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

Complexes MH 2 Cl 2 (P i Pr 3 ) 2 (M = Os ( 1 ), Ru ( 1a )) promote the NH-tautomerization of 2-methylpyridine and stabilize the resulting NH-tautomer to afford the dihydrogen derivatives MCl 2 (η 2 -H 2 ){κ- C -[HNC 5 H 3 Me]}(P i Pr 3 ) 2 (M = Os ( 2 ), Ru ( 3 )), containing the heterocycle coordinated by the C α atom. In dichloromethane under reflux, complex 3 loses the coordinated hydrogen molecule to give the five-coordinate derivative RuCl 2 {κ- C -[HNC 5 H 3 Me]}(P i Pr 3 ) 2 ( 4 ). In contrast to 2-methylpyridine, the reactions of 1 and 1a with pyridine lead to MCl 2 {κ- N -[NC 5 H 5 ]} 3 (P i Pr 3 ) (M = Os ( 5 ), Ru ( 5a )), containing the heterocycle coordinated by the lone pair of the nitrogen. DFT calculations using PMe 3 as a model of P i Pr 3 show that the formation of 2 and the related quinoline complex OsCl 2 (η 2 -H 2 ){κ- C -[HNC 9 H 6 ]}(P i Pr 3 ) 2 ( 6 ) involves an intermolecular osmium to nitrogen hydrogen migration, the subsequent C α −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.