Being two is better than one - Catalytic reductions with dendrimer encapsulated copper - And copper-cobalt-subnanoparticles

Article abstract of DOI:10.1039/c5cc00347d

Copper and copper-cobalt subnanoparticles have been synthesized using 4-carbomethoxypyrrolidone terminated PAMAM-dendrimers as templates. The metal particles were applied in catalytic reduction reactions. While Cu subnanoparticles were only capable of reducing conjugated double bonds, enhancing the Cu particles with Co led to a surprising increase in catalytic activity, reducing also isolated carbon double and triple bonds.

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DOI: 10.1039/C5CC00347D
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Being Two is Better than One — Catalytic Reductions
with Dendrimer Encapsulated Copper- and Copper-
Cobalt - Subnanoparticles
Cite this: DOI: 10.1039/x0xx00000x
Mario Ficker,a Johannes F. Petersen,a Tina Gschneidtner,b AnnꢀLouise
Rasmussen,a Trevor Purdy,a Jon S. Hansen,a Thomas H. Hansen, c Søren Husted,c
Kasper Moth Poulsen,b Eva Olsson,d Jørn B. Christensena *
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
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Copper and Copper-Cobalt subnanoparticles have been
synthesized using 4-carbomethoxypyrrolidone terminated
PAMAM-dendrimers as templates. The metal particles were
applied in catalytic reduction reactions. While Cu
subnanoparticles were only capable of reducing conjugated
double bonds, enhancing the Cu particles with Co, lead to a
surprising increase in catalytic activity, reducing also isolated
carbon double and triple bonds.
The hydrodynamic size of the dendrimers with encapsulated
copper nanoparticle was measured in water with dynamic light
scattering (DLS), exhibiting increasing size with increasing
dendrimer generation (G3 5.57
Ə1.27 nm, G4 7.09Ə1.77 nm
and G5 7.51 2.05 nm). The particles encapsulated by PAMAM
Ə
ꢀPyr dendrimers were also investigated by scanning
transmission electron microscopy (STEM) revealing for the G3
and G4 dendrimers sub nanometer particles with diameters less
than 1 nm. For the G5 dendrimers, the particle diameters
measured in the STEM images are between 2 and 4 nm with a
majority having a diameter of 3 nm. Figure 2 shows a TEM
image of Cu32@PAMAMꢀG5ꢀPyr nanoparticles.
Copper has a long history in organic chemistry from Ullmanꢀ[1,2] and
Sandmeyerꢀreactions[3]
,
CopperꢀChromite reductions[4] at high
pressure to Cu(I)ꢀcatalyzed reactions such as aromatic nucleophilic
substitutions[1,2,5], 1,3ꢀdipolar cycloadditions[6] and CꢀH activation[7],
but what about copper subnanoparticles? Size matters and
nanoparticles with less than 50 atoms (subnanoparticles) can have
quite different properties than larger particles. The aluminium cluster
Al13 have an electron affinity similar to chlorine[8] and behaves
essentially like a nanoelementꢀanalog to the halogens[9,10] and other
examples could be the metaloxides and Ptꢀsubnanoparticles reported
by Yamamoto and coworkers[11,12]. Synthesizing nanoparticles of
this size either needs to take place in the gasꢀphase or by using a
“chaperone” molecule that can serve as protective coating as well as
being the mold in which the subnanoparticles are cast. Such
protective coatings can be porous solids (i.e. as in solid supported
catalysts), surfactants, peptides, thiols or dendrimers depending on
Fig. 2 TEM image of Cu32@PAMAMꢀG5ꢀPyr. The white circles highlight
individual particles. Scale bar corresponds to 20 nm.
the chemical nature of the nanoparticles[13ꢀ16]
Dendrimerꢀencapsulated metal nanoparticles (DENs) have mainly
.
The reduction of methyl cinnamate to methyl 3ꢀ
phenylpropionate with NaBH4 catalyzed by CuꢀDENs (Scheme
1) was used to study the relationship between particle size and
reactivity. Sodium borohydride was applied as an easily
employable hydrogen source with no need for high pressure
equipment; all reactions were performed at atmospheric
pressure and ambient temperature. The reaction follows first
order kinetics and the highest reaction rate was found for the
smallest copper nanoparticles, showing decreasing efficiency as
the particle size increased (Fig. 3). The turnover frequencies are
shown in Table 1.
been reported with noble metals such as Pd, Pt, Rh, Ag and Au[14]
,
with Cu[17,18] and Ni[16] as exceptions. Small Pdꢀ[19] and PtꢀDENs[20]
have high reactivity in Heckꢀ and SuzukiꢀMiyuraꢀreactions[19,20] and
hydrogenations[21]
.
Herein, we report on dendrimerꢀencapsulated nanoparticles
synthesized by complexation of Cu(II) to the tertiary amines in the
interior of PAMAMꢀPyr dendrimers (monitored by the absorption
around 650 nm), followed by reduction to Cu(0) with NaBH4 as
shown in Figure 1.
4ꢀCarbomethoxypyrrolidone
terminated
PAMAMꢀdendrimers
(PAMAMꢀPyr) have excellent solubility in different solvents in
contrast to the commonly used hydroxyꢀterminated PAMAMꢀ
dendrimers[14,15] and the complexation of Cu(II) to the interior of
Scheme 1. Reduction of methyl cinnamate with
0.4 mol% Cun@PAMAMꢀG3ꢀ5ꢀPyr.a
these dendrimers had been studied previously by EPR[22]
.
Fig. 1 Complexation of Cu(II) ions to the dendrimer followed by a NaBH4
reduction to the encapsulated nanoparticle.
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DOI: 10.1039/C5CC00347D
Pyr the reduction was complete within 10 min. giving a mixture of
the reduced product (82 %) and internally shifted isomers (18 %).
a
(1.5 mmol of methyl cinnamate with 0.4 mol% catalyst in 15 mL methanol, addition of 0.45
eq. NaBH4 every 5 min., at room temperature. Monitored by GCꢀMS and 1HꢀNMR.)
Table 1. Turnover frequencies (TOF) for the hydrogenation of methyl
cinnamate with different sized dendrimer encapsulated copper nanoparticles.
Scheme 2. Reduction of 10ꢀundecenꢀ1ꢀol with 0.4 mol%
CunCom@PAMAMꢀG3ꢀ5ꢀPyr.a
Catalyst
TOF [h-1]
(per dendrimer)
TOF [h-1]
(per Cu)
Cu12@PAMAM-G3-
53.4
41.0
26.9
4.5
2.3
0.8
Pyr
Cu18@PAMAM-G4-
Pyr
Cu32@PAMAM-G5-
Pyr
a
(1.5 mmol of 10ꢀundecenꢀ1ꢀol with 0.4 mol% catalyst in 15 mL methanol, addition of 0.45 eq.
NaBH4 every 5 min., at room temperature. Monitored by GCꢀMS and 1HꢀNMR.)
The isolation of some double bond shifted isomers out of the
reaction mixture lead to the thought that the hydrogenation might be
specific for external double bonds, which are more readily available
in a crowded system, like the dendrimer interior. (+)ꢀLimonene was
chosen as another test substrate since it contains two different double
bonds; an external and a highly shielded internal double bond.
Overall the substrate is more rigid then the 10ꢀundecenꢀ1ꢀol. In
Scheme 3 it can be seen that the reduction occurred slower than in
the previous case, especially for the generation five dendrimer, this
most likely due to the steric hindrance of the double bond and the
rigidity of the substrate.
Fig. 3 TEM image of Cu16Co16@PAMAMꢀG5ꢀPyr. White circles highlight
The (+)ꢀlimonene has to overcome a barrier to actually reach the
nanoparticle catalyst within the dendrimer, and converted substrate
needs to diffuse out of the dendrimer. As expected, hydrogenation of
the terminal double bond was the major product (up to 50% for
Cu16Co16@PAMAMꢀG5ꢀPyr in 30 minutes), and hydrogenation of
the internal double bond was the minor product (<3 %). The reaction
rate did not follow first order kinetics and the Cu16Co16@PAMAMꢀ
G5ꢀPyr showed very fast reaction rates, converting more than 35%
of (+)ꢀlimonene to the predicted major product within the first five
minutes.
individual particles. Scale bar corresponds to 20 nm.
Attempts to reduce isolated double bonds with NaBH4 at
atmospheric pressure were unsuccessful in contrast to the recent
report on CuꢀDEN catalyzed hydrogenation with hydrogen under
high pressure, where low conversions (≤20 %, in 12 h with
Cu30@G6ꢀPAMAMꢀOH, 10 bar H2) for the reduction of C=C bonds
were reported[23]
.
By serendipity we discovered that doping the nanoparticles with
small amounts of cobalt made the reduction of isolated double bonds
possible, applying sodium borohydride as the hydrogen source. The
bimetallic CuCoꢀDENs were synthesized by allowing both CuII and
CoII to complex with the dendrimer. Upon reduction with sodium
borohydride the bimetallic nanoparticles were formed. The particles
encapsulated by PAMAMꢀG5ꢀPyr was also investigated by scanning
transmission electron microscopy (STEM) revealing particle sizes
between 0.5 and 1.5 nm with a predominance for 1 nm and smaller
(Fig 3). The relative ratios between Co and Co was checked by ICPꢀ
MS and found to be close to the calculated 1:1.
Scheme 3. Reduction of (+)ꢀlimonene with 0.4 mol%
CunCom@PAMAMG3ꢀ5ꢀPyr.a
For the reduction of an isolated double bond 10ꢀundecenꢀ1ꢀol was
chosen. The reaction was tested with three different generations, to
study a size dependency on the reaction rate. The reduction of 10ꢀ
undecenꢀ1ꢀol was close to first order kinetics (Scheme 2). The
reaction rate was observed to be slightly faster in the initial five to
ten minutes. After that the reaction expressed a nearly constant rate.
It is possible that the slower rates are controlled by the diffusion rate
of substrate/product into/out of the dendrimer. Next to the reduction
of the terminal double bond a shift of the double bond to inner
positions was monitored, even though reduction of the shifted
double bond did not occur rapidly. For the Cu16Co16@PAMAMꢀG5ꢀ
a
(1.5 mmol of (+)ꢀlimonene with 0.4 mol% catalyst in 15 mL methanol, addition of 0.45 eq.
NaBH4 every 5 min., at room temperature. Monitored by GCꢀMS and 1HꢀNMR.)
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DOI: 10.1039J/oC5uCrCn0a0l3N4a7Dme
In conclusion, we have demonstrated that dendrimer encapsulated
Cuꢀ and CuꢀCo subnanoparticles catalyze the reduction of carbonꢀ
carbon double or triple bonds under mild conditions. While Cu
particles alone were only capable of reducing conjugated double
bonds, Co doting improved the catalytic activity significantly. CuCo
subnanoparticles were found to reduce isolated carbonꢀcarbon
double bonds with selectivity for terminal double bonds. A
correlation between size and reactivity was found for the Cuꢀ
subnanoparticles, where the highest catalytic activity was found for
Cu12@PAMAMG3ꢀPyr. For the rigid test substrate (+)ꢀlimonene a
size dependence of the reaction rate on the dendrimer generation was
found. The G5 dendrimer with the densest interior branching showed
the lowest TOF per metal atom and the G3 dendrimer the fastest.
The mechanism of the reaction was found to involve a synꢀaddition
of hydrogen. The two nanoparticlar systems have great potential as
catalysts for hydrogenation reactions, while using sodium
borohydride as convenient and easy employable solid hydrogen and
mild reaction condition as ambient temperature and atmospheric
pressure.
Table 2. Turnover frequencies (TOF) for the hydrogenation of
10ꢀundecenꢀ1ꢀol and (+)ꢀlimonene with dendrimer encapsulated copper
nanoparticles.
Substrate
Catalyst
TOF [h-1]
(per [Cu+Co])
10-undecen-1-ol
10-undecen-1-ol
10-undecen-1-ol
(+)-limonene
Cu6Co6@G3ꢀPyr
Cu9Co9@G4ꢀPyr
Cu16Co16@G5ꢀPyr
Cu6Co6@G3ꢀPyr
Cu9Co9@G4ꢀPyr
Cu16Co16@G5ꢀPyr
30
19
43
28
18
13
(+)-limonene
(+)-limonene
For (+)ꢀLimonene a dependence on dendrimer size on the reaction
rate was found. The bigger generations had lower TOF’s per metal
atom compared to the smaller dendrimers, indicating that larger
dendrimer possess a barrier to reach the nanoparticle within the
dendrimer. (+)ꢀLimonene, as a very rigid molecule, was more
affected by this diffusion barrier then 10ꢀundecenꢀ1ꢀol, which can
also explain the difference in reaction rate between the two
substrates.
Notes and references
a
Department of Chemistry, University of Copenhagen, Thorvaldsensvej
40, DKꢀ1871 Frederiksberg C, Denmark.
Eꢀmail: jbc@chem.ku.dk
To elucidate the possible mechanism for the transfer of hydrogen to
the doubleꢀbond the model compound diphenylacetylene was
chosen. A one step synꢀaddition of hydrogen to the triple bond
would give cisꢀstilbene whereas a twoꢀstep process would give the
thermodynamically more stable transꢀstilbene. The results are shown
in Scheme 4.
b
Department of Chemical and Biological Engineering Chalmers
University of Technology, Kemigården 1, SEꢀ412 96 Göteborg, Sweden.c
d
Department of Plant and Environmental Sciences, University of
Copenhagen, Thorvaldsensvej 40, DKꢀ1871 Frederiksberg C, Denmark.
d
Department of Applied Physics Chalmers University of Technology,
Scheme 4. Reduction of diphenylacetylene with 0.5 mol%
Kemigården 1, SEꢀ412 96 Göteborg, Sweden.
Cu6Co6@PAMAMG3ꢀPyr.a
Electronic Supplementary Information (ESI) available: General
experimental details, preparation of the dendrimers, and nanoparticles,
further characterization and procedures for the catalytic reduction
reactions are included in the supporting information. See
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