(S)-(2-(2-Pyridyl)ethyl)cysteamine and (S)-(2-(2-Pyridyl)ethyl)-D,L-homocysteine as Ligands for the "fac-[M(CO)3]+" (M = Re, 99mTc) Core

The reaction of fac-[NEt4]2[Re(CO)3Br3] with (S)-(2-(2ʹ-pyridyl)ethyl)cysteamine, L1, in methanol leads to the formation of the cationic fac-[Re(CO)3(NSN)][Br] complex, 1, with coordination of the nitrogen of the pyridine, the sulfur of the thioether, and the nitrogen of the primary amine. When fac-[NEt4]2[Re(CO)3Br3] reacts with the homocysteine derivative (S)-(2-(2ʹ-pyridyl)ethyl)-D,L-homocysteine, L2, the neutral fac-Re(CO)3(NSO) complex, 2, is produced with coordination of the nitrogen of the primary amine, the sulfur of the thioether, and the oxygen of the carboxylate group, while the pyridine ring remains uncoordinated. The analogous technetium-99m complexes, 1ʹ and 2ʹ, were also prepared quantitatively by the reaction of L1 and L2 with the fac-[99mTc(CO)3(H2O)3]+ precursor at 70 °C in water. Given that both (S)-(2-(2ʹ-pyridyl)ethyl)cysteamine and homocysteine can be easily N- or S-derivatized by a bioactive molecule of interest, both the NSN or NSO ligand systems could be used to develop target-specific radiopharmaceuticals for diagnosis and therapy.