A screening DFT study of the para-substituent effect on local hyper-softness in bis(phenoxy-imine) titanium complexes to get insights about their catalytic activity in ethylene polymerization

Abstract

Quantum chemical calculations of substituted α-Cumyl and tBu bis(phenoxy-imine) titanium catalysts were performed at the RBP86/SVP level. The impact of para-substitution in the phenoxy-imine ligands on the electronic structure of the titanium complexes was studied within the framework of the Conceptual density functional theory. We found that such a reactivity descriptor as local hyper-softness on titanium correlates well with the Hammett constant for a series of substituents. The effect of the substitution in three para-positions of the phenoxy-imine ligand for the Cumyl catalysts weakens while passing from the α-Cumyl through the salicylic and further to the aniline moiety. By contrast, in the tBu complexes the presence of a substituent in the salicylic part has similar influence on the local hyper-softness on titanium as compared with the aniline moiety. Moreover, placement of the groups with opposite electronic effects at the termini of the phenoxy-imine framework was shown to provide electronic flexibility in both the α -Cumyl and the tBu complexes. Within the framework of the "ligand-oriented" concept, such flexibility increases the activity of a bis(phenoxy-imine) catalyst. In addition, we demonstrated that the hyper-conjugation between the π-system of the phenoxy-imine ligand and the α-Cumyl group raises the energy of the highest occupied molecular orbital keeping the lowest unoccupied orbital almost unchanged. This makes the α-Cumyl complexes chemically softer than their tBu analogs and thereby more active in ethylene polymerization.