Behind the Scenes of Group 4 Metallocene Catalysis: Examination of the Metal–Carbon Bond

Abstract

This contribution provides the first detailed analysis of the nature of the M–C σ-bond of three alkylated, isostructural group 4 (M = Ti, Zr, Hf) metallocenes, thereby elucidating individual peculiarities of each metal center in the catalytic conversion of olefins. Therefore, the subtle electronic differences of the individual M–C σ-bonds, which are considered crucial for several subprocesses in the coordinative polymerization of olefins, were examined by detailed experimental charge density studies. These studies provided measures of the increasing ionic character of the M–C bonds along the group 4 elements (Ti–C < Zr–C < Hf–C). These results are further supported by high-pressure diffraction studies showing that the predominantly ionic Hf–C bond is more compressible than the more covalent Zr–C bond in line with a smaller degree of electron localization in the valence shell of the hafnium relative to the zirconium atom along the M–C bond directions. The Ti–C bond displays the largest degree of electron localization in these group 4 metallocenes as witnessed by a pronounced bonded charge concentration in the valence shell of the titanium atom–a rare phenomenon in transition metal alkyls. All findings were then complemented by experimental and theoretical studies of the kinetic aspects of M–C σ-bond cleavage in group 4 metallocenes. These studies show that the entropy of activation is distinctly more negative for a Zr–C relative to a Hf–C bond dissociation. The combined results of the kinetic and electronic analysis herein shed new light on the different catalytic behavior of group 4 metallocenes with regard to the applied transition metal atom. In this context, deviations between zirconium- and hafnium-based catalysts concerning the catalytic activity and the stereoregularities became clearly explainable, just as the well-known “hafnium-effect” in the production of extraordinarily high molecular weight polypropylenes.

Structure‐Activity Relationships for Bis(phenolate‐ether) Zr/Hf Propene Polymerization Catalysts

“…The need to break the stronger hafnium–carbon bonds would lead to higher insertion barriers, and hence lowered activity relative to zirconium . More recently, Rieger has suggested that differences in ionic/covalent character of M–C bonds as well as in enthalpy and entropy of activation lead to differences in activity, stereoselectivity and molecular weight control of the group IV metals for the same metallocene ligand …”

Preparation of Pincer Hafnium Complexes for Olefin Polymerization

“…We also reported various types of Hf-complexes ([N,P]Hf(CH 2 Ph) 3 , [N,P,N]HfMe 2 , and [N,N]Hf(CH 2 Ph) 3 -type) with tetrahydroquinoline and tetrahydrophenanthroline framework, which were also inferior to I in terms of both activity and α-olefin incorporation capability [45,46,47]. The Hf-C bonding character was significantly ionic when compared to those of Zr-C and Ti-C bonding, and steric congestion around Hf-center might be crucial for the high activity [48]. When steric congestion is not significant, the ionic Hf-C bond becomes strong and the insertion of olefin through the Hf-C bond may be less favorable, leading to lowered activity.…”

Chain Transfer to Solvent and Monomer in Early Transition Metal Catalyzed Olefin Polymerization: Mechanisms and Implications for Catalysis

“…Moreover, preferential aromatic C-H activation, and therefore tolyl rather than benzyl chain ends, would be expected for σ-bond metathesis reactivity [63]. A radical pathway is more consistent with selective toluene activation at the benzylic position [64], and Ti-C bond homolysis is known to be competitive with olefin polymerization, especially at high temperature [65][66][67][68]. The quantitative computational modeling of this reaction required a dedicated benchmark study, which allowed to overcome the difficulties related to a) the 'transferability problem' [69] in the estimates of metal-carbon bond dissociation energies (BDEs) and b) the 1 → 2 particle nature of this reaction that poses a challenge for static DFT methodologies [36].…”