Double deprotonation of ruthenium(II) cations containing 1,2-dimethyl-substituted η6-arenes. Protonation of the resulting exo-coordinated (o-xylylene)ruthenium(0) complexes and X-ray crystal structures of the agostic (η3-pentamethylbenzyl)ruthenium(II)

Treatment of the various ruthenium(II) salts [Ru(ONO2)(eta-6-1,2-dimethylarene)L2]NO3 and [Ru(O2CCF3)(eta-6-1,2-dimethylarene)L2]PF6 with KO-t-Bu or (Me3Si)2NNa in the presence of a ligand L' gives o-xylylene (o-quinodimethane) complexes of zerovalent ruthenium, i.e. Ru{eta-4-(CH2)2C6Me4}L2L' (L = L' = PMe2Ph, P(CD3)2Ph, PMePh2, P(OMe)3, P(OCH2)3CMe; L2 = Ph2PCH2CH2PPh2, L' = PMe2Ph; L2 = (Z)-Ph2PCH=CHPPh2, L' = PMe2Ph, P(CD3)2Ph), Ru{eta-4-(CH2)2C6H2Me2}L2L' (L = L' = PMe2Ph, PMePh2), and Ru{eta-4-(CH2)2C6H4}(PMe2Ph)3, in good to moderate yields. In all cases the o-xylylene group is coordinated through its exo pair of double bonds. The reactions are proposed to proceed via the undetected intermediates Ru(o-xylylene)L2 (L = monodentate P-donor ligand, L2 = bidentate P-donor ligand) in which the ruthenium atom can migrate from the endo to the exo pair of double bonds before ligand L' attacks. On treatment with HPF6, Ru{eta-4-(CH2)2C6Me4}L2L' and Ru(eta-4-(CH2)2C6H4}(PMe2Ph)3 give (eta-3-benzyl)ruthenium(II) salts [Ru{eta-3-(HCH2)(CH2)C6Me4}L2L']PF6 (L = Ph2PCH2CH2PPh2, L' = PMe2Ph (1); L = (Z)-Ph2PCH=CHPPh2, L' = PMe2Ph (2), P(CD3)2Ph (2a); L = L' = PMe2Ph (3), P(CD3)2Ph (3a)) and [Ru{eta-3-(HCH2)-(CH2)C6H4}(PMe2Ph)3]PF6 (4) in which the added proton bridges the metal atom and a terminal methylene group. Crystals of 2 are monoclinic, space group P2(1)/n, with a = 18.88.4 (3) angstrom, b = 18.612 (3) angstrom, c = 12.361 (1) angstrom, beta = 90.40 (1)-degrees, and Z = 4; those of 3 are monoclinic, space group C2/c, with a = 21.220 (8) angstrom, b = 23.412 (10) angstrom, c = 18.580 (7) angstrom, beta = 126.05 (1)-degrees, and Z = 8. The structures were solved by heavy-atom methods and refined by least-squares analysis to R = 0.042 and R(w) = 0.053 for 5787 independent reflections (I greater-than-or-equal-to 3-sigma) (2) and R = 0.053 and R(w) = 0.076 for 5832 independent reflections (I > 3-sigma) (3). Both cations contain a ruthenium atom coordinated in a distorted-octahedral arrangement by a eta-3-pentamethylbenzyl group, which occupies two sites, three phosphorus atoms, and an agostic methyl hydrogen atom that has been directly located in 2 but not 3. The eta-3-benzyl interaction in 2 shows the usual asymmetry, the shortest Ru-C bond being to the terminal CH2 group (Ru-C(22) = 2.164 (5) angstrom, Ru-C(2) = 2.342 (4) angstrom, Ru-C(1) = 2.358 (4) angstrom). The metrical parameters defining the agostic Ru-H-CH2 interaction in 2 are r(Ru-C) = 2.416 (5) angstrom, r(Ru-H) = 1.92 (4) angstrom, r(C-H) = 1.01 (5) angstrom, and angle C-H-Ru = 107 (3)-degrees. The distances from ruthenium to the terminal carbon atoms in 3 (Ru-C(11) = 2.333 (9) angstrom, Ru-C(22) = 2.283 (10) angstrom) are almost equal within experimental error, in contrast with the corresponding distances in 2, and indicate that the solid-state structure of 3 is an average in which either C(11) or C(22) is protonated. Variable-temperature NMR (H-1, P-31) spectra of complexes 1, 2, 2a, 3, 3a, and 4 show these molecules to be fluxional as a consequence of three processes: (1) reversible Ru-H (agostic) bond breaking, which cannot be frozen out, even at -100-degrees-C; (2) reversible eta-3 reversible eta-1 interconversions of the benzyl group, for which the estimated DELTA-G(double dagger) values are ca. 13 kcal/mol at 303 K for 2 and ca