Successive Heterolytic Cleavages of H₂ Achieve N₂ Splitting on Silica-Supported Tantalum Hydrides: A DFT Proposed Mechanism

DFT­(B3PW91) calculations have been carried out to propose a pathway for the N₂ cleavage by H₂ in the presence of silica-supported tantalum hydride complexes [(SiO)₂TaH_x] that forms [(SiO)₂Ta­(NH)­(NH₂)] (<i>Science</i> <b>2007</b>, <i>317</i>, 1056). The calculations, performed on the cluster models {μ-O­[(HO)₂SiO]₂}­TaH₁ and {μ-O­[(HO)₂SiO]₂}­TaH₃, labelled as (SiO)₂TaH_x (x = 1, 3), show that the direct hydride transfers to coordinated N-based ligands in (SiO)₂TaH­(η²-N₂) and (SiO)₂TaH­(η²-HNNH) have high energy barrier barriers. These high energy barriers are due in part to a lack of energetically accessible empty orbitals in the negatively charged N-based ligands. It is shown that a succession of proton transfers and reduction steps (hydride transfer or 2 electron reduction by way of dihydride reductive coupling) to the nitrogen-based ligands leads to more energetically accessible pathways. These proton transfers, which occur by way of heterolytic activation of H₂, increase the electrophilicity of the resulting ligand (diazenido, N₂H^–, and hydrazido, NHNH₂^–, respectively) that can thus accept a hydride with a moderate energy barrier. In the case of (SiO)₂TaH­(η²-HNNH), the H₂ molecule that is adding across the Ta–N bond is released after the hydride transfer step by heterolytic elimination from (SiO)₂TaH­(NH₂)₂, suggesting that dihydrogen has a key role in assisting the final steps of the reaction without itself being consumed in the process. This partly accounts for the experimental observation that the addition of H₂ is needed to convert an intermediate, identified as a diazenido complex [(SiO)₂TaH­(η²-HNNH)] from its ν­(N–H) stretching frequency of 3400 cm^–1, to the final product. Throughout the proposed mechanism, the tantalum remains in its preferred high oxidation state and avoids redox-type reactions, which are more energetically demanding