Spin-trapping studies of the reduction of oxygen and hydrogen peroxide by titanium(III), iron(II), and ruthenium(II) complexes

Abstract

The reductions of H/sub 2/O/sub 2/ and O/sub 2/ by Ti(edta)(H/sub 2/O)/sup /minus//, Ti(H/sub 2/O)/sub 6//sup 3+/, Fe(edta)/sup 2/minus//, Fe(H/sub 2/O)/sub 6//sup 2+/ and Ru(NH/sub 3/)/sub 6//sup 2+/ have been studied by the spin-trapping technique using 5,5-dimethyl-1-pyrroline, N-oxide (DMPO) and N-tert-butyl-/alpha/-phenylnitrone (PBN) radical traps. The resultant radical adducts RDMPO/sup /center dot// and RPBN/sup /center dot// have been characterized by ESR spectroscopy in agreement with literature values. Ti(edta)(H/sub 2/O)/sup /minus//, Fe(edta)/sup 2/minus//, Fe(H/sub 2/O)/sub 6//sup 2+/, and Ru(NH/sub 3/)/sub 6//sup 2+/ reductions of H/sub 2/O/sub 2/ produce HO/sup /center dot// identified by the HO-DMPO/sup /center dot// and HO-PBN/sup /center dot// spectra. When O/sub 2/ is used as the oxidant for Ru(NH/sub 3/)/sub 6//sup 2+/, this reaction known to proceed outer sphere via O/sub 2//sup /minus//, only the dismutation/reduction product (HO/sup /center dot//) is trapped at pH 6.86. Both HO/sub 2//sup /center dot// and HO/sup /center dot// are trapped at pH 2.57 in a 1.0:7.6 ratio. Ti(edta)(H/sub 2/O)/sup /minus// is known to be oxidized inner sphere by O/sub 2/ via coordinated O/sub 2//sup /minus//. No radical adducts for the Ti(edta)(H/sub 2/O)/sup /minus///O/sub 2//radical trap system are observed with or without mediators in the solvent cage. The reduction of O/sub 2/ by eithermore » Fe(edta)/sup 2/minus// proceeds by an inner sphere pathway in which the coordinated O/sub 2//sup /minus// survives long enough to attack an adjacent carboxylate moiety, forming a trappable ligand-based carbon-centered radical, or to attack sacrificial mediators in the solvent cage. 34 refs., 5 figs., 1 tab.« less