A new N-heterocyclic carbene (NHC) ligand that chelates to rhodium under very mild conditions has been synthesised. 

Bis-NHCs are very useful ligands in catalysis because their size and electronic properties can be easily altered to improve catalytic activity. Their use is limited, however, because many bis-NHCs bind to two different metals, producing a non-chelating bis-rhodium species, rather than chelating to just one metal. 

Robert Crabtree's group at Yale University, US, in collaboration with Eduardo Peris at Universitat Jaume I, Spain, believe they have solved this problem. By replacing the carbon chain that links the azole rings in typical bis-NHC ligands with a direct azole-azole bond, Crabtree made a planar bitriazole NHC ligand known as bitz. He found that bitz chelated with rhodium under much milder conditions that for typical NHC ligands and did not form the undesirable bis-rhodium complex. 

'Another advantage of the triazole NHC is that the electron donor power is less extreme than typical NHCs and closer to that of phosphine ligands,' said Crabtree. 'Also, metallation of the azolium ion precursor is easier than in the classical imidazolium NHCs.'

Crabtree also made a surprising dirhodium(II) complex, which is the first reported dirhodium(II) complex to contain a NHC ligand. Both the mono- and dirhodium complexes were catalytically active in transfer hydrogenation reactions. 

Crabtree believes that the bitz ligand can be widely applied to transition metal chemistry. 'We hope that this ligand and analogous NHCs will join the growing series of ligands that has proved so useful in organometallic chemistry and homogeneous catalysis,' he said. 

Joanne Thomson