Reactivity consequences of electronic and spin state effects in organometallic chemistry

有机金属化学中电子和自旋态效应的反应性后果

• 批准号: 261506-2007
• 负责人: McNeil, Stephen
• 金额: $ 1.46万
• 依托单位: University of British Columbia
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2007
• 资助国家: 加拿大
• 起止时间: 2007-01-01 至 2008-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=362380
• 关键词: Reactivity; consequences; electronic; spin; state

I propose an integrated synthetic, mechanistic, and computational study to probe selectively the effects of electronic configuration and spin state on the stoichiometric reactions fundamental to organometallic catalysis.  Careful interplay of metal properties and ligand design will allow for the synthesis of compounds wherein steric and/or electronic modification to the periphery of the ligand sphere will adjust the orbital splitting and pairing energies at the metal centre, either inductively or by adjusting the coordination geometry, thereby selectively tuning the spin state energies of the compound.  This will allow for the synthesis of pairs or series of compounds with identical metals and oxidation states in similar coordination environments, but differing ground state spin, permitting a systematic mechanistic study of how the spin state of a complex alone affects the thermodynamics and kinetics of the reactions that make up organometallic catalytic cycles.The rational design of catalysts for synthetic organic and industrial applications continues to be a principal focus of organometallic chemistry.  The spin state of the catalyst has the potential to play a critical role, in that the presence of unpaired electrons or the need to cross over between surfaces of different spin can have a profound and often unexpected effect on the energies, mechanisms, and rates of reactions.  An increasing number of recently developed catalytic systems, particularly those in the area of alkene polymerization, employ mid-valent compounds of first-row metals such as Cr, Fe, and Co, yielding catalysts or catalyst precursors that not only have paramagnetic ground states, but also may cross between different spin surfaces during the catalytic cycle.  An increasing body of work suggests that spin crossover phenomena will prove to play an enormous role in organometallic catalysis.  The principal goal of this research is therefore the first systematic study to isolate and investigate the means by which spin state alone can affect and control organometallic reactivity generally, and to demonstrate specific instances in which this control might be exploited.Two well-defined systems of relevance to alkene polymerization chemistry are currently under investigation.  These are bis(oxazolinyl)pyridine dialkyl iron(II) (traditional Ziegler-Natta polymerization), and bis(amidinate) and bis(beta-ketoamine) monoalkyl cobalt(III) complexes (atom transfer and organometallic radical polymerization).  Both are five-coordinate, 16 valence electron systems with six d-shell electrons, but in each case there are multiple possible ground state spins, with the most likely configurations being the open-shell quintet and closed-shell singlet.  In each case, the ligands are easily modified to bear a variety of electron donating/withdrawing groups of varying steric profile, which affords control of the exhibited spin properties.  This will permit a side-by-side comparison of fundamental organometallic reactions, whereby one reaction takes place on a single diamagnetic spin surface, while the other takes place on either a wholly paramagnetic surface or requires a crossover from one spin state to another.  Reactions such as coordination, insertion, and beta-elimination are of relevance to Z-N polymerization mechanisms, while single-electron and atom transfer reactions are important in ATRP and OMRP.  Computational studies employing DFT will both guide the choice of systems for experimental investigation, and serve to interpret and support the mechanistic results.This approach of investigation is well suited to the resources associated with current undergraduate research at UBC Okanagan, but will easily be applied to longer-term studies as our graduate programme develops.  The proposed target compounds are a few simple synthetic steps from commercially available and inexpensive materials.  The ligand systems are easily modified, allowing a wide range of compounds to be developed quickly.  The integrated nature of the study will give students experience with organic and organometallic synthesis and characterization, mechanistic studies using UV-visible and NMR spectroscopic techniques, and computational methods.  Despite their relatively simple basis, I believe these studies have dramatic implications, and will yield results that will greatly enhance the rational design of open-shell paramagnetic transition-metal catalysts.

我提出了一项综合的、机械的和计算的研究，有选择地探索电子构型和自旋态对有机金属催化基础化学计量反应的影响。金属性质和配体设计的仔细相互作用将允许合成化合物，其中对配位球外围的空间和/或电子修饰将感应地或通过调整配位几何来调节金属中心的轨道分裂和配对能量，从而选择性地调节化合物的自旋态能量。这将允许在相似的配位环境中合成具有相同的金属和氧化态的化合物对或系列化合物，但基态自旋不同，允许系统地研究络合物的自旋状态如何影响组成有机金属催化循环的反应的热力学和动力学。用于合成有机和工业应用的催化剂的合理设计仍然是有机金属化学的主要焦点。催化剂的自旋状态有可能发挥关键作用，因为不同自旋的电子的存在或需要在不同自旋的表面之间交叉可以对反应的能量、机制和速率产生深刻的、经常是意想不到的影响。随着最近开发的催化系统的数量不断增加，特别是在烯烃聚合领域，利用第一排金属的中价化合物，如铬、铁和钴，产生催化剂或催化剂前体，这些催化剂或催化剂前体不仅具有顺磁基态，而且在催化循环中可能在不同的自旋表面之间交叉。越来越多的工作表明，自旋交叉现象将被证明在有机金属催化中发挥巨大作用。因此，本研究的主要目标是首次系统地分离和研究自旋状态单独影响和控制有机金属反应活性的一般方法。目前正在研究两个与烯烃聚合化学相关的明确定义的体系。它们是双(恶唑啉)吡啶二烷基铁(II)(传统的Ziegler-Natta聚合)，以及双(酰胺)和双(β-酮胺)单烷基钴(III)络合物(原子转移和有机金属自由基聚合)。这两者都是具有六个d壳层电子的五配位16价电子体系，但在每种情况下都有多个可能的基态自旋，最有可能的构型是开壳五重态和闭壳单重态。在每种情况下，配体都很容易被修饰，以承载各种不同空间分布的供电子/脱电子基团，这使得能够控制所展示的自旋性质。这将允许并排比较基本的有机金属反应，其中一个反应发生在单一的抗磁自旋表面上，而另一个反应发生在完全顺磁的表面上，或者需要从一个自旋态到另一个自旋态的交叉。配位、插入和β-消除等反应与Z-N聚合机理有关，虽然单电子和原子转移反应在ATRP和OMRP中很重要。但使用DFT的计算研究将指导实验研究系统的选择，并用于解释和支持机制结果。这种研究方法非常适合与UBC Okanagan大学当前本科生研究相关的资源，但随着我们研究生课程的发展，将很容易应用于长期研究。建议的目标化合物是从商业可获得的廉价材料中进行的几个简单的合成步骤。由于配体和系统很容易修改，这项研究的综合性质将使学生体验有机和有机金属的合成和表征，使用紫外可见和核磁共振光谱技术的机理研究，以及计算方法。尽管基础相对简单，但我相信这些研究具有重大意义，并将产生极大地促进开壳顺磁性过渡金属催化剂的合理设计的结果。