Experimental and Computational Evaluation of Tantalocene Hydrides for C–H Activation of Arenes

Half a century ago, tantalocene hydrides (especially Cp2TaH3, where Cp = η5-C5H5) were reported to catalyze H/D exchange with arenes. However, there has been very little follow-up to the seminal reports, and numerous questions about this chemistry remain unanswered. In an effort to better evaluate the potential of tantalocene hydrides for processes involving C–H activation, we have conducted a series of experimental and computational studies on these complexes. Density functional theory (DFT) calculations support a mechanism for arene C–H activation involving oxidative addition at transient TaIII, rather than a σ-bond metathesis mechanism at TaV. Comparisons were made between thermal and photochemical conditions for the reaction of Cp2TaH3 with benzene-d6, and H/D exchange was found to be moderately faster under thermal conditions. In a reaction with toluene, Cp2TaH3 activates the aromatic C(sp2)–H bonds but not the benzylic bonds. DFT calculations suggest that benzylic C–H activation at TaIII has a barrier similar to aromatic C–H activation, but that formation of a π-complex with Cp2TaH directs preferential aromatic C–H activation. Analogous complexes containing the less labile permethylated ligand Cp* (Cp* = η5-C5Me5) were also evaluated for their ability to catalyze H/D exchange with benzene-d6, but these complexes are less active than Cp2TaH3. DFT calculations indicate that the methyl groups of Cp* disfavor π-coordination of an arene to the TaIII intermediate.

半个世纪前，钽茂氢化物（tantalocene hydrides，尤其指Cp₂TaH₃，其中Cp = η⁵-C₅H₅）被报道可催化芳烃发生H/D交换反应。然而，此类开创性报道后续鲜有跟进研究，该类化学过程仍存在诸多尚未解决的问题。为更全面评估钽茂氢化物在涉及C-H活化的反应中的应用潜力，我们针对这类配合物开展了一系列实验与计算研究。密度泛函理论（Density functional theory, DFT）计算结果表明，芳烃C-H活化的反应机理为在瞬态三价钽（Ta(III)）中心发生氧化加成，而非在五价钽（Ta(V)）中心进行σ键复分解。我们对比了Cp₂TaH₃与氘代苯（benzene-d₆）反应的热解与光化学条件，发现热解条件下的H/D交换反应速率略快。在与甲苯的反应中，Cp₂TaH₃仅活化芳烃的C(sp²)-H键，而不活化苄基C-H键。密度泛函理论计算显示，三价钽中心上的苄基C-H活化能垒与芳烃C-H活化能垒相近，但与Cp₂TaH₃形成的π配合物会导向优先发生芳烃C-H活化。我们还评估了含有更稳定的全甲基化配体Cp*（Cp* = η⁵-C₅Me₅）的类似配合物催化氘代苯发生H/D交换的能力，结果显示这类配合物的催化活性低于Cp₂TaH₃。密度泛函理论计算表明，Cp*配体上的甲基会阻碍芳烃与三价钽中间体形成π配位作用。