Ortho-Metalated Ruthenium Hydrido Dihydrogen Complexes:  Dynamics, Exchange Couplings, and Reactivity

A series of ortho-metalated ruthenium hydrido dihydrogen complexes of the form RuH(H2)(X)(PiPr3)2 (X = 2-phenylpyridine (ph-py) (2), benzoquinoline (bq) (3), phenylpyrazole (ph-pz) (4)) were prepared by adding 1 equiv of the corresponding substrate X−H and 2 equiv of PiPr3 to Ru(COD)(COT) under 3 bar of H2. The analogous complex RuH(H2)(ph-py)(PCy3)2 (1) was prepared by adding 1 equiv of 2-phenylpyridine to RuH2(H2)2(PCy3)2. The ortho-metalated X ligand is coordinated to ruthenium via the nitrogen of one ring and the ortho carbon of a second ring as a result of C−H activation. 2 and 3 were characterized by X-ray diffraction. They are the first examples of complexes displaying exchange couplings between the hydride and the dihydrogen ligands. NMR studies and DFT calculations are used to understand this phenomenon. The series of ortho-metalated model complexes RuH(H2)(X)(PH3)2 was investigated by DFT at the B3PW91 level. The coherent (quantum-mechanical) as well as the incoherent (classical) exchange rates have been determined by line shape analysis, and the activation energies hardly depend on the nature of the X ligand: E a(coherent) = ca. 10 kJ mol-1, E a(incoherent) = ca. 40 kJ mol-1 (ca. 50 kJ mol-1 by DFT). The corresponding transition states (2q TSQEC −4q TSQEC ) for the classical exchange process have been located by DFT at the B3PW91 level. The dihydrogen ligand is now trans to N and perpendicular to the plane of the chelating ligand. These states connect to the isomer with the dihydrogen trans to C through coupled rotation of H2 and proton transfer from H2 to H. The dihydrogen ligand can be substituted easily, and the corresponding complexes RuH(L)(ph-py)(PiPr3)2 with L = N2 (5), O2 (6), CO (7), C2H4 (8) have been isolated and fully characterized by NMR and by crystal structure analyses in the case of 6 and 8. The model systems RuH(L)(ph-py)(PH3)2 (5q−8q) have been optimized at the DFT level (B3PW91). In the case of 8, we could not detect any ethylene insertion in either the Ru−H or the Ru−C bond. Theoretical calculations explain the differences we observed with the Murai type catalysts, which are highly reactive to ethylene insertion.