Electrochemically controlled assembling/disassembling processes with a bis-imine bis-quinoline ligand and the CuII/CuI couple

The bis-iminoquinoline quadridentate ligand L is capable of forming air- and moisture-stable complexes both with CuII and CuI; thus the L/CuII/I set is a bistable system. Owing to its quite rigid preorganized structure, L forms the 1:1 complex [CuIIL]2+ when binding the d9 cation Cu2+, while with the d10 cation Cu+, dimeric complexes of the [CuI2L2]2+ type are formed in which each copper cation is coordinated by two iminoquinoline fragments belonging to two different ligands. Crystal and molecular structure determinations showed that, in [CuIIL](CF3SO3)2, L binds to the metal center in a square-planar fashion, while in [CuI2L2](CF3SO3) 2 the Cu+ cations are coordinated with a tetrahedral geometry, with the two ligands L intertwined in a double helix. On the other hand, in the case of [CuI2L2](ClO4)2 both a helical species and a dimeric nonhelical one were found to coexist in the same crystal cell. However, spectrophotometric and 1H NMR studies demonstrated that, in acetonitrile solution, only two helical forms exist, one of which is more prevalent (87%, at 20°C). The interconversion equilibrium between the two helical forms has been studied in acetonitrile by temperature variable 1H NMR and the pertinent ΔH⊖ and ΔS⊖ values have been determined; these account for the small difference in energy between the two species. Finally, cyclic voltammetry and spectroelectrochemical experiments demonstrated that in acetonitrile solution it is possible to rapidly transform [CuIIL]2+ into the helical [CuI2L2]2+ dimer (or vice versa) by changing the potential applied to the working electrode, that is, it is possible to electrochemically control the self-assembly/disassembly process through the CuII/CuI redox couple. Moreover, it has been shown that self-assembly (reduction)/disassembly (oxidation) cycles can be repeated at will, without any degradation of the system.