Electronic properties and stabilities of methoxy-substituted Lindqvist polyoxometalates [Nb2W4O19CH3]3− by DFT

Abstract

The electronic properties and stabilities of five [Nb2W4O18OCH3]3− isomers have been investigated using a density functional theory method. The results show that the isomer with the methoxy group occupying a bridging position between two tungsten atoms (two tungsten atoms in the plane that contains two niobium atoms) in the [Nb2W4O18OCH3]3− framework is the most stable isomer in acetonitrile. The stability of the one-electron-reduced isomers changes little. The most stable one-electron-reduced isomer has the methoxy group occupying a bridging position between niobium atoms in the [Nb2W4O18OCH3]4− framework. The M-Ob (M = Nb, W; b denotes bridging) bond lengths in anions in which the metal atoms are connected by a methoxy group are longer than those in [Nb2W4O19]4−. The highest occupied molecular orbitals (HOMO) in [Nb2W4O19]4− mainly delocalize over the bridging oxygen atoms of two niobium atoms and two tungsten atoms located in the equatorial plane, and the bridging oxygen atoms on the axial surface. The lowest unoccupied molecular orbitals (LUMO) of [Nb2W4O19]4− are mainly concentrated on the tungsten atoms and antibonding oxygen atoms. Methoxy substitution modifies the electronic properties of the [Nb2W4O18OCH3]3− isomers. The HOMOs in the five isomers formally delocalize over the bridging oxygen atoms, which are distant from the surface containing the methoxy group and four metal atoms. The LUMOs delocalize over the d-shells of the four metal atoms that are close to the methoxy group, and the p-orbitals of oxygen. One-electron reduction occurred at the tungsten atoms, not the niobium atoms.