Reversible carbon–nitrogen bond formation in the isomerisation of triosmium compounds derived from t-butyldiazomethane (ButCHN2); the X-ray crystal structure of [Os3H2(ButCHNNCO)(CO)8(PMe2Ph)]

Abstract

The dihydride [Os₃H₂(CO)₁₀] reacts with the diazoalkane Bu^tCHN₂ to give [Os₃H(Bu^tCHNNH)(CO)₁₀] in the same way as reported for other diazoalkanes. A compound believed to be [Os₃(µ-CH₂Bu^t)(µ-Bu^tCHNNH)(CO)₁₀] is also formed. However, [Os₃H₂(CO)₉(PMe₂Ph)] reacts with Bu^tCHN₂ to give a good yield of a novel type of compound [Os₃H₂(Bu^tCHNNCO)(CO)₈(PMe₂Ph)](X-ray structure reported). Instead of Bu^tCHN₂ addition being followed by insertion into an Os–H bond, the Bu^tCHN₂ molecule couples with CO to give the µ₃-isocyanate ligand RNCO (R = Bu^tCHN) bonded to the Os₃ triangle through N and C atoms. Protonation with CF₃CO₂H leads by C–N cleavage to [Os₃H₂(Bu^tCHNNH)(CO)₉(PMe₂Ph)]⁺. On deprotonation of this cation, the C–N bond reforms to regenerate the Bu^tCHNNCO compound, but the isomer [Os₃H(Bu^tCHNNH)(CO)₉(PMe₂Ph)] is also formed. This monohydride isomerises thermally to an isomer with the same formulation that is also formed by thermal substitution of [Os₃H(Bu^tCHNNH)(CO)₁₀]. In all, three isomers have been identified, two differing in the positions of PMe₂Ph substitution which are clearly established by J_PH values for the hydrido-ligands.