A critical look on the nature of the intra-molecular interligand π,π-stacking interaction in mixed-ligand copper(II) complexes of aromatic side-chain amino acidates and α,α′-diimines

The mixed ligand complexes {[Cu(4Cl-Phe)(bpy)(H2O)]Cl.[Cu(4Cl-Phe)(bpy)Cl].2H(2)O} (1) and [Cu(4HPG)(bpy)].2H(2)O (2) were synthesized and their crystal structures determined by X-ray crystallography (4Cl-Phe = D,L-4-chlorophenylalaninato, 4HPG = D-4-hydroxyphenylglycinato, bpy = 2,2'-bipyridine). The asymmetric unit of crystals of 1 consists of two slightly different Cu(II) complexes-one contains an axial water ligand, whereas the second one has a chloride anion at that position. By contrast, 2 exhibits two close similar molecules, both having axial chloride ligands, and non coordinated water. The geometrical parameters for pi,pi-stacking interactions were determined by an application of PLATON program. Intermolecular pi,pi-interaction between bpy ligands on adjacent Cu(II) moieties results in homo pairs of complexes in the case of 1. In addition, both species of compound 1 exhibit intramolecular interligand pi,pi-interaction between the 4Cl-phenyl ring of 4Cl-Phe and the metalloaromatic Cu-bpy chelate ring. In crystal 2, multi-stacked homo chains are formed. Analogous calculations for previously reported, related compounds, similar to those presented here, revealed that in most cases with intra-molecular interligand pi,pi-interaction there is an efficient "aryl-metal chelate ring'' pi,pi-stacking instead of an aryl-aryl interligand pi,pi-interaction, which indeed is not observed. These findings let us to take a critical look at the nature of the intramolecular interligand pi,pi-stacking interaction in the referred copper(II) mixed-ligand complexes.