Ethylene Oligomerization Using First-Row Transition Metal Complexes Featuring Heterocyclic Variants of Bis(imino)pyridine Ligands

This report examines the replacement of the imine and pyridine functionalities of the ubiquitous bis(imino)pyridine ligand with various heterocycles. The synthesis of a new class of ligand based around thiazole is described; 2,4-bis[1-(arylimino)ethyl]thiazole (aryl = Ph, 1a; Dipp, 1b) and 2,5-bis[1-(arylimino)ethyl]thiazole (aryl = Ph, 1c; Dipp, 1d) have been prepared in good yield and fully characterized. The coordination chemistry of these ligands with chromium, iron, and cobalt is explored, and the potential of these complexes as ethylene oligomerization initiators is assessed. The chromium complex 2a shows an extremely unusual alternating distribution of higher α-olefin products, which has been previously observed on only one occasion. Both series of products, C4n and C4n+2, show Schulz−Flory behavior but with distinctly different k values. The new ligand 2,5-bis[1-(phenylimino)ethyl]-1-methylpyrrole (1e) is reported along with the attempted synthesis of some corresponding iron complexes. The complexation of chromium by 2,5-bis(phenyliminomethyl)thiophene (1f) is also described, and this material was screened for ethylene oligomerization activity, detailed studies indicating that the ligand may be labile under catalytic conditions. A number of other known heterocyclic ligands incorporating pyrazolyl and benzimidazole functionalities have also been explored with iron, and for the first time their potential to facilitate ethylene oligomerization was assessed. All complexes have been tested via activation with MMAO-3A and AlEt3/[Ph3C][Al(OtBuF)4].

本研究考察了通用型双(亚胺)吡啶配体（bis(imino)pyridine）中亚胺（imine）与吡啶（pyridine）官能团被各类杂环（heterocycles）取代的情况。本文报道了一类以噻唑（thiazole）为核心骨架的新型配体的合成：2,4-双[1-(芳基亚胺基)乙基]噻唑（芳基=苯基，1a；2,6-二异丙基苯基（2,6-diisopropylphenyl，缩写Dipp），1b）与2,5-双[1-(芳基亚胺基)乙基]噻唑（芳基=苯基，1c；2,6-二异丙基苯基（2,6-diisopropylphenyl，缩写Dipp），1d）均以较高收率制得，并完成了全面的结构表征。探究了此类配体与铬、铁及钴的配位化学行为，并评估了其配合物作为乙烯齐聚（ethylene oligomerization）引发剂的应用潜力。其中铬配合物2a展现出极为罕见的高级α-烯烃产物交替分布特征，该现象此前仅被报道过一例。两类产物系列（C4n与C4n+2）均表现出舒尔茨-弗洛里行为（Schulz−Flory behavior），但二者的k值存在显著差异。本文还报道了新型配体2,5-双[1-(苯基亚胺基)乙基]-1-甲基吡咯（1e），并尝试合成了部分相应的铁配合物。此外，本文研究了2,5-双(苯基亚胺甲基)噻吩（1f）与铬的配位作用，并对该材料开展了乙烯齐聚活性筛选；详细研究显示，该配体在催化条件下可能发生解离。本文同时探索了多种含吡唑基与苯并咪唑基官能团的已知杂环配体与铁的配位作用，并首次评估了其介导乙烯齐聚的应用潜力。所有配合物均经改性甲基铝氧烷-3A（MMAO-3A）与三乙基铝（AlEt3）/[Ph3C][Al(OtBuF)4]活化后完成了性能测试。