Halide metathesis in overdrive: mechanochemical synthesis of a heterometallic group 1 allyl complex

As a synthesis technique, halide metathesis (n RM + M'X → R M' + n MX) normally relies for its effectiveness on the favorable formation of a metal halide byproduct (MX), often aided by solubility equilibria in solution. Owing to the lack of significant thermodynamic driving forces, intra-alkali metal exchange is one of the most challenging metathetical exchanges to attempt, especially when conducted without solvent. Nevertheless, grinding together the bulky potassium allyl [KA']_∞ (A' = [1,3-(SiMe₃)₂C₃H₃]^-) and CsI produces the heterometallic complex [CsKA'₂]_∞ in low yield, which was crystallographically characterized as a coordination polymer that displays site disorder of the K⁺ and Cs⁺ ions. The entropic benefits of mixed Cs/K metal centers, but more importantly, the generation of multiple intermolecular K^…CH₃ and Cs^…CH₃ interactions in [CsKA'₂]_∞, enable an otherwise unfavorable halide metathesis to proceed with mechanochemical assistance. From this result, we demonstrate that ball milling and unexpected solid-state effects can permit seemingly unfavored reactions to occur.