C(sp₃)-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(sp3)-H bond activation follows the order Al≪Ga<In. Experimental data based on model complexes, XRD studies, and (2)H 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.
Organometallics: Materials and Catalysis Department, Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Université de Rennes 1, Campus de Beaulieu, 35042 Rennes (France).
Nicolas
N
Maudoux
Jian
J
Fang
Thierry
T
Roisnel
Vincent
V
Dorcet
Laurent
L
Maron
Jean-François
JF
Carpentier
Yann
Y
Sarazin
C-H activation; density functional calculations; gallium; hydride abstraction; indium; zwitterions