Structural dichotomy in single-component Ziegler catalyst systems:: Characterization of Zr ••• F and Zr ••• C-bonded structural types of group 4 metallocene [C4H6-B(C6F5)3] betaines

The organometallic Lewis acid B(C6F5)(3) adds to the terminal =CH2 group of the (butadiene)metallocene complexes 5a and 5b to give the ansa-metallocene betaine systems [Me2Si-(C5H4)(2)]Zr[C4H6-B(C6F5)(3)] (6a) and [Me2Si(3-MeC5H3)(2)] Zr[C4H6-B(C6F5)(3)] (6b), respectively, in high yield. Both complexes were characterized by X-ray diffraction. They both contain a substituted eta(3)-allyl ligand F off configuration, and they show a characteristic (ortho aryl)-C-F ... Zr interaction that stabilizes the electron-deficient metal center inside the dipolar structure. B(C6F5)(3) also adds to one butadiene terminus of (s-cis-eta(4)-C4H6) [Me2C(C5H4)-(indenyl)]Zr to give a high yield of a single isomer of the respective ansa-metallocene [C4H6-B(C6F5)(3)] betaine complex 9. The X-ray crystal structure analysis of 9 has revealed that in this case a (Z)-eta(3)-allyl-CH2B(C6F5)(3) ligand is formed. This precluded the (aryl)C-F ... Zr coordination. Instead, the zirconium center in 9 forms a stabilizing internal ion pair interaction between the negatively polarized [B]-C(4)H-2 methylene group and the positive zirconium center. The analogously structured ansa-metallocene [(Z)-C4H6-B(C6F5)(3)] betaine complex 12 is obtained in high yield from B(C6F5)(3) addition to (s-cis-eta(4)-butadiene) [Me2C-(C5H4)(fluorenyl)]Zr. In solution the complexes 6, 9, and 12 exhibit structures that are analogous to those found in the solid state. However, treatment of (butadiene) [Me2Si(C5H4)(2)]-Zr (5a) with B(C6F5)(3) under kinetic control (233 K in toluene-d(8)) quantitatively yields the [Me2Si(C5H4)(2)] Zr [(Z)-C4H6-B(C6F5)(3)] betaine isomer 13, which contains the stabilizing [B]-C(4)H-2... Zr internal ion pair interaction. Subsequent thermally induced rearrangement of the kinetic product 13 (Delta G(rearr)(not equal)(298 K) = 21.5 +/- 0.5 kcal mol(-1)) then results in the formation of the eventually observed thermodynamic ansa-metallocene betaine product 6a, that contains the (E)-C4H6-B(C6F5)(3) ligand and exhibits internal (aryl)C-F ... Zr coordination. A similar reaction sequence was observed during the addition of B(C6F5)(3) to the parent (butadiene)zirconocene system 1: at 213 K the kinetic Cp2Zr[(Z)-(1-3 eta),kappa C-4-C4H6-B(C6F5)(3)] betaine product 14 is formed, which rapidly rearranges at temperatures above 253 K to yield the previously observed stable Cp2Zr[(E)-C4H6-B(C6F5)(3)] betaine system 2, which is characterized by an internal C-F ... Zr bond. The ansa-metallocene betaines 6, 9 and 12 are all active homogeneous single-component Ziegler catalysts for ethene and propene polymerization. They are similarly effective as the usually employed ansa-metallocene dichloride/methylalumoxane catalyst systems.