Solvent influence on the mechanism of a mechanochemical metal-halide metathesis reaction†

Solvents have long been integral to the success of synthetic chemistry, influencing reaction rates, mechanisms, and product selectivity. However, mechanochemistry—typically involving the grinding and mixing of solid reagents—offers an alternative environment that is solvent-free or at least greatly minimizes solvent use, and that drives reactions through mechanical force. In a recent study, the reaction between the salt of a bulky allyl anion, K[A′] (A′ = 1,3-(SiMe₃)₂C₃H₃), and a nickel halide gave very different outcomes depending on whether the reaction was conducted without solvent using a solid material, with a starting solvated complex, [Ni(py)₄Cl₂], modified by a small amount of pyridine, or in pyridine solution. Under certain conditions, halide metathesis occurred, forming the allyl complex in near quantitative yield. Under others, a redox reaction dominated, generating allyl radicals that coupled and left {A′}₂ (1,3,4,6-tetrakis(trimethylsilyl)hexa-1,5-diene) as the major product. To understand these differing outcomes, the formation mechanisms of under varying solvent conditions were investigated here using Density Functional Theory (DFT). The effects of the reaction conditions on factors such as Gibbs free energy change, bond energy behavior, and transition states collectively suggest that electrostatic stabilization dominates in the solvent phase. In the case of solvate-assisted conditions, a complete energy profile diagram of the reaction between [Ni(py)₄Cl₂] and 2K[A′], leading to and KCl, was calculated, with evidence for one of the intermediates ([A′Ni(py)Cl]) being provided by experiments. Calculations confirm that coordination of pyridine to the nickel, whether from the free liquid or (preferentially) from the pyridine solvate, weakens the Ni–Cl bond so that metathesis can proceed easily. If pyridine is absent (i.e., under solvent-free conditions), the redox route will have a kinetic advantage in the reaction. This study provides molecular-level insights for understanding and optimizing solvent-assisted grinding processes.