First‐Generation Bispidine Chelators for (213)Bi(III) Radiopharmaceutical Applications

Hepta‐ and octadentate bispidines (3,7‐diazabicyclo[3.3.1]nonane, diaza‐adamantane) with acetate, methyl‐pyridine, and methyl‐picolinate pendant groups at the amine donors of the bispidine platform have been prepared and used to investigate Bi(III) coordination chemistry. Crystal structure and solution spectroscopic data (NMR spectroscopy and mass spectrometry) confirm that the rigid and relatively large bispidine cavity with an axially distorted geometry is well suited for Bi(III) and in all cases forms nine‐coordinate complexes; this is supported by an established hole size and shape analysis. It follows that nonadentate bispidines probably will be more suited as bifunctional chelators for (213)Bi(III)‐based radiopharmaceuticals. However, two isomeric picolinate‐/acetate‐based heptadentate ligands already show very efficient complexation kinetics with (213)Bi(III) at ambient temperature and kinetic stability that is comparable with the standard ligands used in this field. The experimentally determined hydrophilicities (log D (7.4) values) show that the Bi(III) complexes reported are relatively hydrophilic and well suited for medicinal applications. We also present a very efficient and relatively accurate method to compute charge distributions and hydrophilicities, and this will help to further optimize the systems reported here.