Atomically precise monolayer protected
clusters are molecules comprising
a few-atom cluster core of a noble metal, typically Au or Ag, surrounded
by a protective layer of ligands, exhibiting many special optical,
electrical, catalytic, and magnetic properties, and are emerging as
important materials in biology, medicine, catalysis, energy conversion
and storage, and sensing. The structural diversity of these clusters
or aspicules, as we definitively term them, meaning shielded molecules,
combining the Greek word aspis (shield) with molecule,
is rapidly increasing due to new compositions and modification routes
such as ligand-exchange, alloying, or supramolecular functionalization.
We present a structural analysis of the most stable cluster of this
kind, Au25(SR)18, and propose a Borromean rings
diagram for the cluster, showing its topological configuration of
three interlocked (Au8S6)-rings. This simplified
two-dimensional diagram is used to represent its structure and modifications
via ligand or metal atom substitution uniquely. We enumerate and name
its isomers with two-ligand or metal atom substituents. Among the
several structural insights obtained, the identification of the Borromean
rings-interlocked configuration in Au25(SR)18 may explain its high geometric stability and indicate a possible
general unified structural viewpoint for these clusters without the
division between core and staple motifs. On the basis of our structural
analysis, we developed a structure-based nomenclature system that
can be applied to both describe and understand the structure and modifications
of gold thiolate clusters, Au
M
(SR)
N
, and is adaptable to the general case of
M
M
(X)
N
(M,
metal and X, ligand). The application of structural analysis and diagrams
to Au38(SR)24 and Au102(SR)44, revealing the possible formation of the cluster core by stacking
or growth of rings of metal atoms, is also presented.