Supramolecular Binding of Phosphonate Dianions by Nanojars and Nanojar Clamshells

Despite the widespread use of phosphonates (RPO₃^2-) in various agricultural, industrial, and household applications and the ensuing eutrophication of polluted water bodies, the capture of phosphonate ions by molecular receptors has been scarcely studied. Herein, we describe a novel approach to phosphonate binding using chemically and thermally robust supramolecular coordination assemblies of the formula [RPO₃⊂{cis-Cu^II(μ-OH)(μ-pz)}]^2- (Cu; n = 27-31; pz = pyrazolate ion, C₃H₃N₂^-; R = aliphatic or aromatic group). The neutral receptors, termed nanojars, strongly bind phosphonate anions by a multitude of hydrogen bonds within their highly hydrophilic cavities. These nanojars can be synthesized either directly from their constituents or by depolymerization of [trans-Cu^II(μ-OH)(μ-pz)]_∞ induced by phosphonate anions. Electrospray-ionization mass spectrometry, UV-vis and variable-temperature, paramagnetic ¹H and ³¹P NMR spectroscopy, single-crystal X-ray diffraction, along with chemical stability studies toward NH₃ and Ba²⁺ ions, and thermal stability studies in solution are employed to explore the binding of various phosphonate ions by nanojars. Crystallographic studies of 12 different nanojars offer unprecedented structural characterization of host-guest complexes with doubly charged RPO₃^2- ions and reveal a new motif in nanojar chemistry, nanojar clamshells, which consist of phosphonate anion-bridged pairs of nanojars and double the phosphonate-binding capacity of nanojars.