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

Abstract

Crystallization of atomically precise nanoclusters is gaining increasing attention, due to the opportunity of elucidating both intracluster and intercluster packing modes, and exploiting the functionality of the resulting highly pure crystallized materials. Herein, we report the design and single-crystal X-ray structure of a superfluorinated 20 kDa gold nanocluster, with an Au₂₅ core coated by a shell of multi-branched highly fluorinated thiols (SF₂₇) resulting in almost 500 fluorine atoms, i.e., ([Au₂₅(SF₂₇)₁₈]⁰). The cluster shows a switchable solubility in the fluorous phase. X-ray analysis and computational studies reveal the key role of both intracluster and intercluster F···F contacts in driving [Au₂₅(SF₂₇)₁₈]⁰ crystal packing and stabilization, highlighting the ability of multi-branched fluorinated thiols to endow atomically precise nanoclusters with remarkable crystallogenic behavior.

Fig. 1. DFT characterization of F···F interactions in the F₂₇SH crystal structure.

a Molecular structure of F₂₇SH and graphical representation of intra- and inter-molecular F···F interactions in F₂₇SH. Atom color code: C, gray; 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, c F···F intermolecular interaction energy calculated at M062x/aug-cc-pvtz level of theory with GD3 Grimme’s empirical dispersion corrections and Wiberg bond orders; the inset of b reports the total interacting energy (kcal/mol) for every F···F interacting pair; d non-covalent interactions map calculated at the same level of theory of b and c showing intra and inter non-covalent interacting regions between the branches of two nearby thiol molecules. The reduced density gradient is plotted versus the sign of the second density Hessian eigenvalue multiplied by the density itself (sign(λ₂)ρ); the conventional color code applied uses blue to indicate attractive (negative values of sign(λ₂)ρ) interactions, red for repulsive (positive values of sign(λ₂)ρ) interactions and green for weak van der Waals interactions (sign(λ₂)ρ close to zero); e, f three-dimensional iso-surface plots of non-covalent interactions (isovalue 0.8) calculated in a single thiol molecule and in two nearby thiols visualizing interactions as green surfaces for weak van der Waals, red surfaces for non-bonding repulsive steric interactions. Red, green, blue, and black colors correspond to the classification of F···F interactions indicated in this work.

Fig. 2. Synthesis of AuNC stabilized by F₂₇SH and crystallization of [Au₂₅(SF₂₇)₁₈]⁰.

a Cartoon representation of AuNC stabilized by F₂₇SH thiol. For the sake of clarity, only 6 F₂₇S-ligands have been reported; b schematic representation of crystal formation in solkane solution and colorimetric change upon their dissolution in PFO; c UV–Vis spectra of crude product in solkane and crystals redissolved in PFO; d, e STEM images of crude product showing the presence of small clusters and bigger AuNPs; f, g STEM images of redissolved crystals solution showing the homogeneous presence of small clusters.

Fig. 3. Ultrafast spectroscopy of [Au₂₅(SF₂₇)₁₈]⁰.

a ΔA map of the crystal solution in PFO as a function of probe delay and probe wavelength. b ΔA time traces selected at specific probe wavelengths. c EAS spectra obtained from global analysis fit. d Model of electronic energy relaxation following 400 nm pump excitation. e Extracted coherent oscillations from ΔA time trace measured at 350 nm probe wavelength, after subtraction of a bi-exponential decay and its corresponding Fourier transform spectrum.

Fig. 4. Crystallography of [Au₂₅(SF₂₇)₁₈]⁰ (Mercury CSD 4.2.0).

a Au₂₅ core. Color code: Au_cent in yellow, Au_inn in green, Au_out in pink; b Au₂₅S₁₈ unit, highlighting the six S–Au_out–S–Au_out–S staple motifs and the three planes they lay on. Sulfur atoms are colored in light blue (terminal) and black (bridged), respectively; c Au₂₅S₁₈ unit with C₃ alkyl portion of each thiol. The carbon atoms are colored in gray; d representation of one [Au₂₅(SF₂₇)₁₈]⁰ NC showing the fluorine masking of the gold core; e representation of one [Au₂₅(SF₂₇)₁₈]⁰ NC where the inter-cluster F···F contacts are shown as red dotted lines; f overall crystal packing of four [Au₂₅(SF₂₇)₁₈]⁰ NCs, view along the crystallographic b axis. a–c, e in ball-and-stick model, d, f in space-fill model.

Fig. 5. DFT characterization of F···F interactions in the [Au₂₅(SF₂₇)₁₈]⁰ crystal structure.

a, b F···F Intermolecular interaction energy calculated at M062x/aug-cc-pvtz level of theory with GD3 Grimme’s empirical dispersion corrections as a function of intra- and inter-cluster F···F distances between two cluster F₂₇S- ligands. Color code: red, intra-CF₃; green, F···F intra-branches; blue, F···F inter-branches; and black, F···F inter-F₂₇S-interactions; the inset text box of b reports the total interaction energy (kcal/mol) for every F···F interacting cluster pair; c surface representation of F···F intermolecular interaction energy in the inter-cluster region; color code: −0.30 (red) < −0.15 (white) < 0.00 (blue); d non-covalent interaction map visualized by using promolecular densities showing intra- and inter- non-covalent interacting regions of two nearby clusters. The reduced density gradient is plotted versus the sign of the second density Hessian eigenvalue multiplied by the density itself (sign(λ₂)ρ); color code: blue for attractive (negative values of sign(λ₂)ρ) interactions, red for repulsive (positive values of sign(λ₂)ρ) interactions and green for weak van der Waals interactions (sign(λ₂)ρ close to zero); e magnification of an area of d for inter-cluster F···F interactions; f table of total interacting energy values (kcal/mol) calculated for every F···F interacting pair.