A 4-tert-butylcalix[4] arene tetrahydroxamate podand based on the 1-oxypiperidine-2-one (1,2-PIPO-) chelate. Self-assembly into a supramolecular ionophore driven by coordination of tetravalent zirconium or hafnium(IV)

An octadentate tetrahydroxamic calix[4] arene podand incorporating 1-hydroxypiperidine-2-one (1,2-PIPOH) binding units has been designed as a specific chelator for tetravalent metal cations like Zr4+ or Hf4+. This receptor, which can be considered as the first ever abiotic ligand possessing only cyclic six-membered hydroxamate groups, has been synthesized and characterized in its tetraprotonated form (1H(4)). Contrary to expectation, however, this new chelator did not form a 1 : 1 complex upon reaction with M(acac) 4 (M = Zr and Hf; acac = acetylacetonate), but rather self-assembled into a dimeric species of 2 : 2 stoichiometry. The latter could be characterized in solution by mass spectrometry and NMR spectroscopy as its monopotassium adduct ([M2K(1)(2)](+)), pointing to the ionophoric character of the M-2(1)(2) complex.