Importance of π-Interactions Involving Chelate Rings in Addition to the Tetrel Bonds in Crystal Engineering: A Combined Experimental and Theoretical Study on a Series of Hemi- and Holodirected Nickel(II)/Lead(II) Complexes

Five heteronuclear nickel(II)/lead(II) complexes with two compartmental reduced Schiff base ligands were prepared and characterized. Their structures were confirmed by single crystal X-ray diffraction analyses. In each complex, nickel(II) is placed in the inner N2O2 compartment and lead(II) in the outer O2O2' compartment of reduced Schiff base ligands. Interesting molecular architectures were formed via supramolecular interactions in the solid state of the complexes. A density functional theory study is devoted to analyze unconventional tetrel bonding interactions established between the sigma-hole at the hemicoordinated lead(II) and either the electron-rich thiocyanate or the pi-system of the aromatic ligand. In addition, pi-stacking assemblies between both the aromatic rings and five-membered Pb-chelate rings were described and studied both with regard to their energies and by using the noncovalent plot index.