A combined NMR/DFT study on the ion pair structure of [(PR12R2)Au(?2-3-hexyne)]BF4 complexes

The interionic structure of four gold(i) [small pi]-alkyne ion pairs{,} with general formula [(PR12R2)Au([small eta]2-3-hexyne)]BF4 (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 PR12R2 ligands (particularly{,} 4a) and low for ion pairs with strongly electron donating PR12R2 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.