Unexpected discovery of calcium cryptates with exceptional stability.

A 2,2'-bipyridine-N,N'-dioxide-based cryptand has been found to exhibit exceptional apparent complex stability for Ca2+ and unusually shows very high selectivity for Ca2+ over trivalent lanthanoid cations. The calcium cryptate is kinetically inert to an extent that it even withstands competition experiments with a 77-fold excess of free DOTA ligand, which is currently the chelator with the highest formation constant for calcium.