Mechanistic investigations of Cp-loss reactions in an early-late heterobimetallic complex. Isolation of a cationic tantalum-palladium compound with a free cyclopentadienyl counteranion

Article abstract of DOI:10.1021/om00017a054

This paper reports the synthesis of the early-late heterobimetallic complex Cp₂Ta(μ-CH₂)₂-PdCp (1) and its reactivity with phosphines, including the preparation, structural characterization, and reactivity of a Ta-Pd cationic complex bearing a naked cyclopentadienyl counteranion. Complex 1 was prepared by the reaction of Cp₂Ta(CH₂)(CH₃) with CpPd(C₃H₅). Treatment of 1 with 1 equiv of either PMe₃ or P(OMe)₃ in CH₂Cl₂ resulted in the formation of Cp₂Ta-(μ-CH₂)₂Pd(PR₃)(Cl) (R = Me, 2; R = OMe, 3) and 0.5 equiv of Cp₂(CH₂). Mechanistic studies indicated that the first step in these transformations is the formation of the respective bis-(phosphine) adducts [Cp₂Ta(μ-CH₂)₂Pd(PR₃) ₂]Cl. The reaction of 1 with 2 equiv of PMe₃ or P(OMe)₃ or 1 equiv of Me₂P(CH₂)₂PMe₂ (DMPE) led to the isolation of [Cp₂Ta(μ-CH₂)₂PdL₂]-Cl (L₂ = 2 PMe₃, 4; L₂ = 2 P(OMe)₃, 5; L₂ = DMPE, 6). Complex 6 was characterized by X-ray crystallography. Addition of P(OMe)₃ to 1 in CH₃CN gave the product Cp₂Ta(μ-CH₂)₂Pd-(P(OMe)₃)(CH ₂CN) (7), which again proceeded through a bis(phosphite) adduct. Each of these reactions of 1 with phosphorus compounds implicates the presence of intermediates bearing a free cyclopentadienide ligand. The complex [Cp₂Ta(μ-CH₂)₂Pd(DMPE)]Cp (8) was isolated from the reaction of 1 with DMPE in CH₃CN and was characterized by X-ray crystallography. The shortest distance between the anionic Cp and the bimetallic fragment is 3.46(3) A?; the bonding distances in the bimetallic cation are identical with those of 6. The free Cp anion in 8 undergoes slow deuterium exchange with CD₃CN solvent. Reactions of 8 with electrophilic reagents are similar to those observed for sodium cyclopentadienide. Thus, the addition of FeCl₂ to 8 resulted in the formation of 1/2 equiv of Cp₂Fe and 6. Treatment of 8 with 1,2-dibromoethane led to the quantitative formation of 1/2 equiv of spiro[2.4]hepta-4,6-diene together with the bromide salt of 8.