Valence structures of the diastereomeric complexes meso- and rac-[Ru₂(acac)₄(μ-Q)]ⁿ (n = 2-, 1-, 0, 1+, 2+) with the multiple quinonoid bridging ligand Q = 1,2,4,5-tetraimino-3,6-diketocyclohexane

Abstract

Meso- and rac-configurated diastereoisomers [Ru₂(acac)₄(μ-Q)] have been separated and identified as Ru^II-Q⁰ species through a crystal structure analysis of the meso form. The presence of two redox-active {Ru(acac)₂} groups (acac^- = 2,4-pentanedionate) and quinonoid Q with two equivalent p-conjugated a-diimine chelate sites and one p-quinone function allowed for the full cyclic voltammetric and spectroelectrochemical (UV-vis-NIR, IR, EPR) characterisation of the five accessible states (2-, 1-, 0, 1+ and 2+ forms) for both isomers. Oxidation occurs at the metal ions to produce RuIIRuIII mixed-valent states [Ru₂(acac)₄(μ-Q)]⁺ (K_c≈ 10^4.5) with corresponding EPR features but without detectable intervalence absorption in the near infrared region. IR-spectroelectrochemistry reveals opposite frequency shifts for the ν (CO) and ν (NH) stretching vibrations on reduction and oxidation, in agreement with the assumed electronic structure. Reduction leads to strongly stabilised [Ru₂(acac)₄(μ -Q)]^- states (K_c≈ 10¹¹) which show weak NIR shoulders around 1040 nm. The EPR characteristics are remarkably different for the two isomeric monoanions, reflecting presumably flexible geometry and electronic structure. The observation of broad but detectable EPR resonance at room temperature in solution and the g factor anisotropy in the glassy frozen state at 110 κ suggest a rather evenly metal-ligand mixed singly occupied MO. Together with the ZINDO calculations and the partial experimental results reported previously by Masui et al. (Inorg. Chem., 2000, 39, 141) for [Ru₂(bpy)₄(μ -Q)]ⁿ⁺ (n = 2, 3, 4), the characteristic differences provide an insight into the electronic features such as mixed valency manifestations and the variable extent of mixing of the metal-quinone frontier orbitals of these systems involving RuII-stabilised Q which is unknown as a free ligand.