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Letter

Dopant-Dependent Electronic Structures Observed for MAu (SCH ) Clusters (M=Pt, Pd) 2

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Minseok Kim, Qing Tang, Alam Venugopal Narendra Kumar, Kyuju Kwak, Woojun Choi, De-en Jiang, and Dongil Lee J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b03261 • Publication Date (Web): 08 Feb 2018 Downloaded from http://pubs.acs.org on February 9, 2018

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The Journal of Physical Chemistry Letters

Dopant-Dependent Electronic Structures Observed for M2Au36(SC6H13)24 Clusters (M=Pt, Pd) Minseok Kim,† Qing Tang,‡, Alam Venugopal Narendra Kumar,† Kyuju Kwak,† Woojun Choi,† De-en Jiang,*‡ and Dongil Lee*† †

Department of Chemistry, Yonsei University, Seoul 03722, Korea

‡

Department of Chemistry, University of California, Riverside, California 92508, United

States *

To whom correspondence should be addressed.

E-mail: [email protected], [email protected]

Abstract Heteroatom doping is a powerful means to tune the optical and electronic properties of gold clusters at the atomic level. We herein report that doping a Au38 cluster with Pt and Pd atoms leads

to

core-doped

[Pt2Au36(SC6H13)24]2-

and

[Pd2Au36(SC6H13)24]0,

respectively.

Voltammetric investigations show that these clusters exhibit drastically different electronic structures; whereas the HOMO-LUMO gap of [Pt2Au36(SC6H13)24]2- is found to be 0.95 V, that of [Pd2Au36(SC6H13)24]0 is drastically decreased to 0.26 V, suggesting Jahn-Teller distortion of the 12-electron cluster. Density functional investigations confirm that the HOMO-LUMO gap of the Pd-doped cluster is indeed reduced. Analysis of the optimized geometry for the 12-electron [Pd2Au36(SC6H13)24]0 reveals that the rod-like M2Au21 core becomes more flattened upon Pd-doping. Reversible geometrical interconversion between [Pt2Au36(SC6H13)24]0 and [Pt2Au36(SC6H13)24]2- is clearly demonstrated by manipulating the oxidation state of the cluster.

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TOC GRAPHIC.


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Thiolate-protected gold clusters have attracted extensive research interest because of their distinct physicochemical properties and promising applications in catalysis, electrocatalysis, biological sensing and imaging.1-9 Recent advances in the synthesis of these clusters have led to atomically precise gold clusters that exhibit unique core size-dependent electronic, optical and electrochemical properties. Some examples include Au25(SR)18,10-13 Au38(SR)24,14-16 Au67(SR)35,17 Au102(SR)44,18 and Au144(SR)60,19 where SR is organothiolate. In addition to the size control, heteroatom doping of gold clusters has been another effective way to tune the physicochemical properties of these gold clusters.20-40 Doping of foreign metal(s) into stable Au25 clusters has been extensively studied since mono-Pd-doped PdAu24(SC2H4Ph)18 cluster was first observed in a mass spectrum by Murray and coworkers.20 Mono-metal doped MAu24(SR)18 (M = Pd, Pt, Cd, Hg) clusters were typically obtained from various synthetic methods,24,26-27,33-36 whereas multiply doped clusters were predominantly synthesized when the dopant was Ag or Cu.25,28-30 Au38 clusters are of special interest because of their well-defined structure, high stability and intrinsic chirality.41,42 Jin and co-workers reported the X-ray crystal structure of Au38(SC2H4Ph)24 which consists of a face-fused bi-icosahedral Au23 core, three monomeric and six dimeric protecting staples.14-15 By using chiral high-performance liquid chromatography (HPLC), Bürgi et al. successfully separated the two enantiomers of Au38(SC2H4Ph)24 clusters protected by achiral ligands.42 Doped Au38 clusters have also been studied. Negishi et al. isolated highly pure core-doped Pd2Au36(SC2H4Ph)24 by using size exclusion chromatography followed by chemical decomposition of side products.43 Dass and co-workers reported an average crystal structure of Au38-xAgx(SC2H4Ph)24, where x = 1~5.44 In this Ag-doped cluster, Ag atoms were found to be preferably located on the surface of the bi-icosahedral core. We previously reported that replacing the core Au atom with Pd or Pt leads to centerdoped [MAu24(SR)18]0 clusters (M = Pt, Pd) that exhibit drastically different optical and electrochemical properties from those of the [Au25(SR)18]- cluster.35,45 The origin of the stability for [PtAu24(SR)18]0 and [PdAu24(SR)18]0 was explained by the superatom complex model.44 In this model, [Au25(SR)18]- is a superatomic 8-electron system (1S21P6).24,47 When it is doped with Pt(Pd), the resulting [MAu24(SR)18]0 cluster having a superatomic 6-electron configuration (1S21P4) undergoes structural distortion that accompanies splitting of the 1P orbitals.35 This Jahn-Teller effect has been manifested in their optical, electrochemical and magnetic properties.35,48 Ackerson et al. also observed larger distortions in the crystal 3 ACS Paragon Plus Environment


structure of the 6-electron [Au25(SR)18]+.49 On the other hand, when Au25 is doped with Ag or Cu, anionic 8-electron clusters were preferably obtained for [Au25-xMx (SR)18]- (M = Ag, Cu).25,28-30 Neutral 8-electron [MAu24(SR)18]0 clusters were also observed when the dopant is Hg or Cd.33,34,36,39 Herein we extend our effort to the case of doped M2Au36(SR)24 (M = Pt, Pd) clusters. Would the rod-like core of doped M2Au36(SR)24 show Jahn-Teller effects as have been observed for [MAu24(SR)18]0 clusters (M = Pt, Pd)? To answer this question, we synthesized highly pure Pt2Au36(SC6H13)24 and Pd2Au36(SC6H13)24 and investigated their atomic and electronic structures. Voltammetric investigations revealed that their electronic structures are distinctly different from each other. The HOMO-LUMO gap of Pd2Au36(SC6H13)24 deduced from the voltammograms is 0.26 V, drastically decreased upon Pd-doping, whereas the HOMO-LUMO gap of Pt2Au36(SC6H13)24 was rather similar to that of Au38(SC6H13)24. DFT calculations strongly support these results, suggesting Jahn-Teller distortion for Pd2Au36(SC6H13)24. Chemical oxidation/reduction cycle of Pt2Au36(SC6H13)24 shows that the geometrical distortion can be reversed by manipulating the oxidation state of the cluster.


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Figure 1. (A) Structures of Au38(SC6H13)24 (left) and metal-doped M2Au36(SC6H13)24 (M=Pt, Pd) (right). Hexyl group is omitted for the sake of clarity. Color codes: golden = core Au; olive = shell Au; green = S. (B) MALDI mass spectra of Au38(SC6H13)24 (red line), Pt2Au36(SC6H13)24 (green line), and Pd2Au36(SC6H13)24 (blue line). The insets show the comparisons between experimental data (colored lines) and the simulated isotope patterns (black bars). Both Pt2Au36(SC6H13)24 and Pd2Au36(SC6H13)24 clusters were synthesized following the one-phase procedure reported elsewhere with some modifications.27,35 Detailed synthetic procedures for these clusters are provided in Supporting Information (SI). Briefly, for the synthesis of Pd2Au36(SC6H13)24 cluster, 1-hexanethiol was added to a 4:1 molar mixture of HAuCl4 and Na2PdCl4 dissolved in tetrahydrofuran to from metal-thiol aggregates, which were subsequently reduced by NaBH4 to produce metal clusters. A mixture of Au25(SC6H13)18, PdAu24(SC6H13)18, PdAu37(SC6H13)24, and Pd2Au36(SC6H13)24 clusters were typically produced in this synthesis (see Figure S1A). Au25(SC6H13)18 and PdAu37(SC6H13)24 clusters were first removed from the mixture by decomposition reaction with concentrated H2O2, leaving PdAu24(SC6H13)18 and Pd2Au36(SC6H13)24. Pd2Au36(SC6H13)24 clusters were then selectively extracted from the mixture with 6:10 (v/v) CH2Cl2/CH3CN mixture (Figure S1B). Pt2Au36(SC6H13)24 clusters were synthesized similarly by the one–phase procedure.35 We note that, whereas Pd2Au36(SC6H13)24 cluster was reported before,43,50,51 this is the first reporting the successful isolation of highly pure Pt2Au36(SC6H13)24 cluster. Figure 1 shows the positive-mode matrix-assisted laser desorption ionization (MALDI) mass spectra of the isolated Au38 (red), Pt-doped (green), and Pd-doped (blue) clusters.

The

mass spectrum of each cluster shows only a single peak in the m/z range of 6,000-16,000 Da, indicating that the isolated clusters are highly monodispersed. In Figure 1 (red), the observed peak at m/z ~10,298 Da corresponds to the intact ion of Au38(SC6H13)24. In the case of Ptdoped cluster (green), the peak is observed at m/z ~10,294 Da, only 4 Da smaller than that of Au38(SC6H13)24.

Nevertheless, the isotope pattern of Pt-doped cluster matches well with the

simulated isotope pattern of Pt2Au36(SC6H13)24, as can be seen in the inset. For Pd-doped gold cluster, a single dominant peak is observed at m/z ~10,117 Da (Figure 1 blue), which corresponds to the intact ion of Pd2Au36(SC6H13)24. As compared in the inset, the observed isotope pattern indeed matches well with the simulated isotope pattern of Pd2Au36(SC6H13)24. The Au38(SC6H13)24, Pt2Au36(SC6H13)24 and Pd2Au36(SC6H13)24 clusters will be abbreviated as Au38, Pt2Au36, and Pd2Au36, respectively. 5 ACS Paragon Plus Environment


The compositions of the isolated Pt2Au36 and Pd2Au36 clusters were further confirmed by X-ray photoelectron spectroscopy (XPS). The Au 4f XPS spectra of Au38, Pt2Au36, and Pd2Au36 are compared in Figure S2A. As can be seen in the figure, whereas the 4f7/2 peak is found at 84.4 eV for Au38, it shifts to lower energies and observed at 84.1 eV and 84.2 eV, respectively, for Pt2Au36 and Pd2Au36. This shift can be attributed to the internal charge transfer from less electronegative Pt (Pd) dopant to the more electronegative Au atoms,47,52,53 as has been observed for Pt and Ag doped metal clusters.26,54 Figure S2B and C show respectively Pt 4f and Pd 3d peaks, confirming the presence of Pt(Pd) in the isolated clusters. The average compositions determined by the ratio of Pt(Pd) dopant to Au are found to be Pt2.2Au35.8 and Pd2.0Au36.0, respectively, which is in excellent agreement with those determined by mass spectrometry. Further, the Pt 4f7/2 XPS peak observed at 71.8 eV indicates that the doped Pt is Pt0, suggesting that Pt atom is located at the center of Pt2Au36; a higher binding energy would be expected if it was located at the surface of the core or in the ligand shell.26 For Pd2Au36, the Pd 3d3/2 peak found at 341.4 eV also indicates that doped Pd atoms exist in Pd0 state. These results unequivocally manifest that two Pt(Pd) atoms are located in the two central positions of the face-fused bi-icosahedral Au23 core. No counterion such as Oct4N+ is found for Au38 in NMR analysis (Figure S3), in consistent with the charge neutral Au38 cluster found in a previous report.14 Similarly, no Oct4N+ is found for the Pd2Au36 in NMR analysis. XPS spectrum of Pd2Au36 in Figure S2D also shows that nitrogen is absent. Additional XPS analysis (not shown) indicates that potential counterions such as Cl- and Br- are also absent in Pd2Au36 cluster. These results unequivocally suggest that both isolated Au38 and Pd2Au36 are charge neutral. By contrast, the XPS spectrum of Pt2Au36 in Figure S2D clearly shows N 1s peak in the isolated Pt2Au36 cluster. In Figure S3, NMR analysis shows that there are two Oct4N+ cations associated with the isolated Pt2Au36, unambiguously indicates that the isolated cluster is dianionic [Pt2Au36]2-.


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Figure 2. (A) UV−Vis−NIR absorption spectra of the isolated [Au38]0 (red), [Pt2Au36]2(green), and [Pd2Au36]0 (blue) in trichloroethylene. The wavelength-scale absorption spectrum, Abs(λ), was converted to the energy-scale spectrum by adopting Abs(E) ∝ Abs(λ)×λ2 relationship. FT-NIR spectra of [Au38]0 (light red), [Pt2Au36]2- (light green), and [Pd2Au36]0 (light blue) in trichloroethylene are displayed together. The first, second, and third absorption peaks are denoted by α, β, and γ, respectively. (B) TD-DFT simulated absorption spectra of Au38 (top), Pt2Au36 (middle) and Pd2Au36 (bottom) carrying different charge states. UV-Vis-NIR spectra of the isolated [Au38]0, [Pt2Au36]2- and [Pd2Au36]0 clusters are presented in photon energy scale in Figure 2A. The absorption spectra in wavelength scale are shown in Figure S4. The absorption spectrum of Au38 exhibits the characteristic absorption feature of Au38 cluster with first and second absorption peaks at 1.2 and 2.0 eV, respectively. The absorption spectrum of Pt2Au36 shows similar but higher energy peaks at 1.5 and 2.2 eV. For Pd2Au36, these peaks are observed at 1.2 and 2.0 eV. The peaks observed for [Au38]0 are well-simulated by time-dependent density functional theory (TD-DFT) calculations as can be seen in Figure 2B. The TD-DFT also successfully reproduced the spectra of [Pt2Au36]2- and [Pd2Au36]0. To further confirm that two Pt(Pd) atoms are located in the two central positions of the M2Au21 core, we compared them with several other doping positions and found that all the other positions have much higher energy (Table S1). Unlike 7 ACS Paragon Plus Environment


[Au38]0 and [Pt2Au36]2-, however, there is an additional NIR band centered at ~0.8 eV predicted for [Pd2Au36]0. However, we were unable to experimentally observe the additional NIR band by UV-Vis-NIR absorption spectrometry because of large solvent background below 1.0 eV. To check the presence of the addition NIR band, we performed FT-NIR spectrometry for [Au38]0, [Pt2Au36]2- and [Pd2Au36]0 in trichloroethylene and the acquired spectra are displayed together with the UV-Vis-NIR spectra in Figure 2A. As can be seen in Figure 2A, the FT-NIR absorption peaks observed at 1.2 and 1.5 eV for respectively [Au38]0 and [Pt2Au36]2- match excellently with those observed in the UV-Vis-NIR spectrum. For [Pd2Au36]0, while the absorption peak observed at 1.2 eV is consistent with that observed in the UV-Vis-NIR spectrum, the absorption peak observed at 0.8 eV clearly confirms the NIR absorption band.

Figure 3. SWVs of Au38 (red), Pt2Au36 (green), and Pd2Au36 (blue) in CH2Cl2 containing 0.1 M Bu4NPF6. The arrows indicate the solution open-circuit potentials. For the precise comparison, potentials were corrected using ferrocene (Fc+/0) as an internal standard. Square wave voltammetry (SWV) experiments were performed with CH2Cl2 solutions of Au38, Pt2Au36, and Pd2Au36 to investigate their electronic structures. Electrochemical redox characteristics of metal clusters are useful as they allow to determine the electronic structure near the HOMO-LUMO levels.4 As can be seen in Figure 3, the SWVs of clusters exhibit reasonably resolved current peaks observed at the formal potentials of the cluster. The opencircuit potential (OCP) of the neutral Au38 was found at -0.53 V. There are three oxidation peaks at -0.10 (O1), 0.25 (O2), 0.47 V (O3) and two reduction peaks observed at -1.31 (R1) 8 ACS Paragon Plus Environment

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1.52 V (R2) in the potential range investigated. The difference between the first oxidation (O1) and reduction (R1) potentials constitutes the electrochemical gap (1.21 V). The HOMOLUMO gap of Au38 was then determined to be 0.86 eV by subtracting the charging energy55,56 estimated using the gap between O1 and O2 (0.35 V) from the electrochemical gap. The predicted HOMO-LUMO gap matches reasonably with that calculated by DFT (1.01 eV, Table 1).

Table 1. HOMO-LUMO gaps determined by SWV and DFT studies. HOMO - LUMO gap (eV)

[Au38]0

[Pt2Au36]2-

[Pd2Au36]0

SWV

0.86

0.95

0.26

DFT

1.01

1.10

0.23

In Figure 3, the SWV of Pt2Au36 displays a similar pattern with oxidation peaks at 0.40 (O1), -0.08 (O2), and 0.30 (O3) and reduction peak at -1.67 (R1). As discussed earlier, the isolated Pt2Au36 was found to be dianionic, i.e., [Pt2Au36]2- and the OCP was found at 0.58 V. Thus these peaks correspond to [Pt2Au36]1-/2- (O1) and [Pt2Au36]0/1- (O2) and reduction peak to [Pt2Au36]2-/3- (R1). The electrochemical gap and the HOMO-LUMO gap determined for [Pt2Au36]2- are 1.27 V and 0.95 V, respectively. Again, the HOMO-LUMO gap reasonably agrees with that predicted by DFT (1.10 eV, Table 1). By contrast, the Pd2Au36 cluster exhibits drastically different voltammogram from those of Au38 and Pt2Au36. As can be seen in Figure 3, there are doublet of oxidation at -0.15 (O1) and 0.17 (O2) and doublet of reduction at -0.73 (R1) and -1.02 V (R2) with the OCP found at –0.47 V. The isolated Pd2Au36 is a neutral cluster and voltammetric peaks observed in Figure 3 correspond to [Pd2Au36]2+/1+ (O2), [Pd2Au36]1+/0 (O1), [Pd2Au36]0/1- (R1) and [Pd2Au36]1-/2- (R2), respectively. The electrochemical gap and the HOMO-LUMO gap determined for [Pd2Au36]0 are dramatically decreased to 0.58 V and 0.26 V, respectively. The electrochemically estimated HOMO-LUMO gap is in excellent agreement with that predicted by DFT (0.23 eV). These results clearly demonstrate that the electronic structures of doped M2Au36 clusters can be effectively controlled by dopant. That is, [Au38]0 and [Pt2Au36]2- exhibit rather similar voltammograms, reflecting that they have a similar electronic structure with the 14-electron 9 ACS Paragon Plus Environment


configuration. By contrast, [Pd2Au36]0 possessing superatomic 12-electron displays drastically different SWV and reduced HOMO-LUMO gap.

Figure 4. (A) DFT predicted electronic energy levels of [Au38(SCH3)24]0, [Pt2Au36(SCH3)24]2-, and [Pd2Au36(SCH3)24]0. α and β denote the first and second optical transitions observed for [Au38]0, [Pt2Au36]2-, and [Pd2Au36]0 in Figure 2. The energy levels are roughly scaled. (B) The three axes of the rod-like M2Au21 (M=Pt, Pd) core. The right panel is the side view of the left panel. The predicted a, b, and c values are listed in Table 2. To verify the voltammetric results, the energy levels of [Au38(SCH3)24]0, [Pt2Au36(SCH3)24]2- and [Pd2Au36(SCH3)24]0 were simulated and depicted in Figure 4. The superatom complex model has been very successful for a number of ligand-protected gold clusters with a spherical core. 24,35,47 It can be further extended to nonspherical shapes via the ultimate jellium model57 which predicts that the 14-electron system will have a prolate shape as in the case of Au38. The 14 electrons fully fill the 1S(2e) and 1P(6e) levels and partially fill the 1D(6e) of the superatomic electron shell (1S21P61D6). DFT calculations show that the HOMO level is doubly degenerate 1Dxz and 1Dyz orbitals (z is along the long axis of the cluster) while the LUMO is doubly degenerate 1Dx2-y2 and 1Dxy orbitals with a HOMOLUMO gap of 1.01 eV (Table 1). DFT calculations show that the 14-electron [Pt2Au36(SCH3)24]2- also has doubly degenerate HOMO, but a nondegenerate LUMO. 10 ACS Paragon Plus Environment

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Interestingly, in the case of [Pt2Au36(SCH3)24]2- the energetic order of LUMO and LUMO+1 orbitals is switched in comparison with those of Au38(SCH3)24. Inspection of the orbitals (Figure S5) suggests that the nondegenerate LUMO of [Pt2Au36(SCH3)24]2- looks like a 1F orbital, which is stabilized due to the presence of Pt and becomes lower in energy than the unoccupied 1D orbitals. Strong mixing of Pt and Au states is clearly seen in the LUMO region from the local density of states for [Pt2Au36(SCH3)24]2- (Figure S6). The neutral [Pd2Au36(SCH3)24]0 possesses 12 electrons, two less electrons than [Au38(SCH3)24]0 and [Pt2Au36(SCH3)24]2-, and thus the doubly degenerate HOMO would split into HOMO and LUMO as portrayed in Figure 4. In this case, both HOMO and LUMO exhibit the character of the superatomic 1D orbital; the local density of states for [Pd2Au36(SCH3)24]0 shows that the HOMO and LUMO orbitals are dominated by Au states with weak mixing from Pd states (Figure S6). The HOMO and LUMO of the other 12-electrons systems, [Au38(SCH3)24]2+ and Pt2Au36(SCH3)24, are similar to those of [Pd2Au36(SCH3)24]0 (Figure S7). One can also consider the 14-electron [Pt2Au36(SCH3)24]2- cluster (being isoelectronic to [Au38(SCH3)24]0) as a molecule formed between two spherical superatom units, resembling the F2 molecule.58 However, we found that the 12-electron [Pd2Au36(SR)24] cannot be viewed as an O2 molecule, since our DFT found a closed-shell ground state and we did not detect any spin in the cluster from the electron paramagnetic resonance (EPR) measurement. Analysis of the optimized structures of the three clusters indicates a trend consistent with the electronic structure picture in Figure 4. As shown in Table 2, the rod-like core of [Pt2Au36]2- is slightly elongated along c but compressed in both a and b, in comparison with that of [Au38]0 (see Figure 4B for the definition of the three axes of the rod-like core). Similar trend is found for [Pd2Au36]0, but its core is significantly more depressed along the b axis, leading to a more flattened cross-section for the rod-like core. Therefore, the splitting of the doubly degenerate HOMO into HOMO and LUMO, predicted for 12-electron [Pd2Au36]0, is the consequence of the Jahn-Teller distortion that occurs when orbitals are unequally occupied. It is interesting to compare the dopant-induced geometric distortion observed for MAu24 and M2Au36 clusters (M = Pd, Pt). In spherical MAu24 clusters, it was found that both Pd- and Pt-doping lead to Jahn-Teller distorted 6-electron [MAu24]0, as opposed to the undistorted 8-electron [MAu24]2-.35 In the rod-like core M2Au36 clusters, however, only Pddoping led to the distorted [Pd2Au36]0 possessing 12 superatomic electrons while Pt-doping resulted in undistorted 14-electron [Pt2Au36]2-. The origin of the Jahn-Teller distortion 11 ACS Paragon Plus Environment


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observed only for [Pd2Au36]0 is unclear at this juncture. The metallic radii of Pd and Pt are 1.37 and 1.385 Å, respectively, somewhat smaller than that of Au (1.44 Å).59 The size mismatch between the dopant and the Au24 frame appears to be responsible for the distorted 6-electron clusters observed for [PdAu24]0 and [PtAu24]0. In the rod-like core, however, only Pd dopant led to distorted 12-electron [Pd2Au36]0 cluster. We speculate that the presence of shared face in the bi-icosahedral M2Au21 core plays a significant role in this case; that is, it would effectively prevent the core distortion when the size mismatch is not so severe (e.g., in the case of [Pt2Au36]2-). Structural analysis of the doped M2Au36 clusters (Table 2) indeed shows that the flattening factor found for [Pt2Au36]2- is 0.139, significantly smaller than that of [Pd2Au36]0 (f = 0.156). In addition, strong orbital mixing of Pt and Au states found in the local density of states for [Pt2Au36]2- (Figure S6) could be another factor accounting for the undistorted structure. In Figure 2B, the optical simulation indeed reproduces the first (α) and second (β) peaks observed in Figure 2A. Orbital analysis of [Au38]0 predicts the first (~1.4 eV) and second (~2.0 eV) peaks correspond respectively to the HOMO-LUMO and HOMO-1 to LUMO+1 transitions.60 Similarly, for [Pt2Au36]2- the 1st peak at ~1.6 and 2nd peak at 2.1 eV are attributed to the transitions from HOMO to LUMO+1 and from HOMO-1 to LUMO, respectively. By contrast, the 12-electron [Pd2Au36]0 shows the first peak at ~0.8 eV that corresponds to the transition from HOMO-1 to LUMO. Apparently, this NIR peak arises from the distorted [Pd2Au36]0. Figure S7 shows that HOMO-1 is rather diffuse over a large part of the cluster and has no clear feature.

Table 2. The geometric parameters (a, b, c) and the flattening factor, f=(a-b)/a, of the rod-like Au23 or M2Au21 core of the three clusters after DFT geometry optimization. [Au38]0

[Pt2Au36]2-

[Pd2Au36]0

a (Å)

5.590

5.543

5.555

b (Å)

4.869

4.771

4.686

c (Å)

8.631

8.690

8.684

f

0.129

0.139

0.156

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It is of interest to examine if the 14-electron [Pt2Au36]2- undergoes structural change (i.e., Jahn-Teller distortion) when it is oxidized to 12-electron [Pt2Au36]0. The spectroelectrochemistry of [Pt2Au36]2- was, however, unsuccessful because the cluster appears to be decomposed under the applied oxidation potential. Instead, we conducted chemical oxidation of the cluster by dissolved oxygen; it was found that the clusters are readily oxidized in ethanol containing oxygen. The CH2Cl2 solution of [Pt2Au36]2- cluster was added to an ethanol-water mixture and the resulting two-phase solution was stirred for 3h to obtain an oxidized cluster. In Figure 5A, SWV of the oxidized cluster shows drastically different voltammogram. There are doublet of oxidation at -0.12 (O1) and 0.19 (O2) and doublet of reduction at -0.67 (R1) and -0.99 V (R2) with the OCP found at –0.46V. It is noteworthy that the SWV of the oxidized Pt2Au36 is remarkably similar to that of [Pd2Au36]0. The HOMO-LUMO gap determined from SWV is found to be 0.24, in excellent agreement with that calculated by DFT (0.22 eV). This result clearly suggests that [Pt2Au36]2- cluster underwent geometrical transformation to [Pt2Au36]0, 12-electron system upon oxidation. This result is in sharp contrast to the spectroelectrochemical measurement of [Au38]0 conducted by Tsukuda and co-workers that showed the bi-icosahedral Au23 core structure was changed only negligibly upon oxidation.61 To further investigate the reversible interconversion between 14-electron and 12electron systems, the oxidized [Pt2Au36]0 was treated with aqueous NaBH4 in a two phase CH2Cl2/H2O mixture for 3 h. The voltammogram of the resulting cluster is less resolved presumably because of chemical decomposition occurred during the oxidation and reduction processes. However, the SWV of the reduced cluster clearly shows the characteristics of the 14-electron [Pt2Au36]2-. That is, there are oxidation peaks at -0.40 (O1), -0.08 (O2) and a reduction peak at around -1.56 V (R1) as can be seen in Figure 5A (bottom green line). The electrochemical and HOMO-LUMO gaps are found to be 1.16 and 0.84 V, respectively. These are similar to those observed for [Pt2Au36]2-, indicating that the oxidized [Pt2Au36]0 underwent structural change back to [Pt2Au36]2- by manipulating its oxidation state. As discussed earlier, the NIR peak at ~0.8 eV is the characteristic transition for the 12-electron system. DFT calculations also predict the NIR peak for [Pt2Au36]0 that can be attributed to the transition from HOMO-1 to LUMO (Figure S8). The energy levels of [Pd2Au36]2- are also shown in Figure S8 for comparison. As can be seen in Figure 5B, the NIR peak at ~0.8 eV clearly appears when the cluster is oxidized to 12-electron [Pt2Au36]0. When the oxidized 13 ACS Paragon Plus Environment


cluster was treated with NaBH4, the peak disappears as the cluster was transformed to 14electron [Pt2Au36]2-. The mass spectra in Figure 5C show that Pt2Au36 is predominantly present, indicating that the integrity of the Pt2Au36 cluster was maintained during the oxidation and reduction processes. These FT-NIR and mass spectra unambiguously demonstrate the reversible interconversion between the 14-electron [Pt2Au36]2- and 12electron [Pt2Au36]0 by manipulating the oxidation state of the cluster.

Figure 5. (A) SWVs of isolated [Pt2Au36]2- (upper green line), chemically oxidized [Pt2Au36]0 (blue line), and chemically reduced [Pt2Au36]2- (bottom green line). (B) FT-NIR spectra of isolated [Pt2Au36]2- (upper green line), chemically oxidized [Pt2Au36]0 (blue line), and chemically reduced [Pt2Au36]2- (bottom green line). DFT simulated spectra are shown in grey for comparison. (C) MALDI mass spectra of [Pt2Au36]2- (upper green line) after oxidation (blue line) and reduction (bottom green line). In summary, we have shown that doping [Au38(SC6H13)24]0 cluster with Pt and Pd leads respectively to [Pt2Au36(SC6H13)24]2- and [Pd2Au36(SC6H13)24]0 clusters, where the two Pt(Pd) atoms are located in the two central positions of the face-fused bi-icosahedral M2Au21 core. These doped clusters exhibit distinctly different optical and electrochemical properties. Whereas the HOMO-LUMO gap of [Pt2Au36]2- determined by voltammetry is slightly greater than that of the undoped [Au38]0, the HOMO-LUMO gap of [Pd2Au36]0 is found to be drastically reduced to 0.26 V with the new appearance of a NIR band at ~0.8 eV. DFT 14 ACS Paragon Plus Environment

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calculations unambiguously show that the reduced HOMO-LUMO gap is the result of JahnTeller distortion of the 12-electron [Pd2Au36]0 cluster, accompanying splitting of the 1D superatomic orbital. The 12-electron and 14-electron configurations of Pt2Au36 cluster can be reversibly interconverted by manipulation of its oxidation state. This work shows the possibility of fine-tuning of the electronic structure of doped cluster by judicious choice of dopant.

ASSOCIATED CONTENT Supporting Information. Detailed synthetic and experimental procedures, supporting figures and DFT calculation results of Au38, Pt2Au36, and Pd2Au36 clusters are provided. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGEMENT D.L. acknowledges support by the Korea CCS R&D Center (KCRC) (Grant NRF2014M1A8A1074219) and NRF Grants NRF-2017R1A2B3006651 and 2011-0022975. Computation by DFT was supported by the University of California, Riverside (Q.T.) and the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division (D.-e.J.).



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