Tertiary Hierarchical Complexity in Assemblies of Sulfur-Bridged Metal Chiral Clusters

Self-assembly
of three-dimensional structures with order across
multiple length scaleshierarchical assemblyis of great
importance for biomolecules for the functions of life. Creation of
similar complex architectures from inorganic building blocks has been
pursued toward artificial biomaterials and advanced functional materials.
Current research, however, primarily employs only large, nonreactive
building blocks such as Au colloids. By contrast, sulfur-bridged transition
metal clusters (<2 nm) are able to offer more functionality in
catalytic and biochemical reactions. Hierarchical assembly of these
systems has not been well researched because of the difficulty in
obtaining single-phase clusters and the lack of suitable ligands to
direct structure construction. To overcome these challenges, we employ
a rigid planar ligand with an aromatic ring and bifunctional bond
sites. We demonstrate the synthesis and assembly of 1.2 nm sulfur-bridged
copper (SB-Cu) clusters with tertiary hierarchical complexity. The
primary structure is clockwise/counterclockwise chiral cap and core
molecules. They combine to form clusters, and due to the cap–core
interaction (C–H···π), only two enantiomeric
isomers are formed (secondary structure). A tertiary hierarchical
architecture is achieved through the self-assembly of alternating
enantiomers with hydrogen bonds as the intermolecular driving force.
The SB-Cu clusters are air stable and have a distribution of oxidation
states ranging from Cu(0) to Cu­(I), making them interesting for redox
and catalytic activities. This study shows that structural complexity
at different length scales, mimicking biomolecules, can occur in active-metal
clusters and provides a new platform for investigation of those systems
and for the design of advanced functional materials.