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

摘要

The tetranuclear and pentanuclear mixed-valence coordination compounds Na[(NC)_5Fe~(II)-μ(CN)Pt~(IV)(NH_3)_4-μ(NC)-Fe~(II)(CN)_4-μ(CN)-Ru~(III)(NH_3)_5], or FePtFeRu, and [Ru~(III)(NH_3)_5-μ(NC)-Fe~(II)(CN)_4-μ(CN)Pt~(IV)(NH_3)_4-μ(NC)-Fe~(II)(CN)_4-|ì(CN)-Ru~(III)(NH_3)_5[(OSO_2CF_3)_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 Fe~(II)→Pt~(IV) intervalent charge transfer (IVCT) absorptions in the 400-450 nm range and Fe~(II) 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 Fe~(II)→Pt~(IV) 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 Fe" -. Rum 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.