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

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 dif- ferent 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 1000equivalents of DMAO (based on Al) [activity > 1000 kgPE (mol[Zr] h mol atm)1] or by using a higher pre-cat- alyst 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-cata- lysts were treated with an excess of AliBu2H prior to addition of DMAO, which led to highly isotactic polypropylene Introduction Following the metallocene era of olefin polymerisation, a varie- ty of structurally diverse metal complexes with polydentate li- gands based on nitrogen and/or oxygen and/or sulfur donors have been explored, which sometimes resulted in catalysts that displayed unprecedented features.[1] One of the most suc- cessful class of stereoselective catalysts, also recently intro- duced in commercial production, is based on C1-symmetric ar- ylpyridylamido HfIV complexes (N,N,CAryl)HfR2 (I).[2] Discovered and optimised through high-throughput parallel screening techniques, they have shown outstanding perfor- mance for the production of highly isotactic and high-molecu- lar-weight polypropylene[2] and specialty polyolefins by using solution processes by means of coordinative chain transfer polymerisation.[3] The performance of these catalysts has been [a] Dr. G. Li, Dr. C. Tedesco, Dr. I. D’Auria, Prof. Dr. C. Pellecchia Dipartimento di Chimica e Biologia, Università di Salerno via Giovanni Paolo II 132, 84084 Fisciano (SA) (Italy) E-mail: cpellecchia@unisa.it [b] Dr. G. Li, Dr. C. Zuccaccia, Prof. Dr. A. Macchioni Dipartimento di Chimica, Biologia e Biotecnologie Università di Perugia, via Elce di Sotto 8, 06123 Perugia (Italy) E-mail: alceo@unipg.it ([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 clear- ly showed the formation of dimeric species in which some pyrrolyl or indolyl p-electron density of one unit is engaged in stabilising the metal centre of the other unit, which rele- gates the counterions in the second coordination sphere. The solid-state structure of the dimeric indolyl-pyridyl-ami- domethylzirconium derivative, determined by X-ray diffrac- tion studies, points toward a weak Zr···h3-indolyl interaction. It can be hypothesised that the formation of dimeric cationic species hampers monomer coordination (especially of less reactive a-olefins) and that addition of AliBu2H is crucial to split the homodimers.