Synthesis, structural characterization and the first electroluminescent properties of tris- and bis-cycloiridiated complexes of sterically hindered electron-poor 2-(3,5-bis(trifluoromethyl)phenyl)-4-trifluoromethylpyridine

Abstract

An application of the new sterically hindered electron-poor 2-(3,5-bis(trifluoromethyl)phenyl)-4-trifluoromethylpyridine [HC⁁N] (1) in the one-step high temperature cyclometalation by Ir^IIICl₃ in the presence of Ag^IOC(O)CF₃ resulted in the synthesis of tris-cyclometalated complexes [C⁁N]₂Ir[C⁁C] (3) and [C⁁N]₃Ir (5). A neutral silver cluster with a repeating unit of hexa-silver groups in an infinite chain of (2) was isolated from the above reaction as well. When this cyclometalation was carried out in trimethylphosphate at lower temperature, bis-cyclometalated derivatives [C⁁N]₂Ir(µ-Cl)₂Ir[C⁁N]₂ (6), [C⁁N]₂Ir[η²-(O(C(^tBu))₂CH] (7), and [C⁁N]₂Ir(µ-O-P(OMe)₂-O)₂Ir[C⁁N]₂ (8) were synthesized. According to X-ray analyses complex (3), while trivalent, contains four cyclometalated single Ir–C bonds. One of the Ir–C bonds, next to the nitrogen atom of the C⁁C pyridinium ligand, was found to be the shortest to date (1.977(4) Å) for a single bond between iridium and carbon atoms. The coordination of the C⁁C ligand in (3) to iridium has a decidedly interesting bonding pattern and can be explained by various formulations. The first one is considering this ligand as a monoanionic chelating ligand, in which the second coordination site arises from a carbene or azomethine ylide. Overall the best single picture may be a dianionic ligand making two “normal” Ir–C bonds, in which the ligand just happens to contain a pyridinium function that compensates for one negative charge on the iridium. LEDs constructed with compounds (7) and (8) give blue-green emission with peak electroluminescent efficiency of 15 and 2 cd A⁻¹, respectively. An LED constructed with compound (5) gives a yellowish emission with peak electroluminescent efficiency of 5.5 cd A⁻¹.