Electrochemical assembling/disassembling of helicates with hysteresis

A series of eight tetradentate, ditopic, bisimino bisheterocyclic ligands ( 1-8), and their complexes with Cu-I and Cu-II, have been studied in CH3CN s olution, by means of H-1 NMR, mass, and UV/vis spectroscopy, while the crys tal and molecular structure of the Cu-II complexes [Cu(3)](CF3SO3)(2) and [ Cu(4)](CF3SO3)(2) and of the Cu-I complexes [Cu-2(4)(2)](ClO4)(2) and [Cu-2 (5)(2)](ClO4)(2) have been determined by X-ray diffraction methods. The Cu- II complexes are monomeric, almost square-planar structures, both in soluti on and in the solid state, while the Cu-I complexes are two-metal, two-liga nd dimers which can be both helical and "box-like" in the solid, while they adopt a simple helical configuration in acetonitrile solution. The systems made of ligands 1-8 and copper are bistable, as under the same conditions either the Cu-I helical dimers or the Cu-II monomers can be obtained and ar e stable. The electrochemical behavior of the 16 copper complexes has been studied in acetonitrile solutions by cyclic voltammetry. One reduction and one oxidation wave were found in all cases, which display no return wave an d are separated by a 500-1000 mV interval. Irreversibility is due to the fa st self-assembling process that follows the reduction of [Cu-II(L)](2+) and to the fast disassembling process that follows the oxidation of [Cu-2(I)(L )(2)](2+) (L = 1-8). However, the overall [oxidation+disassembling] or [red uction+self-assembling] processes, i.e., [Cu-2(I)(L)(2)](2+) = 2[Cu-II(L)]( 2+) + 2e(-), are fully reversible. Moreover, CV profiles show that solution s containing copper and L undergo hysteresis on changing the applied electr ochemical potential: in the same potential interval, the systems can exist in solution as either [Cu-2(I)(L)(2)](2+) or [Cu-II(L)](2+), depending on t he electrochemical history of the solution. Moreover, by changing the struc tural or donor features of the ligands it is possible to modulate the poten tials at which the system undergoes a transition from one to the other of i ts two possible states, in the hysteresis cycle. In addition, the spectral properties of the Cu-I and Cu-II complexes of the considered ligands make t hese systems good candidates for storing information in solution, which can be electrochemically written or erased and spectroscopically read.