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

The rare bridging mode of 1,4-bis(2-phenolato)-1,4-diazabutadiene = glyoxalbis(2-hydroxyanil) (L2-) is adopted in {(mu-L2-)[Ru-II(bpy)(2)](2)}(2+) (1(2+)), obtained as bis-perchlorate. Four well accessible redox forms of 1(n) (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 {(mu-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 Ln- in the electron transfer processes. The relative stabilisation of Ru-II by pi-accepting bpy is made responsible for the oxidation of the ligand L2- instead of the metal.