Solvents are multi-hatted players in synthetic chemistry, promoting the interaction of reagents, affecting the rates of reactions, changing the distribution of products, and in many cases dispersing heat. Often the selection of starting materials in organometallic chemistry is dictated by solubility considerations (e.g., when metal iodides are used instead of fluorides) or stability issues (e.g., to avoid unwanted reactivity with ethers). Solvent molecules may also bind strongly to dissolved reagents and products, and change their structure(s) and reactivity. With all these roles for solvents, it should probably not be a mystery that mechanochemical synthesis, performed by the grinding or milling of reagents with little or no solvent, can yield products at variance from those generated in solution. The focus in this presentation is on ‘non-stoichiometric’ behavior in the synthesis of metal complexes of the bulky allyl ligand [A′]– (A′ = [1,3-(SiMe3)2C3H3]–). The reaction of K[A′] (A′= [1,3-(SiMe3)2C3H3]–) with MgBr2 in Et2O, for example, produces the solvated species MgA′2●Et2O. The compound can be desolvated to form a dimer, {MgA2}2. If K[A′] and MgBr2 are simply milled together in a 2:1 ratio, however, the trimetallic species K2MgA′4 is formed, which would make stoichiometric sense if the reagents were in a 4:1 ratio (i.e., 4 K[A′] + MgBr2 → K2MgA′4 + 2 KBr). Analogous differences between the solution and solid-state reactions are observed with Be, Ca, and Sn reagents, although redox behavior is also observed with the latter in the solid state (not in solution). Identification of the specific source(s) for the divergent reactivity between the solution- and mechanochemically generated products will be discussed.

Mechanochemicum mysterium: Why ‘non-stoichiometric’ reactions in the solid state should be expected

256th National Meeting (2018)

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