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

Disulfide-bridged binuclear ruthenium complexes, [{RuCl(P(OMe) 3 ) 2 } 2 (μ-Cl) 2 (μ-S 2 )] ( 1 ), [{RuCl(P(OMe) 3 ) 2 }(μ-Cl) 2 (μ-S 2 ){Ru(CH 3 CN)(P(OMe) 3 ) 2 ] + ( [2] + ), [{Ru(CH 3 CN) 3 (P(OMe) 3 ) 2 } 2 (μ-S 2 )] 3+ ( [3] 3+ ), [{Ru(CH 3 CN)(P(OMe) 3 ) 2 } 2 (μ-Cl) 2 (μ-S 2 )] 2+ ( [4] 2+ ), and [{Ru(CH 3 CN) 3 (P(OMe) 3 ) 2 } 2 (μ-S 2 )] 4+ ( [5] 4+ ), have been synthesized, and their crystal structures have been solved. Compounds 1, [2] +, and [4] 2+ have a triply bridged Ru III (μ-Cl) 2 (μ-S 2 )Ru III core, in which the S 2 2- 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 λ e = 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 g 1 = 2.12, g 2 = 2.05, and g 3 = 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 Ru 3d 5/2 281.0 eV, 3P 3/2 462.4 eV, S(S 2 2- ) 2P 3/2 162.7 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).