The biochemical and industrial synthesis of ammonia relies on the participation of several metal centers to cut the N ≡ N triple bond [1]. This multi-metal cooperation is also the general rule for the reductive cleavage of dinitrogen by molecular systems or surfaces [1].
We have observed N2 cleavage at 250°C and atmospheric pressure by dihydrogen on isolated silica-supported hydride centers of TaIII and TaV, [(≡SiO)2TaIII-H] and [(≡SiO)2TaV-H3]. The reaction product is TaV imido amido [(≡SiO)2Ta(=NH)(NH2)] [2]. The structure of the surface product was determined with atomic-scale precision by infrared and solid NMR spectroscopies, isotopic labeling, EXAFS and theoretical calculations, and by comparison with the direct reaction product of the starting hydrides with ammonia [3].
The reaction intermediates revealed by in situ infrared monitoring and a density functional theoretical (DFT) study revealed a mechanism very different from those previously established for enzymatic, molecular or heterogeneous systems capable of activating N2. The metal dihydrogen adduct plays a central role as a proton source and electron reservoir. A unique molecular role for a supra-stoichiometric dihydrogen molecule is also proposed [4].
