Aminoethylglycine-functionalized Ru(bpy)₃²⁺ with pendant bipyridines self-assemble multimetallic complexes by copper and zinc coordination

Directed self-assembly using inorganic coordination chemistry is an attractive approach for making functional supramolecular structures. In this article, the synthesis and characterization of Ru(bpy)₃²⁺ compounds derivatized with aminoethylglycine (aeg) substituents containing pendant bipyridine (bpy) ligands is presented. The free bpy ligands in these complexes are available for metal chelation to form coordinative cross-links; addition of Cu²⁺ or Zn²⁺ assembles heterometallic structures containing two or three transition-metal complexes. Control over relative placement of metal complexes is accomplished using two strategies: two bipyridine-containing aeg strands tethered to Ru(bpy)₃²⁺ allow intramolecular coordination and result in a dimetallic hairpin motif. Ru(bpy)₃²⁺ modified with a single strand forms intermolecular cross-links forming the trimetallic complex. Each of these is characterized by a range of methods, and their photophysical properties are compared. These data, and comparison to an acetyl aegmodified Ru(bpy) ₃²⁺ complex, confirm that the metal ions cross-link bpy-containing aeg strands. Heterometallic complexes containing bound Cu ²⁺ cause a dramatic reduction in the Ru(bpy)₃²⁺ quantum yields and lifetimes. In contrast, the Ru(bpy)₃²⁺ hairpin with coordinated Zn²⁺ has only a slight decrease in quantum yield but no change in lifetime, which could be a result of steric impacts on structure in the dimetallic species. Analogous effects are not observed in the trimetallic Ru-Zn-Ru structures in which this constraint is absent. Each of these heterometallic structures represents a facile and reconfigurable means to construct multimetallic structures by metal-coordination-based self-assembly of modular artificial peptide units.