Electrochemically controlled assembling/disassembling processes with a bis-imino 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 Cu-II and Cu-I; thus the L/Cu-II/I set is a bistable system. Owing to its quite rigid preorganized structure, L forms the 1:1 complex [(CuL)-L-II](2+) when binding the d(9) cation Cu2+, while with the d(10) cation Cu+, dimeric complexes of the [(Cu2L2)-L-I](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 [(CuL)-L-II](CF3SO3)(2), L binds to the metal center in a square-planar fashion, while in [(Cu2L2)-L-I](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 [(Cu2L2)-L-I](ClO4)(2) both a helical species and a dimeric nonhelical one were found to coexist in the same crystal cell. However, spectrophotometric and H-1 NMR studies demonstrated that, in acetonitrile solution, only two helical forms exist, one of which is more prevalent (87%, at 20 degrees C). The interconversion equilibrium between the two helical forms has been studied in acetonitrile by temperature variable H-1 NMR and the pertinent Delta H-circle minus and Delta S-circle minus 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 [(CuL)-L-II](2+) into the helical [(Cu2L2)-L-I](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 Cu-II/Cu-I 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.