In the UK, the chemicals sector is a key contributor to the UK economy. It adds close to £20 billions of value to the country's economy every year and has an annual turnover of approximately £60 billion, sustaining more than half a million jobs. Within this sector, the manufacture of most chemicals involves the use of a catalyst, which is usually based on the rarest elements on the Earth's crust, such as noble metals (Pd, Rh or Ir). The limited supply of these group of privileged metals, together with their huge environmental footprint (e.g. obtaining 1 kg of pure metallic Rh produces near 32t of carbon dioxide) blocks the development of truly sustainable processes. This is pushing chemists towards the discovery of catalysts based on inexpensive, abundant, and benign base metals (e.g. Ni, Co and Fe). However, reactivity on BM centres often proceeds through one-electron events, resulting in difficulties controlling and maintaining the catalyst function, thus preventing the development of sustainable, efficient, and predictable catalysts.
Among all the strategies employed to control the chemistry of base metals, we were attracted by chemical metal-ligand cooperation, in which actor ligands participate in bond-forming and breaking events. Based on this, our strategy to tame two-electron catalytic cycles and develop predictable catalytic methods with BM will exploit low-valent aluminium-based ligands. Thus, our aim will be furnishing ambiphilic Al-BM units that are capable of (1) binding substrates to the highly electrophilic Al centre and (2) activate them using a nucleophilic base metal centre. Using Al as binding site is not a random choice: this group 13 element is not only benign, but the most abundant metal in the Earth's crust. Furthermore, in its +1 oxidation state presents interesting properties as ligand, becoming a powerful sigma-donor with an empty and accessible p-orbital. These properties have been recently exploited in the field of noble metal catalysis. Nonetheless, heterobimetallic complexes in which Al(I) is paired with another earth-abundant metal remain under-explored and currently limited to stoichiometric activation of small molecules. In these examples, however, the integrity of the Al-BM bond is lost. This represents a major challenge for their implementation in catalytic processes, as ligand dissociation leads to disruption of their cooperative activation ability, resulting in catalytic deactivation. To overcome this issue and achieve rigid and stable structures and Al-BM bonds, we will establish a rational design strategy to obtain bespoke Al-BM complexes that will be built by a delicate selection of ligand backbone, anchor arms, and base metal centre (Co, Ni, Fe). These complexes will be studied using a bottom-up approach based on a stoichiometric-to-catalytic strategy: employing the knowledge gathered from stoichiometric activation studies, infusing nobility to Al-BM units in a catalytic fashion will be within reach. The implementation of Coop-NBM will represent a greener and cheaper alternative to functionalise organic molecules compared to noble metal catalysis, allowing the achievement of environmentally friendly approaches with potential to be applied at industry.
Overall, the importance of this research proposal lies in its potential to provide catalysts based on the most abundant elements of our planet, e.g. Al and Fe. Catalytic use of base metals combined with subvalent Al ligands remains an uncharted territory and an exceptional opportunity to establish a new chemical space that could lead to a dramatic reduction of the environmental footprint of countless organic transformations currently performed by noble metal catalysis. This will certainly make the UK take centre stage in the development of sustainable technologies aiming at retiring noble metals as workhorses of chemical industry.
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