Unexpected alkyl-substituent exchange during the formation of a cationic bis(zirconocene) complex that contains a planar four-co-ordinate carbon atom

Abstract

Treatment of the alkyl Group 4 metallocene cation reagent [ZrCp₂{CH₂Si(CH₃)₃}(THF)]⁺[BPh₄]^– 2b with bis(propynyl)zirconocene 1a yielded the cationic dimetallic complex [(ZrCp₂)₂(µ-η¹∶η²-CH₃CCCH₃){µ-κ²-CCCH₂Si(CH₃)₃}]⁺ 3d with BPh₄^– anion. Complex 3d was characterized spectroscopically and by a crystal structure analysis. It contains a planar four-co-ordinate carbon atom, which is stabilized by the interaction with both Group 4 metal centres. Finding the CH₂Si(CH₃)₃ substituent, that originates from the alkylzirconocene reagent, attached at the µ-acetylide ligand in the final product is unusual. A reversible alkynyl carbometallation sequence is proposed to account for the observed selective formation of 3d. Complex 3d reacted with alkyl isocyanides RNC (R = CMe₂CH₂CMe₃, CMe₃, or cyclohexyl) by replacement of the µ-η¹∶η²-CH₃CCCH₃ ligand to form the µ-isocyanide complexes [(ZrCp₂)₂(µ-η¹-C∶η²-C,N-RNC){µ-κ²-CCCH₂Si(CH₃)₂}]⁺ (with BPh₄^– anion) of which one was characterized by a crystal structure analysis.