‘A lot of people thought that it might not be possible to make a stable uranium nitride and if you tell a chemist something can’t be done that is a very strong motivation to try to do it,’ says Stephen Liddle of the University of Nottingham who led the research team.
A terminal uranium nitride is difficult to construct because the nitrogen is desperate to spread its charge to other metal atoms. Liddle’s team constructed complexes consisting of a single uranium encapsulated within an extremely bulky polydentate triamidoamine ligand. The uranium is almost completely hidden, except for a single small binding pocket into which a nitrogen can fit. When sodium azide (NaN3) is presented to the complex, the powerfully reducing uranium grabs hold of one nitrogen and kicks off the other two. At this point the captured nitrogen is unhappy and thrashes and squirms within its small cavity, trying to grab hold of another metal in the vicinity. However, any nearby uraniums are all well padded sterically and out of reach.
The second key plank of the team’s strategy was to stabilise these reactive intermediates. ‘Here the sodium ions from the azide are crucial,’ says Liddle. ‘These attach to the nitrogen, essentially calming it down and taking the sting out of the situation.’ Two sodiums form a bridge between two of the terminal nitride complexes, creating a less energetic intermediate. In the final step of the process the sodiums are gently prised away through sequestration by a crown ether, leaving a crystalline separated ion pair, with the anion containing the terminal nitride functionality.
‘After many attempts to isolate this elusive bond the beauty of this work is its sheer simplicity,’ Liddle says. ‘This work demonstrates once again the crucial role ancillary ligands play in stabilising unusual bonds.’
Commenting on the study, Jaqueline Kiplinger of the Los Alamos National Laboratory in the US, says: ‘The formation of a terminal uranium nitride complex has been an ultimate goal of synthetic actinide chemists for decades. This is a genuinely beautiful piece of work which significantly adds to our fundamental knowledge of f-element chemistry. The isolation of a terminal nitride likely sets the stage for additional discoveries in this area.’
- DM King et al, Science, 2012, DOI: 10.1126/science.1223488