Structural trends in copper(II) bis(thiosemicarbazone) radiopharmaceuticals

Redox-related changes in biological properties of copper bis(thiosemicarbazone) radiopharmaceuticals are induced by backbone alkylation. To determine whether these changes are mediated by changes in core structural parameters, eight X-ray structures of variously alkylated complexes were determined. The complexes include the hypoxia tracer diacetylbis(4-methyl-3-thiosemicarbazonato) copper(II) (CuATSM). The structures of the nickel analogue NiATSM and the corresponding free ligand ATSMH(2) were also included. Distortions from planarity were slight and only present when there were significant intermolecular interactions (mainly pairs of N-H-N and N-H-S hydrogen bonds). These give rise to cross-linked flat or helical ribbons of complexes. Alkylation at the terminal nitrogen atoms interrupts hydrogen bonding, allowing complexes to become planar, but does not otherwise affect the coordination sphere. Alkylation at the backbone carbon atoms increases the backbone C-C bond length, allowing the metal to fit better into the ligand cavity with shorter Cu-S bonds.