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

Abstract

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.

Controlled metal nanostructures: Fertile ground for coordination chemists

“…Other values reported on single layer systems lead to an average footprint area of 0.214 nm 2 for a thiolate ligand on Au NPs [196] and 0.15nm 2 for an alkynyl one on Ru NPs [197]. Mercaptopropionic acid, a very small bidentate ligand often used as a linker between Au NPs (via its thiol end group) and biomolecules (via its carboxylic acid end group), displays a slightly higher packing density: a recent study indicates an average footprint area of 0.13nm 2 , nearly independent of the size of the Au NPs (from 5 to 100nm) [192 a A c c e p t e d M a n u s c r i p t 26 less densely packed on a MNP surface, form weaker interligand interactions, leave larger "interligand" spacing, which is of interest for catalytic applications as the surface is more accessible [198], [199]. For example, rapid -flip dynamics of phenyl rings of the PPh 3 ligands bound through a Au-P bond onto the Au NP surface have been demonstrated which suggests the presence of well-separated ligands, and thus availability of low-coordinated surface Au atoms in contrast with the densely packed layer formed by ligands with long linear alkyl chains [200].…”

Simple and Efficient Separation of Atomically Precise Noble Metal Clusters

“…The stirring was continued for 15 min after which the solution was orange-red in color. To this solution, 110 μl butanethiol in 500 µl THF (in case of Au25BT18) or a mixture of 110 µl butanethiol and 4.4 mg of calixarene tetrathiol in 500 µl THF (in case of Au25Calix0-3BT6- 18) was rapidly added under vigorous stirring (1200 RPM). The stirring was continued for 2 h during which the solution turned colorless.…”

“…Catalytic and biological applications of such materials are fast evolving. 2,[16][17][18] Along with this development, we have begun to explore the complex chemistry of these systems. [19][20] The different chemically non-equivalent environments at their surfaces offer different possibilities for ligand exchange.…”

Mixed-Monolayer-Protected Au25 Clusters with Bulky Calix[4]arene Functionalities

“…After the first report of calixarene-protected gold nanoparticles by Arduini et al 20 there has been an increased interest towards calixarene-modified gold nanostructures. 21−25 Recently, atomically precise MPCs (Au11) with five phosphine-bound calixarene moieties were realized by de Silva et al 26 and their structure was theoretically verified by Chen et al 27 . Important to note is that the number of calixarene ligands was fixed and their bulkiness led to incomplete capping leaving the gold core partly exposed.…”