Review of the Journal Article “Thermochromic and solvatochromic properties of Lindqvist polyoxometalates”

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!

About me

Hi!

I am Pedro Molina Sánchez, a Postdoctoral Scholar at Oregon State University (USA) and this is my blog. The idea is to write here about polyoxometalates (POMs) in general and in particular to include my reviews on recently published articles in the topic. I like POMs quite a bit so I think I’m gonna enjoy this!

I am interested in aqueous inorganic chemistry, particularly in the formation of anionic, nanosized metal oxides (polyoxometalates), their behavior in solution and their integration into supramolecular functional materials. This fascinating class of compounds helps us to understand natural, fundamental processes, such as ion-association and polyionic speciation, as well as to fabricate functional materials targeted for fuel production, catalysis and energy storage applications. My curiosity and efforts are now focused on discovering new structural motifs in the Nb/Ta polyoxometalate family (polycoltanates), as novel clusters displaying unknown structures will deepen our knowledge of the ion-association between alkali metals and metal oxides and of the ability of these clusters to catalyse a diverse set of reactions under basic conditions.

I got my BSc in Chemistry at the University of Glasgow (UK) and then joined Prof. L. Cronin’s for my PhD work in polytungstate chemistry. Soon after obtaining his PhD, I crossed the pond to join the group led by Prof. M. Nyman to conduct exciting research in the chemistry of polycoltanates (Nb/Ta polyoxometalates). I love cycling and am having a blast riding along the stunning Oregon country roads. My list of scientific publications can be found in ORCID while details on my general interests can be found in my website.