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 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.