BROMINATION OF IMIDAZOLES COORDINATED TO COBALT(III) - KINETICS AND MECHANISM OF BROMINATION OF (NH3)5COIMH3+ SYSTEMS, WHELAND INTERMEDIATES, AND PREASSOCIATION OR DIFFUSION CONTROL

The bromination of RImH3+ and several Me- (2-, 3-, 4-, 5-, and 2, 4-) and Br- (4-, 5-, and 4,5-) substituted imidazole complexes of the pentaamminecobalt(III) ion (R = (NH3)5Co3+) has been studied in aqueous solution at 25.0-degrees-C, I = 1.0 or 0.1 M (NaClO4). Reactions are generally fast and result in polybromination when more than one C site is available, even when less than stoichiometric amounts of Br2 are used. Site reactivity order is C-4 > C-5 >> C-2 for both the neutral (11) and anionic (12) ligands. Br2 is a much more powerful electrophile than Br3-. The pH dependence of monobromination is complex for the neutral ligand 11 and suggests proton abstraction from a Wheland addition intermediate (CoHBr) is rate-determining at acidic pHs; both spontaneous and OH(-)-catalyzed pathways are observed. At more neutral pHs, bromine addition becomes rate-determining. Reaction of the anionic ligand 12 is very fast with rate constants up to 3.4 x 10(10) M-1 s-1 (for R-2,4-Me2Im2+). For such species, preassociation with Br2 before proton abstraction is suggested as an alternative mechanism. For R-3-MeIm3+ (N-methyl derivative), reaction via the ammine conjugate base is suggested for the OH(-)-catalyzed reaction. The effects of deuterium substitution and temperature on the reaction rate are discussed.