Dissociation of CP20 from Iron( II )(cp20) 3 : A Pulse Radiolysis Study

Deferiprone (CP20), a hydroxypyridine‐4‐one, is used in iron‐chelation therapy. For the iron( III )‐/( II )(cp20) 3 couple, a standard electrode potential of –620 mV was measured by cyclic voltammetry [Templeton et al., Inorg. Chim. Acta 1996 , 245 , 199; Merkofer et al., Helv. Chim. Acta 2004 , 87 , 3021]. On the basis of this value and the overall stability constant for the equilibrium of iron( III ) and three CP20 molecules, ligand dissociation ought to take place after reduction of the iron( III )(cp20) 3 complex. By pulse radiolysis, we show that hydrated electrons reduce the iron( III )(cp20) 3 complex with a rate constant of k = (6.4 ± 0.3) × 10 10 M –1  s –1 , and that the iron( II )(cp20) 3 complex aquates rapidly. The dissociation of the first two CP20 molecules takes place within a few μs after the pulse. The dissociation rate constant for the last CP20 ligand is (8 ± 1) × 10 3 s –1 . The observation of a dissociation illustrates that, under dilute conditions, iron( II ) is not fully complexed by CP20 and could potentially participate in redox cycling to produce oxyradicals. The CO 2 · – radical does not reduce iron( III )(cp20) 3 , which indicates that this complex reacts, as expected, very slowly with inner‐sphere reductants. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)