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

Abstract

AbstractThe 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.