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.
ABSTRACT
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 Br2â¢-. Without interference of back-electron transfer, the rate constant for an equal concentration bimolecular reaction of Br2â¢- 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.
ABSTRACT
Photophysical properties such as absorption and emission spectra, lifetimes, and redox potentials of eight ruthenium complexes, Ru(LL)2(MebpyCOOH)2+, where LL represents bpy, phen, Me2bpy, Me4bpy, (MeO)2bpy, (EtO)2bpy, Cl2bpy, and NO2phen, have been measured. The acid dissociation constants of ground and excited states have been determined. The ground-state pKa values were obtained from the pH dependence of the complex absorbance changes. The excited-state pKa* values were extracted from the emission titration curve and corrected for the excited-state lifetime of both protonated and deprotonated species. The largest DeltapKa, pKa*-pKa, found for Ru(Me2bpy)2(MebpyCOOH)2+ and Ru(Me4bpy)2(MebpyCOOH)2+ of 1.7 indicate that MebpyCOOH gains most of the MLCT excited-state electron. The big negative DeltapKa found for Ru(Cl2bpy)2(MebpyCOOH)2+, -4.2, clearly indicates the metal-to-ligand charge transfer to the Cl2bpy ligands.
