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NMR Spectroscopy and X‐Ray Characterisation of Cationic N ‐Heteroaryl‐Pyridylamido Zr IV Complexes: A Further Level of Complexity for the Elusive Active Species of Pyridylamido Olefin Polymerisation Catalysts
Author(s) -
Li Gang,
Zuccaccia Cristiano,
Tedesco Consiglia,
D'Auria Ilaria,
Macchioni Alceo,
Pellecchia Claudio
Publication year - 2014
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201303021
Subject(s) - methylaluminoxane , catalysis , propene , chemistry , cationic polymerization , polymerization , coordination sphere , medicinal chemistry , tacticity , nuclear magnetic resonance spectroscopy , crystallography , stereospecificity , counterion , polymer chemistry , stereochemistry , nuclear chemistry , metallocene , organic chemistry , crystal structure , ion , polymer
New [(N − ,N,N − )ZrR 2 ] dialkyl complexes (N − ,N,N − =pyrrolyl‐pyridyl‐amido or indolyl‐pyridyl‐amido; R=Me or CH 2 Ph) have been synthesised and tested as pre‐catalysts for ethene and propene polymerisation in combination with different activators, such as B(C 6 F 5 ) 3 , [Ph 3 C][B(C 6 F 5 ) 4 ], [HNMe 2 Ph][B(C 6 F 5 ) 4 ] or solid AlMe 3 ‐depleted methylaluminoxane (DMAO). Polyethylene ( M w >2 MDa and M w / M n = 1.3–1.6) has been produced if pre‐catalysts were activated with 1000 equivalents of DMAO (based on Al) [activity >1000 kg PE (mol [Zr] h mol atm) −1 ] or by using a higher pre‐catalyst concentration and a mixture of [HNPhMe 2 ][B(C 6 F 5 ) 4 ] (1 equiv) and Al i Bu 2 H (60 equiv). In the case of propene polymerisation, activity has been observed only if pre‐catalysts were treated with an excess of Al i Bu 2 H 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⋅⋅⋅η 3 ‐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 Al i Bu 2 H is crucial to split the homodimers.