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Understanding the role of paramagnetic organometallic redox centres in oligomerisation catalysis

了解顺磁性有机金属氧化还原中心在低聚催化中的作用

基本信息

批准号：
EP/H023879/1
负责人：
Damien Murphy
金额：
$ 51.15万
依托单位：
CARDIFF UNIVERSITY
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2010
资助国家：
英国
起止时间：
2010 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=EP%2FH023879%2F1
关键词：
Understanding role paramagnetic organometallic redox

项目摘要

Chain growth reactions involving alkenes are very important in Industry. One example of this process, ethylene oligomerization, leads to simple chemicals known as alpha-olefins. In turn, these alpha-olefins can be transformed into a range of commodity chemicals, e.g. surfactants, lubricants, plasticizers, LDPE and polymer additives. The oligomerization process requires homogeneous catalysts, commonly based on Nickel or Chromium compounds. A major problem associated with these catalysts is the formation of an undesirable mathematical distribution of alpha-olefins. One development that solves this problem, which operates via a uniquely different mechanism, is selective ethylene trimerization and tetramerization giving 1-hexene or 1-octene respectively. Recent developments have focussed on designing highly selective catalysts. To date, chromium catalysts account for over 90% of the patent and scientific literature for ethylene oligomerization. The mechanism is generally thought to follow a route involving concerted addition of two ethylene molecules to the metal centre followed by insertion of another ethylene molecule to yield a cyclic metal centred species. With regard to the formal oxidation state of Cr, there is evidence for both Cr(I/III) and Cr(II/IV) couples, and oxidation states of Cr in the catalytic cycle could even be ligand dependent. Several attempts have been made to determine the oxidation states during the catalysis by experimental and computational studies. But the debate still continues, and the precise role of the redox couple in these reactions is still not settled.The current programme of research will attempt to settle this debate by providing a comprehensive insight into the oxidation and spin states involved in the catalytic reaction using EPR spectroscopy. Our goal is less about generating better oligomerisation catalysts, and more about using the N-heterocyclic carben (NHC) ligands to follow the reaction mechanism in detail by EPR. More generally, it will provide a unique opportunity to probe the precise role played by paramagnetic organometallic redox/spin centres in catalysis, an area which is surprisingly poorly researched. The novelty of this work will be the examination of the role played by the paramagnetic spin states in the mechanism. Novel ligands based on NHC's will be synthesised for this purpose in order to stabilise the various oxidation states of the transition metal ions thought to be involved in these reactions. Through precise experimental control of reaction conditions and with advanced EPR spectroscopic techniques, we will peer into the heart of the reaction cycle, and tease out the structure of the paramagnetic reaction intermediates crucial for the successful catalytic cycle. By performing spectroscopic measurements in situ, we can probe the dynamics of the electron spins as the catalyst is activated before and during the reaction. These aims will be achieved through a combination of synthetic chemistry, mechanistic chemistry and advanced spectroscopy (specifically the family of EPR techniques). Ligand design is a key component of the project, focussing on NHC complexes of Cr and group 4 & 5 elements to monitor the reaction. The electronic structure of these new organometallic complexes and intermediates will be thoroughly probed by cw- & pulsed EPR methodologies. While this study is primarily fundamental in scope, we will explore the use of the complexes for ethylene oligomerization. By adopting this unique approach of using pulsed EPR, an unsurpassed glimpse into the redox/spin state, ligand structure and intermediates involved in the catalytic reaction for ethylene oligomerization will be achieved. For the first time pulsed EPR methods will be uniquely used to examine the mechanism of these important industrial reactions.

涉及烯烃的链式生长反应在工业中非常重要。这一过程的一个例子是乙烯齐聚，它导致了被称为阿尔法-烯烃的简单化学物质。反过来，这些α-烯烃可以转化为一系列商品化学品，如表面活性剂、润滑剂、增塑剂、低密度聚乙烯和聚合物添加剂。齐聚过程需要均质催化剂，通常以镍或铬化合物为基础。与这些催化剂相关的一个主要问题是α-烯烃的数学分布不理想。解决这一问题的一个发展是选择性的乙烯三聚和四聚，分别得到1-己烯或1-辛烯，它通过独特的不同机制进行操作。最近的发展集中在设计高选择性催化剂上。到目前为止，铬催化剂占乙烯齐聚专利和科学文献的90%以上。其机理通常被认为遵循这样一条路线，即两个乙烯分子协同加成到金属中心，然后插入另一个乙烯分子以产生环状金属中心物种。关于铬的形式氧化态，有证据表明，铬(I/III)和铬(II/IV)对都存在，而且铬在催化循环中的氧化态甚至可能与配体有关。人们曾多次尝试通过实验和计算研究来确定催化过程中的氧化状态。但争论仍在继续，氧化还原对在这些反应中的确切作用仍未确定。目前的研究计划将试图通过使用EPR光谱对催化反应中涉及的氧化和自旋态进行全面的洞察来解决这一争论。我们的目标不是开发出更好的齐聚催化剂，而是更多地使用N-杂环碳(NHC)配体通过EPR详细跟踪反应机理。更广泛地说，它将提供一个独特的机会来探索顺磁性金属有机氧化还原/自旋中心在催化中所起的确切作用，这是一个研究得令人惊讶的不足的领域。这项工作的新奇之处将是检查顺磁自旋态在机制中所起的作用。为此目的，将合成基于NHC的新型配体，以稳定被认为参与这些反应的过渡金属离子的各种氧化状态。通过对反应条件的精确实验控制和先进的EPR光谱技术，我们将深入反应周期的核心，并梳理出对成功的催化循环至关重要的顺磁性反应中间体的结构。通过在现场进行光谱测量，我们可以探索在反应前和反应过程中催化剂被激活时电子自旋的动力学。这些目标将通过合成化学、机械化学和先进光谱学(特别是EPR技术家族)的组合来实现。配体设计是该项目的关键组成部分，重点是铬和第4&5族元素的NHC络合物以监测反应。这些新型有机金属络合物和中间体的电子结构将用连续波脉冲EPR方法进行深入的研究。虽然这项研究主要是基础性的，但我们将探索使用这些络合物进行乙烯齐聚。通过采用脉冲EPR这种独特的方法，可以无与伦比地一瞥乙烯齐聚催化反应中涉及的氧化还原/自旋状态、配体结构和中间体。脉冲EPR方法将首次被独特地用于研究这些重要工业反应的机理。