Catalyst Decomposition during Olefin Metathesis Yields Isomerization-Active Ruthenium Nanoparticles

Article abstract of DOI:10.1002/cctc.201600738

The second-generation Grubbs catalyst, RuCl₂(H₂IMes)(PCy₃) (=CHPh) [GII; H₂IMes=1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene, Cy=cyclohexyl], is shown to decompose during olefin metathesis to gen

Full text of DOI:10.1002/cctc.201600738

DOI: 10.1002/cctc.201600738
Communications
Catalyst Decomposition during Olefin Metathesis Yields
Isomerization-Active Ruthenium Nanoparticles
+
+
Carolyn S. Higman , Anabel E. Lanterna , M. Luisa Marin, Juan C. Scaiano,* and
Deryn E. Fogg*
[a]
The second-generation Grubbs catalyst, RuCl (H IMes)(PCy )
ing from ring-closing metathesis (RCM) to cross-metathesis
2
2
3
[2,3,8,9]
(
=CHPh) [GII; H IMes=1,3-bis(2,4,6-trimethylphenyl)-4,5-di-
(CM) and metathesis polymerization.
Ruthenium hydride
2
hydroimidazol-2-ylidene, Cy=cyclohexyl], is shown to decom-
complexes generated by catalyst decomposition are widely
viewed as responsible. Until now, only molecular complexes
have been considered as potential culprits, despite the low iso-
pose during olefin metathesis to generate Ru nanoparticles
(RuNPs). These RuNPs appear to contribute significantly to
[10]
competing isomerization during metathesis. Larger, partially
oxidized RuNPs are also observed in commercial GII, but these
exhibit modest isomerization activity. Removal of RuNPs from
the precatalyst does not prevent isomerization, because new,
more reactive NPs are generated by catalyst decomposition
during metathesis.
merization activity documented for leading candidates.
Herein, we show that ruthenium nanoparticles (RuNPs) are
formed by decomposition of GII during metathesis, and that
these are important, hitherto unrecognized contributors to
competing olefin isomerization. Notably, whereas NP formation
is common for low-coordinate Pd catalysts that cycle between
II
0 [11]
Pd and Pd , reports of such behavior for well-defined mono-
ruthenium complexes operating in organic media are rare, out-
6
Ruthenium-catalyzed olefin metathesis is a core tool in organic
side hydrogenation reactions mediated by h -arene complexes
[
1]
[12]
synthesis and an emerging protagonist in the pharmaceutical
of ruthenium. This is the first report of metal NP formation
[
2]
industry. Notwithstanding the importance of these advances,
a number of reports cite challenges arising from competing
by decomposition of a molecular metathesis catalyst.
Olefin isomerization by RuNPs has not, to our knowledge,
previously been reported. Given the activity of such entities in
[
3]
olefin isomerization, the dominant non-metathetical side re-
[
4]
[13]
action. Isomerization is particularly pronounced for the
other catalytic contexts, however, we speculated that they
second-generation Grubbs catalyst (GII), relative to its prede-
cessor GI (Figure 1).
might function as viable isomerization catalysts. This proved to
be the case. RuNPs were prepared by a range of methods (see
[
3]
[14–16]
the Supporting Information)
and were tested for their ac-
tivity toward isomerization of estragole (1). Estragole is an im-
[
17]
portant renewable allylbenzene used in metathesis reactions,
[9,18]
which, as with its congeners,
is readily isomerized. Figure 2
shows the isomerization activity recorded for four different Ru-
containing nanostructures. All are clearly capable of inducing
2
Figure 1. Grubbs catalysts GI and GII. Cy=cyclohexyl, H IMes=1,3-bis(2,4,6-
trimethylphenyl)-4,5-dihydroimidazol-2-ylidene.
1
!2 isomerization. By far most active, however, were the
Chaudret–Philippot NPs (type D), prepared under rigorously
anaerobic conditions, and stabilized by N-heterocyclic carbene
[
14,19]
Tandem metathesis–isomerization or isomerization–metathe-
sis protocols, employed as a deliberate synthetic strategy, can
enable access to targets that are otherwise challenging or inac-
(NHC) ligands.
The dramatically higher isomerization activi-
ty of these NHC-stabilized NPs is consistent with the absence
of oxidized surface species.
[
5–7]
cessible.
More commonly, however, isomerization is an unin-
Given this evidence that RuNPs promote olefin isomeriza-
tion, and prior reports that such side reactions declined if com-
tended, often capricious side reaction that results in variable
control over product selectivity and yields, in processes rang-
+
+
[
a] C. S. Higman, Dr. A. E. Lanterna, M. L. Marin, Prof. J. C. Scaiano,
Prof. D. E. Fogg
Center for Catalysis Research & Innovation
Department of Chemistry and Biomolecular Sciences
University of Ottawa
1
0 Marie Curie, Ottawa ON K1N 6N5 (Canada)
E-mail: dfogg@uottawa.ca
scaiano@photo.chem.uottawa.ca
Figure 2. Isomerization promoted by RuNPs prepared by methods shown in
the Supporting Information. Type A) RuNPs on mesoporous silica MCM-41
(Ru@MCM), B) RuNPs on crystal nanodiamonds (Ru@CND), C) RuNPs stabi-
lized with ethylene glycol, and D) RuNPs stabilized with the NHC 1,3-bis(2,6-
diisopropylphenyl)imidazole-2-ylidene (IPr).
+
[
] These authors contributed equally to this work.
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/
cctc.201600738.
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[
20,21]
mercial GII was chromatographed prior to use,
we ques-
[22]
tioned whether RuNP contaminants might be present in GII,
which trigger competing isomerization during metathesis. We
found that commercial GII catalysts do indeed contain RuNPs,
present as aggregates that agglomerate on isolation to an
average size of >500 nm (see the Supporting Information).
However, the isolated particles induced olefin isomerization
with low efficiency. They required 24 h to reach 45% yield of 2
under the conditions of Figure 2. This is unsurprising given
their large size and partial oxidation, both of which limit the
number of active surface sites.
To determine whether isomerization could be inhibited by
removing the RuNPs present in the precatalyst, we generated
NP-free GII by ultracentrifugation under an atmosphere of N2.
As illustrated in Figure 3, the purified GII effected both meta-
Scheme 1. Ejection of [MePCy ]Cl (A) from the metathesis-active species Ru-
3
1.
state complex GIIm (Scheme 1), attacks the methylidene ligand
[23,24]
of active species Ru-1.
Elimination of the s-alkyl ligand
thus formed occurs by abstraction of a proton (most plausibly
from the H IMes ligand) and bound chloride. This process cul-
2
minates in the extrusion of [MePCy ]Cl (A), a net loss of three
3
ligands per Ru center. Whereas isolation of the putative s-alkyl
intermediate Ru-2 is precluded by its short lifetime, we recent-
ly succeeded in trapping out such a complex in the first-gener-
[25]
ation Grubbs system. The details of NP formation are now
being probed by in situ X-ray absorption studies, but the low-
coordinate Ru species resulting from such “ligand stripping”
represents a plausible starting point.
Figure 3. Performance of NP-depleted (c) versus NP-rich GII (a) in
metathesis of estragole (1). a) Formation and consumption of the self-meta-
thesis product 3. b) Net isomerization (sum of reagent and product isomeri-
zation). See Section S5.2 in the Supporting Information.
Further experimental evidence for RuNP formation during
metathesis comes from electron microscopy. In these experi-
ments, styrene 4 was chosen as the substrate, because the low
solubility of its self-metathesis product 5 facilitates removal of
organic species that otherwise occlude the micrographs. Scan-
ning electron microscopy (SEM, Figure 4a) revealed NP-free
solutions. Likewise, transmission electron microscopy (TEM)
showed no NPs in analysis of multiple samples, down to the
0.2 nm detection level of the instrument. In contrast, abundant
RuNP formation was evident following metathesis of 4, as
shown in Figure 4b.
thesis and isomerization of estragole (1). Thus, yields of meta-
thesis product 3 increased over the first hour of the reaction
but then declined as 3 underwent isomerization (Figure 3a).
Strikingly, the extent of isomerization was only 15% less than
that effected by non-purified GII (Figure 3b). Freshly decom-
posed Ru products thus appear to be important contributors
to isomerization, with a level of activity much higher than that
of the RuNP impurities present in the precatalyst.
Also notable in Figure 3b is the approximately 30 min induc-
tion period that precedes the onset of isomerization. Forma-
tion of NPs over this timescale was confirmed by in situ neph-
elometry experiments, in which the intensity of scattered light
was detected by synchronous wavelength scanning. As with
conventional dynamic light scattering, increases in scattering
intensity indicate NP formation. Intensity changes were moni-
tored in the l=600–700 nm region to eliminate perturbation
arising from absorption by the sample. The intensity of scatter-
ing increased over the first 30 min (see the Supporting Infor-
mation), a change that maps onto the induction period in iso-
merization. In the absence of substrate, scattering was signifi-
cantly reduced.
To examine whether isomerization is promoted by RuNPs
generated by catalyst decomposition during metathesis, or by
molecular species formed at an earlier stage, we performed
This evidence implies that RuNPs are formed by decomposi-
tion of ruthenium species generated during metathesis. We at-
tribute the formation of nanoparticles, as opposed to molecu-
lar Ru products, to the loss of multiple ligands in the process
of catalyst decomposition. Relevant in this context is the estab-
Figure 4. Decomposition of NP-depleted GII into RuNPs during styrene
metathesis. a) SEM image of GII solution prior to metathesis and b) SEM
image after metathesis (COMPO mode). Scale bar: 1 mm. Average particle
size: (100ꢀ25) nm.
lished pathway by which free PCy , liberated from the resting-
3
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mercury-poisoning experiments. Poisoning by elemental mer-
cury is a common test for the involvement of surface-active
a small proportion of a phosphine or phosphite poison, rela-
tive to the total Ru loading, is sufficient to completely shut
down isomerization. On the basis of this cumulative picture,
we propose that RuNPs formed by catalyst decomposition are
important contributors to unwanted isomerization during
olefin metathesis.
[
26–28]
metal(0) sites in catalysis.
As shown in Figure 5, isomeriza-
The context above focuses on unintended isomerization as
a problem encountered during olefin metathesis. Insight into
its origin, however, points toward new opportunities. The reac-
tion conditions explored above were designed for metathesis,
rather than nanoparticle formation or isomerization. Optimiz-
ing the synthesis of ruthenium nanoparticles, as well as the
isomerization conditions, is expected to open new doors for
the design of novel isomerization catalysts.
Figure 5. Impact of added Hg on the isomerization of 1 by initially NP-de-
pleted GII. Conditions as in Figure 3.
Acknowledgements
tion of 1 dropped by approximately 50% in the presence of
Hg. Control experiments indicated that Hg had a negligible
impact on the isomerization activity of common Ru hydride
complexes (see the Supporting Information), or on the forma-
tion of A. Indeed, the Hg test may under-report the contribu-
tion of RuNPs in Figure 5, given the reported instability of the
Work supported by the Natural Sciences and Engineering Re-
search Council of Canada (NSERC). C.S.H. thanks NSERC for
a PGS-D Scholarship. M.L.M. was a Distinguished Visiting Profes-
sor from Universitat Politꢀcnica de Valꢀncia. J.C.S. thanks the
Canada Research Chairs program; D.E.F. thanks Magdalen Col-
lege, Oxford, for a Visiting Fellowship.
[
29]
[26]
Ru–Hg amalgam or adsorbate.
Substoichiometric poisoning experiments (Figure 6) were
performed to further probe the involvement of RuNPs in iso-
merization. Such experiments are predicated on the require-
ment for ꢁ1 equivalent of a poisoning ligand to inhibit cataly-
sis by molecular Ru species, in contrast with the smaller
number of ligands required to inhibit NP catalysis (in which
much of the initial metal charge is inaccessible in the NP core).
Keywords: catalyst
metathesis · nanoparticles · side reactions
decomposition
·
isomerization
·
[
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[
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2
Received: June 18, 2016
Published online on && &&, 0000
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COMMUNICATIONS
Nanoparticles in metathesis: A previ-
ously unrecognized decomposition
pathway is reported for the Grubbs cat-
alyst, which results in the formation of
isomerization-active nanoparticles (NPs).
C. S. Higman, A. E. Lanterna, M. L. Marin,
J. C. Scaiano,* D. E. Fogg*
&& – &&
Catalyst Decomposition during Olefin
Metathesis Yields Isomerization-Active
Ruthenium Nanoparticles
Cy=cyclohexyl, H IMes=1,3-bis(2,4,6-
2
trimethylphenyl)-4,5-dihydroimidazol-2-
ylidene.
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