Synthesis of Cp‘Ru(CO)(L)(η2-olefin)+ Complexes and Kinetic Studies of Olefin Substitution

The ruthenium(II)−olefin complexes [Cp‘Ru(CO)(L)(η2-olefin)]+ (Cp‘ = Cp, Cp*; L = CO, PPh3; olefin = methyl oleate, cis-3-hexene, trans-3-hexene, 1,4-pentadiene) have been synthesized and characterized by IR and 1H and 13C NMR spectroscopy. An X-ray structure of [CpRu(CO)(PPh3)(η2-cis-3-hexene)]+ shows that the olefinic bond is nearly parallel to the plane of the Cp ligand. The olefins in [CpRu(CO)2(η2-trans-3-hexene)]+ and [Cp‘Ru(CO)2(η2-trans-3-hexene)]+ rotate rapidly about the Ru−olefin bond, even at −30 °C, as established by the presence of a single methyl signal for the olefin in the 1H NMR spectrum, whereas olefin rotation in [CpRu(CO)(PPh3)(η2-trans-3-hexene)]+ is slow on the 1H NMR time scale at −25 °C. The [CpRu(CO)(PPh3)]+ unit exhibits a unique diastereoselectivity by binding to only one face of trans-3-hexene, due to steric repulsion between the ethyl groups of the trans-3-hexene and the bulky Cp and PPh3 ligands. Kinetic studies of the substitution of the olefin in [Cp'Ru(CO)2(η2-olefin)]+ by PPh3 show that the lability of the large methyl oleate is similar to that of the smaller cis-3-hexene. Replacement of Cp by Cp* and of cis-3-hexene by trans-3-hexene increases substantially the rate of olefin substitution, due to an increase in steric repulsion.