The characterization of complexes involved in chromium catalysis is challenging due to the paramagnetism of Cr in its common oxidation states. Here, we demonstrate the utility of pulse electron paramagnetic resonance (pulse EPR) techniques in assigning structural features of Cr organometallic complexes relevant to ethylene tetramerization. An S = 3/2, Cr(III) bisaryl-methyl ethylene tetramerization precatalyst (1) has been selected for characterization by CW and pulse EPR spectroscopies. Using an isotopically labeled Cr-CD₃ complex (1-d₃), the methyl ligand was confirmed to remain bound to Cr in solution by detection of ²H couplings in X-band hyperfine sublevel correlation (HYSCORE) spectroscopy. Protonolysis of 1-d₃ led to an S = 3/2, Cr(III) product (2-d₃) that maintained spectroscopic features in HYSCORE for the CD₃ group, indicative of retention of the Cr-alkyl bond. Following protonolysis of 1-h₃ and subsequent reaction with ethylene, an S = 1/2 Cr(I) species with an ethylene-derived ligand was generated, supporting a mechanism involving this Cr oxidation state. Additionally, the pulse EPR characterization of a Cr(I) allyl-diene complex was performed for comparison. This is the first direct observation of hydrocarbyl ligands on Cr using pulse EPR methods. The methods described here are broadly applicable to Cr, first-row transition metals and other open-shell organometallic catalytic systems.

中文翻译：

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乙烯四聚催化研究中脉冲EPR表征Cr-烃基

由于Cr在其常见的氧化态中具有顺磁性，因此涉及铬催化的络合物的表征具有挑战性。在这里，我们证明了脉冲电子顺磁共振（脉冲EPR）技术在分配与乙烯四聚化有关的Cr有机金属配合物的结构特征方面的效用。选择了S = 3/2的Cr（III）双芳基-甲基乙烯四聚预催化剂（1）进行CW和脉冲EPR光谱表征。使用同位素标记的Cr-CD ₃络合物（1-d ₃），通过检测^2可确认甲基配体仍与溶液中的Cr结合X波段超细亚级相关（HYSCORE）光谱中的H耦合。1-d _3的质子分解产生S = 3/2，Cr（III）产物（2-d ₃），该产物在HYSCORE中保持CD ₃基团的光谱特征，表明Cr-烷基键的保留。在1-h ₃进行质子分解并随后与乙烯反应后，S生成了带有乙烯衍生配体的= 1/2 Cr（I）物种，支持了涉及该Cr氧化态的机理。另外，对Cr（I）烯丙基二烯配合物进行脉冲EPR表征以进行比较。这是使用脉冲EPR方法首次直接观察到Cr上的烃基配体。本文所述的方法广泛适用于Cr，第一行过渡金属和其他开壳有机金属催化体系。