One Ligand, Three Roles: L-, X-, and Z-Type Coordination at Nitrogen

Nitrogen-based ligands are central to coordination chemistry but are limited to L- and X-type donor behavior. Here we report a fundamentally new class of nitrogen ligands based on N-heterocyclic nitrenium (NHN) frameworks that exhibit multimodal L-, X-, and Z-type coordination with reversible switching demonstrated between the L- and X-type states. These ligands represent the first examples of nitrogen-based Z-ligands, with the ability to switch between donor and acceptor modes in response to the coordination environment. As such, the formal oxidation state of a transition metal is reversibly changed simply by adding or removing a coordinating solvent introducing a unique concept of "redox-by-coordination". The coordination mode is confirmed by x-ray crystallography and comprehensive computational analysis.