Synthesis and characterization of oxotechnetium(V) mixed-ligand complexes containing a tridentate N-substituted bis(2-mercaptoethyl)amine and a monodentate thiol

A series of 22 mixed-ligand complexes of the general formula TcOL(1)L(2), where L(1)H(2) are N-substituted bis(2-mercaptoethyl)amine ligands, [SN(R)S], and L(2)H are monodentate thiols as coligand, is reported. The complexes were prepared by the ligand exchange method using Tc-gluconate as precursor and equimolar quantities of the two ligands. In all cases the syn stereoisomer was formed in high yield and isolated as a crystalline product. In four cases HPLC analysis demonstrated the presence of the anti stereoisomer in the reaction mixture. Although the yield was less than 1%, one anti isomer, 4a, was successfully isolated as brown crystals. The isolated complexes were characterized by spectroscopic methods and elemental analysis. The formation of the two diastereomers, syn and anti, was expected due to the configuration of the nitrogen substituent (R) with respect to the central TcO core. The X-ray crystallography showed that the coordination geometry of the syn isomers 9, 11, and 18 is trigonal bipyramidal while for the anti isomer 4a it is distorted square pyramidal. This is the first documentation of synlanti isomerism in N-substituted TcO[SN(R)S][S] mixed-ligand complexes.