Abstract
DFT(B3PW91) calculations have been carried out to propose a pathway for the N-2 cleavage by H-2 in the presence of silica-supported tantalum hydride complexes [( SiO)(2)TaHx] that forms [( SiO)(2)Ta(NH)-(NH2)] (Science 2007, 317, 1056). The calculations, performed on the cluster models {mu-O[(HO)(2)SiO](2)}TaH1 and {mu-O[(HO)(2)SiO](2)}TaH3, labelled as ( SiO)(2)TaHx (x = 1, 3), show that the direct hydride transfers to coordinated N-based ligands in ( SiO)(2)TaH(eta(2)-N-2) and ( SiO)(2)TaH(eta(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 H-2, 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)(2)TaH(eta(2)-HNNH), the H-2 molecule that is adding across the Ta-N bond is released after the hydride transfer step by heterolytic elimination from ( SiO)(2)TaH(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 H-2 is needed to convert an intermediate, identified as a diazenido complex [( SiO)(2)TaH-(eta(2)-HNNH)] from its nu(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.