A recyclable ruthenium(ii) complex supported on magnetic nanoparticles: A regioselective catalyst for alkyne-azide cycloaddition

Article abstract of DOI:10.1039/c3cc43048k

A magnetically separable ruthenium catalyst was synthesized through immobilizing a pentamethylcyclopentadienyl ruthenium complex on iron oxide nanoparticles. The catalyst is highly active and selective for the synthesis of 1,5-disubstituted 1,2,3-trizoles

Full text of DOI:10.1039/c3cc43048k

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A recyclable ruthenium(II) complex supported on
magnetic nanoparticles: a regioselective catalyst for
alkyne–azide cycloaddition†
Dong Wang,a Lionel Salmon,b Jaime Ruiza and Didier Astruc*a
Cite this: Chem. Commun., 2013,
49, 6956
Received 24th April 2013,
Accepted 13th June 2013
DOI: 10.1039/c3cc43048k
www.rsc.org/chemcomm
A magnetically separable ruthenium catalyst was synthesized through
The immobilization of homogeneous catalysts to facilitate
immobilizing a pentamethylcyclopentadienyl ruthenium complex on their separation and recycling is a task of great economic and
iron oxide nanoparticles. The catalyst is highly active and selective for environmental importance in catalysis science. Various inorganic
the synthesis of 1,5-disubstituted 1,2,3-trizoles via cycloaddition of and organic supports have been explored, such as inorganic
alkynes and organic azides and can be recycled at least 5 times.
solids,7 polymers,8 fluorous tags,9 and ionic liquids.10
Magnetic nanoparticles (MNPs) have recently emerged as ideal
1,2,3-Triazoles are five-membered nitrogen heterocyclic compounds catalyst supports due to their large surface area, straightforward and
that have been widely used in various research fields including relatively low preparation cost, low toxicity, good stability, as well as
synthetic organic,1 medicinal,2 materials,3 and biological chemistry.4 facile separation by magnetic forces.11 The catalysts supported on
Among numerous methods, catalyzed Huisgen cycloaddition of magnetic nanoparticles combine these advantages of heterogeneous
alkynes and organic azides by complexes of Cu(I) (CuAAC)5 and catalysts (easy recovery and regeneration) and nanocatalysts (such as
[Cp*Ru(II)] (RuAAC)6 are the two most efficient ones that have been a large surface-to-volume ratio relative to bulk materials, excellent
used to assemble the 1,2,3-triazole ring, respectively, forming 1,4- and activity, great selectivity, and high stability), and overcome the
1,5-disubstituted 1,2,3-trizoles selectively (Scheme 1).
aggregation problem of metallic nanoparticles. Moreover, their
Since the latter method (RuAAC) was pioneered by the groups of property of magnetic separability eliminates the requirement of
Fokin and Jia,6a some [Cp*Ru] complexes6 have been successfully catalyst filtration after completion of the reaction.
applied for the catalysis of RuAAC reactions, such as Cp*RuCl(PPh3)2,
Herein, we report simple and efficient synthesis of an iron-
Cp*RuCl(NBD), Cp*RuCl(COD), and [Cp*RuCl]4. However, to date oxide magnetic nanoparticle-supported pentamethylcyclo-
only a handful of reports have been published in the field of RuAAC, pentadienyl ruthenium(II) catalyst and its application in Huisgen
and there has been no publication on recyclable RuAAC catalysts.
cycloaddition of alkynes and organic azides for the fully selective
construction of 1,5-disubstituted 1,2,3-trizoles.
The primary step of this objective was achieved by the synthesis of
magnetic nanoparticles (Scheme 2). Monodisperse silica-coated
g-Fe2O3 nanoparticles (SiO2/g-Fe2O3) 2 were derived by coating
g-Fe2O3 core 1 (which was obtained by a co-precipitation method)
with a silica layer. In this process, tetraethoxysilane (TEOS) and
aqueous NH3 were used as a silica source and a hydrolyzing agent,
respectively.12 Transmission electron microscopy (TEM) images of 2
showed the core–shell structure and the silica coating that has a
Scheme 1 Regioselective copper vs. ruthenium-catalyzed alkyne–azide cycloaddition
reactions (CuAAC and RuAAC).
a
´
ISM, Univ. Bordeaux, 351 Cours de la Liberation, 33405 Talence Cedex, France.
E-mail: d.astruc@ism.u-bordeaux1.fr
b LCC, CNRS, 205 Route de Narbonne, 31077 Toulouse Cedex, France
† Electronic supplementary information (ESI) available See DOI: 10.1039/
c3cc43048k
Scheme 2 Synthesis of SiO2/g-Fe2O3 nanoparticles.
c
6956 Chem. Commun., 2013, 49, 6956--6958
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Fig. 1 (a) TEM image of SiO2/g-Fe2O3; (b) TEM image of Cp*Ru/SiO2/g-Fe2O3
before reaction; (c) TEM image of Cp*Ru/SiO2/g-Fe2O3 after 5 reaction cycles; (d)
Cp*Ru/SiO2/g-Fe2O3 in THF using
a small magnet; (e) Cp*Ru/SiO2/g-Fe2O3
dispersion in a reaction medium; (f) catalyst separation using a small magnet.
uniform thickness of 9 nm (Fig. 1a). The dense silica shell has plenty
of Si–OH units for potential derivatization with different functional
groups, and also prevents leaching of iron from the core under harsh
shaking conditions. The silylated Cp*Ru(II) complex 5 was obtained
through coordination between the cluster (Cp*RuCl2)n and Si(OMe)3-
functionalized triarylphosphine 4.13,14 Then 5 was successfully immo-
bilized on the surface of robust SiO2/g-Fe2O3 via the heterogenization
with the Si–OH binding sites of 2. The TEM images of the targeted
magnetic nanoparticle catalyst (Cp*Ru/SiO2/g-Fe2O3) 6 depicted
relatively uniform core–shell nanoparticles with an average size
of approximately 30 nm (Fig. 1b). The loading of the ruthenium
complex in 6 was calculated by determination of the nitrogen
content (C, H, N elemental analysis), and the result showed that it
was approximately 0.16 mmol gÀ1 (Scheme 3).
The catalytic application of magnetically recyclable catalyst 6 in
cycloaddition of alkynes and organic azides was further investigated
(Table 1). Phenylacetylene and benzyl azide were chosen as the model
substrates, and the cycloaddition was conducted at 65 1C under a
nitrogen atmosphere for 3 h in the presence of 2 mol% of 6. As shown
in Table 1, the corresponding 1,5-disubstituted 1,2,3-trizole 7a was
produced in 91% yield with almost 100% selectivity (99.96% selectivity,
which was determined using both GC and NMR). The catalyst 6
showed excellent magnetic properties, stability (Fig. 1d) and dispersion
(Fig. 1e) in the reaction medium, and can be easily collected using
external magnets after completion of the reactions (Fig. 1f), which
minimizes the loss of catalytically active particles and oxidation of
sensitive ruthenium complexes during separation. The catalyst 6 could
be reused five times by simple magnetic separation with only a
minimum decrease in catalytic activity and selectivity (Table 1). More-
over, the morphology and size of the nanoparticles do not change
much even after five cycles.
Scheme 3 Synthesis of Cp*(PPh3)2Ru/SiO2/g-Fe2O3.
Table 1 Reusability test for the magnetic catalyst 6 in cycloaddition of phenyl-
acetylene and benzyl azidesa
Encouraged by the efficiency of the reaction protocol described
above, the scope of the reaction was examined. Firstly, various
terminal alkynes were investigated in reactions with benzyl azides.
Alkynes containing electron-withdrawing or electron-releasing
groups were suitable cycloaddition partners (Fig. 2). The reactions
of heteroatom-containing alkynes also proceeded smoothly. The
aliphatic alkyne 1-hexyne and ferrocenylacetylene were suitable
substrates producing triazoles in 84% and 93% yield (7h, 7i),
respectively. The organic azide substrates were further investigated,
and the results indicated that both benzyl and alkyl azides reacted
Run
1
91
99.9
2
87
99.5
3
83
99.4
4
86
98.3
5
77
98.3
Yieldb (%)
Selectivityc (%)
a
The reaction was carried out using phenylacetylene (0.5 mmol) and
benzyl azide (0.5 mmol) in the presence of 6 (63 mg) at 65 1C under a
b
nitrogen atmosphere. Isolated yields after column chromatography.
c
Determination using GC or/and NMR.
c
This journal is The Royal Society of Chemistry 2013
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Financial support from the China Scholarship Council (CSC)
(PhD grant to DW), the Universities Bordeaux 1 and Toulouse III,
and the Centre National de la Recherche Scientifique (CNRS) is
gratefully acknowledged.
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´
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6958 Chem. Commun., 2013, 49, 6956--6958
This journal is The Royal Society of Chemistry 2013