Synthesis, X-ray crystal structure, NMR characterization and theoretical calculations on [Cp2Ta(η2-H2)(CO)]+, the first thermally stable group 5 dihydrogen complex

Abstract

Protonation of Cp₂TaH(CO) (Cp  =  C₅H₅, 1a; C₅H₄Bu^t, 1b) by HBF₄·Et₂O at −78 °C in CH₂Cl₂ affords [Cp₂TaH₂(CO)]BF₄ (2, 3) as mixtures of 2 isomers. The minor ones (2a, 2b) contain the known trans-dihydride [Cp₂TaH₂(CO)]⁺ cations whereas the major ones (3a, 3b) are [Cp₂Ta(η²-H₂)(CO)]BF₄, the first group 5 dihydrogen complexes. The crystal structure of the analogous complex 3a·BArf₄ recorded at 120 K confirms the presence of the coordinated dihydrogen ligand, which displays an H–H separation of 1.09(2) Å in agreement with distances calculated from NMR data. Protonation of Cp₂TaH₂(SiMe₂Ph) by (Et₂O)₂ ·HBArf₄ does not lead to an analogous silane derivative but to the new dinuclear complex [(Cp₂TaH₂)₂(μ-H)](BArf₄). Variable temperature NMR studies were carried out on the dihydrogen complex [Cp₂Ta(H₂)(CO)]⁺ (3) and its isotopomers. The high field signal of [Cp₂Ta(HD)(CO)]⁺ (3-d) shows a decoalescence at 208 K in both ¹H and ²D NMR, which allows us to calculate the barrier to rotation of HD (9.6 kcal mol⁻¹). The absence of decoalescence in the signal of 3 down to 173 K and the absence of a large kinetic isotope effect for the classical rotation of H₂ were demonstrated. These results are understood in terms of the presence of very large exchange couplings in a non-rotating dihydrogen ligand. The large barrier of rotation for the dihydrogen ligand in 3 was shown by DFT calculations to arise from a transition state in which the dihydrogen ligand is only coordinated through σ-donation from the H–H bond. The analogous phosphite and phosphine complexes {Cp₂TaH₂[P(OMe)₃]}⁺ (4) and [Cp₂TaH₂(PMe₂Ph)]⁺ (5) were shown to be cis dihydrides, in agreement with DFT calculations on a model compound, to display exchange couplings in NMR and no isotope effect for the classical exchange of the hydride ligands.