Coordination of ethane, pentane and cyclopentane to a cationic osmium complex: comparisons in alkane binding

Abstract

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.