Synthesis and Properties Evolution of a Family of Tiara-like Phenylethanethiolated Palladium Nanoclusters

Tiara-like thiolated group 10 transition metal (Ni, Pd, Pt) nanoclusters have attracted extensive interest due to their fundamental scientific significance and potential application in a number of fields. However, the properties (e.g. the absorption) evolution with the ring size’s increase was not investigated so far to our best knowledge, due to the challenge of obtaining a series of nanocluster analogues. Herein, we successfully synthesized, isolated and identified a family of [Pd(SC2H4Ph)2]n nanoclusters (totally 17 novel clusters, n = 4–20). Their structures were determined to be tiara-like by single crystal X-ray crystallography together with theoretical calculation; their formation mechanism was proposed to be a substitution—polycondensation—ring-closure process based on experimental observations. All of these clusters are rather robust (anti-reductive and anti-oxidative) owing to their tiara-like structures with large HOMO-LUMO gaps. Finally, the optical and electrochemical evolution with the increase of ring size was investigated, and it is found that both optical and electrochemical gaps have a “turning point” at a size corresponding to n = 8 for [Pd(SR)2]n nanoclusters.


Results and Discussion
Synthesis and separation. The homologous series of phenylethanethiolated Pd clusters were synthesized based on a previous method 14 . In a typical synthesis, a deoxygenized acetonitrile solution containing Pd(NO 3 ) 2 •2H 2 O was mixed with 2 equivalents of 2-phenylethanethiol and 2 equivalents of triethylamine (Note: argon atmosphere is employed to avoid any possible interference from air, but it is not essential for this reaction). After continuously stirred for 5 hrs, the reaction mixture was extracted with CH 2 Cl 2 . Then, we thoroughly isolated the components in the crude product via preparative thin layer chromatography (PTLC), which was common in the purification of organic compounds but is rarely used for the isolation of tiara-like metal clusters. Surprisingly and interestingly, the extracted product in our protocol contains at least 17 components as shown by PTLC (see Fig. 1). Of note, at the top of the PTLC plate, there is another band almost indiscernible (marked with an oval, Fig. 1a).
The full separation of the continuous series of [Pd(SC 2 H 4 Ph) 2 ] n (4 ≤ n ≤ 20) is challenging, and the identification of all the 17 compounds is even more challenging. No distinct signal from M/Z of 1000 to 10000 in the MALDI-TOF-MS spectrum was found in either positive or negative ionization mode even if an excellent matrix-trans-2-[3-(4-tert-Butylphenyl)-2-methyl-2-propenyldidene] malononitrile (DCTB) was used 46,47 . Other matrixes including sinapinic acid (SA), 2,5-dihydroxybenzoic acid (DHB), alpha-cyano-4-hydroxycinnamic acid (CHCA), and cesium acetate do not work either. Fortunately, the addition of NaOOCCF 3 assisted in the ionization of the clusters as shown in Fig. 1b and S1 probably due to the inclusion of Na + in the cave of double-crown structure 17 . The major peaks are readily assigned to [[Pd(SC 2 H 4 Ph) 2 ] n + Na] + (4 ≤ n ≤ 20) based on the M/Z value in tandem with the isotopic patterns in the mass spectra (380 × n + 23; 380 is the M/Z value of Pd(SC 2 H 4 Ph) 2 unit, and 23 is the M/Z value of the Na cation). Only one size of thiolated Pd cluster is found in the mass spectrum of every band in PTLC plate (see Fig. 1b and S1), indicating that all the seventeen compounds are well isolated and purified by PTLC.
Of note, Pd 5 and Pd 6 are the two main components (see Fig. 1a). These were obtained at 5%, and 4% yield, respectively. The yield of Pd n is less than that of Pd n+1 (n are odd numbers between 7 and 19), and the yields of Pd n generally decrease as values of n increase (n > 4). Thus, it is difficult to obtain Pd n (n > 10) considering their yields are very low (less than 1%). Furthermore, the products distribution is found to be immune to changes in experimental parameters (for examples, changing the feeding ratio of Pd(NO 3 ) 2 :PhC 2 H 4 SH from 1:2 to 1:6; turning the solvent acetonitrile to n-propanol, replacing the base triethylamine with NaBH 4 , withdrawing the argon atomosphere, etc.). These products can be dried and fully re-dispersed in dichloromethane, chloroform, dichloroethane, toluene, ethylacetate and DMF easily, but they can not dissolve in alcohol, acetonitrile, n-hexane and petroleum ether. All of the Pd n (4 ≤ n ≤ 20) are novel-even the other cluster sizes except [Pd(SR) 2 ] 6 and [Pd(SR) 2 ] 8 have not been reported, to the best of our knowledge. Although [Pd(SR) 2 ] 3 43 and [Ni(SR) 2 ] 3 48 have been theoretically predicted, the practical existence of these species is still problematic because they are less obviously stable than the larger ones 43,48 , and the smallest cluster obtained in this work is Pd 4 . [Ni(μ -StBu)(μ -etet)] 12 was the biggest tiara-like [M(SR) 2 ] n nanocluster synthesized and identified previously, herein we shatter the record by providing larger Pd 20 .

Synthesis mechanism.
An intriguing issue pertains to the mechanism of formation in the continuous series of thiolated Pd clusters. There are two existing mechanisms for the formation of tiara-like [M(SR) 2 ] n : one is termed as "template mechanical assistance" 14,16 , and the other is "addition polymerization of monomer" 45 . However, the two existing mechanisms are not well applied to this case. The "template mechanical assistance" is unsuitable because it is difficult to find the template of all 17 compounds and explain why a series of Pd n (4 ≤ n ≤ 20) is formed from a single template. The "addition polymerization of monomer" is also inconsistent with our finding that NO 3 − and H 2 O exist in the initial Pd-SR complex. Alternatively, we propose a formation mechanism named "substitution-polycondensation-ring-closure".
The entire process could be divided into three steps: first, palladium nitrate dihydrate reacts with 2 equivalents of thiol and immediately yields a yellow precipitate (see Fig. 3 and Eq. 1). The binding energy of Pd3d in the precipitate is 337.6 eV (Fig. S4). This indicates that Pd 2+ is not reduced or oxidized and remains in its dsp 2 orbital hybridization plane after reaction with phenylethanethiol. The peak at 1380 cm −1 in the IR spectrum (Fig. S5) indicates the existence of NO 3 − in the precipitation. The absorption at ~3400 cm −1 in the IR spectrum suggests that one hydrated water may be still there. Thus, it is possible that two phenylethanethiols replace one NO 3 − and one hydrated water molecule. As a result, the formula of the precipitate is PdH(SC 2 H 4 Ph) 2 (NO 3 )(H 2 O)-the acidity of the precipitate indicates phenylethanethiol may not be completely deprotonated ( Fig. 3 and Eq. 1). This is supported by elemental analysis of sulfur (calculated: 13.95%; experimental: 13.80%).
Second, linear condensation of monomer (PdH(SC 2 H 4 Ph) 2 (NO 3 )(H 2 O) (the precipitate of the first step) occurs after base triggering (herein triethylamine is employed as the base). The IR spectra suggest the absence of NO 3 − and H 2 O in the Pd clusters. Thus, the NO 3 − and the proton may be removed in the second step with assistance of triethylamine. Importantly, the fact that the reaction does not proceed smoothly in acidic or neutral media suggests removal of a proton and NO 3 − in this step by the attack of the base (triethylamine). The linear condensation of two monomers leads to the forming of a dimer (Eq. 2), and the condensation of the dimer with a monomer leads to the forming of trimer (Eq. 3). The condensation of two dimmers (Eq. 4) or one trimer with one monomer leads to tetramer, and so on. This process is comparable to the polycondensation of polymers.
Third, the resulting chains close into rings head to tail by detracting the hydrated water molecules (Eq. 5 and Fig. 3). The resulting double-crown structures do not contain hydrated water any longer, which is confirmed by the IR spectra (see Fig. S5). The composition was further supported by elemental analysis of sulfur (calculated: 16.84%; experimental: 16.91%). For the closure of odd Pd clusters, the modification of the configuration was needed before the ring-closure to avoid spatial hindrance. This is why the yields of odd Pd n cluster are lower than those of the adjacent even Pd n+1 clusters.
In brief, the formation of Pd n clusters is a substitution-polycondensation-ring-closure process based on the data. This is consistent with yield of every component in the product. Evolution of optical and electrochemical properties. To systematically compare the property evolutions of the full series of Pd n clusters is meaningful because they have similar tiara-like structures, the same composition, and the same synthesis history. First, the UV/Vis absorption spectra were recorded for comparison. The optical evolution from Pd 4 to Pd 20 is interesting. The absorption spectra are similar, but the shifts in the absorption peaks are dramatic for small Pd n (n is less than 9; see Fig. 4a,b). However, the shifts become static and almost linear when the n in Pd n is larger than 8. Generally speaking, the  Table S2). This is comparable to that of the previous tiara-like Ni 6 (SC 2 H 4 Ph) 12 24 and indicates that the Pd clusters is also anti-reductive, which is confirmed by the reduction experiments (see Fig. S8). Surprisingly, the first oxidation potential of all investigated phenylethanethiolated Pd clusters are almost consistent, and they are remarkably higher than that of Ni 6 (SC 2 H 4 Ph) 12 24 (1.26 vs 0.65 V), indicating that the phenylethanethiolated Pd clusters are very stable to oxidation, which is indeed consistent with the H 2 O 2 oxidation experiments (see Fig. S9). The gaps between the first reduction potentials and the first oxidation potentials of phenylethanethiolated Pd clusters are summarized in Fig. 4d, which is also in agreement with the optical energy gap after considering the charge energy (0.29 V) 49 . The large energy gaps indicate that these clusters are rather robust, which was confirmed by additional experiments: Pd 6 is stable in ambient environment for over 90 days and at 80 °C for over 24 hrs demonstrated by UV/Vis absorption spectra (see Fig. S10,11). A thermogravimetric analysis (TGA) of Pd 6 is shown in Fig. S12, which indicates that Pd 6 is stable until ~200 °C and discomposes completely at ~320 °C with a total weight loss of 62.9 wt% in the temperature range from 50 to 500 °C. However, there is some stability discrepancy at 80 °C between variously sized Pd clusters and it is shown that the stability decreases when the Pd clusters is larger than Pd 8 , see Fig. S11 for a comparison. Previous studies have shown that the energy gaps between the highest occupied molecular orbit and the lowest unoccupied molecular orbit (HOMO-LUMO gap) of some nanoscale semiconductors decrease with size increases 50 . However, in our case, the HOMO-LUMO gaps enlarge as the value of n in Pd n increases from 4 to 8. When the value of n is larger than 8, the gaps slightly decrease and keep unchanged with the further increase of the n value, thus the "turning point" of HOMO-LUMO gaps and the largest gap lie at a size corresponding to n = 8 for Pd n clusters.
In summary, we prepared, isolated and identified a family of novel[Pd(SC 2 H 4 Ph) 2 ] n clusters (totally 17 clusters, n = 4 ~ 20) with tiara-like structures, and propose their formation mechanism to be a substitution-polycondensation-ring-closure process.  20 were investigated, which indicate that the HOMO-LUMO gap "turning point" lie at a size corresponding to n = 8 for [Pd(SC 2 H 4 Ph) 2 ] n clusters: less than that size, the gap is size-dependent and increases with the size's increase; whereas larger than the size, the gap are almost not influenced by size any more.

Method
Synthetic protocols. The reactions are conducted at room temperature under argon atmosphere.
Briefly, Pd(NO 3 ) 2 ·2H 2 O (100 mg, 0.38 mmol) were dissolved in 6 ml acetonitrile in a 25 mL flask with vigorous stirring followed by PhC 2 H 4 SH (120 μ L, 0.9 mmol) and triethylamine (120 μ L, 0.87 mmol). After 5 hrs of stirring, yellow precipitates were collected, washed thoroughly by excess acetonitrile and methanol, and dried under reduced pressure. Mixed tiara-like clusters were extracted with CH 2 Cl 2 and then separated and purified by PTLC (CH 2 Cl 2 : petroleum ether, 1/3 v:v). Needle-like [Pd(SC 2 H 4 Ph) 2 ] 6 single crystals were crystallized from the mixture of toluene and methanol at room temperature after 2 days.
Characterization. The UV/Vis absorption spectra of Pd clusters (dissolved in CH 2 Cl 2 ) were recorded in standard quartz cuvettes on a UV-2550 spectrophotometer (Shimadzu, Japan) at room temperature. Fourier transform infrared (FIIR) spectra were acquired on a Nicolet 8700 (Nicolet, America) spectrometer. MALDI-TOF-MS analyses were performed on an autoflex Speed TOF/TOF mass spectrometer (Bruker, Germany). Trans-2-[3-(4-tert-Butylphenyl)-2-methyl-2-propenyldidene] malononitrile (DCTB) was used as the matrix, and NaOOCCF 3 was added to assist the ionization of clusters. X-ray Photoelectron Spectroscopy (XPS) measurements were conducted on an ESCALAB 250Xi XPS spectrometer (Thermo Scientific, America), using a monochromatized Al Kα source and equipped with an Ar+ ion sputtering gun. All binding energies were calibrated using the C (1 s) carbon peak (284.8 eV). Elemental analyses were performed on vario EL III Elementar analyzer (S mode) (Elementar, Germany). Thermal gravimetric analysis (TGA) (~3 mg sample used) was conducted in a N 2 atmosphere (flow rate ~ 50 mL/min) on a TG/DTA 6300 analyzer (Seiko Instruments, Inc), and the heating rate was 10 °C/min. The single crystal diffraction data of Pd 6 (SC 2 H 4 Ph) 12 was collected on a Bruker APEXDUO X-ray Diffractometer (Bruker, Germany).

Electrochemistry.
A conventional three-electrode system was used for these experiments. A Pt disk electrode (1 mm diameter) was the working electrode (WE). Before use, the WE was polished on emery paper of decreasing grades followed by Al 2 O 3 powders with sizes down to 0.05 μ m. This was cleaned electrochemically with potential-cycling in 0.5 M H 2 SO 4 solution, and the electrode was then rinsed thoroughly with ultrapure water (18.2 MΩ cm). An SCE (with saturated KCl solution) electrode and carbon rods served as the reference (RE) and counter electrode (CE), respectively. The electrode potentials were controlled with a potentiostat (Zahner, Germen). The Pd n clusters were dissolved in 0.1 M Bu 4 NPF 6 that was constantly purged with N 2 (99.99% Nanjing Special Gas Corp.) during the experiments. All electrochemical experiments were carried out at room temperature (ca. 25 °C).
Theoretical calculation. For details of theoretical calculation of structure and UV/Vis spectra, see the supplementary information.