A bioinspired approach for controlling accessibility in calix[4]arene-bound metal cluster catalysts

In enzymes, the electronic and steric environments of active centres, and therefore their activity in biological processes, are controlled by the surrounding amino acids. In a similar manner, organic ligands have been used for the 'passivation' of metal clusters, that is, inhibition of their aggregation and control of their environment. However, the ability of enzymes to maintain large degrees of accessibility has remained difficult to mimic in synthetic systems in which little room, if any, is typically left to bind to other species. Here, using calix[4]arene macrocycles bearing phosphines as crude mimics of the rigid backbones of proteins, we demonstrate the synthesis of gold clusters and the control of their accessibility through an interplay between the sizes of the calixarene ligands and metal cores. For 0.9-nm cores, 25% of all the gold atoms within the cluster bind to the chemisorption probe 2-naphthalenethiol. This accessibility dramatically decreases with 1.1-nm and 4-nm gold cores.