NiII and CuII complexes of a salen ligand bearing ferrocenes in its secondary coordination sphere

Abstract

Herein, we report the synthesis, spectroscopic characterization and electrochemical investigation of the Ni^II and Cu^II complexes of a novel Sal ligand bearing two ferrocene moieties attached at its diimine linker, M(Sal)^Fc. The electronic spectra of M(Sal)^Fc are near identical to its phenyl-substituted counterpart, M(Sal)^Ph, indicating the ferrocene moieties exist in the secondary coordination sphere of M(Sal)^Fc. The cyclic voltammograms of M(Sal)^Fc exhibit an additional two-electron wave in comparison to M(Sal)^Ph, which is assigned to the sequential oxidation of the two ferrocene moieties. The chemical oxidation of M(Sal)^Fc, monitored by low temperature UV-vis spectroscopy, supports the formation of a mixed valent Fe^IIFe^III species followed by a bis(ferrocenium) species upon sequential addition of one and two equivalents of chemical oxidant. The addition of a third equivalent of oxidant to Ni(Sal)^Fc yielded intense near-IR transitions that are indicative of the formation of a fully delocalized Sal-ligand radical (Sal˙), while the same addition to Cu(Sal)^Fc yielded a species that is currently under further spectroscopic investigation. These results suggest the oxidation of the ferrocene moieties of M(Sal)^Fc does not affect the electronic structure of the M(Sal) core, and these are thus in the secondary coordination sphere of the overall complex.