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.