I have recently started my PhD – studying spin-crossover complexes, and my synthetic approach is largely focused on the use of mechanochemistry (1). The general response to me saying that I’m busy is: ‘don’t you just grind stuff?’, and the answer is no because I have analysis to do like everyone else. As mechanochemistry is a (mostly) solid state technique, powder X-ray diffraction is a go-to technique for product analysis. Comparison to starting materials can give an indication that a reaction has taken place, and then the detailed analysis can begin.
Elucidation of the structure of novel compounds which are not compatible with NMR, is often facilitated by single crystal XRD (SCXRD). The data from SCXRD can be used to simulate powder patterns for comparison with the mechanochemically synthesized powders.
Colour changes during grinding can also be indicative of reaction success. Therefore, mechanochemistry can allow for rapid screening of a huge number of different reactions in the search for interesting materials. However, don’t be too quick to dismiss an unsuccessful reaction if you haven’t tried liquid-assisted grinding (LAG). Whilst mechanochemistry is typically a solid-state technique, the addition of a small quantity of appropriate solvent (< 1 equiv. in µL per mg) (2,3) can be useful to get the reaction started.
My first experience with mechanochemistry quickly taught me that retrieving powder from a mortar is not easy, no matter how much it is scraped by a spatula. In the spirit of minimizing waste, the un-scrapeable powder is dissolved in a small amount of appropriate solvent and divided up into numerous vials, where techniques such as vapour diffusion and slow evaporation are used to harvest crystals.
I am still new to the world of mechanochemistry and have a lot to improve on, but it has opened my eyes to a synthetic approach that isn’t regularly seen in undergraduate studies and even research labs. It’s a promising area with a lot of applications, where the methodology could easily be manipulated for manufacturing purposes. However, if you are interested in trying some mechanochemistry, it is important to be aware of the potential hazards associated with the neat grinding of certain compounds, especially those which may explode!
References:
1 J. H. Askew and H. J. Shepherd, Chem. Commun., 2017, 54, 180–183.
2 T. Friić, S. L. Childs, S. A. A. Rizvi and W. Jones, CrystEngComm, 2009, 11, 418–426.
3 D. Tan and F. García, Chem. Soc. Rev., 2019, 48, 2274–2292.
About the Web Writer:
Lee Birchall has recently started his PhD under the supervision of Dr. Helena Shepherd at the University of Kent, where he also completed his MSc under the supervision of Dr. Stefano Biagini. He obtained a first class BSc at University College London. He enjoys music, languages and windsurfing and you can find him on Twitter at @LTBIRCH.
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