CCC–Pincer–NHC Osmium Complexes: New Types of Blue-Green Emissive Neutral Compounds for Organic Light-Emitting Devices (OLEDs)

Novel homoleptic and heteroleptic NHC carbene containing bis­(tridentate) osmium­(II) complexes have been designed, synthesized, and characterized, and their photophysical properties have been studied. The complex OsH6(PiPr3)2 (1) reacts with the tetrafluoroborate salts of 1,3-bis­(3-methylbenzimidazolium-1-yl)­benzene and 1,3-bis­(3-methylimidazolium-1-yl)­benzene, in dimethylformamide, under reflux to afford the hydride–carbonyl derivatives [OsH­(CNHCCarylCbenzimidazolium)­(CO)­(PiPr3)2]­BF4 (2) and [OsH­(CNHCCarylCimidazolium)­(CO)­(PiPr3)2]­BF4 (3), as a result of the direct metalation of one of the NHC units of the salts, the activation of the C–H bond at the 6-position of the bridged aryl group, and metal carbonylation by solvent decarbonylation. In contrast to the BF4 salts, under the same conditions, the iodide salts of 1,3-bis­(3-methylbenzimidazolium-1-yl)­benzene, 1,3-bis­(3-methylimidazolium-1-yl)­benzene, and 1,3-bis­(3-methylbenzimidazolium-1-yl)-5-trifluoromethylbenzene undergo direct metalation of both NHC units and C–H bond activation of the bridged aryl group at the 2-position to give the respective osmium­(IV) dihydrides [OsH2(CNHCCarylCNHC)­(PiPr3)2]I (4a–6a), which by deprotonation with KtBuO yield the osmium­(II) monohydrides OsH­(CNHCCarylCNHC)­(PiPr3)2 (7–9). The reactions of 7 with 1,3-bis­(3-methylbenzimidazolium-1-yl)­benzene tetrafluoroborate and of 9 with 1,3-bis­(3-methylbenzimidazolium-1-yl)-5-trifluoromethylbenzene tetrafluoroborate lead to the homoleptic derivatives Os­(CNHCCarylCNHC)2 (10, 11), whereas the reactions of 9 with the tetrafluoroborate salts of 1,3-bis­(3-methylbenzimidazolium-1-yl)­benzene and 1,3-bis­(3-methylimidazolium-1-yl)­benzene generate heteroleptic Os­(CNHCCarylCNHC)­(CNHCCaryl′CNHC) (12) and Os­(CNHCCaryl′CNHC)­(CNHC′CarylCNHC′) (13). Treatment of 7 with 3,5-bis­(3-methylbenzimidazolium-1-yl)-2,6-dimethylpyridine tetrafluoroborate affords the salt [Os­(CNHCCarylCNHC)­(CNHCCaryl′CNHC)]­BF4 (14), with Caryl′ being a pyridinium group. Its deprotonation generates the neutral heteroleptic derivative Os­(CNHCCarylCNHC)­(CNHCCaryl′CNHC) (15). Complexes 10–13 and 15 are emissive in the blue-green spectral region with high quantum yields in the solid state, which reach 0.62 for 11. OLEDs using this compound as an emitting material show blue emission (CIE coordinates: (0.14, 0.26)). The brightness of the device reaches 10000 cd/m2 at 9.5 V. The maximum external quantum efficiency (EQE) was 19.2% at 1000 cd/m2.