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.