Reactivity of the 30-electron dimolybdenum anion [Mo2(η5-C5H5)2(μ-PCy2)(μ-CO)2]− towards β,γ-unsaturated organic halides: Alkenyl, allenyl and alkoxycarbyne derivatives

The 30-electron dimolybdenum anion [Mo2Cp2(μ-PCy2)(μ-CO)2]− reacts at room temperature with allyl chloride to give the unsaturated σ:π-bonded alkenyl derivative trans-[Mo2Cp2(μ-η1:η2-CMeCH2)(μ-PCy2)(CO)2], this requiring a 2,1-hydrogen shift in the allyl moiety probably induced by the unsaturated nature of the dimetal center. In a similar way, the dimolybdenum anion reacts with trans-1-chloro-2-butene (crotyl chloride) to give a mixture of the alkenyl complexes trans-[Mo2Cp2(μ-η1:η2-CEtCH2)(μ-PCy2)(CO)2] and trans-[Mo2Cp2(μ-η1:η2-CMeCHMe)(μ-PCy2)(CO)2] in a 3:2 ratio, which could not be separated by column chromatography. All these alkenyl species exhibit a dynamic behavior in solution (fast on the NMR timescale even at low temperatures) involving alternative π-bonding of the alkenyl ligand to each metal center. In contrast, the title anion reacts with propargyl chloride (ClCH2–CCH) without further rearrangement of the propargyl moiety, to afford the allenyl derivative trans-[Mo2Cp2{μ-η2:η3-CH2CCH)}(μ-PCy2)(CO)2] as the major species. Acryloyl chloride (ClC(O)–CHCH2) also reacts with the title anion to give a mixture of two products, the carbyne complex [Mo2Cp2{μ-COC(O)CHCH2}(μ-PCy2)(μ-CO)] and the vinyl trans-[Mo2Cp2(μ-η1:η2-CHCH2)(μ-PCy2)(CO)2], in a 1:1 ratio. This reaction is a unique case in which a single electrophile can attack both nucleophilic positions in the dimolybdenum anion, these being located at the O(carbonyl) and metal sites, respectively. The formation of the vinyl derivative requires the decarbonylation of a metal-bound acryloyl group, which proved to be an irreversible reaction, since the addition of CO to the above alkenyl complex gave instead the tricarbonyl vinyl derivative cis-[Mo2Cp2(μ-η1:η2-CHCH2)(μ-PCy2)(CO)3]. The structure of this electron-precise complex was confirmed through a single-crystal X-ray diffraction analysis (Mo−Mo = 3.0858(7) Å).

Rare 2,1-hydrogen shifts or decarbonylation processes occur readily at room temperature in the reactions of the title anion with organic halides having a β,γ-double bond, to give unsaturated alkenyl derivatives. Other reaction pathways, as the electrophilic attack at the O(carbonyl) atoms of the anion to give an alkoxycarbyne derivative, are only marginally competitive.Figure optionsDownload as PowerPoint slide