Protonation versus oxidation in the reactions of trifluoroacetic acid with dinuclear osmium(I) complexes: molecular structure of [Os2(MeCo2)2(µ-H)(CO)4(PMe2Ph)2][PF6]

Abstract

The room temperature reaction between trifluoroacetic acid and the osmium(I) dimer [Os₂(MeCO₂)₂(CO)₆] gives the osmium(II) compound [Os(CF₃CO₂)₂(CO)₃] as a single osmium product, which is also very easily prepared by the direction reaction of CF₃CO₂H with [Os₃(CO)₁₂] at above 160 °C. However, this acid protonates the more basic derivatives [Os₂(MeCO₂)₂(CO)₄L₂](L = PMe₂Ph, PMePh₂, PPh₃, or pyridine), to give the hydrido complexes [Os₂(MeCO₂)₂(µ-H)(CO)₄L₂]⁺, isolated as the [(CF₃CO₂)₂H]^– or [PF₆]^– salts, which are stable to an excess of CF₃CO₂H. The X-ray structure of [Os₂(MeCO₂)₂(µ-H)(CO)₄(PMe₂Ph)₂][PF₆] indicates that the overall geometry of the molecule is preserved on protonation although the Os ⋯ Os distance is increased to 3.075(1)Å. Protonation of [Os₂(MeCO₂)₂(CO)₄(PMe₂Ph)(PPh₃)] leads to only a small reduction of the phosphorus–phosphorus nuclear spin–spin coupling constant from ³J= 59.3 to ⁴J= 46.0 Hz. The direct Os–Os bond is replaced by an interaction via a bridging hydride which substantially preseves the P–P coupling.