Peeling the Core–Shell Au25 Nanocluster by Reverse Ligand-Exchange

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Peeling the Core-Shell Au25 Nanocluster by Reverse Ligand-Exchange Man-Bo Li, Shi-Kai Tian, Zhikun Wu, and Rongchao Jin Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04907 • Publication Date (Web): 01 Feb 2016 Downloaded from http://pubs.acs.org on February 4, 2016

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Peeling g the C Core-Shell Au25 Nanoc cluster by Rev verse LigandL Exchan nge Man-Bo Lii,† Shi-Kai Tian, T ‡ Zhikuun Wu*,† andd Rongchaoo Jin*,§ †

Key Laboratory of Materiaals Physics, Anhui A Key Labboratory of Naanomaterials and a Nanotechhnology, Instituute of Solid State S ( Hefei 230031, Chinna, ‡Departmennt of Chemisttry, University y of Science annd Physics, Chinnese Academyy of Sciences (CAS), Technology of o China, Hefeei, Anhui 2300026, China, §Department D oof Chemistry, Carnegie C Mellon University y, Pittsburgh, Pennsylvaniaa 15213, Uniteed States. ABSTRACT:: The developm ment of novel synthesis s meth hod is of great importance forr the nanoclustter research. Here, H we introduuce a reverse ligand-exchange method m to peel core-shell Au25(SC2H4Phh)18- (Au25C). A complete transformatioon from Au255C to SC2H4Ph)2+ (Auu11) and finallyy biicosahedraal Au25(PPh3)100(SC2H4Ph)5Cll22+ (Au25B) viaa Au13(PPh3)8(S SC2H4Ph)32+ (A Au13) Au11(PPh3)8(S intermediate is observed. Auu13, Au11 and Au A 25B can be reespectively obttained as majorr product by coontrolling the reaction r param meters. mportant impliccations to the tailoring of com mpositions, struuctures and prooperties of golld nanoclusterss. This work proovides some im

Owing to their t well-definned compositiions (structurees) and intriguing prooperties, gold nnanoclusters have drawn exttensive interests from m a number of fields such ass solid state phhysics, catalysis and biomedicine, b n only for fuundamental sciientific not 1 studies but alsso for applicattion purposes.1-9 In the reseaarch of gold nanoclussters, synthesiss is the key step, s which prrovides materials for the t subsequentt research on structures, s properties and applicatioons.2,4,6 Currenntly there are two well-recognized synthetic methhods for gold nanoclusters: n o is the direcct synone thesis startingg from gold saalt,10-21 the otheer is the transforma2 tion of gold nanoclusters under variouus conditions,22-38 in which the ligaand-exchange ddeveloped by Hutchison, Tsukuda, Jin, et al. is an a very effectiive method. Foor the latter method, m strong nucleo ophilic ligandss are generallly employed to exchange with thhe weak ones on o the surface of gold core.222-36 For instance, phossphine-passivaated gold nanooclusters were transformed to thioolated gold nannoclusters by ligand-exchange due to stronger Au u-SR interactioon compared with w Au-PR3 innteraction,22-24,26-29 The T reverse prrocess (i.e. thee strong nucleoophilic ligands were replaced r by thee weak ones) was w rarely repoorted.39 As indicated above, the devvelopment of novel synthessis methods is of greeat importance for the nanoclluster research. Here, we introduce a reverse ligaand-exchange method to taillor the compositions (structures) off gold nanocluusters. As an illlustraA 25(SC2H4Ph)18 tion, phenyletthanethiolated core-shell Au 1 (abbreviated as Au A 25C) was “ppeeled”by PP Ph3 (see Figuree 1) to Au13(PPh3)8(S SC2H4Ph)32+ (A Au13), which was w further etcched to Au11(PPh3)8(S SC2H4Ph)2+ (abbbreviated as Au11) and finally to biicosohedral Au25(PPh3)10(SC ( 2H4Ph)5Cl22+ (abbreviated as Of note, onne of the Au25B). products(i.e. SC2H4Ph)5Cl222+) and one off the transform mations Au25(PPh3)10(S (i.e. f from A Au11(PPh3)8(SC C2H4Ph)2+ to Au25(PPh3)10(S SC2H4Ph)5Cl222+) in this workk are similar too those reported by Tssukuda and cow workers.28,29

Figure 1. Illlustration of thhe peeling pro ocess of core--shell Au25. Carbon, hydrogen, suulfur and phossphorus atomss are A om mitted for clarrity.

Figure 2. UV/vis/NIR U speectral evolutioon for the reaaction beetween Au25C and a PPh3.

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Figure 3. UV/vis/NIR U an nd correspondin ng ESI-MS sp pectra of P1 (A A, B), P2 (C, D) D and P3 (E, F). F Inset: the ex xperimental an nd simulated isotop pe patterns of P1, P P2 and P3.. UV/vis/NIR sp U pectrum as thaat of stage 2 was w obtained after A 25C and PPh3 were reacted for 5 h (Figurre 3C); a prolo Au onged reeaction for 24 hours in DCM M yielded a product p (denoteed as P3) with similarr absorption ass that of stage 3 (Figure 3E). The P2 and P3 weere identified bby electrospraay ionization mass pectrometry (E ESI-MS) to bee Au11(PPh3)8(S SC2H4Ph)2+ (Figure sp 2+ 44,45 3D D) and Au u25(PPh3)10(SC C2H4Ph)5Cl2 (Figure ( 3F),29,446 resp pectively. Thee P1 cluster waas very unstab ble and care had to bee taken in ESII-MS analysis,, shown in Fig gure 3B. Unlik ke the afforementioned d two cases, seerious fragmen ntation is obseerved inn the mass speectrum of P1. The distinct peak with the maxim m mum m/z assigned value at 2534.7 iss to A 13(PPh3)8(SC Au C2H4Ph)32+ (Figgure 3B),37,38,477 which could d correespond to the molecular ionn since we do not find otherr distinct peaks at hiigher m/z valuues under all th he investigated d conher two obviouus fragments which w are assiigned diitions. The oth too Au13(PPh3)7(SC2H4Ph)32+ aand Au13(PPh3)6(SC2H4Ph)32++ proviide strong evidence for the assignment off the moleculaar ion peeak (Figure 3B B). Beside the ab bove mentioneed major produ ucts, a colorlesss byprroduct of conssiderable amouunt can also be b observed du uring thhe etching by PPh P 3, and the aamount of the by-product b inccreasess with the exteension of reacttion time to so ome extent, ind dicatinng that the by-p product is rathher stable and can c be continuo ously yiielded in the reeaction processs. Its composition is identifiied to bee Au(PPh3)2+ by b ESI-MS (Figure 4). Thuss, the tandem transt foormation in DCM D can be suummarized as follows: Firstt, the A 12 shell of Au25C is peeleed by PPh3, forming Au133 and Au + A Au(PPh age 1); then, thhe Au13 core is further etcheed by 3)2 (sta PPh3 to form Au A 11 and Au(P PPh3)2+ (stage 2); finally, Au u11 is A 25B (stage 3)).29 The wholee tandem proceess is trransferred to Au prresented in Figure 5. It is believed b that Au-thiol A bondiing is sttronger than Au-phosphine A bonding, thus such a ligand d exch hange is reverrse to the com mmon ligand exchange-a e weeakly prrotecting ligan nd is exchangged by a stron ngly protectin ng li-

Au25C is thee most extensiively studied gold g nanoclustter owing to its eassy preparation,, well-defined d structure and d intriguing propertiies,3,40-43 thus it is chosen ass the starting nanocn luster in the present p work. PPh3 is a weaker nucleoph hilic ligand compareed with pheny ylethanethiol, and a is chosen as the attacking ligan nd. In a typicaal transformatio on experiment,, Au25C was dissolved d in dichloromeethane (DCM)), and 20 equiv valents of PPh3 (per mole m of Au25C) were added to t the solution n dropwise. The reaaction mixturee was stirred at a room tempeerature and monitored by UV/vis//NIR spectrosccopy. Based on o the ution, three maain stages were observed an nd each spectral evolu stage was stab ble for some tiime (Figure 2)). First, the characteristic peaks off Au25C at 400 0 and 680 nm disappeared, leaving l only one charracteristic peak k at 445 nm. Then, T the peak at 445 nm blue-shifteed to 415 nm slowly, s with a new n absorption n band at 380 nm ap ppeared in the UV/vis/NIR spectra s in the meantime. Thirdly,, two new peaaks at 440 an nd 670 nm weere observed with th he peaks at 4155 and 380 nm unchanged. u Fro om the UV/vis/NIR spectral evolutiion, one can co onclude that Au A 1144,45 dral Au25B29,46 are formed in n stage 2 and sttage 3, and biicosahed respectively. While the thiolate-for-ph t hosphine exchaange process is i normal, the reverrse process (i.ee. phosphine-fo or-thiolate exch hange) is unexpected. To gain insig ght, we tried to isolate the clusters c produced at diifferent stages.. We note that it was howeveer very challenging to o isolate the cllusters in stagees 1 and 2 duee to the fact that the products p are labbile in DCM. Fortunately, by b controlling the reaaction time and d selecting thee reaction solveent, we were able to selectively s obttain three diffeerent nanoclussters as major productts. In toluene, a product (den noted as P1) exhibite ing similar ab bsorption of stage s 1 (see above) a was issolated when Au25C and a PPh3 weree reacted for 1 h (Figure 3A). 3 In CH3CN, the main m product (denoted ( as P2 2) showing a similar s 2

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gand,22-36 and herein it may be named as “reverse ligandexchange”. Such a process can occur, indicating that it is thermodynamically and kinetically favorable in our conditions. Due to the simultaneous transformation of nanocluster’ structure during the process of ligand exchange as well as the reaction medium influence, one cannot simply predict the ligand exchange direction by comparing the bonding energy between Au-thiol and Au-phosphine bonding. At least, two factors influence the reaction direction. One is the solvent as shown above (see Figure 3) and by our additional experiments (see figure S1-3), and the other is the relative abundance of different ligands (PPh3 vs PhCH2CH2SH) in the reaction system: When the PPh3/Au25C molar ratio is less than 6/1, i.e., the PPh3/PhCH2CH2SH molar ratio is less than 6/18, the transformation from Au25C to Au13, Au11 or Au25B is not observed; only further increasing the PPh3/PhCH2CH2SH molar ratio can lead to the notable transformation demonstrated by the UV/vis/NIR in our control experiments (see Figure S1-3). Of note, too excessive PPh3 (e.g. 100 equivalents per mole of Au25C) results in decomposition of the formed Au13 (in CH3CN) or Au25B (in DCM).

MS. It is expected that this work may have some important implications to the tailoring of compositions, structures and properties of gold nanoclusters by the reverse ligand-exchange strategy.

ASSOCIATED CONTENT Supporting Information. Detailed information about the isolation and purification of Au13, Au11, Au25B and Au(PPh3)2+; UV/Vis/NIR spectral change of Au25C after the addition of various amounts of PPh3 in different solvent. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected], [email protected].

ACKNOWLEDGMENT M.L. owes thanks to Natural Science Foundation of China (21501182), Anhui Provincial Natural Science Foundation (1608085QB31); Z.W. would like to thank National Basic Research Program of China (Grant No. 2013CB934302), Natural Science Foundation of China (No. 21222301, 21171170), Ministry of Human Resources and Social Security of China, the Innovative Program of Development Foundation of Hefei Center for Physical Science and Technology (2014FXCX002), Hefei Science Center, CAS (user of potential: 2015HSCUP003), the CAS/SAFEA International Partnership Program for Creative Research Teams and the Hundred Talents Program of the Chinese Academy of Sciences for financial support. R.J. acknowledges financial support from the U.S. Department of Energy-Office of Basic Energy Sciences, Grant DE-FG02-12ER16354, and Natural Science Foundation of China (Overseas, Hong Kong & Macao Scholars Collaborated Researching Fund, No. 21528303).

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Figure 4. ESI-MS spectrum of Au(PPh3)2

Figure 5. The tandem etching process.

In summary, a reverse ligand-exchange method is introduced for the first time to “peel” the core-shell Au25C nanocluster. A complete transformation from Au25C to Au11 and finally Au25B via Au13 has been observed, and the process consists of three main steps: First, the Au12 shell of Au25C is peeled by PPh3, leading to the production of Au13 and Au(PPh3)2+; second, the remained Au13 core is continually etched by PPh3 to form Au11 and Au(PPh3)2+; finally, Au11 is transformed to Au25B. Au13, Au11 and Au25B can be respectively obtained as major product by controlling the reaction time and selecting the reaction medium, and these primary products, together with the colorless by-product, are clearly identified by ESI3

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