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

• Main Authors: Melyssa L. Grieve, Patrick R. W. J. Davey, Craig M. Forsyth, Brett M. Paterson
• Format: Article
• Language: English
• Published: MDPI AG 2021-06-01
• Series: Molecules
• Subjects: thiosemicarbazone; macrocycle; transition metal; cyclen; gallium-68; coordination chemistry
• Online Access: https://www.mdpi.com/1420-3049/26/12/3646

A 1,4,7,10-tetraazacyclododecane (cyclen) variant bearing two thiosemicarbazone pendant groups has been prepared. The ligand forms complexes with Mn²⁺, Co²⁺ and Zn²⁺. X-ray crystallography of the Mn²⁺, Co²⁺ and Zn²⁺ complexes showed that the ligand provides a six-coordinate environment for the metal ions. The Mn²⁺ and Zn²⁺ 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 Co²⁺ 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 Mn²⁺ and Co²⁺ 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.