Molecular s-block Assemblies for Redox-active Bond Activation and Catalysis: Repurposing the s-block as 3d-elements

The moderation of a reaction's energy demands through catalysis plays a vital role in addressing the kinetic viability of a vast array of commodity, pharmaceutical and fine chemical processes. The current level of catalyst utilisation enables ca. $190 billion p.a. of domestic revenue for the UK, a value that it is estimated will rise to $227 bn by 2025. It is also clear that the future prosperity of a truly circular and sustainable economy will require investment in our fundamental approaches to catalytic science. Innovative modes of catalysis and, more generally, chemical synthesis, therefore, will continue to be primary drivers for the advancement of science and technology and a seed corn for new applications to meet the objectives for a green recovery in the aftermath of the COVID-19 pandemic. Homogeneous catalysis, in particular, continues to be dominated by platinum group metals (PGMs, e.g. Rh, Ir, Pd, Pt). The continued and growing implementation of these elements, however, is threatened by a combination of high cost, vulnerable supply and their relatively high toxicity. These factors have stimulated a contemporary focus on the identification of alternative catalytic vectors resulting from systems derived from the more earth abundant lighter transition metals and main group (s- and p-blocks) of elements. The fulfilment of this objective, however, requires a massive global effort to devise innovative means to productively harness this broad palette of highly disparate chemistry. The geological availability of, in particular, magnesium and calcium ensures that these elements are orders of magnitude less expensive than PGMs, while their appeal is underscored by their generally benign environmental impact and biocompatibility. Although these systems now encompass a wide range of multiple bond heterofunctionalization and cross coupling catalyses, without exception the individual bond activation processes occur in defined two electron steps and through the maintenance of the extremely stable Ae(II) oxidation state. Although significant variability arises with adjustments to the radii and polarizability of the Ae(II) cations, this latter feature precludes any possibility of the redox activity and tuneable HOMO-LUMO energies that arise from the available manifold of nd valence orbitals of catalytic PGMs.

This proposal builds on the applicants' recent discovery (J. Am. Chem. Soc. 2021, 143, 17851) that a heterobimetallic assembly comprising a low oxidation state Mg-Mg bonded unit and two sodium cations is readily accessible by reduction of a conventional molecular Mg(II) bis-anilide. The electronic structure and chemistry of the {Mg2Na2} assembly is dependent not only on the presence of the formally Mg(I) centres but also on the cooperative interaction of the Mg-Mg bond with the sodium cations. In this proposal, we present a joint synthetic and computational study to exploit this general design principle and access a family of formally low oxidation state group 1 / group 2 heterobimetallics. The resultant compounds will possess narrow and manipulable frontier orbital energies that will allow unprecedented access to s-block systems that display reversible redox behaviour. These compounds will facilitate an exploration of their reactivity toward small molecules such as H2, CO, CO2 and N2, while their potential redox activity will enable the construction of catalytic manifolds, which display a higher degree of commonality with those derived from the nd orbital configurations of transition metals than those of isolated group 1 and 2 centres.