One Scaffold, Three Chelation Modes: 2-(1 H -Tetrazol-1-yl)pyridine as a Dynamic Ligand for Cationic and Neutral Cyclometalated Iridium(III) Complexes

Abstract

The 2-(1H-tetrazol-1-yl)pyridine scaffold (L1) is exploited as a versatile ligand in heteroleptic iridium(III) complexes. Depending on the reaction conditions, ligand L1 reacts with the prototypical iridium(III) dimer [Ir(ppy)2(μ-Cl)]2 (where Hppy = 2-phenylpyridine) leading to the formation of distinct complexes. As a neutral N∧N ancillary ligand, it forms the cationic complex A, otherwise, after deprotonation of its tetrazole ring, as a monoanionic C∧N ligand it can lead the neutral complex B. Finally, after N-methylation to form the tetrazolium salt L2, it behaves as a N-heterocyclic carbene (N∧C: chelation mode), forming the cationic complex C. This trimodal coordination versatility is further enriched by the serendipitous discovery of the byproduct complex D, arising from partial ring-opening of the tetrazolium salt. The electrochemical redox gap of the complexes reflects the different σ-donor and π-acceptor character of each coordination mode. Complex A shows a broad metal-to-ligand charge-transfer (3MLCT) emission band at 298 K, while complexes B–D display an intricate interplay between ligand-centered (3LC) and 3MLCT states, as confirmed by DFT calculations. This work establishes a novel class of multifunctional ligands and provides a comprehensive structure–property investigation for improving the photophysical performances of tetrazole-based iridium(III) emitters for optoelectronic applications.