Mechanistic studies of nickel(II) alkyl agostic cations and alkyl ethylene complexes:: Investigations of chain propagation and isomerization in (α-diimine)Ni(II)-catalyzed ethylene polymerization
Mechanistic studies of nickel(II) alkyl agostic cations and alkyl ethylene complexes:: Investigations of chain propagation and isomerization in (α-diimine)Ni(II)-catalyzed ethylene polymerization
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DOI：
10.1021/ja021071w
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发表时间：
2003-03-12
影响因子：
15
通讯作者：
Brookhart, M
中科院分区：
文献类型：
--
作者：
Leatherman, MD;Svejda, SA;Brookhart, M
提出了通过相应的 (α-二亚胺)NiBr2 前体的格氏烷基化合成一系列 (α-二亚胺)NiR2 (R = Et, Pr-n) 配合物。这些物质在低温下被氧鎓酸 [H(OEt2)(2)](+)[BAr'(4)](-) 质子化，产生阳离子 Ni(II) β-激动烷基络合物，该络合物模拟镍催化烯烃聚合反应中存在的相关中间体。使用 NMR 谱线展宽技术可以定量地解决这些激动人心的烷基阳离子的高动态性质。用乙烯捕获这些络合物提供了阳离子 Ni 烷基乙烯物种，其用于确定乙烯插入伯碳中心和仲碳中心的速率。 Ni 不规则烷基阳离子也被 CH3CN 和 Me2S 捕获，产生 Ni(R)(L)(+) (L = CH3CN, Me2S) 配合物，并讨论了这些物质在不同 [L] 存在下的动态行为。从这些实验中获得的动力学数据用于呈现(α-二亚胺)Ni催化剂的乙烯聚合机理的全貌，包括反应温度和乙烯压力对催化剂活性、聚乙烯支化和聚合物结构的影响。对这些系统与之前提出的类似钯催化剂进行了详细比较。
The synthesis of a series of (alpha-diimine)NiR2 (R = Et, Pr-n) complexes via Grignard alkylation of the corresponding (alpha-diimine)NiBr2 precursors is presented. Protonation of these species by the oxonium acid [H(OEt2)(2)](+)[BAr'(4)](-) at low temperatures yields cationic Ni(II) beta-agostic alkyl complexes which model relevant intermediates present in nickel-catalyzed olefin polymerization reactions. The highly dynamic nature of these agostic alkyl cations is quantitatively addressed using NMR line broadening techniques. Trapping of these complexes with ethylene provides cationic Ni alkyl ethylene species, which are used to determine rates of ethylene insertion into primary and secondary carbon centers. The Ni agostic alkyl cations are also trapped by CH3CN and Me2S to yield Ni(R)(L)(+) (L = CH3CN, Me2S) complexes, and the dynamic behavior of these species in the presence of varied [L] is discussed. The kinetic data obtained from these experiments are used to present an overall picture of the ethylene polymerization mechanism for (a-diimine)Ni catalysts, including effects of reaction temperature and ethylene pressure on catalyst activity, polyethylene branching, and polymer architecture. Detailed comparisons of these systems to the previously presented analogous palladium catalysts are made.