A combined NMR/DFT study on the ion pair structure of [(PR1 2R2)Au(η2-3-hexyne)]BF4 complexes

The interionic structure of four gold(i) π-alkyne ion pairs, with general formula [(PR¹<inf>2</inf>R²)Au(η²- 3-hexyne)]BF<inf>4</inf> (R¹ = R² = ^tBu, 1a; R¹ = ^tBu, R² = o-diphenyl, 2a; R¹ = R² = 2,4,6-trimethoxyphenyl, 3a; R¹ = R² = 2,4-di-tert-butylphenoxy, 4a), was studied by ¹⁹F, ¹H-HOESY NMR spectroscopy. In all the cases the anion locates mainly close to the alkyne, but the degree of specificity of location strongly depends on the P-ligand and it is high for ion pairs bearing poorly electron donating PR¹<inf>2</inf>R² ligands (particularly, 4a) and low for ion pairs with strongly electron donating PR¹<inf>2</inf>R ² ones (such as 3a). This result is rationalized through relativistic DFT calculations, showing that the electronic properties of the P-ligand finely tune the charge accumulation on the alkyne and, consequently, its ability of attracting the anion. © 2013 The Royal Society of Chemistry.