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.