Isomerization in bent phosphametallocenes: Combining rotational barriers and the intramolecular slip-inversion-slip mechanism to control stereo-conformation

Four new phosphazirconocenes, [(4,5,6-trihydro-1-phosphapentalenyl)₂ZrCl₂] (8), [(4,5,6,7-tetrahydro-1-phosphindolyl)₂ZrCl₂] (9), [(4,5,6,7,8-pentahydro-1-phosphazulenyl)₂ZrCl₂] (10), (2,4-dimethylphospholyl)₂ZrCl₂ (11), were prepared and characterized. A crystal structure of [(4,5,6,7-tetrahydro-1-phosphindolyl)₂ZrCl₂] (9), was obtained that revealed disorder (rac and meso isomers in the same crystal) and demonstrates unambiguously the meso configuration in the bent early transition metal phosphametallocenes. Lacking a spectroscopic probe of the rotational dynamics in phosphametallocenes, propylene polymerization was chosen to provide a record of the stereoselective insertions and by inference insight into the phosphametallocene dynamics. Phosphazirconocenes 8-11 were evaluated, and the rac/meso mixture of [(4,5,6,7-tetrahydro-1-phosphindolyl)₂ZrCl₂] (9) was found to produce a mixture of isotactic (iPP) and atactic (aPP) polypropylenes. ¹³C NMR spectral analysis of the stereo-errors contained in the iPP fraction indicated enantiomorphic site control as the source of the stereoselectivity. The rotational dynamics of the η⁵-phospholyl ligand of the parent phosphazirconocene, [(C₄H₄P)₂ZrCl₂], was examined as a first order surrogate of reasonable computing complexity for the catalytically active species. The phosphazirconocene was examined using a restricted Hartree-Fock method with a split basis set of 6-311+G(d,p) for C, H, Cl, P and of LANL2DZ for Zr. The rotational potential energy surface was found to be unsymmetrical with a maximum barrier of 8.8 kcal mol⁻¹, which is about an order of magnitude greater than that reported for Cp₂ZrCl₂. The projection of the calculated rotational dynamics onto a 1-phosphatetrahydroindenyl system is illustrated. This model leads to an interpretation of the polymerization results and the rotational dynamics in phosphametallocenes.     

Synthesis, characterization and reactivity of tetramethylphospholyl complexes of scandium

Reaction of 2 equiv. of (C₄Me₄P)Li(tmeda) (tmeda = tetraethylenediamine) with 1 equiv. of ScCl₃(THF)₃ gave the new compound (η⁵-C₄Me₄P)₂ScCl₂Li(tmeda) (1), which was characterized by X-ray crystallography. A phospholyl moiety in 1 is labile, as demonstrated by reactions of 1 with LiCH(SiMe₃)₂ and Cp^∗Li (Cp^∗ = C₅Me₅) to afford, respectively, (η⁵-Me₄C₄P)Sc[CH(SiMe₃)₂]Cl₂Li(tmeda) (4) and (η⁵-Me₄C₄P)Cp^∗ScCl₂Li(tmeda) (5). Attempts to generate alkyl derivatives of the general type (η⁵-C₄Me₄P)₂ScR (R = alkyl) were unsuccessful.