Controlled Synthesis of Diphosphine-Protected Gold Cluster Cations Using Magnetron Sputtering Method

We demonstrated, for the first time, atomically precise synthesis of gold cluster cations by magnetron sputtering of a gold target onto a polyethylene glycol (PEG) solution of 1,3-bis(diphenylphosphino)propane (Ph2PCH2CH2CH2PPh2, dppp). UV-vis absorption spectroscopy and electrospray ionization mass spectrometry revealed the formation of cationic species, such as [Au(dppp)n]+ (n = 1, 2), [Au2(dppp)n]2+ (n = 3, 4), [Au6(dppp)n]2+ (n = 3, 4), and [Au11(dppp)5]3+. The formation of [Au(dppp)2]+ was ascribed to ionization of Au(dppp)2 by the reaction with PEG, based on its low ionization energy, theoretically predicted, mass spectrometric detection of deprotonated anions of PEG. We proposed that [Au(dppp)2]+ cations thus formed are involved as key components in the formation of the cluster cations.


Introduction
Ligand-protected gold clusters with atomically-defined sizes constitute a distinct class of nanomaterials, showing size-specific optical and catalytic properties [1][2][3][4][5][6]. Their conventional synthetic method is based on the chemical reduction of Au(I)-ligand complexes. Key processes involved in the cluster production include the competition between nucleation/cluster growth and passivation with the ligands. However, the actual processes are much more complicated, such that the choice of precursor complexes is a critical factor that determines the products. For example, chemical reduction of Au(PPh 3 )L (L = Cl, NO 3 , SR) yielded [Au 11 (PPh 3 ) 8 Cl 2 ] + [7], [Au 9 (PPh 3 ) 8 ] 3+ [8], and Au 13 (PPh 3 ) 8 (SR) 3 [9], respectively. As such, the atomically-controlled, targeted synthesis of ligand-protected Au clusters remains challenging.
A simple approach for targeted synthesis is to control the kinetics of the nucleation/aggregation of naked Au atoms in the presence of the ligands. One efficient way to supply naked Au atoms is through the magnetron sputtering method [10,11]. Since the first report of the production of silver and iron nanoparticles in silicon oil by Wagner [12], various metal nanoparticles have been efficiently prepared by magnetron sputtering onto liquids containing surfactants and ligands [13,14] or ionic liquids [15][16][17][18][19]. Au clusters smaller than 2 nm have been successfully obtained utilizing thiols as protective ligands [20].

Formation and Characterization of Au Clusters Synthesized by Magnetron Sputtering
The dppp-protected Au clusters were prepared using an auto fine coater designed to prepare specimens for scanning electron microscopy by coating them with the conductive layer (see the materials and methods part for more detail). In short, Au atoms and clusters generated by the magnetron sputtering were deposited onto the PEG solutions of dppp with a concentration of x mM. The sputtering current and sputtering time of the auto fine coater were set to 10 mA and 600 s, respectively, so that the total amount of Au atoms deposited was comparable for all the samples. The dppp-protected Au clusters produced were washed with the aqueous solution of NaNO 3 to collect them by precipitation as nitrate salts. The Au clusters thus obtained are hereafter referred to as Au-dppp-x.
In order to obtain a whole picture of the cluster growth process after the magnetron sputtering, we quantitatively compared the UV-vis absorption spectra of another set of the Au-dppp-x samples. Figure 2a,b show photo images and the corresponding absorption spectra of the PEG solutions after magnetron sputtering as a function of x, respectively. The color of the PEG solution was pale blue at x = 21 due to the absorption peak at 584 nm originated from [Au 6 (dppp) 4 ] 2+ [25]. The blue color became more intense with the reduction of x to 15, in accordance with increase of the peak intensity at 584 nm. This behavior indicated that the production of [Au 6 (dppp) 3,4 ] 2+ was promoted by the reduction of dppp concentration. At x = 12, the brownish ring was observed in the photo image and another peak at 420 nm emerged. This trend grew more pronounced at x = 9. This result indicated that the growth to [Au 11 (dppp) 5 ] 3+ was promoted by further reduction of x. The ring patterns implied that the Au 6 and Au 11 clusters were distributed nonhomogeneously in the unstirred, viscous PEG solutions. At x = 6 and 4, the PEG solutions became brownish, but did not exhibit an absorption peak at~520 nm due to the localized surface plasmon resonance. This result indicated the formation of Au clusters smaller than 2 nm in Audppp-6 and Au-dppp-4, which was confirmed by transmission electron microscopy (TEM) (Figure 2c). The above results implied that the yield of specific clusters could be maximized by optimizing the dppp concentration.

Mechanism of the Formation of Au Cluster Cations by Magnetron Sputtering
In our magnetron sputtering source, major species deposited onto the PEG solution were neutral Au atoms rather than Au + ions, since the Au target placed on the electrode was biased to negative voltage with respect to the stage for the PEG solution. Thus, the initial step was the capture of Au atoms by the dppp ligands dissolved in the PEG. Nevertheless, we detected the cationic species [Au(dppp) 2 ] + as the major ionic species (Figure 1a), indicating that Au-dppp complexes were spontaneously ionized in the PEG. Similar phenomena were observed by introducing the vapor of Au(0) atoms into the solution of phosphine ligands [27,28]. For example, [Au(PPh 3 ) 2 ] + was obtained by evaporating Au atoms into the solution of PPh 3 and KSCN [27]. To gain insight into the driving force of the formation of cationic clusters, we conducted density functional theory (DFT) calculations on the ionization energy of Au(dppp) 2 . The geometric structures of [Au(dppp) 2 ] + and Au(dppp) 2 were optimized using BP86 functional, starting from a crystal structure of [Au(dppp) 2 ] + in which two dppp ligands coordinated to an Au atom tetrahedrally (Figure 3a [29]). The DFT optimized structures of [Au(dppp) 2 ] + and Au(dppp) 2 are shown in Figure 3b,c. The optimized structure of [Au(dppp) 2 ] + qualitatively reproduced the crystal structure [29]; the average Au-P bond length calculated was longer than the experimental length de-termined by~5%. The adiabatic ionization energy of Au(dppp) 2 calculated is listed in Table 1 together with those of Au and Cs atoms. The ionization energy of Au(dppp) 2 was significantly smaller than that of Au and grew smaller than that of Cs. The significant reduction of the ionization energy of Au(dppp) 2 was ascribed to much stronger bonding energies of dppp to Au + than to Au.  It is known that alkali metal atoms like Cs are ionized by reactions with hydroxy groups of organic compounds. We hypothesized that [Au(dppp) 2 ] with lower ionization energy is ionized by the reaction with PEG according to the following equation: Formally, Au(dppp) 2 is ionized by the abstraction of an electron by the proton formed by the deprotonation of PEG. This hypothesis is supported by the detection of deprotonated anions of PEG in the negative-mode ESI-MS of the PEG solution after magnetron sputtering ( Figure 4). Namely, the mixing of sputtered PEG solution with aqueous solution of NaNO 3 resulted in an anion exchange process from [H(OCH 2 CH 2 ) n O] − to NO 3 − , leading to the precipitation of the Au-dppp clusters.

Preparation of Au Clusters by Sputtered Deposition
The magnetron sputtering of the gold target was carried out on a JEC-3000FC auto fine coater (JEOL Ltd., Tokyo, Japan). An argon gas cylinder was connected to the auto fine coater with a stagnation pressure of 0.04 MPa. First, a PEG solution of dppp (5 mL) with a concentration of 4-21 mM was poured into a Petri dish with a diameter of 60 or 55 mm for quantified spectral measurement or mass spectral analysis, respectively. The distance between the Au target and the solution surface in the Petri dish was set at 30 mm. Then, the sputtering chamber was evacuated for at least 30 min by pumping with a rotary pump (100 L·h -1 ). The chamber was flushed by Ar gas at least 3 times to sweep out the remaining air. Finally, the sputtering process was initiated within a small flow of Ar reaching 7 Pa. The sputtering current was set to 10 mA. After 300 s of sputtering, the PEG solution was taken out of the chamber and stirred homogeneously. Then, the PEG solution was sputtered again using the same procedure; the total sputtering time was 600 s.

Purification of Au Cluster Products
The magnetron-sputtered PEG solution was mixed with a saturated NaNO 3 aqueous solution to reach a total volume of 50 mL. The liquid mixture was centrifuged at 3500 rpm for 1 h. After removal of the supernatant, the precipitate was then washed by deionized water, hexane, toluene, and diethyl ether successively to remove NaNO 3 and excessive dppp.

Characterization
Ultraviolet-visible (UV-vis) absorption spectra of the products in acetonitrile were recorded on a V-670 spectrometer (JASCO Corporation, Tokyo, Japan) with a quartz cell. Electrospray ionization (ESI) mass spectra of the products were obtained on a JMS-T100LC time-of-flight mass spectrometer (JEOL Ltd.). The specimens were prepared by dissolving the samples in acetonitrile with the concentration of 0.1 mg·mL -1 . ESI mass spectra of PEG were obtained by dissolving pure PEG and magnetron sputtered PEG solution in methanol and the ratio of PEG and methanol was 1:9 (v/v).
Transmission electron microscopy (TEM) images of the products were acquired on a JEM-ARM200F NEOARM Atomic Resolution Analytical Electron Microscope (JEOL Ltd., Tokyo, Japan). The specimens were prepared by drop-casting methanol solutions of the samples onto a thin carbon-coated copper grid (SHR-C075, Okenshoji, Tokyo, Japan).

Theoretical Calculation
All density functional theory (DFT) calculations were conducted using the Turbomole software package [31]. The initial structure of the tetrahedral Au(dppp) 2 was constructed from the reported crystal data [29]. Structural optimization of Au(dppp) 2 , [Au(dppp) 2 ] + , dppp and Au were conducted with the BP86 functional and the def-SV(P) basis sets with def-ecp effective core potentials for Au. The vibrational frequencies for the optimized structures of Au(dppp) 2 and [Au(dppp) 2 ] + were calculated to confirm that those structures were located at local minima of the potential energy surfaces.

Conclusions
In this work, [Au 6 (dppp) 4 ] 2+ and [Au 11 (dppp) 5 ] 3+ cluster cations were successfully synthesized by supplying magnetron-sputtered Au atoms into a PEG solution of dppp. The relative population of [Au 6 (dppp) 4 ] 2+ and [Au 11 (dppp) 5 ] 3+ could be tuned by controlling the kinetic balance between cluster growth and passivation by dppp. We proposed that Au(dppp) 2 was ionized by the reaction with PEG based on its small ionization energy theoretically predicted and mass spectrometric detection of deprotonated anions of PEG. The results suggested that the [Au(dppp) 2 ] + cations thus formed were key building components of the cluster cations observed. This work provided a simple synthetic route of ligand-protected Au clusters alternative to the conventional chemical synthesis.
Supplementary Materials: The following is available online, Figure S1: Expanded views of positivemode ESI mass spectra of (a) Au-dppp-20 and (b) Au-dppp-5. The theoretical isotope pattern of each species is shown as black bars.