Novel oxorhenium and oxotechnetium complexes from an aminothiol[NS]/thiol[S] mixed-ligand system

The simultaneous action of a bidentate aminothiol ligand, (LH)-H-n, (n = 1: (CH3CH2)(2)NCH2CH2SH and n = 2: C5H10NCH2CH2SH) and a monodentate thiol li gand, LH (LH: p-methoxythiophenol) on a suitable MO (M = Re, Tc-99g) precur sor results in the formation of complexes of the general formula [MO(L-n)(L )(3)] (1, 2 for Re and 5, 6 for Tc-99g). In solution these complexes gradua lly transform to [MO(L-n)(L)(2)] complexes (3, 4 for Re and 7, 8 for Tc-99g ). The transformation is much faster for oxotechnetium than for oxorhenium complexes. Complexes 1-4, 7, and 8 have been isolated and fully characteriz ed by elemental analysis and spectroscopic methods. Detailed NMR assignment s were made for complexes 3, 4, 7, and 8. X-ray studies have demonstrated t hat the coordination geometry around rhenium in complex 1 is square pyramid al (tau = 0.06), with four sulfur atoms (one from the (LH)-H-1 ligand and t hree from three molecules of p-methoxythiophenol) in the basal plane and th e oxo group in the apical position. The (LH)-H-1 ligand acts as a monodenta te ligand with the nitrogen atom being protonated and hydrogen bonded to th e oxo group. The four thiols are deprotonated during complexation resulting in a complex with an overall charge of zero. The coordination geometry aro und rhenium in complex 4 is trigonally distorted square pyramidal (tau = 0. 41), while in the oxotechnetium complex 7 it is square pyramidal (tau = 0.1 6). In both complexes LIH acts as a bidentate ligand. The NS donor atom set of the bidentate ligand and the two sulfur atoms of the two monodentate th iols define the basal plane, while the oxygen atom occupies the apical posi tion. At the technetium tracer level (Tc-99m), both types of complexes, [(T cO)-Tc-99m(L-n)(L)(3)] and [(TcO)-Tc-99m(L-n)(L)(2)], are formed as indicat ed by HPLC. At high ligand concentrations the major complex is [(TcO)-Tc-99 m(L-n)(L)(3)], while at low concentrations the predominant complex is [(TcO )-Tc-99m(L-n)(L)(2)]. The complexes [(TcO)-Tc-99m(L-n)(L)(3)] transform to the stable complexes [(TcO)-Tc-99m(L-n)(L)(2)]. This transformation is much faster in the absence of ligands. The complexes [(TcO)-Tc-99m(L-n)(L)(2)] are stable, neutral, and also the predominant product of the reaction when low concentrations of ligands are used, a fact that is very important from the radiopharmaceutical point of view.