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CHAPTER 4_2
Organometallic Approach for the Synthesis of Noble Metal Nanoparticles: Towards Application in Colloidal
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4.4.5 Ionic Liquid Stabilized Nanoparticles as Catalysts
Providing more environmentally-friendly conditions than traditional solvents, the use of ionic liquids (ILs), acting as both the solvent and the stabilizer, is emerging as an alternative for the preparation of nanocatalysts.
The size of Ru NPs is governed by the degree of self-organization of the imidazolium-based ionic liquid. Since the 3D organization of the ionic liquid is better maintained at low temperature, the Ru nuclei are confined inside to a greater degree, giving smaller nanoparticles as a result.
This observation was further confirmed by using a series of imidazoliumderived ionic liquids: [RMIm][NTf2] (R = CnH2n11 with n = 2, 4, 6, 8, 10), [R2Im][NTf2] (R = Bu) and [BMMIm][NTf2] (with BMMIm = 1- butyl2,3-dimethylimidazolium) (see Figure 4.6).
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Figure 4.6 Influence of the ionic liquid chain length on the size of the RuNPs
What are Ionic Liquids?
Ionic Liquid
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26
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Negligible vapor pressure.
High thermal stability
Nonflammable nature
Metling temperature and
hydrophilicity/hydrophobicity can be fine-tuned.
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The positive effects of ionic liquids on catalysis
The catalysts often become more reactive in the presence of ionic liquids.
Formation of More Reactive Catalytic Species in Ionic Liquids.
Stabilization of Reactive Intermediates in Ionic Liquids.
Stabilization of Transition State
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The range of reactions in ionic liquids gives a flavour of what can be achieved in these neoteric solvents. Because the properties and behavior of the ionic liquid can be adjusted to suit an individual reaction type, they can truly be described as designer solvents. We have shown that, by choosing the correct ionic liquid, high product yields can be obtained, and a reduced amount of waste can be produced in a given reaction. Often the ionic liquid can be recycled, and this leads to a reduction of the costs of the processes. It must be emphasized that reactions in ionic liquids are not difficult to perform and usually require no special apparatus or methodologies. The reactions are often quicker and easier to carry out than in conventional organic solvents.
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4.5 Carbon–carbon Coupling Reactions
Pd nanoparticles have attracted a great deal of interest as catalysts for C–C coupling reactions, especially in Heck and Suzuki couplings. The first Heck type coupling reaction that was catalyzed by Pd NPs stabilized with tetraoctylammonium bromide was reported by Beller et al. Such Pd NPs were also effective for Suzuki coupling reactions as well as the ones protected with dendrimers in the work of Narayanan et al. Despite the progress in asymmetric catalysis, there are few examples of nanocatalysts displaying an enantioselective catalytic activity. Pt(Pd)/cinconidine and Pd/BINAP colloids were used for the hydrogenation of ethyl pyruvates and the asymmetric hydrosilylation of styrene, respectively.
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R-X + R'-SnR3 Pd(0) R-R' + XSnR3 aryl or
vinyl halide
organotin
R-X + R'-ZnR Pd(0) R-R' + XZnR aryl or
vinyl halide
organozinc
R-X + R'-B(OH)2 Pd(0) R-R' + XB(OH)2 aryl or
vinyl halide
organoboron vinyl halide
R-X + Pd(0) + HX
aryl or vinyl halide
alkyne
R' R R'
Stille coupling:
Negishi coupling:
Suzuki coupling:
Sonogashira coupling:
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4.5.1 Pd Nanoparticles Stabilized by Chiral Diphosphite Ligand
Pd NPs stabilized by the chiral xylofuranoside diphosphite ligand, L1, were achieved by decomposition of [Pd2(dba)3] (THF; P(H 2)=3 bar; r.t.)and were
investigated in the allylic alkylation of rac-3-acetoxy-1,3-diphenyl1-propene (rac- I) with dimethyl malonate (see Figure 4.7). With the Pd/L1 nanocatalyst, this
reaction proceeded with only one enantiomer of the racemic substrate (> 95%
ee), while the corresponding molecular species stabilized with the same ligand yielded both enantiomers. These results demonstrated a very high degree of kinetic resolution with Pd/L1 NPs
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Figure 4.7 Chiral diphosphite ligands and the corresponding organometallic synthesis of PdNPs.
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Table 4.1 Asymmetric allylic alkylation of rac‐3‐acetoxy‐1,3‐diphenyl‐1‐propene (rac‐I) catalyzed by molecular and colloidal Pd systems containing chiral diphosphite L1.
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A new family of hybrid pyrazole derived ligands containing alkylether, alkylthioether or alkylamino moieties was used in the preparation of Pd NPs and their corresponding molecular complexes in order to compare their catalytic performance in Suzuki– Miyaura and homocoupling reactions (see Figure 4.8).
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Figure 4.7 Synthesis of PdNPs stabilized by pyrazole derived ligands with [Lx]–Pd=1.0
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40
Pd/L3 nanocatalyst gave a remarkable increase in the yield of homocoupled product with a full and 70% conversion to toluene for 4- iodotoluene and 4-bromotoluene as a substrate, respectively.
Homocoupling reactions in the absence of the phenylboronic acid using Pd/L3 NPs led to a 70% and full conversion to toluene for 4-bromotoluene and 4-iodotoluene, respectively. (Table 4.2)
For all Pd/pyrazole nanocatalysts, catalytic activity was observed both for the C–C coupling reaction and the dehalogenation of the substrate. The dependence of the chemoselectivity of the reaction on the nature of the catalyst strongly suggested that the homocoupling reaction takes place on the surface of the heterogeneous systems.
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Table 4.2 C–C coupling reaction using Pd/pyrazole colloidal solutions
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+ CO + H2 H +
linear (normal) branched (iso)
R R
R
R
alkene isomerization alkene hydrogenation
R
side reactions
*
* Largest homogeneous catalytic process
* > 15 billion pounds of aldehydes (alcohols) per year
• Commercial catalysts are complexes of Co or Rh
• Selectivity to linear (normal) aldehyde important
Otto Roelen (1897-1993)
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In collaboration with Castillo´n and Claver et al., RhNPs were prepared by decomposition of [Rh(3-C3H5)3] and [Rh(-OMe)(COD)]2, in the presence of diphosphite ligands (1 or 2; Rh–L=1 : 0.2; THF; P(H2)=3 bars; r.t.) (see Figure 4.8). For both, TEM micrographs of Rh1 and Rh2 NP samples derived from [Rh(3-C3H5)3] revealed good dispersion of spherical NPs with similar small sizes (Rh1: ~3 nm and Rh2: ~2 nm). Rh3 and Rh4 NPs synthesized from [Rh(-OMe)(COD)]2 showed large and sponge-like spherical superstructures (Rh3 NPs: ~50 nm and Rh4 NPs: ~35 nm) with a tendency to agglomerate. Images at higher magnification revealed small individual NPs in the superstructures (~4 nm).
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44
Figure 4.7 Synthesis of Rh NPs from [Rh(3‐
C3H5)3] and [Rh(‐OMe)(COD)]2 in the
presence of chiral diphosphite ligands 1 and 2
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Rh3 and Rh4 NPs were investigated in the asymmetric hydroformylation of styrene (see Table 4.3). Rh3 NPs showed low conversions while Rh4 ones were more active. The addition of free ligand in the reaction media led to less active but more regioselective catalytic systems. The enantioselectivity was also affected by the addition of the free ligand: from 0% to 40% (S) for Rh3 NPs and from 13% to 24% (S) for Rh4 NPs.
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Table 4.3 Styrene hydroformylation with diphosphite‐stabilized RhNPs
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4.7 Conclusion and Perspective
In this chapter, we presented an overview of the major results obtained in the synthesis of metal nanoparticles from organometallic complexes and their investigation in catalysis for various reactions. Despite the poor handling of metal–organic precursors which are highly sensitive towards oxygen and moisture, the organometallic approach is still a versatile method in the preparation of both soluble and supported MNPs. It allows us to tune the nature of the MNPs for the precise control of size, shape, composition and surface state according to the aim of the catalytic reaction.
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48
In solution, the control of the characteristics of the MNPs is governed by a ligand that is stabilizing by nature and by controlling the key parameters of the syntheses such as the rate of nucleation and the rate of growth. With the experience gained from homogeneous catalysis,the ligands already known to orient catalytic properties, such as phosphine and carbene derivatives, can also be used for very small nanocatalysts. Moreover, ligands can be specifically chosen for their advantageous properties such as solubility or enantioselectivity.
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For instance, water soluble NPs are prepared using amphiphilic ligands.
Enantioselective catalysis could also be achieved by using oxazoline or diphosphite ligands, but keeping in mind the low chiral induction.
Concerning the immobilization of MNPs onto supports, the most promising results evidenced the important role of the functionalization of the support to increase the anchorage of the particles and consequently their catalytic performance by increasing the selectivity and the recovery of the catalyst.
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50
The preparation of nanocatalysts displaying desirable catalytic performance is one of the challenges which remain to be met in nanocatalysis.
Although numerous efforts with various capping agents have been made over the past few years by researchers all over the world, future progress will probably necessitate the design of molecules more appropriate for the metal surface to induce higher catalytic properties in terms of selectivity.
The recycling and the recovery of the nanocatalysts are still to be improved at this point.