Bimetallic Supported Catalysts from Single-source Precursors - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 64

3.3 Clusters as Precursors

3.3.1 Definition
The term 'cluster' to define a multinuclear molecular complex presenting
M-M bonds was first introduced by Cotton. 28 In this chapter, we will refer to
clusters as molecular species presenting at least three metal atoms bonded
by a minimum of two M-M bonds and surrounded by a layer of organic
ligands. These ligands confer them solubility, hence these compounds are
amenable to NMR, infrared and mass spectrometry characterizations.
The definite nuclearity and geometry of the metal core as well as three-
dimensional arrangement of the ligands can only be precisely determined by
crystallography. Clusters can be homometallic (i.e., presenting only metal
atoms of one particular nature) or heterometallic. For the purpose of the
present chapter, we will mainly focus on the latter.
Clusters are thus intermediate species between coordination compounds
('complexes') and free-standing nanoparticles, which used to be called
'colloids'. Hence confusion arises in the literature because the term 'cluster'
is often used in very general terms for a small agglomerate of a few metal
atoms, in the gas phase or immobilized on a support. This is not the same
definition as presented here, because these agglomerates are devoid of lig-
ands and are usually not monodisperse. They are not molecular species that
can be solubilized, purified and crystallized. A definition that has appeared 29
concerning molecular clusters is connected to this, stating that clusters are
colloids that can be crystallized and characterized by single-crystal X-ray
diffraction. Moreover, spectroscopic characterization by IR, NMR and mass
spectrometry showed that the solid-state structure is usually retained in
solution. A crystal structure is not obtained for random agglomerates.
However, the border between very large molecular metal clusters and very
small ligand-stabilized metal nanoparticles ('colloids') is dicult to draw. 29
Indeed, the number of metal atoms in their cores can be very similar. The
largest molecular carbonyl-only metal cluster that has been characterized up
to now is [Ni 32 Pt 24 (CO) 56 ] 6 in which the size is ca. 2.1 nm in diameter. 30
These two fields will probably merge, because one of the major current goals
of nanochemistry is obtaining atomically monodisperse ultra-small metal
nanoparticles (smaller than 5 or even 3 nm). 29,31
By increasing the number of metal atoms, a transition from an electron-
precise molecular species with covalent bonds to a metallic state should
occur. 31 From a theoretical point of view, this corresponds to a transition
of the electronic state of the cluster from discrete molecular eigenstates
(localized electrons) at low nuclearities to highly delocalized electrons sub-
ject to quantum size effects at high nuclearities. This means that as the
nuclearity increases, the HOMO-LUMO gap should decrease, and eventually
a continuum of energy levels should appear as the bulk state is reached.
Molecular clusters should therefore undergo insulator-to-metal transitions
with increasing nuclearity, passing through a semiconductor regime. How-
ever, ligand coordination partially quenches the metallization process.

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