Inter- and intramolecular thermal activation of sp3 C-H bonds with ruthenium bisallyl complexes

Complexes of type [{R2P(CH2)(n)PR2}Ru(2-Me-all)(2)] (2-Me-all = 2-methylpropenyl; R = Cy, n = 1-3, 5a-c; R = Me, n = 2, 6; R-2 = -(CH2)(4)-, n = 2, 7) have been synthesized from the reaction of the corresponding electron-rich diphosphines with [C(cod)Ru(2-Me-all)(2)] (4) at 50-70 degrees C. The new complexes were fully characterized by multinuclear NMR spectroscopy and mass spectroscopic techniques. Reacting 4 with Cy2P(CH2)(n)PCy2 containing hydrocarbon bridges with n = 3 (1c) and n = 4 (1d) at 95 and 50 degrees C, respectively, led to [kappa(2)P,P'-{(eta(3)-C6H8)CyP(CH2)nPCy(2)}Ru(eta(3)-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 scCO(2) as the reaction medium resulted in insertion of CO2 into the Ru-allyl moiety, yielding catalytically inactive ruthenium carboxylates.