Anion-π Interactions in Hollow Crystals of a Copper(II)-Cyamelurate Coordination Complex

A novel cyamelurate-based copper(II) coordination compound has been designed from a computational approach to show anion-p interactions between the s-heptazine core and perchlorate anions; therefore, a complex of formula {[Cu(pmta)](3)cyam}.(ClO4)(3) (1) [pmta = N,N,N',N'',N''-pentamethyl-diethylenetriamine and cyam = cyamelurate ligand] was synthesized. The cationic molecule stabilizes in the solid state with two perchlorate anions one above and the other below the cyamelurate aromatic rings in a polar conformation, crystallizing in the R3c noncentrosymmetric space group with a close cubic packing of the cations. The binding energies were calculated to be ca. -175 kcal/mol for the two species 1:OClO3- and 1:O3ClO-, and the anion-p contribution could also be calculated, being ca. -10 kcal/mol. The dielectric and magnetic properties were analyzed showing a semiconductor behavior in the temperature range studied (300-458 K) and a weak antiferromagnetic interaction among the three Cu(II) ions. The crystals of 1 show a hollowed hexagonal prismatic morphology, with hollow diameters up to 300 mu m. A mechanism based on oriented growth, dissolution, and recrystallization of the outer shell was proposed to explain these hollowed structures