FERROCENE METALLOCYCLAM CONJUGATES - NEW REDOX SYSTEMS WHOSE 2-ELECTRON ACTIVITY CAN BE MODULATED THROUGH THE MEDIUM

The two conjugate systems 1 and 2, in which a ferrocene subunit and a nickel-cyclam fragment are covalently linked, have been prepared and investigated for their redox behavior, using voltammetric techniques. 1 has been synthesized through the reaction of Ni(II) with the free ligand 3, prepared from the reaction of (ferrocenylmethyl)trimethylammonium iodide with a 5-fold excess of cyclam. 2 has been obtained by a very convenient one-pot template procedure, in which ferrocenesulfonamide acts as a locking fragment in closing an open-chain tetraamine, in the presence of formaldehyde. Both 1 and 2 undergo a two-electron oxidation process through two consecutive reversible one-electron steps, the first one being ascribed to the oxidation of the metallorganic moiety and the second one to the Ni(II) to Ni(III) change. The difference between the two corresponding potentials, DELTAE = E(NiL) - E(Fc), decreases with the increasing donor tendencies of the medium, reflecting the stabilization of the Ni(III) state by the axially bound solvent molecules. Such an effect is not experienced by the coordinatively saturated ferrocene moiety. A ferrocene fragment has also been appended to 1,4,7-triazacyclononane, The ferrocene-functionalized triaza macrocycle 4 complexes Ni(II) according to a 2:1 stoichiometry. The two appended organometallic subunits display independent redox behaviors and release two electrons at the same potential, whereas a further electron is released by the Ni(II) center at a very positive potential, due to the strong repulsive electrostatic effects exerted by the proximate ferrocenium subunits. Ferrocene-metallocyclam conjugates represent a novel class of versatile multielectron-redox systems, whose activity can be controlled from the outside (solvent, background electrolyte).