Reactions of Polypyridylchromium(II) Ions with Oxygen: Determination of the Self-Exchange Rate Constant of O2/O2-

Article abstract of DOI:10.1021/ja00223a025

Reductive quenching of the ²E excited states of CrL3³⁺ complexes, where L is 2,2'-bipyridine (bpy), 1,10-phenantroline (phen), and their substituted analogues, by disodium ethylenediaminetetraacetate and sodium oxalate was studied by use of laser flash photolysis.The quenching by both compounds results in the rapid, irreversible formation of CrL3²⁺ with a quantum yield that can approach a value of 2.0.This novel method for the generation of CrL3²⁺ was utilized to study the reactions of CrL3²⁺ with ground-state dioxygen.The rate constants for these reactions show a linear dependence on ΔG^o as predicted by Marcus theory and range from 2.5E5 M^-1 s^-1 (Cr(5-Clphen)3²⁺) to 2.5E7 M^-1 s^-1 (Cr(Me2phen)3²⁺).It is postulated that the reactions proceed by outer-sphere electron transfer to yield superoxide ion as the initial product, as supported by the quantity of H2O2 determined after its disproportionation.The electrostatically corrected Marcus cross-relation yielded k₂₂ = 2 +/- 1 M^-1 s^-1 for the self-exchange rate constant of the O2/O2^- couple.A close inspection of the data obtained in this work along with the literature data strongly suggests that the true value of k₂₂ lies in the range 1-10 M^-1 s^-1.