Chelating carbene ligand unveiled

The planar chelating bitriazole molecule shown is the latest addition to the versatile family of N-heterocyclic carbene (NHC) ligands. Created by Robert H. Crabtree of Yale University; Eduardo Peris of University Jaume I, in Spain; and colleagues, the bitriazole is an analog of the popular 2,2' -bipyridine ligand and expands the range of NHC electronic and steric properties (Chem. Commun. 2007, 2267). The ligand forms in situ when a precursor bitriazole reacts with a standard rhodium complex. Depending on reaction conditions, catalytically active rhodium complexes containing one or two of the chelating ligands are produced, including an unusual dirhodium complex in which the two rhodium atoms and the ligands form unprecedented six-membered chelate rings. NHCs typically are strong electron donors to transition metals, but the new ligand's donating power is significantly weaker, placing it slightly lower than that of 2,2′-bipyridine and bis(dimethylphosphino)ethane, Crabtree points out. This property and the rigid, planar structure that arises from the N–N bond linking the azole rings may increase the utility of NHC ligands, he says.