MECHANISTIC INVESTIGATIONS OF CP-LOSS REACTIONS IN AN EARLY-LATE HETEROBIMETALLIC COMPLEX - ISOLATION OF A CATIONIC TANTALUM PALLADIUM COMPOUND WITH A FREE CYCLOPENTADIENYL COUNTERANION

This paper reports the synthesis of the early-late heterobimetallic complex CP2Ta(mu-CH2)2PdCp (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(mu-CH2)2Pd(PR3)(CI) (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(mu-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(mu-CH2)2PdL2]Cl (L2 = 2PMe3, 4; L2 = 2P(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(mu-CH2)2Pd-(P(OMe)3)(CH2CN)(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(mu-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) angstrom; 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.