Chemists Isolate Elusive U≡N Complex

Chemists have continued a recent string of successes in actinide chemistry by synthesizing and obtaining the first crystal structure of a uranium complex containing a terminal nitride ligand (Science, DOI: 10.1126/science.1223488). The accomplishment by Stephen T. Liddle of the University of Nottingham, in England, and coworkers provides welcome data on the structure-bonding-reactivity relationships of actinides. Organometallic chemists are intrigued by thorium, uranium, and plutonium complexes because their combination of 5f and 6d valence orbitals allows them to bind and activate small molecules in ways not observed with transition-metal catalysts. In addition, these elements are important as nuclear fuels. For example, ceramic pellets based on UO₂ are the nuclear fuel of choice at the moment, but nitride-based ceramics, (U≡N)_n, are being explored as potential fuel because of the nitride’s enhanced thermal conductivity over oxides. Before now, terminal uranium nitrides had been glimpsed only spectroscopically, and little is known about the U≡N functional group. Liddle’s team began by reacting a uranium(III) complex bearing a triamidoamine ligand with NaN₃, forming a disodium-bridged uranium(V) nitride dimer. The researchers then trapped sodium with a crown ether to isolate the monomeric U≡N complex shown.