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

As a synthesis technique, halide metathesis (n RM + M'X(n) → R(n)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-(SiMe(3))(2)C(3)H(3)](–)) 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(…)CH(3) and Cs(…)CH(3) 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.