Discrete RhIII/FeII and RhIII/FeII/CoIII Cyanide-Bridged Mixed Valence Compounds

The heterotrinuclear complexes trans- and cis-[{cis-VI-L15RhIII(μ-NC)}{trans-III-L14SCoIII(μ-NC)}FeII(CN)4]2+ are unprecedented examples of mixed valence complexes based on ferrocyanide bearing three different metal centers. These complexes have been assembled in a stepwise manner from their {trans-III-L14SCoIII}, {cis-VI-L15RhIII}, and {FeII(CN)6} building blocks. The preparative procedure follows that found for other known discrete assemblies of mixed valence dinuclear CrIII/FeII and polynuclear CoIII/FeII complexes of the same family. A simple slow substitution process of [FeII(CN)6]4− on inert cis-VI-[RhIIIL15(OH)]2+ leads to the preparation of the new dinuclear mixed valence complex [{cis-VI-L15RhIII(μ-NC)}FeII(CN)5]− with a redox reactivity that parallels that found for dinuclear complexes from the same family. The combination of this dinuclear precursor with mononuclear trans-III-[CoIIIL14SCl]2+ enables a redox-assisted substitution on the transient {L14SCoII} unit to form [{cis-VI-L15RhIII(μ-NC)}{trans-III-L14SCoIII(μ-NC)}FeII(CN)4]2+. The structure of the final cis-[{cis-VI-L15RhIII(μ-NC)}{trans-III-L14SCoIII(μ-NC)}FeII(CN)4]2+ complex has been established via X-ray diffraction and fully agrees with its solution spectroscopy and electrochemistry data. The new species [{cis-VI-L15RhIII(μ-NC)}{trans-III-L14SCoIII(μ-NC)}FeII(CN)4]2+ and [{cis-VI-L15RhIII(μ-NC)}FeII(CN)5]− show the expected electronic spectra and electrochemical features typical of Class II mixed valence complexes. Interestingly, in the trinuclear complex, these features appear to be a simple addition of those for the RhIII/FeII and CoIII/FeII moieties, despite the vast differences existant in the electronic spectra and electrochemical properties of the two isolated units.