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

Abstract

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 η3-allyl ligand F of E 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-η4-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)-η3-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)H2 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-η4-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-d8) quantitatively yields the [Me2Si(C5H4)2]Zr[(Z)-C4H6−B(C6F5)3] betaine isomer 13, which contains the stabilizing [B]−C(4)H2···Zr internal ion pair interaction. Subsequent thermally induced rearrangement of the kinetic product 13 (ΔG⧧rearr(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)-(1−3η),κC4-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.