Combination of chemical and redox metal-ligand cooperativity. Most approaches focus in exploring metal-ligand cooperativity mechanisms via aromatisation-dearomatisation mechanisms focus on metal-carbonyl species, which control the spin state of the metal centres. Nevertheless, the same CO ligands are usually not substitutionally labile and therefore can block active sites at the metal centre.

We have investigated for the first time the combination between chemical and redox MLC in the context of spin unrestricted iron complexes (i.e. Fe-N2 species), where ligand non-innocence and a flexible changes between spin states facilitate new reaction pathways. For this, we have used PNN ligand platform which incorporates a redox-active pyridineimine fragment. The compression of the spin-ladder in such complexes allowed us to discover a new mode of dinitrogen activation, which, in contrast, to literature precedence, is triggered by a physical stimulus. Vacuum triggered dissociation of terminal N2 ligands determines a significant labilisation of the remaining bridging N2 ligand, which was previously only achieved using chemical stimuli (reducing agent or Lewis acids). Deprotonation of the benzylic side-arm induces a spin-state change (singlet-to-triplet), enhancing the reactivity of the resulting species towards small molecules.

Diazinediimine redox-active ligands. A key aspect of enzyme catalysis is their flexibility to change conformation based on specific requirements. This allows the regulation of their catalytic activity based on the presence of an external effector which determines conformational changes of the active site by interacting with a remote site in the enzyme (allosteric regulation). Our research programme aims to transfer this concept to molecular complexes. We have demonstrated that the spin-state of iron complexes can be controlled by the Lewis acid trigger used to functionalise the distal nitrogen atom of the purposely developed pyrazinediimine redox-active ligands. We have extended these investigations to pyrimidin-diimine redox-active ligands which show a strong increase of π-acidity compared to their pyridine analogues. The desymmetrisation of the N-heterocyclic ring triggers an unsymmetric distribution of spin-density, with consequences in magnetic and catalytic properties of the resulting complexes.