Engineering ligand chemistry on Au25 nanoclusters: from unique ligand addition to precisely controllable ligand exchange

Au25 nanoclusters (NCs) protected by 18 thiol-ligands (Au25SR18, SR is a thiolate ligand) are the prototype of atomically precise thiolate-protected gold NCs. Studies concerning the alteration of the number of surface ligands for a given Au25SR18 NC are scarce. Herein we report the conversion of hydrophobic Au25PET18 (PET = 2-phenylethylthiolate) NCs to Au25SR19 [Au25PET18(metal complex)1] induced by ligand exchange reactions (LERs) with thiolated terpyridine-metal complexes (metal complex, metal = Ru, Fe, Co, Ni) under mild conditions (room temperature and low amounts of incoming ligands). Interestingly, we found that the ligand addition reaction on Au25PET18 NCs is metal dependent. Ru and Co complexes preferentially lead to the formation of Au25SR19 whereas Fe and Ni complexes favor ligand exchange reactions. High-resolution electrospray ionization mass spectrometry (HRESI-MS) was used to determine the molecular formula of Au25SR19 NCs. The photophysical properties of Au25PET18(Ru complex)1 are distinctly different from Au25PET18. The absorption spectrum is drastically changed upon addition of the extra ligand and the photoluminescence quantum yield of Au25PET18(Ru complex)1 is 14 times and 3 times higher than that of pristine Au25PET18 and Au25PET17(Ru complex)1, respectively. Interestingly, only one surface ligand (PET) could be substituted by the metal complex when neutral Au25PET18 was used for ligand exchange whereas two ligands could be exchanged when starting with negatively charged Au25PET18. This charge dependence provides a strategy to precisely control the number of exchanged ligands at the surface of NCs.


Synthesis of 4'-[4-(Methylthio
)phenyl]-2,2';6',2''-terpyridine (tpy-SMe) [1] In a glovebox, a flame-dried two-necked flask (500mL) was charged with 5.6g potassium tert-butoxide (t-BuOK) and 75mL anhydrous THF.After vigorously stirring for half hour, 3.55mL 2-acetylpyridine was added dropwise by syringe, and the color turned yellow.Then 2mL 4-(methylthio)benzaldehyde was injected to the flask slowly and the color changed to ruby red during stirring.The mixture was continued reacting for about 60h.Afterwards, 38g ammonium acetate suspended in 270mL ethanol was added, followed by refluxing at inert atmosphere for 5h and the color changed to yellow.Subsequently, the reaction mixture was cooling down and the solvent was concentrated to approximately 150mL, followed by filtering to get a yellow solid.The solid was washed by warm ethanol, filtered again and recrystallized in DCM/Ethanol mixture.The filtration from the reaction was repeated according to the above procedure to obtain more product. 1H NMR and 13 C NMR Spectra of tpy-SMe were shown in Supplementary Figure S6. 1

Preparation of 4'-[4-(mercapto)phenyl]-2,2';6',2''-terpyridine (tpy-SH)
500mg tpy-SMe, 1.2g t-BuOK and 10mL anhydrous DMF were added to a completely dried two-necked flask (25mL).After stirring for several minutes, 0.8mL t-BuSH was added to the flask.Then freezepump-thaw was repeated three times to remove the gas and the suspension was refluxed for 12h at 130 ℃ and 3h at 150℃.After cooling in an ice-bath, the red solution was mixed with 30mL MilliQ water followed by filtration to get the filter liquor.Subsequently, 75mL saturated ammonium chloride aqueous solution was added and yellow precipitation appeared.The precipitation was collected by filtering and washed several times with water and ethanol. 1 H NMR and 13 C NMR spectra of tpy-SH is shown in Supplementary Figure S7. 1 [2] 20 mg tpy-SH was added to the suspension of tpy(M)Cl 2 or tpy(M)Cl 3 (1.0 equiv) in methanol (12mL).

Synthesis of metal complex
Three drops of N-ethylmorpholine were added (for Ru complex) to the mixture, followed by flowing Ar gas for 1h to keep inert atmosphere.Then refluxing was applied overnight.After cooling down, the mixture was filtered through filter paper.After that excess amount of ammonium hexafluorophosphate was added to precipitate all Ru complexes.The solid was collect by filtration and washed with water.
Metal complex was purified by silica gel column chromatography (eluents: acetonitrile/0.2Maqueous potassium nitrate solution 4:1).The second fraction was collected and the solvent was evaporated.Then excess aqueous NH 4 PF 6 solution was added to exchange the counterions.Finally, the product was obtained by filtration and washed with excess water, ethanol and diethyl ether. 1 H NMR spectra of Ru complex is shown in Supplementary Figure S2. 1  Luminescence spectra and quantum yields were measured with a FluoroLog 3-1iHR (PMT R5509-73) in combination with an integrating sphere (G8, GMP).The luminescence spectra were corrected for the instrument sensitivity according to the fluorimeter manufacturer.The correction function of the integrating sphere was obtained comparing the fluorimeter lamp profile using the integrating sphere and a Spectralon reflector as reference.Contributions of the inherent emission from the integrating sphere were subtracted (scaled for the difference in scattered light between empty and loaded sample).

Ligand exchange reactions
Purified neutral Au 25 PET 18 in DCM was mixed with metal complex (in acetonitrile) at different Au NCs/complex molar ratios (final nanocluster concentration 1mg/mL) and stirred in glovebox.Ligand exchange reactions took place at room temperature and HRESI-MS of the samples after reaction for different time was performed without purification (unless otherwise noted).Time-course UV-Vis absorption spectra after LER with different metal complexes for different time at nanocluster/complex ratio 1:2 was shown in Supplementary Figure S8.
Supplementary Figure S1.(a) UV-Vis absorption spectrum of Au 25 PET 18 in DCM and (b) HR-ESI MS of Au 25 PET 18 and comparison of experimental and simulated isotopic patterns of Au 25 PET 18 .