A general approach to mono- and bimetallic organometallic nanoparticles

Article abstract of DOI:10.1039/c4sc01231c

A comprehensive methodology to prepare nanometric size organometallic particles (ONPs) containing rhodium(i), iridium(i) and nickel(0) with ROMP-derived polycycloocta-1,5-diene (pCOD) by a controlled single chain collapse mechanism was developed. The polymeric complexes could be produced via direct exchange of the respective labile ligands of metal complexes by the 1,5-hexadiene elements in pCOD, or via in situ reduction of metal ions in the presence of the polymer. These well-defined π-bound polymeric complexes were characterized by UV-Vis spectroscopy, dynamic light scattering (DLS) and size exclusion chromatography (SEC) measurements and the resulting polymer sizes were found to be inversely proportional to the amount of metal added due to concomitant single chain collapse. Moreover, these procedures were readily extended to the synthesis of organobimetallic nanoparticles containing two metals; which could be added in commutative order and specific metal ratios. The embedded metal elements were found to be readily accessible for applications in catalysis, where the close proximity of the catalytic centers led to distinctive reactivity compared to the isolated complexes.

Full text of DOI:10.1039/c4sc01231c

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A general approach to mono- and bimetallic
organometallic nanoparticles†
Cite this: Chem. Sci., 2014, 5, 4196
*
Sudheendran Mavila, Illya Rozenberg and N. Gabriel Lemcoff
A comprehensive methodology to prepare nanometric size organometallic particles (ONPs) containing
rhodium(I), iridium(I) and nickel(0) with ROMP-derived polycycloocta-1,5-diene (pCOD) by a controlled
single chain collapse mechanism was developed. The polymeric complexes could be produced via direct
exchange of the respective labile ligands of metal complexes by the 1,5-hexadiene elements in pCOD, or
via in situ reduction of metal ions in the presence of the polymer. These well-defined p-bound
polymeric complexes were characterized by UV-Vis spectroscopy, dynamic light scattering (DLS) and
size exclusion chromatography (SEC) measurements and the resulting polymer sizes were found to be
inversely proportional to the amount of metal added due to concomitant single chain collapse.
Moreover, these procedures were readily extended to the synthesis of organobimetallic nanoparticles
containing two metals; which could be added in commutative order and specific metal ratios. The
embedded metal elements were found to be readily accessible for applications in catalysis, where the
close proximity of the catalytic centers led to distinctive reactivity compared to the isolated complexes.
Received 29th April 2014
Accepted 18th June 2014
DOI: 10.1039/c4sc01231c
www.rsc.org/chemicalscience
emission, dramatically enhancing the quantum yield,11 and met-
allosupramolecular polymers have been used to make special
Introduction
materials with the ability to heal themselves aer sustaining
damage.12 Recently, Gladysz et al. ingeniously polymerized an h4-
benzene iridium complex, taking advantage of the uncoordinated
double bond of the benzene ring.13 This methodology afforded an
unprecedented polyacetylene framework where iridium atoms
were bound along the butadiene fragments of the main chain.
Another recent example from the Manners group14 shows how by
simply changing the reaction conditions a ring-opening reaction of
a methylsilaferrocenophane affords very different polymers. Thus,
the creation of a new metallopolymer family and its correct char-
acterization can lead to accurate structure–activity relationship
studies and to the design of more complex and useful materials.
Well-dened polymer bound complexes may be also used as
soluble polymer supported catalysts under homogeneous condi-
tions.15–17 In general, these polymer bound catalysts may be
recovered and reused by precipitating them in a poor polymer
solvent.18 In this context, complexes of rhodium and iridium have
received considerable attention because of their large potential to
carry out various organic transformations with high degrees of
stereo- and regio-selectivity.19,20
The advancement of polymer science embraces the invention of
molecular constructs that provide functional applications. As a
result, the development of novel methodologies for the production
of polymers and complex macromolecular architectures1–6 stands
out as an appealing goal enthusiastically pursued by the synthetic
chemistry community. In this regard, metallopolymers offer a
great advantage, as they may combine the catalytic/functional
properties of the metal complex with the material/structural facets
of polymers. Polymers containing metals may be classied by the
type of ligand used to bind the metal, by the type of metal used to
dope the polymer, or by the way the metal is bound to the polymer
chain (i.e. as a metallic substituent in the side chain, or a main
chain metal polymer; linear, dendritic or star-shaped architec-
tures; dynamic or static binding modes, etc.).7 Even though they
have been known for more than three decades,8,9 only since the
mid-1990s have techniques that allow the synthesis and charac-
terization of well-dened metallopolymers been mature enough to
be practically useful. Modern metallopolymers are used for a great
variety of applications, including catalysis, sensing, OLED
constituents, optics, information storage, and even stimuli-
responsive materials.10 For example, in emissive layers, the intro-
duction of iridium into the polymer allows phosphorescence
An attractive process to regulate the physical properties of
macromolecules is by their transformation to well-dened
organic nanoparticles by intramolecular cross-linking. Three
main approaches stand out as the most common methods to
induce chain collapse in polymers; irreversible covalent,
reversible covalent (dynamic) or non-covalent cross-linking.21
The strategies for chain collapse can also be classied according
to the type of chemistry used to generate the nanoparticle.22 The
Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva 84105,
Israel. E-mail: lemcoff@bgu.ac.il
† Electronic supplementary information (ESI) available: Detailed experimental
procedures and characterization of ONPs including: 1H and 13C NMR, SEC
spectra and GC-MS chromatograms of reaction mixtures. See DOI:
10.1039/c4sc01231c
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rst examples of intramolecular cross-linking date back forty [RhCl(C2H4)2]2 (1) as a precursor cross-linker,32 we set out to
years to the seminal work by Walsh et al.23 Martin continued the determine the generality of this concept by extending it to other
study of intramolecularly cross-linked polymers, both experi- metals and bimetallic structures. Thus, single chain collapse of
mentally and theoretically, in the early 1980s24 and demon- pCOD was attempted via the intramolecular binding of 1,5-
strated the relationship between the cross-link density and the hexadiene fragments to iridium(I) under dilute conditions.
contraction factor. The eld laid more or less dormant for the
In order to obtain an understanding of the exchange process,
next 20 years, until the turn of the century where new chemis- the model monomers 1,5-cyclooctadiene (COD), 1,5-hexadiene
tries were developed for cross-linking strategies, such as the and (perhaps as a more realistic framework) a mixture of 4,8-
benzocyclobutane (BCB) method25 and olen metathesis,26,27 dodecadienes were mixed with [IrCl(COE)2]2 (2) (the tetrakis-
among others. [For recent examples of intramolecularly cross- ethylene complex of iridium was unstable and could not be used
linked polymers and dendrimers see ref. 28–31.]
for this purpose). The facile exchange process afforded
Herein, we introduce a wide-ranging strategy for the exploi- compounds 3, 4 and 5, as conrmed by NMR spectroscopy (see
tation of organometallic bonds to induce single chain collapse the ESI†).
of ROMP derived pCOD. Thus, rhodium, iridium and nickel
Having shown that the model dienes readily exchanged the
organometallic nanoparticles were made and characterized. In COE ligands in 2,33 this complex was tested as a cross-linker for
addition, we disclose that this strategy may be used to form pCOD. Indeed, pCOD (Mn z 50 000 g molꢀ1, PDI ¼ 1.02)
nanoparticles containing both rhodium and iridium ions in any quickly reacted with 2 (10 mol% with respect to the 1,5-diene
chosen ratio. Moreover, as a proof of concept and to detail the unit) in THF and exchanged COE at room temperature to afford
potential of these materials, we show how the catalytic proper- Ir-ONPs (Scheme 1).
ties of the embedded metal may be transformed by changing
the architecture of the organometallic nanoparticle.
The appearance of a new broad signal at d ¼ 4.56–4.72 ppm
corresponding to olen protons bound to an iridium metal
1
center in the H NMR spectrum strongly supported the forma-
tion of Ir-ONPs via the exchange of COE by 1,5-hexadiene units
(Fig. S16, ESI†). In addition, we sought a more practical method
to monitor the formation of ONPs and decided to probe UV-Vis
spectroscopy. Pleasingly, the resulting Ir-ONPs showed an
absorption maximum at 446 nm, similar to that observed for
the COD complex (3) (451 nm) (Fig. 1c),34 while the absorption
maximum seen for complex 2 at 420 nm disappeared; strongly
supporting the occurrence of binding events of 1,5-hexadiene
units to Ir(I). UV measurements were also extended to the Rh-
ONPs with similarly positive results (vide infra). Finally, the
most compelling evidence for the formation of well-dened
organometallic nanoparticles of iridium was acquired by
measuring the size of the polymer obtained aer adding the
metal. The SEC overlay of Ir-ONPs produced by using 10 and 2
mol% of 2 (with respect to 1,5-hexadiene units of pCOD)
together with the starting polymer material provides compelling
evidence for a well-controlled single chain collapse (Fig. 1a),
indicating an almost 30% decrease in hydrodynamic radius
according to the triple-detector system used.35 Supporting the
notion that chelation is the main driving force behind the single
Results and discussions
Synthesis of organometallic nanoparticles of iridium(I)
Having recently disclosed a convenient and straightforward
synthesis of pCOD complexes of rhodium(I) by using
Scheme 1 Schematic illustration of the preparation of Ir-ONPs.
Fig. 1 (a) Overlay of the SEC traces obtained for Ir-ONPs at varying concentrations of Ir(I) and pCOD. (b) Overlay of DLS traces obtained for Ir-
ONPs at varying concentrations of Ir(I) and pCOD. (c) UV-Vis spectra of pCOD, 2, 3, 5 and Ir-ONPs (10 mol% Ir(I)).
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Table 1 SEC and DLS measurements of Ir-ONPs with varying iridium
contenta
Entry Ir(I)b [mol%] Mn [ꢁ104 g molꢀ1
]
PDI Rg [nm] Rhd [nm]
c
c
1
2
3
4
0
2
5
4.96
5.01
5.32
5.45
1.02 10.1
10.1
9.4
8.6
7.9
1.01
1.06
1.06
9.4
8.4
7.3
10
a
b
Conditions: solvent ¼ THF, t ¼ 16 h, T ¼ 35 ꢂC, Ir(I) ¼ [IrCl(COE)2]2.
c
Relative to original COD content. Determined by triple-detector
SEC in THF. d Determined by DLS in THF.
Fig. 2 Plot of hydrodynamic radius (measured by DLS) of Ir-ONPs vs.
iridium(I) content.
chain collapse at high dilution, treatment of ROMP derived
poly(cis-cyclooctene) (pCOE) with Ir complex 2 in dilute THF
solution did not afford a well-dened organometallic nano-
particle. Instead, an insoluble cross-linked material precipi-
tated from solution, reminiscent of the intermolecular cross-
linking observed in the rhodium case.32
The control of intramolecular chain collapse was also probed
by systematically varying the percentage of iridium(I) intro-
duced onto the polymer chain. As in the case of Rh-ONPs, the
Scheme 2 Schematic illustration of the synthesis of Ni-ONPs.
newly generated Ir-ONPs exhibited
a linear dependence
between iridium(I) (cross-linker) added and the reduction in the
size of the particles. Therefore, upon increasing the metal
content from 2 mol% to 10 mol%, a regular decrease in the
hydrodynamic radius was observed by both size exclusion
chromatography (SEC) and dynamic light scattering (DLS)
studies in THF (Fig. 1 and Table 1).
Moreover, UV-Vis spectroscopy was also used to monitor the
varying concentrations of Ir(I) in the formation of Ir-ONPs. As
expected, a gradual rise in the intensity of the absorption band
at 446 nm consistent with the increase of iridium(I) content in
the polymer chain was observed (Fig. S26, ESI†).
As shown in Table 1, the hydrodynamic radii obtained by
using triple-detector SEC in THF showed excellent correlations
with those obtained by DLS measurements. As expected, the
intrinsic viscosity was also reduced with increasing amount of
metal incorporated into the polymer chain (Fig. S27a, ESI†). For
convenience, a plot of the results obtained in the DLS
measurement of the hydrodynamic radius vs. the iridium
content of the ONPs is shown in Fig. 2.
As an additional characterization method for the Ir-ONPs
(10 mol% Ir(I)), cryogenic transmission electron microscopy
(TEM) analysis was carried out. The spherical particles imaged,
of about 20 nm diameter, nicely correlate with the results
obtained by DLS and triple-detector SEC (Fig. S28a and b, ESI†).
The amount of iridium embedded into the polymer chain was
quantied by ICP-MS analysis giving an iridium content of 125
mg gꢀ1 of the pCOD; conrming the successful cross-linking.
This drawback makes the procedures used for rhodium and
iridium incompatible for nickel. Thus, nickel(II) acetylacetonate
(6) had to be used as the precursor cross-linker to generate the
ONP. Auspiciously, the in situ reduction of 6 with DIBAL-H
in the presence of pCOD afforded the desired Ni-ONPs
(Scheme 2).37
Again, the polymer solution was kept highly diluted (z10ꢀ5
M) in order to avoid any possible intramolecular cross-linking.
SEC and DLS measurements revealed the smooth formation of
Ni-ONPs showing a systematic reduction in the hydrodynamic
radius upon increasing the nickel content in the polymer chain,
reproducing the linear dependence found between the amount
of metal added and the hydrodynamic radius of the polymer
(Fig. 3 and Table 2). Similarly, a reduction in the measured
intrinsic viscosity was observed also for Ni-ONPs upon
increasing the amount of incorporated metal (Fig. S27b, ESI†).
In contrast to the rhodium and iridium polymer complexes,
the Ni-ONPs had to be kept under inert atmosphere in order to
avoid decomposition (gradual decomposition was observed
over 24 h under air). Nonetheless, these nanoparticles were
stable enough to be measured by SEC and DLS analyses.
Synthesis of organometallic nanoparticles of nickel(0)
Having secured the straightforward synthesis of both Ir-ONPs
and Rh-ONPs, we turned our attention to produce pCOD
complexes of nickel. Although nickel(0) has been shown to bind
1,5-dienes,36 its complexes are usually less stable compared to
Fig. 3 (a) Overlay of the SEC traces obtained for Ni-ONPs at varying
concentrations of Ni(II). (b) Overlay of DLS traces obtained for Ni-ONPs
the corresponding rhodium(I) and iridium(I)/diene complexes. at varying concentrations of Ni(II).
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Table 2 SEC and DLS measurements of Ni-ONPs with varying nickel
contenta
Ni(II)b [mol%]
Mn [ꢁ104 g molꢀ1
]
PDI
Rhd [nm]
c
Entry
1
2
3
4
0
10
20
30
4.40
3.60
3.00
2.55
1.03
1.01
1.05
1.15
10.5
9.6
9.1
8.0
a
Reaction conditions: solvent ¼ THF, t ¼ 2 h, T ¼ ꢀ78–0 ꢂC, Ni(II) ¼
b
[Ni(acac)2], DIBAL-H ¼ 2.5 equiv. w.r.t. Ni(II). Relative to original
c
COD content.
Determined by triple-detector SEC in THF.
d
Determined by DLS in THF.
Fig. 4 (a) Overlay of SEC traces obtained for Rh–Ir-ONPs by the
addition of 5 mol% Rh(I) followed by 5 mol% Ir(I). (b) Overlay of DLS
traces obtained for Rh–Ir-ONPs by the addition of 5 mol% Rh(I) fol-
lowed by 5 mol% Ir(I). (c) Overlay of SEC traces obtained for Ir–Rh-
ONPs by the addition of 5 mol% Ir(I) followed by 5 mol% Rh(I). (d)
Overlay of DLS traces for Ir–Rh-ONPs obtained by the addition of
5 mol% Ir(I) followed by 5 mol% Rh(I).
As further evidence for bimetallic ONP formation, the newly
generated Rh–Ir-ONPs showed absorption maxima corre-
sponding to both Rh/diene and Ir/diene fragments respectively
at 368 nm (compared to 365 nm for Rh-ONPs) and 443 nm
(compared to 446 nm for Ir-ONPs), presenting a convenient
additional spectroscopic handle for the characterization of
these structures (Fig. 5). The formation of bimetallic complexes
of pCOD was also monitored by 1H NMR analyses.
The appearance of broad peaks at d ¼ 4.31–4.43 ppm and d ¼
4.56–4.72 ppm corresponding to the olenic protons bound to
Rh and Ir metal centers provides good evidence of complex
formation (Fig. S19, ESI†). Furthermore, TEM analysis of Rh–Ir-
ONPs containing 5 mol% rhodium and 5 mol% iridium showed
spherical particles of about 20 nm in diameter (Fig. S28c and d,
ESI†).
Scheme 3 Schematic illustration of the synthesis of organobimetallic
ONPs of rhodium(I) and iridium(I) (Rh–Ir-ONPs and Ir–Rh-ONPs).
Synthesis of organobimetallic nanoparticles of rhodium(I)
and iridium(I)
The facile procedure by which the addition of metal ions
induced the formation of the ONPs persuaded us to attempt the
synthesis of bimetallic nanoparticles. Organobimetallic nano-
particles have the enormous potential to achieve cooperative or
tandem catalysis, where a single ONP may combine the catalytic
efficiency of both embedded metals.38–41
In addition, we surmised that lodging different metals in
close proximity, and in any desired ratio, could have implica-
tions even beyond catalyst development.
Thus, cross-linkers 1 and 2 were sequentially introduced to a
solution of pCOD (Scheme 3). Indeed, bimetallic nanoparticle
formation could be ascertained by both SEC and DLS Table 3 SEC and DLS measurements of organobimetallic nano-
particles containing rhodium(I) and iridium(I)a
measurements (Fig. 4). For example, the addition of 1 to pCOD
reduced the hydrodynamic radius of the parent polymer from
Rh(I)b
[mol%]
Ir(I)b
[mol%]
Mn
Rh
c
d
11.0 nm to 9.6 nm, followed by a subsequent reduction to 8.7
nm aer the addition of the second complex (2) (Table 3).
A comparative kinetic study was conducted for both cross-
linkers 1 and 2 using the stopped-ow technique. The analysis
showed complete ligand exchange of 1 and 2 with pCOD in less
than one minute (Fig. S1, ESI†).
To conrm that both metals are actually independently
interacting with the polymer, the sequence of cross-linker
addition was reversed. Thus, addition of complex 2 to pCOD
reduced the hydrodynamic radius of the particles from 11.0 nm
to 9.6 nm, which was then decreased to 8.4 nm when complex 1
was added (Table 3).
Entry
[ꢁ104 g molꢀ1
]
PDI
[nm]
pCOD
Rh-ONPs
—
2.5
5
2.5
5
—
—
7.5
5
—
—
—
7.5
5
2.5
5
2.5
5
5.71
4.63
5.52
7.18
6.05
4.20
4.50
7.13
5.72
1.02
1.01
1.04
1.12
1.11
1.09
1.07
1.11
1.03
11.0
10.4
9.6
8.7
8.7
10.6
9.6
8.7
Rh–Ir-ONPs
Ir-ONPs
Ir–Rh-ONPs
8.4
a
Conditions: solvent ¼ THF, t ¼ 16 h, T ¼ 35 ꢂC, Rh(I) ¼ [RhCl(C2H4)2]2,
b
c
Ir(I) ¼ [IrCl(COE)2]2. Relative to original COD content. Determined
triple-detector SEC in THF. d Determined by DLS in THF.
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Fig.
5 Overlay of UV-Vis spectra obtained for organobimetallic
nanoparticle (Rh–Ir-ONPs), Rh-ONPs and Ir-ONPs.
Scheme 4 Reduction of benzyl benzamide (8) by using Ir-ONPs.
hydrophobic volume of the nanoparticle may bring about
unexpected reactivities.
Rhodium and iridium/diene catalysed cross-coupling reac-
tions are well established synthetic methods in organic
synthesis,42–48 and diene ligands are considered as viable alter-
natives to phosphines in many important transformations.44
Compelled by this notion, we probed the chemical accessibility
of iridium(I) in Ir-ONPs by mirroring a recently reported
reduction of secondary amides by an iridium silane complex (7)
(Scheme 4).49
In this successful example, the ONPs were employed just as
the metal supplier for a known catalytic reaction. Encouraged
by this result, we set out to probe the catalytic performance of
the Ir-ONPs in a reaction where the Ir(I)/1,5-diene moiety
remains intact (no detachment of Ir from the ONP is expected).
As recently reported by Barker et al.,46 no catalysis was observed
when the iridium complexes were not ligated to dienes in
ketone allylations. Thus, this reaction would offer a good
assessment for the premise that catalysis must proceed within
the ONP space and not by the formation of a freestanding Ir unit
in solution. For this purpose, Ir-ONPs were tested in a bench-
mark allylation of acetophenone (Scheme S2, ESI†). Indeed, the
allylation reaction of acetophenone (11) with allyl boronic acid
pinacol ester (12) in the presence of 2 mol% of Ir-ONPs (2 mol%
of Ir(I)/1,5-diene units with respect to substrates) smoothly
afforded the expected allylated product 13 in 97% yield in 24 h
(Table S1, ESI†). Aer the reaction the Ir-ONPs were evaporated,
washed and reused three times without losing any of their
catalytic activity. The recycled Ir-ONPs showed more than 95%
conversion even aer the third recycling (Table S1, ESI†).
Having shown that the ONPs can carry out reactions with
Fig. 6 Plot of hydrodynamic radius against varying ratios of rhodium
and iridium.
The nal rhodium (34 mg gꢀ1) and iridium (49 mg gꢀ1
)
contents were quantied by ICP-MS analysis conrming the
successful synthesis of the bimetallic ONPs.
Finally, we decided to probe whether different metal ratios
could also be used to form ONPs, and what effect this would
have on the size of the particle. As observed in Fig. 6, ONPs
retained approximately the same dimensions irrespective of the
ratio of rhodium and iridium incorporated into the polymer
chain, only the total amount of metal dictated the nal size.
Therefore, one or more metals in any desired ratio can be
readily inserted into the polymer chain to achieve both size and
composition control.
Preliminary applications of Rh(I) and Ir(I)-ONPs in catalysis
One of the most attractive potential uses for the organometallic
nanoparticles is their potential to provide accessible metal ions similar efficiency as their small molecule models, we sought to
to the surrounding media for catalytic applications. Thus, the determine whether the peculiar arrangement of metal
polymer may act as a type of “metal sponge”, releasing active complexes within the polymer matrix may lead to different
species into solution when the appropriate conditions are met. catalytic behaviour.
The use of ONPs for chemical catalysis is expected to offer
Two factors may inuence the reactions catalyzed by the
several advantages. First, unlike dendrimers and other complex ONPs, the rst is that if the reaction is taking place inside the
macromolecular architectures which oen demand lengthy and ONP, then the polarity environment will be different than that
expensive procedures, these polymer-based systems are inex- of the bulk solution. The second factor that may cause reactions
pensive and may be synthesized very easily under mild reaction to behave differently is that the distribution of the catalytically
conditions. Second, aer the reaction, the product could be active metal fragments is not homogeneous throughout the
readily isolated by simple precipitation of the nanoparticles. solution because the ions are supposed to be concentrated
Most importantly, dense packing of active metal species into the within the nanoparticle. Thus, 4-nitrobenzaldehyde (14a) and
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Scheme 5 Rh(I) mediated cross-coupling reaction between 14 and 15.
Table 4 Cross-coupling reaction of 14 and 15 using Rh(I) catalystsa
Entry
Catalyst
[Aldehyde] [mmol Lꢀ1
]
[Rh(I)] [mmol Lꢀ1
]
t [h]
Conv.b [%]
16b [%]
17b [%]
1
2
3
4
5
6
7
Rh-ONPsc
18
18
18
10.0
10.0
10.0
3.0
2.0
10.0
10.0
0.5
0.5
10.0
3.0
0.1
0.5
48
16
16
16
16
48
16
40
>99
>99
>99
>99
26
—
>99
—
8
16
30
>99
7
>99
92
84
70
—
18
Rh-ONPsc
18
0.5
>99
93
a
Conditions: solvent ¼ THF, T ¼ 80 ꢂC, boronic acid ¼ 2 equiv. per aldehyde. b Determined by GC-MS analysis. c Rh(I) ¼ 10 mol% w.r.t the 1,5-
hexadiene elements in the polymer chain, aldehyde ¼ 14a (entries 1–5) and 14b (entries 6 and 7).
phenyl boronic acid (15) were chosen as the standard coupling (Table 4, entry 4). The amount of biphenyl formation could be
partners (Scheme 5). The results of the cross-coupling reactions further increased from 16% to 30% (Table 4, entry 5) when the
employing Rh-ONPs and the control reactions using isolated standard reaction ratios (i.e., using 5 mol% catalyst relative to
complex [RhCl(COD)]2 (18) at different reaction conditions are aldehyde) were kept and the reaction was carried out under high
detailed in Table 4. The reaction using 5 mol% of 18 resulted in dilution (5 times); maybe simulating events that occur in a
the complete conversion of the starting materials to the expec- nanoparticle where the rhodium quickly reacts with boronic
ted cross-coupled product 16a (Table 4, entry 2). In contrast, the acid and does not have a signicant effective concentration of
reaction using Rh-ONPs gave moderate conversions and a aldehyde in the surroundings. These results support a view
negligible yield of cross-coupled product (Table 4, entry 1). where the homo-coupling reaction is occurring either within or
Interestingly, the major product formed in this case was on the surface of the ONPs, where the effective concentration of
biphenyl.50,51 In order to understand the underlying mecha- Rh(I)/diene is much higher than in the neighboring environ-
nism, various control reactions were carried out. We assumed ment (Scheme 6). In order to probe whether the highly non-
that the higher effective concentration of Rh(I)/diene within the polar nature of the ONPs might be the cause for the observed
ONPs might play a role in the outcome of the reaction. Indeed, behaviour, a less polar aldehyde, 14b, was used instead of 14a.
by simply raising the concentration of 18, a larger amount of However, biphenyl was still the only product observed (Table 4,
biphenyl was produced (Table 4, entry 3).
entry 6) and the conversion was even lower. In contrast, the
Moreover, when the reaction was performed under more control reaction using complex 18 afforded cross-coupled
dilute conditions, maintaining an equimolar ratio of substrate product 16b as the major product (Table 4, entry 7).
to catalyst, the amount of biphenyl formed was even higher
Although similar types of homo-coupling have been reported
elsewhere in the literature,52 selective homo-couplings under
conditions favorable to cross-coupling have not been reported.
This alluring property of ONPs may have a signicant impact on
the outcome of other ONP catalyzed reactions and we plan to
continue to study this outstanding behaviour.
Since formation of biphenyl was observed with a closely
packed Rh(I) environment within the nanoparticle, we asked
ourselves whether partial unfolding of ONPs by the introduc-
tion of coordinating NHCs could shi the catalytic outcome of
the ONPs from homo- to cross-coupling.
Transition metal complexes bearing N-heterocyclic carbenes
(NHCs) have had an enormous impact in organic synthesis.53,54
Chaudret et al. have recently shown a very nice example of how
NHCs can inuence the formation and properties of metallic
nanoparticles.55
Scheme 6 Schematic proposal for the biphenyl production catalysed
by Rh-ONPs.
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NHC-ONPs) (Table 5). Notably, a substantial improvement in
the formation of cross-coupled product was observed (Table 5,
entry 2). Moreover, complete unfolding of the ONP by using
stoichiometric amounts of NHC (with respect to the metal)
further improved the selectivity to achieve 70% cross-coupled
product (Table 5, entry 3) (naturally, the rhodium is still
attached to the polymer, but does not cross-link it as it only
needs one diene ligand).
Scheme 7 Preparation of Rh-ONPs bearing NHCs.
In brief, the catalytic activity of the ONP could be switched in
a controlled manner via the addition of NHC ligands.
Conclusions
We have shown that the synthesis of organometallic nano-
particles based on ROMP derived pCOD may be readily
extended to other transition metals. The ONPs were fully char-
acterized by SEC, DLS and TEM analyses. The sizes of all ONPs
could be controlled by the amount of cross-linking metal
complex added and the formation of the new metal–polymer
hybrids could be established by simple UV-Vis spectroscopy
measurements. Moreover, the facile and mild synthetic proce-
dures allow also for the generation of nanoparticles containing
more than one metal in any desired ratio. The associative
character of the bimetallic ONP formation process strongly
suggests that the metals are randomly distributed within the
polymer framework, although no direct evidence of the spatial
distribution of the metals within the ONPs could be obtained
until now. We surmise, based on our ndings, that any metal
that can bind to 1,5-cyclooctadiene has the potential ability to
be incorporated into these pCOD nanoparticle frameworks. The
ONPs synthesized were found to be catalytically active in several
reactions, and we showed that the crowded ONP environment
gives rise to novel catalytic performance. Research efforts are
ongoing to advance tandem/cooperative catalytic methods with
the bimetallic ONPs, study the bulk electronic properties of
different ONPs with different metals, as well as making use of
block copolymers to enhance the stability of these novel mate-
rials in other solvents.
Fig. 7 SEC overlay of pCOD, Rh-ONPs and Rh–NHC-ONPs bearing
3 mol% NHC.
In general, NHC complexes may be easily obtained by ligand
exchange processes.56 Within the ONP, the introduction of NHC
ligands would be expected to replace a diene–metal bond and
change the electronic environment of the metal and increase
the polymer size (Scheme 7).
For this purpose, 3 mol% (with respect to the 1,5-hexadiene
elements in the polymer chain) 1,3-bis(2,4,6-trimethylphenyl)-
imidazolium chloride was mixed with the Rh-ONPs containing
10 mol% of Rh(I) and potassium tert-butoxide was added to
generate the NHC in situ. Indeed, the introduction of an NHC
ligand led to a signicant increase in size, probably because of
the relatively large size of the NHC (Fig. 7) in addition to the
severance of a cross-linking bond.
Acknowledgements
The United States–Israel Binational Science Foundation
(2010047) is acknowledged for partial funding of this study. The
authors thank Dr Sharon Vanounou and Dr Yael Levi-Kalisman
(Ilse-Katz Institute for Meso- and Nano-scale Science and
Technology, Ben-Gurion University of the Negev) for technical
assistance in dynamic light scattering data analysis and TEM
imaging. We also wish to thank Dr Alexandra Massarwa for
assistance with the stopped-ow instrument and Dr Sarit Yer-
Having shown that the introduction of NHC ligands led to
less compact ONPs, the cross-coupling reaction of 14a and 15
was repeated using NHC containing Rh-ONPs as catalysts (Rh–
Table 5 Cross-coupling reaction of 14a and 15 using Rh–NHC-ONPs
as catalysta
Entry Catalyst
Rh(I)b [mol%] NHCb [mol%] 16ac [%] 17c[%] ushalmi for assistance with ICP-MS measurements.
1
2
3
4
Rh-ONPs
Rh–NHC-ONPs 10
Rh–NHC-ONPs 10
10
0
3
10
—
—
19
70
>99
>99
81
30
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—
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2 P. J. M. Stals, Y. Li, J. Burdynska, R. Nicola¨y, A. Nese,
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Conditions: solvent ¼ THF, T ¼ 80 ꢂC, catalyst ¼ 5 mol%. b Rh(I) ¼ 10
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c
Determined by GC-MS analysis.
4202 | Chem. Sci., 2014, 5, 4196–4203
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