Iridium(III) Complexes with Phenyl-tetrazoles as Cyclometalating Ligands

Ir­(III) cationic complexes with cyclometalating tetrazolate ligands were prepared for the first time, following a two-step strategy based on (i) a silver-assisted cyclometalation reaction of a tetrazole derivative with IrCl₃ affording a bis-cyclometalated solvato-complex <b>P</b> ([Ir­(ptrz)₂­(CH₃CN)₂]⁺, Hptrz = 2-methyl-5-phenyl-2<i>H</i>-tetrazole); (ii) a substitution reaction with five neutral ancillary ligands to get [Ir­(ptrz)₂L]⁺, with L = 2,2′-bypiridine (<b>1</b>), 4,4′-di-<i>tert</i>-butyl-2,2′-bipyridine (<b>2</b>), 1,10-phenanthroline (<b>3</b>), and 2-(1-phenyl-1<i>H</i>-1,2,3-triazol-4-yl)­pyridine (<b>4</b>), and [Ir­(ptrz)₂L₂]⁺, with L = <i>tert</i>-butyl isocyanide (<b>5</b>). X-ray crystal structures of <b>P</b>, <b>2</b>, and <b>3</b> were solved. Electrochemical and photophysical studies, along with density functional theory calculations, allowed a comprehensive rationalization of the electronic properties of <b>1</b>–<b>5</b>. In acetonitrile at 298 K, complexes equipped with bipyridine or phenanthroline ancillary ligands (<b>1</b>–<b>3</b>) exhibit intense and structureless emission bands centered at around 540 nm, with metal-to-ligand and ligand-to-ligand charge transfer (MLCT/LLCT) character; their photoluminescence quantum yields (PLQYs) are in the range of 55–70%. By contrast, the luminescence band of <b>5</b> is weak, structured, and blue-shifted and is attributed to a ligand-centered (LC) triplet state of the tetrazolate cyclometalated ligand. The PLQY of <b>4</b> is extremely low (<0.1%) since its lowest level is a nonemissive triplet metal-centered (³MC) state. In rigid matrix at 77 K, all of the complexes exhibit intense luminescence. Ligands <b>1</b>–<b>3</b> are also strong emitters in solid matrices at room temperature (1% poly­(methyl methacrylate) matrix and neat films), with PLQYs in the range of 27–70%. Good quality films of <b>2</b> could be obtained to make light-emitting electrochemical cells that emit bright green light and exhibit a maximum luminance of 310 cd m^–2. Tetrazolate cyclometalated ligands push the emission of Ir­(III) complexes to the blue, when compared to pyrazolate or triazolate analogues. More generally, among the cationic Ir­(III) complexes without fluorine substituents on the cyclometalated ligands, <b>1</b>–<b>3</b> exhibit the highest-energy MLCT/LLCT emission bands ever reported