Internally Phosphine-Stabilized Zirconocene Cations Employing Substituted ((Diarylphosphino)methyl)cyclopentadienyl Ligand Systems

The dimethylzirconocene complex [(Cp−CMe 2 −PAr 2 ) 2 ZrMe 2 ], 2a (Ar = p -tolyl), was treated with 1 molar equiv of B(C 6 F 5 ) 3 to yield the salt [(Cp−CMe 2 −PAr 2 ) 2 ZrMe + MeB(C 6 F 5 ) 3 - ], 3a . The X-ray crystal structure analysis of 3a shows that both −PAr 2 units are intramolecularly coordinated to zirconium in a close to C 2 -symmetric arrangement with the [Zr]−CH 3 group placed in the central position in the bent metallocene σ-ligand plane. Treatment of 2a with 2 equiv of B(C 6 F 5 ) 3 generates the highly reactive dication system [(Cp−CMe 2 −PAr 2 ) 2 Zr 2+ ] ( 4 with two MeB(C 6 F 5 ) 3 - anions). The highly electrophilic cation 4 abstracts chloride from, e.g., dichloromethane solvent to yield [(Cp−CMe 2 −PAr 2 ) 2 Zr-Cl + ] ( 5, with MeB(C 6 F 5 ) 3 - anion). The same cation ( 5 ‘, with ClB(C 6 F 5 ) 3 - anion) was obtained from the reaction of [(Cp−CMe 2 −PAr 2 ) 2 ZrCl 2 ] ( 1a ) with B(C 6 F 5 ) 3 . The C 2 -symmetric, internally −PAr 2 -stabilized dication adds acetonitrile or 2,6-dimethylphenyl isocyanide to give the respective C 2 -symmetric donor-ligand adducts in which the −PAr 2 coordination to zirconium is retained. The complexes 3a (Δ G ⧧ enant (300 K) = 14.0 ± 0.5 kcal/mol) and 5 (Δ G ⧧ enant (360 K) = 17.5 ± 0.5 kcal/mol) show dynamic NMR spectra due to an intramolecular enantiomerization process proceeding with a rate-determining cleavage of the Zr−P linkages. Complex 5 was also characterized by an X-ray crystal structure analysis.