Reactions of an excited state of carbon- and sulfur-centered radicals with transition metal complexes

Huston, Patrick
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The kinetics of the reductive quenching of the [superscript]2E excited state of CrL[subscript]3[superscript]3+ (L = 2,2[superscript]'-bipyridine, 1,10-phenanthroline and their substituted analogues) by Ti(III) were investigated at 25°C in 1.0 M LiCl-HCl. The dependence of the rate on hydrogen ion concentration indicates that both Ti(H[subscript]2O)[subscript]6[superscript]3+ and (H[subscript]2O)[subscript]5Ti(OH)[superscript]2+ quench. With both Ti[superscript]3+ and TiOH[superscript]2+ the rates of electron transfer depend on the E[subscript]sp*3/2∘ of the different polypyridine complexes. The quenching rate constants for Ti[superscript]3+ are in the range (0.26-3.7) x 10[superscript]7 L mol[superscript]-1s[superscript]-1 and those for TiOH[superscript]2+ are in the range (0.81-9.7) x 10[superscript]8 L mol[superscript]-1s[superscript]-1 using a value for K[subscript] a of 4.6 x 10[superscript]3 M. The formation of a stable (>100 [mu]s) Cr(II) product was observed spectrophotometrically, indicating back electron transfer to the ground state complex, CrL[subscript]3[superscript]3+, occurs too slowly to measure on the laser time scale. This is explained by the instability of the immediate product of back electron transfer, TiO[superscript]+, with respect to TiO[superscript]2+;Alkyl radicals, generated from the photohomolysis of organocobalt complexes, were allowed to react in aqueous solution with (H[subscript]2O)[subscript]2CrL[superscript]2+ (L = 1,4,8,11-tetraazacyclopentadecane or (15) aneN[subscript]4). The reaction rates were evaluated by laser flash photolysis, using the known reaction between R· and the methyl viologen radical cation as a kinetic probe. Data were obtained for reactions of 13 radicals with (H[subscript]2O)[subscript]2CrL[superscript]2+. The rate constants span a narrow range, (6-19) x 10[superscript]7 L mol[superscript]-1s[superscript]-1 at 25°C. The complexes with R = CH[subscript]2OH and CH[subscript]2OCH[subscript]3 are quite stable, showing <10% decay over 6 hours. Equilibrium constants for radical binding, calculated from forward and reverse rate constants, are log K[subscript]298 = 11.23 (CH(CH[subscript]3)[subscript]2) and 12.19 (CH[subscript]2C[subscript]6H[subscript]5). The kinetic and equilibrium data are discussed in terms of a reaction model featuring a concurrent Cr-C and Cr-OH[subscript]2 bond making and bond breaking;Reactions of thiyl radicals (RS·) have been studied in aqueous solution using laser flash photolysis. One method used in this study involved generating alkyl radicals by photohomolysis of the cobalt-carbon bond in RCo((14) aneN[subscript]4)[superscript]2+ complexes with visible light. The carbon-centered radicals abstract a hydrogen atom from a thiol to form thiyl radicals. This reaction, known as the repair reaction, has been studied for methyl and ethyl radicals with ethane thiol, cysteine and glutathione. The kinetics of the oxidation of transition metal complexes by C[subscript]2H[subscript]5S· have been studied using the kinetic probes ABTS[superscript]2- and TMPD. The reaction of C[subscript]2H[subscript]5S· with V(H[subscript]2O)[subscript]6[superscript]2+ proceeds by an outer-sphere mechanism, while the reactions with Cr(H[subscript]2O)[subscript]6[superscript]2+, Fe(H[subscript]2O)[subscript]6[superscript]2+, CoL[superscript]2+ (L = (14) aneN[subscript]4, Me[subscript]6(14) aneN[subscript]4, Me[subscript]6(14) dieneN[subscript]4) and vitamin B[subscript] 12r proceed by an inner-sphere mechanism. The formation of C[subscript]2H[subscript]5SCr(H[subscript]2O)[subscript]5[superscript]2+ has been directly observed spectrophotometrically.

Chemistry, Inorganic chemistry