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) Complexes [Ru{η³-(HCH₂)(CH₂)C₆Me₄}{(Z)-Ph₂PCH=CHPPh₂}(PMe₂Ph)]PF₆ and [Ru{η³-(HCH₂)(CH₂)C₆Me₄}(PMe₂Ph)₃]PF₆

Treatment of the various ruthenium(II) salts [Ru(ONO₂)(η⁶-1,2-dimethylarene)L₂]NO₃ and [Ru-(O₂CCF₃)(η⁶-1,2-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{η⁴-(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{η⁴-(CH₂)₂C₆H₂Me₂}L₂L′ (L = L′ = PMe₂Ph, PMePh₂), and Ru{η⁴-(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{η⁴-(CH₂)₂C₆Me₄}L₂L′ and Ru{η⁴-(CH₂)₂C₆H₄)(PMe₂Ph)₃ give (η³-benzyl)ruthenium(II) salts [Ru{η³-(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{η³-(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 P2₁/n, with a = 18.884 (3) Å,b = 18.612 (3) Å,c = 12.361 (1) Å, β = 90.40 (1)°, and Z = 4; those of 3 are monoclinic, space group C2/c, with a = 21.220 (8) Å,b = 23.412 (10) Å, c = 18.580 (7) Å, β = 126.05 (1)°, 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 ≥ 3σ) (2) and R = 0.053 and R_W = 0.076 for 5832 independent reflections (I > 3σ) (3). Both cations contain a ruthenium atom coordinated in a distorted-octahedral arrangement by a η³-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 η³-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) Å, Ru-C(2) = 2.342 (4) Å, Ru-C(1) = 2.358 (4) Å). The metrical parameters defining the agostic Ru-H-CH₂ interaction in 2 are r(Ru-C) = 2.416 (5) Å, r(Ru-H) = 1.92 (4) Å, r(C-H) = 1.01 (5) Å, and ∠C-H-Ru = 107 (3)°. The distances from ruthenium to the terminal carbon atoms in 3 (Ru-C(11) = 2.333 (9) Å, Ru-C(22) = 2.283 (10) Å) 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 (¹H, ³¹P) 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 °C; (2) reversible η³ ⇄ η¹ interconversions of the benzyl group, for which the estimated ΔG^‡ values are ca. 13 kcal/mol at 303 K for 2 and ca. 10 kcal/mol at 243 K for 3; (3) reversible C-H bond breaking in the Ru-H-CH₂ bond, for which limiting high-temperature spectra cannot be reached owing to sample decomposition. For complexes 1-3, a combination of these processes enables the RuL₃ fragment to circumnavigate the six-membered ring.