Complex formation followed by internal electron transfer: the reaction of [ethylenebis(biguanide)]silver(III) with ascorbic acid

Abstract

[Ethylenebis(biguanide)]silver(III)[Ag(H₂L)]³⁺1 has been found to oxidize ascorbic acid (H₂asc) in acidic aqueous solution via an inner-sphere mechanism to give dehydroascorbic acid, silver(I) and free ethylenebis(biguanide). The 1 : 1 adduct formed as an intermediate between 1 and ascorbic acid decomposes after a non-separable two-electron-transfer reaction. The decomposition of the three complexes actually formed, co-ordinating different numbers of protons, occurs through two parallel pathways: one via decomposition of the unprotonated adduct [Ag^III(H₂L)(H₂asc)], another via the deprotonated adduct [Ag^III(H₂L)(Hasc)], while the protonated adduct [Ag^III(H₃L)(H₂asc)] appears to be rather stable. The equilibrium constant for the adduct formation has been found to be 4.08 ± 0.45 dm³ mol^–1, whereas the acid and base constants of this complex are 0.41 mol dm^–3 and 2.86 dm³ mol^–1, respectively. The two rate constants for the decomposition are 25 ± 3 and 220 ± 25 s^–1 for [Ag^III(H₂L)(H₂asc)] and [Ag^III(H₂L)(Hasc)], respectively. The possibility of an outer-sphere mechanism is discussed.