Solvate-Assisted Grinding: Metal Solvates as Solvent Sources in Mechanochemically Driven Organometallic Reactions

Abstract

Solvent effects in synthetic chemistry can be analyzed in various ways, but a possibly counterintuitive approach is to remove the solvent entirely from the reaction environment. Mechanochemical initiation (grinding or milling of solid reagents) can then be used to replace the mixing and energy that would be supplied by a solvent. The effect of complete solvent elimination can be unpredictable, however, and even partial removal, as found in the realm of liquid-assisted grinding (LAG), can alter reaction outcomes profoundly. Reduced quantities of solvents can also be present in the form of solvates, and although their use has been classified under the category of “neat” or “dry” grinding, “solvate-assisted grinding” (SAG) might be a more descriptive term, as the outcome of such reactions can differ from those of both LAG and neat grinding, with the latter term more properly restricted to the use of anhydrous or unsolvated reagents only. This study examines the mechanochemically driven synthesis of the bis­(allyl)metal complexes [MA′2] (M = Cr, Fe, Co, Ni; A′ = 1,3-(SiMe3)2C3H3) via halide metathesis. Particular emphasis is given to the nickel-based system, whose synthesis from nickel halide solvates is compared with the same reaction using the anhydrous metal halide and either large or LAG quantities of solvent (a few microliters of solvent per milligram of reagents). The reactions employing solvate-assisted grinding, either alone (e.g., with [Ni­(py)4Cl2]) or with LAG amounts of a different solvent (e.g., with [Ni­(dme)­Br2] and THF as the LAG solvent) yield the nickel allyl complex the most efficiently, suggesting that the intimate interaction of coordinated solvents with the metal centers is an important variable in determining the outcome of the metathesis reactions. Although they were developed in the context of mechanochemically driven reactions, these results have implications for solution-based synthesis as well, where the use of solvated reagents can strongly affect outcomes.

Mechanochemical Grignard Reactions with Gaseous CO2 and Sodium Methyl Carbonate**

“…[19] Probably, this positive effect of the additive THF was due to a stabilization of the organometallic intermediate by the Lewis basic ether as observed by Grignard himself, [10a] further investigated by Schlenk and Schlenk, [20] as well as others. [ 21 , 22 ]…”

Mechanochemical Grignard Reactions with Gaseous CO2 and Sodium Methyl Carbonate**

“…As a result, the yield of 2 a increased to 25 % (Table 1, entry 4) [19] . Probably, this positive effect of the additive THF was due to a stabilization of the organometallic intermediate by the Lewis basic ether as observed by Grignard himself, [10a] further investigated by Schlenk and Schlenk, [20] as well as others [21, 22] …”

Toward Mechanistic Understanding of Mechanochemical Reactions Using Real-Time In Situ Monitoring

“…Such autocatalytic behavior, so far studied in the mechanochemical formation of cocrystals 67,68 and coordination compounds, offers an explanation for the general observation that solvated starting materials are more reactive than the corresponding nonsolvated systems. 69 Our team has reported the in situ observation of such autocatalytic behavior as a serendipitous discovery during real-time monitoring of the formation of coordination polymers involving CdCl 2 and cyanoguanidine (cnge) (Figure 9). 70 Preparation of the reaction Once CdCl 2 was fully depleted the released water remained in the mixture leading to autocatalytic reaction acceleration.…”