Monomer capture in Brookhart's Ni(II) diimine olefin polymerization catalyst: Static and dynamic quantum mechanics/molecular mechanics study

A combination of quantum mechanics and molecular mechanics QM/MM has been u sed to study the capture of ethylene by Brookhart's Ni diimine catalysts of the type (ArN=C(R')-C(R')=NAr)Ni-II-propyl(+) with (1) R' = H and Ar = H, (2) R' = H and Ar = 2,6-C6H3(i-Pr)(2), or (3) R' = CH3 and Ar = 2,6-C6H3(i- Pr)(2) The study made use of both conventional "static" density functional theory (DFT) based calculations as well as slow growth first principle mole cular dynamics (FPMD) DFT methods to examine the capture of ethylene. Exami nation of the static potential energy surface of all. three catalyst models 1, 2, and 3 reveals that there is no enthalpic barrier to the capture proc ess. However, both the static and molecular dynamics simulations suggest th at there is an entropic barrier to the-association that originates from the loss of rotational and translational entropies upon association. The;FPMD QM/MM slow growth barriers were calculated to be 7.5, 10.3, and 10.8 kcal/m ol at 300 K for catalysts 8, 2, and 3 , respectively. An analysis suggests that the trend in the barriers can be related to the size of the; active si te. The free energy barrier for the pure QM model of 1 has also been estima ted from a series of frequency calculations. This approach provides a barri er of 7.7 kcal/mol (and 6.8 kcal/mol without quantum dynamical contribution s), which is in fair agreement with the 7.5 kcal/mol barrier (without quant um dynamical contributions) calculated from the slow growth simulations. An alysis of the estimate from the frequency calculations suggests that this b arrier estimate represents an upper limit, since the components of the vibr ational entropy that compensate the loss of rotational and translational en tropy upon association are partially neglected in the treatment.