Can the Disproportion of Oxidation State III Be Favored in RuII-OH2/RuIV:O Systems?

Three new Ru-aqua complexes containing a mixed carbene and pyridylic ligands with general formulas [Ru(CNC)(bpy)(H₂O)](PF₆)₂ (1) (CNC is 2,6-bis(butylimidazol-2-ylidene)pyridine; bpy is 2,2′-bipyridine) and cis-/trans-[Ru(CNC)(nBu-CN)(H₂O)](PF₆)₂ (cis–2 and trans–2) (nBu-CN is 2-(butylimidazol-2-ylidene)pyridine) have been prepared and structurally characterized both in the solid state (monocrystal X-ray diffraction analysis for 1 and for the related complex trans-[Ru(Br)(CNC)(nBu-CN)](PF₆)) and in solution (for all of them) through NMR. The electrochemical properties of these three Ru-aqua complexes have been investigated by cyclic voltammetry, differential pulse voltammetry and Coulombimetric techniques. It is found that, for complex 1 at pH 7, the difference between the IV/III and the III/II redox couples (E_1/2) is 50 mV, which is the smallest ever reported for this type of complex. On the other hand, for complexes cis–2 and trans–2, the oxidation state III is unstable with respect to disproportionation to II and IV. The reactivity of their Ru=O species has been tested toward cis–methylstyrene oxidation, and it has been compared to [Ru(O)(trpy)(bpy)]²⁺. An inverse correlation between the degree of cis/trans-epoxide isomerization and E_1/2 is found. In particular, for complexes cis–2 and trans–2, which have a E_1/2 < 0, the epoxidation is highly stereoselective, yielding only cis-epoxide.