Investigating the Bismuth Complexes with Benzoazacrown Tri- and Tetra-Acetates

The complexation properties towards Bi³⁺ of benzoazacrown ligands with five (H₃BA3A) and six (H₄BATA) heteroatoms and corresponding number of acetic arms have been investigated. Complexation constants were determined via potentiometric titration for BA3ABi complex (logβ=26.2) and refined for BATABi complex (logβ=31.7). According to NMR, X‐ray and EXAFS results, chelation of cation by H₃BA3A appears to be outside the macrocyclic cavity both in the solid state and in the aqueous solution. This out‐cage location despite the high stability constant causes serum and in vivo instability of complex of smaller azacrown H₃BA3A in contrast to H₄BATA. Whereas the larger macrocycle and four pendant arms of H₄BATA provide long‐range coordination effectively shielding the cation thereby ensuring high inertness in vitro, low accumulation of the radionuclide in the organs, and fast clearance.