UK scientists have studied a greener way to make microporous materials that could lead to new types of metal-organic frameworks.

Stuart James and Anne Pichon, at Queen's University Belfast, investigated a wide range of solvent-free mechanochemical reactions. Mechanochemistry is the initiation of chemical reactions by grinding two or more solids together using, for example, a mechanical ball-mill or pestle and mortar. Since mechanochemistry avoids the use of solvents, it could be a greener alternative to solution reactions and more efficient and convenient for lab-scale synthesis.

A ball-mill can be used to make microporous crystalline products

'I was fascinated to learn at a conference a few years ago that reactions between solid organic compounds could occur quite rapidly in a ball mill,' says James. 'It seemed an interesting extension to try to make microporous materials this way.'

James and Pichon surveyed 60 metal complexation reactions between 12 different metal salts and five bridging ligands using a ball-mill to grind the solid components together. They found that many of the mixtures were highly reactive and gave crystalline products within a few minutes. The study revealed some interesting trends, including a general inverse correlation between ligand melting point and reactivity, which could prompt further investigation. They also obtained new types of structures than those formed using solvents.

'Mechanochemistry is actually a very old method,' explains James. 'However, most chemists are not tempted to try it because it is so strongly counter-intuitive that two solids can react with each other. Solvents clearly expand the possibilities of synthetic chemistry but it's comparatively recent that the impetus for avoiding them has become strong enough to start making chemists think again.'

Graham Bowmaker, an expert in mechanochemical synthesis from the University of Auckland, New Zealand, advises that 'the wider acceptance of mechanochemical synthesis in chemistry requires a better knowledge than we currently possess of the range of applicability of this method'. He says that James' study demonstrates 'how such knowledge can be gained using an array-based approach, which also provides clues about the possible mechanisms of such reactions, which in turn will greatly enhance further exploitation of the method'.

Ruth Doherty

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