Mechanistic aspects of ethylene polymerization by iron(II)-bisimine pyridine catalysts: A combined density functional theory and molecular mechanics study

We present an extensive theoretical study of the iron(II)-bisimine pyridine based ethylene-polymerization catalysts {[2,6-((R)N=C(R'))(2)-C5H3N]FeC3H7 }(+) (R = R' = H, 1a; R = 2,6-C6H4(i-Pr)(2), R' = CH3, 1A) recently develop ed by the groups of Brookhart and Gibson. The study was based on density fu nctional theory (DFT) for the "generic" model system la and a combined DFT and molecular mechanics approach for the "real" system 1A. It is shown that the rate-determining step for both termination and propagation in the "rea l" system is the capture of ethylene by 1A. The steric bulk introduced by R = 2,6-C6H4(i-Pr)(2) was found to suppress ethylene capture for the termina tion step and increase the rate of insertion. Termination takes place on th e singlet potential energy surface (PES). For propagation the singlet and t riplet PES's are close in energy and spin-state change is possible. The qui ntet states are too high in energy to play any role in polymerization. The model system la was found to form an ethylene complex that is too stable fo r any further chemical transformation to take place.