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

Abstract

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.

Nichtkoordinierende Anionen – Traum oder Wirklichkeit? Eine Übersicht zu möglichen Kandidaten

“…Eine verwandte neutrale Brønsted‐Säure ist das dipolare Pyrroliumborat (H 5 C 4 N)B(C 6 F 5 ) 3 , das [Cp 2 ZrMe 2 ] unter Freisetzung von Methan zu [Cp 2 ZrMe] + [(H 4 C 4 N)B(C 6 F 5 ) 3 ] − protoniert [Gl. (22)] 149 …”

Noncoordinating Anions—Fact or Fiction? A Survey of Likely Candidates

“…A related neutral Brønsted acid is the dipolar pyrroliumborate (H 5 C 4 N)B(C 6 F 5 ) 3 that protonates [Cp 2 ZrMe 2 ] with liberation of methane and formation of [Cp 2 ZrMe] + [(H 4 C 4 N)B(C 6 F 5 ) 3 ] − [Eq. (22)] 149 …”

Synthesis and Reactivity of (C6F5)3BN‐heterocycle Complexes, 2

“…Erker has shown that the products formed from the reaction between the pyrrole complex 1 and different dimethylmetallocene complexes are active propylene polymerization catalysts 16. We show here that the B‐N complexes obtained from the five‐membered aromatic NH heterocycles described above are also efficient stoichiometric activators for both ethylene polymerization and ethylene‐propylene copolymerization, in some cases better than MAO (at Al/Zr = 500), and clearly better than the known dimethylmetallocene‐B(C 6 F 5 ) 3 catalysts 14a,b…”

“…Erker and coworkers reported the synthesis of the 5 H ‐pyrrole · B(C 6 F 5 ) 3 complex 1 by protonation of the Li salt of the [B(pyrrolyl)(C 6 F 5 ) 3 ] − borate (Scheme , top) and have shown that this complex can be used as a cocatalyst for metallocene‐catalyzed olefin polymerization. The methylene group in 5 is sufficiently acidic to protonate a CH 3 of the metallocene dimethyl catalyst with the formation of methane, the [B(C 6 F 5 ) 3 (1‐pyrrolyl)] borate anion and the metallocenium methyl cation 16…”