Reactions of Coordinated Ligands. The Bromination of 2,4-dimethyl-3H-imidazole and 3H-imidazole Coordinated (at N1) to Pentaamminecobalt(III)

Abstract

The pH dependence of the bromination of R-2,4-Me₂ImH³⁺ [R=(NH₃)₅Co] by Br₂ in acidic aqueous solution, giving R-2,4-Me₂-5-BrImH³⁺, suggests that proton abstraction from a bromine-substituted Wheland intermediate is rate determining for pH < 3 (both OH^-- catalysed and spontaneous paths are observed), while in the pH range 3-5, bromine addition is rate determining. For pH > 5 the reaction may be interpreted to occur through rate-determining Br₂ addition to the conjugate base from of the reactant. Rate constants for bromine addition to R-2,4-Me₂ImH³⁺ and R-2,4-Me₂Im²⁺ are respectively 1.1×10⁴ and 3.4×10¹⁰ dm³ mol^-1 s^-1 at 25.0°C, I = 1.0 (NaClO₄). Bromination of RimH³⁺, to give R-4-BrImH³⁺ initially, appears to occur largely via the conjugate base form of the reactant (k₂ = 3.6×10⁹ dm³ mol^-1 s^-1), but for Ph < 2, addition of Br₂ to RImH³⁺ contributes to the observed rate (k₁ = 0.68 dm³ mol^-1 s^-1). E_a = 61±2 kJ mol^-1 for the former process, and the primary kinetic isotope effect, k_H/k_D, increases from 1.3 in 0.05 M H+ to 2.4 in 5.42 M HClO₄. These observations are discussed in terms of diffusion-controlled, or preassociation -type mechanisms.