XIE An , WANG Zhi , WU Qiaoyu , CHENG Liping 2,, LUO Genggeng ,＊, SUN Di Key Laboratory of Functional Materials and Applications of Fujian Province, School of Materials Science and Engineering, Xiamen University of Technology, Xiamen 61024, Fujian Province, P. R. China.

2 Key Lab of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, P. R. China.

3 College of Materials Science and Engineering, Huaqiao University, Xiamen 361021, Fujian Province, P. R. China.

Abstract: Crystalline silver cluster compounds are highly interesting owing to their intriguing structure and potential technological application in luminescence, semiconductivity,and as precursors for nonlinear optical materials. Typically, the synthesis of silver clusters involves the use of protecting ligands such as thiolates, phosphines, and alkynes, which have been found to be critical for cluster size and shape tuning. Among these nanoclusters,pentacosanuclear silver clusters (Ag25) have only been reported with either thiolate ligands or phosphine ligands, while those bearing mixed protecting ligands are rather rare. In the course of our exploration of novel silver clusters, a new silver thiolate precursor(iPrC6H4SAg)n was used to construct nanosized silver clusters. In the presence of 1,3-bis(diphenyphosphino)propane (dppp) and CF3SO3Ag, the pentacosanuclear silver cluster [Ag25(SC6H4Pri)18(dppp)6](CF3SO3)7·CH3CN (designated as Ag25, HSC6H4Pri =4-t-isopropylthiophenol) ligated both by thiolate and phosphine ligands was obtained under ultrasonic reaction conditions. Yellow block crystals were isolated from the solution, whose molecular structure was elucidated by single-crystal X-ray analysis. The skeleton of the Ag25 cluster comprises a sandwich-like motif containing two structurally very similar cylinders sharing a metal-cluster plane. The core of each cylinder presents the overall shape of a twisted hexagonal cylinder made of two connected Ag3S3 units, with six sulfur atoms and six silver atoms alternating on a puckered drum-like surface. The metal-cluster plane contains one type of pure-Ag tetragons showing significant Ag···Ag argentophilic interactions. The optical properties of Ag25 were investigated in the solid state. A band gap of ～2.5 eV was estimated for Ag25 from the optical absorption spectrum, suggesting this cluster to be a potential wide-gap semiconductor. This Ag25 cluster was also found to emit green luminescence at λ = 505 nm and room temperature.

Key Words: Silver cluster; Crystal structure; Optical property; Luminescence; Argentophilicity

1 Introduction

Increasing attention has been paid to the construction of novelsilver-sulfur clusters because of their intriguing structures and fascinating optical properties1–3. The assembly of such large discrete molecules from small building blocks still confused the synthetic chemists for a long time, although many examples from the largest Ag(I)cluster, Ag4904, to the largest Ag(0)-containing nanocluster,Ag3745have been witnessed. Considering the available ligand types, three kinds of ligand, thiolates, phosphines, and alkynes are prefer to bind toward silver atoms, whereas metal acetylides are explosive, so the former two were widely used in the assembly of silver clusters6. Recently, we isolated a series of anion-templated Ag/S/P clusters with some novel characteristics7, however, silver atoms aggregations and mixed ligand combinations are intensively influenced by solution assembly conditions8, thus further explorations on the synthesis of such kind of silver clusters are urgently needed. In addition, a few 25 nuclear silver clusters only containing single thiolate ligands or phosphine ligands have been reported. Prominent examples are[Ag25(SPhMe2)18]9, [Ag25(2,4-DMBT)18]-(2,4-DMBT:2,4-dimethylbenzenethiol)9and [Ag25H22(DPPE)8]3+(DPPE:1,2-bis(diphenylphosphino)ethane)10, while other 25 nuclear silver clusters with mixed ligands (thiolates and phosphines) are rather rare. Gratifyingly, our attempts leaded to a novel 25 nuclear silver thiolate cluster[Ag25(SC6H4Pri)18(dppp)6] (CF3SO3)7·CH3CN (Ag25,HSC6H4Pri= 4-t-isopropylthiophenol, and dppp =1,3-bis(diphenyphosphino)propane) with phosphine as co-ligand. Herein we report the synthesis, crystal structure and optical properties of the molecular cluster Ag25. The skeleton of the cluster Ag25can be viewed as a sandwich-like structure which has two structurally very similar cylinders sharing a pure metal-cluster plane. Finally,the optical properties of the Ag25nanocluster are explored.

2 Experimental

2.1 Chemicals and general methods

1,3-bis(diphenyphosphino)propane (99%, Sigma-Aldrich),4-t-isopropylthiophenol (99%, Sigma-Aldrich), CF3SO3Ag(99%, Sigma-Aldrich), ethanol (HPLC grade, Alfa Aesar),acetonitrile (HPLC grade, Alfa Aesar). All chemicals were used as received.

The elemental analyses (C, H, and N contents) were determined on a Vario EL III analyzer (German). The diffusereflectance spectra were performed on UV-Vis spectrophotometer (Evolution 220, ISA-220 accessory, Thermo Scientific,USA) using a built-in 10 mm silicon photodiode with a 60 mm Spectralon sphere. Photoluminescence spectra were obtained using a Thermo Scientific Lumina fluorescence spectrophotometer (USA). FT-IR spectra were recorded on a PerkinElmer Spectrum Two in the frequency range of 4000–400 cm-1(German). Powder X-ray diffraction (PXRD) data were collected on a Philips X’Pert Pro MPD X-ray diffractometer(Philips Corporation, The Netherlands) with Cu Kαradiation equipped with an X’Celerator detector.

2.2 Synthesis of Ag25

The mixture of (iPrC6H4SAg)n (0.05 mmol, 13 mg) and 1,3-bis(diphenyphosphino)propane (dppp, 0.05 mmol, 20.6 mg)were dissolved in a mixed solution of ethanol and acetonitrile(5 mL , volume ratio 3/2) and treated under the ultrasonic condition (160 W, 40 kHz) for 10 min. at room temperature.Then CF3SO3Ag (0.05 mmol, 12.85 mg) was added to above mixture and ultrasound for another 10 min., the yellow solution was filtered and the corresponding filtrate was left to evaporate slowly for two weeks in the dark, yellow block crystals were isolated in a yield of 25%. Elemental analysis cacld (%) for C333H357Ag25F21NO21P12S25: C 44.55, H 4.01, N 0.16; found(%) C 44.37, H 3.89, N 0.09. Selected IR peaks (cm-1): 3057(w), 2963 (w), 1484 (w), 1434 (m), 1401 (w), 1260 (m), 1218(m), 1156 (m), 1098 (m), 1018 (s), 957 (w), 818 (m), 739 (m),692 (s), 634 (s), 572 (w), 538 (m), 514 (s), 472 (m).

2.3 X-ray single-crystal crystallography

Single crystal of Ag25with appropriate dimension was chosen under an optical microscope and quickly coated with high vacuum grease (Dow Corning Corporation) to prevent decomposition. Intensity data and cell parameters were recorded at 123 K on a Bruker Apex II single crystal diffractometer, employing a Mo Kα radiation (λ = 0.071073 nm)and a CCD area detector. The raw frame data were processed using SAINT and SADABS to yield the reflection data file11.The structure was solved using the charge-flipping algorithm,as implemented in the program SUPERFLIP12and refined by full-matrix least-squares techniques against Fo2using the SHELXL program13through the OLEX2 interface14.Hydrogen atoms at carbon were placed in calculated positions and refined isotropically by using a riding model. Appropriate restraints or constraints were applied to the geometry and the atomic displacement parameters of the atoms in the cluster. All structures were examined using the Addsym subroutine of PLATON15to ensure that no additional symmetry could be applied to the models. Pertinent crystallographic data collection and refinement parameters are collated in Table S1 (Supporting Information). Selected bond lengths and angles are collected in Table S2. CCDC number: 1579565.

3 Results and Discussion

Typically, the synthesis of [Ag25(SC6H4Pri)18(dppp)6](CF3SO3)7·CH3CN (designated as Ag25) involves the reaction of equivalent molar amounts of (iPrC6H4SAg)n,1,3-bis(diphenyphosphino) propane (dppp) and CF3SO3Ag in a mixture of ethanol and acetonitrile under the ultrasonic condition. The resulting yellow solution was filtered and evaporated slowly in darkness at room temperature. The bulk sample of Ag25was collected by picking up the crystals manually under the microscope and its phase purity was justified by compared PXRD patterns shown in Fig. S1(Supporting Information). The yellow block Ag25suitable for single-crystal X-ray crystallography was obtained after two weeks (See inset of Fig. 1). The molecular structure of Ag25 is determined from single-crystal X-ray diffraction data at low temperature (123 K). Single-crystal X-ray structural analysis revealed that the molecular cluster Ag25crystallized in the monoclinic P21/n space group with a cell volume as large as about 39.607 nm3. Its asymmetric unit comprises one complete cationic silver cluster protected by dppp andiPrC6H4S-ligands.The total charge neutrality is finished by seven CF3SO3-anions in the lattice.

Fig. 1 The molecular structure of the cationic cluster[Ag25(SC6H4Pri)18(dppp)6]7+ (denoted as Ag25).The CF3SO3- and solvent molecules are omitted. Color legend: pink, Ag;yellow, S; orange, P; grey, C. Inset: the yellow block crystals of Ag25.

Fig. 2 The sandwich-like skeleton of Ag25 (left) was made up of two cylinders sharing one metal-cluster plane as shown in the right.Color legend: pink, Ag; yellow, S; orange, P.

As displayed in Fig. 1, the overall cationic part of the cluster is comprised of twenty-five independent Ag(I) ions, six diphosphine ligands dppp and eighteen bridgingiPrC6H4S-ligands. The cationic cluster skeleton is depicted in Fig. 2. The cluster skeleton can be well described as a sandwich-like structure: two structurally very similar cylinders, i.e., cylinder A (head) and cylinder B (tail), sharing a metal-cluster plane C(middle). The Ag6S6 core of cylinder A has an overall shape of a twisted hexagonal cylinder made up of two connected Ag3S3units, with six sulfur atoms and six silver atoms alternating on a puckered drum-like surface. The Ag3S3units are distinctly crown-shaped, having chair-like conformations and are staggered with respect to each other. The silver ions in the cylinder A can be geometrically divided into two categories.The first kind contains three silver atoms (Ag5, Ag6 and Ag7),and the coordination environment around each silver atom is three-coordinate form and coordinated by two thiolate sulfur atoms (Ag-S 0.2460(6)–0.2502(7) nm) and one phosphorous atom of dppp (Ag-P 0.2398(7)–0.2423(9) nm). The other kind consists of three silver atoms (Ag8, Ag9 and Ag13) and each silver atom is tetragonally surrounded by four sulfur atoms(Ag-S 0.2427(7)–0.2684(6) nm). It should be noted that there are another independent three silver atoms (Ag11, Ag12 and Ag14) with each linked to the cylinder A by thiolate sulfur atom. Each silver ion is two-coordinate mode by thiolate sulfur atom (Ag-S 0.2424(6)–0.2440(6) nm) and the other phosphorous atom of dppp (Ag-P 0.2352(7)–0.2376(7) nm)occupying the second coordination site. The sulfur atoms of theiPrC6H4S-ligands in the Ag6S6 core display two distinct bridging modes: μ2and μ3. Similar findings were also observed in the cylinder B. The plane C shared by cylinder A and cylinder B consists of seven silver atoms. An important structural feature is that this plane contains one type of pure-Ag tetragon, which contains Ag1, Ag2, Ag3 and Ag4. In addition,Ag10, Ag15 and Ag21 are attached to the tetragon by Ag1, Ag2 and Ag4, respectively. The Ag(I)···Ag(I) distances in the metal-cluster plane range from 0.2856(4) to 0.3086(3) nm,which are shorter than twice the van der Waals radius of the silver atom (0.344 nm)16, indicating the significance of argentophilicity promoting the aggregation of silver centers. It should be noted that a 25 nuclear silver nanocluster with an atomically precise molecular formula [Ag25(SPhMe2)18]-has been reported9. Structurally, compared with the present Ag25,the previously reported [Ag25(SPhMe2)18]-cluster only contains single thiolate ligands and has a 13-atom Ag icosahedral core.

Fig. 3 (a) Solid-state UV-Vis absorption spectrum of Ag25 measured at 298 K; (b) Diffuse reflectance UV-Vis spectrum of the Kubelka-Munk function vs energy (eV) of Ag25.

Fig. 4 Solid-state emission spectrum of the cluster Ag25 upon excitation at λ = 350 nm at 298 K.

Solid-state UV-Vis absorption spectrum and photoluminescence of the cluster Ag25 were carried out at room temperature. As depicted in Fig. 3a, the electronic absorption spectrum of Ag25in the solid state exhibits intense absorption bands spanning from 350 to 440 nm in the UV-Vis region and low-energy shoulders tailing to ～500 nm. The high-energy bands below 350 nm are assigned as IL (π→π*) transitions,which are typical foriPrC6H4S–ligands. The absorptions centered at ～350–420 nm can be assigned to electronic transitions from the σ(Ag―S) to empty antibonding π*orbitals. The tails extending down to 500 nm may arise from the cluster-centered(MC) transitions. The solid state optical diffuse reflection spectrum of Ag25 was also measured at room temperature to determine the band gap (Eg, photoresponse wavelength region).The Eg term is defined as the intersection point between the energy axis and the line extrapolated from the linear portion of the adsorption edge in a plot of the Kubelka-Munk function F versus energy E. Absorption data were obtained from the reflectance using the Kubelka-Munk function (Fig. 3b). The band gap of Ag25 was estimated to be ～2.5 eV, which indicates the nature of the silver cluster is a potential wide-gap semiconductor.

As shown in Fig. 4, the cluster Ag25shows green emission with maximum peak at ～505 nm upon excitation at 350 nm in the solid state at room temperature. The corresponding photoluminescence quantum yield is determined to be 0.85%.The green luminescence of Ag25 is believed to mainly originate from the S 3p to Ag 5s charge transfer (ligand-to-metal-chargetransfer, LMCT) transition with metal-centered (MC) d10→d9s1transition, which is characteristic of the silver clusters with metallophilic interactions17.

4 Conclusions

In conclusion, a new 25 nuclear silver thiolate cluster (Ag25)with mixed protecting ligands of thiolate and phosphine has been synthesized by using polymeric silver-sulfur(iPrC6H4SAg)nprecursor and silver salt (CF3SO3Ag) as well as auxiliary dppp ligand under the ultrasonic reaction condition.The molecular cluster Ag25 has a sandwich-like geometry structure, namely, two structurally very similar cylinders sharing a metal-cluster plane. Finally, it is found that the cluster Ag25 exhibits band gap of 2.5 eV and photoluminescence centered at λ = 505 nm at room temperature.