Tuning the Kinetic Inertness of Bi3+ Complexes: The Impact of Donor Atoms on Diaza-18-Crown-6 Ligands as Chelators for 213Bi Targeted Alpha Therapy.

The radionuclide 213Bi can be applied for targeted α therapy (TAT): a type of nuclear medicine that harnesses α particles to eradicate cancer cells. To use this radionuclide for this application, a bifunctional chelator (BFC) is needed to attach it to a biological targeting vector that can deliver it selectively to cancer cells. Here, we investigated six macrocyclic ligands as potential BFCs, fully characterizing the Bi3+ complexes by NMR spectroscopy, mass spectrometry, and elemental analysis. Solid-state structures of three complexes revealed distorted coordination geometries about the Bi3+ center arising from the stereochemically active 6s2 lone pair. The kinetic properties of the Bi3+ complexes were assessed by challenging them with a 1000-fold excess of the chelating agent diethylenetriaminepentaacetic acid (DTPA). The most kinetically inert complexes contained the most basic pendent donors. Density functional theory (DFT) and quantum theory of atoms in molecules (QTAIM) calculations were employed to investigate this trend, suggesting that the kinetic inertness is not correlated with the extent of the 6s2 lone pair stereochemical activity, but with the extent of covalency between pendent donors. Lastly, radiolabeling studies of 213Bi (30-210 kBq) with three of the most promising ligands showed rapid formation of the radiolabeled complexes at room temperature within 8 min for ligand concentrations as low as 10-7 M, corresponding to radiochemical yields of >80%, thereby demonstrating the promise of this ligand class for use in 213Bi TAT.

Tuning the Separation of Light Lanthanides Using a Reverse-Size Selective Aqueous Complexant.

Efficiently separating the chemically similar lanthanide ions into elementally pure compositions is one of the greatest scientific challenges of the 21st century. Although extensive research efforts have focused on the development of organic extractants for this purpose, the implementation of aqueous complexants possessing distinct coordination chemistries has scarcely been explored as an approach to enhancing intralanthanide separations. In this study, we investigate the lanthanide coordination chemistry of macrophosphi, a novel analogue of the reverse-size selective expanded macrocycle macropa. Our studies reveal that substitution of the pyridyl-2-carboxylic acid pendent arms of macropa with pyridyl-2-phosphinic acid arms of macrophosphi gives rise to a dramatic enhancement in the ability to discriminate between light lanthanides, reflected by a binding affinity of macrophosphi for La3+ that is over 5 orders of magnitude higher than that for Gd3+. Furthermore, upon implementation of macrophosphi as an aqueous complexant in a biphasic extraction system containing the industrial extractant bis(2-ethylhexyl)phosphoric acid, separation factors of up to 45 were achieved for the Ce/La pair. These results represent a remarkable separation of adjacent lanthanides, demonstrating the significant potential of reverse-size selective aqueous complexants in lanthanide separation schemes.