Theoretical approach to the protonation of Anderson type polyoxometalate [XM6O24]n- (X = I, Te, and Sb, M = Mo and W, and n=5,6,7)

Theoretical studies were conducted to study the protonation of Anderson type polyoxometalates (POMs) that have a general formula of [XM6O24]n- (X = I, Te, and Sb, M = Mo and W, and n = 5, 6, 7) and the formation of defective structures due to the departure of a water molecule. Density functional theory (DFT) was used to investigate these processes in both gas phase and acetonitrile. The goal is to determine the most basic POM, and in addition determine the most basic sites within these POMs. The results of our calculations determined that in the gas phase, the central oxygen atoms were the more energetically favorable for all POMs, while in acetonitrile, the most basic site shifts to a bridging oxygen for IMo. Furthermore, successive protonation until the fully protonated neutral POM is obtained revealed that central and bridging oxygen atoms were more susceptible for protonation, and the negative charge was not a reliable indicator of the next protonation site. Factors such as a strong oxygen-metal bond, high symmetry, and high negative charge were all deterministic of the most basic sites. Finally, the formation of a defective POM was found to result from double protonation on the same oxygen atom. Our calculations revealed that double protonation on central or bridging oxygen atoms was more energetically favorable, but the removal of a terminal oxygen atom was more favorable for all the considered POMs. Notably, the removal of a terminal oxygen atom in TeMo POM resulted in a deformed structure with unique characteristics.