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

Abstract

Mechanisms of chain propagation and β-hydride transfer (BHT) chain termination stages of poly- and oligomerization of ethylene by catalysts of general formula [2,6-(CR1N((2-R2)(4-R4)(6-R3)C6H2)2C5H3N]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)2C5H3N]FeCH3 +, and one of the catalytic (“high steric bulk”, HSB) systems studied experimentally, [2,6-(CMeN(2,6-iPr2C6H3))2C5H3N]FeCH3 +. We find that two axial ligands are required in order for the d z 2 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 z 2 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 z 2 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.

Diene-Containing Half-Sandwich MoIII Complexes as Ethylene Polymerization Catalysts: Experimental and Theoretical Studies

“…It has also been shown by MO/MM studies that the steric effect of bulky ligands lowers this activation barrier. [67,72,79,81] Therefore, the calculations are consistent with [CpMo(h 4 -C 4 H 6 )R] being a good model for one of the catalytically active sites for our 2/MAO system. As is generally found, the relative stability of the agostic propyl product goes as b ) a b g. The parameters in Table 4 reveal that this trend parallels the strength of the agostic interaction (a greater stabilization is associated with shorter MoÀH and longer CÀH distances for the agostic H atom).…”

“…[77,78] Theoretical calculations on the Brookhart/ Gibson catalysts also indicate the possible involvement of more than one spin state along the olefin polymerization process. [72,79] Therefore, we have extended our study to both spin doublet and quartet hypersurfaces for the Mo system, including the MECP.…”

Iron-Based and Cobalt-Based Olefin Polymerisation Catalysts

“…The calculations for the two extremes of steric bulk are in agreement with the observed suppression of β -hydride transfer termination upon increasing the steric bulk of the ligand l . For cobalt(I) alkyl Polymer complexes, experiment and theory have been shown to be in agreement and support a stepwise pathway for the reaction of the alkyl complexes with 1-alkenes, reacting by b -hydride transfer via a cobalt hydride intermediate [115] . Such cobalt alkyls have also been shown to contain low-spin cobalt(II) antiferromagnetically coupled to a ligand radical anion.…”

“…Musaev, Morokuma and co-workers have been concerned with understanding the mechanisms of chain propagation and b -hydride transfer termination for a series of iron catalysts [115] . By varying the steric bulk of the systems studied, it was found that two axial ligands are required in order for the d z2 orbital to be destabilised (with the tridentate bis(imino)pyridine ligand defining the equatorial xy plane) and for the singlet to be the ground state, a situation found for the b -hydride termination species.…”

Olefin Polymerization with Non-metallocene Catalysts (Late Transition Metals)

“…5. The active species of the polymerization is initially considered to be cationic alkyl metal species, similar to the metallocene polymerization [178,179]. Detailed characterization of the active species and theoretical studies, however, clarified unique polymerization mechanism.…”