Oxidation of Bromide to Bromine by Ruthenium(II) Bipyridine-Type Complexes Using the Flash-Quench Technique.

Abstract

Six ruthenium complexes, [Ru(bpy)3]2+ (1), [Ru(bpy)2(deeb)]2+ (2), [Ru(deeb)2(dmbpy)]2+ (3), [Ru(deeb)2(bpy)]2+ (4), [Ru(deeb)3]2+ (5), and [Ru(deeb)2(bpz)] 2+ (6) (bpy: 2,2'-bipyridine; deeb: 4,4'-diethylester-2,2'-bipyridine; dmbpy: 4,4'-dimethyl-2,2'-bipyridine, bpz: 2,2'-bipyrazine), have been employed to sensitize photochemical oxidation of bromide to bromine. The oxidation potential for complexes 1-6 are 1.26, 1.36, 1.42, 1.46, 1.56, and 1.66 V vs SCE, respectively. The bimolecular rate constants for the quenching of complexes 1-6 by ArN2+ (bromobenzenediazonium) are determined as 1.1 × 109, 1.6 × 108, 1.4 × 108, 1.2 × 108, 6.4 × 107, and 8.9 × 106 M-1 s-1, respectively. Transient kinetics indicated that Br- reacted with photogenerated Ru(III) species at different rates. Bimolecular rate constants for the oxidation of Br- by the Ru(III) species derived from complexes 1-5 are observed as 1.2 × 108, 1.3 × 109, 4.0 × 109, 4.8 × 109, and 1.1 × 1010, M-1 s-1, respectively. The last reaction kinetics observed in the three-component system consisting of a Ru sensitizer, quencher, and bromide is shown to be independent of the Ru sensitizer. The final product was identified as bromine by its reaction with hexene. The last reaction kinetics is assigned to the disproportionation reaction of Br2-• ions, for which the rate constant is determined as 5 × 109 M-1 s-1. Though complex 6 has the highest oxidation potential in the Ru(II)/Ru(III) couple, its excited state fails to react with ArN2+ sufficiently for subsequent reactions. The Ru(III) species derived from complex 1 reacts with Br- at the slowest rate. Complexes 2-5 are excellent photosensitizers to drive photooxidation of bromide to bromine.