The Synthesis of a Bis(thiosemicarbazone) Macrocyclic Ligand and the Mn(II), Co(II), Zn(II) and (68)Ga(III) Complexes

A 1,4,7,10-tetraazacyclododecane (cyclen) variant bearing two thiosemicarbazone pendant groups has been prepared. The ligand forms complexes with Mn(2+), Co(2+) and Zn(2+). X-ray crystallography of the Mn(2+), Co(2+) and Zn(2+) complexes showed that the ligand provides a six-coordinate environment for the metal ions. The Mn(2+) and Zn(2+) complexes exist in the solid state as racemic mixtures of the Δ(δ,δ,δ,δ)/Λ(λ,λ,λ,λ) and Δ(λ,λ,λ,λ)/Λ(δ,δ,δ,δ) diastereomers, and the Co(2+) complex exists as the Δ(δ,δ,δ,δ)/Λ(λ,λ,λ,λ) and Δ(λ,λ,λ,δ)/Λ(δ,δ,δ,λ) diastereomers. Density functional theory calculations indicated that the relative energies of the diastereomers are within 10 kJ mol(−1). Magnetic susceptibility of the complexes indicated that both the Mn(2+) and Co(2+) ions are high spin. The ligand was radiolabelled with gallium-68, in the interest of developing new positron emission tomography imaging agents, which produced a single species in high radiochemical purity (>95%) at 90 °C for 10 min.