NH-Tautomerization of Quinolines and 2-Methylpyridine Promoted by a Hydride-Iridium(III) Complex: Importance of the Hydride Ligand

The reactions of the iridium complexes IrHCl2(PiPr3)2 (1) and IrCl(η2-C8H14)(PiPr3)2 (2) with quinoline, 8-methylquinoline, 2-methylpyridine and benzo[h]quinoline (Hbq) have been studied. Complex 1 promotes the NH-tautomerization of quinoline and 8-methylquinoline and stabilizes the resulting NH-tautomers to afford IrHCl2{κ-C-(HNC9H6)}(PiPr3)2 (3) and IrHCl2{κ-C-(HNC9H5CH3)}(PiPr3)2 (4), while the respective reactions of 2 lead to IrH{CH2CH(CH3)PiPr2}Cl{κ-N-(NC9H7)}(PiPr3) (5) and IrHC1(CH2C9H6N)(PiPr3)2 (6). Complex 1 also tautomerizes 2-methylpyridine and stabilizes the resulting tautomer to give IrHCl2{κ-C-(HNC5H3CH3)}(PiPr3)2 (7). However, in the presence of 2, the tautomerization does not occur. Treatment of 2 with 2-methylpyridine leads to a mixture of unidentified nontautomer derivatives. The products from the reactions of 1 with benzo[h]quinoline depend upon the metal/heterocycle ratios used. Treatment of 1 with 1.2 equiv of benzo[h]quinoline leads to the NH-tautomer derivative IrHCl2{κ-C-(HNbq)}(PiPr3)2 (8; 15%) and the metalated species IrHCl{κ-N,C-(bq)}(PiPr3)2 (9; 85%). However, when a 1:3 molar ratio is used, 9 (60%) and the salt [HNHbq][IrHCl3(PiPr3)2] (10, 40%; HNHbq = benzo[h]quinolinium) are formed. Complexes 4, 5, 6, 7, 9, and 10 have been characterized by X-ray diffraction analysis. The mechanism of the NH-tautomeration is also reported.