The “steric acidity” of the zirconium methyl amide complexes: A theoretical study

Density functional theory calculations have been used to study the acidity of the N-H proton in the zirconium methyl amide complexes. A process containing two steps has been proposed by us to understand the acidity of the N-H proton in these amide complexes: the first step is their deprotonation; and the second one is the reactions of the resultant anions from the deprotonation step with the Li(THF)n cation to form the lithium zirconimidate complexes. The present calculations show that the acidity of the N-H proton in the zirconium methyl amide complexes is determined by the deprotonation step since the reactions of the resultant anions from the deprotonation with the LiTHF cation are barrierless. Solvation does not influence the deprotonation step. Thus, the acidity of these zirconium methyl amide complexes can be compared directly. In the deprotonation step, all these N-H bond lengths in these zirconium methyl amide complexes are 1.02 Å and the difference in the structures of these complex are these substituents on the two CP ligands. The repulsion between these substituents on the two CP ligands can be decreased by the loss of the N-bound proton. Thus, the steric strain, including the repulsion between these substituents on the two CP ligands as well as the angle stain of the linking group (CH2-CH2) that connect the two CP ligands coordinating to the Zr center, should be responsible for the acidity strength order of the zirconium methyl amide complexes. The zirconium methyl amide complexes represent a rare “steric acidity”. The proposed process can account well for the experimental observation. This work is very helpful for experimentalist to better understand the “steric acidity” of the zirconium methyl amide complexes.