Synthesis, characterization and reactivity of a Au(III)-NHC complex

Abstract

The research on N-heterocyclic carbene (NHC) ligands has been tremendously expanded in organometallic and catalysis fields during the last two decades. In fact, they have been widely used for the design of highly catalytic active specie for several processes, including cross-coupling, metathesis, dehydrogenation, hydrogenation, among other reactions. This wide applicability is mainly due to the fact they can be easily prepared and are highly tunable, as well as they form very -strong bonds with practically any transition metal. Currently, gold NHC complexes are taking an important role in medicine and catalysis. In the realm of catalysis there are several topologies described with a wide range of applications, for example: halogenation of aryls, cycloisomerization of propargylic amides, the Meyer-Schuster reaction (rearrangement of secondary and tertiary propargylic alcohol to α, β-unsaturated ketones), [3+3] rearrangement and double hydroarylation of diyne diether. On the one hand, in medicine, the use of gold (III) species for the treatment of cancer covers more importance, due to gold can be eliminated easily form our body. In our goal of synthesizing a pincer Au(III) complex bearing a NHC-aryl-NHC, we have achieved the synthesis and characterization of the desired a NHC-aryl-NHC bisazolium salt and the corresponding a new bimetallic Au(I)-NHC complex 3, where each gold center is bonded two NHC moieties, and the arene moiety is not affected. We noticed that the ligand structure with n-butyls is more soluble. In order to get the proximal arene moiety into play, the oxidation of the bimetallic Au(I) complex with Br2 was performed and a bimetallic Au(III)-NHC complex (4), where each gold(III) center is bonded to two Br- and two NHC moieties. Also we tried the substitution of the coordinated bromide anions with pyridine or benzoic acid to activate the reactivity of the Au(III) centers, but with no success, probably due to the high stability conferred by the Br anions. We finally tested complex 4 as catalyst for the arylation of aldehydes and the cycloisomerization of 4-pentanoic acid, but the catalysis were not successful in either case. We can conclude that the reactivity of complex 4 can only be activated for catalysis if a reliable methodology for removing the coordinated halides is developed in the future