Electrochemical Assembling/Disassembling of Helicates with Hysteresis

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Abstract
A series of eight tetradentate, ditopic, bisimino bisheterocyclic ligands (1−8), and their complexes with CuI and CuII, have been studied in CH3CN solution, by means of 1H NMR, mass, and UV/vis spectroscopy, while the crystal and molecular structure of the CuII complexes [Cu(3)](CF3SO3)2 and [Cu(4)](CF3SO3)2 and of the CuI complexes [Cu2(4)2](ClO4)2 and [Cu2(5)2](ClO4)2 have been determined by X-ray diffraction methods. The CuII complexes are monomeric, almost square-planar structures, both in solution and in the solid state, while the CuI complexes are two-metal, two-ligand 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 CuI helical dimers or the CuII monomers can be obtained and are 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 and are separated by a 500−1000 mV interval. Irreversibility is due to the fast self-assembling process that follows the reduction of [CuII(L)]2+ and to the fast disassembling process that follows the oxidation of [CuI2(L)2]2+ (L = 1−8). However, the overall [oxidation+disassembling] or [reduction+self-assembling] processes, i.e., [CuI2(L)2]2+ = 2[CuII(L)]2+ + 2e-, are fully reversible. Moreover, CV profiles show that solutions containing copper and L undergo hysteresis on changing the applied electrochemical potential: in the same potential interval, the systems can exist in solution as either [CuI2(L)2]2+ or [CuII(L)]2+, depending on the electrochemical history of the solution. Moreover, by changing the structural or donor features of the ligands it is possible to modulate the potentials at which the system undergoes a transition from one to the other of its two possible states, in the hysteresis cycle. In addition, the spectral properties of the CuI and CuII complexes of the considered ligands make these systems good candidates for storing information in solution, which can be electrochemically written or erased and spectroscopically read.