Zirconocene Catalysts: Ion-pairs, Zwitterions, or Weakly Bound Molecules?

✍ Scribed by Dario Braga; Fabrizia Grepioni; Emilio Tedesco; Maria José Calhorda

• Book ID: 101299408
• Publisher: John Wiley and Sons
• Year: 2000
• Tongue: German
• Weight: 194 KB
• Volume: 626
• Category: Article
• ISSN: 0372-7874
• DOI: 10.1002/(sici)1521-3749(200002)626:2<462::aid-zaac462>3.0.co;2-a

✦ Synopsis

The bonding between electron deficient zirconocene complexes such as [(g 5 -C 5 Me 5 ) 2 ZrR] + and the counterions [(C 6 F 5 ) 3 BR] ± (R = CH 3 , 1, R = H, 2) has been investigated by DFT and extended Hu È ckel calculations. The molecular bonding analysis has been carried out in the light of the structural information on the inter-ion interactions available for these and other similar electron deficient coordinatively unsaturated zirconocene complexes. It has been shown that the ªanion-cationº interactions in 1 and 2 are different manifestations of the same need to provide electron density to the metal centre. In 1 this is achieved via the -C±H r-bonds and has been described as a pseudo-agostic interaction, whereas in 2 electron donation occurs via the phe-nyl F-atoms. The study of the covalent component of the bonding between the ionic components has shown that the weak electron donation, though not sufficient to saturate the metal centre, stabilises the unsaturated species and allows it to have a longer life time. It has been inferred that in the homogenous Ziegler-Natta catalytic process the approach of a strongly coordinating molecule, such as an olefin in the polymerisation, destroys the weak interaction and allows the reaction to proceed whilst the ion-pairing maintains the counterion in proximity of the complex.