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

Ammonia N–H bond activation and dinitrogen N≡N cleavage with dihydrogen on an isolated metal atom have been achieved with the silica‐grafted TaIII and TaV hydrides [(≡SiO)2TaH] (2a) and [(≡SiO)2TaH3] (2b), accessed through surface organometallic chemistry (SOMC). The synthesis of the starting tantalum hydrides 2a and 2b by grafting tris(neopentyl)neopentylidenetantalum(V), Ta(=CH–tBu)Np3, on silica yields well‐defined, isolated tantalum atoms. Silsesquioxane molecular modelling shows that the mechanism of the grafting reaction implies a tetraalkyl intermediate [(≡SiO)TaNp4]. The starting hydrides 2a and 2b react stoichiometrically and catalytically with alkanes in reactions such as alkane metathesis, cross‐metathesis between ethane and toluene, and methane coupling to form ethane. Mechanistic studies show the relevance of tantalum carbenes and Chauvin‐like metallacyclobutane intermediates in most of these reactions. Finally, the stoichiometric N2 cleavage and NH3 activation to the final imido amido tantalum(V) complex [(≡SiO)2Ta(NH)(NH2)] (3) are reviewed and discussed mechanistically. In the N≡N cleavage reaction, dihydrogen adducts on silica‐grafted isolated tantalum atoms appear to play a central role. The ammonia reaction occurs by bifunctional activation through the Lewis acid/Lewis base couple formed by a metal centre and a coordinated nitrogen atom, the so‐called “NH effect”. Such bifunctional activation is also observed for the heterolytic cleavage of H2 by [(≡SiO)2Ta(NH)(NH2)] (3). Isolated TaIII and TaV hydride centres [(≡SiO)2TaH] and [(≡SiO)2TaH3] supported on a silica surface cleave N2 with H2 at 250 °C and atmospheric pressure and activate NH3 at room temperature to yield the TaV amido imido product [(≡SiO)2Ta(=NH)(NH2)]. Their synthesis by surface organometallic chemistry (SOMC), reactivity toward C–H and C–C bonds of alkanes and this new reactivity toward the N≡N and N–H bonds are reviewed from a mechanistic point of view.