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. 2022 Nov 4;2(11):2627–2634. doi: 10.1021/jacsau.2c00519

Supervalence Bonding in Bi-icosahedral Cores of [M1Au37(SC2H4Ph)24]− (M = Pd and Pt): Fusion-Mediated Synthesis and Anion Photoelectron Spectroscopy

Emi Ito †, Shun Ito †, Shinjiro Takano †, Toshikazu Nakamura ‡, Tatsuya Tsukuda †,*
PMCID: PMC9709954  PMID: 36465538

Abstract

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Au38(PET)24 (PET = SC2H4Ph) is known to have a bi-icosahedral Au23 core consisting of two Au13 icosahedrons by sharing three Au atoms. Previous theoretical studies based on a supervalence bond (SVB) model have demonstrated that the bonding scheme in the Au23 core is similar to that in the F2 molecule. The SVB model predicted that the electron configuration of the Au23 core with 14 valence electrons is expressed as (1Σ)2(1Σ*)2(1Π)4(2Σ)2(1Π*)4 where each orbital is created by the bonding and antibonding interactions between the 1S and 1P superatomic orbitals of the icosahedral Au13 units. Therefore, the bi-icosahedral Au23 can be viewed as a di-superatomic molecule. To validate the SVB model, we herein conducted anion photoelectron spectroscopy (PES) on [M1Au37(PET)24]− (M = Pd and Pt), which are isoelectronic and isostructural with Au38(PET)24. To this end, the neutral precursors [M1Au37(PET)24]0 were first synthesized by fusion reactions between hydride-doped clusters [HAu9(PPh3)8]2+ and [M1Au24(PET)18]−. The formation of bi-icosahedral M1Au22 cores with open electronic structure in [M1Au37(PET)24]0 was confirmed by single-crystal X-ray diffraction analysis and electron paramagnetic resonance measurement. Then, the target anions [M1Au37(PET)24]− were obtained by reducing [M1Au37(PET)24]0 with NaBH4, and isoelectronicity with [Au38(PET)24]0 was confirmed by optical spectroscopy and density functional theory calculations. Finally, anion PES on [M1Au37(PET)24]− observed two distinctive peaks as predicted by the SVB model: one from the nearly degenerate 1Π* orbitals and the other from the nearly degenarate 1Π and 2Σ orbitals.

Keywords: superatomic molecules, supervalence bond (SVB) model, bi-icosahedral core, fusion-mediated synthesis, single-crystal X-ray diffraction, anion photoelectron spectroscopy, density functional theory calculation

Introduction

Since pioneering works by Teo,1 an increasing number of ligand-protected gold clusters with anisotropic structures have recently been reported.2−14 Single-crystal X-ray diffraction (SCXRD) analysis revealed that their Au cores are composed of small Au cluster units that are called superatoms based on the similarity of their quantized electronic structures compared with those of conventional atoms. Table 1 lists typical examples of core structures of ligand-protected anisotropic Au clusters composed of a variety of superatomic units and bonding patterns. For instance, different types of dimer and trimer of the icosahedral Au13 units were formed by sharing a facet3 or vertex atom(s).2,5,7,9 Hydrides mediate bonding between the facets of the Au12 units and those of the Au11 units to form Au24 and Au22, respectively.11,12 The hydride-mediated bonding between Au9 and Au11 was also found in the Au20 heterodimer.10 Because the structures of the superatomic units are retained, these anisotropic clusters can be viewed as “superatomic molecules”.15−22

Table 1. Examples of Au Superatomic Moleculesa.

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a

The ligand layer is omitted for simplicity.

The bonding theory of superatomic molecules is an interesting subject based on an analogy of the chemical bonding theory of conventional molecules.23 A supervalence bond (SVB) model has been proposed for the bi-icosahedral Au23 core of a thiolate-protected Au cluster Au38(PET)24 (referred to as Au23 hereafter, PET = SC2H4Ph).16,17,22 According to the SVB model, the electron configuration of Au23 with 14 electrons is expressed as (1Σ)2(1Σ*)2(1Π)4(2Σ)2(1Π*)4: the 1Σ and 1Σ* orbitals are constructed by the bonding and antibonding interactions between the 1S superatomic orbitals (SOs) of the Au13 units, respectively; the 1Π/1Π* and 2Σ/2Σ* orbitals were constructed by the tangential and coaxial overlaps of the 1P SOs of the Au13 units.16,17 A comparison with conventional diatomic molecules showed that such a bonding scheme in Au23 is similar to that in F2 molecules. The extension of this idea to Pd2Au36(PET)24 (Pd2Au21) with 12 electrons suggests that the ground state is triplet, similar to O2 molecules. However, the superconducting quantum interference device (SQUID) magnetometer measurement demonstrated that Pd2Au21 was not a triplet species but a singlet species.24 This phenomenon was explained by the breaking of the degeneracy of the highest occupied molecular orbitals (HOMOs) by lowering the core symmetry, implying that a Hund’s-type rule is not applicable for Pd2Au21. It was also found that the 1P SOs of the Pd@Au12 units in Pd2Au21 overlap in a tilted configuration.24 Thus, the bonding theory of superatomic molecules is still in its infancy.

A promising experimental approach for validation of the SVB model is gas-phase photoelectron spectroscopy (PES) that allows us to probe the absolute energy levels and the density of states of the occupied SOs with reference to the vacuum level.25−28 A monoanion of Au23 is an ideal target for PES because it allows mass selection prior to the PE measurement. However, a stable monoanion of Au23 could not be produced even with the strong reducing agent NaBH4 because of the closed electronic structure of Au23. To circumvent this problem, we used [Pd1Au37(PET)24]− (Pd1Au22–) and [Pt1Au37(PET)24]− (Pt1Au22–) obtained by reducing the corresponding neutrals Pd1Au37(PET)24 (Pd1Au22) and Pt1Au37(PET)24 (Pt1Au22), respectively: the formation of bi-icosahedral M1Au22 cores with an open electronic structure in M1Au22 was confirmed by SCXRD analysis and electron paramagnetic resonance (EPR) measurement. In addition, the isoelectronicity of M1Au22– (M = Pd and Pt) and undoped Au23 was confirmed by optical spectroscopy and density functional theory (DFT) calculations. The PE spectra of the mass-selected M1Au22– (M = Pd and Pt) were recorded in the gas phase. Two peaks in the PE spectra were assigned to the PE detachment from the 1Π, 2Σ, and 1Π* orbitals with the help of DFT calculations.

Results and Discussion

Details of the synthesis and characterization are provided in the Materials and Method section. Neutral precursor clusters M1Au22 (M = Pd and Pt) were synthesized using the fusion reactions between the hydride-doped Au cluster [HAu9(PPh3)8]2+ (Au9-H) and the PET-protected clusters [M1Au24(PET)18]− (M1Au12–), respectively (Table 2).29−31 First, Au9-H was prepared by adding an equal amount of NaBH4 to [Au9(PPh3)8]3+.29 Independently, M1Au12– (M = Pd and Pt) was synthesized according to the reported methods.24,30,31 Then, the Au(I)-PET oligomer was prepared by mixing Au(I)ClSMe2 and PET with 2:1 equivalence. Finally, two DCM solutions containing the equivalent amount of Au9-H and M1Au12– (M = Pd and Pt) were mixed, followed by the addition of 6 equiv of Au(I)-PET (entries 1 and 2, Table 2). Similarly, Au23 was synthesized as a reference by the fusion reaction between Au9-H and Au25(PET)18 (Au13) (entry 3, Table 2).32,33 The products were isolated by chromatography.

Table 2. Synthesis of M1Au22 (M = Pd, Pt, and Au).

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      Isolated product
Entry Hydride-doped clustersa PET-protected clustersb Formulac Yield (%)
1 Au9-H Pd1Au12– Pd1Au22 17
2 Au9-H Pt1Au12– Pt1Au22 13
3 Au9-H Au13 Au23 11
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a

Au9-H: [HAu9(PPh3)8]2+.

b

Pd1Au12–: [Pd1Au24(PET)18]−, Pt1Au12–: [Pt1Au24(PET)18]−, and Au13: Au25(PET)18.

c

Pd1Au22: Pd1Au37(PET)24, Pt1Au22: Pt1Au37(PET)24, and Au23: Au38(PET)24.

The charge state of the isolated products of entries 1–3 was assigned to be neutral because no ion signal was observed in attempts to measure the mass spectrum using electrospray ionization (ESI). Thus, they were characterized by positive-ion matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (Figure 1a), EPR spectroscopy (Figure 1b), UV–vis absorption spectroscopy (Figure 1c), and SCXRD (Figure 1d, Table S1). The MALDI mass spectrum of the isolated product of entry 1 (green trace in Figure 1a) indicates the successful synthesis of Pd1Au22. Although the MALDI mass spectrum of the product of entry 2 (pink trace in Figure 1a) exhibits the mass peak at m/z ∼ 10770 close to that of Pt1Au22 (calculated m/z 10776.25), this analysis alone could not exclude the possibility of assignment to the undoped Au23 (m/z 10778.14). To facilitate the assignment, the EPR spectrum of the product of entry 2 was recorded in a frozen DCM at 5 K (Figure 1b). The EPR profiles were fitted with the following parameters: g tensor (g1, g2, g3), anisotropic hyperfine coupling tensor (A1, A2, A3) between two Au (I = 3/2) nuclei with natural abundances, and line width. The fitted parameters are listed in Table S2. The results show that both products of entries 1 and 2 are paramagnetic and have an open electronic structure, whereas Au23 is diamagnetic. Our EPR spectrum agreed well with that of Pt1Au22 recently synthesized by a co-reduction method.34 We conclude from these results that the products of entries 1 and 2 are assigned to Pd1Au22 and Pt1Au22, respectively. The product of entry 3 was unambiguously assigned to Au23 based on the MALDI MS and the characteristic UV–vis spectrum.35−38 Under optimized synthesis conditions, the isolated yields of Pd1Au22, Pt1Au22, and Au23 were 17, 13, and 11%, respectively, based on the amount of Pd, Pt, and Au. Synthesis of M1Au22 (M = Pd and Pt) was attempted by reactions between the hydride-doped alloy clusters [HPd1Au8(PPh3)8]+ (Pd1Au8-H) or [HPt1Au8(PPh3)8]+ (Pt1Au8-H) and Au13.24,30 However, the reaction was dominated by the core growth reactions of M1Au8-H toward M1Au12, and the yields of M1Au22 were lowered (Table S3). These results show that Au9-H is more reactive toward the fusion reactions than M1Au8-H.24

Figure 1.

Figure 1

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(a) Positive-ion MALDI mass spectra of the products of entries 1–3. Peaks with an asterisk are assigned to laser-induced fragments (Au4(PET)4 loss). The inset shows the expanded mass spectra. (b) Experimental (colored, solid) and simulated (black, dotted) X-band EPR spectra of the products of entries 1 and 2. (c) Optical absorption spectra of the products of entries 1–3 in CHCl3 as a function of the photon energy. The spectra as a function of the wavelength are shown in Figure S1. (d) Crystal structures of the products of entries 1 and 2. Metal and sulfur atoms are shown as spheres. Color code: yellow, Au; teal, Pd; pink, Pt; red, S; and gray, C. Organic residues are depicted as gray sticks, and H atoms and disordered parts are omitted for clarity. In the products of entries 1 and 2, the central position is treated as half-occupancy of the Pd/Au and Pt/Au atoms, respectively.

The effects of Pd or Pt doping on the geometric and electronic structures were examined by comparing M1Au22 (M = Pd and Pt) and Au23. The optical spectra of M1Au22 exhibited a similar profile to that of Au23, except for an additional band at a low energy region of <1 eV (Figure 1c). The additional bands are assigned to the electronic transition to a singly occupied molecular orbital (SOMO) and are another indication of the open electronic shells. This result indicates that the electronic structures of Au23 and M1Au22 are similar, except that the electronic shell is closed or open. The effect of Pd/Pt doping on the geometric structures was investigated by SCXRD. Figure 1d represents the SCXRD results for Pd1Au22 and Pt1Au22 obtained at 103 and 100 K, respectively. The absence of counterions supported that Pd1Au22 and Pt1Au22 are neutral in charge state and electronically open. Spin density analysis by a single-point DFT calculation of Pd1Au22 showed that the unpaired electron was delocalized over the Pd1Au22 core (Figure S2). The structures look nearly identical to that of Au23: they are composed of bi-icosahedral M1Au22 (M = Pd and Pt) cores protected by three Au1(PET)2 and six Au2(PET)3 oligomers.3,24 The location of the Pd dopant in Pd1Au22 was identified at one of the centers of two icosahedrons,24,39 whereas that of the Pt dopant in Pt1Au22 was not identified because Au and Pt could not be distinguished by SCXRD. Nevertheless, it is most likely that the Pt dopant occupied either the center of the bi-icosahedrons based on the previous examples in which Pt exclusively took the central site of Au superatoms.24,30,31,34,40−45 A qualitative comparison of the geometric structures of M1Au22 (M = Pd, Pt, and Au) is summarized in Table 3. Although a slight elongation in the interdimer distance Di–i was observed, the bond lengths within Au23 and M1Au22 (M = Pd and Pt) are comparable. In summary, the structures of M1Au22 (M = Pd and Pt) are nearly identical to that of Au23.

Table 3. Selected Structural Information on M1Au22 (M = Pd, Pt, and Au) Coresa.

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Cluster Di–ib De–ec Drbd Dsbe
Au23f 4.087 8.427 2.751–2.835 2.756–3.025
Pd1Au22 4.094 8.433 2.744–2.833 2.758–3.126
Pt1Au22 4.112 8.453 2.743–2.814 2.797–3.030
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a

From SCXRD analysis.

b

Au–M distance in Å.

c

Averaged distance of both ends in Å.

d

Radial bond distribution of each icosahedron in Å.

e

Surface bond (<3.3 Å) distribution of each icosahedron in Å.

f

Reproduced from ref (23).

Finally, M1Au22 (M = Pd and Pt) was reduced by the reaction with NaBH4.31,46 Negative-ion ESI mass spectra after the reduction exhibit a single peak (Figure 2a). Analysis of the isotope patterns confirmed the assignment to M1Au22–. Since the structures of M1Au22– could not be determined by SCXRD, the structural information was obtained by optical spectroscopy and DFT calculations. The UV–vis spectra of M1Au22– showed similar profiles with the corresponding M1Au22 except that the lowest energy peak observed in M1Au22 disappeared due to suppression of the optical transition to SOMO upon the reduction (Figure 2b). DFT calculations were conducted on model systems [Pd1Au37(SCH3)24]− (Pd1Au22(m)–), [Pt1Au37(SCH3)24]− (Pt1Au22(m)–), and Au38(SCH3)24 (Au23(m)) by simplifying PET to methane thiolate. The DFT-optimized structures are presented in Figure S3. The DFT-optimized structure of Au23(m) reproduced the SCXRD structure of Au23, demonstrating the reliability of the theoretical method used here. The optimized structures of M1Au22(m)– were similar to the crystal structures of corresponding neutrals M1Au22 (Figure 1d). These results indicate that M1Au22 retains the geometric structures upon the reduction.

Figure 2.

Figure 2

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(a) Negative-ion ESI mass spectra. Insets compare the experimental (colored) and simulated (black) isotope patterns. (b) Comparison of UV–vis spectra of Pd1Au22– (blue, solid) and Pt1Au22– (red, solid) to that of Au23 (black, solid) in THF. Dotted lines show the spectra of Pd1Au22 (green) and Pt1Au22 (pink) before the reduction. (c) Energy diagram of the selected KS orbitals of Pd1Au22(m)– (left) and Au23(m) (right). (d) Selected KS orbitals of Pd1Au22(m)–. Isodensity value was set to 0.015.

Interestingly, the spectral profiles of M1Au22– are very similar to that of Au23 except for the slight blue shift for M = Pt (Figure 2b). The spectral similarity between M1Au22– and Au23 is rationalized by the similar energy diagrams of their Kohn–Sham orbitals, although the KS orbitals of M1Au22– are upshifted with respect to the corresponding orbitals of Au23 due to the negative charge (Figures 2c and S4). The contribution of atomic orbitals to each KS orbitals of M1Au22(m)– was analyzed (see the Materials and Method section for details). The projected density of states thus obtained (Figure S5) indicates that the frontier KS orbitals are mainly composed of sp orbitals of the metals in the M1Au22 core. The shapes of the frontier KS orbitals from LUMO+2 to HOMO–4 are presented in Figures S6 and S7, and selected orbitals are shown in Figure 2d. It appears that the orbitals from LUMO+2 to HOMO–4 of M1Au22(m)– and Au23(m) are divided into five types: 2Σ*, 1Δ, 1Π*, 2Σ, and 1Π. The occupied orbitals are energetically divided into two subsets (Figure 2c): (1) two 1Π* orbitals and (2) one 2Σ and two 1Π orbitals. The nature of the occupied orbitals of M1Au22(m)– agrees with the theoretical prediction for Au23(m) based on the SVB model.16,17 The NBO charge analysis of M1Au22(m)– and Au23(m) revealed that the charge distribution is not uniform in the hetero-bi-icosahedral M1Au22 core. As shown in Table S4, the charge distribution at the M1Au12 side of the core of M1Au22(m)– was negatively biased by ∼0.3e compared to the Au13 side because of the lower valency of Pd and Pt than Au. Nevertheless, we can conclude that the electron configurations of M1Au22– (M = Pd and Pt) and Au23 are both expressed as (1Σ)2(1Σ*)2(1Π)4(2Σ)2(1Π*)4.

The PE spectra of the mass-selected M1Au22– (M = Pd and Pt) were recorded by a home-built apparatus consisting of an ESI source, a quadrupole linear ion trap, a time-of-flight mass spectrometer (TOF MS), and a magnetic bottle-type photoelectron spectrometer (MB PES) (Figure S8).28 Briefly, M1Au22– was introduced into the vacuum apparatus through the ESI source, was mass selected, and then was irradiated with the fourth harmonics of a Nd:YAG laser (266 nm) or the output of an ArF excimer laser (193 nm). The laser fluence was carefully adjusted to detect only a single-photon electron detachment process (M1Au22– + hν → M1Au22 + e–) and to avoid two-photon processes.28 The detached electrons were collected by an inhomogeneous magnetic field and detected by the MB PES. The electron binding energy (Eb) was determined by subtracting the electron kinetic energy from the photon energy and then was calibrated using the PE spectra of I–. The representative PE spectra of M1Au22– (M = Pd and Pt) are shown in Figure 3a. The PE spectra of both clusters at 193 nm (1 mJ cm–2) exhibited two peaks I and II centered at ∼3.8 and ∼4.3 eV along with intense broad bands at the higher Eb of >4.5 eV. The PE spectra at 266 nm showed peak I and a rising edge of peak II (Figure 3a). Based on the onset energies of peak I, the adiabatic electron affinities of M1Au22 were determined to be 3.53 ± 0.06 and 3.60 ± 0.06 eV for M = Pd and Pt, respectively. The PE spectra of M1Au22(m)– (M = Pd and Pt) were theoretically simulated by assuming the identical photoelectron detachment probability from all the states. The theoretical PE spectra shown in Figure 3b predicted that the vertical detachment energies of M1Au22(m) (M = Pd and Pt) were 3.64 and 3.72 eV, respectively, and reproduced the experimental spectral features shown in Figure 3a. The results of Figure 3a,b suggest that peak I is assigned to the PE detachment from nearly degenerate 1Π* orbitals, whereas peak II is assigned to that from nearly degenerate 2Σ and 1Π orbitals. These PE results support the SVB theory.

Figure 3.

Figure 3

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(a) Gas-phase PE spectra of Pd1Au22– (top) and Pt1Au22– (bottom). Blue and red traces at 266 nm were measured with a laser fluence of 0.5 and 1 mJ cm–2, respectively. Arrows show the adiabatic electron affinities. (b) Calculated density of states of Pd1Au22(m)– (top) and Pt1Au22(m)– (bottom). Each states were broadened with a full width half-maximum of 0.35 eV.

Conclusions

In summary, a SVB model proposed for a bi-icosahedral Au23 core in Au38(PET)24 was tested by gas-phase PES of [M1Au37(PET)24]− (M = Pd and Pt) having similar electronic structures. Targeted anions were obtained by the reduction of neutral [M1Au37(PET)24]0 synthesized by fusion reactions between two superatoms, [HAu9(PPh3)8]2+ and [M1Au24(PET)18]−. SCXRD and EPR measurements revealed that [M1Au37(PET)24]0 possessed open electronic structures and bi-icosahedral M1Au22 cores with geometric structures similar to the bi-icosahedral Au23 core of Au38(PET)24. The anion photoelectron spectra of [M1Au37(PET)24]− showed two peaks with the electron binding energies of ∼3.8 and ∼4.5 eV, which were assigned to the PE detachment from the 1Π* and 1Π/2Σ orbitals. This study not only demonstrates the wide applicability of the fusion reactions of superatoms for the targeted synthesis of superatomic molecules but also provides experimental support for the SVB model.

Materials and Method

Chemicals

All commercially available chemicals were used without further purification. Sodium borohydride (NaBH4), trimethylamine (NEt3), acetone, ethanol (EtOH), methanol (MeOH), dichloromethane (DCM), hexane, chloroform, and chlorobenzene were obtained from FujiFilm Wako Pure Chemical Corporation. 2-Phenylethanethiol (PET-H) and trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) were purchased from Tokyo Chemical Industry. Tetrahydrofuran (THF; dehydrated and stabilizer free) and DCM (dehydrated) were purchased from Kanto Chemical Company. 1, 2-Dichlorobenzene was obtained from Merck. AuClSMe2 complex, M1Au24(PET)18 (M = Pd and Pt), [Au9(PPh3)8](NO3)3, Au25(PET)18, and M2Au36(PET)24 (M = Pd and Pt) were synthesized according to the reported procedures, respectively.24,28,30,47,48

General Procedure

UV–vis optical spectra of the products were measured by a Jasco V-670 spectrophotometer using 2 and 10 mm cuvettes. Electrospray ionization (ESI) mass spectra were recorded on a Bruker compact ESI-Q-TOF-MS. Matrix-assisted laser desorption/ionization (MALDI) mass spectra were recorded by a Shimadzu AXIMA-CFR. DCTB was used as a matrix. The sample (0.1 mg) and DCTB (1.5 mg) were dissolved in toluene (20 μL), and a few microliters of the mixed solution were spotted on the MALDI plate. Gel permeation chromatography was conducted using a YMC-GPC T30000 column with toluene as an eluent. X-band CW-EPR measurement was carried out using a Bruker E500 spectrometer with a 4He flow cryostat. The DCM solution was placed in a quartz tube (4 mmφ) and rapidly frozen by immersing in liquid N2. The frozen solution was quickly transferred to the precooled cryostat, and measurement was conducted at several temperatures. The spectrum shown in this paper was recorded by using the following parameters: microwave frequency = 9.67 GHz, microwave power = 0.1 mW, and modulation amplitude = 0.500 mT. The microwave power dependency indicated that the condition was far less than saturation (data not shown). The EPR spectrum was simulated using the EasySpin toolbox in MATLAB 9.6.0.5. The range of 150–550 mT was used for fitting.

Synthesis

[Pd1Au37(SC2H4Ph)24]0 (Pd1Au22)

A vial was charged with Pd1Au24(PET)18 (9.1 mg, 1.25 μmol), followed by DCM (1.1 mL). Ethanolic solution containing 1 equiv of NaBH4 (0.1 M, 12.5 μL) was added to the vial (solution A). [Au9(PPh3)8](NO3)3 (1 equiv, 5.0 mg, 1.25 μmol) was dissolved in DCM (275 μL) in another vial, followed by the rapid addition of ethanolic solution containing 1 equiv of NaBH4 (0.1 M, 12.5 μL) (solution B). Solution B was added dropwise to solution A, and the mixture was stirred at room temperature for 20 min. AuClSMe2 (12 equiv, 4.4 mg, 15 μmol) was added to THF (250 μL) in another vial to obtain a white suspension. PET (6 equiv, 1.01 μL, 7.5 μmol) was added dropwise to the suspension while stirring to obtain a pale yellow solution (solution C). Solution C was added dropwise to the mixture of solutions A and B, and the mixture was stirred at room temperature for more than 130 min. Then, 12 equiv of NEt3 (2.08 μL, 15 μmol) was added to the solution, and the mixture was stirred for 10 min. Finally, MeOH was added, and the solution was evaporated to dryness. The residue was washed with EtOH and extracted with DCM. The extract was evaporated and then purified by running Al2O3 column chromatography. DCM and n-hexane mixed in different volume ratios were used as the mobile phase. First, a column was built, and the crude product was loaded with the least polar solvent (DCM/n-hexane = 1:1). Then, the eluent was changed to a more polar solvent (DCM/n-hexane = 2:1). In the end, the most polar eluent (DCM/n-hexane = 6:1) was used. Green bands eluted first, and then the color of the bands gradually changed to brown. The first few fractions turning to brown were discarded because their UV–vis spectra suggested the contamination by a small amount of Au23. The latter brown part of the bands was collected and further purified with PTLC. PTLC was developed with the mixture of DCM and n-hexane to obtain green and brown bands. A brown band was collected and extracted with a volume ratio of DCM/MeOH = 50:1. The brown bands were collected and dried in vacuo. Yield: 2.3 mg (17% with respect to Pd). The residue was recrystallized after purification with GPC.

[Pt1Au37(SC2H4Ph)24]0 (Pt1Au22)

A vial was charged with Pt1Au24(PET)18 (9.2 mg, 1.25 μmol), followed by DCM (1.1 mL). Ethanolic solution containing 1 equiv of NaBH4 (0.1 M, 12.5 μL) was added to the vial (solution A′). [Au9(PPh3)8](NO3)3 (1 equiv, 5.0 mg, 1.25 μmol) was dissolved in DCM (275 μL). Ethanolic solution containing 1 equiv of NaBH4 (0.1 M, 12.5 μL) was rapidly added to the vial (solution B). Solution B was added dropwise to solution A′, and the mixture was stirred at room temperature for 20 min. AuClSMe2 (12 equiv, 4.4 mg, 15 μmol) was put in THF (250 μL) in another vial to obtain a white suspension. PET (6 equiv, 1.01 μL, 7.5 μmol) was added dropwise to the suspension while stirring to obtain a pale yellow solution (solution C). Solution C was added dropwise to the mixture of solutions A′ and B, and the mixture was stirred at room temperature for 130 min. Then, 12 equiv of NEt3 (2.08 μL, 15 μmol) was added to the solution, and the mixture was stirred for 10 min. Finally, MeOH was added, and the solution was evaporated to dryness. The residue was washed with EtOH and extracted with DCM. The extract was evaporated and then purified with Al2O3 column chromatography using the same procedure as that for Pd1Au22. The isolated yield was 1.8 mg (13% with respect to Pt).

[Au38(SC2H4Ph)24]0 (Au23)

A vial was charged with Au25(PET)18 (9.2 mg, 1.25 μmol) and DCM (1.1 mL) (solution A″). In another vial, 1 equiv of [Au9(PPh3)8](NO3)3 (5.0 mg, 1.25 μmol) was weighed and dissolved in DCM (275 μL). Ethanolic solution containing 1 equiv of NaBH4 (0.1 M, 12.5 μL) was rapidly added to the vial (solution B). Solution B was mixed dropwise with solution A″, and the mixture was stirred at room temperature for 20 min. AuClSMe2 (12 equiv, 4.4 mg, 15 μmol) was prepared in the third vial to which THF (250 μL) was added to make a white suspension. PET (6 equiv, 1.01 μL, 7.5 μmol) was added dropwise to the suspension while stirring to obtain a pale yellow solution (solution C). Solution C was added dropwise to the mixture of solutions A″ and B, and the mixture was stirred at room temperature for more than 100 min. Then, 12 equiv of NEt3 (2.08 μL, 15 μmol) was added to the solution, and the mixture was stirred for 10 min. Finally, MeOH was added, and the solution was evaporated to dryness. The residue was washed with EtOH and subsequently extracted with DCM. The extract was evaporated and then purified using silica gel column chromatography. The mixtures of DCM and n-hexane with a 1:1 volume ratio were used as an eluent. The brown band was collected and dried in vacuo. Yield: 1.8 mg (11% with respect to Au).

X-ray Crystallography

General

The crystals were gradually cooled by a cold N2 stream at ∼10 K/min to minimize the structural disorder in the ligand shells. Diffraction experiments were performed using a Rigaku VariMax dual diffractometer with a Pilatus 200K hybrid pixel array detector with MoKα radiation. The data were corrected for Lorentz polarization, and the absorption correction was performed numerically using the CrysAlisPro software. The initial phase trials were conducted by a direct method using SHELXS-2014, and the structures were refined using the full-matrix least-squares method on F2 by SHELXL-2018.49 All non-hydrogen atoms, except for the solvent molecule, were refined anisotropically, and the hydrogen atoms were treated as riding models. All aromatic rings were treated using rigid constraints (AFIX 66). Disordered organic residues in the ligands were treated by splitting the moieties into several parts to build a reasonable model. Special treatment for structure refinement, CheckCif reports, Cambridge Crystallographic Data Center (CCDC) depository numbers, and structural notes are listed as follows.

Pd1Au22

Crystals for X-ray diffraction experiments were prepared by mixing a chlorobenzene solution of clusters (∼10 mg/mL) and n-hexane with a volume ratio of 1:7 in a test tube at room temperature for 1 week. Since the cluster consists of one Pd atom, the sum of half-occupancy of Pd atom and Au atom at the center of the icosahedron was assumed for the refinement using the same positional and displacement factors of each elements. The CheckCIF program did not generate any A and B level alerts (CCDC depository no. 2194621).

Pt1Au22

Crystals for X-ray diffraction experiments were prepared by mixing a 1,2-dichlorobenzene solution of clusters (∼10 mg/mL) and n-hexane with a volume ratio of 1:7 in a test tube at room temperature for 3 weeks during which n-hexane was added several times. The half-occupancy of Pt atoms at the center of each icosahedrons was assumed. The CheckCIF program generated B level alerts regarding some missing reflections below theta(min) and relatively high residual electron densities. The former is due to the interference of the direct beam stopper caused by MoKα radiation with low crystal symmetry. The latter is due to an imperfect absorption correction derived from the anisotropic crystal shape and highly disordered nature of the crystal. Although these alerts were found, the conclusion was not affected by these alerts since the total structural motif is unequivocal (CCDC depository no. 2194620).

Gas-Phase PES

PES was conducted using a custom-built apparatus equipped with an electrospray ionization (ESI) source, a time-of-flight mass spectrometer (TOF-MS), and a magnetic-bottle-type photoelectron spectrometer (MB-PES) (Figure S8).28M1Au22– was prepared by reducing 0.2 mg of M1Au22 dissolved in THF (20 μL) with 1 equiv of NaBH4 dissolved in cold EtOH (0.01 M, 1.86 μL) and then diluted with THF (1 mL). M1Au22– was introduced into the apparatus via the ESI source and desolvated by a heated capillary. Ions were transferred to a linear quadrupole ion trap (LQT) through a skimmer (1 mm diameter). Continuously flowing ions were accumulated in the LQT for 99 ms and released 300 μs before the acceleration of the TOF-MS. The released ions were accelerated by a set of Wiley–McLaren-type electrodes and were temporally and spatially separated by the TOF-MS with a mass resolution (Δm/m) of ∼600. Pulsed third or fourth harmonics of an Nd:YAG nanosecond laser (Quanta-Ray GCR130, Spectra-Physics, US) or pulsed ArF excimer laser (PSX-100, MPB Communications Inc.) clipped by a pinhole were irradiated to the mass-selected ion beam during the TOF-MS. The emitted photoelectrons were collected by an inhomogeneous magnetic field generated by a permanent magnet and a coil. The kinetic energies of the photoelectrons were determined by the TOF of the electrons and then converted to the electron binding energies (EBEs). The EBEs were calibrated with the PE spectrum of I– produced by the dissociative electron attachment to CH3I. The energy resolution was approximately 110 mV for an electron with a kinetic energy of 1 eV. The obtained PE spectra were smoothed by averaging the data points within ±50 meV. The adiabatic electron affinity was determined by the energy of the spectral onset where the tangential line at the inflection point intersected the X-axis. Laser fluences were measured at the exit of the Nd:YAG laser.

DFT Calculations

Density functional theory (DFT) calculation was conducted on Au38(SCH3)240 (Au23(m)), [Pd1Au37(SCH3)24]− (Pd1Au22(m)–), and [Pt1Au37(SCH3)24]− (Pt1Au22(m)–) with B3LYP functionals and 6-31G(d) basis sets for H, C, and S atoms and LanL2DZ basis sets for Pd, Pt, and Au atoms by using the Gaussian 16 program.50 All structures were created from crystal structures by substituting the PET ligands with SCH3 and structural optimization was performed. Frequency calculations on the optimized structures did not show any imaginary frequencies, indicating that the obtained structures were at local minima. Vertical detachment energies were calculated by subtracting the total energy of the structure-optimized Pd1Au22(m)– and Pt1Au22(m)– from that of the neutrals with the fixed geometries without considering the zero-point energies. Spin density of Pd1Au22 was examined by single-point calculation on the crystal structure at the same level of calculations. The contributions of atomic orbitals to each Kohn–Sham orbital were analyzed based on Mulliken population analysis (MPA) using the GaussSum program.51 NBO charge analysis was conducted by the NBO program implemented in the Gaussian.52

Acknowledgments

This research was financially supported by JST, CREST (grant no. JPMJCR20B2), and JSPS KAKENHI (grant nos. JP20H00370 and JP21J20631). Part of theoretical calculations was performed using the Research Center for Computational Science, Okazaki, Japan. Part of this work was conducted at Institute for Molecular Science, supported by nanotechnology platform program <molecule and material synthesis> (JPMXP09S20MS1047) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.2c00519.

  • Optical absorption spectra of the products of entries 1–3 in Table 2; KS orbitals of Pd1Au22(m)–, Pt1Au22(m)–, and Au23(m); schematic illustration of the PES apparatus; fitting results of EPR data; synthetic results; and crystal data (PDF).

  • C204H235Au37ClPdS24 (CIF)

  • C192H216Au37PtS24 (CIF)

  • PtAu37PET24 at 100 K (PDF)

  • PdAu37PET24 at 103 K (PDF)

Author Contributions

E.I. and S.I. equally contributed to the work. E.I. conducted the synthesis and basic characterization. S.I. conducted PES measurement and DFT calculations. S.T. advised on the synthesis and conducted SCXRD analysis. T.N. conducted EPR measurement. T.T. conceived the idea and supervised the project. The manuscript was drafted by E.I., S.I., and T.T. and was approved by all the authors. CRediT: Emi Ito data curation, formal analysis, investigation, visualization, writing-original draft; Shun Ito data curation, formal analysis, investigation, methodology, visualization, writing-review & editing; Shinjiro Takano data curation, methodology, supervision, validation, writing-review & editing; Toshikazu Nakamura investigation, writing-original draft; Tatsuya Tsukuda conceptualization, supervision, validation, visualization, writing-original draft, writing-review & editing.

The authors declare no competing financial interest.

Supplementary Material

au2c00519_si_001.pdf (9.7MB, pdf)
au2c00519_si_002.cif (4.1MB, cif)
au2c00519_si_003.cif (7.1MB, cif)
au2c00519_si_004.pdf (258.7KB, pdf)
au2c00519_si_005.pdf (251.3KB, pdf)

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

au2c00519_si_001.pdf (9.7MB, pdf)
au2c00519_si_002.cif (4.1MB, cif)
au2c00519_si_003.cif (7.1MB, cif)
au2c00519_si_004.pdf (258.7KB, pdf)
au2c00519_si_005.pdf (251.3KB, pdf)

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