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.