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Syntheses and Crystal Structures of Disulfide-Bridged Binuclear Ruthenium Compounds:  The First UV−Vis, Raman, ESR, and XPS Spectroscopic Characterization of a Valence-Averaged Mixed-Valent RuIIISSRuII Core

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posted on 17.04.1996, 00:00 by Kazuko Matsumoto, Takaya Matsumoto, Masaki Kawano, Hiroshi Ohnuki, Yushi Shichi, Toshikazu Nishide, Toshio Sato
Disulfide-bridged binuclear ruthenium complexes, [{RuCl(P(OMe)3)2}2(μ-Cl)2(μ-S2)] (1), [{RuCl(P(OMe)3)2}(μ-Cl)2(μ-S2){Ru(CH3CN)(P(OMe)3)2]+ ([2]+), [{Ru(CH3CN)3(P(OMe)3)2}2(μ-S2)]3+ ([3]3+), [{Ru(CH3CN)(P(OMe)3)2}2(μ-Cl)2(μ-S2)]2+ ([4]2+), and [{Ru(CH3CN)3(P(OMe)3)2}2(μ-S2)]4+ ([5]4+), have been synthesized, and their crystal structures have been solved. Compounds 1, [2]+, and [4]2+ have a triply bridged RuIII (μ-Cl)2(μ-S2)RuIII core, in which the S22- ligand bridges the two Ru atoms in a cis configuration. Compounds [3]3+ and [5]4+ have a singly bridged trans-RuSSRu core, whereby [3]3+ corresponds to a one-electron reduced form of [5]4+. Compound [3]3+ is the first example of a well-characterized mixed-valent compound with a trans-MSSM core, where M is any metal. All the compounds have intense absorption bands at around 700 nm, which can be explained for [3]3+ and [5]4+ as a π−π* transition of the distinct trans-RuSSRu core. Resonance Raman spectroscopy of 1, [2]+, and [3]3+ and comparison with several literature values for cis-RuSSRu compounds show that only [3]3+ exhibits a strong ν(S−S) Raman band, when excited by λe = 647.1 nm, whereas all the others show strong to medium ν(Ru−S) and very weak ν(S−S) bands. The ESR spectrum of [3]3+ shows a rhombic signal with g1 = 2.12, g2 = 2.05, and g3 = 1.995. This anisotropy is unusually small, compared to most mononuclear and binuclear Ru(III) compounds with various ligands. Analysis of the g values by use of the matrix of spin−orbit coupling Hamiltonian has revealed a very small spin−orbit coupling constant of 100 cm-1, which is a result of the extensive covalency of the metal−disulfide bond. The X-ray photoelectron spectrum of [3]3+ did not give any of the expected double peaks of the Ru(II) and Ru(III) components; the observed peaks are Ru3d5/2 281.0 eV, 3P3/2 462.4 eV, S(S22-) 2P3/2 162.7 eV. Compound [3]3+ does not give any intervalence-transition band in the longer-wavelength visible to near-IR region, other than the UV−vis band similarly observed in the one-electron oxidized compound [5]4+. These characteristics are reasonably understood, if [3]3+ is regarded as a mixed-valent complex with valence-averaged ground state (class III of the Robin and Day classification).