Anion Recognition by a Dicopper(II) Cryptate

The dicopper(II) complex of the bistren cage 1, in an aqueous solution buffered at pH = 8, incorporates polyatomic anions (NCO-, NCS-, N-3(-), SO42-, HCO3-, CH3COO-) according to a 1:1 stoichiometry. Log K values range over the 2-5 interval and show a sharp peak selectivity with respect to the anion 'bite', i.e. the distance between the two proximate donor atoms of each anion. The peak maximum lies at about 2.3 Angstrom, a bite length corresponding to the N-3(-) anion. Thus, the dicopper cryptate does not recognize anion size or coordinating tendencies, but simply recognizes the bite length.