Photocatalytic Oxidation of Bromide to Bromine

Three consecutive bimolecular reactions are employed to photocatalyze bromide oxidation to bromine. The system consists of a ruthenium(II) complex, [Ru(deeb)₂(dmbpy)]²⁺ (deeb = 4,4'-diethylester-2,2'-bipyridine; dmbpy = 4,4'-dimethyl-2,2'-bipyridine), 4-bromobenzenediazonium tetrafluoroborate (ArN₂BF₄), and Br^-. Varying reagent concentrations allowed us to optimize the sequence of reactions for product formation. The electronically excited ruthenium complex (*Ru) reacts first with ArN₂BF₄ to produce a ruthenium(III) (Ru^III) intermediate, triggering a subsequent reaction with Br^-. Transient absorption measured at 486 and 380 nm provides insight into the time-dependent concentrations of *Ru, Ru^III, and Br₂^•-. Without interference of back-electron transfer, the rate constant for an equal concentration bimolecular reaction of Br₂^•- was determined to be 5 × 10⁹ M^-1 s^-1. The final products, bromine and tribromide, were spectroscopically characterized, and the turnover number (TON) was 230.