Transition Metal-Catalyzed and MAO-Assisted Olefin Polymerization; Cyclic Isomers of Sinn’s Dimer Are Excellent Ligands in Iron Complexes and Great Methylating Reagents

Kaidi Yang,Rainer Glaser

doi:10.3390/catal12030312

Kaidi Yang, Rainer Glaser

Open Access

https://doi.org/10.3390/catal12030312

Copy DOI

Abstract

Methylaluminoxane (MAO) is the most commonly used co-catalyst for transition metal-catalyzed olefin polymerization, but the structures of MAO species and their catalytic functions remain topics of intensive study. We are interested in MAO-assisted polymerization with catalysts L(R2)FeCl2 (L = tridentate pyridine-2,6-diyldimethanimine; imine-R = Me, Ph). It is our hypothesis that the MAO species is not merely enabling Fe–Me bond formation but functions as an integral part of the active catalyst, a MAO adduct of the Fe-precatalyst [L(R2)FeCl]+. In this paper, we explored the possible structures of acyclic and cyclic MAO species and their complexation with pre-catalysts [L(R2)FeCl]+ using quantum chemical approaches (MP2 and DFT). We report absolute and relative oxophilicities associated with the Fe ← O(MAO) adduct formation and provide compelling evidence that oxygen of an acyclic MAO species (i.e., O(AlMe2)2, 4) cannot compete with the O-donor in cyclic MAO species (i.e., (MeAlO)2, 7; MeAl(OAlMe2)2, cyclic 5). Significantly, our work demonstrates that intramolecular O → Al dative bonding results in cyclic isomers of MAO species (i.e., cyclic 5) with high oxophilicities. The stabilities of the [L(R2)FeClax(MAO)eq]+ species demonstrate that 5 provides for the ligating benefits of the cyclic MAO species 4 without the thermodynamically costly elimination of TMA. Mechanistic implications are discussed for the involvement of such Fe–O–Al bridged catalyst in olefin polymerization.

Highlights

Results for the methylaluminoxane [5–7] (MAO) species are collected in Table
We have presented the results of computational studies of the association of precatalysts [L(Ph)2FeCl]+ with three MAO species
We have shown that the oxygen in cyclic MAO species is a much better ligand in iron complexes compared to the oxygen in acyclic iron species

Summary

Introduction

Ziegler-Natta polymerization [1,2] is the dominant method for the production of polyolefin [3,4]. Ziegler-Natta catalysts usually contain transition metal complexes (i.e., TiCl4) and partially hydrolyzed aluminum alkyls as co-catalyst, among which methylaluminoxane [5–7] (MAO) is most commonly used today [8–10] It was a major advance in the field when Brookhart discovered that Fe(II) complexes with di-nitrogen or tri-nitrogen ligands can serve as pre-catalyst with satisfying catalytic activity [11–13]. Sun and co-workers explored structurally similar bidentate bis(imino)pyridyl Fe(II) complexes [14], and tridentate 2,8-bis(imino)quinoline Fe(II) complexes [15], and related systems with nickel [16–18], cobalt [19,20], and titanium [21,22] have been studied. These kinds of pre-catalysts require the presence of a very large excess of MAO as cocatalyst (>2000:1) and the structure and the function of MAO have not been clarified at all [23]. Theoretical studies by Ziegler et al [28], by Hall and co-workers [29], and by Linnolahti et al [30,31] explored possible species with ladder and cage structures derived from dimethylaluminum hydroxide (DMAH)

Methods

Results

Conclusion