Allosteric effects in a ditopic ligand containing bipyridine and tetra-aza-crown donor units

Artificial allosterism: A ditopic ligand, which contains both bipyridine and tetra‐aza‐crown binding sites, coordinates one CuII through all four N‐donors of the tetradentate aza‐crown unit. Reaction at the bipyridine site with another CuII allosterically changes the aza‐crown from being a tetradentate to a tridentate N‐donor unit, a change in coordination mode that causes the successive binding of two CuII ions to proceed with severe negative cooperativity (see scheme).