The electrooxidation-induced structural changes of gold di-superatomic molecules: Au₂₃vs. Au₂₅

Abstract

The gold cluster compounds Au₃₈(SC₂H₄Ph)₂₄ and [Au₂₅(PPh₃)₁₀(SC₂H₄Ph)₅Cl₂]²⁺ are known to possess bi-icosahedral Au₂₃ and Au₂₅ cores, respectively, inside their ligand shells. These Au cores can be viewed as quasi-molecules composed of two Au₁₃ superatoms sharing three and one Au⁺ atoms, respectively. In the present work, we studied the structural changes of these gold di-superatomic molecules upon electrooxidation via spectroelectrochemical techniques, X-ray absorption fine structure analysis, and density functional theory calculations. The Au₂₃ core was electrochemically stable, but the Au₂₅ core underwent irreversible structural change. This marked difference in the stability of the oxidized states is ascribed to differences in the bonding scheme of Au₁₃ units and/or the bonding nature of the protecting ligands.

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Introduction

The Au₁₃ icosahedron is a ubiquitous structural motif that can be found in a variety of ligand-protected gold clusters, such as [Au₁₃(PMe₂Ph)₁₀Cl₂]³⁺,¹ [Au₂₅(SC₂H₄Ph)₁₈]⁻,^2,3 [Au₁₃(PPh₂C₂H₄PPh₂)₅Cl₂]³⁺,⁴ [Au₁₉(C≡CPh)₉(PPh₂NHPPh₂)₃]²⁺,⁵ and [Au₁₃(PPh₂C₂H₄PPh₂)₅(C≡CPh)₂]³⁺.⁶ All of these Au₁₃ cores formally accommodate eight valence electrons with a closed (1S)²(1P)⁶ electronic configuration, where S and P represent orbitals with angular momentum values of 0 and 1, respectively (Scheme 1).^7,8 Thus, the Au₁₃⁵⁺(8e) core can be viewed as a stable, rare gas-like superatom.⁸

Scheme 1 Schematic representation of the bonding scheme for (a) Au₂₃⁹⁺(14e) and (b) Au₂₅⁹⁺(16e) from their superatomic units (ref. 18 and 19). The energy scales are arbitrary and the energy splitting of superatomic orbitals (ref. 20 and 21) is ignored. Optical transitions A, B, and C correspond to the peaks in the optical spectrum (Fig. 1a).

The Au₁₃ core serves as the building unit for a new class of quasi-molecules known as superatomic molecules^9,10 in a manner analogous to the formation of molecules from atoms. To date, two different bonding modes have been revealed by single-crystal X-ray crystallographic studies. One is a face-sharing mode found in the Au₂₃⁹⁺(14e) core of Au₃₈(SR)₂₄.¹¹ The other is a vertex-sharing mode found in the Au₂₅⁹⁺(16e), Au₃₇¹³⁺(24e), and Au₆₀²⁰⁺(40e) cores of [Au₂₅(PR₃)₁₀(SR′)₅Cl₂]²⁺,¹² [Au₃₇(PR₃)₁₀(SR′)₁₀Cl₂]⁺,¹³ and [Au₆₀Se₂(PR₃)₁₀(SeR′)₁₅]⁺,¹⁴ respectively. A recent computational study suggests that the Au₂₃⁹⁺ core can be viewed as a dimer of halogen-like Au₁₃⁶⁺(7e) superatoms formed by sharing three Au⁺ atoms (Scheme 1a):⁹

Au₂₃⁹⁺(14e) = 2 × Au₁₃⁶⁺(7e) − 3Au⁺ (1)

The electronic configuration of Au₂₃⁹⁺(14e) may be described as (1σ)²(1σ*)²(1π)⁴(2σ)²(1π*)⁴. The 1σ and 1σ* orbitals are constructed from the 1S superatomic orbitals of Au₁₃⁶⁺(7e), whereas the 1π, 2σ, and 1π* orbitals result from the 1P superatomic orbitals of Au₁₃⁶⁺(7e). This super valence bond model can also be applied to bonding in the vertex-sharing Au₂₅⁹⁺, which may be considered as a dimer generated by rare gas-like Au₁₃⁵⁺(8e) superatoms sharing an Au⁺ atom (Scheme 1b):^12,15–17

Au₂₅⁹⁺(16e) = 2 × Au₁₃⁵⁺(8e) − Au⁺. (2)

The formal bond orders in Au₂₃⁹⁺ and Au₂₅⁹⁺ can be calculated to be one and zero, respectively.

In terms of the bonding scheme, the Au₂₃⁹⁺(14e) and Au₂₅⁹⁺(16e) cores correspond to an F₂ molecule and a van der Waals dimer of Ne, respectively. These di-superatomic molecules, thus, provide an interesting opportunity to study similarities and differences of the fundamental properties between conventional molecules and superatomic molecules. This study aims to compare the structural changes of superatomic molecules induced by electrooxidation with those of conventional molecules by ionization. In the case of molecules, the bond distance is the unique structure parameter that can be changed. For example, the F–F bond of F₂ is shortened from 1.44 to 1.33 Å upon ionization owing to the removal of an electron from the highest occupied molecular orbital (HOMO) of anti-bonding nature.²² The internuclear distance of Ne₂ is significantly reduced from 3.09 to 1.7 Å upon ionization owing to the charge-resonance stabilization in Ne₂⁺.²³ In the case of superatomic molecules, the structural change of superatomic units themselves may be induced in addition to the elongation/reduction of their distance. In the present study, we investigated how the geometric structures of the Au₂₃⁹⁺ and Au₂₅⁹⁺ cores are altered upon electrooxidation using spectroelectrochemical methods,²⁴ X-ray absorption fine structure (XAFS) analysis and density functional theory (DFT) calculations.

Experimental and computational methods

Synthesis of samples

All of the chemicals used in this study were commercially available. Samples of Au₃₈(SC₂H₄Ph)₂₄ (1⁰) and [Au₂₅(PPh₃)₁₀(SC₂H₄Ph)₅Cl₂]²⁺ (2²⁺) were synthesized according to literature procedures, but with slight modifications.^12,25,26 The details of the syntheses are described in the ESI.† Successful synthesis of both compounds was confirmed by matrix-assisted laser desorption ionization (MALDI) mass spectrometry and optical absorption spectroscopy.

Electrochemical measurements

Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) of 1 and 2 were performed using an electrochemical analyzer (BAS, ALS720B) at room temperature under an Ar atmosphere. The electrolyte solution used for these measurements was 0.1 M tetrabutylammonium hexafluorophosphate (NBu₄PF₆) in 1,2-dichloroethane (DCE) purged with Ar. The working, reference, and counter electrodes were a 1 mm glassy carbon disk, Ag/AgNO₃ (−0.21 V vs. Fc/Fc⁺), and a Pt wire, respectively. The potential scan was initiated at open circuit potential (0.01 and −0.27 V for 1 and 2, respectively) in the negative direction.

The optical absorption spectra of 1 and 2 in the UV-vis-NIR region were recorded under electrochemical redox conditions. These spectroelectrochemical measurements were conducted in a DCE solution containing 0.1 M NBu₄PF₆ that was purged with Ar prior to measurements using an electrochemical analyzer (BAS, ALS720B) and two spectrophotometers (Jasco, V-670 for 1 and Agilent, Agilent 8453 for 2). A quartz cell (1 mm width) was used for spectroelectrochemistry. The working, reference, and counter electrodes were an optically transparent Pt mesh electrode, an Ag/AgNO₃ electrode, and a Pt wire electrode, respectively. The Pt mesh electrode was set in the light path of the spectroelectrochemical cell.

Bulk electrolysis

The bulk electrolysis of 2 was carried out in a spectroelectrochemical 10 mm quartz cell (HX-701Y, Hokuto Denko) with an electrochemical analyzer (BAS, ALS720B). The working, reference, and counter electrodes were a Pt mesh electrode, an Ag/AgNO₃ electrode, and a Pt coil electrode, respectively. The electrolysis was conducted in a DCE solution containing 0.1 M NBu₄PF₆, which was purged with Ar prior to measurements. The completion of the bulk electrolysis was established by optical absorption spectroscopy.

XAFS spectroscopy

Au L₃-edge XAFS measurements were performed on the BL01B1 beamline at the SPring-8 facility of the Japan Synchrotron Radiation Institute (proposal nos. 2012B1986, 2014B1430, and 2014A1680). A Si(311) two-crystal monochromator was used for the incident beam and all spectra were recorded in transmission mode using ion chambers as the I₀ and I detectors. Samples of 2 diluted with boron nitride and 2 electrolyzed and diluted with NBu₄PF₆ were placed in the cryostat for measurements at 8 or 10 K. Energy scale was calibrated using Cu foil and data analysis was carried out employing the REX2000 Ver. 2.5.92 software package (Rigaku Co.). The k³-weighted χ spectra in the k range of 3.0–16.0 Å⁻¹ were Fourier-transformed (FT) into r-space. The curve fitting analysis was performed for Au–S (P, Cl) and Au–Au bonds over the r range of 1.5–3.2 Å. In the curve fitting analysis, the phase shifts and back-scattering amplitude functions for the Au–S and Au–Au bonds were extracted from Au₂S (ICSD#78718) and Au metal (ICSD#44362), respectively, using the FEFF8 program.²⁷ The phase shifts and back-scattering amplitude functions of Au–S were used for those of Au–P and Au–Cl because the electron densities of P and Cl atoms are close to that of S atom.

Transmission electron microscopy

Transmission electron microscopy (TEM) images were recorded by using a Hitachi HF-2000 microscope operated at 200 kV. A dispersion of the electrooxidized sample of 2 was dropped onto a carbon-coated copper grid. The sample grid was dried at room temperature in air.

Computational methods

The GPAW software,²⁸ a projector-augmented wave (PAW) and real-space grid DFT package, was employed to inspect the redox behavior of 1 and 2. Structural optimizations were performed in the gas phase environment with consideration of the full ligands, until the residual forces acting on the atoms reached 0.05 eV Å⁻¹ or below. The spin-polarized DFT simulations were performed with a real-space grid spacing of 0.2 Å and included the scalar relativistic effect for PAW setups. Linear response time-dependent DFT based on Casida's formulation²⁹ was employed to analyze the optical absorption spectra of 2²⁺ and 2³⁺. All the calculations in this work adopted the generalized gradient-corrected exchange–correlation functional developed by Perdew, Burke and Ernzerhof (PBE).³⁰

Results and discussion

Redox behavior of Au₃₈(SC₂H₄Ph)₂₄

The successful synthesis of Au₃₈(SC₂H₄Ph)₂₄ (1⁰) was confirmed by MALDI mass spectrometry (Fig. S1, ESI†). An intense peak at m/z = 1.078 × 10⁴ was assigned to intact Au₃₈(SC₂H₄Ph)₂₄, whereas a smaller peak at m/z = 9.33 × 10³ was attributed to a fragment with the formula Au₃₄(SC₂H₄Ph)₁₉S.²⁵ The optical absorption spectrum of 1⁰ (Fig. 1a) shows distinct peaks at 1.18 (peak A), 1.65 (peak B), 1.97 (peak C), 2.39, and 2.57 eV, in agreement with previously reported data.³¹ The peaks A, B and C have been theoretically assigned to the electronic transitions HOMO(1π*) → LUMO(2π*), HOMO−1(2σ) → LUMO+1(2σ*), and HOMO(1π*) → LUMO+3, respectively (Scheme 1a).¹⁸

Fig. 1 (a) Optical absorption spectrum and (b) CV curves of 1⁰.

Fig. 1b shows the CV curves of 1⁰ recorded at different scan rates; an arrow indicates the open circuit potential. These plots exhibit two pairs of redox peaks for the 0/+1 and +1/+2 couples at the same voltages, regardless of the scan rates. The peak-to-peak separations of the two couples are 58 and 61 mV, respectively, at a scan rate of 100 mV s⁻¹.³² This reversible behavior indicates that 1 and its oxidized forms 1¹⁺ and 1²⁺ are stable in the electrolyte solution. The formal potentials of the redox couples 1^0/1+ and 1^1+/2+ were determined to be 0.07 and 0.32 V, respectively. These values are smaller by several tens of meV than reported values obtained from CV using acetonitrile : benzene (1 : 1) as a solvent of the electrolyte solution.³³ Based on these results, it is evident that 1¹⁺ and 1²⁺ can be synthesized by the electrolysis of 1⁰ at 0.20 and 0.60 V, respectively.

The optical absorption spectra of 1⁰, 1¹⁺, and 1²⁺ are displayed in Fig. 2a. The positions of peaks A, B, and C of 1⁰ (Fig. 1a) are unchanged following electrolysis whereas the intensities of these peaks change depending on the charge states of 1. The intensities of peaks A and C decreased with oxidation. The reduction of the peak intensities upon oxidation to +1 and +2 is ascribed to the reduced probability of transition from the HOMO(1π*) orbital from which the electron(s) is/are removed. In contrast, the intensity of peak B did not change upon oxidation. This behavior can be understood by considering that the HOMO(1π*) orbital is not involved in the optical transition. Interestingly, the absorbance of the valley between peaks B and C increased upon oxidation, suggesting that a new optical transition became accessible in the oxidized state. According to a previously reported theoretical study,¹⁸ the increased absorbance at 1.8 eV can be attributed to an optical transition to the HOMO(1π*) orbital, in which one or more holes are created by oxidation.

Fig. 2 (a) Optical absorption spectra of 1⁰, 1¹⁺, and 1²⁺ and (b) spectra of 1¹⁺ and 1²⁺ following subtraction of the spectrum of 1⁰.

Fig. 2b shows the spectra obtained by subtracting the spectrum of 1⁰ from those of 1¹⁺ and 1²⁺. The peak positions evidently did not change upon oxidation, indicating that the energy levels of the superatomic orbitals of 1 do not change regardless of the charge state. Given that the electronic structures of small clusters are sensitive to slight difference in the geometric structure,³⁴ this result indicates that the geometric structure of 1 is not changed appreciably by the redox reactions. Namely, the bond length between the Au₁₃ units within 1 was not changed even after the formal bond order is increased from 1 to 2. This behavior is in sharp contrast to that of F₂, a molecular analogue of Au₂₃⁹⁺, since it is known that the F–F bond length in F₂⁺ (1.33 Å) is shorter than that in F₂ (1.44 Å).²² The negligible structural change of Au₂₃⁹⁺, even after 2-electron oxidation, thus demonstrates the structural rigidity of the face-sharing bi-icosahedral Au₂₃.

Furthermore, the DFT structural optimizations of 1⁰ and 1²⁺ support this finding showing that the bond distances of core atoms do not change significantly at different charge states (Fig. S3 and Table S1, ESI†). One of the HOMO(1π*) orbitals in 1⁰ is promoted to the LUMO orbital in 1²⁺, but the HOMO orbitals of these structures still retain the same symmetry of 1π* (Fig. S4, ESI†). Hence, there is no distinctive structural effect induced by the electronic factor. The rigidity of Au₂₃⁹⁺ may partly arise from the protection by the rigid Au–SR oligomers with different lengths: especially three –SR–Au–SR– “staples” bridge across the waist of two Au₁₃ icosahedra.³⁵

Redox behavior of [Au₂₅(PPh₃)₁₀(SC₂H₄Ph)₅Cl₂]²⁺

The synthesis of [Au₂₅(PPh₃)₁₀(SC₂H₄Ph)₅Cl₂]²⁺ (2²⁺) was confirmed by optical spectroscopy (Fig. 3a). The spectrum shows distinct peaks at 1.84, 2.98, 3.30, and 3.76 eV, which reproduces the reported data.^12,36 The CV curves of 2²⁺ shown in Fig. 3b exhibit irreversible waves in both the oxidative and reductive potential ranges. This result indicates that cluster 2²⁺ is unstable upon one-electron oxidation.

Fig. 3 (a) Optical absorption spectrum and (b) CV curves of 2²⁺.

Fig. 4a presents the optical absorption spectra obtained during one-electron oxidation of 2²⁺ at 0.60 V. The spectral profile changes with time while exhibiting three isosbestic points, as indicated by arrows. The peak at 1.84 eV drops in intensity and the optical gap is reduced during the electrooxidation reactions. This result implies that the electrooxidation of 2²⁺ yielded a certain product having less structured absorption spectrum than 2²⁺.

Fig. 4 (a) Optical absorption spectra of 2²⁺ following electrolysis at 0.60 V. (b) Deconvoluted spectra of oxidized species assuming that the spectrum after 40 min contains 20% (red) and 24% (blue) of 2²⁺. The black curve shows the spectrum of 2²⁺ for reference.

In order to gain insight into the electronic structures of the one-electron oxidation product, we attempted to extract its optical spectrum by assuming that 2²⁺ is converted to a single species upon electrooxidation. Namely, we assume that the spectra in Fig. 4a represent a linear combination of the spectra of unoxidized 2²⁺ and the oxidation product. Because the population of unoxidized 2²⁺ at a certain time is unknown, we further assume that the oxidation product does not have sharp peaks at the same position with those of 2²⁺. The optical spectra of the oxidation product extracted based on these assumptions are shown in Fig. 4b. Although there is an ambiguity in the spectral profile depending on the assumed concentration of unoxidized 2²⁺, we can conclude that the oxidation product is less structured as compared to 2²⁺ and optical onset is shifted to lower energy.

DFT calculations were carried out to study the geometric structure and optical properties of the one-electron oxidation product. Structural optimization of 2³⁺ starting from vertex-sharing biicosahedral cores with eclipsed and staggered configurations (Fig. S5, ESI†) both yielded the eclipsed motif, which is similar to that of 2²⁺ (Fig. 5a). The optimization results do not suggest that a staggered (twisted bi-icosahedral core)³⁷ configuration is preferred for 2³⁺, upon oxidation of 2²⁺. Since the HOMO of 2²⁺ has anti-bonding nature according to the previous theoretical calculations (Scheme 1b),^15,17 one would expect that the distance between the two Au₁₃ units to become smaller upon oxidation. In contrast to the prediction from Scheme 1b, the distance between the two Au₁₃ units is slightly increased upon oxidation: the average distance between the nearest neighboring Au atoms between the Au₁₃ units are 3.23 ± 0.018 and 3.27 ± 0.015 Å for 2²⁺ and 2³⁺, respectively (Fig. 5a). This unexpected trend is due to the fact that the HOMO of 2²⁺ obtained by the present calculation has bonding nature (Fig. S6, ESI†). Inspection of the shape of the HOMO suggests that it is constructed via bonding interaction between 1P superatomic orbitals of Au₁₃ moieties. Based on Scheme 1b, the HOMO of 2²⁺ corresponds now to a 2σ state. Meanwhile, both the HOMO−1 and HOMO−2 correspond to 1π* states; and the HOMO−3 refers to a 2σ* state (Fig. S6, ESI†). Therefore, the ordering of the energetic states of HOMO and HOMO−3 has been interchanged. We consider this as a ligand effect: the energetically closely spaced orbitals, which are of superatomic 1P character, are sensitive to the influence of localized p-orbital states of individual sulfur atoms at the cluster waistline, and this lowers the energy of the 2σ* state.

Fig. 5 (a) Optimized core structures and (b) optical absorption spectra of 2²⁺ and 2³⁺. For simplicity, the ligands are omitted. The individual transition values have been multiplied by a factor of 20, and the full range of optical absorption spectra are available in Fig. S7 (ESI†). There is no significant change between the core structures optimized in different charge states, with a slight increment for distances between the nearest neighboring Au atoms of the Au₁₃ moieties.

Fig. 5b shows the calculated optical absorption spectra of 2²⁺ and 2³⁺. The calculated spectrum of 2²⁺ reproduces well characteristic features of the experimental spectra (Fig. 3a). The spectrum of 2³⁺ appears to be similar to that of 2²⁺. Especially, the characteristic peak at ∼1.7 eV for 2²⁺ is still present for 2³⁺ (odd number of electrons, with spin-polarization), which is inconsistent with the experimental observation (Fig. 5b). This contradiction suggests that oxidized form 2³⁺ is not stable in solution and is transformed into another structure or structures.

The electrooxidation sample of 2²⁺ was examined by TEM. In the TEM images (Fig. S8, ESI†), only Au clusters smaller than 2 nm were observed although the sample contained unoxidized 2²⁺. This observation and the absence of a surface plasmon resonance band in the oxidized sample (Fig. 4b) suggest that aggregation of 2²⁺ is not induced upon oxidation. The oxidation product obtained after bulk electrolysis of 2²⁺ was further examined using XAFS. Fig. 6 shows the Au L₃-edge extended X-ray absorption fine structure (EXAFS) oscillations and FT-EXAFS spectra of 2²⁺ and the oxidation product. The results of the EXAFS analysis are summarized in Table 1. From these data, it is evident that the coordination numbers of the Au–Au and Au–ligand bonds did not change appreciably upon electrooxidation.

Fig. 6 (a) Au L₃-edge EXAFS oscillations and (b) FT spectra of 2²⁺ (blue) and the one-electron oxidation product (red).

Table 1 Fitted EXAFS parameters for the electrooxidation product

Sample	Atom^a	C.N.^b	r (Å)	D.W.^d	R (%)
a Bonding atom. b Coordination number. c Bond length. d Debye–Waller factor. e R factor.
2 ^2+	S (P, Cl)	1.1(2)	2.324(11)	0.0061(27)	9.9
	Au1	2.4(1.2)	2.744(3)	0.0045(7)
	Au2	4.2(7)	2.875(3)	0.0050(5)
One-electron oxidation product	S (P, Cl)	0.9(1)	2.306(4)	0.0040(15)	8.9
	Au1	1.5(2)	2.727(20)	0.0040(10)
	Au2	4.4(5)	2.860(22)	0.0076(12)

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The experimental and theoretical results reported above suggest that Au₂₅⁹⁺ with the vertex-sharing biicosahedral motif undergoes irreversible structural change upon oxidation. It is reported that phosphine-protected Au clusters such as [Au₁₁(PPh₃)₈Cl₂]⁺ are electrochemically unstable,³⁸ whereas thiolate-protected Au clusters such as [Au₂₅(SR)₁₈]⁻ are electrochemically stable.^38,39 Thus, difference in the electrochemical instability between Au₂₃⁹⁺ and Au₂₅⁹⁺ superatomic molecules may be associated with that in the protecting ligands. The Au₂₃⁹⁺ core is protected by three –SR–Au–SR– and six –SR–(Au–SR)₂– oligomers, whereas the Au₂₅⁹⁺ core is protected by five RS, ten PR₃ and two halides. One possible pathway is the isomerization of the Au₂₅ core induced by the release of the phosphine ligand upon electrooxidation. Such electrochemically irreversible structural isomerization has been observed in [Au₈(Ph₂P(CH₂)₃PPh₂)₄]ⁿ⁺ (n = 2 and 4).⁴⁰

Conclusions

The structural changes of Au₃₈(SC₂H₄Ph)₂₄ and [Au₂₅(PPh₃)₁₀(SC₂H₄Ph)₅Cl₂]²⁺ upon electrooxidation were studied by spectroelectrochemical and XAFS measurements and DFT calculations. The face-sharing Au₂₃⁹⁺ biicosahedron protected by Au–SR oligomers retains the structure upon oxidation, indicating that oxidation-induced geometric relaxation is negligibly small in contrast to the corresponding molecule F₂. In contrast, the vertex-sharing Au₂₅⁹⁺ biicosahedron protected by thiolates, phosphines and halides underwent irreversible structural change. This electrochemical instability is ascribed to the bonding scheme between two Au₁₃ units and/or the bonding nature of the protecting ligands.

Acknowledgements

We thank Prof. Hiroshi Nishihara (The University of Tokyo) for providing us with access to the TEM apparatus. This research was financially supported by the Elements Strategy Initiative for Catalysis & Batteries (ESICB) and by a Grant-in-Aid for Scientific Research (no. 26248003) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan. Financial support from the Centres of Excellence Program (Project 284621) by the Academy of Finland and the graduate student scholarship by Magnus Ehrnrooth foundation are gratefully acknowledged. Computational resources were provided by CSC – the IT Center for Science Ltd Espoo, Finland.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5cp06969f

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