Why is a dicationic digallene so reactive towards activation of strong covalent bonds? Scope and mechanistic...

Herein, we investigate the reactivity of the trans-bent Ga⇄Ga double bond in the dicationic digallene [{Ga(dcpe)}₂]²⁺ (dcpe = bis(dicyclohexylphosphino)ethane) as its [pf]^- salt ([pf]^- = [Al(OR^F)₄]^-; R^F = C(CF₃)₃), which is formed in situ within seconds. Unusually, this digallene is highly reactive towards covalent bonds and oxidatively adds even to strong E-Y σ-bonds, e.g., H-O, H-N, H-C and C-F bonds, under mild conditions, often at room temperature. Their bond activation at any cationic subvalent group 13 compound is unprecedented and the C-H bond activation is the first oxidative addition reported between any subvalent gallium compound and a neutral substrate. The scope and mechanism of the bond activation reactions were experimentally investigated by interaction with selected substrates and via isotope labelling experiments, as well as using high-level quantum chemical calculations. Mechanistically, the pronounced reactivity of the digallene can be attributed to an easily accessible asymmetric conformer with one (Lewis-acidic) planarized and one (Lewis-basic) pyramidalized reactive Ga-site, allowing for cooperative E-Y bond cleavage. In addition, the [2 + 2] cycloaddition of the Ga⇄Ga bond to C[double bond, length as m-dash]C double and triple bonds was studied: it follows a stepwise, non-concerted reaction mechanism, which allows for the catalytic isomerization of cis-olefins and may serve as the basis for follow-up functionalization reactions.