Theoretical study on bis(imino)pyridyl-Fe(II) olefin poly- and oligomerization catalysts. Dominance of different spin states in propagation and beta-hydride transfer pathways

Mechanisms of chain propagation and beta -hydride transfer (BHT) chain term ination stages of poly- and oligomerization of ethylene by catalysts of gen eral formula [2,6-(CR1=N((2-R-2)(4-R-4)(6-R-3)C6H3)(2)C5H3N] FeCl2 were stu died theoretically. Density functional (B3LYP) and integrated molecular orb itals + molecular mechanics (IMOMM) methods were applied respectively to a model ("low steric bulk", LSB) system, [2,6-(CH=NH)(2)C5H3N]FeCH3+, and one of the catalytic ("high steric bulk", H[SB) systems studied experimentally , [2,6(CMe=N(2,6-(Pr2C5H3)-Pr-i))(2)C5H3N]FeCH3+. We find that two axial li gands are required in order for the d(z2) orbital (with the trichelating li gand defining the equatorial xy plane) to be destabilized and for the singl et to be the ground state and that this is realized in BHT chain terminatio n related species. In contrast, in the chain propagation region of potentia l energy surface (PES) only one axial ligand is present, where, consequentl y, the d(z2) orbital is singly occupied and the singlet becomes a low lying excited state. Our calculations on the LSB system place the lowest (single t) BHT transition state (TS) 5.7 kcal/mol lower than the lowest (quintet an d singlet) chain propagation TSs. Inclusion of both zero point energy and e ntropy corrections, namely, the Gibbs free energy, notably favors higher sp in states, in which metal-ligand antibonding orbitals are occupied. This ef fect should be of general character for highly coordinated open shell trans ition metal complexes. On the Gibbs free energy surface of the LSB system, the lowest singlet BHT TS is only 1.0 kcal/mol lower than the lowest quinte t chain propagation TS. In the HSB system, the axial positions are sterical ly destabilized. The main effect of increasing the steric bulk in axial pos ition is the differentiation of the two ways of "saturating" the d(z2) orbi tal, one by destabilizing it, as in singlet species, and the other by popul ating it with Fe's d electron, in favor of the latter. On the PES of the HS B system, the lowest BHT TS lies 17.6 kcal/mol higher than the lowest chain propagation TS. This is in agreement with the experimentally observed supp ression of BHT chain termination upon increase in steric bulk.