Preparation of Phosphorescent Osmium(IV) Complexes with N,N′,C- and C,N,C′-Pincer Ligands

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.