Di- versus Trinuclear Copper(II) Cryptate for the Uptake of Dicarboxylate Anions

Searching for receptors selective for the binding of dicarboxylate anions, the copper(II) complexes of the known ditopic octaazacryptand (t2pN8), derived from bistren [tren = tris(2-aminoethyl)amine] linked by p-xylyl spacers, were re-examined, with the expectation of observing a selective binding of oxalate or malonate by bridging the two copper centers of the [Cu2(t2pN8)(H2O)2]4+ receptor. Solution studies involving the supramolecular species formed by the receptor and oxalate (oxa2–), malonate (mal2–), and succinate (suc2–) anions are reported. The determined association constants revealed the unexpected formation of a 3:1:1 Cu/t2pN8/anion stoichiometry for the cascade species with oxa2– and mal2–, and the single crystal X-ray structural characterization confirmed the presence of tricopper(II) complexes, with an unusual binding mode for the dicarboxylate anions. Each of the two copper atoms binds four nitrogen donor atoms of the t2pN8 cryptand and one additional hydroxide group, which bridges to the th...