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

Background

In 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 Approach

Recently 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.