The theme of most of our research programme is the study of the reactivity of transition metal catalysts, and their use in reactions that provide us with fuels, materials, fine chemicals, drugs, and agrochemicals. Without these, we would not enjoy the standard of living that we currently do. We want to access molecules that we currently cannot make, or access known molecules more quickly, more cost effectively, and with reduced environmental impact.
Our current research programme focusses on three key areas:
- Nickel-Catalysed Cross-Coupling. Nickel has the potential to change the way in which we conduct a number of reactions, particularly due to the ability of nickel(0) to oxidatively add to more reluctant electrophiles, but many of the mechanistic aspects of nickel catalysis remain a mystery. We are quantifying the rates of key processes that occur during nickel-catalysed reactions, and studying structure/activity relationships to elucidate the scope and limitations of nickel catalysis and how ligand structure affects reactivity.
In (Organometallics, 2017, 36, 1662 [DOI]) we report kinetic studies of oxidative addition to a model Ni(0) complex, which reacts with a number of aryl (pseudo)halides at different rates, forming Ni(I) complexes as the ultimate products. Our plan is now to use this reactivity scale to delineate the scope and limitations of nickel catalysis with different catalysts, and explore how we can control nickel-catalysed reactions of highly-functionalised substrates.
In (Chem. Eur. J., 2017, 23, 16728 [DOI]) we showed that halide abstraction mechanisms can play an important role in nickel catalysis. The competition between oxidative addition to [Ni(PR3)3] and halide abstraction by this species leads to a mixture of Ni(I) and Ni(II) products. Our calculations are qualitatively and quantitatively consistent with the results of Kochi's seminal study of oxidative addition to nickel.
Further work on oxidative addition has shown that [Ni(NHC)2] complexes undergo oxidative addition to aryl halides, if the NHC is sufficiently small. For larger NHCs, a competing halide abstraction pathway leads instead to [NiX(NHC)2] complexes. Our calculations are again consistent with experimental results from others working in the field. Full details can be found in (Chem. Commun., 2018, 54, 10646 [DOI])
- Ruthenium-Catalysed C-H Functionalisation. C-H activation can enable the reactions of the future because it can preclude the need to first functionalise substrates with halogens or with boron, tin, zinc, or magnesium species. Achieving sufficient reactivity yet tight control of regioselectivity is a challenge due to the low reactivity and ubquity of C-H bonds in organic molecules. Our work in this area is focussing on understanding the rules that govern directing groups, which can effectively guide the catalyst to the desired site of reaction.
In (Catal. Sci. Technol., 2018, In Press [DOI]) we have developed the first scale for directing group power in Lewis base directed ruthenium-catalysed C-H arylation. A full and detailed DFT study shows that the loss of the p-cymene ligand is essential for reactivity, and that we can predict which substrates react most readily.
- Ligand Design and Quantitative Characterisation.
Ligand choice can have profound effects on catalysis by ensuring that the catalyst has the appropriate electronic and steric properties. We synthesise, study, and deploy new ligands - particularly N-heterocyclic carbenes (NHC)s - for use in various processes. For example, we have recently disclosed 'IPaul', which has spatially-defined steric impact and leads to highly active copper complexes for the hydrosilylation of ketones (Dalton Trans. 2016, 45, 11772 [DOI]). The image below shows the X-ray crystal structure of this copper complex, and the associated steric map that shows how the steric impact is distributed (looking along the Cl-Cu bond).Subsequent work in this area has led to the synthesis of the saturated analogue ("SIPaul") and its complexes (Z. Anorg. Allg. Chem. In Press [DOI]).
We make use of a number of tools and techniques :
- Organometallic Chemistry: Our ability to synthesise, handle, and characterise very unstable (e.g. air-sensitive) organometallic complexes allows us to prepare new catalysts and prepare models for intermediates in catalysis.
- Organic Synthesis: Our end goal is the synthesis of organic molecules, and we have the necessary equipment and skills to prepare, purify, and characterise such compounds.
- Physical Organic Chemistry: The application of mechanistic studies, and particularly kinetic studies that allow us to monitor processes and measure their rates, will allow us to develop a true understanding of the reactions we study. We can then use this as a platform for the design and development of catalytic reactions without resorting to extensive and tedious screening of catalysts, ligands, solvents, and additives.
We are grateful to a number of organisations for supporting our research.