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

Ross F Koby,Nicholas R Rightmire,Timothy P Hanusa,Nathan D Schley,William W Brennessel

doi:10.3762/bjoc.15.181

Ross F Koby, Nicholas R Rightmire + Show 3 more

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https://doi.org/10.3762/bjoc.15.181

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Abstract

As a synthesis technique, halide metathesis (n RM + M'Xn → RnM' + 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.

Highlights

Halide metathesis is a broadly useful synthetic technique in organometallic chemistry, applicable to elements across the entire periodic table
What happens when the energy difference between M'X and MX becomes small? Here we describe the application of mechanochemistry in an organometallic context to examine alkali metal halide exchange unassisted by solvents
Several metal compounds of the allyl anion [A']– were potential candidates for the present study, the need for a base-free, unsolvated complex that preferably had been crystallographically characterized limited the choice to the potassium complex [KA']∞

Summary

Introduction

Halide (or ‘salt’) metathesis is a broadly useful synthetic technique in organometallic chemistry, applicable to elements across the entire periodic table. Intra-alkali metal exchange ( not halide metathesis) has been conducted with the [A']– anion, but always in the presence of a solvent to help drive the process (Equation 4) [19]. From all the grinds of [KA'] with the alkali metal iodides (excepting CsI), the potassium allyl was the only recoverable material; extracted with toluene, it crystallized from solution as the solvate.

Results

Conclusion