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

Mechanisms of chain propagation and beta -hydride transfer (BHT) chain termination stages of poly- and oligomerization of ethylene by catalysts of general formula [2,6-(CR1=N((2-R-2)(4-R-4)(6-R-3)C6H3)(2)C5H3N] FeCl2 were studied theoretically. Density functional (B3LYP) and integrated molecular orbitals + 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 ligands are required in order for the d(z2) orbital (with the trichelating ligand defining the equatorial xy plane) to be destabilized and for the singlet to be the ground state and that this is realized in BHT chain termination related species. In contrast, in the chain propagation region of potential energy surface (PES) only one axial ligand is present, where, consequently, 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 (singlet) BHT transition state (TS) 5.7 kcal/mol lower than the lowest (quintet and singlet) chain propagation TSs. Inclusion of both zero point energy and entropy corrections, namely, the Gibbs free energy, notably favors higher spin states, in which metal-ligand antibonding orbitals are occupied. This effect should be of general character for highly coordinated open shell transition 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 quintet chain propagation TS. In the HSB system, the axial positions are sterically destabilized. The main effect of increasing the steric bulk in axial position is the differentiation of the two ways of "saturating" the d(z2) orbital, one by destabilizing it, as in singlet species, and the other by populating it with Fe's d electron, in favor of the latter. On the PES of the HSB 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 suppression of BHT chain termination upon increase in steric bulk.