Kinetics and mechanism of reactions of S-protected dithiol monoaminemonoamide (MAMA) ligands with technetium: Characterization of a technetium—thiolate—thioether—MAMA complex, a kinetic intermediate of the reaction

The exchange reactions of S-protected dithiol monoaminemonoamide (MAMA) ligands with Tc(V)-gluconate were investigated. Protection of the mercaptide sulfur atoms with acid, base and metal labile groups permitted complex formation of the MAMA ligands at a range of pHs. In general, the rate of complex formation was faster with the MAMA ligands than with the corresponding diamide dithiol (DADS) ligands. The rate of Tc complex formation depended on the nature of the sulfur protecting groups and on the position of the amine group with respect to the other donor groups in the ligands. Two isomeric ligands showed different mechanisms of complex formation. The isomer which gave the final Tc-dithiolate-MAMA complex in higher yield was shown to form a Tc-thioether-thiol-MAMA complex as an intermediate prior to metal-assisted S-dealkylation. The formation of the Tc-thioether complex intermediate at a lower temperature may account for the enhanced kinetics of chelation compared to the isomer which did not form the intermediate complex.