NH-Tautomerization of 2-Substituted Pyridines and Quinolines on Osmium and Ruthenium: Determining Factors and Mechanism

Complexes MH2Cl2(PiPr3)2 (M = Os (1), Ru (1a)) promote the NH-tautomerization of 2-methylpyridine and stabilize the resulting NH-tautomer to afford the dihydrogen derivatives MCl2(η2-H2){κ-C-[HNC5H3Me]}(PiPr3)2 (M = Os (2), Ru (3)), containing the heterocycle coordinated by the Cα atom. In dichloromethane under reflux, complex 3 loses the coordinated hydrogen molecule to give the five-coordinate derivative RuCl2{κ-C-[HNC5H3Me]}(PiPr3)2 (4). In contrast to 2-methylpyridine, the reactions of 1 and 1a with pyridine lead to MCl2{κ-N-[NC5H5]}3(PiPr3) (M = Os (5), Ru (5a)), containing the heterocycle coordinated by the lone pair of the nitrogen. DFT calculations using PMe3 as a model of PiPr3 show that the formation of 2 and the related quinoline complex OsCl2(η2-H2){κ-C-[HNC9H6]}(PiPr3)2 (6) involves an intermolecular osmium to nitrogen hydrogen migration, the subsequent Cα−H bond activation of the protonated heterocycle, and the dihydride−dihydrogen tautomerization of the resulting dihydride. The structures of 2 and 5 have been determined by X-ray diffraction analysis.