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) (RuIII) intermediate, triggering a subsequent reaction with Br–. Transient absorption measured at 486 and 380 nm provides insight into the time-dependent concentrations of *Ru, RuIII, 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.