Synthesis, spectroscopy, electrochemistry, and photochemistry of cyano-bridged mixed-valence coordination compounds containing two different types of intervalent charge-transfer bands

The tetranuclear and pentanuclear mixed-valence coordination compounds Na[(NC)5FeII-μ(CN)-PtIV(NH3)4-μ(NC)-FeII(CN)4-μ(CN)-RuIII(NH3)5], or FePtFeRu, and [RuIII(NH3)5-μ(NC)-FeII(CN)4-μ(CN)-PtIV(NH3)4-μ(NC)-FeII(CN)4-μ(CN)-RuIII(NH3)5](OSO2CF3)2, or RuFePtFeRu, were synthesized and characterized by IR and UV–Vis spectroscopy, electron microprobe analysis (EPMA), inductively coupled plasma (ICP), and cyclic voltammetry (CV). Both molecules exhibit FeII → PtIV intervalent charge transfer (IVCT) absorptions in the 400–450 nm range and FeII → RuIII transition(s) between 750 and 950 nm. The energies, intensities, and half-widths of these transitions correspond well with those of model compounds. The cyclic voltammogram of FePtFeRu between 0.00 and 0.90 V versus SCE exhibits two quasi-reversible Fe waves at 0.56 and 0.74 V versus SCE, while that for RuFePtFeRu has only one Fe redox event at 0.72 V versus SCE. When the potential of the working electrode is scanned negative of −0.38 V versus SCE, however, both complexes undergo an ECE (electrochemical–chemical–electrochemical) mechanism whereby the electrochemical reduction of Ru(III) is followed by a double electron transfer to reduce Pt(IV) to Pt(II). Upon reduction to Pt(II), the cyanide bridges break and the complexes dissociate into smaller fragments. Irradiation of the FeII → PtIV IVCT transition in both compounds leads to a photolysis solution that contains dissociated Fe(II)–Ru(III) as one of its products. Irradiation of the FeII → RuIII IVCT transition yields a similar UV–Vis spectrum, suggesting that the same intermediate is common to both photolysis mechanisms. The implications of this research within the larger context of multiple electron transfer are also discussed.