Trithiacyclononane as a ligand for potential technetium and rhenium radiopharmaceuticals: Synthesis of [M(9S3)(SC2H4SC2H4S)][BF4] (M=Tc-99, Re,Re-188) via C-S bond cleavage

Chemical or electrochemical reduction of the 1,4,7-trithiacyclononane (9S3) complexes [M-II(9S3)(2)][BF4](2) (M = Re (3a) or Tc (3b)) results in instantaneous C-S bond cleavage to yield ethene and the stable M-III thiolate complexes [M-III(9S3)L][BF4] (hl = Re (4a) or Tc (4b), L = SCH2CH2SCH2CH2S). Compounds 4 have been characterized by H-1 NMR spectroscopy, and the pseudo-octahedral geometry of 4b has been confirmed by X-ray crystallography. Upon electrochemical reduction 4a loses ethene, while 3b can be reversibly reduced to [Tc-II(9S3)L], which is then further reduced to Tc(I) with loss of ethene. Successive ethene loss is observed in the mass spectra of compounds 3 and 4. The radiosynthesis of 4a with Re-188 can be comfortably completed within 10 min starting with (ReO4-)-Re-188 from a W-188/Re-188 generator, with a radiochemical yield in excess of 90%. and thus represents a practical approach to the preparation of stable Re-188 (and Tc-99m) thioether co lex derivatives/conjugates for clinical use. Crystal data: 4b, C10H20S6Tc, orthorhombic Pbca, a = 12.233(2) b = 14.341(2) Angstrom, c = 20.726(3) Angstrom, Z = 8.