Understanding the role of paramagnetic organometallic redox centres in oligomerisation catalysis

烯烃链生长反应在工业上是非常重要的。这一过程的一个例子是乙烯低聚化，它产生了被称为α -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，例如表面活性剂、润滑剂、增塑剂、LDPE和聚合物添加剂。低聚过程需要均相催化剂，通常基于镍或铬化合物。与这些催化剂有关的一个主要问题是α -烯烃的数学分布不理想。解决这一问题的一种方法是选择性乙烯三聚化和四聚化，分别产生1-己烯或1-辛烯。最近的发展集中在设计高选择性催化剂上。迄今为止，铬催化剂占乙烯低聚反应专利和科学文献的90%以上。一般认为，这一机制遵循的路线是，在金属中心协同添加两个乙烯分子，然后插入另一个乙烯分子，以产生环状金属中心物质。关于Cr的形式氧化态，有证据表明Cr（I/III）和Cr（II/IV）对都存在，并且Cr在催化循环中的氧化态甚至可能依赖于配体。通过实验和计算研究，已经进行了多次尝试来确定催化过程中的氧化态。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态和自旋态提供全面的见解来解决这一争论。我们的目标不是制备更好的低聚化催化剂，而是利用n -杂环碳（NHC）配体通过EPR详细跟踪反应机理。更一般地说，它将提供一个独特的机会来探测顺磁性有机金属氧化还原/自旋中心在催化中所起的精确作用，这是一个令人惊讶的研究很少的领域。这项工作的新颖之处在于对顺磁自旋态在机制中所起作用的检验。为了稳定被认为参与这些反应的过渡金属离子的各种氧化态，将为此目的合成基于NHC的新型配体。通过对反应条件的精确实验控制和先进的EPR光谱技术，我们将深入到反应循环的核心，梳理出对成功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，我们可以探测到催化剂在反应前和反应过程中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）的结合来实现。配体设计是该项目的关键组成部分，重点关注Cr和4、5族元素的NHC配合物来监测反应。这些新的有机金属配合物和中间体的电子结构将通过连续波和脉冲EPR方法进行彻底的探测。虽然这项研究主要是基础的范围，我们将探索配合物的乙烯低聚化的使用。通过采用这种独特的脉冲EPR方法，将实现对乙烯低聚催化反应中涉及的氧化还原/自旋状态，配体结构和中间体的无与伦比的一瞥。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。

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.