The role of binuclear rhodium complexes as possible intermediates in the formation of cyclopentadienone-rhodium complexes. Reactions between (η-C₅H₅),Rh₂(CO)(CF₃C₂CF₃) and symmetrically disubstituted alkynes

Abstract

A stepwise pathway for the formation of cyclopentadienone complexes (η-C₅H₅)Rh{C₄R₄CO} from (#-C₅H₅)Rh(CO)₂ and RC≡CR has been elucidated. When solutions of (η-C₅H₅)Rh(CO)₂ are heated, there is loss of CO and (η-C₅H₅),Rh₂(CO)₃ is formed. This reacts with alkynes to give binuclear complexes such as (η-C₅H₅),Rh₂(CO)₂(CF₃C₂CF₃) gives (ηC₅H₅)₂Rh₂(CO)(Bu^tC₂Bu^t). Removal of CO from (η-C₅H₅)₂Rh₂(CO)₂(CF₃C₂CF₃) gives (η-C₅H₅)₂Rh₂(CO)(CF₃C₂CF₃) and this monocarbonyl complex reacts with more alkyne, R'C≡CR', to form the bis-alkyne complex (η-C₅H₅)₂Rh₂(CO)(CF₃C₂CF₃)(R'C₂R')(R' = SiMe₃) and then the pentadienone complex (η-C₅H₅)₂Rh₂{C₄(CF₃)₂R'₂CO} (R' = =Me, Me₃Si, CF₃). In some instances (R' = Ph, Bu^t, CO₂Me), this reaction gives equimolar amounts of the metallodiene complex (η-C₅H₅)₂Rh₂{C₄{CF₃)₂R'₂} plus the dicarbonyl complex (η-C₅H,)₂Rh₂(CO)₂(CF₃C₂CF₃). The binuclear pentadienone complexes (η-C₅H₅),Rh₂{C₄R₂R'₂CO) (R = CF, and R' = Me; or R = R' = Et) can be converted into mononuclear cyclopentadienone complexes (η-C₅H₅)Rh{C₄R₂R'₂CO} by heating in the presence of CO or alkyne, or both.