Hydride-Mediated Controlled Growth of a Bimetallic (Pd@Au8)2+

Communication Cite This: J. Am. Chem. Soc. 2018, 140, 12314−12317

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Hydride-Mediated Controlled Growth of a Bimetallic (Pd@Au8)2+ Superatom to a Hydride-Doped (HPd@Au10)3+ Superatom Shinjiro Takano,† Haru Hirai,† Satoru Muramatsu,† 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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only the chemical properties of the clusters toward the controlled growth but also the geometric and electronic structures of the clusters. A recent theoretical study proposed that the H atom interstitially doped into the Au core of Au25(SCH3)18 contributes to the electron counting.13 Other studies reported that the adsorption of hydrides or hydrogens on small nonplasmonic Au clusters induced localized surface plasmon resonance due to electron donation.14,15 In the present study, we focused on doping of H− into the oblate (Pd@Au8)2+ superatomic core of [PdAu8(PPh3)8]2+ (1) having a coordinatively unsaturated Pd atom at the center. The structure of the (HPd@Au8)+ core of [HPdAu8(PPh3)8]+ (2) was studied by nuclear magnetic resonance (NMR) techniques and density functional theory (DFT) calculations. Hydridedoped cluster 2 was fully converted to [HPdAu10(PPh3)8Cl2]+ (4) via [HPdAu9(PPh3)8Cl]+ (3) by the sequential addition of AuCl units. The H dopant survived during the growth, in sharp contrast to the release of H+ from [HAu9(PPh3)8]2+ (eq 1). Single-crystal X-ray diffraction (SCXRD) analysis showed that the (HPdAu10)3+ core of 4 was appreciably distorted compared to the (Au11)3+ core due to the presence of an interstitial H atom. This study not only provides a molecular-level understanding of the effect of H− doping on the structures and properties of Pd−Au bimetallic clusters but also presents an atomically precise bottom-up method toward the synthesis of new Au-based superatoms. Figure 1a,b shows the positive-ion electrospray ionization mass spectra (ESI-MS) before and after the addition of an ethanol solution containing 1 mol equiv of NaBH4 to an ethanol−tetrahydrofuran solution of 1.16,17 Figure 1b demonstrates that the reaction of 1 and NaBH4 yielded 2 as the dominant ionic species. The reaction of 1 and NaBH4 (1 mol equiv) was also monitored by 1H NMR spectroscopy. Three bands in the 1H NMR chart (Figure 1c) assigned to the protons of PPh3 ligands of 1 were significantly shifted after the reaction with NaBH4 (Figure 1d), which are assigned to the proton signals of the PPh3 ligands of 2. The absence of other new peaks in the 1H NMR chart in Figure 1d and the 31P{1H} NMR chart (Figure S1a)17 indicates that cluster 2 is the major product. Notably, a multiplet centered at 12.4 ppm appears in Figure 1d. This band is assigned to a hydride dopant in 2 based on the intensity ratio of the multiplet to the proton signals of PPh3. The 1H{31P} decoupling experiment (Figure S1b)17 demonstrates that the eight phosphorus nuclei coupling with

ABSTRACT: A hydride (H−)-doped bimetallic superatom (HPdAu8)+ was produced by reacting BH4− with an oblate (PdAu8)2+ superatom protected by PPh3. The H atom in (HPdAu8)+ survived during the sequential addition of Au(I)Cl to form an (HPdAu10)3+ superatom, in sharp contrast to the proton release from a H−-doped pure gold superatom (HAu9)2+ in the growth process to (Au11)3+. Single-crystal X-ray diffraction analysis and density functional theory calculations on (HPdAu10)3+ showed that the interstitially doped H atom induced a notable deformation of the core.

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igand-protected clusters of metal, especially gold, provide an ideal opportunity to attain an atomic-level understanding of how the structures evolve with the size and how the properties are tuned by doping or alloying.1−4 Atomically precise synthesis of the ligand-protected metal clusters is the foundation of the research and has been mainly based on topdown approaches (e.g., size focusing via chemical etching) and high-resolution separation techniques.1−4 In contrast, precise synthesis by a bottom-up approach has been limited to a few examples due to the difficulty in controlling the kinetic balance between the fast clustering of the metal (0) atoms and the surface passivation by the ligands.5−8 Recently, we demonstrated that a hydride (H−)-doped gold cluster [HAu9(PPh3)8]2+, formed by reacting [Au9(PPh3)8]3+ with BH4−, was selectively converted to [Au11(PPh3)8Cl2]+ by the addition of two AuCl units according to the following reactions.9 [HAu 9(PPh3)8 ]2 + + AuCl → [Au10(PPh3)8 Cl]+ + H+ (1)

[Au10(PPh3)8 Cl]+ + AuCl → [Au11(PPh3)8 Cl 2]+

(2)

This growth process can be viewed as the transformation of an oblate (Au9)3+ superatom with an electron configuration of (1S)2(1P)4 (ref 10) to a spherical (Au11)3+ superatom with a closed electron configuration of (1S)2(1P)6 (ref 11) via a hydride-doped superatom of (HAu9)2+. Hydride-mediated core growth to [PtAu8(PPh3)8]2+ has also been reported in a Pt−Au bimetallic cluster [HPtAu7(PPh3)8]2+.12 These observations suggest that the addition of Au atoms to the seed clusters is initiated by electron donation via H− adsorption followed by the replacement of H+ with Au+ species rather than direct adsorption of the reduced Au(0) species. Hydride affects not © 2018 American Chemical Society

Received: June 27, 2018 Published: September 4, 2018 12314

DOI: 10.1021/jacs.8b06783 J. Am. Chem. Soc. 2018, 140, 12314−12317

Communication

Journal of the American Chemical Society

simplification of the PPh3 ligand to PMe3 does not affect the core motif. The electron configuration of 1m is expressed as (1S)2(1Px)2(1Py)2 because the high-lying 1Pz superatomic orbital in the oblate core is not occupied. Two optimized structures, 2m-a and 2m-b, with comparable stability (ΔE = 0.027 eV) were obtained as shown in Figure 2b. The less-stable structure 2m-b is similar to that of [HAu9(PPh3)8]2+ reported previously:9 the H atom is bonded to the terminal site of the Pd atom (Pd−H bond length = 1.64 Å). In the most stable structure, 2m-a whose counterpart was not reported in the previous study of [HAu9(PPh3)8]2+,9 the H atom occupies the bridging site of one of the eight Au−Pd bonds (Pd−H bond length; 1.75 Å). Figure 2b shows that the H− adsorption induces spherization of the cores of 2m-a and 2m-b and occupation of three nearly degenerate 1P orbitals. Even if the H atom exclusively occupies the Pd−Au bridging site in 2 according to the relative stability of 2m-a and 2m-b, the nonet feature of the 1H NMR chart (Figure 1d) suggests that the H atom in 2 in ambient solution migrates to all the bridging sites equivalently. In addition, occupation of the terminal site by H cannot be excluded because the energy difference between 2ma and 2m-b is so small that it can be reversed depending on the level of calculation and affected by the simplification of the phosphine ligand. We speculate that the H atom in 2 migrates between the bridging and terminal sites around the Pd atom in association with the structural fluctuation of the Au−PPh3 framework on the NMR time scale. Regardless of the location of the H atom, we can conclude that the hydride doping of the (PdAu8)2+ superatom with six electrons affords the more spherical (HPdAu8)+ superatom with eight electrons. The growth reaction of the (HPdAu8)+ core of 2 was monitored by mass spectrometry. The time course of the ESIMS of the mixture of 2 and AuClPPh3 (Figure 3) shows that

Figure 1. Positive-ion ESI-MS of (a) 1 and (b) the reaction products with NaBH4. Insets compare the experimental and theoretical isotope patterns. 1H NMR charts (400 MHz, C2D5OD-THF-d8, 288 K) of (c) 1 and (d) the reaction products with NaBH4 in C2D5OD.

the proton on the Pd atom of the (PdAu8)2+ core of 2 are equivalent on the NMR time scale. To our knowledge, cluster 2 is the first example of a hydride-doped Pd−Au bimetallic cluster, whereas the corresponding hydride-doped Pt−Au bimetallic cluster [HPtAu8(PPh3)8]+ was reported previously.18,19 The optical spectrum of 2 (Figure S2)17 shows that the electronic structure of 1 is significantly affected by the addition of H− and that cluster 2 is converted back to 1 under an ambient environment. DFT calculations were conducted on the model systems [PdAu8(PMe3)8]2+ (1m) and [HPdAu8(PMe3)8]+ (2m). The crown motif of the (PdAu8)2+ core of 120,21 was reproduced by the optimized structure of 1m (Figure 2a), indicating that

Figure 3. Time-resolved ESI-MS during the growth reaction of 2. Mass peaks #1 and #2 are assigned to [HPdAu9(PPh3)8]2+(BH4−) and [PdAu9(PPh3)7Cl2]+, respectively.

cluster 2 undergoes growth to [HPdAu10(PPh3)8Cl2]+ (4) via [HPdAu9(PPh3)8Cl]+ (3) by consecutive addition of AuCl units. The formal number of valence electrons remains at eight during this growth. Interestingly, the H atom remained throughout the growth processes (Figure S3),17 in sharp contrast to the case of the growth of [HAu9(PPh3)8]3+,9 where the proton was released in the first addition step of AuCl (eq

Figure 2. Geometric and electronic structures of (a) 1m and (b) two isomers of 2m 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. 12315

DOI: 10.1021/jacs.8b06783 J. Am. Chem. Soc. 2018, 140, 12314−12317

Communication

Journal of the American Chemical Society

slowing down of the motion of the Au−PPh3 framework (Figure S6).17 On the other hand, the nonet peak due to adsorbed H remained sharp even at 204 K (Figure S6).17 This observation suggests that the H atom migrates even after the Au−PPh3 framework is frozen (Figure S7).17 In summary, the H atom in 4 either in the solid state (∼90 K) or in ambient solution is not attached to the surface of the metallic core as a ligand as observed in polyhydride Ag and Cu clusters,24,25 but is directly incorporated into the metallic core as a building component. The above results indicate that the core growth from 1 to 4 is induced exclusively via hydride-doped cluster 2. Here, we discuss the building-up scheme of 2 → 3 → 4 based on molecular orbitals calculated for their model systems of 2m, [HPdAu9(PMe3)8Cl]+ (3m) and 4m: 2m-b rather than 2m-a was chosen as a structure of 2m because of its more symmetrical structure. Because the growth from 2 to 3 can be viewed as electrophilic addition of an AuCl unit to 2, we consider the interaction of LUMO of AuCl and an occupied orbital of 2m-b (Figure 5a). Among the three nearly

1). This behavior is consistent with a higher affinity of H to Pd than to Au. Product 4 was formed previously by coreduction of PPh3 complexes of Au and Pd, but its structure remained unknown.22 Figure 4a shows the structure of 4 determined

Figure 4. X-ray structures (left) and the core structures (right) of (a) 4 and (b) [Au11(PPh3)8Cl2]+ (ref 23). Phenyl rings are depicted as gray sticks and hydrogen atoms are omitted for clarity. (c) Optimized structures of two isomers of 4. Methyl groups are depicted as sticks. The color codes except light green (Cl) are the same as those in Figure 2.

by SCXRD analysis (Figure S4a).17 The location of the doped H could not be determined due to its weak X-ray scattering power. However, we noticed that the structure of the metallic core of 4 was significantly deformed as compared to that of the Au11 core of [Au11(PPh3)8Cl2]+ determined crystallographically (Figure 4b).23 One of the Au−Au surface bonds in 4 is significantly longer (3.88 Å) than the corresponding Au−Au bond in [Au11(PPh3)8Cl2]+ (3.11 Å), while the lengths of the radial bonds agree within 2% (Figure S5 and Table S1).17 This marked difference in the core structures is due to the interaction of the H atom with the metallic core. To test this hypothesis, we conducted DFT calculations on a model system [HPdAu10(PMe3)8Cl2]+ (4m). Two optimized structures, 4ma and 4m-b, with comparable stability (ΔE = 0.112 eV) were obtained (Figure 4c). The H atoms in 4m-a and 4m-b are located at the bridging site between the central Pd atom and a surface Au atom bonded to Cl and PPh3, respectively. The Pd−H bond lengths in 4m-a and 4m-b are 1.92 and 1.83 Å, respectively, longer than that in 2m-a (1.75 Å). The elongated Au−Au bond in 4 (3.88 Å) was not reproduced by that in 4ma (3.05 Å), but by that in 4m-b (4.05 Å). Therefore, 4m-b is a plausible model for 4 in the solid state. However, a nonet signal at 5.28 ppm in the 1H NMR chart of 4 in ambient solution (Figure S4c)17 suggests that eight phosphorus nuclei coupling with the proton are equivalent on the NMR time scale. The 1H and 31P NMR signals of PPh3 ligands are broadened by reducing the temperature to 204 K due to the

Figure 5. (a) Calculated energy diagrams and selected orbitals of AuCl and 2m-b. (b) Optimized structures and the 1Pz orbitals of two isomers of 3. Methyl groups are depicted as sticks. The color codes are the same as those in Figure 4.

degenerate 1P orbitals of 2m-b, the 1Pz orbital is most likely involved in the bonding with AuCl because of its higher accessibility along the z-axis through an open space of the ligand layer compared to the 1Px and 1Py orbitals (Figure 2b). By bonding AuCl directly to the H atom of 2m-b and to the Pd atom of 2m-b from the opposite side of the H atom, two optimized structures, 3m-a and 3m-b, with comparable stability (ΔE = 0.128 eV) were obtained (Figure 5b), 12316

DOI: 10.1021/jacs.8b06783 J. Am. Chem. Soc. 2018, 140, 12314−12317

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(10) Yamazoe, S.; Matsuo, S.; Muramatsu, S.; Takano, S.; Nitta, K.; Tsukuda, T. Inorg. Chem. 2017, 56, 8319. (11) Walter, M.; Akola, J.; Lopez-Acevedo, O.; Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Whetten, R. L.; Grönbeck, H.; Häkkinen, H. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 9157. (12) Kanters, R. P. F.; Bour, J. J.; Schlebos, P. P. J.; Bosman, W. P.; Behm, H.; Steggerda, J. J.; Ito, L. N.; Pignolet, L. H. Inorg. Chem. 1989, 28, 2591. (13) Hu, G.; Tang, Q.; Lee, D.; Wu, Z.; Jiang, D.-e. Chem. Mater. 2017, 29, 4840. (14) Ishida, R.; Yamazoe, S.; Koyasu, K.; Tsukuda, T. Nanoscale 2016, 8, 2544. (15) Ishida, R.; Hayashi, S.; Yamazoe, S.; Kato, K.; Tsukuda, T. J. Phys. Chem. Lett. 2017, 8, 2368. (16) Ito, L. N.; Felicissimo, A. M. P.; Pignolet, L. H. Inorg. Chem. 1991, 30, 988. (17) For details, see Supporting Information. (18) Bour, J. J.; Schlebos, P. P. J.; Kanters, R. P. F.; Schoondergang, M. F. J.; Addens, H.; Overweg, A.; Steggerda, J. Inorg. Chim. Acta 1991, 181, 195. (19) Pignolet, L. H.; Aubart, M. A.; Craighead, K. L.; Gould, R. A. T.; Krogstad, D. A.; Wiley, J. S. Coord. Chem. Rev. 1995, 143, 219. (20) Ito, L. N.; Johnson, B. J.; Mueting, A. M.; Pignolet, L. H. Inorg. Chem. 1989, 28, 2026. (21) Matsuo, S.; Takano, S.; Yamazoe, S.; Koyasu, K.; Tsukuda, T. ChemElectroChem 2016, 3, 1206. (22) Kurashige, W.; Negishi, Y. J. Cluster Sci. 2012, 23, 365. (23) McKenzie, L. C.; Zaikova, T. O.; Hutchison, J. E. J. Am. Chem. Soc. 2014, 136, 13426. (24) Jordan, A. J.; Lalic, G.; Sadighi, J. P. Chem. Rev. 2016, 116, 8318. (25) Bootharaju, M. S.; Dey, R.; Gevers, L. E.; Hedhili, M. N.; Basset, J.-M.; Bakr, O. M. J. Am. Chem. Soc. 2016, 138, 13770. (26) Nishigaki, J.; Koyasu, K.; Tsukuda, T. Chem. Rec. 2014, 14, 897. (27) Tomalia, D. A.; Khanna, S. N. Chem. Rev. 2016, 116, 2705. (28) Jena, P.; Sun, Q. Chem. Rev. 2018, 118, 5755.

respectively. Figure 5b shows that the lobes of the 1Pz orbitals of 3m-a and 3m-b are distributed along the z-axis. Structure 4m-b in Figure 4c may be constructed by bonding interaction of LUMO of another AuCl unit with the 1Pz orbitals of 3m-b. The maximum number of the AuCl units added to 1 is limited to two probably due to steric hindrance in the organic layer. In summary, we elucidated the hydride-mediated growth of the oblate (Pd@Au8)2+ superatom to a novel hydride-doped (HPd@Au10 )3+ superatom. SCXRD analysis and DFT calculations showed that the (HPdAu10)3+ superatom was significantly distorted due to the presence of an interstitial H atom. This study provides an atomically precise bottom-up synthesis of new Au-based superatoms.26−28

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b06783.

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Synthesis details and additional data (PDF) Data for crystal structure of 4 (CIF)

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Shinjiro Takano: 0000-0001-9262-5283 Tatsuya Tsukuda: 0000-0002-0190-6379 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Profs. M. Shionoya and S. Tashiro (UTokyo) for their generous assistance of with temperature-dependent NMR measurement. This research was financially supported by the Elements Strategy Initiative for Catalysts & Batteries (ESICB) and by the “Nanotechnology Platform” (Project No. 12024046) of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan and a Grant-in-Aid for Scientific Research (A) (Grant No. 17H01182) from the Japan Society for the Promotion of Science (JSPS). Calculations were partly performed using the Research Center for Computational Science, Okazaki, Japan.

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REFERENCES

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DOI: 10.1021/jacs.8b06783 J. Am. Chem. Soc. 2018, 140, 12314−12317