Ligand stabilized clusters

The ligand-stabilized cluster [Ru3(CO)7(y 3, rf x/ -4,6,8-lrimelhylazulene)] reacts with PhMe2Si-H to yield a new cluster containing a partially hydrogenated azu-lene ligand Ru3(CO)7(//3, if //5-4,5-dihydro-4,6,8-lrimethylazulene). Both of these complexes are efficient catalyst precursors for the hydrosilylation of aceto-... 

Further indications that capping ligands stabilize clusters toward redox reactions is found with [Co3(/i,3-E)(/i.3-E )Cp3] (76), which adopt a structure similar to 66. Two reversible one-electron oxidations and one reduction are found for 76 (E = E = S), and the monocation and monoanion of 76 (E = S, E = CS) 149) were detected by cyclic voltammetry. 

Ligand-stabilized clusters could be suited to solve several problems in catalysis simultaneously by virtue of their uniform size, their solubility, their use on different supports, and the control of their selectivity by varying their ligand shells. 

Summarizing these experiments, the use of different ligand molecules, covering identical metal clusters, can lead to remarkable differences in activity and selectivity. The fact that ligand-stabilized clusters can be synthesized, modified, and investigated in advance of their immobilization on supports distinguishes these catalysts in a decisive manner from established metal catalysts. 

Figure 3. Schematic diagram of the ligand-stabilized cluster Au5s(PPh3)i2Cl2. The metal core, a regular polyhedron, has a diameter of 1.4 nm and it is shrouded by umbrella-shaped PPhs ligands to give a total diameter of 2.1 nm (reproduced, with permission, from Ref. 7).

To avoid this problem Chi et al. reported STS on AU55 monolayers prepared on various substrates (Fig. 5) by means of a two-step self-assembly (SA) process and by a combined Langmuir-Blodgett/SA-process. The spectroscopy gives clear evidence of Coulomb blockade at room temperature originating from the double barrier at the ligand-stabilized cluster as the central electrode. 

A. Schnepf (2004) Angewandte Chemie International Edition, vol. 43, p. 664 - Novel compounds of elements of group 14 ligand stabilized clusters with naked atoms . 

These very preliminary results of catalytic reactions with supported large ligand stabilized clusters are promising. The present experiments indicate a considerably increased activity compared with usual systems. For the present, one may interpret these results in terms of the uniformity of the cluster molecules used because the influence of the ligands is still not understood. 

A new class of metal colloids has recently been established in which the surface of the particle is covered by relatively small ligand molecules such as sulfonated triphenylphosphine. [30] These materials, which will be discussed in due course below, overlap with the giant ligand-stabilized clusters mentioned in Section 6.1.3, and covered in detail elsewhere in this volume. 

Condensed clusters separated by nonmetal atoms, or ligand stabilized clusters in crystal or pellet forms, and composed of appropriate elements, may lead to important new magnetic materials. 

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