High-resolution crystal structure of a 20 kDa superfluorinated gold nanocluster

The design, synthesis, and high-resolution single-crystal X-ray structure of a superfluorinated 20 kDa gold nanocluster was achieved. The nanocluster presents an Au25 core coated by a shell of multi-branched highly fluorinated thiols (SF27) resulting in almost 500 fluorine atoms.

Atomically precise gold nanoclusters (AuNC) are ultrasmall nanomaterials (< 2 nm) with molecule-like electronic and optical properties, arising from the quantum confinement of free electrons to the discrete electronic state. Gold nanoclusters are endowed with fluorescence and catalytic activities, features that, along their extra small size, promoted their application in several high-end fields, ranging from imaging, nanomedicine, biosensing, catalysis, and energy conversion. For these reasons, the search for new synthetic routes and stabilizing agents for novel atomically precise nanoclusters is growing rapidly.

In the last few years, our research group has developed several gold nanoparticle (AuNPs) systems stabilized by fluorinated thiols. The presence of fluorine in stabilizing ligands leads to enhanced hydrophobicity and stiffness of the ligand shell, and can promote self-assembly via F···F contacts. Despite the established role of fluorinated compounds in both crystal engineering and functional materials design, very few examples of atomically precise metal nanoclusters having a high content of fluorine atoms in their ligand-shells have been reported, to now.

In the attempt to extend the use of fluorinated ligands to the stabilization of AuNC, we designed a bulky multi-branched highly fluorinated thiol (C20H14F27O4SH and hereafter F27SH; Fig. 1a) with strong crystallization tendency. To clarify the self-assembly features of the newly-developed ligand and investigate the nature of F···F contacts occurring in F27SH, both intra and intermolecularly, bond order (BO) analysis and Non-Covalent Interactions studies were performed. Computational data revealed the attractive nature of F···F contacts occurring both intra- and inter-branches, as well as inter-thiol interactions. These interactions, although independently weak, are numerous and act additively, creating a network of weak but locally stabilizing interactions, assisting F27SH crystallization.

Figure 1: a) Molecular structure of F27SH and graphical representation of intra- and inter-molecular F···F interactions in F27SH. Atom color code: C, grey; O, red; F, light yellow; S, dark yellow and H, white.  F···F interactions color code: red, intra-CF3; green, F···F intra-branches; blue, inter-branches; and black, F···F inter-F27SH; b) Schematic representation of crystal formation in solkane solution and colorimetric change upon their dissolution in PFO; c) Representation of one [Au25(SF27)18]0 NC where the inter-cluster F∙∙∙F contacts are shown as red dotted lines. Color code: Aucent in yellow, Auinn in green, C in grey, F in green.


Encouraged by F27SH self-assembly features highlighted by computational studies, we reasoned that, by binding the gold core, F27S- ligand would achieve an optimal spatial distribution that could favour the F···F contacts as key supramolecular tools for the assembly of fluorinated AuNC in the solid state. We therefore developed a modified Brust procedure that afforded fluorinated AuNC (F-AuNC), which were easily dispersed in 1,1,1,3,3-pentafluorobutane (solkane) yielding a brown solution. UV-Vis and Scanning Transmission Electron Microscopy (STEM) techniques revealed that the crude synthetic product presented [Au25(SF27)18]- species together with bigger nanoparticles. After two months, the sample, stored at room temperature, showed the formation of dark crystals that were scarcely soluble in a partially fluorinated solvent like solkane, but dissolved well in a fluorous medium such as perfluorooctane (PFO), forming a green solution (Fig 1b). UV-Vis and MALDI studies clarified that the [Au25(SF27)18]- specie had spontaneously oxidized yielding, in the form of dark crystals, [Au25(SF27)18]0 nanoclusters. Single Crystal X-Ray analysis (Fig 1c) confirmed that the dark crystal was composed of [Au25(SF27)18]0, which, bearing 486 F atoms, represents the most fluorinated nanoobject ever described in the crystalline state.

In summary, our study reveals the ability of a multi-branched superfluorinated thiol to stabilize atomically precise nanoclusters and drive their crystallization through F···F interactions. Notably, the use of fluorinated ligands not only provides the clusters with valuable self-assembly properties, but also endows them with additional features, such as solubility in the fluorous phase.

For more details on this work, please read the full paper in Nature Communications at https://www.nature.com/articles/s41467-022-29966-2.





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