Dissociative and Nondissociative Pathways in the endo to exo Isomerization of Tetramethyl-o-xylylene Complexes of Ruthenium and Osmium, ML₃{η⁴-o-C₆Me₄(CH₂)₂} (M = Ru, L = PMe₃; M = Os, L = PMe₃, PMe₂Ph). Formation of Hexamethylbenzene-1,2-diyl Complexes by Ligand Addition to the exo-Osmium Complexes

Treatment of the (η⁶-hexamethylbenzene)ruthenium(II) and -osmium(II) salts [M(O₂CCF₃)L₂(η⁶-C₆Me₆)]PF₆ (M = Ru, L = PMe₃; M = Os, L = PMe₃, PMe₂Ph) in the presence of L with KO-t-Bu gives exclusively the endo- (tetramethyl-o-xylylene)metal(0) complexes ML₃{η⁴-endo-o-C₆Me₄(CH₂)₂}, endo-1, -2, and -3, respectively, in high yield; these are protonated by an excess of triflic acid (CF₃SO₃H, TfOH) to give the (η⁶-hexamethylbenzene)metal(II) salts [ML₃(η⁶-C₆Me₆)](OTf)₂ [M = Ru, L = PMe₃ (4); M = Os, L = PMe₃ (5); M = Os, L = PMe₂Ph (6)). Complexes 4−6 revert to endo-1−3 on treatment with KO-t-Bu, whereas for M=Ru, L=PMe₂Ph the complexes [ML₃(η⁶-C₆Me₆)]²⁺ and [M(O₂CCF₃)L₂(η⁶-C₆Me₆)]⁺/L react with KO-t-Bu to give exclusively the exo isomer, Ru(PMe₂Ph)₃{η⁴-exo-o-(CH₂)₂C₆Me₄} (exo-7). The endo complexes 1−3 are converted quantitatively into the corresponding exo isomers in toluene in the temperature range 65−106 °C, the process being first order in endo complex. Kinetics studies in the presence of PMe₃ (for 1 and 2) or PMe₂Ph (for 3) indicate that two pathways are available:  one depends on initial dissociation of L and proceeds through a bis(ligand) intermediate or intermediates, e.g., ML₂{endo-o-C₆Me₄(CH₂)₂} and ML₂{exo-o-(CH₂)₂C₆Me₄}, and the other does not. The dissociative mechanism is predominant for M = Ru, L = PMe₃, whereas the nondissociative or direct mechanism plays the dominant, possibly exclusive, role for M = Os, L = PMe₃. The osmium(0) compound exo-2 adds PMe₃ irreversibly to give the σ-bonded (hexamethylbenzene-1,2-diyl)osmium(II) complex Os(PMe₃)₄{κ²-o-(CH₂)₂C₆Me₄} (8), whereas the corresponding PMe₂Ph derivative 9 is in equilibrium with exo-3 and PMe₂Ph and cannot be isolated; the ruthenium(0) compound exo-1 is inert toward PMe₃. Density functional calculations on the model compounds ML₃{η⁴-exo-o-(CH₂)₂C₆H₄} and ML₄{κ²-o-(CH₂)₂C₆H₄}(M = Ru, Os; L = PH₃) correctly reflect the observed stability order Os > Ru for the diyl complex but predict the latter to be more stable than the η⁴ complex for both elements. In this case, the usual computational simplification of replacing a tertiary phosphine by PH₃ is probably unjustified. The molecular structures of the η⁴ complexes endo-3, exo-3, and exo-1 and of the diyl complex 8 have been determined by X-ray crystallography. The endo- to exo-o-xylylene isomerizations are compared with the intramolecular migrations that occur in Fe(CO)₃(η⁴-polyene) and Cr(CO)₃(η⁶-substituted-naphthalene) complexes.