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Hydride-doped Gold Superatom (AuH) : Synthesis, Structure and Transformation Shinjiro Takano, Haru Hirai, Satoru Muramatsu, and Tatsuya Tsukuda J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b03880 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 20, 2018
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Journal of the American Chemical Society
Hydride-doped Gold Superatom (Au9H)2+: Synthesis, Structure and Transformation 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 Catalysis and Batteries (ESICB), Kyoto University, Katsura, Kyoto 615-8520, Japan.
Supporting Information Placeholder ABSTRACT: Doping of a hydride (H–) into an oblate-
shaped gold cluster [Au9(PPh3)8]3+ was observed for the first time by mass spectrometry and NMR spectroscopy. Density functional theory calculations for the product [Au9H(PPh3)8]2+ demonstrated that the (Au9H)2+ core can be viewed as a nearly spherical superatom with a closed electronic shell. The hydride-doped superatom (Au9H)2+ was successfully converted to the well-known superatom Au113+, providing a new atomically precise synthesis of Au clusters via bottom-up approach.
The interaction of hydrogen with coinage metal (Cu, Ag, and Au) clusters has been of great interest since it significantly affects the physicochemical properties. Small nonplasmonic Au clusters exhibit localized surface plasmon resonance in the presence of NaBH4 due to electron donation from the adsorbed H atoms.1,2 It was proposed that hydrides within Cu clusters play a crucial role in selective electrochemical reduction of CO2.3 In order to understand how the novel properties of coinage metal clusters will be affected by H atoms, information on their binding motif is essential. The structures of Cu polyhydride clusters have been studied extensively.4–9 Neutron diffraction studies on [Cu20H11{S2P(Oi-Pr)2}9] (Ref. 6) and Cu32H20{S2P(Oi-Pr)2}12 (Ref. 8) revealed that H– prefers the bridging binding mode. This binding mode of H– was also suggested in [Cu18H7{1,2S(C6H4)PPh2}10(I)] (Ref. 7) and [Cu25H22(PPh3)12]+ (Ref. 9) based on density functional theory (DFT) calculations and Xray diffraction data. A similar bridging mode was confirmed in Ag monohydride clusters by means of X-ray and multinuclear nuclear magnetic resonance (NMR) techniques.10 Recently, the synthesis of Ag polyhydride clusters, such as [Ag18H16(PPh3)10]2+, [Ag25H22(DPPE)8]3+ (DPPE = 1,2-bis(diphenylphosphino)-ethane), and [Ag26H22(TFPP)13]2+ (TFPP = tris(4-fluorophenyl)phosphine), was confirmed by mass spectrometry and NMR spectroscopy.11 The sequential loss of H2 from the Cu and Ag polyhydride clusters was observed in the gas phase.12,13 In contrast, less is known about the interaction between H and Au clusters because hydrogen has not been experimentally observed in protected Au clusters.14–17 Instead, an analogy between Au and H has been demonstrated experimentally and
theoretically on bare Au clusters.18,19 For example, photoelectron spectroscopy revealed that the electron affinities of HAun agreed quantitatively with those of Aun+1.18 Theoretical calculation predicted that the Au atom on the surface can be replaced with H while retaining the whole structure.19 A recent theoretical study proposed that an H atom behaves as an Au atom in the thiolate-protected Au cluster Au25(SR)18
and contributes its 1s electron to the superatomic electron count.20 This
unique behavior of H in the Au cluster suggests that H can be interstitially doped into the Au clusters and that its superatomic electronic structure21–23 can be tuned. In the present study, we observed the formation of [Au9H(PPh3)8]2+ by doping H– into [Au9(PPh3)8]3+ (1), a 6e gold superatom having a coordinatively unsaturated site (Scheme 1). DFT calculations revealed that the core of the [Au9H(PPh3)8]2+ product can be viewed as a nearly spherical superatom (Au9H)2+ with a closed electronic shell (8e). We also demonstrated that this superatom (Au9H)2+ can be transformed into Au113+ by the sequential addition of AuCl units. The [Au9(PPh3)8]3+ cluster (1) was synthesized according to the method in Ref. 24 with slight modification.25 After the addition of an ethanol solution containing 1 molar equivalent of NaBH4 at room temperature, the color of a dichloromethane solution of 1 changed quickly from light orange to deep red (Figure S1). The reaction was monitored in situ by positive-ion electrospray ionization mass spectrometry (ESI-MS). Figure 1(a) shows the mass spectrum of 1 before the addition of NaBH4, exhibiting the mass peaks of [Au9(PPh3)8]3+ and its complex with NO3–. After the addition of NaBH4, the peak of [Au9(PPh3)8]3+ disappeared almost completely and a new peak assigned to the adduct of [Au9(PPh3)8]3+ and hydride (H–) ([Au9H(PPh3)8]2+ (2)) was observed as a dominant peak (Figure 1(b)). This assignment was validated by an isotope labeling experiment. The same reaction was performed using an ethanold6 solution of NaBD4 instead of an ethanol solution of NaBH4. Figure 1(c) shows that the mass peak of the product was increased by 1 Da. The cross isotope labeling experiment indicated that the source of the hydride in 2 was the added BH4– or BD4– and that the hydride in 2 did not undergo exchange with the surrounding protic medium (Figure S2). Mass spectrometry showed that although product 2 was detected as a dominant product for several minutes after the reaction with NaBH4, it gradually decomposed (Figure S3). This result suggests a metastable nature of 2.
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The formation of 2 by addition of 1 molar equivalent of NaBH4 in ethanol-d6 changed the charts dramatically (Figures 2(c) and (d)). The phenyl protons of 2 are mainly separated into three groups. The broad signal centered at 7.25 ppm is assigned to the o-proton of PPh3 in which a triplet due to the spin coupling with the neighboring m-proton and phosphorous nuclei is smeared out. The other triplets at 6.62 and 7.05 ppm are assigned to m- and p-protons of the PPh3, respectively. The most notable result is the appearance of a multiplet centered at 15.1 ppm. The multiplet can be assigned to the hydride of 2 bound at the central gold atom due to the following reasons. Firstly, the relative intensities of the five lines experimentally observed (approximately 1.1:2.0:2.5:2.1:1.0) agreed well with those of the five main peaks calculated for the nonet (1:2:2.5:2:1). Secondly, the multiplet was converted to a singlet when the peak at 53.5 ppm in the 31P{1H} NMR chart (Figure 2(d)) was selectively decoupled (Figure 2(e)). This result indicates that the multiplet originated from the spin-spin coupling with eight equivalent phosphorous nuclei resulting from fast exchange of the peripheral Au atoms. Thirdly, the relative ratio of the integrated area of the multiplet to that of the o-, m-, and p-proton signals of PPh3 (1:49.7:27.0:47.4) corresponds to the number ratio of proton nuclei (1:48:24:48) calculated for the composition of 2 (Figure 1). Small triplet and singlet peaks marked with circles were observed in the 1H and 31P{1H} NMR chart just after the mixing of 1 and NaBH4 (Figures 2(c) and 2(d)). These peaks grew in intensity with time (Figure S4) in parallel to the mass peak of [Au8(PPh3)7]2+ in the ESI-MS data (Figure S3). Based on these results, we assigned these peaks to [Au8(PPh3)7]2+ produced by the decomposition of 2. The hydride peak of 2 in the 1H chart was shifted significantly to a lower field (15.1 ppm) as compared to that of [Pt(H)Au8(PPh3)8]+ (5.4 ppm)28,29 and other metal hydride complexes (
