Mechanistic studies on the oxidation of hydroquinone by an oxo-bridged diiron(III,III) complex in weakly acidic aqueous media

The kinetics of oxidation of hydroquinone (H(2)Q) by a mu-oxo-bridged diiron(III,III) complex, Fe-2(mu-O)(phen)(4)(H2O)(2)](4+) (1) has been investigated in aqueous media at 25.0 degrees C in presence of an excess of 1,10-phenanthroline (phen). The overall redox rate increases with increase in [H+]. The title complex (1) and its conjugate bases, [Fe-2(mu-O)(phen)(4)(OH)(2)](3+)(2) and [Fe-2(mu-O)(phen)(4)(OH)(2)](2+) (3), participate in the reaction with H2O as the only kinetically reactive reducing species. Rate constants (in dm(3)supercript stop mol(-1) s(-1)) for the parallel reactions (1) + H2O -> Products, (2) + H2O -> Products and that for (3) + H2O -> Products are, respectively, 500 +/- 40, 100 +/- 6 and 30 +/- 2. Substantial rate retardation in D2O media in comparison to that in H2O media suggests that electron transfer is coupled with proton movements in the rate-determining step.