In situ detection of an unstable C,N-Au(III) chelate by 15N NMR methods

Abstract

Au(III)-NHC-oxazolyl complexes are prepared by oxidation and anion exchange of the corresponding Au(I)Cl-NHC-oxazolyl precursors, which are synthesized from appropriate imidazolium pre-ligands. As the Au(III) complexes are too unstable for proper isolation and spectroscopic characterization, selective 15N NMR techniques provide valuable knowledge of N-coordination to gold by formation of N-ligated Au(III) complexes. The changes in 15N-shift values (Δδ15N), observed by 1H,15N-HMBC 2D NMR studies, going from Au(I)Cl, via Au(III)Cl3 to the C,N-Au(III) NHC-oxazolyl complexes, afford important information of the oxidation and the anion exchange processes. In particular, the huge up-field shift of the oxazoline-N by anion exchange (Δδ15NAE: −71.3 ppm), represents significant evidence that oxazoline-N-coordination to Au(III) takes place by anion exchange, and, hence, that the target six-membered bidentate C,N-Au(III)-NHC-oxazoline chelated complex is formed.