Ancillary ligand determination of the spin location in both oxidised and reduced forms of diruthenium complexes bridged by bis-bidentate 1,4-bis(2-phenolato)-1,4-diazabutadiene

Abstract

The rare bridging mode of 1,4-bis(2-phenolato)-1,4-diazabutadiene = glyoxalbis(2-hydroxyanil) (L^2-) is adopted in {(μ -L^2-)[Ru^II(bpy)₂]₂}²⁺ (1²⁺), obtained as bis-perchlorate. Four well accessible redox forms of 1ⁿ (n = 4+, 3+, 2+, +) have been characterised by UV-VIS-NIR spectroelectrochemistry. The (3+) and (+) intermediates have also been investigated by EPR, both showing radical-type signals close to g = 2. This observation stands in stark contrast to EPR results previously obtained for the related {(μ -L)[Ru(acac)₂]₂}ⁿ, n = + and -, both of which exhibit metal-centred spin. In combination with the UV-VIS-NIR spectra these results suggest the preferential involvement of the multistep ligand redox system L^n- in the electron transfer processes. The relative stabilisation of Ru^II by π-accepting bpy is made responsible for the oxidation of the ligand L^2- instead of the metal.