Metal ion – biomolecule interactions. Part 16. C(2)-H isotopic exchange in Co(III)-coordinated imidazoles

Transition-metal-bound imidazoles are suitable models for evaluating the roles of metal ions in biomolecules having the imidazole moiety and similar heterocyclic residues as part of their structure. Such studies provide useful insights into metal–biomolecule interactions in biological systems, especially when the lability of the metal–ligand bond is substantially reduced, such that the identity of the metal–ligand complex is preserved during the course of the reaction under investigation. The present paper reports on a kinetic study of tritium exchange from the C(2) position of the imidazole moiety in the substitution-inert complex cations [Co(NH3)5[2-3H]-imidazole]3+ (1) and [Co(NH3)5-1-methyl-[2-3H]-imidazole]3+ (2). Rate–pH profiles have been determined in aqueous solution at 60 °C. Both substrates are believed to react through rate-determining attack of hydroxide ion (kM+ pathway) at C(2)-T. Dissection of the kinetic data reveals an additional pathway for 1 consequent upon deprotonation of its pyrrole...