CH Bond Activation with Triel Metals: Indium and Gallium Zwitterions through Internal Hydride Abstraction in Rigid Salan Ligands

The hydropyrimidine salan (salan=N,N′‐dimethyl‐N,N′‐bis(2‐hydroxyphenyl)methylene]‐1,2‐diaminoethane) proteo‐ligands with a rigid backbone {ON^(CH₂)^NO}H₂ react with M(CH₂SiMe₃)₃ (M=Ga, In) to yield the zwitterions {ON^(CH⁺)^NO}M^?(CH₂SiMe₃)₂ (M=Ga, 2 ; In, 3 ) by abstraction of a hydride from the ligand backbone followed by elimination of dihydrogen. By contrast, with Al₂Me₆, the neutral‐at‐metal bimetallic complex {ON^(CH₂)^NO}AlMe]₂ ( 1]₂ ) is obtained quantitatively. The formation of indium zwitterions is also observed with sterically more encumbered ligands containing o‐Me substituents on the phenolic rings, or an N (CHPh) N moiety in the heterocyclic core. Overall, the ease of C?H bond activation follows the order Al?Ga<In. Experimental data based on model complexes, XRD studies, and ²H NMR spectroscopy show that the formation of the Ga/In zwitterion involves rapid release of SiMe₄ followed by evolution of H₂, and suggest the formation of a transient metal‐hydride species. DFT calculations indicate that the systems {ON^(CH₂)^NO}H₂+M(CH₂SiMe₃)₃ (M=Al, Ga, In) all initially lead to the formation of the neutral monophenolate dihydrocarbyl species through a single protonolysis. From here, the thermodynamic product, the model neutral‐at‐metal complex 1 , is formed in the case of aluminum after a second protonolysis. On the other hand, lower activation energy pathways lead to the generation of zwitterionic complexes 2 and 3 in the cases of gallium and indium, and the formation of these zwitterions obeys a strict kinetic control; the computations suggest that, as inferred from the experimental data, the reaction proceeds through an instable metal‐hydride species, which could not be isolated synthetically.