Rhenium(I) and Technetium(I) Tricarbonyl Complexes with [NSO]‐Type Chelators: Synthesis, Structural Characterization, and Radiochemistry

The reactions of the NSO ligands 2-[2-(pyridin-2-yl)ethylthio]acetic acid (HL1), 3-[2-(pyridin-2-yl)ethylthio]propanoic acid (HL2), 2-(carboxymethylthio)-3-(1H-imidazol-4-yl)propanoic acid (H2L3), and 2-(carboxyethylthio)-3-(1H-imidazol-4-yl)propanoic acid (H2L4) with [NEt4]2[ReBr3(CO)3] are presented. Ligands HL1, H2L3, and H2L4 act as tridentate NSO chelators and readily generate the fac-[Re(NSO)(CO)3]complexes Re-L1, Re-HL3, and Re-HL4. Ligand HL2 acts as NSO tridentate chelator only in the presence of base to give complex Re-L2, while without base it coordinates as a NS bidentate chelator to generate complex fac-[ReBr(NS)(CO)3], Re-HL2. All complexes were isolated and characterized by elemental analysis, IR and 1H, 13C NMR spectroscopy. Complex Re-L1, Re-HL2, and Re-HL3 were characterized also by X-ray crystallography. Furthermore, the analogous technetium complexes fac-[99mTc(NSO)(CO)3]+, 99mTc-L1, 99mTc-L2, 99mTc-HL3, and 99mTc-HL4 were synthesized by reacting ligands HL1–H2L4 with the fac-[99mTc(OH2)3(CO)3]+ precursor for 30 min at 75 °C. The tracer complexes 99mTc-L1–99mTc-HL4 were found to be stable in L-cysteine and L-histidine challenge experiments. The bifunctional chelating agents that bear a second carboxylate group are promising for the development of targeted 99mTc radiopharmaceuticals.