Ligand Structure Effects on the Stability of Cationic Zirconocene Dimethylaniline Complexes: Insights from Density Functional Theory

The formation of dimethylaniline complexes during zirconocene- and borate-mediated cationic polymerization significantly impacts catalyst performance. Using density functional theory at the M06-2X level, we investigated how ligand structure, particularly Cp substituents, influences the stability of these complexes through a comprehensive analysis of bonding energetics and electronic structure. Our findings reveal two key factors controlling complex stability: First, sterically demanding Cp substituents weaken the Zr-N bond by increasing bond distance and lowering dissociation energy while simultaneously stabilizing the overall complex by restricting phenyl group rotation. Second, the electron density at the metal center critically determines the bond strength, with less electron-deficient Zr centers forming stronger Zr-N bonds through enhanced orbital overlap and charge transfer. Noncovalent interaction analysis further shows that smaller Cp ligands promote attractive Zr-N interactions, while larger substituents increase repulsive interactions between Cp rings and the aniline ligand. These insights into the interplay between steric and electronic effects provide clear design principles for developing more efficient polymerization catalysts with a reduced tendency to form unwanted dimethylaniline complexes.