Augmentation of Alkaline Earth Reactivity: An FLP Analogy

Catalysis, the acceleration of chemical transformation, is the key to realising environmentally friendly and economical processes for the conversion of both conventional (fossil) and alternative (e.g. biomass and carbon dioxide) chemical feedstocks. Catalysts act by reducing the energy required for a reaction to proceed and will, thus, occupy a key role in the world's energy future. Many of the most useful soluble and solid catalysts incorporate precious metals such as rhodium, palladium, platinum and ruthenium. These metals are expensive and their supply is limited. There is, therefore, a need for the development of non-precious-metal catalysts as replacements.

In response to these requirements, the last decade has seen emergence of s- and p-block compounds as inexpensive and ecologically benign catalytic reagents. The applicant's previous research has led the way in the development of group 2 reagents, particularly those based on magnesium and calcium (the eight and four most earth-abundant elements respectively) for a wide variety of catalytic transformations. Although this has included some of the most practically desirable transformations, the usefulness of currently available group 2 reagents is limited by their absolute reactivity.

A detailed mechanistic approach to the study of these reactions has allowed the emergence of more generalised models for the chemical reactivity of these previously under-appreciated elements. This understanding is based on a polarisation model intrinsic to the bonding of most substrate molecules to a highly electropositive group 2 centre. The charge separation induced across a substrate molecule by these species may be viewed as resulting from reactivity closely related to another emerging field, 'frustrated' Lewis pair chemistry. In this latter case, a wide variety of small molecules may be activated through a cooperative interaction of a high energy vacant molecular orbital and a low energy filled atomic orbital. This proposal wishes to extrapolate this hydpothesis to apply the FLP concept to design new group 2 species of unparalleled reactivity. In so doing the new reagents will enable catalytic systems that combine the environmental and fiscal benefits of group 2 chemistry with the enhanced and readily manipulated FLP reactivity.

Since our initial report of the first molecular catalysis based on an alkaline earth element, the calcium-mediated intramolecular hydroamination of aminoalkenes, the application of complexes of the heavier group 2 elements (Mg, Ca, Sr and Ba) for the catalysis of atom-efficient chemical transformations has become something of a 'hot topic' in contemporary synthetic chemistry. Indeed, the area has been highlighted as such several times in recent review and perspective articles , and has even been afforded its own dedicated chapter in the 2013 compendium 'Comprehensive Inorganic Chemistry II' (CIC II, 'Alkaline Earth Chemistry: Applications in Catalysis', 2013, vol 1, Pages 1189-1216, M. Arrowsmith, M.S. Hill). Stimulated by our initial publications, a number of groups across the United States, Europe and Japan have now become active in the area and competition is growing rapidly. It can be deduced, therefore, that our previous work has been highly influential and has provided a high academic impact.

In parallel to our research in group 2-based reactivity, the chemistry of 'frustrated' Lewis pairs (FLPs) has stimulated enormous interest and a plethora of bond activation and catalytic reactions have now been devised (and notably provides a further new addition as a separate chapter in CIC II, 'Frustrated Lewis Pairs: Activation of H2 and Other Small Molecules' 2013, vol. 1. Pages 2013, Pages 1069-1103, D.W. Stephan). The two areas, alkaline earth- and FLP-based reactivity, however, illustrate how main group reactivity is packaged into convenient but divisive sub-classes. The central thesis of the current proposal suggests that enormous untapped 'amplified' reactivity may be attained by cutting across these semantic differences and through a cross-fertilisation between the two fields. Furthermore, the hypothesis that FLP and s-block reactivity may be treated as one and the same thing is further illustrated by the final WP of the proposal, which suggests that many bimetallic systems currently employed for the stoichiometric activation of even the most non-acidic C-H bonds may be considered in an analogous fashion. We, thus, suggest that these analogies provide the tip of the iceberg and that all main group reactivity may be analysed similarly. Taken to its ultimate conclusion, the implications of this suggestion would pave the way for the unification of the current rather disparate threads of s- and p-block chemistry. The academic impact of the uptake of this hypothesis should, thus, be self-evident and would lead the way toward the deliberate design of main group systems for the incorporation of even the most thermodynamically most stable and most kinetically robust X-Y bonds in productive catalytic cycles.

Our own previous work and the research to be developed in this project has been unashamedly academic. The widespread uptake of the broader perspective provided by this project will undoubtedly influence chemists world-wide who are engaged in the development of catalytic species derived from the more abundant and non-toxic main group elements. In the longer term, however, this demonstration will undoubtedly attract the attention of industrialists (a primary motivation for the 'forum' activities described in the Pathways to Impact document). Around 75% of all chemicals require catalysts in their manufacture and the chemical industry contributes ca. 21% of UK GDP. While the uptake of even one new industrial process over the next 20 years as a direct consequence of the activities of this project would provide a more than tangible economic impact, we suggest that this should represent a very limited ambition in the longer 30-50 year time scale. The implication of the ideas described signify that replacement of the entire palette of current and future large scale homogeneous precious metal catalysts by inexpensive main group species is not only desirable, but is also a realistic proposition.