The effect of particle size and ligand configuration on the asymmetric catalytic properties of proline-functionalized Pt-nanoparticles

Article abstract of DOI:10.1039/c5cc06990d

The effect of particle size on the asymmetric catalytic properties of supported ligand-functionalized nanoparticles (NPs) was investigated for the first time and found to alter significantly the activity but surprisingly not the stereoselectivity. These results suggest that the stereoselectivity of these complex systems is primarily determined by the ligand-reactant interaction, whereas the activity is determined by the particle size.

Full text of DOI:10.1039/c5cc06990d

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DOI: 10.1039/C5CC06990D
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COMMUNICATION
The Effect of Particle Size and Ligand Configuration on the
Asymmetric Catalytic Properties of Proline-functionalized
Pt-Nanoparticles
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Imke Schradera, Sarah Neumanna, Rieke Himstedta, Alessandro Zanab, Jonas Warnekea, Sebastian
Kunz*a
www.rsc.org/
The effect of particle size on the asymmetric catalytic properties of particle size. Therefore, it has to be considered that the ligands on a
supported ligand-functionalized nanoparticles (NPs) was
investigated for the first time and found to alter significantly the
activity but surprisingly not the stereoselectivity. These results
suggest that the stereoselectivity of these complex systems is
primarily determined by the ligand-reactant interaction, whereas
the activity is determined by the particle size.
particle surface may be chemically different. As a result rather
unpredictable effects of particle size on the activity and
stereoselectivity may be expected. Especially the influence of the
surface on the asymmetric induction, which has so far not been
studied, is of fundamental importance in order to determine
guidance for achieving high stereoselectivities with ligand-
functionalized NPs.
Understanding and the development of new strategies for the
control of selectivity is one of the major research aims for this
century’s catalysis research.1 Even though the use of ligands for this
task is a well-established strategy in homogeneous catalysis their
exploration in heterogeneous catalysis has just begun revealing a so
far unknown potential. By binding ligands to a nanoparticle (NP), the
surface becomes electronically and geometrically modified. As a
result the reactant activation may be affected which in turn leads to
a change in selectivity.2-4 It has furthermore been shown that the use
of chiral ligands enables for the induction of asymmetry and hence
to achieve stereoselectivity.5 This evidences that beyond the
potential of ligands to modify the surface properties it is possible to
achieve ligand-reactant interactions as known from homogeneous
catalysis. Ligands may hence be applicable as a selectivity controlling
element perpendicular to the surface of a NP.
As the potential of ligands for asymmetric catalysis with
supported NPs has just recently been discovered, only a few systems
are known yet, and reported enantiomeric excesses (ee) are quite
low (max. 14 % ee).6 The finding of highly stereoselective systems will
hence be an important and time consuming research task for the
next decade, just like in the beginning of asymmetric metalorganic
catalysis.7
In the present study, we show the first step towards higher
stereoselectivities (34 % ee for 1C in Fig.1) by hydrogenating
ethylacetoacetate (EAA) over Pt NPs (1.2 nm in size) functionalized
with L-proline (PRO). Using this system, which is right now the actual
benchmark for stereoselectivity with supported ligand-
functionalized NPs, we performed a study on the effect of particle
size and ligand configuration that enables to gain some fundamental
insights into what determines the catalytic properties of these
materials.
A metalorganic catalyst consists of a single metal atom with a
geometrically well-defined ligand sphere. In contrast, a particle
exhibits a surface composed of atoms that are geometrically and
electronically different and its properties depend furthermore on the
a.Institute for Applied and Physical Chemistry (IAPC), Faculty 2, University of
Bremen, Leobenerstraße, 28359 Bremen, Germany.
Figure 1. Investigated reaction: Hydrogenation of ethylacetoacetate
(EAA, 1A) leads to the formation of (S)- and (R)-Ethyl-3-
hydroxybutyrate (1B and 1C, respectively).
b.Nano-Science Center, Department of Chemistry, University of Copenhagen,
Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark.
Electronic Supplementary Information (ESI) available: Experimental details,
representative chromatograms of catalytic experiments, representative TEM
images and particle size analysis, IR spectra of PRO-N-functionalized Pt NPs before
and after exposure to catalytic conditions, scheme of N-H assisted reaction pathway,
formation rates of “unprotected”, PRO-, and N-methyl-PRO-functionalized Pt NPs,
all formation rates normalized to the number of ligand-free surface atoms, long-
term stability tests. See DOI: 10.1039/x0xx00000x
For the material preparation we followed a specific strategy
established and presented in detail previously (see Section 1 in ESI
for detailed syntheses protocols).5 Briefly, so-called “unprotected” Pt
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NPs are first synthesized and functionalized with the desired ligand representative discussion, the formation rates were VnieowrmArtaiclilezeOdnlitnoe
DOI: 10.1039/C5CC06990D
in a separate step. The particles are then deposited by adding the the total number of surface atoms, determined from the average
support material to the colloidal dispersion, followed by solvent particle size of the TEM size distributions given in Figure S1 (see
removal. As the particle size is preserved in each preparation step, reference for details on dispersion estimation).5
the approach enables for the independent control of particle, ligand,
When comparing the steady state formation rates, it is found
that for both, “unprotected” and PRO-functionalized Pt NPs, the
rates change with particle size. Over larger particles the activities are
generally higher. Especially the “unprotected” Pt NPs show a strong
size dependence with an increase by a factor of 2.7 as the particle
size is increased from 1.2 to 2.1 nm. PRO-functionalized NPs still
show a 1.7 fold enhancement. For Pt clusters electronic structure
induced changes of the catalytic properties already start to vanish at
cluster size of 20 atoms.11 As the number of atoms for the small
particles (1.2 nm) investigated here is already around 64 (see ESI
Section 3 for further details), a change of the electronic structure that
is of catalytic relevance can be excluded for the two investigated
sizes. The obtained increase of the steady state formation rates with
particle size is however a known phenomenon for the hydrogenation
of C=O compounds over supported Pt NPs and has been related to
decarbonylation of carbonlys by low coordinated Pt surface atoms.12
The CO formed by this reaction then poisons these highly reactive
surface atoms so that they cannot further contribute to the catalytic
reaction. In order to demonstrate the presence of CO on the
particles, PRO-functionalized particles were investigated by IR
spectroscopy before and after exposure to catalytic conditions (see
and support and hence to study their influence systematically.5 The
size of “unprotected” Pt NPs can be controlled to some extend by
varying the pH value of the reaction mixture.8 In the present study
we used this approach and prepared Pt NPs of 1.2 nm (± 0.3) and 2.1
nm (± 0.5) that were subsequently functionalized with L- and D-
proline (see Fig. S1 – S2 in ESI for size distributions and TEM images).
The two particle sizes are within the mitohedrical region (0.8 to 4 nm)
where the ratio of low to highly coordinated surface atoms changes
distinctively with size. Above 6 nm atoms with high coordination
numbers (in low-index planes) predominate and particle size has
approximately no influence.9, 10 On the basis of the model of van
Hardeveld and Hertog the ratio of low to highly coordinated surface
atoms changes from 2 to 0.7 when increasing the particle size from
1.2 to 2.1 nm (see ESI Section 3 for further details). Thus, significant
changes in the catalytic properties can be expected for these two
types of particles if size effects are of importance for PRO-
functionalized NPs.
125
1.2 nm
2.1 nm
Section
4 in ESI). The resulting spectra do not reveal any
100
75
50
25
0
characteristic feature for CO on Pt before exposure to reaction
conditions, but a distinctive band appears above 2000 cm-1 after
exposure that is characteristic for CO linearly adsorbed on Pt.13
For the small size the rates normalized to total number of
surface atoms are almost the same over “unprotected” and PRO-
functionalized particles, even though ligands are expected to block
catalytic sites. Activation energies for reactions over Pt have been
demonstrated to be not affect by amine ligands.14 An electronic
effect of amine ligands on the catalytic properties of Pt can hence be
excluded.15 Primary and secondary amines are however known to
induce an alternative reaction pathway compared to the purely
metal catalyzed reaction that may be related to the so-called N-H
effect that is known from homogeneous catalysis (see Scheme S1 in
ESI).16 This effect can also proceed on ligand-functionalized Pt NPs as
recently discussed and superpose the blocking of active surface.6 An
appropriate test for the presence of this effect is the use of a tertiary
amine as ligand.17 Because a tertiary amine does not exhibit any
hydrogen substituents, such ligands are not able to induce a N-H
assisted reaction path, but must cause an activity inhibition.
Performance of this test for a N-H effect using N-methyl-L-proline (N-
Me-PRO) as the simplest tertiary amine derivative of PRO was indeed
found to be positive for the investigated reaction (see Section 6 in
ESI). A N-H induced acceleration for the investigated reaction over
PRO-functionalized NPs is hence concluded to occur that is making
up the losses in activity through blocking of catalytic surface atoms
by ligand binding.
P
t-N
P
P
t-N
P
t-N
P
P
t-N
t-N
-P
t-N
-P
P
P
"
"
O
O
d
d
ro-P
ro-P
R
R
te
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-P
-P
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c
-P
-P
D
D
e
e
rot
L
L
rot
p
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u
"
"
Figure 2. Formation rates normalized to the total number of surface
atoms on “unprotected” (red), L-PRO-Pt (blue), and D-PRO-Pt NPs
(green). Particles of 1.2 nm size are shown on the left, 2.1 nm
particles on the right.
Catalytic activities for the hydrogenation of EAA over
“unprotected” Pt NPs of 1.2 and 2.1 nm and the same particles
functionalized with L- and D-proline (PRO) are shown in Figure 2 and
Table 1, as steady state formation rates. In order to allow a
2 | J. Name., 2012, 00, 1-3
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On 2.1 nm particles the presence of PRO leads to a slight ligand is bound is able to adsorb a further molecule sVuiecwhAartsicHle2O.nFlioner
DOI: 10.1039/C5CC06990D
reduction of the rate normalized the total number of surface atoms oxygen covered Pt surfaces it has been reported that dissociative
of around 30 % (see Fig. 2), when compared to “unprotected” NPs of hydrogen activation can occur on a site pair formed from a free Pt
the same size. Hydrogenation of EAA over a bare metal surface is surface atom and an adjacent oxygen binding Pt atom.19 H2 can
expected to require adjacent surface atoms as both, EAA and adsorb molecularly on the free surface atom and then dissociate by
hydrogen, have to be activated and to be adjacent to each other in interaction with the adjacent oxygen binding Pt atom. We therefore
order to react.18 For all PRO-functionalized samples the ligand propose that for the N-H assisted reaction pathway over a Pt NP, a
coverages are so high (0.8 to 0.96, see Table 1) that the remaining ligand-free surface atom has to be available, adjacent to a ligand-
ligand-free surface atoms can be considered as being isolated sites. blocked surface atom. Such a site pair is then suitable facilitate H2
We therefore propose that a purely metal-catalyzed reaction cannot activation via dissociation. Due to the high ligand coverages of PRO-
occur on PRO-Pt NPs, but the N-H assisted reaction path. For the N- Pt NPs their activity has then to be considered as being limited by the
H mechanism the carbonyl does not have to adsorb directly on a bare availability of ligand-free surface atoms to form the proposed active
metal atom but H2 (see Scheme S1 in ESI). This coordinated molecular site. This reasoning enables for explaining why on large particles in
hydrogen is then activated via heterogeneous dissociation, induced contrast to the small ones a slight decrease in activity is obtained by
by interaction with the nitrogen atom of the adjacent ligand that acts binding PRO, as the ligand coverage on the larger particles is higher
as a base.16 This means that a free adsorption site on the metal has than on small (Table 1).
to be available close enough to the ligand for H2 activation to
proceed, which leads to formation of the active species that adsorbs
and hydrogenates the carbonyl reactant.
Table 1. Catalytic experiments performed in THF at T = 25°C, PH₂ = 20
bar. A negative ee corresponds to an excess of (S)-Ethyl-3-
hydroxybutyrate (1B in Fig.1) a positive ee to an excess of (R)-Ethyl-
3-hydroxybutyrate (1C in Fig.1).
Rate per
NP
size
(nm)
Ligand
cover-
age
Rate per
surface
ee
(%)
free
surface
atom (h-1)
39
101
273
1675
200
Sample
atom (h-1)
Pt NPs
Pt NPs
L-PRO-Pt
L-PRO-Pt
D-PRO-Pt
D-PRO-Pt
1.2
2.2
1.2
2.2
1.2
2.2
0
0
0.85
0.96
0.8
0.93
39
101
41
67
40
0
0
34
30
-30
-33
70
1000
In order to illustrate the effect of ligand acceleration, when
considering that the availability of ligand-free surface atoms
determines the actual activity, the formation rates were normalized
to the total number of ligand-free surface atoms (see Figure S5 in ESI
and Table 1). A 5 to 7 fold increase in activity can be obtained for
small and
a 10 to 17-fold increase for the large NPs by
functionalization with PRO (see Section 5 in ESI for further details and
discussions).
Comparison of the stereoselectivies reveals that even though
the ratio of low to highly coordinated surface atoms changes
significantly from 1.2 to 2.1 nm, there is no distinctive change in
selectivity, by taking the experimental error into account (± 2%). This
differentiates these systems from the use of chiral modifiers for
supported Pt NPs, for which particle size dependent
stereoselectivities have been reported.20 The absence of any
significant particle size effect on the stereoselectivity may be
explained in two ways. The ligand-reactant interaction is not strongly
influence by the structure of the underlying metal surface, because
according to the N-H assisted reaction pathway (Scheme 1) the
organic reactant is not directly adsorbed on the metal surface for its
activation. Alternatively, all low coordinated Pt atoms may be
Scheme 1. N-H assisted reaction path with a ligand-free surface
atom adjacent to a ligand-blocked as active site. The C=O reactant
can interact with Pt bound hydrogen and an amine bound proton,
to become activated (Step 1) and hydrogenated (Step 2). The
catalyst is reformed by coordinating molecular hydrogen (Step 3)
that is heterogeneously dissociated by interacting with a ligand-free
surface atom and the ligand nitrogen (Step 4).
The ligand sphere of a metalorganic catalyst is flexible and
coordinated solvent molecules can desorb to leave open sites for H2
adsorption. In contrast, a surface atom exhibits a rigid ligand sphere.
It seems thus very unlikely that a single surface atom to which a
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poisoned by CO due to decarbonylation of the reactant and the
reaction proceeds only over highly coordinated surface atoms that
are similar for the two particle sizes.
6. I. Schrader, J. Warneke, J. Backenköhler and S. KVuienwz,ArJti.cAlemOn.line
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stereoselectivity was probed. From homogeneous catalysis it is
known that the product configuration can be inverted while 9. G. C. Bond, Platin. Met. Rev.., 1975, 19, 126-134.
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maintaining the ee when using the opposite enantiomer of a
metalorganic complex.21 Similar results have been reported for the
use of chiral modifiers for supported Pt NPs.22 Comparison of the
stereoselectivities obtained for different particle sizes and the two
PRO enantiomers shows that with regard to the experimental error
the absolute value of the enantiomeric excess does not change, while
the product configuration is inverted. It is hence concluded that with
ligand-functionalized NP catalysts the stereochemistry of the
product can be controlled by using the appropriate ligand
configuration.
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255-257.
,
The fact that the absolute value of the ee neither depends on
the particle size nor on the ligand configuration suggests that for
ligand-functionalized NPs the surface morphology is not of significant
importance for the stereoselectivity. Instead, it is primarily
determined by the ligand-reactant choice similar as in homogeneous
catalysis,17 whereas the activity is a matter of particle size. If this
finding does not merely hold for the present case but in general the
search for ligand-functionalized Pt NP catalysts with higher
stereoselectivities can be reduced to the search for appropriate
ligand-reactant combinations while the activity can be optimized via
tuning the particle size. We next aim to perform a broader study with
PRO derivatives as ligands and different reactants in order to further
validate this hypothesis.
16. S. E. Clapham, A. Hadzovic and R. H. Morris, Coord. Chem.
Rev., 2004, 248, 2201-2237.
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The authors gratefully acknowledge the “Fonds der
Chemischen Industrie” (FCI) for financial support.
Notes and references
‡ Footnotes relating to the main text should appear here. These
might include comments relevant to but not central to the matter
under discussion, limited experimental and spectral data, and
crystallographic data.
§
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etc.
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