American Chemical Society
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Dissociative and Nondissociative Pathways in the endo to exo Isomerization of Tetramethyl-o-xylylene Complexes of Ruthenium and Osmium, ML34-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

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journal contribution
posted on 1998-07-21, 00:00 authored by Martin A. Bennett, Mark Bown, David C. R. Hockless, John E. McGrady, Harold W. Schranz, Robert Stranger, Anthony C. Willis
Treatment of the (η6-hexamethylbenzene)ruthenium(II) and -osmium(II) salts [M(O2CCF3)L26-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 ML34-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 [ML36-C6Me6)](OTf)2 [M = Ru, L = PMe3 (4); M = Os, L = PMe3 (5); M = Os, L = PMe2Ph (6)). Complexes 46 revert to endo-13 on treatment with KO-t-Bu, whereas for M=Ru, L=PMe2Ph the complexes [ML36-C6Me6)]2+ and [M(O2CCF3)L26-C6Me6)]+/L react with KO-t-Bu to give exclusively the exo isomer, Ru(PMe2Ph)34-exo-o-(CH2)2C6Me4} (exo-7). The endo complexes 13 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)42-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 ML34-exo-o-(CH2)2C6H4} and ML42-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)34-polyene) and Cr(CO)36-substituted-naphthalene) complexes.