Tuesday, November 27, 2012

Polyoxometalates are cationic, not anionic

N.V. Izarova, N. Vankova, A. Banerjee, G.B. Jameson, T. Heine, F. Schinle, O. Hampe, U. Kortz, Angewandte Chemie International Edition 2010, 49, 7807-7811 (Paywall)
Contributed by Marcel Swart

Polyoxometalates are clusters of metals connected together through oxygens, and can be giant molecules such as {(MoVI)MoVI5O21}12(MoV2O4)30]12- (also known as Mo132) as shown by Bo and Miró in Dalton Transactions recently[1]. These clusters bear a total anionic charge, for instance -12 in the aforementioned example. However, this is not the complete picture.

Ulrich Kortz (Univ. Bremen) visited Girona a couple of months ago and reported an interesting example of how theory can be useful. His group was working on a "A Noble-Metalate Bowl", and when trying to reproduce the 51V NMR spectrum computationally, it was impossible to get good agreement. By introducing Na/K cations they did get more and more reasonable results, and only obtained good agreement after having introduced 7 potassiums (or alternatively 6 potassiums and 1 sodium).

At first, the experimental people did not believe the calculations, because they had their mind set on the fact that polyoxometalates carry a large negative charge, and not a positive one. However, after doing electrospray mass spectrometry, indeed they observed mainly two peaks:

1) a singly charged molecular cation {K7[Pd7V6O24(OH)2]}+ (with seven potassiums)
2) a singly charged molecular cation {Na1K6[Pd7V6O24(OH)2]}+ (with six potassiums)

Therefore, contrary to popular belief, polyoxometalates are cationic!

[1] C. Bo, P. Miró, Dalton Trans. 2012, 41, 9984-9988

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