Golden Opportunity: A Clickable Azide-Functionalized [Au25(SR)18]-Nanocluster Platform for Interfacial Surface Modifications

Abstract

Ultrasmall atomically precise monolayer-protected gold thiolate nanoclusters are an intensely researched nanomaterial framework, but there is a lack of a system that can be directly synthesized and undergo interfacial surface chemistry. We report an [Au<sub>25</sub>(SCH<sub>2</sub>CH<sub>2</sub>-<i>p</i>-C<sub>6</sub>H<sub>4</sub>-N<sub>3</sub>)<sub>18</sub>]<sup>-</sup> nanocluster platform with azide moieties appended onto <i>each </i>surface ligand. The structure of this surface reactive cluster has been confirmed by single-crystal X-ray crystallography, mass spectrometry and UV-Vis, IR and NMR spectroscopies. We show that <i>all</i> surface azide moieties are amenable to cluster-surface strain-promoted alkyne-azide cycloaddition (CS-SPAAC) chemistry with a strained cyclooctyne, opening this as a new <i>platform </i>to allow functional, post-assembly surface modifications to this very prominent nanocluster.

Size-Controlled Synthesis of Modifiable Glycine-Terminated Au Nanoclusters as a Platform for Further Functionalization

“…With a further increase in the concentration of the reducing agent (0.32 M NaBH 4 and above), rapid precipitation takes place and the solution turns clear immediately (i.e., no clusters remain in solution). We have previously shown that Au 25 (SR) 18 NCs are much more stable in high concentrations of NaBH 4 than larger NCs; 7 we believe that higher NaBH 4 concentrations lead to preferential precipitation of larger NCs during the synthesis, which may be either due to the reductive removal of thiolates off their surfaces or the poorer solubility of the larger NCs. Similar results have been previously reported for the Au 25 (MUA) 18 system (where MUA = mercaptoundecanoic acid).…”

“…We have shown that using higher concentrations of the reducing agent (NaBH 4 ) in the original synthesis of Fmoc-Gly-CSA-functionalized Au NCs leads to smaller NCs and narrow size distributions. At a concentration of 0.30 M NaBH 4 , Fmoc-Gly-CSA-functionalized Au 25 (SR) 18 NCs have been synthesized which have characteristic UV−vis absorption peaks at 690, 440, and 390 nm and core sizes of 1.1 ± 0.2 nm. Fmoc deprotection gives stable Au NCs with primary amine functional groups on the surface with no significant change of the NC size or UV−vis spectral signature.…”

“…17 Also, Gunawardene et al reported the surface modification of [Au 25 L 18 ] − NCs with azide moieties using azide-amide cycloaddition chemistry. 18 Such methods open up the possibility of functionalization and surface modification of very prominent Au 25 NCs.…”

Seeing Ligands on Nanoclusters and in Their Assemblies by X-ray Crystallography: Atomically Precise Nanochemistry and Beyond

“…With the identification of every ligand in the ligand shell, it is possible to precisely functionalize the surface of Au 25 NCs by site-specific ligand exchange. 58 Recently, a clickable azidefunctionalized [Au 25 (SR) 18 ] − was reported (Figure 2i), 48 in which the N α 's of the azide moieties ligated on the phenyl rings make an average C−C−N α angle of ∼122.7°, indicating the primarily sp 2 character of the N α , and the N α −N β −N γ is almost linear, as indicated by SCXRD. Such a configuration enables the full amenability of azide moieties to strain-promoted alkyne−azide cycloaddition chemistry.…”

Surface Menshutkin SN2 Reaction on Basic Gold Clusters Provides Novel Opportunities for the Cationization and Functionalization of Molecular Metal Clusters

“…Besides these methods, surface reactions have also received significant attention in recent years; however, there are few reports on such reactions, two of which are azido-and amide-bond formation reactions. 18,19 The Menshutkin S N 2 reaction is a common method to obtain the quaternary ammonium salt ([−N(CH 3 ) 3 ] + ) by reacting a tertiary amine with an alkylating agent. 20 In this study, we demonstrate the Menshutkin (methylation) reaction of the surface pyridyl (Py) moieties on basic Au 25 (4-PyET) 18 clusters (4-PyET = −SCH 2 CH 2 Py) for their successful transformation into cationized Au 25 (4-PyET-CH 3 + ) x (4-PyET) 18-x (x ≈ 18) clusters (Scheme 1).…”