Adjusting Ortho-Cycloalkyl Ring Size in a Cycloheptyl-Fused N,N,N-Iron Catalyst as Means to Control Catalytic Activity and Polyethylene Properties

Mingyang Han,Irina V Oleynik,Yanping Ma,Ivan I Oleynik,Gregory A Solan,Tongling Liang,Wen-Hua Sun,Hongyi Suo,Qiuyue Zhang

doi:10.3390/catal10091002

Abstract

Five examples of bis(arylimino)tetrahydrocyclohepta[b]pyridine dichloroiron(II) complex, [2-{(Ar)N=CMe}-9-{N(Ar)}C10H10N]FeCl2 (Ar = 2-(C5H9)-4,6-(CHPh2)2C6H2Fe1, 2-(C6H11)-4,6-(CHPh2)2C6H2Fe2, 2-(C8H15)-4,6-(CHPh2)2C6H2Fe3, 2-(C12H23)-4,6-(CHPh2)2C6H2Fe4, and 2,6-(C5H9)2-4-(CHPh2)C6H2Fe5), incorporating ortho-pairings based on either benzhydryl/cycloalkyl (ring sizes ranging from 5 to 12) or cyclopentyl/cyclopentyl groups, have been prepared in reasonable yield by employing a simple one-pot template strategy. Each complex was characterized by FT-IR spectroscopy, elemental analysis, and for Fe3 and Fe5 by single crystal X-ray diffraction; pseudo-square pyramidal geometries are a feature of their coordination spheres. On treatment of Fe1–Fe5 with modified methylaluminoxane (MMAO) or methylaluminoxane (MAO), a range in catalytic activities for ethylene polymerization were observed with benzhydryl/cyclopentyl-containing Fe1/MMAO achieving the maximum level of 15.3 × 106 g PE mol−1 (Fe) h−1 at an operating temperature of 70 °C. As a key trend, the activity was found to drop as the ortho-cycloalkyl ring size increased: Fe1C5H9/CHPh2~Fe5C5H9/C5H9 > Fe2C6H11/CHPh2 > Fe3C8H15/CHPh2 > Fe4C12H23/CHPh2. Furthermore, strictly linear polyethylenes (Tm > 126 °C) were formed with molecular weights again dependent on the ortho-cycloalkyl ring size (up to 55.6 kg mol−1 for Fe1/MAO); narrow dispersities were a characteristic of all the polymers (Mw/Mn range: 2.3–4.7), highlighting the well-controlled nature of these polymerizations.

Highlights

The transition metal-catalyzed polymerization of ethylene is one of the most important carbon–carbon bond-forming reactions and it is widely used in the chemical industry [1,2].Catalysts 2020, 10, 1002; doi:10.3390/catal10091002 www.mdpi.com/journal/catalysts ortho-/para-substitution pattern or even more dramatic structural changes to the N,N,N-ligand core itself have seen improvements in the catalytic performance and molecular weight of the polyolefin.Of note, iron catalysts bearing carbocyclic-fused bis(imino)pyridine ligands have shown a capacity to 10, display Catalysts1002 optimal productivity at temperatures in the range of 50–80 °C
We revisit the singly-fused bis(arylimino)tetrahydrocyclohepta[b]pyridine skeleton shown in iron-containing D and E, with a view to introducing N-aryl ortho-pairings based on benzhydryl/cyclopentyl, benzhydryl/cyclohexyl, benzhydryl/cyclooctyl and benzhydryl/cyclododecyl; for comparative purposes, we report the ortho-combination cyclopentyl/cyclopentyl (Scheme 1), as it is a common feature to all iron complexes to be synthesized is the presence of a para-benzhydryl group
Molecular weights and molecular weight distributions of the polymers were determined with an Agilent PL 220 gel permeation chromatography (GPC) instrument operating at 150 ◦ C with

Summary

Introduction

The transition metal-catalyzed polymerization of ethylene is one of the most important carbon–carbon bond-forming reactions and it is widely used in the chemical industry [1,2].Catalysts 2020, 10, 1002; doi:10.3390/catal10091002 www.mdpi.com/journal/catalysts ortho-/para-substitution pattern (i.e., steric and electronic effects) or even more dramatic structural changes to the N,N,N-ligand core itself have seen improvements in the catalytic performance and molecular weight of the polyolefin.Of note, iron catalysts bearing carbocyclic-fused bis(imino)pyridine ligands have shown a capacity to 10, display Catalysts1002 optimal productivity at temperatures in the range of 50–80 °C. Catalysts 2020, 10, 1002; doi:10.3390/catal10091002 www.mdpi.com/journal/catalysts ortho-/para-substitution pattern (i.e., steric and electronic effects) or even more dramatic structural changes to the N,N,N-ligand core itself have seen improvements in the catalytic performance and molecular weight of the polyolefin. Iron catalysts bearing carbocyclic-fused bis(imino)pyridine ligands have shown a capacity to 10, display Catalysts. Bis(imino)pyridines bearing doubly or singly fused seven-membered rings have provided robust ligand frameworks for a range of iron(II) precatalysts (B–E, Scheme 1) [19,20,21,22,23,24]. Bearing relatively small ortho-alkyl substituents displayed excellent effective catalysts in ethylene polymerization [9,10,11,12,13]; others involving the first row d-block centers such −1. In particular those based on iron and cobalt [3,4,5,6,7,8], have emerged as performance, fused B [19]

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Conclusion