Electron-rich phenanthroline bearing N-heterocyclic imine substituents: synthesis, optical properties, metal coordination

Polypyridines functionalized with π-donating groups constitute a class of electron-rich ligands with significant relevance in coordination chemistry and catalysis. The incorporation of strongly basic guanidinyl substituents, however, often introduces multiple binding sites, with coordination typically favoring the guanidyl nitrogen atoms. Herein, we report the synthesis and characterization of a new electron-rich 1,10-phenanthroline ligand featuring bulky NHI groups that define a well-structured coordination cavity. Protonation studies and the preparation of a zinc(II) complex reveal that Lewis acids preferentially coordinate at the phenanthroline nitrogen atoms rather than the NHI moiety. The electronic and photophysical properties of the new ligand and its complexes are explored through a combination of computational and experimental methods, demonstrating that its emission and absorption characteristics are highly sensitive to protonation, concentration, and metal coordination.