Inter- and Intramolecular Thermal Activation of sp³ C−H Bonds with Ruthenium Bisallyl Complexes

Complexes of type [{R 2 P(CH 2 ) n PR 2 }Ru(2-Me-all) 2 ] (2-Me-all = 2-methylpropenyl; R = Cy, n = 1−3, 5a − c; R = Me, n = 2, 6; R 2 = − (CH 2 ) 4 −, n = 2, 7 ) have been synthesized from the reaction of the corresponding electron-rich diphosphines with [(cod)Ru(2-Me-all) 2 ] ( 4 ) at 50−70 °C. The new complexes were fully characterized by multinuclear NMR spectroscopy and mass spectroscopic techniques. Reacting 4 with Cy 2 P(CH 2 ) n PCy 2 containing hydrocarbon bridges with n = 3 ( 1c ) and n = 4 ( 1d ) at 95 and 50 °C, respectively, led to [κ 2 P,P‘-{(η 3 -C 6 H 8 )CyP(CH 2 ) n PCy 2 }Ru(η 3 -C 8 H 13 )] ( n = 3, 4; 8c,d ) via intramolecular C−H bond activation and concomitant hydride transfer to cyclooctadiene. The molecular structure of 8c was unambiguously assigned by multinuclear 1D and 2D NMR spectroscopy and confirmed by single-crystal X-ray diffraction. The new complexes were tested as homogeneous catalyst precursors in thermal intermolecular C−H activation processes. In dehydrogenation of cyclooctane (coa), an initial turnover frequency of 1.9 h - 1 was observed using complex 5a under refluxing conditions without the need of a hydrogen scavenger. A maximum total number of 5 catalytic turnovers was achieved after 48 h. Ligand degradation by dehydrogenation was detected under catalytic conditions, presumably initiated via intramolecular C−H activation as in species of type 8 . Attempts to utilize complexes 5 for C−H activation in scCO 2 as the reaction medium resulted in insertion of CO 2 into the Ru−allyl moiety, yielding catalytically inactive ruthenium carboxylates.