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 (oxa^2-), malonate (mal^2-), and succinate (suc^2-) 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 oxa^2- and mal^2-, 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 third copper. The square planar environment of this one is complete with two oxygen atoms from the oxalate (or the malonate). The two copper centers bound to the tren heads are ∼6.5 Å apart, each one at about 3.5 Å from the third Cu center. These studies were complemented by SQUID magnetization measurements and DFT calculations. The magnetic susceptibility measurements of the oxalate cascade complex showed a strong magnetic coupling (J = - 210 cm^-1) between the Cu centers at a short distance (3.5 Å), while the coupling between the two equivalent Cu atoms (∼6.5 Å) was only -70 cm^-1. This result was well reproduced by DFT calculations.