Steric and Electronic Effects of Zirconocenealkyl-Borate Ion Pairs on Catalyst Activation: A Theoretical Study

The effectiveness of metallocene catalysts in the cationic ring-opening polymerization (cationic ROP) of ε-caprolactone (CL) is influenced by the choice of metallocene/borate systems, particularly their bulkiness. Recent research examines this effect on the initiation and propagation stages of cationic ROP. We conducted a density functional theory study on the precatalyst activation of cationic CL ROP by zirconocene/borate catalysts, where four models of zirconocene precatalysts (Cp₂ZrMe₂ (a), (Me₅Cp)CpZrMe₂ (b), (Me₅Cp)₂ZrMe₂ (c), and Ind₂ZrMe₂ (d)) were combined with boron cocatalysts B(C₆F₅)₃ and [X⁺][B(C₆F₅)₄ ^-] (X⁺ = Ph₃C⁺ or PhMe₂NH⁺). We modeled and predicted their thermodynamic, steric, and electronic properties during ion-pair formation and separation. The calculated Gibbs energies of ion-pair formation correlated with the positive charge at the metal center and steric congestion of the catalyst. All catalyst precursors showed exothermic and exergonic insertion of CL, toluene solvent, and contact ion pairs; solvent-separated ion pairs were the preferred activation pathway. Catalyst c showed the most stable ion pair in precatalyst activation, with the lowest separation energy, aided by methyl group bulkiness and toluene solvent. We evaluated Cp'-based ligands using percent buried volume (%V _Bur) trends. Noncovalent interaction analysis indicated weak interactions at ion-pair contacts. This study enhances our understanding of cationic ROP and could aid in developing new polymerization catalysts for polyester synthesis.