Expanded ring and functionalised expanded ring N-heterocyclic carbenes as ligands in catalysis

Abstract

The synthesis of new functionalised 6- and 7-membered NHC (N-heterocyclic carbene) precursors bearing anisidyl or pyridine N-substituents, both symmetrically and non-symmetrically substituted is reported. Their corresponding rhodium(I) and iridium(I) complexes, M(COD)(NHC)Cl, were also prepared and characterised. The unusual Rh(III)/Rh(I) salt, [Rh(η²-NHC-py)₂Cl₂][Rh(COD)Cl₂], was obtained with one of the pyridyl-functionalised NHC ligands. Single-crystal X-ray analyses have been obtained for the majority of the complexes and NHC salts. The activity of these complexes was tested in the hydrogenation of a range of substrates with molecular hydrogen, including 1-cyclooctadiene and 2-methyl styrene, where they show enhanced activity and stability in comparison to non-functionalised NHC analogues, operating under exceptionally mild conditions (ambient temperature and atmospheric pressure).