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 I -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 I -coordination of an arene to the TaIII intermediate.