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-d 6, and H/D exchange was found to be moderately faster under thermal conditions. In a reaction with toluene, Cp2TaH3 activates the aromatic C(sp 2 )–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-d 6, 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.

Although late transition metals are well-known to activate alkane C–H bonds through oxidative addition, this mechanistic step is atypical for early transition metals. Instead, prior examples of intermolecular C(sp3)–H activation at early transition metals tend to proceed through σ-bond metathesis or 1,2-addition mechanisms. Recent theoretical work suggested that tantalocenes may be capable of activating aliphatic C–H bonds by oxidative addition. Herein, we demonstrate that monoalkyl-substituted tantalocenes R Cp2TaH3 undergo H/D exchange on the alkyl substituent “R” in the presence of C6D6, indicating that intramolecular C(sp3)–H activation takes place. Moreover, Cp2TaH3 was found to catalyze H/D exchange between H2 and octane-d 18 and methylcyclohexane-d 14, an indication of the involvement of an intermolecular C(sp3)–H activation step. Density functional theory calculations support C(sp3)–H oxidative addition at transient Ta­(III), a mechanistic step that has not been previously seen for intermolecular activation of alkanes at tantalum.