Ni(II) and Cu(II) complexes of a salen ligand bearing ferrocenes in its secondary coordination sphere

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(II)Fe(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.