Ferrocene-metallocyclam conjugates: new redox systems whose two-electron activity can be modulated through the medium

The two conjugate systems 1and 2, in which a ferrocene subunit and a nickel-cyclam fragment are covalently linked, havebeenpreparedandinvestigatedfortheirredoxbehavior,usingvoltammetrictechniques. 1hasbeensynthesized throughthereactionofNil1with thefreeligand3,prepared fromthereactionof(ferrocenylmethy1)trimethylammonium iodidewitha5-foldexcessofcyclam. 2hasbeenobtainedbyaveryconvenientone-pottemplateprocedure,inwhich ferrocenesulfonamideactsasalockingfragmentinclosinganopen-chaintetraamine,inthepresenceofformaldehyde. 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 Nil1to Nil1[change. The difference between the two corresponding potentials, AE = E N ~-LE F ~d,ecreases with the increasing donor tendenciesofthemedium,reflectingthestabilizationoftheNil1[statebytheaxiallyboundsolventmolecules. Such aneffectisnotexperiencedbythecoordinativelysaturatedferrocenemoiety. Aferrocenefragmenthasalsobeen appended to 1,4,7-triazacyclononane.The ferrocene-functionalizedtriaza macrocycle 4 complexes Nil1according toa2:1stoichiometry. Thetwoappendedorganometallicsubunitsdisplayindependentredoxbehaviorsandrelease two electrons at the same potential, whereas a further electron is released by the Nil1center at a very positive potential,duetothestrongrepulsiveelectrostaticeffectsexertedbytheproximateferroceniumsubunits. Ferrocene- metallocyclam conjugates represent a novel class of versatile multielectron-redox systems, whose activity can be controlled from the outside (solvent, background electrolyte)