Taming Sustainable Homogeneous Catalysts Through Tailored Diphosphine / Multi-dentate Ligand Design - A Combined Experimental / Computational Study

背景技术在催化竞技场中，有相当大的动力来取代贵金属，特别是Pd、Pt、Rh、Ir和Ru的使用，这不仅是基于成本，而且还因为它们的供应在未来几年将变得越来越有限。这导致了对第一排d区元素的研究和使用的复兴，特别是钛、钒、铬、锰、铁、钴、镍和铜，它们是地壳中最丰富的金属。在这方面，几种钴配合物，无论是明确定义的分子系统或原位形成的配合物的形式，正在受到越来越多的关注，从化学界在一系列的催化应用。与较重的同系物相比，钴的价格要便宜得多，而且更容易获得，因为钴是铜和镍开采/提取的副产品。虽然使用更轻的、地球上丰富的d区元素（如钴）是有吸引力的，但它们的化学性质比贵金属更复杂。通常，基于金属如钴的（均相）催化剂缺乏对它们的反应性的可预测性和控制，这主要是由于这些第一行金属可以容易地进入各种不同的自旋状态，以及它们经历竞争性单电子转移过程的倾向。为了尝试和“驯服”钴基和其他第一行金属基催化剂的反应性并理解它们的反应性，多齿配体的使用尤其重要。这些类型的金属支架可用于调节和操纵决定反应性的空间和/或电子因素。考虑到催化C-C和C-X键形成反应在商品和精细化学合成中的重要性，当今地球上丰富的金属的有限应用反映了获得催化相关氧化还原和几何歧管的困难。实验方法最近，Dyer小组已经表明，即使金属配位几何形状的微小差异（由必要的二膦施加）也会控制氧化还原行为，从而控制催化C-C和C-X键形成所需的必要的氧化加成，还原消除和transmethalo步骤。这种新的见解将被用来指导创新的约束几何形状，电子（平衡σ-供体/π-受体）和空间调谐的双膦和相关的多齿配体的设计。例如，与刚性、庞大的骨架结合，空间要求高的富电子环状取代基将在磷处部署，以限制金属中心的几何形状，进而影响金属的电子构型。再加上对地球上丰富的主族和过渡金属的配位和氧化还原化学的理解，将获得新的多功能，可持续的双膦催化剂系统。DFT研究将用于评估金属-配体键合的性质，以及这如何影响几何结构和氧化还原化学/金属自旋态，这反过来将推动迭代配体设计。将使用X射线晶体学、NMR、IR和NIR-UV-维斯光谱、循环伏安法、光谱电化学和催化研究对结果进行实验评估。该项目将探索一系列钴（II）和（I）配合物的合成，表征和反应性，这些配合物具有双齿配体（例如二膦和α-二亚胺）和定制的多齿配体，用于潜在的催化应用，包括C-C/C-X键形成和选择性烯烃低聚。重点将是阐明结构-反应性的相关性，以及这些参数如何影响获得潜在的催化相关的CoI和CoII氧化还原状态。

BackgroundIn the catalysis arena there is a considerable impetus to replace use of the precious metals, particularly Pd, Pt, Rh, Ir, and Ru, not only on a cost basis, but also because their supply will become increasingly limited in the coming years. This has led to a renaissance in the study and use of first row d-block elements, especially titanium, vanadium, chromium, manganese, iron, cobalt, nickel, and copper, which are some of the most abundant metals in the earth's crust. In this respect, several cobalt complexes, in the form of either well-defined molecular systems or in situ-formed complexes, are receiving increasing attention from the chemistry community in a range of catalytic application. Compared to its heavier congeners, cobalt is much less expensive and more available because of its production as a by-product of copper and nickel mining/extraction. While the use of the lighter, earth-abundant d-block elements such as cobalt is attractive, their chemistry is more complex than that of the noble metals. Often (homogeneous) catalysts based on metals such as cobalt lack predictability and control over their reactivity, largely as a result of the ease with which these first row metals can access various different spin states coupled with their propensity to undergo competitive single electron transfer processes. In order to try and "tame" the reactivity of cobalt- and other first row metal-based catalysts and to understand their reactivity, the use of multidentate ligands is especially important. These types of metal scaffold can be used to moderate and manipulate steric and/or electronic factors, which dictate reactivity. Given the importance of catalytic C-C and C-X bond-forming reactions in both commodity and fine chemical syntheses, today's limited application of Earth-abundant metals in reflects the difficulty of accessing catalytically-relevant redox and geometric manifolds. Experimental ApproachRecently the Dyer group have shown how even very small differences in metal coordination geometry (imposed by the essential diphosphine) governs redox behaviour, which in turn controls essential oxidative addition, reductive elimination, and transmetallation steps required for catalytic C-C and C-X bond formation. This new insight will be used to direct the design of innovative constrained-geometry, electronically- (balancing sigma-donor/pi-acceptor) and sterically-tuned diphosphine and related multi-dentate ligands. For example, in combination with a rigid, bulky backbone, sterically-demanding, electron-rich cyclic substituents will be deployed at phosphorus in order to constrain the geometry at the metal centre, in turn influencing the metals electronic configuration. Coupled with understanding of the coordination and redox chemistry of Earth-abundant main group and transition metals, new versatile, sustainable diphosphine catalyst systems will be accessed. DFT studies will be used to assess the nature of metal-ligand bonding and how this affects geometry and redox chemistry/metal spin state(s), which in turn will drive iterative ligand design. Results will be assessed experimentally using X-ray crystallography; NMR, IR and NIR-UV-Vis spectroscopies; cyclic voltammetry, spectro-electrochemistry and catalysis studies. This project will explore the synthesis, characterisation, and reactivity of a range of cobalt(II) and (I) complexes bearing bidentate ligands (e.g. diphosphines and alpha-diimines) and tailored multi-dentate ligands for potential catalytic applications including C-C/C-X bond-formation and selective olefin oligomerisation. Emphasis will be upon elucidating structure-reactivity correlations and how such parameters impact on access to potentially catalytically relevant CoI and CoII redox states.