Quantum chemical investigation on the structural and electronic properties of zinc–salphen complex: DFT and QTAIM analysis

To characterize the structural, thermochemical and electronic aspects in complexation of zinc cation with N, N′-bissalicylidene phenylenediimine dianion (salphen) Schiff base ligand, a Density Functional Theory (DFT) study has been conducted in combination with Quantum Theory of Atoms in Molecules (QTAIM) analysis. Comparison of our calculated structural parameters of Zn–salphen complex with the available X-ray crystallographical data reveals that both functionals (B3LYP and M06) can well reproduce X-ray structure of the complex with a near accuracy. Then, we have examined the effect of three solvents, benzene, DMF and methanol, on thermochemistry of complexation by calculating the enthalpy and Gibbs free energy changes. In the next step, QTAIM method has been utilized to explore the nature of various possible interactions in Zn–salphen complex in terms of Bond Critical Points (BCPs). Topological study of electron density and its Laplacian clearly predicted that the bond strength of imine (CN) and phenoxide (CPhO) moieties in salphen ligand decreases through the complexation. This behavior has been also investigated via QTAIM charge analysis. Moreover, it has been demonstrated that there are two HH bond interactions with the stabilizing effect on the complexation process. On the other hand, based on QTAIM electronic energy indicators we have discussed the electrostatic character of interactions between zinc cation with N and O atoms of salphen in the coordination sphere.