Oxidation or Reduction State of Au Stabilized by an MOF: Active Site Identification for the Three-Component Coupling Reaction

As one of the most efficient catalysts for the three-component coupling reaction of aldehyde, amine, and alkyne (A(3) coupling), Au has attracted extensive attention, but there is a debating issue on whether Au delta+ (delta > 0, representing the oxidation state of Au species) or Au-0 is the better active site. According to previous reports, both Au delta+ and Au-0 can catalyze the A(3) coupling reaction. Therefore, the establishment of a suitable comparison model to identify the better active site is highly desired. In this work, an ideal model to identify the efficient active sites for the A(3) coupling reaction based on a metal-organic framework (MOF) platform is rationally fabricated. The Au delta+ in oxidation state can be well bound to the MOF, MIL-101-NH2, via postsynthetic modification, and the Au-0 catalyst with retained MOF skeleton can be formed via a subsequent one-step reduction, which provide ideal comparison models with the only difference in the Au oxidation state. Strikingly, the two catalysts exhibit significant activity difference in the A(3) coupling reaction. The activity of Au delta+ catalyst is 11-fold higher than that of Au-0, which reveals that Au delta+ serves as a much more effective active site in the A(3) coupling reaction.