[Fe2(µ-O)(phen)4(H2O)2]4+ (1) (Fig. 1, phen = 1,10-phenanthroline) equilibriates with [Fe2(µ-O)(phen)4(H2O)(OH)]3+ (2) and [Fe2(µ-O)(phen)4(OH)2]2+ (3) in aqueous solution in the presence of excess phen, where no phen-releasing equilibria from 1, 2 and 3 exist. 1 quantitatively oxidizes ascorbic acid (H2A) to dehydroascorbic acid (A) in the pH range 3.00–5.50 in the presence of excess phen, which buffers the reaction within 0.05 pH units and ensures complete formation of end iron product ferroin, [Fe(phen)3]2+. The reactive species are 1, 2 and HA− and the reaction proceeds through an initial 1 : 1 inner-sphere adduct formation between 1 and 2 with HA−, followed by a rate limiting outer-sphere one electron one proton (electroprotic) transfer from a second HA− to the ascorbate-unbound iron(III).
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