Osmium and Ruthenium Complexes Containing an N-Heterocyclic Carbene Ligand Derived from Benzo[h]quinoline

Benzo[h]quinoline (Hbq) undergoes 1,2-hydrogen shift from the carbon at the 2-position to the nitrogen in the presence of the complexes MH2Cl2(PiPr3)2 (M = Os (1), Ru (1a)). The coordination of the undressed carbon atom of the resulting NH tautomer (HNbq) to the metal center of the osmium and ruthenium complexes gives rise to the formation of the osmium-elongated dihydrogen species OsCl2{κ-C-[HNbq]}(η2-H2)(PiPr3)2 (2; H−H = 1.22 Å) and to the five-coordinate ruthenium derivative RuCl2{κ-C-[HNbq]}(PiPr3)2 (3), respectively. Under a hydrogen atmosphere, complex 3 is in equilibrium with the dihydrogen RuCl2{κ-C-[HNbq]}(η2-H2)(PiPr3)2 (4; H−H = 0.91 Å). The X-ray structure of 2 and the 1H NMR spectra of 2−4 suggest that a hydrogen bond involving the NH group of the heterocycle and a chloride ligand plays an important role in the stabilization of these complexes. Both 2 and 4 undergo deprotonation to afford species containing the usual metalated benzo[h]quinoline group (bq). Treatment of 2 with Et3N gives OsCl{κ2-N,C-[bq]}(η2-H2)(PiPr3)2 (5; H−H = 1.29 Å), whereas the reaction of 4 with the amine leads to RuCl{κ2-N,C-[bq]}(η2-H2)(PiPr3)2 (6; H−H = 1.01 Å). Complexes 5 and 6, which have been characterized by X-ray diffraction analysis, can be directly obtained by reaction of the corresponding dihydride-dichloro starting materials with benzo[h]quinoline in the presence of Et3N.