Tantalocene trihydride

Tantalocene trihydride, or bis(η5-cyclopentadienyl)trihydridotantalum, is an organotanalum compound in the family of bent metallocenes consisting of two cyclopentadienyl rings and three hydrides coordinated to a tantalum center. Its formula is TaCp2H3, and it is a white crystalline compound that is sensitive to air. It is the first example of a molecular trihydride of a transition metal.

Synthesis
The synthesis of tantalocene trihydride was first reported by Green, McCleverty, Pratt, and Wilkinson in 1961. Tantalum pentachloride was added to a solution of sodium cyclopentadienide in tetrahydrofuran and an excess of sodium borohydride with yields reaching 60%, although the authors report that the preparation does not always succeed. A more reliable and reproducible method was reported by Green and Moreau in 1978. A suspension of tantalocene dichloride in toluene was reacted with NaAlH2(OCH2CH2OCH3)2 and then hydrolyzed to form tantalocene trihydride, though with a lower yield of 42%.

Characterization
The high-field signals in the 1H NMR spectrum corresponding to the hydrides appear at τ = 11.63 ppm (δ = -1.63 ppm, 1H, t, J = 9 Hz) and τ = 13.02 ppm (δ = -3.02 ppm, 2H, d, J = 9 Hz). The peak splitting pattern is characteristic of A2B groupings, which means that there are two equivalent hydrides, and one non-equivalent hydride. The signal for the hydrogen atoms on the cyclopentadienyl rings appear at τ = 5.24 ppm (δ = 4.76 ppm, 10H, s).

A strong, sharp absorption band can be seen in the infrared spectra of TaCp2H3 at 1735 cm−1, which corresponds to the Ta-H bond stretching frequency.

As opposed to other metallocene hydrides, such as ReCp2H, MoCp2H2, and WCp2H2, TaCp2H3 does not behave as a base, even in trifluoroacetic acid. It is decomposed by aqueous acids. This is consistent with the fact that the tantalum center does not have any lone pairs, since all orbitals have been utilized in bonding with the ligands.

The two cyclopentadienyl rings are in a bent conformation as confirmed by neutron diffraction studies where the ring-to-tantalum-to-ring bending angle is 139.9°. The three hydrides lie in the same plane as the tantalum center with the three Ta-H bond distances being essentially equal (1.769(8) Å, 1.775(9) Å, and 1.777(9) Å).

Reactivity
Tantalocene trihydride has been found to be capable of activating C-H bonds by oxidative addition as seen through hydrogen/deuterium exchange, involved in the insertion of phosphines, and capable of forming post transition metal ethyl adducts.

Catalysis of hydrogen–deuterium exchange
Barefield, Parshall, and Tebbe discovered that when TaCp2H3 was heated at 100 °C in benzene-d6 under a hydrogen atmosphere, HD and D2 were detected along with H2 in the vapor phase in a ratio of 41.1 to 41.6 to 17.0 (H2:HD:D2). This indicates that there is catalytic exchange, and that the complex is able to cleave the C-D bonds of the solvent. In another study by Foust et al., when TaCp2H3 was photolyzed for 36 h at 15 °C in benzene-d6, analysis of the evolved gases revealed that there was a mixture of H2, HD, and D2. If carbon monoxide was present in the reaction with toluene as a solvent, the CO containing product TaCp2(CO)H was formed through the intermediate species TaCp2H.

Activation of Csp3-H bonds by oxidative addition
Neufeldt et al. explored the activation of aliphatic C-H bonds by TaCp2H3 and related monosubstituted cyclopentadienyl rings experimentally and computationally. In order to go through oxidative addition, there must be an initial loss of H2 from TaCp2H3. Then, the monohydride complex can form a π-complex with an unsaturated solvent, such as benzene. Finally, the complex oxidatively adds to the C-H bond. Intramolecular and intermolecular C-H activation was found to be possible.

A σ-complex will form instead if the solvent used is aliphatic, such as octane. The authors observed a change in the hydride NMR signals due to H/D exchange when TaCp2H3 was heated to 120 °C for 48 h in octane-d18 and methylcyclohexyl-d14. The loss of another hydrogen molecule from the products can lead to β-hydride elimination, which forms complexes of the TaCp2(H)L, with L being an unsaturated π-ligand, having their own reactivity.

Phosphine insertion
The first phosphido derivative of tantalocene was obtained by the insertion of ClPPh2 into the Ta-H bond, resulting in the precipitation of the white ionic compound [TaCp2H2(PHPh2)]Cl. Deprotonation of this compound results in pale yellow crystals of the dihydride phosphido complex TaCp2H2PPh2. Through X-ray diffraction studies, the Ta-P bond distance was 2.595(3) Å, which is typical of a single bond between tantalum and phosphorus. ClPPh2 has been shown to insert into the niobium analogue, NbCp2H3. However, the deprotonation step results in the monohydride phosphido complex NbCp2H(PHPh2) instead. The authors of this article theorize that stabilization of hydrido phosphide complexes of the third row transition metals is due to higher M-H bond energy when compared to those of the second row.

Lewis acid-base adducts
TaCp2H3 can form Lewis acid-base adducts with AlEt3, GaEt3, ZnEt2, and CdEt2 at the unique hydride. As opposed to promotion of catalysis of olefin reactions with Lewis acids like AlEt3 such as in Ziegler-Natta catalysts, triethylaluminium seems to deactivate the hydride ligand toward ethylene insertion.