NMR Spectroscopy and X‐Ray Characterisation of Cationic N‐Heteroaryl‐Pyridylamido ZrIV Complexes: A Further Level of Complexity for the Elusive Active Species of Pyridylamido Olefin Polymerisation Catalysts |
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| Authors: | Dr Gang Li Dr Cristiano Zuccaccia Dr Consiglia Tedesco Dr Ilaria D'Auria Prof?Dr Alceo Macchioni Prof?Dr Claudio Pellecchia |
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| Affiliation: | 1. Dipartimento di Chimica e Biologia, Università di Salerno via Giovanni Paolo?II 132, 84084 Fisciano (SA) (Italy);2. Dipartimento di Chimica, Biologia e Biotecnologie, Università di Perugia, via Elce di Sotto 8, 06123 Perugia (Italy) |
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| Abstract: | New (N?,N,N?)ZrR2] dialkyl complexes (N?,N,N?=pyrrolyl‐pyridyl‐amido or indolyl‐pyridyl‐amido; R=Me or CH2Ph) have been synthesised and tested as pre‐catalysts for ethene and propene polymerisation in combination with different activators, such as B(C6F5)3, Ph3C]B(C6F5)4], HNMe2Ph]B(C6F5)4] or solid AlMe3‐depleted methylaluminoxane (DMAO). Polyethylene (Mw>2 MDa and Mw/Mn = 1.3–1.6) has been produced if pre‐catalysts were activated with 1000 equivalents of DMAO (based on Al) activity >1000 kgPE (molZr] h mol atm)?1] or by using a higher pre‐catalyst concentration and a mixture of HNPhMe2]B(C6F5)4] (1 equiv) and AliBu2H (60 equiv). In the case of propene polymerisation, activity has been observed only if pre‐catalysts were treated with an excess of AliBu2H prior to addition of DMAO, which led to highly isotactic polypropylene (mmmm]>95 %). Neutral pre‐catalysts and ion pairs derived from their activation have been characterised in solution by using advanced 1D and 2D NMR spectroscopy experiments. The detection and rationalisation of intercationic NOEs clearly showed the formation of dimeric species in which some pyrrolyl or indolyl π‐electron density of one unit is engaged in stabilising the metal centre of the other unit, which relegates the counterions in the second coordination sphere. The solid‐state structure of the dimeric indolyl‐pyridyl‐amidomethylzirconium derivative, determined by X‐ray diffraction studies, points toward a weak Zr???η3‐indolyl interaction. It can be hypothesised that the formation of dimeric cationic species hampers monomer coordination (especially of less reactive α‐olefins) and that addition of AliBu2H is crucial to split the homodimers. |
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| Keywords: | dimerization NMR spectroscopy polymerization X‐ray diffraction zirconium |
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