Stacking interaction potential energy surfaces of square-planar metal complexes containing chelate rings

Stacking interactions of metal chelate rings, chelate-aryl and chelate-chelate stacking, have been recognized by analyzing crystal structures in the Cambridge Structural Database, while the energies of the interactions have been obtained by high level quantum chemical calculations, including the CCSD(T)/CBS level, that is considered to be the gold standard in quantum chemistry. In this review we present data on calculated potential energy surfaces of metal chelate ring stacking interactions for nickel, copper, zinc, palladium, and platinum, and two chelate ligands, acac-type and dithiolene. The data show that both, the nature of the metal atom and the nature of the coordinated chelate ligand, have significant influence on the geometries, as well as on the energies of the interactions. The most stable geometries of the chelate-aryl and chelate-chelate stacking geometries are various parallel-displaced geometries, in both cases. The calculated aryl-chelate stacking interaction energies of minima on potential curves are quite strong, from − 5.36 (for Pt-acac type chelate) to − 7.52 kcal/mol (for Zn-acac type chelate). These interactions are significantly stronger than stacking interaction between two benzene molecules (− 2.73 kcal/mol). The chelate-chelate stacking interactions are even stronger, from − 9.21 (for Pd-acac type chelate) to − 10.34 kcal/mol (for Ni-dithiolene chelate). The data on metal chelate stacking interactions indicate that the strength of the stacking interactions can be varied by varying metals and ligands, which is important for crystal engineering, material science and other supramolecular structures, including biological systems