REACTIONS OF COORDINATED LIGANDS - THE BROMINATION OF 2,4-DIMETHYL-3H-IMIDAZOLE AND 3H-IMIDAZOLE COORDINATED (AT N-1) TO PENTAAMMINECOBALT(III)

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 x 10(4) and 3.4 x 10(10) dm3 mol-1 s-1 at 25.0-degrees-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 x 10(9) 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.