A DFT study on the effect of metal, anion charge, heteroatom and structure upon the relative basicities of polyoxoanions

In many reactions catalyzed by polyoxoanions, the first step involves attaching a proton at the anion so that the activated species can react with the substrate. The present study focuses on this first step and makes a detailed analysis of the factors that affect the basicity of the distinct oxygen sites in a polyoxometalate. Several prototypical polyoxoanions were studied in solution using the conductor-like screening model (COSMO) to model the solution. The nucleophilicity of a given site depends mainly on the coordination type, the metal to which the oxygen is linked, the charge of the anion and the bonding angle in the bridging oxygen sites. The coordinated metal and the coordination type are the most important. We discuss in detail the effect of the charge and of the charge density of the anion on the basicity of the polyoxoanion.

DFT calculations carried out for a series of polyoxotungstates show that the protonation energy of a given oxygen site depends on the number and nature of metal centers directly bonded to an oxygen site, the charge of the anion and the bonding angle in bridging sites. Figure optionsDownload as PowerPoint slide