Theoretical comparative survey on the structure and electronic properties of first row transition metal substituted Keggin type polyoxometalates

Abstract

In the present study, molecular geometry, electronic structure, first row transition metal-polyoxometalate interactions, transition metal-H2O bonding, and simulated infrared (IR) spectrum of H2O ligated/unligated transition metal substituted Keggin type polyoxometalates ([PW11O39(MII/MIIOH2)]5-, MII ​= ​Cr, Mn, Fe, Co, Ni, Cu and Zn) have been studied through Density Functional Theory (DFT) calculations. From the results, it has been predicted that the interaction of H2O ligand with MII leads to a stable [PW11O39(MII-H2O)]5- complex, without spin crossing during water attachment. The calculations have also revealed that interaction of H2O ligand with [PW11O39(MII)]5- could be discussed in two different classes: when MII is Cr, Mn and Fe, the H2O molecule interacts directly to MII from O side, while for MII ​= ​Co, Ni, Cu and Zn, the H-bond interaction between H2O and O atoms around the M, leads to the stabilization of POM-H2O system. Moreover, it has been predicted that the binding properties of H2O strikingly depends on axial 3dz2 orbital of MII. In this regard, it has been exhibited that the affinity of H2O for MII in [PW11O39(MII)]5- with a filled 3dz2 orbital, is strongly reduced as measured by increasing in calculated MII-ligand distance. Also, strong contributions of MII 3d as well as the 2p orbitals of five oxygen donor atoms in polyoxometalate cluster results in the bonding interaction between MII and lacunary Keggin polyoxometalate. The DFT-derived IR spectra showed displacement and splitting in four previously known vibrational bands of Keggin-type polyoxometalate, α-[PW12O40]3-, owing to transition metal substitution and H2O ligation.