Mausolate Complexes of the Alkaline Earth Metals

The capacious and rigid mausolate ligand [1]2- has the capacity to adaptively coordinate some of the largest metal cations on the periodic table, such as those which are utilized in targeted alpha-particle therapy. One such ion, Ra2+, has proven challenging to keep securely contained by other chelators. Here we investigate the binding behaviour of the Ba2+ (96% the size of Ra2+ but conveniently non-radioactive) with the mausolate [1]2- along with the other alkaline earth metals Mg2+, Ca2+, and Sr2+, principally utilizing real time electrospray ionization mass spectrometry monitoring. We also report the syntheses of several functionalized mausolate derivatives, [3-5]2-, and compare their binding abilities with previously reported variants. We find that the mausolates display a distinct preference for larger alkaline earth metals, with Ba2+ showing extremely rapid uptake and promising stability in a number of physiologically relevant conditions. The incorporation of a reactive bromine atom in the 4-pyridine position of the mausolate ([3]2-) can be utilized to introduce further functionalization through a nucleophilic substitution pathway without compromising coordination ability.