Electrosynthesis of Atomically Precise Au Nanoclusters

Abstract

Innovation in synthesis methodologies is crucial for advancing the discovery of new materials. This work reports the electrosynthesis of a [Au₁₃(4-^tBuPhC≡C)₂(Dppe)₅]Cl₃ nanocluster (Au₁₃ NC) protected by alkynyl and phosphine ligands. From simple precursor, HAuCl₄ and ligands, the whole synthesis is driven by a constant potential in single electrolytic cell. X-ray crystallography determines its total structure. Control experiments, cyclic voltammetry, Proton Nuclear Magnetic Resonance (¹H NMR), gas chromatography, and other characterizations demonstrate that a critical tetranuclear Au(I) complex defines the electrochemical redox behavior of the reaction solution. The critical role of a base (e.g., triethylamine) is to suppress the hydrogen evolution reaction at the cathode, paving the way for the reduction of Au ions. To resolve the problem of over-reduction and deposition of Au on the cathode, pulsed electrolysis, which is specific to electrosynthesis is employed. It significantly improves the reaction rate and the isolated yield of Au₁₃. To extend the application scope, another four NCs protected by different ligands, [Au₁₃(4-FPhC≡C)₂(Dppe)₅]Cl₃, [Au₈(2-CF₃PhC≡C)₂(Dppp)₄](PF₆)₂, [Au₁₁(Dppp)₅]Cl₃, and [Au₈(SC₂H₄Ph)₂(Dppp)₄]Cl₂ are synthesized electrochemically, demonstrating the versatility of the strategy.

Keywords: Au nanoclusters; electrosynthesis; pulsed electrolysis.

Figure 1

Illustration of the electrosynthesis of [Au₁₃(4‐ ^t BuPhC≡C)₂(Dppe)₅]Cl₃ (Au₁₃). Ligands: 4‐tert‐butylphenylacetylene (TBA) and 1,2‐bis‐ (diphenylphosphino)ethane (Dppe). Anode: graphite. Cathode: Ni electrode. Color code: yellow, Au; pink, P; gray, C.

Figure 2

Total structure a) and core structure b) of Au₁₃ . c) Distribution and coordination patterns of phosphine ligands. d) Mass spectrum of Au₁₃ . Inset: experimental (black trace) and simulated (red trace) isotopic patterns of the molecular ion peak for [Au₁₃(4‐ ^t BuPhC≡CR)₂(Dppe)₅]Cl₃. e) UV−vis absorption spectra of the Au₁₃ solution before (black line) and after 12 h electrolysis (red line). The inset is photographs of the vessel before (left) and after (right) electrolysis.

Figure 3

a) UV−vis spectra of the solutions from divided cell. The spectrum of Au₁₃ is included for comparison. Insert: the photograph of the reaction vessel after 12 h electrosynthesis. The right part of the vessel is the cathodic compartment. b) Cyclic voltammograms (CV) of the solution at different feeding stages (scan rate = 100 mV s⁻¹). Me₂S is dimethyl sulfide, TBA is 4‐tert‐butylphenylacetylene and Dppe is 1,2‐bis‐ (diphenylphosphino)ethane). c) The CV comparison of the solutions containing different components. They show evident resemblance. d) The crystal structure of Au₄(Dppe)₂Cl₄. e) The powder XRD patterns of the dried reaction solution (before electrolysis, red line) and the one simulated from the single crystal of Au₄(Dppe)₂Cl₄ (black line). f) UV–vis absorption spectra of Au nanoclusters synthesized by electrosynthesis (red line) and by NaBH₄ (black line).

Figure 4

a) UV−vis spectra of the reaction solution after electrolysis with different reactants. Inset: corresponding photographs. b) ¹H NMR spectra of reaction solution after electrosynthesis using HAuCl₄ and Et₃N (red line) and the spectrum of Et₃N (blue line). c) High‐resolution mass spectrum of the reaction solution the same as that in (b). d) Linear sweep voltammetry (LSV) curves of reaction solution in the presence of different amounts of Et₃N. Scan rate, υ = 5 mV s⁻¹. Working electrode: Ni plate; counter electrode: graphite; reference electrode: saturated calomel electrode. e) Gas chromatography (GC) analysis: the signal response of H₂ during electrosynthesis with different amount of Et₃N. f) XRD pattern of the dried reaction solution using NaOH as a base. Inset: UV−vis spectrum and the photograph of vessel after reaction.

Figure 5

a) (I) represents the results obtained under constant potential. The photo on the left is the Ni electrode before and after electrosynthesis. The one in the middle is the reaction vessel after 4 h of electrosynthesis. The one on the right is the single crystal of Au₁₃ . (b) and (c) are corresponding results under pulsed potentials. Pulse width is 60 s for (II) and 10 seconds for (III). d) XRD patterns of the used Ni electrodes. e) UV–vis spectra of the solutions formed by rinsing the used Ni electrodes with CH₂Cl_2.

Figure 6

Scope of application. The structure anatomy of [Au₁₃(4‐FPhC≡C)₂(Dppe)₅]Cl₃ (Au₁₃‐2) a). [Au₈(2‐CF₃PhC≡C)₂(Dppp)₄](PF₆)₂ (Au₈‐1) b). [Au₁₁(Dppp)₅]Cl₃ (Au₁₁ ) (c) and[Au₈(SC₂H₄Ph)₂(Dppp)₄]Cl₂ (Au₈‐2) d). Color code: yellow, Au; pink, P; gray, C; green, F; blue, S. All hydrogen atoms are omitted for clarity.