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 K
t
BuO 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.