Self-assembly of tricuprous double helicates: Thermodynamics, kinetics, and mechanism

We report in this paper the coordination and kinetic properties of two oligobipyridine strands, which contain three 2,2′-bipyridine subunits separated by oxydimethylene bridges, the 4,4′-bis(CONET₂)-substituted L and the 4,4′-bis(CO₂Et)-substituted L′. Spectrophotometric measurements allowed the characterization of thermodynamic complexes and kinetic intermediates* which are involved in the self-assembly process of L₂Cu₃ and L₂Cu₃ helicates. The reaction presents positive cooperativity for the binding of two 2,2′-bipyridine strands to the cuprous cations. While reactive kinetic intermediates* present distorted coordination geometries around Cu¹, the final rearrangement of the tricuprous bistranded helicates allows more closely tetrahedral coordination of each cation and reduces the interactions. Differences in the bulkiness and electronic properties of the L and L′ substituents do not affect significantly the stability of the corresponding helicates, but greatly influence binding rates in the self-assembly process.