Hydrocarbon complexes of iron, ruthenium, and osmium. Part 12. Triruthenium complexes of pentalene derived from cyclo-octatetraenes and cyclo-octatrienes

Abstract

Cyclo-octatetraenes C₈H₇R (R = H, Me, Ph, or SiMe₃) and cyclo-octatrienes C₈H₈,(SiMe₃)₂ and C₈H₇(SiMe₃)₃ react with [Ru₃(CO)₁₂] or [Ru₂(SiMe₃)₂(CO)₈] to give, among other products, triruthenium pentalene compounds [Ru₃(CO)₈(pentalene)]. Complexes containing unsubstituted or symmetrically substituted pentalene exist as two isomers in which the hydrocarbon is either edge- or face-bonded with respect to an Ru₃ triangle. Hydrogen-1 n.m.r. studies establish that in solution these isomers are in equilibrium, with the predominant edge-bonded form also undergoing an oscillatory fluxional process. The combined effect of these processes is to achieve migration of the pentalene ligand over the faces and edges of the triruthenium cluster. Unsymmetrically substituted pentalene complexes exist exclusively as edge-bonded isomers which do not undergo dynamic behaviour. Trifluoroacetic acid readily cleaves trimethylsilyl groups from complexed pentalene, but other attempted electrophilic substitution reactions have been unsuccessful, as were attempts to isolate pentalenes or pentalene dimers by oxidation of the complexes. Treatment of [Ru₃(CO)₈{C₈H₃(SiMe₃)₃-1,3,5}] with H₂(1 atm) gave a tetraruthenium complex [Ru₄H₂(CO)₉{C₈H₃(SiMe₃)₃}].