In situ formation of reactive (di)gallenes for bond activation

Access to reactive low-valent main-group complexes capable of bond activation commonly requires multistep syntheses, limiting options for electronic or steric fine-tuning. Here we present a one-pot synthesis of highly reactive Ga(I) complex cations for the activation of strong bonds that works analogously to the in situ generation of active transition-metal catalysts. Commercially available phosphines, selected by computational screening, react with the easily accessible salt [Ga(PhF)_2-3]⁺[Al(OR^F)₄]⁻ (PhF = C₆H₅F; R^F = C(CF₃)₃) to form ambiphilic gallene cations in situ. Their dimerization tendency is reduced or even inhibited by variation of the electronic and steric properties of the chiral or achiral ligands. As an example, the reactivity of the in situ formed gallene [Ga(dipf)]⁺ (dipf = 1,1-bis(diisopropylphosphino)ferrocene) was studied. The cation reversibly dimerizes to the respective digallene in solution. Carbon–carbon multiple bonds in acyclic alkynes and alkenes undergo [2π + 2π] cycloadditions with the digallene [{Ga(dipf)}₂]²⁺. The resulting digallacyclobutanes form reversibly at room temperature, yielding an equilibrium between the Ga(I) and Ga(II) species. Importantly, gallene [Ga(dipf)]⁺ inserts into H–Ga, H–Si and H–B bonds. Finally, reactivities of the dicationic digallene and cationic gallene are analysed by density functional theory and compared to neutral Al(I) and Ga(I) ambiphiles.