Mechanochemical Research

Specific Aims

Solvent-free/mechanochemical organometallic synthesis to generate otherwise unavailable or base-free compounds for both stoichiometric and catalytic reactions.

Generation of catalysts from earth-abundant main-group metals (alkali/alkaline-earth metals); this involves special ligand design to put metals in unusual coordination environments

Investigation of mechanochemical fluorination reactions, specifically development of fluorinated organometallics through C-H/C-F exchange

Background The assumption is of such long standing that the alchemists had a Latin phrase for it: “Corpora non agunt nisi soluta” (bodies do not react unless dissolved). The adage is an oversimplification, yet it remains true that overwhelming majority of reactions in chemistry, including virtually all of biological importance, take place in a solvent, whether water or an organic liquid. Solvent-free reactions, in fact, are sufficiently distinct that they are a characteristic feature of mechanochemistry, which uses mechanical action (e.g., impact and friction) to generate the energy needed for chemical reactions. Although some heat may be involved, mechanochemical reactions are not simply thermal reactions in disguise. The cracking of crystals, the magnitude of the electric field near the tip of a mobile crack, and the high defect density introduced in crystals by grinding can fracture bonds and create radicals in ways that do not occur in solution. Not surprisingly, the outcome of mechanochemical reactions need not be the same as in solution; as long ago as the end of the 19th century, the American chemist Carey Lea demonstrated that silver halides decompose under grinding, but melt when heated, and that Cu(II) chloride is reduced to Cu(I) when heated, but does not respond to grinding. Such differences helped mechanochemistry to earn recognition as a distinct field of investigation, separate from other branches of chemistry.

Solvent-based reactions remain the overwhelming norm in chemistry, however, and for good reason: solvents promote intimate mixing and interaction of reagents and allow for dispersion of heat in exothermic reactions. There are significant issues to contend with when solvents are not used: many reactions are simply not amenable to the solvent-free approach, particularly on a large scale, where exothermic reactions can be difficult to control. Efficient mixing can be a problem on any scale, and solvents are still often required for extraction, separation and purification of products. Thus solvent-free chemistry is not necessarily a completely “green” approach to synthesis. Nevertheless, solid state, solvent-free (or at least reduced solvent) syntheses provide the opportunity to investigate new compounds not attainable when a solvent is present. This might be because available solvents interfere with the interaction of the reagents, or because solvent molecules may bind irreversibly to the product, and change its structure and reactivity.

The use of solvent-free chemistry in inorganic chemistry is not particularly exotic, as a considerable variety of metal complexes, coordination polymers and MOFs have been prepared using this method. Its use in organometallic chemistry is much rarer, however; one of the early examples, the solid-state synthesis of ferrocene, dates only from 1999. Solvent-free organometallic reactions, although far less common than solvent-mediated versions, can encompass a variety of transformations, including isomerization, ligand rearrangement and replacement, and oxidative addition/reductive elimination. Our interest in solvent-free chemistry is for the possibilities it provides for access to new types of compounds not available through conventional synthetic routes. We are particularly interested in low-coordinate metal complexes, as they can be expected to display higher reactivity than solvated species.

Collaborative research

Design and synthesis of MOCVD precursors for metal oxide dielectrics