Enhanced kinetic inertness in the electrochemical interconvertion of Cu(I) double helical – Cu(II) monomeric complexes

Three new ligands made of two iminoquinoline halves separated by an (R,R)-trans-1,2-cyclohexenediyl spacer have been synthesized. These ligands feature –OR functions appended in the 8-positions of the quinoline rings (R = n-alkyl). The ligands display a behaviour similar to that of their analogues that contain unsubstituted quinolines, forming a bistable system with copper. [Cu2L2]2+ helicates are obtained with Cu+ and [CuL]2+ monomers with Cu2+, as shown by UV/Vis titrations, determination of complex formation constants, mass and NMR measurements, and X-ray crystallographic analysis. The OR groups are found to be noncoordinating, but the presence of such substituents on the quinoline rings slows the electrochemical interconversion of [Cu2L2]2+ into [CuL]2+. In particular, oxidation of [Cu2L2]2+ gives a reversible two-step profile in cyclic voltammetry experiments, due to the formation of the Cu2+ helicate [Cu2L2]4+, that does not evolve into [CuL]2+in the CV experiment time scale