Cation assisted binding and cleavage of dinitrogen by uranium complexes

The role of alkali promoters in N 2 cleavage by metal complexes remains poorly understood despite its relevance to the industrial production of ammonia from N 2 . Here we report a series of alkali bound-oxo-bridged diuranium( iii ) complexes that provide a unique example of decreasing N 2 binding affinity with increasing cation size (from K to Cs). N 2 binding was found to be irreversible in the presence of K. A N 2 complex could be isolated in the solid state in the presence of the Rb cation and crystallographically characterized, but N 2 binding was found to be reversible under vacuum. In the presence of the Cs cation N 2 binding could not be detected at 1 atm. Electrochemical and Computational studies suggest that the decrease in N 2 binding affinity is due to steric rather than electronic effects. We also find that weak N 2 binding in ambient conditions does not prevent alkali assisted N 2 cleavage to nitride from occurring. More importantly, we present the first example of cesium assisted N 2 cleavage leading to the isolation of a N 2 derived multimetallic U/Cs bis-nitride. The nitrides readily react with protons and CO to yield ammonia, cyanate and cyanide. N 2 binding affinity decreases markedly in a series of isostructural U( iii )-alkali ions complexes with increasing cation size. N 2 binding is undetectable in the Cs analogue, but the first example of cesium-assisted N 2 cleavage to bis-nitride was observed at ambient condition.