Kinetics and mechanisms of the oxidation of iodide and bromide in aqueous solutions by a trans-dioxoruthenium(VI) complex

The kinetics and mechanisms of the oxidation of I^- and Br ^- by trans-[Ru^VI(N₂O₂)(O) ₂]²⁺ have been investigated in aqueous solutions. The reactions have the following stoichiometry: trans-[Ru^VI(N ₂O₂)(O)₂]²⁺ + 3X^- + 2H⁺ → trans-[Ru^IV(N₂O₂)(O) (OH₂)]²⁺ + X₃ ^- (X = Br, I). In the oxidation of I^- the I₃ ^- is produced in two distinct phases. The first phase produces 45% of I₃ ^- with the rate law d[I₃ ^-]/dt = (k_a + k _b[H⁺])[Ru_VI][I^-]. The remaining I₃ ^- is produced in the second phase which is much slower, and it follows first-order kinetics but the rate constant is independent of [I^-], [H⁺], and ionic strength. In the proposed mechanism the first phase involves formation of a charge-transfer complex between Ru ^VI and I^-, which then undergoes a parallel acid-catalyzed oxygen atom transfer to produce [Ru^IV(N₂O ₂)(O)(OHI)]²⁺, and a one electron transfer to give [Ru^V(N₂O₂)(O)(OH)]²⁺ and I ^.. [Ru^V(N₂O₂)(O)(OH)]²⁺ is a stronger oxidant than [Ru^VI(N₂O₂)(O) ₂]²⁺ and will rapidly oxidize another I^- to I^.. In the second phase the [Ru^IV(N₂O ₂)(O)(OHI)]²⁺ undergoes rate-limiting aquation to produce HOI which reacts rapidly with I^- to produce I₂. In the oxidation of Br^- the rate law is -d[Ru^VI]/dt = {(k _a2 + k_b2[H⁺]) + (k_a3 + A _b3[H⁺]) [Br^-]}[Ru^VI][Br ^-]. At 298.0 K and I = 0.1 M, k_a2 = (2.03 ± 0.03) × 10^-2 M^-1 s^-1, k_b2 = (1.50 ± 0.07) × 10^-1 M^-2 s^-1, k _a3 = (7.22 ± 2.19) × 10^-1 M^-2 s^-1 and k_b3 = (4.85 ± 0.04) × 10₂ M^-3 s^-1. The proposed mechanism involves initial oxygen atom transfer from trans-[Ru^VI(N₂O₂)(O) ₂]²⁺ to Br to give trans-[Ru^IV(N ₂O₂)(O)(OBr)]⁺, which then undergoes parallel aquation and oxidation of Br^-, and both reactions are acid-catalyzed. © 2008 American Chemical Society.