Thermal and photochemical reduction of aqueous chlorine by ruthenium(II) polypyridyl complexes

Studies are reported on the reactions of aqueous chlorine with a series of substitution-inert, one-electron metal-complex reductants, which includes [Ru(bpy)(3)](2+), [Ru(4,4'-Me(2)bpy)(3)](2+), [Ru(4,7-Me(2)phen)(3)](2+), [Ru(terpy)(2)](2+), and [Fe(3,4,7,8-Me(4)phen)(3)](2+). The reactions were studied by spectrophotometry at 25 degrees C in acidic chloride media at mu = 0.3 M. In general the reactions have the stoichiometry 2[ML3](2+) + Cl-2 --> 2[ML3](3+) + 2Cl(-). In the case of [Ru(bpy)(3)](2+), the reaction is quite photosensitive; the thermal reaction is so slow as to be practically immeasurable. The reactions of [Ru(4,4'-Me(2)bpy)(3)](2+) and [Ru(4,7-Me(2)phen)(3)](2+) are also highly photosensitive, giving pseudo-first-order rate constants that depend on the monochromator slit width in a stopped-flow instrument; however, the thermal rates are fast enough that they can be obtained by extrapolation of k(obs) to zero slit width. The reactions of [Ru(terpy)(2)](2+) and [Fe(3,4,7,8-Me(4)phen)(3)](2+) show no appreciable photosensitivity, allowing direct determination of their thermal rate laws. From the kinetic effects of pH, [Cl-2](tot), and [Cl-] it is evident that all of the thermal rate laws have a first-order dependence on [ML3](2+) and on [Cl-2]. The second-order rate constants decrease as E degrees for the complex increases, consistent with the predictions of Marcus theory for an outer-sphere electron-transfer mechanism. Quantum yields at 460 nm for the reactions of [Ru(4,4'-Me(2)bpy)(3)](2+) and [Ru(4,7-Me(2)phen)(3)](2+) exceed 0.1 and show a dependence on [Cl-2] indicative of competition among spontaneous decay of *Ru, nonreactive quenching by Cl-2, and reactive quenching by Cl-2.