Kinetics and mechanism of oxidation of iodide with a (μ‐oxo)diiron(III,III) complex in weakly acidic media

Abstract

2+ (2a) coexist in rapid equilibria in the range pH 4. 23-5.35 in the presence of excess phenanthroline (pK a1 = 3.71 ± 0.03, pK a2 = 5.28 ± 0.07). The solution reacts quantitatively with I − to produce [Fe(phen) 3 ] 2+ and I 2 . Only 1 but none of its hydrolytic derivatives is kinetically active. Both inner and outer sphere pathways operate. The observed rate constants show second-order dependence on the concentration of iodide, while the dependence on [H + ] is complex in nature. Added Cl − inhibits the formation of adduct with I − and thus retards the rate of inner sphere path, leading to a rate saturation at high [Cl − ], where only the outer sphere mechanism is active. Kinetic data indicate that simultaneous presence of two I − in the vicinity of diiron core is necessary for the reduction of 1. C 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: [737][738][739][740][741][742][743] 2005

Iridium(III) catalyzed oxidation of iodide ions in aqueous acidic medium

“…However, due to the instability of this oxidant in solution, related oxidants like [Fe III (bpy) 2 (CN) 2 ] + have been shown to be highly effective as sensitizers in photochemical cells of the Gra¨tzel type [24][25][26]. Oxidation of iodide ion by (loxo) diiron (III, III) complexes in weakly acidic medium [27] and by various iron(III) complexes in acetonitrile has been reported and it has been shown that trace levels of copper ions are the potent catalysts for the reaction [28,29]. Oxidation of iodide ions by hexacyanoferrate(III) has already been reported by us in which first order kinetics with respect to [K 3 Fe(CN) 6 ] and [H + ] ions and second order with respect to [I ) ] was observed, when the reaction is catalyzed by palladium(II) in hydrochloric acid [30] and ruthenium(III) in sulphuric acid medium [31].…”