Discrimination of cis-trans isomers by dinuclear metal cryptates at physiological pH: selectivity for fumarate vs. maleate.

Cryptand L (L = N[(CH2)2NHCH2(2,6-C10H6)CH2NH(CH2)2]3N) and its dinuclear metal cryptates Zn2L4 (1) and Cu2L4 (2) have been prepared, and the binding properties of the cryptates with fumarate and its cis isomer maleate were investigated using fluorescent spectra, (1)H NMR titrations and single crystal X-ray diffraction analysis for [(Cu2L)(fum)][ClO4]2 (3) (fum = fumarate). Thanks to the size and shape matching effect, the cryptates can selectively recognize fumarate at physiological pH, with an association constant almost 18-fold larger than that of maleate, forming a cradle-like cascade complex.