Protonation of the Heterocyclic Cp-Anion Equivalent [Pyrrolyl-B(C6F5)3]Li − Formation of a Useful Neutral Brønsted Acid for the Generation of Homogeneous Metallocene Ziegler Catalysts

N-pyrrolyllithium (1) adds to B(C6F5)3 to yield [(C4H4N)B(C6F5)3]Li (3). The corresponding monoetherate (3·OEt2) was characterized by X-ray diffraction and shows an η5 coordination of the pyrrolyl moiety to the lithium cation. Treatment of 3 with HCl in ether leads to H+ addition at a pyrrolyl α-position to yield the neutral dipolar Brønsted acid system [(C4H5N)B(C6F5)3] (4). The reagent 4 can be used to protonate Cp2Zr(CH3)3 to yield [Cp2Zr(CH3)+(C4H4N)B(C6F5)3−] (6) and methane. H+ transfer from 4 to (butadiene)zirconocene results in the formation of the (π-allyl)metallocene salt [(C4H7)ZrCp2+(C4H4N)B(C6F5)3−] (8). In analogous reactions, the neutral Brønsted acid was employed as a protonating activator component for the generation of homogeneous Ziegler catalysts derived from a variety of (butadiene)- or (dimethyl)ansa-metallocenes. The systems generated by treatment with 4 show comparable activities in ethene or propene polymerization reactions as conventionally generated Group 4 metallocene Ziegler catalysts.