When is a Pyridine Not a Pyridine? Benzannulated N-Heterocyclic Ligands in Molecular Materials Chemistry

The C=N bond is a critical structural piece of many N-donor ligand scaffolds and is central to the properties and reactivity of important coordination complexes. For example, C=N units play a key role in the ‘redox non-innocence’ of α-diimine complexes and in making charge-transfer excited-state character available to complexes of N-heterocyclic ligands such as bipyridine. In N-heterocycles like pyridine, benzannulation can be used to extend the conjugated C=N containing π-system to quinoline (2,3-benzopyridine) to acridine (2,3-benzoquinoline). This stabilizes the lowest unoccupied molecular orbital (LUMO) of the molecule and boosts its electron-accepting properties, but the position of the benzannulation matters. For example, phenanthridine (3,4-benzoquinoline), an asymmetric isomer of acridine, bears a similarly electronically accessible extended π-system but more chemically isolated, ‘imine-like’ C=N moiety. This Award Paper presents an overview of our work investigating the impact of such site-selective benzannulation on the chemistry and properties of phenanthridine as a molecule and ligand.