Bimetallic Supported Catalysts from Single-source Precursors - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 65

3.3.2 Synthesis
Many synthetic routes exist to obtain mono- or heterometallic clusters. 32 The
most common reactions for the build-up of cluster species involve reduction
of metal salts in the presence of potential ligands. As many transition metal
clusters are carbonylated species, these reactions are very often simply per-
formed under a carbon monoxide atmosphere. Again, it should be noted
that this parallels metal nanoparticle syntheses that also very often involve
metal salts and reducing agents. For clusters, and especially mixed-metal
compounds, other methods exist that can start from salts, mononuclear
complexes or preformed clusters. Redox condensation has been widely used,
whereby two fragments containing one different metal each merge together
with concomitant exchange of electrons. Other types of condensation re-
actions, with thermal or high pressure assistance also lead to cluster for-
mation. In most cases, these reactions are not predictable and can yield
unexpected products. This is due to the very small energetic differences
between similar geometries and/or nuclearities, especially when the nucle-
arity increases. Isomerization of clusters in solution is a highly facile process
that gives rise to fluxionality on the NMR timescale. To construct hetero-
metallic clusters, strategies involving bridging ligands have been more
selective, as well as using electrophilic capping groups. However, designing
a synthetic procedure for a given cluster structure is not yet feasible.
Nevertheless, since this field bloomed in the 1980s, many species, with
variable nature of metallic atoms, type of ligands, nuclearities and
stoichiometries, have appeared over the years, that can be reliably obtained
in a pure form.

d n 9 r 4 n g | 5

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3.3.3 Clusters in Catalysis
The first application of clusters in catalysis was called 'the cluster-surface
analogy', and consisted in finding parallels between ligands reactivity within
clusters and transformations of organic molecules on metallic surfaces. 33 It
arises from the observations that the metal packing in clusters often re-
sembles that of the bulk metallic state. 29 In addition, rearrangements of
metal cluster cores have been connected to the hopping mechanism of
adatom diffusion on metal surfaces. The cluster-surface analogy seems to be
valid for structural considerations of ligand bonding. However, the reactivity
observed on surfaces and with heterogeneous catalysts is not always the
same as on clusters. 29
Cluster compounds have also been tested quite successfully as organo-
metallic compounds in homogeneous catalysis. 34 The great challenge in this
area was to prove that the active species is indeed the intact cluster and not
decomposition products, like mononuclear complexes or colloidal nano-
particles formed in situ. This can be studied by using various poisoning tests,
kinetics and spectroscopic characterizations. Proofs of intact cluster cata-
lysis have been gained from observed synergistic effects with heterometallic
clusters or very stable capped triangular clusters. Also, asymmetric induction