Dinuclear alkoxide-bridged ruthenium(II) complexes with class III mixed-valence states: a structural and spectroelectrochemical study

Abstract

Reaction of [Ru(bipy)₂Cl₂](bipy = 2,2′-bipyridine) with NaOMe or KOH in methanol at reflux afforded high yields of the dinuclear complex [{Ru(bipy)₂}₂(µ-OMe)₂][PF₆]₂1. Reaction of 1 with NaOEt in ethanol resulted in exchange of the bridge to give [{Ru(bipy)₂}₂(µ-OEt)₂][PF₆]₂2. The crystal structures of 1 and 2 showed that the complex cations have similar structures in which two Ru(bipy)₂²⁺ fragments, which for steric reasons have the same absolute configuration, are linked by two alkoxide bridges. Each dinuclear complex cation is therefore chiral, and there are equal numbers of each enantiomer in the crystals of both 1 and 2. Electrochemical measurements showed that there is a strong electrochemical interaction between the metal centres, with separations of 0.55 and 0.57 V respectively between the two Ru^II–Ru^III couples corresponding to K_c values of 3 × 10⁹ and 6 × 10⁹ for the mixed-valence states of 1 and 2. A spectroelectrochemical study of complex 1 showed that oxidation of the mixed-valence state results in a new transition in the electronic spectrum at ca. 1800 nm with ε= 5000 dm³ mol^–1 cm^–1, which disappears on further oxidation to the Ru^III state. The observations that this transition is not solvatochromic, and that the half-width of the peak is much narrower than the value predicted by Hush theory for vectorial intervalence charge-transfer bands, both point to a class III (fully delocalised) mixed-valence state; delocalisation between the d_π orbitals of the two metal centres is facilitated by good overlap with oxygen p_π orbitals in the Ru₂O₂ core. The magnitude of the electronic interaction V_ab is estimated to be 2800 cm^–1, similar in magnitude to that of the Creutz–Taube ion.