A combined NMR/DFT study on the ion pair structure of [((PR2R2)-R-1)Au(eta(2)-3-hexyne)]BF4 complexes

The interionic structure of four gold(I) pi-alkyne ion pairs, with general formula [((PR2R2)-R-1)Au(eta(2)-3-hexyne)]BF4 (R-1 = R-2 = Bu-t, 1a; R-1 = Bu-t, R-2 = o-diphenyl, 2a; R-1 = R-2 = 2,4,6-trimethoxyphenyl, 3a; R-1 = R-2 = 2,4-di-tert-butylphenoxy, 4a), was studied by F-19, H-1-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 (PR2R2)-R-1 ligands (particularly, 4a) and low for ion pairs with strongly electron donating (PR2R2)-R-1 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.