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

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• 作者：Xavier Solans-Monfort ; Catherine Chow ; Eric Gour茅 ; Yasemin Kaya ; Jean-Marie Basset ; Mostafa Taoufik ; Elsje Alessandra Quadrelli ; Odile Eisenstein
• 刊名：Inorganic Chemistry
• 出版年：2012
• 出版时间：July 2, 2012
• 年：2012
• 卷：51
• 期：13
• 页码：7237-7249
• 全文大小：579K
• 年卷期：v.51,no.13(July 2, 2012)
• ISSN：1520-510X

文摘

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 [(鈮iO)₂TaH_x] that forms [(鈮iO)₂Ta(NH)(NH₂)] (Science2007, 317, 1056). The calculations, performed on the cluster models {渭-O[(HO)₂SiO]₂}TaH₁ and {渭-O[(HO)₂SiO]₂}TaH₃, labelled as (鈮iO)₂TaH_x (x = 1, 3), show that the direct hydride transfers to coordinated N-based ligands in (鈮iO)₂TaH(畏²-N₂) and (鈮iO)₂TaH(畏²-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^{鈥?/sup>, and hydrazido, NHNH₂鈥?/sup>, respectively) that can thus accept a hydride with a moderate energy barrier. In the case of (鈮iO)₂TaH(畏2-HNNH), the H₂ molecule that is adding across the Ta鈥揘 bond is released after the hydride transfer step by heterolytic elimination from (鈮iO)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 [(鈮iO)₂TaH(畏2-HNNH)] from its 谓(N鈥揌) stretching frequency of 3400 cm鈥?, 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.}