Synthesis and structural characterization of technetium and rhenium complexes containing derivatized amino acids

Abstract

A novel N₂S tridentate ligand system (H₂Lⁿ) for the formation of Re^VO, ⁹⁹Tc^VO and ⁹⁹Tc^VN complexes has been synthesized. N-Protected amino acids conjugated with S-methyl 2-methyldithiocarbazate gave chelate complexes [MO(Lⁿ)Cl] (M = Tc or Re) and [TcN(Lⁿ)(PPh₃)] in which H₂Lⁿ are doubly deprotonated. All the compounds have been characterized by elemental analysis, IR and NMR spectroscopy and for [TcO(L²)Cl] and [TcN(L⁴)(PPh₃)] crystal structures determined. Both possess a distorted square-pyramidal geometry with the oxygen or nitrogen atom in the apical position, the tridentate ligand and a Cl^– ion or PPh₃ group forming the basal plane respectively. A study of reactivity shows that the formation of complexes can be related to the N-protecting groups, the alkyl substituent on the C_α atom and the metal precursors. Circular dichroism measurements on Re^VO complexes have been performed in order to investigate possible conformational variations in solution. Finally, although the presence of isomers is possible, NMR spectroscopy has shown that they are absent at room temperature.