Supramolecular assembly in Cu(II) and Zn(II) compounds with pyridine and anthraquinone-1,5-disulfonate: Experimental and theoretical analysis

Two new multi-component coordination compounds of Cu(II) and Zn(II) viz. [Cu(py)2(H2O)4]ADS·2H2O (1) and [Zn(4-CNpy)2(H2O)4]ADS·2H2O (2) (py = pyridine, 4-CNpy = 4-cyanopyridine, ADS = anthraquinone-1,5-disulfonate) have been synthesized in aqueous media at room temperature and characterized using elemental and spectroscopic analyses (FT-IR, electronic), TGA and single crystal X-ray diffraction techniques. Both the compounds crystallize as metal–organic multi-component compounds containing complex cationic moieties along with uncoordinated anionic ADS moieties in the crystal structures. Crystal structure analysis of 1 reveals the stabilisation of the layered assembly of the compound assisted by CH⋯O, OH⋯O hydrogen bonding and π-stacking interactions. Antiparallel CN⋯CN and aromatic π-stacking interactions provide rigidity to the crystal structure of compound 2. In addition, enclathration of lattice water molecules within the supramolecular cavity of 2 provides additional reinforcement to the crystal structure. We have carried out theoretical investigations to analyze π-stacking, H-bonding and antiparallel CN···CN non-covalent interactions observed in the compounds using DFT calculations, quantum theory of atoms in molecules (QTAIM) and non-covalent interaction (NCI) plot index computational tools. The binding energies of the supramolecular assemblies are quite significant due to the ion-pair nature of the interactions which lack directionality; however, the aforementioned weaker non-covalent interactions play crucial roles in the final arrangement of molecules in the solid state due to their directional nature.