The catalytic mechanism of intramolecular alkylation of α-diimine by rare earth complexes: A DFT study

We presented a theoretical study on the mechanism of the selective alkylation reaction of CH=N bond of a-diimine catalyzed by the rare earth complexes (1-3): [2,6-R₂Ph-N=CH-CH=N-PhR₂-2,6]Ln(CH₂SiMe₃)₃(THF) → [2,6-R₂Ph-N-CH₂-C(CH₂SiMe₃)=N-PhR₂-2,6]Ln(CH₂Si-Me₃)₂(THF) (where R = Me, THF is tetrahydrofuran and the compounds 1, 2 and 3 denotes Ln = Sc, Y and Lu, respectively). All the quantum chemistry calculations were performed by employing the density functional theory (DFT) M06/sdd method with the THF solvation effects. The calculated results showed that the alkylation reaction is stepwise for all the three rare earth complexes, with a CH₂SiMe₃ group selectively inserting into a specific one of the two CH=N bonds in the first step and a hydrogen transferring in the second step. The calculated reaction barrier heights indicated that the CH₂SiMe₃ insertion is the rate determining step, which was also confirmed by further rate constant calculation. The half life period for 1, which was selected as example for rate constant calculation, is 1.38 s at room temperature, which was in consistent with the experimental observations. The selectivity of the CH₂SiMe₃ insertion was found originated from steric effects of the coordination ligands and the substituent groups in the α-diimine. The results also showed that the M06/sdd method could provide good prediction for both energetic and geometric properties for the chemical reaction system involving organometal compounds like the rare earth complexes.