Coordination of Ethane, Pentane and Cyclopentane to a Cationic Osmium complex: Comparisons in Alkane Binding

When a solution of [η⁵‑CpOs(CO)₃]⁺[Al(OC(CF₃)₃)₄]^- is photolyzed in the presence of ethane, pentane or cyclopentane, photo-liberation of carbon monoxide occurs and the corresponding metal-alkane s‑complex, [η⁵‑CpOs(CO)₂(alkane)]⁺ (where alkane = ethane, pentane and cyclopentane), forms. Here we report the NMR spectroscopic and computational investigations into the structure, reactivity, lifetimes and binding energies of the osmium-centred alkane σ‑complexes [η⁵‑CpOs(CO)₂(C₂H₆)]⁺, [η⁵‑CpOs(CO)₂(n‑C₅H₁₂)]⁺ and [η⁵‑CpOs(CO)₂(c‑C₅H₁₀)]⁺. The fragment [η⁵‑CpOs(CO)₂]⁺ binds alkanes tightly and forms remarkably stable complexes with ethane, n-pentane and cyclopentane. The effective half-life for [η⁵-CpOs(CO)₂(n-C₅H₁₂)]⁺ and [η⁵-CpOs(CO)₂(c-C₅H₁₀)]⁺ are 0.95 and 0.21 h at -50 ^oC, making these amongst the most stable metal-alkane complexes in solution reported to date. Different isomers of the n-pentane complexes are observed and the relative amount of each in solution is strongly dependent on the presence of photo-irradiation. When irradiated, the methyl bound (C1) isomer is the major product and in the absence of irradiation the system equilibrates, and the methylene bound isomers (C2 and C3) are the major products.