Comparison of the Reactivity and Structures for the Neutral and Cationic Bis(imino)pyridyl Iron and Cobalt Species by DFT Calculations

Zilong Li,Wen-Hua Sun,Yanping Ma

doi:10.3390/catal10121396

Abstract

Density Functional Theory (DFT) method was adopted to investigate and compare the reaction mechanisms of ethylene polymerization catalyzed by neutral, cationic bis(imino)pyridyl (PDI) iron and cobalt derivatives. The electronic structure and the oxidation states of the metal center and the PDI ligand were analyzed by taking spin states, natural bond orbital (NBO) charge distribution, etc. into consideration, revealing that the reactivity is closely related to the valence electron numbers instead of the charge numbers. The neutral Co(0) had the lowest reactivity as it possessed the most electrons. During the formation of the cationic Co(+)/Fe(+), one electron was mainly lost from PDI ligand rather than the metal center while the metal center maintained +II valence state through the process. Moreover, a special unsymmetrically bidentate N^N coordination manner was found to provide the deficient metal surroundings with 14e, which may initiate the reactivity of some unsymmetrical species with rich electrons. Finally, an anion [AlMe4]− participating process was proposed to explain the presence of the experimentally observed LCo(+)B(C2H4). A special intermediate, Co(+)B(C2H4) [AlMe4]− with Co in +I and absence of Co–C σ bond, was obtained. These calculation results may provide fundamental information for further understanding and designing the ethylene polymerization catalysts.

Highlights

Much attention was focused on late transition metal complexes by researchers and engineers sinceBrookhart’s and Gibson’s research groups independently found that bis(imino)pyridine cobalt and iron complexes had high activities toward ethylene polymerization in 1998 [1,2]
To gain insight into the nature of the active species formed upon treatment of the corresponding metal dihalide complexes with MAO, various alkyl species based on bis(imino)pyridyl iron and cobalt complexes have been isolated in the experiments [2,27,42]
The activities and structures of the isolated species are related to the methodologies and additives which the researchers used during their alkylation synthesis

Summary

Introduction

Brookhart’s and Gibson’s research groups independently found that bis(imino)pyridine cobalt and iron complexes had high activities toward ethylene polymerization in 1998 [1,2]. Against this backdrop, the late transition metal complexes have made tremendous progress on ethylene polymerization by modifying the catalyst structures [3,4,5,6,7,8,9,10]. Numerous studies focusing on the relationship between catalyst structure and their catalytic activity have gradually been reported [26,27,28,29,30,31,32,33,34,35,36,37].

Methods

Results

Conclusion