Theoretical insight into dihydrogen activation with β-diketiminato ligand supported Group 13 and 14 elements: mechanism and activity difference

Abstract

On the basis of density functional theory (DFT) calculations within the framework of the ONIOM method, here we report a new bimolecular mechanism for understating dihydrogen activation using the β-diketiminate (BDI)-ligated Group 13 metal complexes. In parallel with the termolecular Lewis acid–base cooperative mechanism proposed previously (J. Catal., 2019, 373, 1–12), this bimolecular stepwise mechanism, consisting of the initial intramolecular 1,4′-addition of dihydrogen to the BDI-ligated gallium complex followed by the intermolecular 4′,4-H-shift, could be also employed to rationalize the experimental observation. On the other hand, mechanisms and activity difference between the BDI-supported Group 13 and 14 elements, as well as the solvent effect on the aspect of the dihydrogen activation were also comparatively investigated. Differing from the aluminum and gallium complexes, dihydrogen activation over the silylene or germylene carbene analogue tends to occur via the termolecular cooperative mechanism. However, silicon and germanium ligated BDI analogues are demonstrated to be less effective in dihydrogen activation, owing to the smaller atomic radius, the weaker hydride affinity and the larger nonbonded repulsion effect from the intrinsic lone-pair electrons. Moreover, it was found that solvent media have a moderate impact on dihydrogen activation. The non-polar solvent might be more beneficial for the H–H bond cleavage and the intermolecular proton migration in the gallium system.