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.