The role of bulky substituents in Brookhart-type Ni(II) diimine catalyzed olefin polymerization: A combined density functional theory and molecular mechanics study

The role of the bulky ligands in Ni(II) diimine catalyzed ethylene polymerization has been examined with a combined density functional theory quantum mechanics and molecular mechanics (QM/MM) model. Specifically, we have examined the catalytic center of the type (ArN=C(R)-C(R)=NAr)Ni(II)- R'⁺, where R = Me and Ar = 2,6-C₆H₃(i-Pr)₂. The Ar and R groups were treated by a molecular mechanics force field while density functional theory was applied to the remainder of the system. The chain propagation, chain branching, and chain termination processes have been investigated with the hybrid method and found to have barriers of ΔH(≠) = 11.8, 15.3, and 18.4 kcal/mol, respectively, which is in excellent agreement with experiment in both absolute and relative terms. This is in stark contrast to the pure QM model in which the influence of the bulky Ar and R groups was neglected and the established order of the barriers is not even reproduced. The role played by the bulky substituents is dual in nature. First, the Ar and R groups act to sterically hinder the axial coordination sites of the Ni center. This has the most dramatic destabilizing effect on the resting state and termination transition states, in which both axial positions are occupied. In addition to the steric factor, we find that the electronic preference for the aryl rings to orient themselves in a coplanar fashion with the diimine ring results in a stabilization of the insertion transition state relative to the resting state. These two factors act to both lower the propagation barrier and increase the termination barrier compared to the 'naked' pure QM model system.