Transition Metal Alkane Sigma Complexes by Solid-Gas Synthesis Routes: Defining and Exploiting a New Area of Organometallic Chemistry

Alkanes are cheap and readily available chemicals and a long-term goal of chemical research has been to develop methods to exploit these species directly as chemical feedstocks. This would greatly improve the overall efficiency of chemical synthesis used in the production of commodity and fine chemicals that underpin the chemical and pharmaceutical industries. In order to achieve this goal the means to "activate" (i.e. break) the notoriously unreactive C-H bond is required. While examples of such C-H activation with transition metal complexes have been reported in recent years, much more information on this process is required before the integration of C-H activation into chemical synthesis is fully realised. Much useful insight could be gained by studying the way an alkane interacts with a metal centre prior to actually cleaving the C-H bond. This requires the means to prepare and study such so-called 'transition metal alkane sigma complexes'; however, until recently, examples of such species have been extremely limited due to the lack of straightforward and predictable general methods of preparation.

Very recently the Weller (experimental chemistry) and Macgregor (computational modelling) research groups have reported a breakthrough in the synthesis and characterisation of transition metal alkane sigma complexes (see Science 2012, 337, 1648). Our approach was to react a transition metal alkene precursor in the crystalline solid-state with hydrogen gas. This produces the equivalent alkane sigma-complex directly in the crystalline state, allowing for its full characterisation by a variety of experimental and complementary computational techniques. This represents a step-change in current state-of-the-art for the synthesis and characterization of transition metal alkane sigma-complexes, opening up the exciting prospect that, for the first time, such species could be routinely synthesized and their chemistry fully understood.

This research proposal seeks support for the Weller and Macgregor groups to fully explore and generalise the new gas-solid synthetic methodology for the synthesis of transition metal alkane sigma-complexes. Our approach will involve experimental synthesis and characterisation allied with the computational modelling; with the latter also providing structural data that are difficult to routinely obtain by experimental means. In addition computational modelling can be used to predict how strong the transition metal-alkane interaction will be. In this way the calculations will be able to target the most promising combinations for further study experimentally, thus significantly enhancing the overall productivity of the project. From our understanding of what makes a transition metal alkane sigma complex stable in the solid state we will then move to develop systems that are also stable in solution in order to provide definitive data on the properties and reactivity (i.e. C-H activation) of such species. Another exciting possibility will be to explore what other chemical transformations occur in the gas-solid regime; for example the commercially important catalytic hydrogenation of alkenes may occur in the solid state, without the need to revert to the use of solvents.

The research in this project is fundamental in nature, however it will deliver significant long-term legacy through developing the chemistry of transition metal alkane sigma-complexes and how such species relates to the key C-H activation process. It will also explore the development of solid-state organometallic chemistry, both in terms of making new organometallic supramolecular complexes, but also, more generally, by exploring the potential of new catalytic processes (especially solid-gas reactions) in the solid state.

Who will benefit from this research?
In addition to the academic beneficiaries described in the previous section, three additional groups of beneficiaries can be identified, namely: in the short term, (i) the PDRA co-workers working on the project, and, in the longer term, (ii) UK industry and (iii) UK society.
How will they benefit from this research?
(i) PDRAs. These will experience the most immediate impact of the work by receiving a first-class training in laboratory and analytical techniques (at Oxford) or an outstanding training in the modern computational chemistry (at Heriot-Watt). Both aspects of the project will expose these workers to a very broad range of techniques. The project will provide the opportunity to develop their professional skills, including communication and presentation skills, organisational skills and team working. These skills will be further enhanced by both PDRAs playing a central role in the organisation of the meeting on "sigma-complexes and C-H activation" planned for Year 2 of the project. These benefits will begin to accrue immediately the project is underway.
(ii) UK Industry. Our proposed work will contribute to the development of atom efficient processes which are based on catalytic CH activation. This will benefit the UK Chemicals Sector by responding to growing environmental pressures for sustainable synthesis and moving away from current more wasteful methods based on preactivated feedstocks, e.g. those containing C-halogen rather than C-H bonds. The development of such technology on an industrial scale is likely to be on an 8-15 year timescale. Our experience indicates that for this type of fundamental, exploratory research project, setting up industrial collaborations is most sensibly performed once exceptional properties of interest are clearly demonstrated, as realistic specific potential uses can then be forecast. This project will also result in two highly trained PDRAs who we expert to contribute in further ways to the UK, through employment in the chemical industry or in education.
(iii) UK Society. The development of sustainable chemical synthesis based on C-H activation should also lead to benefits for the wider public. This approach will lessen the environmental impact associated with the production of chemical commodities. As well as bulk chemical production, such processes also have the potential to underpin a range of fine chemicals found in pharmaceuticals, agrochemicals and other technological materials that society consumes in increasing volumes, while demanding the means of production be placed on a sustainable footing. Such potential impact is on a longer timescale (10-20 years).