An Rh-105 complex of tetrathiacyclohexadecane diol with potential for formulating bifunctional chelates

1,5,9,13-Tetrathiacyclohexane-3,11-diol (16S 4-diol), a sulfur crown ether analog, was studied as a potential chelating agent to complex no-carrier-added (NCA) grade 105Rh(III) in high yield at low ligand concentrations. trans-[RhCl 2(16S 4-diol)]χ (χ = Cl, PF 6) was prepared using nonradioactive RhCl 3 · 3H 2O and characterized by UV-Vis, nuclear magnetic resonance (NMR) and X-ray crystallography. It was shown to have a +1 charge with the Rh(III) metal center coordinated to the four S atoms equatorially and two Cl atoms in trans axial positions. The 105Rh-16S 4-diol complex prepared with NCA 105Rh(III)-chloride reagent was found to exhibit identical chromatographic properties as trans-[Rh(III)Cl 2(16S 4-diol)] + (including silica and C-18 thin-layer chromatography [TLC] and electrophoresis). The preparation of 105Rh-16S 4-diol complex formation optimized for conditions of pH, temperature, time, % ethanol and quantity of 16S 4-diol resulted in yields >90%. Very low quantities of 16S 4-diol (3 nmol) complex NCA 105Rh(III) under relatively mild reaction conditions (heating at 64 °C for 90 min) in the presence of ethanol (10%), yielded the high specific activity 105Rh-16S 4-diol complex as a single cationic species. The 105Rh-16S 4-diol complex was shown to be stable for ⩾4 days in physiological buffers at room temperature and in human serum at 37 °C.