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aromatic diimine complexes. l 1 The compound Fe(GMI)32+can be oxidized by strong oxidizing agents (e.g., Ce(1V)). We now present some evidence for the reaction between HOz. radicals and Fe(GMI)32+. In Figure l the changes in concentration of hydrogen peroxide (Figure la), complex (Figure lb), and evolution of oxygen (Figure IC) with time are shown. The rate of decomposition of hydrogen peroxide decreases as the concentration of complex increases, and Fe(II1) decreases. When all the colored species are destroyed, the decomposition of hydrogen peroxide catalyzed by iron(II1) is observed. The rate of oxygen evolution decreases as the concentration of complex increases. This slower rate of oxygen evolution can be easily explained in the light of the mechanism proposed by Barb, et a1.,12for the decomposition of hydrogen peroxide by Fe(1II) ions.
+ HsOn+Fe(I1) + HOn. + H+ Fe(I1) + HsOI --+Fe(I1I) + HO. + HO+ HO. + Hz01 +HOs. + HIO H O I . + Fe(II1) +H+ + Fe(I1) + O? H 0 2 . + Fe(I1) HO?- + Fe(II1) Fe(II1)
(1) (2)
(3)
(4) (5 )
By this mechanism, the reaction between H O z . radicals and Fe(II1) is responsible for the oxygen evolution (eq 4). To slow the rate of oxygen evolution, the complex must compete with Fe(II1) for H o p . radicals. Thus the complex is oxidized to Fe(GMI)33+, regenerating hydrogen peroxide ( c j eq 5) and acting as an H 0 2 . radical trap. The ferric complex so formed can undergo an internal redox reactiong producing a radical at the ligand. The c~mplex-radicals~~ which are generated may react with HOz. or H O . radicals to form the labile ligand-oxidized species, As the formal electrode potential of the couple HOs.IH20sis approximately 1.5 V,14 it is reasonable to assume that the oxidation of Fe(GMI)32+is by HOz. radicals. This reaction must be very fast since the rate of electron exchange in analogous systems, e.g., between F e ( ~ h e n ) ~and ~ + F e ( ~ h e n ) ~is~very + highlj ( 3 X lo8 M-I sec-l) (cf: Figure Ib). Attempts to oxidize Fe(GMI)32+with H O . radicals gave a very low yield of the labile ligand-oxidized complexes. However, these experiments are not completely analogous since the acid concentration is much higher (0.5 M HzSO,) than that of the iron(I1l) experiments (pH 2.4). These radicals were generated by treating hydrogen peroxide with excess iron(II)I6 (in the presence of complex). The rate laws deduced, including a reaction between the complex and H O . radicals and the reactions of the Barb, et al., mechanism,lz seem not to be compatible (IO) W. W. Brandt, F. P. Dwyer, and E. C. Gyarfas, Chem. Rea., 54, 959 (1954).
( I I ) P. I
