Redox-driven intramolecular anion translocation between transition metal centres

In a two-component system containing two transition metal centres, M-1 and M-2, an anion X- coordinated to M-1 can be translocated to M-2, if i) the latter metal is redox active (through the M-2(n+)/M-2((n+1)+) change) and ii) the affinity towards X- decreases along the series: M-2((n+1)+)>M-1>M-2(n+). In these circumstances, when the M-1 similar to M-2 system is in its reduced form, X- stays on M-1; on oxidation X- moves to M-1. The above-mentioned model has been verified with the covalently linked two-component system i, in which a tripodal tetramine subunit (tren) hosts a Cu-II ion, and a tetramine macrocyclic subunit (cyclam) encircles a nickel centre, which is redox active through the Ni-II/Ni-III couple. Binding tendencies of inorganic anions towards the Cu-II, Ni-II and Ni-III ions, in an MeCN solution, were investigated and compared with those involving the separate components [Cu-II(2)](2+) and [Ni-II,Ni-III(3)](2+/3+). In general, affinity towards X- decreases along the series: Ni-III>Cu-II>Ni-II. Thus, we observed through spectroelectrochemical techniques that in the reduced form of the two-component system Cu(II)similar to Ni-II, the X- anion (Cl-, NCO-) is located on the Cu-II centre, whereas on Ni-II-to-Ni-III oxidation it is translocated to the Ni-III centre. The translocation is quickly reversible and, in the case of the oxidation resistant chloride anion, can be carried out indefinitely through consecutive oxidation and reduction processes, in a controlled potential electrolysis experiment. The intramolecular nature of the redox-driven anion translocation in the Cu(II)similar to Ni-II,Ni-III system is discussed and substantiated by considering the pertinent thermodynamic functions Delta H degrees and Delta S degrees, obtained by temperature dependent voltammetric studies. We conclude that the intramolecular Cl- translocation from Cu-II to Ni-III prevails over any other intermolecular process, due a more favourable entropy contribution