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 σ-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 respectively at -50 °C, 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.