Redox reactivity of bis(1,4,7-triazacyclononane)iron(II/III) complexes in alkaline solution and characterization of a deprotonated species: Amidoiron(III) vs aminyliron(II) ground-state formulation. EPR, kinetic, pulse radiolysis, and laser photolysis study

The redox reactivity of the low-spin complexes (FeL2)3+ and (FeL22+ (L = 1,4,7-triazacyclononane, C6H15N3) in alkaline aqueous solution has been investigated. Orange (FeIIIL2)3+ is reversibly deprotonated in alkaline solution to yield a deep blue species, which has been characterized by its frozen-solution EPR spectrum as the low-spin amidoiron(III) complex, (FeIIIL(L-H))2+ (LL-H) is the N-deprotonated form of the ligand L). The aminyl radical iron(II) formulation, (Feiil(.L-H))2+, is proposed to be an electronically excited state (ligand-to-metal charge transfer). (FeIIIL(L-H))2+ disproportionates slowly under anaerobic conditions to yield 50% (FeIIL2)2+ and probably 50% (LFeII(OH2)3)2+ and a two-electron-oxidation product of one 1,4,7-triazacyclononane ligand, which has not been characterized. In the presence of an external oxidant (H2O2 or oxygen) the blue species decomposes to produce quantitatively 1 equiv of oxidized macrocycle and (FeIIIL(OH2)3)3+. The kinetics of both reactions have been measured. Pulse radiolysis experiments have shown that the oxidation of (FeL2)2+ with Br2.-, I2.-, and (SCN)2.- and oxygen-centered radicals such as OH. and the phenoxyl radical yield (FeIIIL2)3+ at nearly diffusion controlled rates in the pH range 4-10. At higher pH values (11-13) the blue amidoiron(III) species is formed. (FeIIIL(L-H))2+ oxidizes ascorbate(2-) to give (FeIIL2)2+ and the ascorbate(1-) radical. 51 refs., 8 figs., 2 tabs