Noncovalent bonding: Stacking interactions of chelate rings of transition metal complexes

Abstract

Analysis of crystal structure data deposited in the Cambridge Structural Database (CSD) has shown that aromatic rings tend to stack with square planar transition metal complexes when they contain chelate rings. In these interactions, the orientation between chelate and aryl ring is a parallel-displaced orientation, like stacking interactions between aromatic molecules. In fused systems containing chelate and aryl rings, the aryl rings prefer to stack with the chelate rather than with other aryl rings. Quantum chemical calculations show that chelate-aryl stacking is stronger than aryl-aryl stacking. Interaction energies of six-membered chelates of the acetylacetonato type with benzene exceed -6 kcal/mol (CCSD(T)/CBS), more that twice as strong as that for two benzene molecules. Further analysis of the CSD has shown that chelate rings, both isolated and fused stack with other chelate rings. These chelate-chelate stacking interactions can have both face-to-face and parallel-displaced geometries, unlike the stacking interactions between aromatic molecules, for which face-to-face geometry is not typical. Chelate-chelate stacking is stronger than aryl-aryl stacking and stronger even than chelate-aryl stacking. Stacking energies between six-membered chelates of acetylacetonato type exceed -9 kcal/mol, while those between five-membered dithiolene chelates are even stronger. Calculated interaction energies and analysis of supramolecular structures have shown that chelate-chelate and chelate-aryl stacking must be considered in understanding the packing and organization of supramolecular systems and crystal engineering.