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 st ereochemical arrangements). In a multicomponent system, the conversion of o ne state to the other can modify a given property of a nearby subunit or ca n induce drastic changes in the system topology. In this sense, the metal b ehaves as a switch, which can be operated through an external input (the va riation 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 centre d redox couple (e.g. Ni-II/Ni-III); (ii) the pH driven motion of an aminoal kyl side chain in a Ni-II scorpionate complex, which is signalled by the va riation of the light emission intensity of an appended anthracenyl fragment ; (iii) the pH controlled translocation of a Ni-II ion within a multidentat e ligand containing two compartments of different coordinating tendencies a nd (iv) the intramolecular translocation of a Cl- anion between two pre-pos itioned metal centres Cu-II and Ni-II, within a ditopic receptor, electroch emically driven through the Ni-II/Ni-III redox change. (C) 1999 Elsevier Sc ience S.A. All rights reserved.