Conspectus
Chemical research in synthesizing metal nanoparticles has been
a major topic in the last two decades, as nanoparticles can be of
great interest in many fields such as biology, catalysis, and nanotechnology.
However, as their chemical and physical properties are size-dependent,
the reliable preparation of nanoparticles at a molecular level is
highly desirable. Despite the remarkable advances in recent years
in the preparation of thiolate- or p-MBA or PA-protected
gold and silver nanoclusters (p-MBA = p-mercaptobenzoic acid; PA = phenylalkynyl), as well as the large
palladium clusters protected by carbonyl and phosphine ligands that
initially dominated the field, the synthesis of monodispersed and
atomically precise nanoparticles still represents a great challenge
for chemists.
Carbonyl cluster compounds of high nuclearity
have become more
and more part of a niche chemistry, probably owing to their handling
issues and expensive synthesis. However, even in large size, they
are known at a molecular level and therefore can play a relevant role
in understanding the structures of nanoparticles in general. For instance
the icosahedral pattern, proper of large gold nanoparticles, is also
found in some Au–Fe carbonyl cluster compounds.
Rh clusters
in general can also be employed as precursors in homo-
and heterogeneous catalysis, and the possibility of doping them with
other elements at the molecular level is an important additional feature.
The fact that they can be obtained as large crystalline species, with
dimensions of about 2 nm, allows one to place them not only in the
nanometric regime, but also in the ultrafine-metal-nanoparticle category,
which lately has been attracting growing attention. In fact, such
small nanoparticles possess an even higher density of active catalytic
sites than their larger (up to 100 nm) equivalents, hence enhancing
atom efficiency and reducing the cost of precious-metal catalysts.
Finally, the clusters’ well-defined morphology could, in principle,
contribute to expand the studies on the shape effects of nanocatalysts.
In this Account, we want to provide the scientific community with
some insights on the preparation of rhodium-containing carbonyl compounds
of increasing nuclearity. Among them, we present the synthesis and
molecular structures of two new heterometallic nanoclusters, namely,
[Rh23Ge3(CO)41]5– and [Rh16Au6(CO)36]6–, which have been obtained by reacting a rhodium-cluster precursor
with Ge(II) and Au(III) salts. The growth of such clusters is induced
by redox mechanisms, which allow going from mononuclear complexes
up to clusters with over 20 metal atoms, thus entering the nanosized
regime.
