Design of redox systems for the selective transport of electrons across liquid membranes: nickel(II,III) tetraaza macrocyclic complexes

NiLX2 (L = N-cetylcyclam; X = Cl, ClO4) were prepd. to be used as carriers for the transport of electrons across a CH2Cl2 bulk liq. membrane, through the NiII/NiIII redox change. Two-phase (H2O/CH2Cl2) expts. showed that aq. peroxydisulfate ion is able to oxidize NiLCl2, but not NiL(ClO4)2, due to the very large difference of the NiIII,II redox potentials. On the other hand, [NiLCl2]Cl in the org. layer was reduced by aq. cationic reducing agents, according to a rate sequence (TiIII > CrII > FeII > [CoII cage] complex) that does not correlate with their redox potential values. The anionic reducing agent I- does not reduce [NiLCl2]Cl under 2-phase conditions, whereas it does in homogeneous conditions. Three-phase expts. were performed in which electrons are transported from an aq. reducing phase, contg. the metal-centered agents mentioned above, to an aq. oxidizing phase, contg. peroxydisulfate and chloride, across a CH2Cl2 membrane contg. the [NiLCl2]/[NiLCl2]Cl redox couple. A counterflow of Cl- is coupled to the flow of electrons. Under the conditions employed, the electron transport is complete in a time ranging from minutes (TiIII) to several hours ([CoII cage] complex).