Rhenium and Technetium Complexes with Pentadentate Thiocarbamoylbenzamidines: Steps toward Bioconjugation

Thiocarbamoylbenzimidoyl chlorides, PhC­(Cl)N–(CS)–NR1R2, react with 2-(iminodiacetic acid)­benzylamine under formation of the potentially pentadentate ligands H3L (R1, R2 = Et) and H3L–COOEt (R1 = Me, R2 = C6H4-4-COOEt) in high yields. Hydrolysis of H3L–COOEt in NaOH/MeOH gives quantitatively another benzamidine ligand H3L–COOH. The novel ligands readily react with (NBu4)­[MOCl4] (M = Re, Tc) under formation of stable complexes with the general composition [MO­(L)], in which they are triply deprotonated and fully occupy the remaining five coordination positions of the {MO}3+ cores. In a “proof-of-principle” reaction for possible bioconjugations, the complex [ReO­(L–COOH)] has been labeled with triglycine ethyl ester in high yields.