The ionic mononuclear and trinuclear Au(I)-complexes ligated by phosphine-functionalized ionic liquids: Synthesis, characterization, and catalysis to hydration of phenylacetylene

A series of ionic Au(I)-complexes ligated by the phosphine-functionalized ionic liquids were synthesized and characterized, which are composed of the Au(I)-complex cation and the counteranion of OTf^-, AuCl ₄^-, or PF₆^-, respectively. The single crystal X-ray diffraction analyses show that the Au(I)-centered vector in 1A, 1B, 2A, 3A, and 4A all possess the slightly twisted linear geometry, and in each individual the Au(I)-center is coordinated by one chlorine and one imidazolium-based phosphine. The aggregation of 1B in acetone can lead to the formation of the trinuclear Au(I)-complex of 1C due to the aurophilic Au(I)-Au(I) interaction, the electrostatic attraction, and the steric preference. When these ionic Au-complexes were employed as precatalysts for hydration of phenylacetylene in aqueous-methanol media, the reaction proceeded selectively according to Markovnikov's rule with moderate to high yields of acetophenone. The highest activities were achieved over 2A with hydrophobic PF₆^- as the counteranion and the trinuclear 1C under mild conditions (75 °C, 2 h), in conjunction with the additive of proton acid H₂SO₄. The ion-pair effect on the catalytic performance of the corresponding Au(I)-complexes, coming from the phosphine-ligated cations and the counteranions, were investigated.