Ruthenium nanoparticles, ligands

In the case of the particles accommodating amine ligands, a new phenomenon has been evidenced, namely, a dynamic exchange at the NMR timescale between free and coordinated amines. It has been correlated to the TEM and HREM results, which show that, at the early stage of the reaction, the particles display a spherical aspect and a small size (ca. 2-3 nm), and that after a few hours, the particles coalesce into elongated wormlike particles, still constituted of pure, unoxidized hep ruthenium. This NMR observation is particularly interesting since it evidences for these particles a fluxionality similar to that of molecular clusters, which is well documented. The ruthenium nanoparticles contain coordinated mobile surface hydrides, as recently demonstrated by a combination of NMR techniques in solution, gas phase, and in the solid state. ... 

Nanoparticles can be protected by the HDA (hexadecyiamine) ligand to prevent reactions between each other and in order to stabilize them in solution [69]. This stabilization mode presents several advantages such as their behavior as molecular systems, control of the size, size distribution and adjustment of the surface state (Fig. 18.13). Solution C NMR studies evidenced a fast exchange on the NMR time scale between amine ligands free and coordinated to mthenium . In this section, the preparation of HDA-protected ruthenium nanoparticles, the study of the size of the nanoparticles and their characterization will be described. 

The structural details of ruthenium nanoparticles are first characterized. Figure 3.12 depicts a representative H NMR spectrum of the Ru=CH-Fc nanoparticles after the metal core is dissolved by dilute KCN. The singlet peak at 4.77 ppm (left dashed box) is attributed to the methine proton of the carbene ligands, and the peaks between 4.0 and 4.4 ppm (right dashed box) are ascribed to the combined contribution of the ferrocenyl protons and the a-methylene protons of the carbene ligands. The ratio between the integrated areas of these two peaks is then used to estimate the surface coverage of Fc on the particle surface, which is summarized in Table 3.3 as sample 111. Nanoparticles with other ferrocene surface concentrations (5%-20%) are prepared and characterized in a similar fashion, and the results are also included in Table 3.3. 

Since the achievements of Grubbs on metathesis, N-heterocyclic carbene ligands (NHCs) are strongly related to ruthenium in molecular chemistry. However, these ligands had not been used for the stabilization of ruthenium nanoparticles despite the work of Tilley et al. on AuNPs [81]. It was therefore of interest, following a comprehensive study of phosphine coordination on nanoparticles, to investigate the interaction between RuNPs and NHCs [82]. The ruthenium nanoparticles were prepared by decomposition of [Ru(COD)(COT)] in pentane (3 bar H2 RT) in the presence of a carbene, i.e. l,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) or N,N-di(ferf-butyl)imidazol-2-ylidene (l Bu), as shown in Pig. 11. 

In summary, we have over the years investigated a wide range of hgands on the surface of ruthenium nanoparticles. The first conclusion is that the behavior of ligands is similar on these nanoparticles and on molecular complexes. Thus, hydrides and CO coordinate on the surface of these nanoparticles, as well as olefins and methyl groups. The hydrides are always fluxional whereas CO appears only fluxional when the surface of the particles is free. Amines are also fluxional and do... 

The coordination of ligands at the surface of metal nanoparticles has to influence the reactivity of these particles. However, only a few examples of asymmetric heterogeneous catalysis have been reported, the most popular ones using a platinum cinchonidine system [65,66]. In order to demonstrate the directing effect of asymmetric ligands, we have studied their coordination on ruthenium, palladium, and platinum nanoparticles and the influence of their presence on selected catalytic transformations. 

Scheme 18.4 Ruthenium carbonyl cluster chemisorbed on the silica surface (as a model of the type of bonding between a surface oxygen atom and a nanoparticle (b)) another example of bonding is represented (c) in which the surface siloxy group behaves as a 4-electron ligand to the particle.

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