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

背景在催化领域，有相当大的推动力来取代贵金属的使用，特别是Pd、Pt、Rh、Ir和Ru，这不仅是因为在成本上，而且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块元素的研究和使用的复兴，特别是钛、钒、铬、锰、铁、钴、镍和铜，这些都是地壳中含量最丰富的金属。在这方面，一些钴的络合物，无论是明确的分子体系还是原位形成的络合物，在一系列催化应用中都受到了化学界的越来越多的关注。与较重的同类产品相比，钴的价格要低得多，而且更容易获得，因为它是铜和镍开采/提取的副产品。虽然使用更轻、富含地球的d-块元素，如钴是有吸引力的，但它们的化学成分比贵金属更复杂。通常，以钴等金属为基础的(均相)催化剂缺乏可预测性和对其反应活性的控制，这主要是因为这些第一排金属很容易进入各种不同的自旋态，加上它们容易经历竞争性的单电子转移过程。为了尝试和“驯服”钴和其他第一排金属催化剂的反应性，并了解它们的反应性，使用多齿配体尤为重要。这些类型的金属支架可用于调节和操纵决定反应性的立体和/或电子因素。鉴于催化C-C和C-X成键反应在商品和精细化学合成中的重要性，目前地球上丰富的金属在化学合成中的应用有限，反映了获取与催化相关的氧化还原和几何流形的困难。实验方法最近，Dyer小组展示了金属配位几何结构的微小差异(由基本的双膦造成)如何控制氧化还原行为，这反过来又控制催化C-C和C-X键形成所需的基本氧化加成、还原消除和跨金属步骤。这一新的见解将被用于指导创新的约束几何、电子(平衡sigma供体/pi受体)和空间调谐双膦以及相关的多齿配体的设计。例如，与刚性、体积大的主链相结合，在磷上将部署空间要求高、电子丰富的环取代基，以约束金属中心的几何结构，进而影响金属的电子构型。再加上对地球上丰富的主要基团和过渡金属的配位和氧化还原化学的了解，将获得新的多功能、可持续的双膦催化剂系统。DFT研究将被用来评估金属-配体成键的性质以及这如何影响几何和氧化还原化学/金属自旋态(S)，这反过来将推动迭代配体设计。将使用X射线结晶学、核磁共振、红外和近红外-紫外可见光谱、循环伏安、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含双齿配体(如双膦和α-二亚胺)和定制多齿配体的钴(II)和(I)配合物的合成、表征和反应活性，以用于潜在的催化应用，包括C-C/C-X键形成和选择性烯烃齐聚。重点将放在阐明结构-反应性相关性以及这些参数如何影响获得潜在催化相关的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.