Molecular Events Switched by Transition Metals

Transition metals can typically give rise to two (or more) distinct states of comparable stability (two consecutive oxidation states; two different stereochemical arrangements). In a multicomponent system, the conversion of one state to the other can modify a given property of a nearby subunit or can induce drastic changes in the system topology. In this sense, the metal behaves as a switch, which can be operated through an external input (the variation of the pH or of the redox potential). Recent examples of molecular switching by transition metals are reviewed: (i) the quenching/enhancement of the emission of a luminescent fragment effected by a nearby metal centred redox couple (e.g. Ni-II/Ni-III); (ii) the pH driven motion of an aminoalkyl side chain in a Ni-II scorpionate complex, which is signalled by the variation of the light emission intensity of an appended anthracenyl fragment; (iii) the pH controlled translocation of a Ni-II ion within a multidentate ligand containing two compartments of different coordinating tendencies and (iv) the intramolecular translocation of a Cl- anion between two pre-positioned metal centres Cu-II and Ni-II, within a ditopic receptor, electrochemically driven through the Ni-II/Ni-III redox change.