Selective EPR Detection of Primary Amines in Water with a Calix[6]azacryptand-Based Copper(II) Funnel Complex

A water-soluble calix[6]arene-based azacryptand was synthesized. The corresponding tren [tris(2-aminoethyl)amine] cap grafted at the small rim coordinates strongly a copper(II) ion over a wide range of pH. The host-guest properties of the complex were explored by EPR spectroscopy. Due to second coordination sphere effects and the hydrophobic effect ascribed to the calixarene cavity, this funnel complex selectively binds neutral molecules (alcohols, nitriles, amines) versus anions in water near physiological pH. Among the coordinating guests, hydrophobic primary amines are preferentially recognized thanks to the combined effect of the better metal-ligand interaction and hydrogen bonding to the oxygen atoms present at the small rim. Hence, this Cu(II) calix[6]arene-based funnel complex behaves as a sensitive and selective EPR probe for primary amines, including biologically important molecules such as tyramine and tryptamine, in water, over a large pH window.