Hydride Doping of Chemically Modified Gold-Based Superatoms

Nov 14, 2018 - Biography. Shinjiro Takano earned his Ph.D. at the University of Tokyo in 2016 under the supervision of Prof. Tatsuya Tsukuda. He is cu...
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Cite This: Acc. Chem. Res. 2018, 51, 3074−3083

Hydride Doping of Chemically Modified Gold-Based Superatoms Published as part of the Accounts of Chemical Research special issue “Toward Atomic Precision in Nanoscience”. Shinjiro Takano,† Shingo Hasegawa,† Megumi Suyama,† and Tatsuya Tsukuda*,†,‡ †

Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo Bunkyo-ku, Tokyo 113-0033, Japan Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Katsura, Kyoto 615-8520, Japan

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‡

CONSPECTUS: Atomically size-selected gold (Au) clusters protected by organic ligands or stabilized by polymers provide an ideal platform to test fundamental concepts and size-specific phenomena, such as the superatomic concept and metal-tononmetal transition. Recent studies revealed that these stabilized Au clusters take atomlike quantized electronic structures and can be viewed as chemically modified Au superatoms. An analogy between Au and hydrogen (H) atoms is an interesting proposal made for bare Au clusters: a Au atom at a low-coordination site of a Au cluster can be replaced with a H atom while retaining the structural motif and electronic structure. However, this proposal has not been experimentally proved in chemically modified Au superatoms while a recent theoretical study predicted the formation of [HAu25(SR)18]0 (RS = thiolate). This Account summarizes our recent studies on the interaction of hydride(s) with two types of chemically modified Au-based superatoms: (1) the Au cores of [Au9(PPh3)8]3+ and [PdAu8(PPh3)8]2+ formally described as (Au9)3+ and (PdAu8)2+, respectively, and (2) Au34 cluster stabilized by poly(N-vinyl-2-pyrrolidone) (PVP). The (Au9)3+ and (PdAu8)2+ cores correspond to oblate-shaped superatoms with six electrons and a coordinatively unsaturated site at the center, whereas the Au34 cluster in PVP is viewed as a nearly spherical superatom having a closed electronic structure with 34 electrons and multiple uncoordinated sites on the surface. Through this study, we aimed to deepen our understanding on the role of a hydride in the formation processes of Au superatoms, the effect of adsorbed hydride(s) on the electronic structure of Au superatoms, and the activity of adsorbed hydrogen species for hydrogenation catalysis. Mass spectrometry and nuclear magnetic resonance spectroscopy demonstrated that a single hydride (H−) was selectively doped to (Au9)3+ and (PdAu8)2+ upon reactions with BH4− to form (HAu9)2+ and (HPdAu8)+, respectively. Density functional theory (DFT) calculations showed that (HAu9)2+ and (HPdAu8)+ were more spherical than the original superatoms and had a closed electronic structure with eight electrons. The hydride-doped (HAu9)2+ was selectively converted to the well-known (Au11)3+ by electrophilic addition of two Au(I) units whereas (HPdAu8)+ was converted to a new hydride-doped (HPdAu10)3+. A two-step mechanism was proposed for hydride-mediated growth of Au-based superatoms: closure of the electronic structures by adsorption of a hydride, followed by the addition of two Au(I) units. The selective formation of Au34 superatoms in PVP is also explained by assuming that hydride-doped Au clusters with 34 electrons were involved as key intermediates. The Au34 superatom exhibited the localized surface plasmon resonance (LSPR) band by reacting with BH4− due to the electron donation by multiply adsorbed hydrides. The LSPR band disappeared by exposing hydride-doped Au34 to dissolved O2, but reappeared by reaction with BH4−. Catalysis for hydrogenation of CC bonds was generated by doping a single Pd or Rh atom to Au34. The results reported here demonstrate that the hydride doped to chemically modified Au superatoms mimics Au− in terms of electron count. The hydride-mediated growth processes observed will contribute to the development of an atomically precise, bottom-up method of synthesizing new artificial elements in a periodic table for nanoscale materials. The interaction of hydride(s) with Au superatoms will find application in hydrogenation catalysis and hydrogen sensing.

1. INTRODUCTION A series of ligand (phosphines, thiolates, alkynyls, and halides)protected gold clusters with atomically precise sizes provides us not only an expanding library of novel building units of © 2018 American Chemical Society

Received: August 7, 2018 Published: November 14, 2018 3074

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two types of Au-based superatoms as targets and BH4− as a source of a hydride.22 The first target is phosphine-protected Au-based clusters of [Au 9 (PPh 3 ) 8 ] 3+ (Au9) 2 3 and [PdAu8(PPh3)8]2+ (PdAu8)24 whose crystal structures are depicted in Figure 1. The formal cores (Au9)3+ and (PdAu8)2+

functional materials,1−3 but also an ideal platform to test the fundamental concepts of nanoscale materials science. One of such examples is the superatomic concept.4 Valence electrons in a cluster of simple metals are confined in a jelliumlike potential field created by an assembly of positively charged nuclei.5,6 A potential well creates a series of superatomic orbitals, 1S, 1P, 1D, 2S, 1F, 2P, 1G, 2D, and so forth, in the order of energy, where S, P, D, F, and G, represent, respectively, the angular momenta, L = 0, 1, 2, 3, and 4. The valence electrons originating from the 6s orbital of individual Au atoms are accommodated in these orbitals. According to this concept, a spherical [AuN]Z cluster gains high stability when the total number of valence electrons (n), calculated by the following equation, reaches to 8, 20, 40, 58, 92, and so forth due to the closure of the electronic shells, n=N−Z

(1)

where N and Z represent the number of Au constituent atoms and the net charge of the cluster, respectively. Electronic shell closure has been confirmed by photoelectron spectroscopy on a size-selected AuN− beam:7 the energy gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of AuN clusters with N = 8, 20, 34, and 58 are discontinuously larger than those of the neighbors.8 The stability of ligand (L)-protected Au clusters [AuN(L)M]Z (M: number of ligands) can also be understood by this concept with slight modification.9 The formal number of valence electrons (n*) confined in the AuN core of [AuN(L)M]Z is calculated by the following equation: n* = N − Z − NL = n − NL

Figure 1. Au-based superatoms used in the present study. PPh3 ligands for Au9 and PdAu8 are shown by wire frame,23,24 and PVP stabilizers for Au34, Pd1Au33, and Rh1Au34 are omitted for simplicity.33,34

can be viewed as oblate superatoms with an electron configuration of (1S)2(1P)4 (n* = 6).25,26 Adsorption of a hydride to the coordinatively unsaturated site at the center of the (Au9)3+ and (PdAu8)2+ superatoms (Figure 1) and transformation to larger superatoms with n* = 8 have been studied.27,28 The second target is a Au cluster stabilized by poly(N-vinyl-2-pyrrolidone) (PVP) which exhibits high catalytic activity for aerobic oxidation reactions.29 Mass analysis showed that Au34 is contained as the dominant species30 due to the closure of the electronic shells (1S)2(1P)6(1D)10(2S)2(1F)14 (n* = 34).8 Therefore, the PVP-stabilized Au cluster (Au34) can be viewed as a Au34 superatom whose surface is only partially covered by PVP (Figure 1) and is suitable for studying the effect of multiple hydrides on the electronic structures of the superatoms31,32 and the activity of hydrogen species for catalytic usage for hydrogenation reactions. PVP-stabilized Pd1Au33 and Rh1Au34 (Pd1Au33 and Rh1Au34, respectively) were also studied (Figure 1) to reveal the effect of single atom doping of Pd and Rh on hydrogenation catalysis.33,34

(2)

where NL is the total number of electrons withdrawn by the ligands: thiolate, alkynyl, or halide takes one electron, whereas phosphine does not. Thus, the primary role of the ligands is adjustment of the number of valence electrons in the Au core in addition to steric protection of the Au core from aggregation. Upon photoirradiation, these ligand-protected Au clusters do not exhibit localized surface plasmon resonance (LSPR), but show distinct absorption bands reflecting the quantized electronic structures.10 In this regard, ligandprotected Au clusters can be viewed as chemically modified Au superatoms9,11 and are promising candidates for artificial elements in a new periodic table of nanoscale materials.12−14 Another interesting proposal made for bare Au clusters is an analogy between Au and H owing to the similar electron configuration: a Au atom at a low-coordination site of the Au cluster can be replaced with an H atom while retaining the structural motif and electronic structure.15,16 However, less is known about the interaction between hydrogen species and chemically synthesized Au clusters, although it has been theoretically proposed that an H atom in [HAu25(SR)18]0 (RS = thiolate) contributes to the superatomic electron count.17 This situation is in sharp contrast to those of polyhydride clusters of the other coinage metals (Cu and Ag) where hydrides ligate at the bridging sites on the cluster surface.18−20 Interstitial hydrides were also observed in a Cu polyhydride20 in which they play a crucial role in the selective electrochemical reduction of CO2.21 The aim of the present study was to investigate the interaction between hydride(s) and chemically modified Au superatoms to develop new types of hydride-doped Au superatoms, which will be useful for a controlled growth and catalytic application in hydrogenation. To this end, we chose

2. HYDRIDE DOPING OF GOLD-BASED SUPERATOMS PROTECTED BY PHOSPHINE LIGANDS 2.1. Formation and Structures of (HAu9)2+ and (HPdAu8)+ Superatoms

Figure 2a and b shows the electrospray ionization mass spectra (ESI-MS) in the positive-ion mode after the reaction of [Au9(PPh3)8]3+ (Au9) and [PdAu8(PPh3)8]2+ (PdAu8), respectively, with an equimolar amount of NaBH4 or NaBD4.27,28 Soon after the addition of NaBH4, mass peaks assigned to [HAu9(PPh3)8]2+ (HAu9) and [HPdAu8(PPh3)8]+ (HPdAu8) are observed, respectively, indicating the adsorption of a hydride from BH4−, as demonstrated for Ag clusters19 and proposed for Au nanoparticles.22 The binding environment of the hydride in HAu9 and HPdAu8 and their abundances in solution were studied by 1H and 31P NMR spectroscopy. Figure 3a and b represents the 1H NMR and 31P{1H} NMR charts of Au9, respectively, and Figure 3c and d shows the corresponding charts of HAu9. 3075

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Figure 2. ESI-MS: (a) HAu9 and DAu9; (b) HPdAu8 and DPdAu8. 31

P signals in Figure 3g and h indicate that HPdAu8 is the main product in solution. Figure 3g illustrates that a hydride in HPdAu8 exhibits a nonet at 12.4 ppm as in the case of HAu9 with the coupling constant3J(31P−1H) of 19.3 Hz. These results indicate that the hydride in HPdAu8 is also bonded to the central Pd atom and that the eight phosphorus nuclei of the PPh3 ligands are equivalent on the NMR time scale. The hydride peaks of HAu9 (15.1 ppm) and HPdAu8 (12.4 ppm) are shifted significantly to a lower field as compared to that of [Pt(H)Au8(PPh3)8]+ (5.4 ppm).36 This downfield shift is possibly due to the shielding of an external magnetic field induced by the paratropic current of the delocalized electrons in superatomic cores. Geometric and electronic structures of HAu9 and HPdAu8 were studied by DFT calculations on the model systems, [HAu 9 (PMe 3 ) 8 ] 2+ (HAu9-m) and [HPdAu 8 (PMe 3 ) 8 ] + (HPdAu8-m), in which the PPh3 ligand is simplified to PMe3. Figure 4a compares the structures of HAu9-m and [Au9(PMe3)8]3+ (Au9-m), a model of pristine Au9. Au9-m reproduces the crown motif of the (Au9)3+ core of Au9 which was experimentally resolved.23,26 The electron configuration of the oblate (Au9)3+ core of Au9-m is (1S)2(1Px)2(1Py)2 due to the destabilization of the 1Pz superatomic orbital.25,26 Two isomers (HAu9-m(a) and HAu9-m(b)) with comparable stability (ΔE = 0.12 eV) are obtained as the optimized structures of HAu9-m (Figure 4a). The H atom in HAu9m(b) forms a linear substructure with the peripheral Au atom and the PPh3 ligand: the distance between H and the central Au atoms is 1.95 Å. In contrast, the H atom in HAu9-m(a) is bonded to the terminal site of the central Au atom with the distance of 1.69 Å. The more stable structure HAu9-m(b) will be destabilized significantly by reversing back the model PMe3 ligand with the PPh3 ligand due to steric repulsion between them. Thus, we conclude that HAu9-m(a) gives a more plausible model for HAu9. The core motif of HAu9-m(a) becomes more isotropic than that of Au9-m. As a result, the three 1P superatomic orbitals in HAu9-m(a) become nearly degenerate, forming a closed electron configuration (1S)2(1Px)2(1Py)2(1Pz)2. Molecular orbital analysis of HAu9m(a) suggests that the 1Pz orbital is constructed mainly by the bonding interaction between the 1s orbital of H and the 1Pz orbital of Au9-m. The natural bond orbital (NBO) charge on the H atom is almost zero, similar to that on the peripheral Au atoms, indicating that the H atom mimics the Au atom by contributing its 1s electron to the superatomic electron count. In conclusion, the core of HAu9-m(a) can be viewed as a

Figure 3. 1H NMR and 31P{1H} NMR charts of (a, b) Au9 and (c, d) HAu9 in CD2Cl2 at 288 K and (e, f) PdAu8 and (g, h) HPdAu8 in C2D5OD-THF-d8 mixture at 288 K. Symbol codes: triangle, Au9; square, HAu9; circle, PdAu8; star, HPdAu8.

Almost complete shifts of the peaks in Figure 3c and d indicate that HAu9 is the main reaction product. Notably, a nonet centered at 15.1 ppm observed for HAu9 (Figure 3c) is assigned to the adsorbed hydride because the intensity ratio of the nonet to that of the proton signals of PPh3 (1:124) agrees with the number ratio of proton nuclei (1:120) calculated for HAu9 (Figure 2a). The splitting is due to the spin−spin coupling with eight equivalent phosphorus nuclei resulting from fast exchange of the peripheral Au atoms, indicating that the hydride is bound to the central Au atom. The coupling constant of 3J(31P−1H) in HAu9 was 20.6 Hz, comparable to that reported for [Pt(H)Au8(PPh3)8]+ (14.3 Hz).35 The 1H NMR and 31P{1H} NMR charts of PdAu8 and HPdAu8 are represented in Figure 3e/f and g/h, respectively. Again, 1H and 3076

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Figure 4. Geometric structures and the energy diagrams of (a) Au9-m, HAu9-m, and (b) PdAu8-m, HPdAu8-m obtained by DFT calculations. Color codes: yellow, Au; dark green, Pd; blue, P; red, H. Methyl groups are depicted as sticks. Kohn−Sham orbitals are depicted with isodensity values at the 0.025e level.

nearly spherical (HAu9)2+ superatom with an electronically closed shell. Similar results were obtained for PdAu8-m and HPdAu8-m (Figure 4b). The optimized structure of PdAu8-m also reproduces the crown motif of the (PdAu8)2+ core of PdAu8.24,25 HPdAu8-m has two optimized structures HPdAu8-m(a) and HPdAu8-m(b) with similar stability (ΔE = 0.027 eV) (Figure 4b). The H atoms in HPdAu8-m(a) and HPdAu8-m(b) occupy the terminal site on the central Pd atom of the (PdAu8)2+ core (Pd−H bond length: 1.64 Å) and the bridging site of a Au−Pd bond (Pd−H bond length: 1.75 Å), respectively. Given that relative stability of HPdAu8-m(a) and HPdAu8-m(b) will be altered by the level of calculation and the simplification of the phosphine ligand, we speculate that the H atom in HPdAu8 in ambient solution migrates between the bridging and terminal sites around the Pd atom on the NMR time scale. Regardless of the location of the H atom, we can infer that the hydride doping of the (PdAu8)2+ superatom with the (1S)2(1Px)2(1Py)2 configuration affords the hydride-doped (HPdAu8)+ superatom with the (1S)2(1P 2 2 2 x) (1Py) (1Pz) configuration. Figure 4 implies that a hydride can be doped specifically to the naked site where the ligands are not bonded (Figure 1). In contrast, Lee and Jiang theoretically predicted that H can be doped inside the (Pt@Au12)6+ core (n* = 6) of methyl thiolate-protected PtAu24(SCH3)18 whose 12 surface Au atoms are completely passivated by six Au2(SCH3)3 units with a staple motif.36 In order to test experimentally the possibility that a hydride can be doped inside a superatomic core whose surface atoms are completely ligated, we studied the reaction of PtAu24(SC2H4Ph)18 (PtAu24) with BH4−. Optical spectroscopy (Figure 5) showed that the absorption band of PtAu24 at the lowest energy (∼1.1 eV) disappeared completely by the addition of 1 mol equiv of NaBH4. This spectral change is similar to that observed in the electrochemical reduction of

Figure 5. UV−vis spectra of PtAu24 before (black) and after (red) the addition of 1 mol equiv of NaBH4. The inset shows the UV−vis spectra of PtAu24 (black) and electrochemically synthesized [PtAu24]2− (green) adapted with permission from ref 37. Copyright 2015 American Chemical Society.

[PtAu24SC6H13)18] to [PtAu24(SC6H13)18]2− (n* = 8).37 However, the optical spectrum of the product does not completely match with that of [PtAu24(SC6H13)18]2−,37 suggesting the formation of another superatom with n* = 8. At this moment, we proposed a hydride-doped superatom [HPtAu24(SC2H4Ph)18]− (n* = 8) as the most plausible candidate, although further characterization such as singlecrystal X-ray diffraction (SCXRD) and 1H NMR is necessary to make a decisive conclusion. We believe that hydride doping is possible for an electronically nonclosed superatom regardless of whether the terminal or bridging binding sites on the surface of the superatom are available. 3077

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Figure 6. Time-resolved ESI-MS recorded after mixing AuClPPh3 (2 mol equiv) with (a) HAu9 and (b) HPdAu8.

2.2. Growth Reactions of (HAu9)2+ and (HPdAu8)+ Superatoms

example of size-controlled synthesis based on the bottom-up approach. This selective growth is associated with the adsorption of a single hydride and two Au+ units to (Au9)3+. The second hydride is not adsorbed because the electronic shell is closed by the adsorption of a single hydride and the third AuCl unit is not adsorbed because the ligand shell is sterically closed by the adsorption of two AuCl units. Figure 6b shows the ESI-MS spectra during the reaction of HPdAu8 with AuClPPh3. The data illustrates that cluster HPdAu8 undergoes consecutive growth to HPdAu10 via HPdAu9 while retaining the n* value at 8. The hydrogen atom remained throughout the growth processes, in sharp contrast to the case of the growth of HAu9. The structure of the final product HPdAu10, synthesized previously by the coreduction method,41 is determined by SCXRD analysis (Figure 7a). The

It is known that the hydrogen atoms adsorbed on platinum group metal colloids can reduce metal ions and leads to deposition of the metal atoms (sacrificial core growth).38 This process suggests that (HAu9)2+ and (HPdAu8)+ can be grown to larger superatoms by taking advantage of the reducing ability of the adsorbed hydride. To test this hypothesis and attain a molecular-level understanding of the growth mechanism, we monitored in situ the reactions of HAu9 or HPdAu8 with a Au(I) complex by ESI-MS. Figure 6a shows the time-resolved ESI-MS during the reaction of HAu9 with AuClPPh 3 . New peaks assigned to hydride-free [Au10(PPh3)8Cl]+ (Au10) and [Au11(PPh3)8Cl2]+ (Au11) appear after the addition of AuClPPh3 to HAu9 and grow gradually with the reaction time. These results indicate that cluster HAu9 undergoes consecutive growth to the well-known Au11 via Au10 while retaining the n* value at eight. The hydride doped to HAu9 is released in the form of proton in the first electrophilic addition of AuCl. Scheme 1 shows the Scheme 1. Hydride-Mediated Growth of (Au9)3+ to (Au11)3+

Figure 7. (a) X-ray structure (left) and core structure (right) of HPdAu10, and (b) DFT-optimized structure (left) and core structure (right) of HPdAu10-m. Phenyl rings and methyl groups are depicted as gray sticks and hydrogen atoms except for the hydride (red) are omitted for clarity. (c) UV−vis spectra of HPdAu10 and Au11.

(HPdAu10)3+ core of HPdAu10 was significantly deformed as compared to that of the (Au11)3+ core of Au11 determined crystallographically:40 one of the Au−Au surface bonds in HPdAu10 (3.88 Å) is significantly longer than the corresponding Au−Au bond in Au11 (3.11 Å). This marked difference in bond lengths is attributable to the interaction of the hydrogen atom with the metallic core based on the DFT calculation on a model system [HPdAu10 (PMe 3 ) 8 Cl2 ] + (HPdAu10-m) (Figure 7b). Although the structure of HPdAu10-m reproduces that of HPdAu10 in the solid state,

hydride-mediated growth mechanism of (Au9)3+ to (Au11)3+ based on the crystal structures of Au1039 and Au11.40 Overall, the growth process is composed of two steps: (1) reduction of (Au9)3+ (n* = 6) to (HAu9)2+ (n* = 8) by hydride doping; (2) replacement of proton with Au+ to convert (Au10)2+ (n* = 8), followed by the addition of Au+ to form (Au11)3+ (n* = 8). The hydride-mediated growth of Au9 to Au11 represents a unique 3078

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structures of HPdAu928 and HPdAu10 (Figure 7b). As in the case of Scheme 1, the growth process is divided into two steps: (1) reduction of (Pd@Au8)2+ (n* = 6) to (HPd@Au8)+ (n* = 8) by hydride doping; (2) sequential additions of two Au+ units to form (HPdAu10)3+ (n* = 8). Again, the selective conversion is associated with the closure of the electronic shell in (HPd@Au8)+ and the ligand shell in (HPdAu10)3+.

we cannot exclude the possibility that the hydrogen atom in HPdAu10 in ambient solution migrates among different interstitial sites in the core due to the structural fluxionality.28 The differences in the optical spectra between HPdAu10 and Au11 (Figure 7c) reflect those of the central components (PdH vs Au) and of the core geometries due to the presence of hydrogen atom. Scheme 2 shows the hydride-mediated growth mechanism of (Pd@Au8)2+ to (HPdAu10)3+ based on DFT-optimized

3. HYDRIDE DOPING OF GOLD SUPERATOMS STABILIZED BY POLYMERS

Scheme 2. Hydride-Mediated Growth of (Pd@Au8)2+ to (HPd@Au10)3+

3.1. Hydride-Mediated Preferential Formation of Au34 Superatom

A mixture of Au34 and Au43 clusters stabilized by PVP (Au34 and Au43) was obtained as the main product by homogeneous mixing of an aqueous solution of HAuCl4 and that of NaBH4 using a microfluidic mixer in the presence of PVP, as revealed by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (Figure 8a).30,33,34 Although Au34 and Au43 were detected as the adducts of Cl, the Cl adsorbates may not act as electron-withdrawing ligands because the electron affinity of Au34 (3.4 eV)8 is comparable to that of Cl (3.6 eV). The high abundance of Au34 can be explained by the closure of superatomic shells as in the case of bare Au34 clusters, although the origin of the high abundance of Au43 is not clear at present.8 It should be emphasized that a magic Au34 superatom is produced selectively by a bottom-up process without a topdown, postsynthetic step such as etching. It is reasonable to speculate that preferential formation of Au34 is realized by

Figure 8. (a) Typical MALDI-MS of Au34, the optimized structures of (b) Au34 and (c) Au32(H−), and (d) the electronic structures of Au34 and Au32(H−). 3079

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of the LSPR band suggests that over-reduced clusters AuN(H−)M with n* ≫ 34 are formed at the initial stage. Preferential formation of Au34 under such conditions may be due to the geometrical shell closure of Au34: it was reported that the second atomic layer is closed in Au34 and the next atom goes to the third layer in Au35.45

hydride-mediated growth reactions similar to those depicted in Scheme 1: sequential additions of AuCl units to electronically closed hydride-doped superatoms generated by the interaction with BH4−. In those reactions, AuN(H−)M with 34 electrons (n* = N + 2M = 34) should be involved as follows: AuN (H−)M + 2M AuCl = Au34 Cl 2M + M H+

(3)

3.2. Hydride-Induced Localized Surface Plasmon Resonance in Au34 Superatom

Here, putative building units of AuCl may be formed by partial reduction of AuCl4−. To test the validity of this hypothesis, we conducted DFT calculations on the simplest systems of Au32(H−) and Au34. The optimized structure of Au34 (Figure 8b) reproduces the Au4@Au30 core−shell structure proposed previously.42,43 Figure 8c shows the optimized structure of Au32(H−) obtained by placing H− at a bridging site on the previously reported structure of Au32.44 The electronic structures of Au 32 (H − ) and Au 34 were very similar, demonstrating that H− mimics Au− and contributes to the electron count (Figure 8d). Careful investigation revealed that the LSPR band appeared at the initial stage of the synthesis of Au34, remained for about 30 min, and then rapidly disappeared due to the oxygen in air (Figure 9). Observation

The LSPR band appeared when Au34 reacted with BH4−. The mechanism of this phenomenon was studied by monitoring the electronic structures of Au34 by in situ measurements of X-ray absorption spectroscopy (XAS) at the Au L3-edge and UV−vis spectroscopy. The reaction was initiated by the rapid injection of an aqueous solution of NaBH4 into the dispersion of Au34 ([BH4−]:[Au] = 0.25:1) and a mixed hydrosol in a vessel was flowed in a closed circulatory system equipped with two cells for the measurement of XAS and UV−vis spectroscopy.32 The cluster size was retained during the reaction with BH4−, as confirmed by EXAFS analysis at the Au L3-edge. Figure 10a compares time courses of the intensities of the white line at the Au L3-edge (11918 eV), due to electronic transition from the occupied Au 2p orbital to the unoccupied Au 5d orbital, and the LSPR band (520 nm). The intensities of the white line and the LSPR band both start to increase soon after the mixing, take the maximum values, and gradually diminish simultaneously from ∼230 s. These results indicate that the enhancement of LSPR intensity is closely related to that of the vacancy of the d orbitals.46 The enhancement of the vacancy of d orbitals is ascribed to the formation of unoccupied Au 5d−H 1s antibonding orbitals (Figure 10a) and thus indicates the bonding interaction between the H 1s orbital and Au 5d band. The synchronous enhancement of intensity of the LSPR band and the white line (Figure 10a) shows that the occupancy of sp orbitals of Au34 is increased by the hydride-doping and the resulting hydride-doped Au34 mimics a plasmonic Au nanoparticle with a diameter of >2 nm. The unique feature of hydride-mediated plasmonic Au34 is that it can be reversibly converted to nonplasmonic Au34 by removal of hydrides by dissolved O2.

Figure 9. Time course of absorption spectra during the preparation of Au34. Inset shows the change in absorbance at 509 nm versus time. Adapted with permission from ref 31. Copyright 2016 Royal Society of Chemistry.

Figure 10. (a) Changes of absorption intensities at 11918 eV (A) and 520 nm (B) over time during the reaction of Au34 with NaBH4. (b) Schematic illustration of electronic structures of Au cluster before and after hydride adsorption. Adapted with permission from ref 32. Copyright 2017 American Chemical Society. 3080

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Accounts of Chemical Research 3.3. Hydrogenation Catalysis of Pd1Au33 and Rh1Au34 Superatoms

by PVP. The former are viewed as oblate-shaped superatoms with six electrons and a coordinatively unsaturated site at the center, whereas the latter is a nearly spherical superatom having the closed electronic structure (34 electrons) and multiple uncoordinated sites on the surface. A hydride is selectively bonded to the unligated Au and Pd site of (Au9)3+ and (PdAu8)2+ to generate (HAu9)2+ and (HPdAu8)+, respectively, having nearly spherical shape and closed electronic structure (eight electrons). (HAu9)2+ is converted to (Au11)3+ by the addition of two Au(I) complexes by releasing a proton, whereas (HPdAu8)+ is converted to a new (HPdAu10)3+ superatom having an interstitial hydrogen atom. This hydride-mediated growth proceeds in two steps: reduction to electronically closed states by hydride adsorption, followed by the addition of Au+ components. The selective formation of the Au34 superatom in polymer is also explained by the involvement of hydride-doped Au clusters. Upon the reaction with BH4−, the Au34 superatom exhibits the LSPR band due to the increase in the number or density of electrons by multiple adsorption of hydrides. Hydride-doped plasmonic Au34 is converted to nonplasmonic Au34 by removal of hydrides by dissolved O2. Hydrogens adsorbed on Au34 are not reactive toward hydrogenation of CC bonds, but can be used efficiently for the hydrogenation by doping a single atom of Pd or Rh to Au34. These results demonstrate that the gold−hydrogen analogy proposed for bare Au clusters can be applied to chemically modified Au clusters. The hydride-mediated growth not only sheds light on the important role of hydrides in the chemical synthesis of Au superatoms, but also suggests a new atomically precise, bottom-up method of synthesizing new superatoms. The adsorption of hydride(s) on Au superatoms paves the way for hydrogenation catalysis and hydrogen sensing.

The LSPR band was also induced by the reaction between H2 and Au34, indicating that H 2 undergoes dissociative adsorption on Au34. Catalytic application of Au34 for hydrogenation reactions was tested using 1-dodecene, styrene, and cyclooctene as model substrates. Table 1 shows that Au34 Table 1. Hydrogenation Catalysisa

reaction

catalyst

H2 (MPa)

conversion (%)

(a)

Au34 Pd1Au33 Rh1Au34 Au34 Pd1Au33 Rh1Au34 Au34 Pd1Au33 Rh1Au34

0.1