Novel Methods for the Formation of Free Carbyne Radicals in Solution

Carbyne free radicals (RC) are usually associated with high-energy processes, thus research on their preparation, chemical reactivity, and prevalence under mild conditions is scarce. Recently, it was reported that metallo-complexes containing CR ligands may undergo spontaneous degradation in aqueous solutions to produce free RC radicals. These highly reactive species may react with each other to form the corresponding alkyne and other products. The reaction of 1,1,1 halo-carbo-hydrates with Cr(II) ions also forms RC radicals under mild conditions. Herein, we report two additional synthetic routes that produce free RC radicals under mild conditions. First, the reaction between metallic zinc and acetic anhydride produces 2-butyne and several other C₂, C₃, and C₄ compounds. Isotopic-labeling experiments indicate that the formation of 2-butyne results from an inter-molecular reaction in which two RC moieties from two acetic anhydride molecules combine in solution. In addition, the degradation of the tri-molybdenum complex [Mo₃(H₃CC≡CCH₃)(OAc)(H₂O)₂Br₇], in which a 2-butyne ligand is coordinated to the Mo₃ framework in a μ₃-η₂ (⊥) binding mode in aqueous solution produces several C₂, C₃, C₄ and C₅ molecules. This indicates the formation of free CH₃C radicals by a homolytic cleavage of the carbon-carbon triple bond.