Superior Base Catalysis of Group 5 Hexametalates [M6O19]8– (M = Ta, Nb) over Group 6 Hexametalates [M6O19]2– (M = Mo, W)

Brønsted and Lewis base catalysis of hexametalate clusters of group 5 metals [M­(V)6O19]8– (M­(V) = Ta, Nb) and group 6 metals [M­(VI)6O19]2– (M­(VI) = Mo, W) was studied using Knoevenagel condensation and CO2 fixation reaction, respectively, as test reactions. It was found from mass spectrometry and elemental analysis that tetrabutylammonium salts of [M­(V)6O19]8– were partially protonated to form [H4M­(V)6O19]4– under ambient conditions, whereas those of [M­(VI)6O19]2– were not. Base catalytic activity increased in the order of [Mo6O19]2–, [W6O19]2– ≪ [H4Nb6O19]4– < [H4Ta6O19]4–, which is consistent with the order of the average NBO charges on the surface O atoms. This trend suggests that the highest activity of [H4Ta6O19]4– is due to the large amount of negative charges on surface O atoms. Theoretical calculations on [H n Ta6O19](8–n)– (n = 1–4) demonstrated that protonation of the O atoms at the edges is energetically favorable regardless of n and that the NBO charges on the remaining unprotonated O atoms in [H4Ta6O19]4– are still more negative than those of pristine [Nb6O19]8–, [W6O19]2–, and [Mo6O19]2–. Theoretical calculations also predicted that CO2 can be reductively activated at all of the surface O atoms of [H4Ta6O19]4– regardless of their locations. This work demonstrates that group 5 polyoxometalates are promising candidates for active base catalysts.