Synthesis and characterization of Au(I) and Au(III) complexes containing N-heterocyclic carbene ligands derived from amino acids.

A series of [Au(NHC)2] (1a?4a) complexes supported by NHC ligands derived from glycine, alanine, methionine and phenylalanine (1?4 respectively) were prepared via a direct transmetalation reaction of their respective silver complexes. The Au complexes were characterized by ESI-MS and NMR spectroscopy in solution. These compounds exhibit instability when the solvent is removed; they displayed a strong tendency to form colored solutions in the order 4a > 3a > 2a > 1a, which is associated with gold nanoparticles. 1a and 2a undergo oxidative addition of elemental bromine, yielding [Au(NHC)2Br2] (1b) for 1a and a mixture of [Au(NHC)2Br2] (2b) and [Au(NHC)Br3] (2c) for 2a. 2b and 2c were characterized by single crystal X-ray diffraction.