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

Abstract

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.

C(sp3)–H Oxidative Addition at Tantalocene Hydrides

“…Each complex shown in Figure was subjected to heating at 100 °C in benzene- d 6 (Table ). Under these conditions, tantalocenes are known to effect H/D exchange between the metal and the deuterated solvent, leading to replacement of the tantalum hydrides with deuterium. ,, We tracked the rates of H/D exchange by monitoring the disappearance of the hydride signals by 1 H NMR relative to an internal standard (cyclohexane). Furthermore, when a hydride on tantalum is replaced with deuteride, changes to the splitting pattern of the neighboring hydrides can be observed, and these changes served as qualitative verification that the decreased level of integration of the hydride signals is due, at least in part, to H/D exchange rather than some other type of decomposition.…”

“…We undertook DFT calculations to analyze the energetics of benzene activation by these complexes (Table ). The DFT method (see the footnotes of Table and the Supporting Information for details) was chosen on the basis of benchmarking reported in our previous study of the reaction of Cp 2 TaH 3 with arenes . An oxidative addition mechanism was calculated, as previously reported .…”

“…The DFT method (see the footnotes of Table and the Supporting Information for details) was chosen on the basis of benchmarking reported in our previous study of the reaction of Cp 2 TaH 3 with arenes . An oxidative addition mechanism was calculated, as previously reported . This mechanism involves initial loss of H 2 from R Cp 2 TaH 3 to generate a high-energy Ta­(III) monohydride A .…”

“…However, in cases of intermolecular activation of aliphatic C–H bonds promoted by early transition metals, C–H cleavage tends to take place through a σ-bond metathesis or related mechanism , or through 1,2-addition across metal–ligand multiple bonds. , In these reactions, there is no change to the oxidation state of the high-valent (typically d 0 ) metal. However, recent computational studies in our group hinted that tantalocenes may be capable of activating aliphatic C–H bonds through an oxidative addition pathway . Herein we describe experimental studies, together with additional computational work, demonstrating that tantalocene hydrides can activate C (sp3) –H bonds in both an intra- and intermolecular fashion.…”

“…Methyl lithium (1.6 M in Et 2 O) and n -butyl lithum (2.5 M in hexanes) were obtained from Sigma-Aldrich in Sure/Seal bottles and used as received. Cp 2 TaH 3 ( 1 ) was prepared according to a literature procedure . Unless otherwise noted, all operations were performed in a nitrogen-filled glovebox or using air-free techniques in oven-dried glassware.…”