Isomerism of [Ru(η³-allyl)Cl(CO)(PPh₃)₂]

Abstract

Treatment of [RuHCl(CO)(PPh3)3] with CH2CCMe2 produced the new allyl complex [Ru(η3-CH2CHCMe2)Cl(CO)(PPh3)2], which slowly isomerizes to [Ru(η3-CH2CMeCHMe)Cl(CO)(PPh3)2] in both solution and the solid state. While both endo and exo isomers can be observed by solution NMR for the allyl complex [Ru(η3-CH2CHCMe2)Cl(CO)(PPh3)2], only the endo isomer was detected for other related analogous allyl complexes [Ru(η3-allyl)Cl(CO)(PPh3)2] (e.g. allyl = CH2CMeCHMe, CH2CHCHPh). The isomeric behavior has been investigated by computational chemistry. The theoretical calculations show that the metal−η3-allyl interaction in an endo isomer of [Ru(η3-allyl)Cl(CO)(PR3)2] complexes is stronger than that in an exo isomer, because the structural arrangement provides an optimal situation to maximize the Ru(d)-to-CO(π*) back-bonding interaction. However, substituents at the η3-allyl ligand also play an important role in determining the relative stability. It was found that an anti substituent (with respect to the central allylic substituent) at one of the terminal carbons destabilizes the endo isomer more significantly and therefore makes the endo and exo isomers comparable in terms of their relative stabilities. A syn substituent at one of the terminal carbons is, however, found to have a negligible effect on the relative stability. The endo−exo interconversion was found to proceed by intervention of η1-allyl intermediates instead of a direct η3-allyl rotation.