Authors: S. Herrmann, J. T. Margraf, T. Clark and C. Streb
DOI: 10.1039/C5CC05730B
This paper caught my attention during my journal browsing in July. The article was published in Chemical Communications (RSC) by Carsten Streb’s group based in Ulm University (Germany). Carsten occasionally mentored me during my undergrad and initial postgrad work and I have fond memories of his support in the lab and of our interesting chats about chemistry. I hope I’m not too positively biased!
The article reports the solvatochromic and thermochromic properties of a series of hexametalate clusters. The discussed clusters display the classical Lindqvist structure and have W, Mo or V/W or V/Mo in their composition. The authors’ main thesis is that the change in color of the clusters as a function of the temperature (thermochromism) is caused by changes in the dynamics of the cluster structures and they support this claim by computational analysis.
The solvatochromism of one of the clusters ({V2W4}) is extensively characterized for a fair number of solvents. The authors observe a variation in the number of absorption bands in the UV depending of whether the solvent is protic (1 band) or aprotic (2 bands). They ascribe this effect to hydrogen-bonding networks formed between clusters and protic solvents in solution and support this by DFT calculations. Interestingly, the plot of the molar absorption coefficient of the solutions vs. the relative permitivity of the solvents show an unusual bell-curve shape. A plot of the absorption maxima of the solutions vs. the Gutman acceptor number of the solvents (a measure of the Lewis acidity of the solvent) does not seem to show a clear trend.
Cooling five of these clusters down to 77K results in a significant change of color in two of them, {V2W4} and {VMo5}, as shown by digital images. This thermochromic effect is characterized in more detail for {V2W4} by solid-state UV-vis spectroscopy. 51V NMR experiments show a linear trend in the chemical shift of {V2W4} vs temperature, an effect naturally assigned to the modification of the chemical environment of the clusters yet no further rationalized.
In summary, a nice, well-written paper on fundamental properties of POMs including a handy synthetic procedure to make POM salts soluble in most solvents. I think I’ll be using that soon! I miss a deeper discussion on the relationship between the solvent properties and the absorption of the cluster solutions. I would have liked to see the characterization of the solvatochromism of {VMo5}, the only other cluster showing a significant degree of thermochromism, even though the authors claim it behave similarly to {V2W4}. This may be outside the scope of a brief communication so I guess I’m happy with what I get from the article just as it is!
Polycoltanate 