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(C 6 F 5 ) 3 adds to the terminal CH 2 group of the (butadiene)metallocene complexes 5a and 5b to give the ansa -metallocene betaine systems [Me 2 Si(C 5 H 4 ) 2 ]Zr[C 4 H 6 −B(C 6 F 5 ) 3 ] ( 6a ) and [Me 2 Si(3-MeC 5 H 3 ) 2 ]Zr[C 4 H 6 −B(C 6 F 5 ) 3 ] ( 6b ), respectively, in high yield. Both complexes were characterized by X-ray diffraction. They both contain a substituted η 3 -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(C 6 F 5 ) 3 also adds to one butadiene terminus of ( s - cis -η 4 -C 4 H 6 )[Me 2 C(C 5 H 4 )(indenyl)]Zr to give a high yield of a single isomer of the respective ansa -metallocene [C 4 H 6 −B(C 6 F 5 ) 3 ] betaine complex 9 . The X-ray crystal structure analysis of 9 has revealed that in this case a ( Z )-η 3 -allyl-CH 2 B(C 6 F 5 ) 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 )-C 4 H 6 −B(C 6 F 5 ) 3 ] betaine complex 12 is obtained in high yield from B(C 6 F 5 ) 3 addition to ( s - cis -η 4 -butadiene)[Me 2 C(C 5 H 4 )(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)[Me 2 Si(C 5 H 4 ) 2 ]Zr ( 5a ) with B(C 6 F 5 ) 3 under kinetic control (233 K in toluene- d 8 ) quantitatively yields the [Me 2 Si(C 5 H 4 ) 2 ]Zr[( Z )-C 4 H 6 −B(C 6 F 5 ) 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 (Δ G ⧧ 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 )-C 4 H 6 −B(C 6 F 5 ) 3 ligand and exhibits internal (aryl)C−F···Zr coordination. A similar reaction sequence was observed during the addition of B(C 6 F 5 ) 3 to the parent (butadiene)zirconocene system 1: at 213 K the kinetic Cp 2 Zr[( Z )-(1−3η),κ C 4 -C 4 H 6 −B(C 6 F 5 ) 3 ] betaine product 14 is formed, which rapidly rearranges at temperatures above 253 K to yield the previously observed stable Cp 2 Zr[( E )-C 4 H 6 −B(C 6 F 5 ) 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.