Stability and lability of dicopper double-stranded helicates in solution

Stability constants have been measured for a series of ligands based on a 2,2'-(pyridine-2,6-diyl)bis[1H-benzimidazole] unit which forms dinuclear double-stranded helical complexes with copper(I). Variation of different structural parameters confirms the importance of the coordinate bond, the stacking interactions, and the weakly bridging pyridine units observed by X-ray crystallography. The stabilities of the complexes depend strongly on the solvent, and in MeCN, which is a good solvent for copper(I), the complexes are less stable and assemble in a stepwise manner. The interconversion of the enantiomers may be followed by (1)H-NMR and takes place on a millisecond time scale around room temperature. The trends in lability are similar to those found for the stability of the complexes.