Mechanistic studies on oxidation of L-ascorbic acid by an oxo-bridged diiron complex in aqueous acidic media

[Fe-2(mu-O)(phen)(4)(H2O)(2)](4+) ( 1) (Fig. 1, phen = 1,10-phenanthroline) equilibriates with [Fe-2(mu-O)(phen)(4)(H2O)(OH)](3+) ( 2) and [Fe-2(mu-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 (H(2)A) 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).