One of the most important and challenging tasks in modern chemistry is the reduction of dinitrogen and its transformation into ammonia (NH3) or value-added N-containing organic compounds. Metal nitrides are of high interest because of their role as intermediates in dinitrogen fixation and their potential application in N-atom transfer reactivity to substrates forming N-H or N-C bonds. In particular, uranium nitride materials (UN) are attracting an increasing number of studies because of their property to act as catalysts in the Haber-Bosch process and they are potential candidates for application as nuclear fuels. Moreover, molecular uranium nitrides are of interest for the fundamental study of U-N multiple bonding and magnetic communication. Only few bridging or terminal uranium nitrides have been reported and straightforward synthetic route remain elusive, but these systems showed high reactivity towards a variety of small molecules such as N2, H2, CO, CO2, CS2, and CN-. Here, the synthesis and the reactivity of uranium nitride complexes with different binding modes will be presented. First, the role of the coligands in the reactivity of mono(µ-nitride) di-uranium(IV) complexes is presented, showing how the supporting ligands greatly affect, in non-anticipated manner, the reactivity of the nitride moiety. We will also report the synthesis of a bis(µ-nitride) di-uranium(V) which effects the remarkable oxidative cleavage of H2 in mild conditions to afford a bis-imido bridged diuranium(IV) that can be further protonated to afford ammonia in 73% yield. This provides a good model for hydrogen cleavage by metal nitrides in the Haber–Bosch process. The measured magnetic data for the bis-nitride and derivatives show an unusually strong antiferromagnetic coupling between uranium(V) ions demonstrating that nitrides are attractive linkers for promoting magnetic communication in uranium complexes. Finally, we will present the unprecedented photochemical synthesis of a uranium(VI) terminal nitride, showing that a careful tuning of the supporting ligands and of the reaction conditions allows the isolation of the nitride. The photochemically generated nitride is stable in ambient conditions but displays high reactivity towards electrophiles to yield cyanate upon reaction with CO and 100% ammonia in presence of a strong acid.

Synthesis and reactivity of uranium nitride complexes

ACS Spring Meeting 2021

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