Full-shell clusters

The next available full-shell cluster that has been investigated by MoBbauer spectroscopy was the four-shell cluster Pt309phen 3603o+io (phen = 4,7-p-C6H4S03Na substituted 1,10-phenanthroline) [17]. Its inner core consists of 147 atoms. However, since platinum is not MoBbauer-active, the cluster sample had to be irradiated with thermal neutrons to transfer a fraction of the Pt... 

The introduction of organometaUic ligands as stabilizers by Schmid et al. in 1981 [48] and Vargaftik et al. in 1985 [49] paved the way for the precise molecular definition of metallic full shell clusters, which are well defined in size and shape [58,59]. P, N, S donors have been exploited extensively over the years (cf. Sections 3.1 and 3.2) [60,61]. 

The metal nuclei of the so-called full-shell clusters, also referred to as magic number clusters, generally exhibit hexagonal or cubic close packing (ccp). Figure 6 shows the stepwise formation of the first magic-number clusters built by 13, 55, 147, and 309 atoms. 

Figure 6. Stepwise formation of full-shell clusters, 13 (1 + 12), 55 (13 + 42), 147 (55 + 92), and 309 (147 + 162). Generally lOn -for the nth shell. (Reproduced with kind permission of G. Schmid, University Essen-Duisburg, Germany.)...

One strategy adopted to synthesize large full-shell clusters exists in the generation and aggregation of metal atoms in the presence of a few but geometrically appropriate... 

Exercise 3.16. Calculate the total number of atoms and the % surface atoms in full-shell clusters of the type represented by two-shell Au55(PR3)i2Cl6 for 3,4 and 5 shells. 

The formation of full-shell clusters is a consequence of their thermodynamic stability. The ideal shape of such particles makes them unusually stable species with respect to chemical reactions. Thus, ligand-free AU55 clusters cannot be oxidized... 

Conditions for the successful synthesis of full-shell clusters are first, the possibility of unhindered collisions of the metal atoms, and second, the prevention of a local ligand excess, leading to smaller clusters and complexes. The only method of fulfilling these conditions hitherto described is the use of diborane(6) as a reducing agent for metal halides and additionally as a Lewis acid to bind phosphine to such an extent that only very small amounts of free ligand molecules are present in solution. 

Figure 1 illustrates the structural principles of some existing full-shell clusters. 

Scheme 1 describes the synthesis of some palladium full-shell clusters with different ligands. The ligand molecules influence not only the cluster size but, as will be shown, especially the catalytic selectivity [6]. 

Example Pd2057Ph i 84O"-i600 [15] Figure 1. Examples of ligand-stabilized full-shell clusters. 

Much larger clusters have, however, been found in a series of giant metal-cluster molecules, the metal cores of which are members of the series of so-called magic-number (full-shell) clusters depicted in Fig. They are obtained by surrounding... 

Full-shell clusters (no. of shells) Total Number of Atoms Surface Atoms (%)... 

Figure 3.106 Organization of cubo-octahedral full-shell clusters with lOn -I- 2 atoms in the nth shell.

Figure 3.107 TEM images of full-shell clusters, (a) A Pt309 four-shell cluster (b) A Pdsgi five-shell cluster.

In the past, metal nanoparticle synthesis has been mainly focused on a limited size distribution, with the value of any new procedure consisting mainly of improvements in size distribution. Except for a few examples of discrete size (as in the full-shell clusters M55, M309 or M561), particles of 10% in size are generally also referred to as monodisperse. ... 

The structure of the Ni2i framework can be understood as an intermediate on the way to a series of full-shell clusters in which a central cuboctahedron is surrounded by n shells each of which contain (lOn + 2) metal atoms (for n > 1). This principle, as proposed by G. Schmid, is exemplified by dusters of the type [Mj5(PPh3)i2Q6] (see Chapter 3.3). The atoms Nil-Ni4 and Nil -Ni4 can be regarded as being part of a second sheU which can contain a maximum of 42 Ni atoms. 

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