Cationic NHC-gold(I) complexes: Synthesis, isolation, and catalytic activity

The reaction of [(NHC) AuCl] complexes (NHC = N-heterocyclic carbene) with a chloride abstractor of the type AgX, where X is a non-coordinating anion, led, in the presence of a neutral coordinating solvent S, to a series of cationic gold(I) complexes of formulae [(NHC)Au(S)] X. Hence, different cationic NHC-gold(I) species bound to acetonitrile, pyridine, 2-Br-pyridine, 3-Br-pyridine, norbornadiene, and THF could be synthesized and characterized by H-1 and C-13 NMR spectroscopies. Among these, the results of X-ray diffraction studies for [(IPr)Au(NCMe)]SbF6, [(IAd)Au(NCMe)]PF6, [(IPr)Au(pyr)]PF6, [(IPr) Au(2-Br-pyr)]PF6, [(IPr)Au(3-Br-pyr)]PF6 are discussed. As special feature, the structure of [(IPr)Au(2-Br-pyr)] PF6 presented a secondary interaction between the gold and bromine atoms. Additionally, while attempting to obtain crystals of [(IPr)Au(nbd)]PF6, we crystallized a decomposition product featuring a very rare PF4- anion as bridging ligand with formulae [(mu-PF4)((IPr)Au)(2)]PF4. The observation of a possible P-F bond activation has important implications for cationic Au-based homogeneous catalysis. Finally, we compared the catalytic activities of the different cationic [(NHC) Au(S)] X complexes in the allylic acetate rearrangement reaction and notably observed the inertness of pyridine-based catalysts. (C) 2008 Elsevier B.V. All rights reserved.