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).