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((PPr3)-Pr-i)(2) (1) and IrCl(eta(2)-C8H14)((PPr3)-Pr-i)(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{kappa-C-(HNC9H6)}((PPr3)-Pr-i)(2) (3) and IrHCl2{kappa-C-(HNC9H5CH3)}((PPr3)-Pr-i)(2) (4), while the respective reactions of 2 lead toIrH{CH2CH(CH3)(PPr2)-Pr-i}Cl{kappa-N-(NC9H7)}((PPr3)-Pr-i) (5) and IrHCl(CH2C9H6N)((PPr3)-Pr-i)(2) (6). Complex 1 also tautomerizes 2-methylpyridine and stabilizes the resulting tautomer to give IrHCl2 {kappa-C-(HNC5H3CH3)} ((PPr3)-Pr-i)(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 I 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{kappa-C-(HNbq))((PPr3)-Pr-i)(2) (8; 15%) and the metalated species IrHCl{kappa-N,C-(bq))((PPr3)-Pr-i)(2) (9; 85%). However, when a 1:3 molar ratio is used, 9 (60%) and the salt [HNHbq][IrHCl3((PPr3)-Pr-i)(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.