Influence of the Bite Angle of Dianionic C,N,C-Pincer Ligands on the Chemical and Photophysical Properties of Iridium(III) and Osmium(IV) Hydride Complexes

The study of the
reactions of IrH5(PiPr3)2 (1) with 2,6-diphenylpyridine and
2-phenoxy-6-phenylpyridine and the photophysical characterization
of some of the resulting compounds, as well as their osmium counterparts,
reveal that the bite angle of dianionic C,N,C-pincer ligands has a
marked influence on the chemical and photophysical properties of iridium­(III)
and osmium­(IV) hydride complexes. Complex 1 promotes
a double o-CH bond activation of both disubstituted
pyridines. The reaction with 2,6-diphenylpyridine directly leads to
IrH­{κ3-C,N,C-(C6H4-py-C6H4)}­(PiPr3)2 (2), whereas
the activation of 2-phenoxy-6-phenylpyridine is sequential and slower.
Initially, the phenyl activation gives IrH2{κ2-C,N-(C6H4-py-OPh)}­(PiPr3)2 (3), which subsequently evolves to IrH­{κ3-C,N,C-(C6H4-py-OC6H4)}­(PiPr3)2 (4). Complexes 2 and 4 are Brønsted bases, which react with HBF4·OEt2. The protonation of the hydride position is
elusive; the proton of the acid is selectively added to one of the
metalated substituents. Thus, the reaction with 2 affords
[IrH­{κ3-C,N,(C–H)-(C6H4-py-Ph)}­(PiPr3)2]­BF4 (5), in which
the C–H bond formed remains coordinated. The protonation of 4 selectively occurs at the phenoxy substituent to give the
five-coordinate cationic complex [IrH­{κ2-C,N-(C6H4-py-OPh)}­(PiPr3)2]­BF4 (6) bearing a free OPh group. In contrast to 5, complex 6 inserts acetylene and phenylacetylene into the Ir–H
bond to yield [Ir­{(E)-CHCHR}­{κ2-C,N-(C6H4-py-OPh)}­(PiPr3)2]­BF4 (R = H (7), Ph (8)). Complexes 2 and 4 and their osmium­(IV) counterparts OsH2{κ3-C,N,C-(C6H4-py-C6H4)}­(PiPr3)2 (A) and OsH2{κ3-C,N,C-(C6H4-py-OC6H4)}­(PiPr3)2 (B) are phosphorescent emitters, which display emission wavelengths
between 473 and 619 nm and quantum yields between 0.03 and 0.96 depending
upon the metal center and the presence or absence of an oxygen atom
in the pincer ligand.