Redox-active di(heteroaryl)methenes as ligands for electron transfer reactions
As a consequence of the discovery of ligand non-innocence, the last few decades have seen the chemistry of coordination complexes being re-evaluated. Compared to their counterparts, non-innocent ligands are actively involved in chemical reactions, so accessing unusual reaction pathways. In such a way, high-energy barriers associated with the formation of unstable intermediates are bypassed through ligand participation and innovative reactions can be undertaken. Non-innocent ligands can be divided in three main categories: hemilabile ligands that dissociate one or more donor functionalities, providing a reactive vacant coordination site on the metal centre; cooperative ligands that interact directly with the substrate, providing a reactive partner to the metal centre and favouring intramolecular reactivity; redox-active ligands that act as electron acceptors or donors, limiting access to unstable metal oxidation states. This Thesis describes the design, synthesis, characterisation and application in catalysis of new redox-active ligands belonging to the di(heteroaryl)methene family, in which two heterocyclic substituents are linked by a methine bridge. Chapter 1 reviews important advances in the field of non-innocent ligands, focusing on redox-active ligands and highlighting their main applications in synthetic chemistry and catalysis. Carbon dioxide valorisation is also introduced and the main reduction pathways described. Particular attention is given to electroreduction, with an overview of the state-of-the-art homogeneous electrocatalytic systems. Recent studies in redox-active di(heteroaryl)methene systems are also presented, highlighting their individual features and potential applications. Chapter 2 describes the synthesis, characterisation and reactivity of a bis(iminothienyl)methane HLNS. This compound is readily deprotonated at the meso position to obtain the fully conjugated anion LNS−. Introduction of the thiophene functionalities impacts the electronic properties of the anion, enabling access to a rich oxidation chemistry. As such, the unusually stable acyclic neutral radical LNS• is obtained upon single-electron oxidation. This radical can act as a neutral L type ligand towards transition metals, as shown in the synthesis and characterisation of a unique dinuclear copper(I) complex of LNS•. Alternatively, the use of the stronger oxidant silver(I) tetrafluoroborate triggers three sequential one-electron oxidations to form the radical dication LNS•2+. Detailed UV-vis and EPR spectroscopic, electrochemical and single-crystal X-ray crystallographic studies are discussed. Chapter 3 describes the effect that simple ligand engineering has on the electronic properties of additional members of the di(heteroaryl)methene family. The synthesis and characterisation of compounds bearing aryl substituents or furan rings to replace the imine functionalities or the thiophene heterocycles, respectively, are reported. Unexpectedly, both structural modifications cause a significant stabilisation of the corresponding radicals formed by single-electron oxidation. Additionally, the presence of strong donors is found imperative to impart kinetic stability to the anion and hamper spontaneous oxidation. Combined experimental and computational spectroscopic, crystallographic and electrochemical investigations unveil the unusual properties of these compounds, highlighting their potential in electroreduction reactions. Chapter 4 reports the use of the compounds described in Chapters 2 and 3 in carbon dioxide reduction. Different outcomes are observed upon reaction of carbon dioxide with di(heteroaryl)methene anions. Despite this, strong evidence for a spontaneous electroreduction process is observed in all cases. Additionally, the coordination of the redox-active organic scaffold to a redox-inactive metal cation is deemed essential to achieve reactivity. A rational interpretation of the experimental data and a potential reaction mechanism are proposed. Chapter 5 reports the ongoing development of alternative redox-active ligands based on the findings described in the previous chapters. The first fully core-substituted anionic porphyrin system is described, supported by spectroscopic and crystallographic characterisation, as well as computational investigation.