Ru(III)(edta) complexes as molecular redox catalysts in chemical and electrochemical reduction of dioxygen and hydrogen peroxide: inner-sphere versus outer-sphere mechanism

The reduction of molecular oxygen (O(2)) and hydrogen peroxide (H(2)O(2)) by [Ru(II)(edta)(pz)](2−) (edta(4−) = ethylenediaminetetraacetate; pz = pyrazine) has been studied spectrophotometrically and kinetically in aqueous solution. Exposure of the aqua-analogue [Ru(II)(edta)(H(2)O)](2−) to O(2) and H(2)O(2) resulted in the formation of [Ru(III)(edta)(H(2)O)](−) species, with subsequent formation of the corresponding Ru(V)[double bond, length as m-dash]O complex. A working mechanism for the O(2) and H(2)O(2) reduction reactions mediated by the Ru(II)(edta) complexes is proposed. The role of the coordinated water molecule (by its absence or presence in the primary coordination sphere) in controlling the mechanistic pathways, outer-sphere or inner-sphere, is discussed.