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.

Four different pincer ligands, which prove that the disposition of the donor atoms in pincer ligands allows them to develop a marked ability to form less common coordination polyhedra and to favor unusual metal oxidation states, have been generated starting from OsH6(PiPr3)2 and 2-phenyl-6-(1H-pyrazol-3-yl)­pyridine (Hpz-py-Ph) in order to stabilize phosphorescent, pentagonal-bipyramidal osmium­(IV) emitters. Complex 1 reacts with the disubstituted heterocycle to give OsH2{κ3-N,N′,C-(pz-py-C6H4)}­(PiPr3)2 (2), as a result of the activation of a phenyl o-CH bond and the pyrazolyl NH of the pyridine substituents. The addition of methyl trifluoromethanesulfonate (MeOTf) to 2 produces the methylation of the free N atom of the pz group and the formation of the salt [OsH2{κ3-N,N′,C-(Mepz-py-C6H4)}­(PiPr3)2]­OTf (3), which undergoes deprotonation of the metal center in the presence of KOtBu. The resulting monohydride OsH­{κ3-N,N′,C-(Mepz-py-C6H4)}­(PiPr3)2 (4) is unstable and evolves into OsH2{κ3-C,N,C′-(Mepz-py-C6H4)}­(PiPr3)2 (5) as a consequence of the oxidative addition of the C–H bond at position 4 of the Mepz group to the osmium atom of 4. The addition of MeOTf to 5 produces the methylation of the free N atom of the Mepz group, which is converted into a remote N-heterocyclic carbene, to form [OsH2{κ3-C,N,C′-(Me2pz-py-C6H4)}­(PiPr3)2]­OTf (6). Complex 1 also reacts with 2-(1H-pyrazol-3-yl)­pyridine (Hpz-py). The reaction gives rise to the trihydride derivative OsH3{κ2-N,N-(pz-py)}­(PiPr3)2 (7), which leads to the salt [OsH3{κ2-N,N-(Mepz-py)}­(PiPr3)2]­OTf (8) by addition of MeOTf. The deprotonation of 8 affords the osmium­(II) dihydride OsH2{κ2-N,N-(Mepz-py)}­(PiPr3)2 (9), which is stable, in contrast to 4. Complexes 2, 3, 5, 7, and 8 are emissive in the green to red region upon photoexcitation.

The preparation of a N,N,N-osmaligand, its coordination to iridium to afford an efficient catalyst precursor, and the catalytic activity of the latter in dehydrogenation reactions of hydrogen carriers based on alcohols are reported. Complex OsH 2 Cl 2 (P i Pr 3 ) 2 (1) reacts with 3-(2-pyridyl)pyrazol to give the osmium(II) complex 2H, which contains an acidic hydrogen atom. Deprotonation of the latter by the bridging methoxy groups of the dimer [Ir(μ-OMe)(η 4 -COD)] 2 (COD = 1,5-cyclooctadiene) leads to Ir(2)(η 4 -COD) (3), where the osmaligand 2 has a free-nitrogen atom. Iridium complex 3 catalyzes the dehydrogenation of secondary and primary alcohols to ketones and aldehydes or esters, respectively, and the dehydrogenation of diols to lactones. Cyclooctatriene is detected during the catalysis by GC-MS, suggesting that the true catalyst of the reactions is a dihydride IrH 2 (2)species with the osmaligand 2 acting as N,N,N-pincer. The presence of a phenyl group in the substrates favors the catalytic processes. The dehydrogenative homocoupling of primary alcohols to esters appears to take place via the transitory formation of hemiacetals.