Bond Activation and Catalysis by Main Group Systems

The activation of relatively inert non-polar chemical bonds is key to numerous catalytic functionalization processes generating high value-added chemical products. In the region of 75% of all chemicals currently require catalysts at some stage in their manufacture, and typical catalysts feature heavier late Transition Metals, reflecting their amenability to bond modifying redox processes. Issues relating to the sustainable availability/cost of such elements and the incorporation of heavy metals into products, mean that the search for alternative catalytic platforms is at the cutting edge scientifically and economically. Main Group metals, by contrast, are inexpensive, abundant, and in the cases of the lighter elements (such as the germanium compounds ultimately targeted here) less of an issue with regard to toxicity.

In recent years, research into Main Group element compounds has highlighted the accessibility of low-valent derivatives with vacant coordination sites, and frontier orbitals with relatively small energy gaps able to facilitate bond activation by oxidative addition. Thus, a fundamental mode of reactivity typical of late Transition Metals has been opened up, and small molecule activation under mild conditions can be envisaged. Complementary strategies utilising redox inert metals (e.g. Ca2+) in constant oxidation state catalysis (e.g. via sigma-bond metathesis) have also emerged. Thus, the opportunity to exploit Main Group elements, once perceived as catalytically inert, as novel catalysts is not only at the cutting edge scientifically, but also offers huge potential for growth. With regard to redox-based processes, Main Group systems capable of effecting oxidative activation of E-H bonds are now known (e.g. for E = H, C, N, O, Si). Reagents capable of such insertion chemistry, however, are often highly reactive sub-valent species, and E-H bond activation processes typically generate products in thermodynamically very stable oxidation states. Catalytic turnover via reductive regeneration of the active species similar to late d-block catalysis is thus difficult to effect. However, our recent work and related research into Group 15 systems gives encouragement that catalytic cycles based on n/n+2 oxidation states for Main Group elements are indeed viable.

The step change in homogenous catalysis which this proposal seeks to bring about is to open up catalytic bond modification processes based on redox chemistry (oxidative addition/reductive elimination) to Main Group metals. Our approach will be built on exciting preliminary results for tin systems, while ultimately targeting catalysts based around germanium, which is more environmentally benign, but a more challenging redox prospect. Our goals for the lifetime of this project are not necessarily to produce immediate replacements for existing Transition Metal systems in societally important catalytic transformations, but rather to establish the fundamental ground rules for catalyst design in what is an entirely new area of endeavour.

The aim of this proposal is to establish the ground rules for a step-change in homogenous catalysis. We aim to establish the fundamental science underpinning redox-based bond modifying catalysis using Main Group elements (rather than Transition Metals) as the active site. While these are adventurous goals (albeit de-risked to a significant degree by extensive preliminary results), we believe that the potential impact especially in the medium to long term is very high. Catalysis in general is acknowledged to be critical to the delivery of future growth in the manufacturing sector: ca. 75% of all chemicals currently require catalysts at some stage in their manufacture, with catalytic processes generating £700 billion in products worldwide. In the US, for example, catalysis and catalytic processes account for ca. 20 % of GDP, with 30 of the 50 largest volume chemicals currently produced via catalytic routes. The development and fundamental understanding of innovative new catalyst systems therefore has clear, direct and long-term benefits to the chemical manufacturing sector and to the broader knowledge-based economy. Moreover, reflecting its national importance, the catalysis research area has been identified as a 'growth area' in the EPSRC portfolio, and a priority area in its own right, as exemplified by the recent launch of the £13 million Harwell-based 'Catalysis Hub'.

The selectivity and mild reaction conditions implicit in homogenous catalysts, allied to their amenability to both mechanistic investigation and systematic tuning mean that such systems are widely employed in key industrial processes. A significant proportion of currently employed industrial catalysts use expensive late Transition Metals (e.g. Rh, Ir, Pd, Pt). Increasing scarcity, exacerbated by global industrialization, has led to a sharp increase in the cost of such commodities, and leads to critical questions concerning, for example, their long term sustainability. Current prices of Rh (US$ 65,000/kg) and Pt (US$ 55,000/kg), for example, mean that precious metal recovery has become a major growth sector. By contrast, catalysts utilizing inexpensive and abundant s and p-block elements have recently begun to emerge. Drawing on this theme, the current proposal is centred around elements costing a small fraction (ca. 2-3%) of widely used precious metals, and seeks to develop viable processes based on single-site Main Group catalysts. Thus, the opportunity to exploit Main Group elements, once perceived as catalytically inert, as novel catalysts is not only at the cutting edge scientifically, but also offers huge potential for growth.

While our initial approach will be built on exciting preliminary results for tin compounds, we ultimately target catalysts based around more environmentally benign germanium systems. Our goals for the lifetime of this project are therefore to establish the fundamental ground rules for catalyst design in what is an entirely new area of endeavour. As such, we feel that the proposed work fits within the remit of 'Structural and kinetic studies to understand the molecular mechanisms involved in catalytic reactions, preparation of novel or improved catalysts and the development of new catalytic processes' as defined by the EPSRC's explicit interpretation of catalysis in its research portfolio. In the wider scheme, the delivery of a new library of catalytic systems has longer term relevance in the manufacture of commodity and fine chemicals, as well as being a significant enabling discipline to EPSRC priority areas such as innovative and sustainable future energy systems, solar technologies, synthetic biology, water and environment, sustainable chemistry, manufacturing and healthcare and grand challenges of urgent societal need such as sustainable energy and new functional materials by design.