Sulphonated NHC-gold(I) complexes in aqueous catalysis. An experimental and DFT computational analysis

A series of sulphonated NHC gold(I) complexes, with different steric bulk around the metal center, [1,3-bis(2,6-diisopropyl-4-sodium sulfonatophenyl)imidazol-2-ylidene] gold(I) chloro (C1), [1-(2,6-diisopropylphenyl)-3-(3-sodium sulfonatopropyl)imidazol-2-ylidene] gold(I) chloro (C2), [1-mesityl-3-(3-sodium sulfonatopropyl)imidazol-2-ylidene] gold(I) chloro (C3), [1-methyl-3-(3-sodium sulfonatopropyl)imidazol-2-ylidene] gold(I) chloro (C4) and [(3-sulfonatopropyl)imidazol-2-ylidene gold(I) chloro] (C5), have been studied in the hydration of phenylacetylene in aqueous media, resulting to be active and even recycled. 

All reactions were performed in water and water:methanol. Notably, except C1, there was no reaction with any of other complexes (C2-C5) in water. In contrast to these results, all the reactions carried out in water:methanol were positive. In order to explain the experimental results, we decided to study the reactivity of these complexes applying Density Functional Theories (DFT) methods. For this purpose, we considered complexes C1 and C5 as representative compounds taking into account that their structure and reactivity are completely different. Thus, C1 has two bulky substituents (2,6-diisopropyl-4-sodiumsulfonatopheny) attached to the nitrogen atoms, while C5 has a less bulky moiety as an alkyl chain (sulfonatopropyl). The 100% conversion of phenylacetylene to acetophenone was obtained after drastically different induction periods: 30 min for C1 and 100 h for C5. The DFT computational study support the experimental results showing that the alkyl chain folds generate a strong steric hindrance around the metal center causing a decrease of the catalytic effect.