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

Abstract

Complexes of type [{R2P(CH2)nPR2}Ru(2-Me-all)2] (2-Me-all = 2-methylpropenyl; R = Cy, n = 1−3, 5a−c; R = Me, n = 2, 6; R2 = −(CH2)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 Cy2P(CH2)nPCy2 containing hydrocarbon bridges with n = 3 (1c) and n = 4 (1d) at 95 and 50 °C, respectively, led to [κ2P,P‘-{(η3-C6H8)CyP(CH2)nPCy2}Ru(η3-C8H13)] (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 scCO2 as the reaction medium resulted in insertion of CO2 into the Ru−allyl moiety, yielding catalytically inactive ruthenium carboxylates.