Sterically Stabilized Homoleptic Copper and Gold Allyl Complexes

Well-defined allyl derivatives of the coinage metals remain rare, often owing to their thermal and redox sensitivity. We report the isolation of a potassium bis-(allyl)-cuprate, [KCuA'₂] (A' = 1,3-(SiMe₃)₂C₃H₃), prepared with either solution or mechanochemical methods, which is stable at room temperature in both the solid state and solution. In the solid state, it crystallizes as a dimer ([{KCuA'₂}₂]), featuring a planar K₂Cu₂ core and η¹-bound allyl ligands. In neat hexanes, [{KCuA'₂}₂] enables conjugate (1,4)-allylation of α,β-unsaturated ketones. It does not transmetallate when treated with lithium or sodium salts but converts to the crystallographically authenticated, but less stable neutral tetramer, [{CuA'}₄], which possesses μ-η:¹η² allyl bridges and a puckered Cu₄ ring. Gold analogs of both the anionic and neutral complexes have also been prepared ([{KAuA'₂}₂] and [{AuA'}₄], respectively) and are structurally similar to their copper counterparts. However, the potassium bis-(allyl)-aurate does not perform the same addition chemistry as its copper analogue. DFT calculations on the unsubstituted di-(allyl) cuprates (i.e., [{(Li,K)-Cu-(C₃H₅)₂}₂]) suggest that both the trimethylsilyl groups and the substitution of Li⁺ with K⁺ influence the aggregation of potassium bis-(allyl)-cuprate. These findings provide structural and reactive benchmarks for coinage-metal allyl complexes.