pd@1*.
7r gave the alkenes 8q,r in excellent yields (96%, 98%) with
good Z/E-ratios (both 95/5). Over-hydrogenation products
were only identified in traces.
Also, functional groups on the alkyl substituents like methyl
ester or nitrile 7o,p are tolerated to give the alkenes 8o,p in
good yields (96%, 97%) and Z-selectivities (96:4, 95:5). Further-
more, the selective reduction of the bisalkyl substituted al-
kynes was studied. The semi-hydrogenation of alkynes 7q and
Finally, we investigated the semi-hydrogenation of the termi-
nal alkynes 7s and 7t. As expected for steric reasons, these
substrates were found to be more reactive, and the catalyst
loading was decreased to 1 mol%. Although over-hydrogena-
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Conclusion
We introduced a low-cost photoactive copolymer that can be
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Acknowledgements
We thank the Deutsche Forschungsgemeinschaft (SFB 858) for
funding this work. Sebastian Lamping and Prof. Dr. Bart Jan
Ravoo (both WWU Mꢁnster) are acknowledged for performing
XPS-measurements. Michael Holtkamp and Prof. Dr. Uwe Karst
(both WWU Mꢁnster) are acknowledged for performing TXRF
measurements.
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after 2 min of irradiation, whereas PdNP formation in pure DMF was ob-
served after 6 min of irradiation, providing support that 1 is the reduc-
tant in the current system.
Keywords: heterogeneous catalysis · palladium nanoparticles ·
photoactive polymers · radicals · semi-hydrogenation
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Received: July 12, 2016
Published online on && &&, 0000
Chem. Eur. J. 2016, 22, 1 – 6
www.chemeurj.org
5
ꢂ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
FULL PAPER
A facile light-mediated preparation of
small palladium nanoparticles (PdNPs)
with a diameter of 1.3 nm was achieved
by using low-priced and readily pre-
pared photoactive polymers. The PdNP–
polymer hybrid materials are efficient
hydrogenation catalysts that show high
activity and Z-selectivity in the semi-hy-
drogenation of alkynes.
&
Heterogeneous Catalysis
F. Mꢀsing, X. Wang, H. Nꢁsse, J. Klingauf,
A. Studer*
&& – &&
Facile Light-Mediated Preparation of
Small Polymer-Coated Palladium-
Nanoparticles and Their Application as
Catalysts for Alkyne Semi-
Hydrogenation
&
&
Chem. Eur. J. 2016, 22, 1 – 6
www.chemeurj.org
6
ꢂ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!