Anne Pichon and Stuart James from Queen's University, Belfast, UK, reveal some of the intriguing reactions that can be carried out without solvents

Grinding together solid reactants in the absence of solvent has been known for at least 100 years as an effective way to do reactions. Despite this, the overwhelming majority of chemical syntheses are currently carried out in solvents, which are normally volatile organic species. Over the past few decades, however, driven by increasing environmental and health and safety concerns, the search for greener processes based on using solvents such as water, supercritical carbon dioxide or ionic liquids, has surged. Organic reactions in the complete absence of solvent have also become more keenly researched since the 1980s. Surprisingly, modern systematic studies of the solvent-free preparation of coordination complexes have only begun to emerge in the past few years. 

There are various ways to carry out solvent-free reactions. Grinding together the starting materials, a method known as mechanochemistry, is being investigated by several groups. Mechanochemical methods range from simply grinding reactants manually, with a mortar and a pestle, to the more reproducible (and effort-free) use of ball mills, in which reactants are ground by a ball bearing inside a shaken or rotated vessel. What is remarkable is that these reactions are sometimes faster than the original solvent-based syntheses, even when solid starting materials are used. The idea that reactants, especially when they are in the solid state, should react together in the absence of solvent is initially quite surprising. It is possible for such reactions to proceed via a melt phase, but in some cases there is no evidence for bulk melting. Mechanochemical reactions between solids can involve the formation of product on the interface between crystals. The product layer then falls away to expose fresh surfaces for the reaction to continue.

The recent resurgence of this topic has revealed that a wide range of coordination compounds, including mononuclear complexes, cages and polymers, can be obtained quickly by mechanochemical methods. For example, the complex [Ni(phenanthroline)₃]²⁺ was reported to form in only two minutes by manually grinding nickel nitrate with phenanthroline, and even relatively inert metal salts such as platinum(II) chloride will react with phosphines in a ball mill. More elaborate supramolecular host structures, such as a tetraplatinum square and a nano-scale bowl-shaped hexapalladium cage, have also been found to self-assemble with remarkable efficiency. Also, crystalline one-dimensional polymer chains have been formed from bridging diamine ligands and silver and copper salts. Porous coordination polymers - sometimes called metal-organic frameworks - have become of special interest recently because of their absorption characteristics, which could lead to molecular storage or separation applications. The mechanism of the formation of their pores has normally been assumed to involve solvent molecules as templates. It is therefore intriguing that a porous, crystalline, metal-organic framework has been prepared by simply grinding copper(II) acetate with an organic bridging ligand (isonicotinic acid, NC₅H₄CO₂H) in the absence of solvent, suggesting that the acetic acid by-product might act as a pore template in this case. This solvent-free synthesis also competes very favourably with solvent-based synthesis in terms of materials, energy and time (ten minutes versus several hours or days). In parallel to mechanochemical methods, other approaches involving gas-solid reactions or co-sublimation of reactants are also attracting considerable interest, and intriguing reversible reactions are being discovered. 

Although this resurgence in solvent-free synthesis is still in its infancy, and the applicability of this approach still needs to be considered on a case-by-case basis, solvent-free methods can be viewed as promising alternatives to solvent-based ones. They could offer a greener alternative to some syntheses, and can even be simpler and faster to carry out. Together with the need to explore and understand reactivity in the solvent-free context, this provides much stimulation for both basic and applied research.

Read the full tutorial review on 'Solvent-free synthesis of metal complexes' in issue 6, 2007 of Chemical Society Reviews.