Observation of a Tungsten Alkane σ-Complex Showing Selective Binding of Methyl Groups Using FTIR and NMR Spectroscopies

The alkane sigma-complex (HEB)W(CO)(2)-(pentane) (HEB = eta(6)-hexaethylbenzene) is produced from the UV photolysis of (HEB)W(CO)(3) in alkane solvents at low temperature. IR and H-1 and C-13 NMR spectroscopic data are reported, representing the first NMR data for a goup 6 alkane complex. Only binding of the methyl functionality of the pentane ligand was observed in (HEB)W(CO)(2)-(pentane). This contrasts with the previously reported binding of pentane to rhenium fragments, wherein both methylene and methyl groups were observed to bind, with a slight preference for binding of the former. The reason for the preference for binding through the methyl group is investigated, and the steric requirement for the pentane to adopt an unfavorable gauche conformation when bound via a methylene is identified as a contributing factor.