Diffusion NMR measurements on cationic linear gold(I) complexes

Abstract

In the last years, it has been demonstrated that the anion plays a key role in gold(I) catalysis, affecting yield, regio- and stereo-selectivity of processes. In order to perform such activity, necessarily the anion has to locate itself close to the reaction center. In this contribution, the level of aggregation of cationic linear gold complexes bearing different ligands, such as phosphines and carbenes, is studied by diffusion PGSE (Pulsed field Gradient Spin Echo) NMR spectroscopy as a function of concentration and solvent. It is found that functional groups, which establish specific interactions with the anion, such as –NH or polarized –CH moieties, strongly influence the self-aggregation of gold(I) complexes: ion pairs are the predominant species in solution, but ion quadruples also form in apolar solvents only when –NH or polarized –CH moieties are present in the ligand. In the absence of those functional groups free ions are present in solution with a small amount of ion pairs. Interestingly, the presence of an extended aromatic group on the cation leads to dicationic adducts and ion triples, which are held together by π–π stacking interactions. When more than one functional group is present, 19F, 1H HOESY (Heteronuclear Overhauser Effect SpectroscopY) NMR experiments and DFT coulomb potential maps are used to check which group establishes the strongest interaction with the anion.