Hydrocarbon complexes of iron, ruthenium, and osmium. Part 15. Stereochemical non-rigidity of hexacarbonyldiruthenium complexes of monosubstituted cyclo-octatetraenes

Abstract

Complexes [Ru₂(CO)₅(C₈H₇R)] are obtained in good yield upon heating [Ru₃(CO)₁₂] with the cyclo-octatetraenes C₈H₇R (R = Me, Ph, SiMe₃, SiMePh₂, SiPh₃, or GeMe₃), and are readily carbonylated to give hexacarbonyls [Ru₂(CO)₆(C₈H₇R)]. Variable-temperature ¹H and ¹³C n.m.r. spectra reveal that in the latter the substituent R is attached to an unco-ordinated olefinic bond; however, its presence does not repress stereochemical non-rigidity of the kind established for the parent [Ru₂(CO)₆(C₈H₈)]. Each of the complexes [Ru₂(CO)₆(C₈H₇R)] exists at low temperatures as a single isomer, but on warming a ‘twitch’ of the Ru₂(CO)₆ unit relative to the hydrocarbon occurs to generate another isomer in equilibrium. At ambient temperatures rapid interconversion of the isomers provides time-averaged chemical shifts for the cyclo-octatetraene ring protons.