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

• Published in: Journal of organometallic chemistry Vol. 694; no. 4; pp. 551 - 560
• Main Authors: de Frémont, Pierre, Marion, Nicolas, Nolan, Steven P.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.02.2009
• Subjects: Allylic rearrangement; Cationic; Gold; N-Heterocyclic carbene; Pyridine; Gold; Allylic rearrangement; Pyridine; Cationic; N-Heterocyclic carbene
• ISSN: 0022-328X; 1872-8561
• DOI: 10.1016/j.jorganchem.2008.10.047

A series of cationic gold(I) complexes of formulae [(NHC)Au(L)]X, where L = MeCN, THF, nbd, pyr, 2-Br-pyr, 3-Br-pyr and X = BF 4, PF 6, SbF 6, FABA, has been synthesized, isolated and characterized. The catalytic activity of these complexes was then tested in the Overmann rearrangement. 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 1H and 13C NMR spectroscopies. Among these, the results of X-ray diffraction studies for [(IPr)Au(NCMe)]SbF 6, [(IAd)Au(NCMe)]PF 6, [(IPr)Au(pyr)]PF 6, [(IPr)Au(2-Br-pyr)]PF 6, [(IPr)Au(3-Br-pyr)]PF 6 are discussed. As special feature, the structure of [(IPr)Au(2-Br-pyr)]PF 6 presented a secondary interaction between the gold and bromine atoms. Additionally, while attempting to obtain crystals of [(IPr)Au(nbd)]PF 6, we crystallized a decomposition product featuring a very rare PF 4 - anion as bridging ligand with formulae [(μ-PF 4)((IPr)Au) 2]PF 4. 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.