Enhanced kinetic inertness in the electrochemical interconversion of Cu(i) double helical to Cu(ii) monomeric complexes

Abstract

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 non-coordinating, 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.