Effect of Ion-Pair Strength on Ethylene Oligomerization by Divalent Nickel Complexes
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Effect of Ion-Pair Strength on Ethylene Oligomerization by Divalent Nickel Complexes Deepak Chandran • Seong Jin Byeon Hongsuk Suh • Il Kim
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Received: 21 March 2013 / Accepted: 2 May 2013 / Published online: 15 May 2013 Ó Springer Science+Business Media New York 2013
Abstract Detailed ethylene oligomerizations were performed over highly active nickel complexes ligated by N-((4-butyl-6-phenylpyridin-2-yl)methylene)-2,6-diisopropylaniline derivatives. The activity trend according to the structure of the complex was attempted to understand using the concept of ion-pair formation during activation process. The active species were characterized by comparing the oligomerization results with the UV–Vis spectroscopic and the cyclic voltammetry experiments. Keywords Catalyst Ethylene Oligomerization Nickel Transition metal chemistry
1 Introduction Alkyl or hydride anion abstraction reactions using strong Brønsted or Lewis acids from transition metal complexes are the pivotal activation step in homogeneous ‘single site’ Ziegler–Natta catalysis [1–7]. This abstraction process mediated by Lewis acidic reagents and reaction conditions yields highly electrophilic and mononuclear ion pairs, exhibiting unprecedented olefin polymerization activity and selectivity [8]. A detailed investigation on the formation and strength at various reaction conditions for these
ion-pairs are important in understanding and optimizing the performances of various types of complexes in catalysis. Most of such studies reported so far are based on early transition metal complexes [1–5]. Even though a large number of complexes of N^N and N^N^N ligands bearing various late transition metals such as Ni, Pd, Co, Fe and Cr are widely employed catalysts in ethylene polymerization/ oligomerization [6, 9–13] attempts to study their performance based on the ion-pair strength are hardly made. We have recently reported the synthesis of a series of Ni-diimine catalysts based on N-((4-butyl-6-phenylpyridine2-yl)methylene)-2,6-diisopropylaniline derivatives [14]. Apart from the interesting observation and information obtained during their synthesis, these complexes were observed with high catalytic potential in ethylene oligomerization (EO) which serves them suitable candidates to study the ion-pair formation and strength at various reaction conditions. Three different complexes having the same core structure has been adopted in order to investigate this effect in this study. We found that, as in the case of early transition metal complexes, the ion-pair formation and its stability are equally influential in the cationic late transitions metal complexes. Here, attempts have been made to study EO by using the three Ni(II) complexes based on the ion-pair concept.
2 Experimental D. Chandran S. J. Byeon I. Kim (&) The WCU Center for Synthetic Polymer Bioconjugate Hybrid Materials, Department of Polymer Science and Engineering, Pusan National University, Busan 609-735, Korea e-mail: [email protected] H. Suh Department of Chemistry and Chemistry Institute
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