Platinum(II)-Crosslinked Single-Chain Nanoparticles: An Approach towards Recyclable Homogeneous Catalysts

Article abstract of DOI:10.1002/anie.201700718

We introduce the synthesis and in-depth characterization of platinum(II)-crosslinked single-chain nanoparticles (Pt^II-SCNPs) to demonstrate their application as a recyclable homogeneous catalyst. Specifically, a linear precursor copolymer of styrene and 4-(diphenylphosphino)styrene was synthesized via nitroxide-mediated polymerization. The triarylphosphine ligand moieties along the backbone allowed for the intramolecular crosslinking of single chains via the addition of [Pt(1,5-cyclooctadiene)Cl₂] in dilute solution. The successful formation of well-defined Pt^II-SCNPs was evidenced by size exclusion chromatography, dynamic light scattering, nuclear magnetic resonance (¹H, ³¹P{¹H}, ¹⁹⁵Pt), and diffusion-ordered spectroscopy. Finally, the activity of the Pt^II-SCNPs as homogeneous, yet recyclable catalyst was successfully demonstrated using the example of the amination of allyl alcohol.

Full text of DOI:10.1002/anie.201700718

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201700718
German Edition: DOI: 10.1002/ange.201700718
Recyclable Catalysts
Platinum(II)-Crosslinked Single-Chain Nanoparticles: An Approach
towards Recyclable Homogeneous Catalysts
+
+
Nicolai D. Knçfel , Hannah Rothfuss , Johannes Willenbacher, Christopher Barner-Kowollik,*
and Peter W. Roesky*
Abstract: We introduce the synthesis and in-depth character-
multi-chain nano-aggregates, SCNPs consist of only a single
ization of platinum(II)-crosslinked single-chain nanoparticles
polymer chain, which is crosslinked via intramolecular
II
[5–7]
(
Pt -SCNPs) to demonstrate their application as a recyclable
bonds.
In the past, bioinspired SCNPs have been typically
homogeneous catalyst. Specifically, a linear precursor copoly-
mer of styrene and 4-(diphenylphosphino)styrene was synthe-
sized via nitroxide-mediated polymerization. The triarylphos-
phine ligand moieties along the backbone allowed for the
intramolecular crosslinking of single chains via the addition of
synthesized via hydrogen- or covalent bonding in order to
[8,9]
mimic biomacromolecules.
More recently, additional
approaches employ external linker molecules, for instance
metal complexes, to induce the chain collapse via dynamic
[10–13]
bonding.
While metal complexes, in the first place,
[
Pt(1,5-cyclooctadiene)Cl ] in dilute solution. The successful
operate as triggers for the chain-collapse, their embedding
into the SCNP-structure additionally allows them to function,
2
II
formation of well-defined Pt -SCNPs was evidenced by size
exclusion chromatography, dynamic light scattering, nuclear
magnetic resonance ( H, P{ H}, Pt), and diffusion-ordered
[14–17]
for example, as catalytic centers.
research has been carried out to exploit SCNP systems for
At present, only little
1
31
1
195
II
[18]
spectroscopy. Finally, the activity of the Pt -SCNPs as
advanced catalysis.
The advantages of SCNP systems in
homogeneous, yet recyclable catalyst was successfully demon-
strated using the example of the amination of allyl alcohol.
comparison to established catalysts are yet to be fully
established. In a recent study, Lemcoff et al. described the
complexation of ROMP-derived polycycloocta-1,5-diene
(pCOD) with Ir- and Rh-complexes, which were successfully
I
n recent years, single-chain nanoparticles (SCNPs) have
[15]
emerged as versatile nanostructures based on their outstand-
ing characteristics combined with an array of applications in
for example, information storage, catalysis, or drug delivery
systems, since their properties can be readily varied and
applied in bimetallic cross-coupling reactions,
Pomposo and co-workers employed Cu -SCNPs for the
selective oxidative coupling of terminal alkynes. Further,
our team demonstrated the formation of SCNPs via Pd ions
whereas
II
[16]
II
[
1–4]
[17]
adapted to finely selected conditions.
In comparison to
and applied them as catalyst in a Sonogashira coupling.
Nevertheless, the most intrinsic disadvantage in homogene-
ous catalysis, the separation of the catalysts from the reaction
mixture, has only been studied to a certain extent by applying
SCNP chemistry. To date, organometallic complexes are still
the most effective homogeneous catalysts for a wide range of
[
+]
[
*] N. D. Knçfel, Prof. Dr. P. W. Roesky
Institute for Inorganic Chemistry
Karlsruher Institute of Technology (KIT)
Engesserstrasse 15, 76131 Karlsruhe (Germany)
E-mail: roesky@kit.edu
[19,20]
chemical reactions.
However, their separation from the
[
+]
reaction mixture is a highly challenging task, often demanding
extensive purification methods and sometimes the catalyst
simply remains in the final product. Heterogeneous catalysts
allow for simple separation, yet often suffer from lower
H. Rothfuss, Prof. Dr. C. Barner-Kowollik
Institut fꢀr Technische Chemie und Polymerchemie
Karlsruhe Institute of Technology (KIT)
Engesserstrasse 18, 76131 Karlsruhe (Germany)
and
[21]
catalytic activity and require harsher reaction conditions.
Institute for Biological Interfaces, Karlsruhe Institute of Technology
II
We herein introduce Pt -SCNPs that are able to bridge the
(
KIT)
Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen
Germany)
gap between homogeneous activity and heterogeneous recy-
II
clability. Pt ions are implemented, since they are very
(
efficient homogeneous catalysts for numerous transforma-
and
[
22–24]
195
School of Chemistry, Physics and Mechanical Engineering, Queens-
land University of Technology (QUT), 2 George Street, Brisbane, QLD
001, (Australia)
E-mail: christopher.barner-kowollik@kit.edu
christopher.barnerkowollik@qut.edu.au
tions.
Moreover, the NMR active Pt isotope offers the
[25]
possibility for in-depth solution studies. We demonstrate
that our current system allows for post-catalytic isolation and
reuse of the Pt -SCNPs, an important advantage given the
4
II
high cost and toxicity of Pt complexes (Scheme 1).
Dr. J. Willenbacher
Materials Research Laboratory (MRL); University of California, Santa
As basis for the single-chain nanoparticles, a linear
precursor copolymer was synthesized via nitroxide-mediated
polymerization (NMP; Figure 1). Following the “repeat unit
Barbara
Santa Barbara, CA 93106 (USA)
[
26,27]
+
[
approach”,
the monomers styrene and 4-(diphenylphos-
] These authors contributed equally to this work.
phino)styrene were statistically copolymerized, incorporating
the functional units directly into the polymer chain (Fig-
ure 1A). Phosphine moieties were introduced for their
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201700718.
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Scheme 1. Illustration of a single-polymer-chain collapse into a nano-
particle via addition of the metal complex [Pt(cod)Cl ]. The nano-
2
particles are further employed as homogeneous recyclable catalysts for
the amination of allyl alcohol.
capability of forming stable 2:1 complexes via dynamic
II
bonding with Pt ions. From a series of copolymers in
different ratios and chain lengths (Table S2 in the Supporting
Information), copolymer P1 was selected as the most suitable
for SCNP chemistry based on chain length, polydispersity, and
functional group distribution. The copolymer composition
3
1
1
was determined via P{ H} NMR inlet measurements with
Pd(PPh ) Cl ] as a reference substance (Figure S19) and for
[
3
2
2
P1 resulted in 5 mol% of phosphine units. Size exclusion
chromatography (SEC) analysis [RI, THF] of P1 indicated an
À1
Mn close to 40600 gmol and a narrow distribution with
a dispersity index of ꢀ ꢀ 1.15. The intramolecular collapse of
copolymer P1 into SCNPs was induced by addition of the
platinum precursor complex [Pt(cod)Cl ], (COD = cyclooc-
2
tadiene). In this case, COD can readily be substituted by two
phosphine ligands. The folding of the copolymer P1 into well-
defined SCNPs was initially confirmed by SEC analysis. In
II
comparison to the copolymer P1, the SEC trace for the Pt -
SCNPs is completely shifted towards longer retention times
(
Figure 2A), indicating a smaller hydrodynamic radius and
thus pointing to the exclusive formation of SCNPs (ꢀ ꢀ 1.22)
without any intermolecular side reactions. In addition, the
decrease of the particlesꢀ mean hydrodynamic radius was
confirmed by diffusion ordered spectroscopy (DOSY). The
mean hydrodynamic radius of P1 resulted in Rh,DOSY = 7.4 nm,
Figure 1. A) Synthesis of copolymer P1 via NMP of styrene and 4-
(
diphenylphosphino)styrene. The subsequent addition of [Pt(cod)Cl2]
II
results in SCNP formation. B) The complete complexation of the
phosphine units in the polymer to platinum(II) ions is confirmed by
while a radius of Rh,DOSY = 4.9 nm was determined for the Pt -
SCNPs (Table S1). Furthermore, the transition of the linear
copolymer into a more compact structure was demonstrated
by dynamic light scattering (DLS). Figure 2B shows the
3
1
1
P{ H} NMR spectroscopy. The resonance of free phosphine, apparent
in the spectrum of P1 (top), is replaced by new characteristic reso-
II
nances for platinum complexed phosphine species in Pt -SCNPs (bot-
II
number-weighted size distributions of P1 and the Pt -SCNPs.
tom).
For the copolymer, a mean hydrodynamic diameter of
1
5.2 nm (Rh,DLS = 7.60 nm) was obtained, whereas 8.50 nm
decrease in the hydrodynamic radius, determined by DOSY
and DLS measurements, are in excellent agreement. As
II
(Rh,DLS = 4.25 nm) was estimated for the Pt -SCNPs. Thus, the
2
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already present in the unfolded chain. Within the boundaries
of NMR spectroscopy, a cis/trans-configuration ratio of 9:1 of
II
[29]
the two phosphine ligands is assumed for the Pt -SCNPs.
The preference of a cis-configuration in the folding unit is
presumably due to the predetermined coordination geometry
3
1
1
of the cis-configured precursor [Pt(cod)Cl ]. The P{ H}
2
1
95
NMR data (Figure 2C) were further confirmed by Pt NMR
1
spectroscopy. The main resonance at d = À4413 ppm (t, J
Pt-
=
3745 Hz), can be assigned to the cis-coordinated platinum
P
moiety. Another less-intense resonance at d = À4023 ppm (t,
1
J
= 2621 Hz) is associated with a trans-coordinated plati-
PtP
num species.
II
Having synthesized Pt -SCNPs, we investigated their
activity as homogeneous, yet reusable catalyst in the amina-
tion of allyl alcohol (Figure 3).
[30]
In comparison to the
[
24]
established monomeric complex cis-[Pt(PPh ) Cl ],
we
3
2
2
II
used the Pt -SCNPs as a catalyst. Various aniline derivatives
were employed as starting materials. The conversions and
1
selectivity of the reactions were determined by H NMR
spectroscopy and are collated in Table 1. With aniline (1a),
the allyl alcohol was converted nearly quantitatively within
2
4 h. The major product was the allyl-substituted amine 1b
(ca. 91%), while the di-substituted side-product 1c was only
formed to a small percentage (ca. 4%). Using the halide-
substituted aniline derivatives 3-chloroaniline, 3-fluoroani-
line, and 4-chloroaniline (2a–4a), the conversion was still
high (91–98%) with nearly identical yields for the corre-
sponding products 2b–4b (79–86%). Employing 2,6-dime-
thylaniline (5a), the conversion of the allyl alcohol, as well as
the yield of 5b, decreased significantly (ca. 35%), a result of
II
steric hindrance. To quantify the catalytic activity of the Pt -
SCNPs, analogous reactions were carried out with the
molecular platinum complex cis-[Pt(PPh ) Cl ] (Figure S10–
3
2
2
S14). Although the metals are anchored to a polymer support,
II
the activity and selectivity of the Pt -SCNPs are similar to
those observed for cis-[Pt(PPh ) Cl ], which showed an allyl
3
2
2
alcohol conversion of 90–97% for 1a–4a and yields of 79–
5% for the corresponding products 1b–4b (Table 1). Hence,
8
the polymeric system still allows for ready access of the
substrates to the catalytic platinum centers. Slightly higher
Figure 2. The decrease of the hydrodynamic radius for the SCNPs in
comparison to copolymer P1 is shown by A) SEC [RI, THF] and
B) DLS. C) The cis- and trans-configuration of the phosphine units at
conversions were achieved by the addition of SnCl as a Lewis
acid co-catalyst (Figure S29).
2
The applied reaction conditions not only enabled high
conversion rates, but also ensured the ability to quantitatively
1
95
the platinum centers is further evidenced via Pt NMR spectroscopy.
II
recover the Pt -SCNPs after catalysis by applying dialysis in
II
depicted in Figure 1, the complete complexation of the
methanol. To determine the post-catalytic Pt -SCNP struc-
3
1
1
31
1
phosphine units in P1 to platinum was confirmed by P{ H}
ture, further SEC and P{ H} NMR measurements were
carried out. The SEC traces of the nanoparticles after their
NMR spectroscopy, since no resonance for free phosphine
II
(
d = À6.22 ppm) is detected in the spectrum of the Pt -
application as catalyst match closely with the traces of the
II
SCNPs, instead a new set of resonances appears downfield
original Pt -SCNPs, indicating
a
still folded structure
(
d ꢀ 0-30 ppm). The most intense resonance at d = 13.4 ppm
(Figure 3). In repeated measurements a certain degree of
1
95
1
II
and the corresponding Pt satellites (d, J = 3706 Hz) can
additional compactation of the recycled Pt -SCNPs was
P-Pt
3
1
1
be assigned to a square-planar coordinated ion with two
triarylphosphine moieties in cis-configuration. The data is in
close agreement with the complex cis-[Pt(PPh ) Cl ] (d =
1
1
detected by SEC analysis. The corresponding P{ H} NMR
spectrum still points to the quantitative coordination of the
phosphine ligands to the platinum ions, since no resonance for
free phosphine moieties is detected (Figure S17). Interest-
3
2
2
1
[28]
4.3 ppm; JP-Pt = 3673 Hz).
A less-intense signal at d =
II
9.5 ppm can be assigned to the trans-coordinated phos-
ingly, the Pt -SCNPs show an intense resonance at d =
II
phine-platinum linker unit. Another resonance at d =
9.1 ppm is assigned to a phosphine oxide species, which is
17.9 ppm, most likely associated with trans-coordinated Pt -
2
phosphine units. The change in geometry is presumably
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II
Table 1: Amination of allyl alcohol, catalyzed by Pt SCNPs and cis-
[
a]
[
Pt(PPh ) Cl ].
3 2 2
[
b]
Subst.
Catalyst
Conv.
Product,
yield [%]
Side product,
yield [%]
[
c]
[c]
[c]
[%]
Pt-SCNPs
99
97
91
85
3.8
3.2
[
[
Pt(PPh ) Cl ]
3
2
2
Pt-SCNPs
Pt(PPh ) Cl ]
96
92
86
79
2.8
2.4
3
2
2
Pt-SCNPs
Pt(PPh ) Cl ]
98
90
79
82
3.3
2.6
[
[
3
2
2
Pt-SCNPs
Pt(PPh ) Cl ]
91
96
79
84
2.5
3.0
3
2
2
Pt-SCNPs
Pt(PPh ) Cl ]
35
34
32
22
–
–
[
3
2
2
II
Figure 3. Top: Catalytic cycle of the amination of allyl alcohol with Pt -
SCNPs as reusable homogeneous catalyst. Bottom: The SEC [RI,
DMAc] analysis of the SCNPs after catalysis demonstrates their folded
because the SEC traces of the used and unused SCNPs overlap.
[a] Conditions: 0.24 mol% catalyst, SnCl (0.96 mol%), C D , 24 h,
2 6 6
1008C, molecular sieves (3 ꢁ). [b] Aniline derivatives were used in excess
1
(2 equiv). [c] Calculated by H NMR spectroscopy with ferrocene as
internal standard.
related to the high temperature during catalysis, which allows
a cis/trans-isomerization of the folding unit. Thus, explaining
the slightly altered SEC traces and the different solubility
behavior of the SCNPs after catalysis. The SEC analysis,
combined with the P{ H} NMR spectroscopy data, reveal the
still folded state of the SCNPs after catalysis. Independently,
freshly synthesized platinum(II) nanoparticles, which are
predominantly cis-configured, were heated to 1008C and thus
alcohol. In contrast to many heterogenized molecular cata-
lysts, our system is as active and as selective as the homoge-
neous reference catalyst. We show—to our knowledge for the
first time—the recovery of a Pt -SCNP system, which retains
its folded structure and remains catalytically active after being
employed as catalyst.
II
3
1
1
transformed mainly to a trans-geometry (Figure S18). These Acknowledgements
particles show nearly the same catalytic activity as the cis-
II
isomer (Figure S24). The recyclability of the recovered Pt -
SCNPs was investigated in a second amination cycle of allyl
alcohol with aniline (1a). With an allyl alcohol conversion of
approximately 70%, the SCNPs retain their high catalytic
activity (Figure S15), thus confirming them to be a recyclable
homogeneous catalyst, obtaining high conversions while
applying only 0.24 mol% of catalyst.
C.B.-K. and P.W.R. acknowledge support from the SFB 1176
(Project A2) funded by the German Research Council
(DFG). H.R.ꢀs and N.K.ꢀs PhD studies are additionally
funded by the Fonds der Chemischen Industrie (FCI). C.B.-
K. acknowledges continued support by the Helmholtz asso-
ciation from the STN and BIFTM programs.
In summary, we introduce the synthesis and in-depth
II
characterization of Pt -SCNPs, which was confirmed by SEC, Conflict of interest
1
31
1
195
NMR spectroscopy ( H, P{ H}, Pt), DLS, and DOSY
II
measurements. Furthermore, the Pt -SCNPs were found to be
The authors declare no conflict of interest.
active homogeneous catalysts for the amination of allyl
4
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Keywords: homogeneous catalysis · nanoparticles · platinum ·
recyclable catalysts · single-chain nanoparticles (SCNPs)
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Final Article published: && &&, &&&&
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Communications
Recyclable Catalysts
Keeping in shape: Recyclable metal-con-
taining single-chain nanoparticles
N. D. Knçfel, H. Rothfuss,
J. Willenbacher, C. Barner-Kowollik,*
P. W. Roesky*
(SCNPs) retain their form and function
when used in homogeneous catalysis,
paving the way as a new class of
&&&& — &&&&
advanced catalytic systems. The novel
platinum(II)-SCNP catalyst thus com-
bines the advantages of homogeneous
activity with heterogeneous recyclability.
Platinum(II)-Crosslinked Single-Chain
Nanoparticles: An Approach towards
Recyclable Homogeneous Catalysts
6
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