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

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(III)Cl3 in the presence of Ag(I)OC(O)CF3 resulted in the synthesis of tris-cyclometalated complexes C--N]2IrC--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(mu-Cl)2IrCN]2 (6), C--N]2Ireta2-(O(C((t)Bu))2CH] (7), and C--N]2Ir(mu-O-P(OMe)2-O)2IrC--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 CC pyridinium ligand, was found to be the shortest to date (1.977(4) angstroms) 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).