Syntheses, structures and applications of new weakly coordinating anions to catalytic olefin polymerization.

摘要

Weakly coordinating anions have been the subject of extensive research because of their important effects on catalytic olefin polymerization. While catalytic systems involving the [BMe(C6F5) 3]- anion, formed by reacting e.g. dimethyl metallocenes with B(C6F5)3, are very useful, catalysts employing the anion [B(C6F5)4]- are generally much more effective because [B(C6F 5)4]- is more weakly coordinating. The latter anion competes less effectively with monomer molecules for active site(s) on a metal. Many attempts to form alternative weakly coordinating anions have involved the syntheses of more complex fluoroarylborate anions, which are similar but which exhibit greater delocalization of charge and hence less inclination to coordinate. This approach often involves complicated synthetic procedures, and we have successfully explored an alternative strategy. Since the [BMe(C6F5)3]- anion coordinates to metal ions via mu-methyl linkages, we hypothesized that replacing one or more of the methyl hydrogen atoms with alkyl groups should result in sterically hindered anions with much lower proclivities to bind to metal sites. We have accordingly, via reactions of B(C6F5)3 with the corresponding alkyllithium compounds, prepared a series of salts of the borate anions [BEt(C6F5)3] -, [B(iPr)(C6F5)3 ]- and [B(CH2CMe3)(C 6F5)3]-. Metathesis reactions of these with trityl chloride (Ph3CCl) and chloride salts of (Ph 3P)2N+ and PMePh3+ proceed with varying degrees of success, as shown by NMR spectroscopy and electrospray mass spectrometry, and the compounds [PPN][EtB(C6F 5)3], [PMePh3][NpB(C6F5) 3] have been characterized crystallographically. The anions assume the expected pseudotetrahedral structures with normal B-aryl bond lengths and R-B-aryl and aryl-B-aryl bond angles. A series of anilinium salts of [HNR 2R'][B(CH2CMe3)(C6F 5)3], (R, R' = Me, Ph), have been synthesized and characterized using various methods. This dissertation will discuss the syntheses and the characterization of these new borate anions, as well as their effectiveness as counteranions during olefin polymerization by cationic metallocene catalysts of the type [Cp'2ZrMe] + (Cp' = substituted cyclopentadienyl). These new cocatalysts react with Cp2ZrMe2, (Indenyl) 2ZrMe2 and SBIZrMe2, SBI = rac-dimethylsilylbis(1-indenyl), to give propylene polymerization catalysts which are significantly more active and give higher molecular weight polypropylene than do the catalysts obtained using B(C6F5)3.