Dinuclear Cyano-Bridged Co^III−Fe^II Complexes as Precursors for Molecular Mixed-Valence Complexes of Higher Nuclearity

The preparation and characterization of a series of trinuclear mixed-valence cyano-bridged Co^III−Fe^II−Co^III compounds derived from known dinuclear [{L_nCo^III(μ-NC)}Fe^II(CN)₅]^- complexes (L_n = N₅ or N₃S₂ n-membered pendant amine macrocycle) are presented. All of the new trinuclear complexes were fully characterized spectroscopically (UV−vis, IR, and ¹³C NMR). Complexes exhibiting a trans and cis arrangement of the Co−Fe−Co units around the [Fe(CN)₆]^4- center are described (i.e., cis/trans-[{L_nCo^III(μ-NC)}₂Fe^II(CN)₄]²⁺), and some of their structures are determined by X-ray crystallography. Electrochemical experiments revealed an expected anodic shift of the Fe^III/II redox potential upon addition of a tripositively charged {Co^IIIL_n} moiety. The Co^III/II redox potentials do not change greatly from the di- to the trinuclear complex, but rather behave in a fully independent and noncooperative way. In this respect, the energies and extinction coefficients of the MMCT bands agree with the formal existence of two mixed-valence Fe^II−CN−Co^III units per molecule. Solvatochromic experiments also indicated that the MMCT band of these compounds behaves as expected for a class II mixed-valence complex. Nevertheless, its extinction coefficient is dramatically increased upon increasing the solvent donor number.