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

• Published in: Inorganica Chimica Acta 2009-04, Vol.362 (6), p.1701-1708
• Main Authors: Pfennig, Brian W., Norris, Michael R., Zimmerman, Nicole, Gallagher, Jessica R., McCloskey, Alexander I.
• Format: Article
• Language: English
• Subjects: Coordination chemistry; Electrochemistry; Electron transfer; Mixed valence; Photochemistry
• ISSN: 0020-1693; 1873-3255
• DOI: 10.1016/j.ica.2008.07.023

The spectroscopy and electrochemistry of two polynuclear mixed-valence compounds containing both Fe(II) → Pt(IV) and Fe(II) → Ru(III) IVCT bands was investigated. 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-μ(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 → Ru III 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 II → Ru III 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.