01 December 2010
UK scientists have coupled two carbon monoxide (CO) molecules using an uranium(III) amide at low pressure and temperature and found that the resulting ynediolate reacts further with heat to form a second C-C bond and a C-H bond. Both steps have overcome challenges in CO coupling reactions.
Polly Arnold's group at the University of Edinburgh, in collaboration with catalyst company Sasol Technology UK, discovered that exposing a uranium tris(amide) complex UN''3 (N'' = N(SiMe3)2) to CO at ambient temperature and pressure produced N''3U-O-C(triple bond)C-O-UN''3. 'All of a sudden we saw beautiful golden crystals that had grown in the flask. We couldn't believe our luck when we solved the crystal structure. Crystallography was the only thing that would let us believe what we had done,' says Arnold.
Previous reports had concluded that UN''3 was not able to coordinate to CO at ambient temperature and pressure, although no details of solvent were included. During her research, Arnold saw that the alkyne formation worked in hexanes or toluene, but not in donor solvents like THF or pyridine.
Many metal complexes have catalysed insertion of CO into organic substrates, but few have demonstrated C-C coupling of two metal-coordinated CO ligands
No further reactivity of the ynediolate complex was anticipated since the coupled C2O2 fragment product is sterically protected. Indeed, no reaction was observed when the ynediolate was treated with dihydrogen, silanes, boranes or haloboranes. However, the team discovered that heating the complex in benzene resulted in the addition of a C-H bond from one of the methyl groups on the (SiMe3) ligands across the triple bond to form a seven-membered metallacyclic ring. 'The complex ended up reacting with itself, but these are still pleasing reactions because just by heating it up, we form a second C-C bond and a C-H bond. Both of these in their own right are recognised as being difficult reactions to do in the context of carbon monoxide homologation,' explains Arnold. 'It is the first time that reductively coupled or homologated CO has been able to form bonds to normal sp3 carbon and hydrogen atoms.'
'Uranium(III) is actually quite difficult to work with because it's very reactive to oxygen so it's nice to see that they could do this with such a simple ligand,' says Annie Gorden, an expert in uranium chemistry at Auburn University, US. 'I've seen uranium(III) complexes coordinated to CO, but this is unique in that they can do the CO coupling.'
Arnold's goal is to develop a uranium-based catalyst and she is working towards identifying reagents to replace the O-U bond with something else, ideally O-H, to complete the catalytic cycle.