Bis(imino)pyridine Iron Alkyls Containing β-Hydrogens: Synthesis, Evaluation of Kinetic Stability, and Decomposition Pathways Involving Chelate Participation

Bis(imino)pyridine iron alkyl complexes bearing β-hydrogens, (iPrPDI)FeR ((iPrPDI = 2,6-(2,6-iPr2−C6H3NCMe)2C5H3N; R = Et, nBu, iBu, CH2cycloC5H9; 1-R), were synthesized either by direct alkylation of (iPrPDI)FeCl (1-Cl) with the appropriate Grignard reagent or more typically by oxidative addition of the appropriate alkyl bromide to the iron bis(dinitrogen) complex, (iPrPDI)Fe(N2)2 (1-(N2)2). In the latter method, the formal oxidative addition reaction produced (iPrPDI)FeBr (1-Br), along with the desired iron alkyl, 1-R. Elucidation of the electronic structure of 1-Br and related 1-R derivatives by magnetic measurements, structural studies and NMR spectroscopy established high spin ferrous compounds antiferromagnetically coupled to chelate radical anions. Thus, the formal oxidative process is bis(imino)pyridine ligand-based (one electron is formally removed from each chelate, not the iron) during oxidative addition. The kinetic stability of each 1-R compound was assayed in benzene-d6 solution and found to produce a mixture of the corresponding alkane and alkene. The kinetic stability of the iron alkyl complexes was inversely correlated with the number of β-hydrogens present. For example, the iron ethyl complex, 1-Et, underwent clean loss of ethane over the course of three hours, whereas the corresponding 1-iBu compound had a half-life of over 12 h under identical conditions. The mechanism of the decomposition was studied with a series of deuterium labeling experiments and support a pathway involving initial β-hydrogen elimination followed by cyclometalation of an isopropyl methyl group, demonstrating an overall transfer hydrogenation pathway. The relevance of such pathways to chain transfer in bis(imino)pyridine iron catalyzed olefin polymerization reactions is also presented.

带有β-氢原子的双(亚胺基)吡啶(Bis(imino)pyridine)烷基铁配合物(iPrPDI)FeR【其中(iPrPDI)=2,6-双[(2,6-二异丙基苯基)亚胺基甲基]吡啶；R=乙基(Et)、正丁基(nBu)、异丁基(iBu)、环戊基甲基(CH2cycloC5H9)，记为1-R】，可通过两种路径合成：其一为将(iPrPDI)FeCl（记为1-Cl）与对应格氏试剂(Grignard reagent)直接进行烷基化反应；更常用的路径则是将相应烷基溴化物氧化加成(oxidative addition)至双(二氮)铁配合物(iron bis(dinitrogen) complex)(iPrPDI)Fe(N2)2（记为1-(N2)2）中。在后一种路径中，形式上的氧化加成反应会生成(iPrPDI)FeBr（记为1-Br）与目标烷基铁配合物1-R。通过磁测量、结构表征与核磁共振波谱法(NMR spectroscopy)对1-Br及相关1-R衍生物的电子结构进行解析后，确认此类配合物为高自旋亚铁化合物与螯合自由基阴离子发生反铁磁耦合的体系。由此可见，该形式氧化过程在氧化加成阶段是以双(亚胺基)吡啶配体为电子受体（形式上每个螯合配体失去一个电子，而非铁中心）。在氘代苯(benzene-d6，苯-d6)溶液中对各1-R配合物的动力学稳定性进行测试后发现，它们会分解为相应烷烃与烯烃的混合物。该类烷基铁配合物的动力学稳定性与所含β-氢原子数目呈负相关。例如，乙基铁配合物1-Et可在3小时内完全消除乙烷，而相同条件下对应的异丁基铁配合物1-iBu的半衰期则超过12小时。通过一系列氘标记实验对该分解路径进行研究后证实，其反应历程为先发生β-氢消除(β-hydrogen elimination)，再对异丙基的甲基发生环金属化(cyclometalation)，最终证明整体为转移氢化(transfer hydrogenation)路径。本文同时探讨了此类反应路径与双(亚胺基)吡啶铁催化烯烃聚合(olefin polymerization)反应中链转移过程的相关性。