Dinitrogen Dissociation on an Isolated Surface Tantalum Atom

Abstract

Both industrial and biochemical ammonia syntheses are thought to rely on the cooperation of multiple metals in breaking the strong triple bond of dinitrogen. Such multimetallic cooperation for dinitrogen cleavage is also the general rule for dinitrogen reductive cleavage with molecular systems and surfaces. We have observed cleavage of dinitrogen at 250 degrees C and atmospheric pressure by dihydrogen on isolated silica surface-supported tantalum(III) and tantalum(V) hydride centers [(identical with Si-O)2Ta(III)-H] and [(identical with Si-O)2Ta(V)H3], leading to the Ta(V) amido imido product [(identical with SiO)2Ta(=NH)(NH2)]: We assigned the product structure based on extensive characterization by infrared and solid-state nuclear magnetic resonance spectroscopy, isotopic labeling studies, and supporting data from x-ray absorption and theoretical simulations. Reaction intermediates revealed by in situ monitoring of the reaction with infrared spectroscopy support a mechanism highly distinct from those previously observed in enzymatic, organometallic, and heterogeneous N2 activating systems.

Ammonia and Dinitrogen Activation by Surface Organometallic Chemistry on Silica‐Grafted Tantalum Hydrides

“…(10) Heating to 250°C leads to a decrease in the intensity of this N 2 coordination band and the appearance of new bands consistent with ν(N-H) around 3400 cm -1 , indicative of an intermediate containing Ta (N 2 H x ) with reduced N-N bond order (diazenido or hydrazido). At this point, addition of H 2 leads to the formation of the final product.…”

“…Furthermore, DFT calculations on a model compound supported on a cristobalite (100) surface ( Figure 5) have yielded optimized geometries, calculated IR stretching frequencies and NMR resonances comparable with experimental data. [10] DFT periodic calculations also provided (9) thermodynamic insight into the nitrogen cleavage reaction by [(ϵSiO) 2 TaH] and [(ϵSiO) 2 TaH Preliminary mechanistic studies on the formation of the imido amido species by sequential addition of N 2 followed by H 2 support an initial reversible end-on coordination of N 2 to the tantalum centre at room temperature, as evidenced by a new band in the IR spectrum at 2280 cm -1 . Such coordination is more likely to occur at the electronpoor Ta V d 0 species [(ϵSiO) 2 TaH 3 ] (2b), given the weak redshift with respect to free dinitrogen and the weak backdonation from the metal centre.…”

“…[10] This paper will review these recent advances around silica-supported tantalum hydrides in the context of this unique NϵN triple bond cleavage with H 2 on an isolated metal atom and try to offer an explanation for the lack of similar precedents in molecular, heterogeneous or enzymatic systems. In particular, it will show that the ability of SOMC to anchor isolated metal atoms on a surface distinguishes it both from heterogeneous and molecular systems by creating single-centre catalyst sites that are able to overcome some of the limitations of either heterogeneous or homogeneous catalysts.…”

“…[60] The tantalum imido amido complex [(ϵSiO) 2 Ta-(=NH)(NH 2 )] (3), reported above from the reaction of 2 with ammonia, can also be synthesized through reaction of a 1:1 mixture of dinitrogen and dihydrogen with surface hydrides 2 at 250°C [Equation (9)]. [10] As in the case of ammonia, the products of the reaction have been characterized by IR spectroscopy, 2D HETCOR and DQ NMR spectroscopy, EXAFS and elemental analysis. Furthermore, DFT calculations on a model compound supported on a cristobalite (100) surface ( Figure 5) have yielded optimized geometries, calculated IR stretching frequencies and NMR resonances comparable with experimental data.…”

A Biomass-Based Model to Estimate the Plausibility of Exoplanet Biosignature Gases

“…The Haber process is the principle industrial method for producing ammonia 7 . More efficient catalysts are known, which can catalyze the formation of ammonia from elemental nitrogen and hydrogen gases at 500 K and standard pressure (Yue et al 2006;Avanier et al 2007). Others catalyze the formation of ammonia from nitrogen gas and a proton (Yandulov & Schrock 2003;Shilov 2003;Weare et al 2006) or from an activated nitrogen molecule and hydrogen gas (Nishibayashi et al 1998), in water at room temperature and pressure.…”