Cyclometalation of 6-Phenyl-2,2′-Bipyridine and Iridium: Synthesis, Characterization, and Reactivity Studies

Heating the potential tridentate 6-(4-R-phenyl)-2,2'-bipyridine ligand la (R = H), and derivatives 1b (R = CMe3) and 1c (R = OH), with IrCl3 hydrate in 2-methoxyethanol or acetone/H2O gave the unexpected bidentate cyclometalated NC dinuclear complexes [Ir(NC)Cl-2(C5H5N)](2) (2a-cPy), as the major product. Altering the ligand/metal ratio from 1:1 to 2:1 produced a mixture of bis-cyclometalated complexes, Ir(NNC)(NC)Cl (3a,b), with tridentate and bidentate binding modes. Using discrete Ir-I synthons, such as Ir(dmso)(3)Cl or [Ir(cyclooctene)(2)Cl](2), gave a complicated mixture of products. However, when [Ir(C2H4)(2)Cl](2) was used, then the desired tridentate cyclometalated Ir(NNC) complex Ir(NNC)Et(CH4)Cl (4) was synthesized cleanly. The dinuclear complex 2a-Py was converted to the corresponding mononuclear dichloride complexes Ir(NC)(NN)Cl-2 (5a) upon refluxing with 4,4'-di-tert-butylbipyridine in N,N-dimethylacetamide (DMA). Treatment of 5a with ZnMe2 gives Ir(NC)(NNtBu)MeCl (6a). Abstraction of the chloride with AgOTF yields Ir(NC)(NNtBu)MeOTf (7a). Complex 7a undergoes stoichiometric CH activation with arenes and shows catalytic activity for the H/D exchange between benzene and ( trifluoro)acetic acids.