Water dissociation on alpha1-hafnium and ytterbium substituted Dawson polyoxotungstates: a density functional theory study

Density functional theory (DFT) calculations were devised to get insight into Lewis acidic catalysis by POMs, especially on the intriguing activation of complexed water molecules that was observed in some experimental cases. Computationally, it appears that deprotonation is feasible with alpha(1)-Hf(H(2)O)P(2)W(17)O(61)](6-), but not with alpha(1)-Yb(H(2)O)P(2)W(17)O(61)](7-). This reflects the difference of the electronic structures (diamagnetic for hafnium POM, paramagnetic for ytterbium POM). From a mechanistical point of view, indirect Br?nsted catalysis cannot be excluded in the hafnium case, especially for Mannich reactions. But our calculations show that catalysis by alpha(1)-Yb(H(2)O)P(2)W(17)O(61)](7-) (and presumably all the lanthanide series) proceeds through direct complexation of the substrates to the POM.