Highly Conducting Neutral Coordination Polymer with Infinite Two

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A highly conducting neutral coordination polymer with infinite two-dimensional silver-sulfur networks Xing Huang, Haisheng Li, Zeyi Tu, Liyao Liu, Xiaoyu Wu, Jie Chen, Yingying Liang, Ye Zou, Yuanping Yi, Junliang Sun, Wei Xu, and Daoben Zhu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b07921 • Publication Date (Web): 12 Sep 2018 Downloaded from http://pubs.acs.org on September 12, 2018

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A highly conducting neutral coordination polymer with infinite two-dimensional silver-sulfur networks Xing Huanga,c, Haisheng Lib, Zeyi Tua,c Liyao Liua,c, Xiaoyu Wua,c, Jie Chena,c, Yingying Lianga,c, Ye Zoua,c , Yuanping Yia,c, Junliang Sunb*, Wei Xua,c*, and Daoben Zhua,c* a. Beijing National Laboratory for Molecular Sciences CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 (China). b. College of Chemistry and Molecular Engineering, Peking University, 100871 Beijing, China c. University of Chinese Academy of Sciences, Beijing 100049 (China).

ABSTRACT: Highly crystalline films of a silver-based coordination polymer, [Ag5(C6S6)]n (Ag-BHT, BHT =benzenehexathiol), have been prepared. The structure of Ag-BHT, solved by combining rotation electron diffraction and powder X-ray diffraction techniques, indicates that it has a lamellar structure with alternatively stacked two-dimensional Ag-S networks and layers composed of one-dimensional metal-dithiolene polymers. In addition, the polycrystalline Ag-BHT film shows high electrical -1 conductivity of up to 250 S·cm at 300 K. The ultraviolet-photoelectron spectroscopy and electronic band structure calculations reveal that this can be attributed to the partially filled valence band and the unique two-dimensional Ag-S networks.

The lack of structural diversity would limit the exploration of exotic physical properties and the applications of this kind of CPs. Furthermore, the limitation in structure diversity will also put a great challenge on their structural analysis since one of the most feasible ways to solve a new material’s 14 structure is using the existing structure as template. Therefore, it is of great interest to search for BHT-like ligands based CPs with novel structural topology. Attribute to the diversity of silver ion’s coordination 15-16 modes, silver-based CPs with the formation of Ag-Ag interaction, Ag-S chain or other typical structures have been 16-17 well documented. Taking these in consideration, [Ag5(C6S6)]n (Ag-BHT, BHT =benzenehexathiol) was prepared through an interface reaction between BHT/toluene and silver(I) nitrate/H2O (as illustrated in

Recently, the development of coordination polymers (CPs) based on highly symmetrical multi-dentate planar ligands, 1-6 such as benzenehexathiol (BHT), 7-11 2,3,6,7,10,11-hexaiminotriphenylene (HITP), 12 hexaiminobenzene (HIB) and so on, has drawn wide attentions because this subclass of CPs presents high electrical conductivity and physical properties which 3, 12 conventional CPs have’t shown, such as metallic behaviour 3 and superconductivity . Furthermore, these progresses have broadened the applications of CPs as active materials in 1 11 fields of field effect transistors, gas sensors, 9 4 supercapacitors , transparent electrodes , and 2, 8 electrochemical catalysis . In electrical conductive CPs based on the BHT-like ligands, the metal-dithiolene motif connects the aromatic ligand units and the metal ions to form a two dimentional π−d conjugated plane and thus promote long-range electron 1, 6 delocalization. Most of them have two particular 6, 10, 12 structures: the hexagonal honeycomb structure and 1, 5 fully-filled honeycomb structure (as shown in Figure S1). In fact, the electronic states of metal-dithiolene-based complex are quite sensitive to the different molecular 13 arrangement, orientation as well as the crystal structure.

Figure 1. a) Scheme of the interfacial method used for synthesizing Ag-BHT film, b) the SEM image of cross-section of Ag-BHT film, c) the TEM image of Ag-BHT nanocrystal and the corresponding TEM-EDS results, d) HRTEM image of Ag-BHT nanocrystal, the upper right inset is the amplified image and the lower right is the corresponding FFT image.

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Figure 1a). The as-prepared film is free-standing. As shown in -1 Figure S2, the strong Raman signal at 2500 cm ascribed to the S-H stretching vibration of BHT completely disappeared in Ag-BHT, indicating that all the thiol groups of BHT have participated in the coordination with Ag ions. Combining the X-ray photoelectron spectroscopy (XPS), electron probe microanalysis (EPMA), and inductively coupled plasma (ICP) results, the formula of Ag-BHT is calculated to be [Ag5(C6S6)]n (the calculation details can be found in SI), which is totally different from other BHT-based CPs ever 1 reported, such as [Cu3(C6S6)]n for Cu-BHT and [Ni3(C6S6)2]n 6 for Ni-BHT . As shown in Figure S3, the XPS peaks of Ag 3d are asymmetric, suggesting that there should be two kinds of Ag with different chemistry environments. The Scanning Electron Microscope (SEM) image of the cross section of an Ag-BHT film (Figure 1b) presents that the film is composed of needle-like nanocrystals. After sufficient ultrasonic dispersion, Ag-BHT nanocrystals can be exfoliated from the film and dispersed in ethanol. The Transmission Electron Microscope (TEM) image of Ag-BHT nanocrystal

(Figure 1c) shows that they have prism-structure with length of 1µm and width of ~100 nm. The elemental mappings with TEM-EDS confirm the compositional homogeneity of Ag-BHT nanocrystals. In the HR-TEM image of Ag-BHT nanocrystal (Figure 1d), two series of crystal lattice fringes with the d-space of 3.4 Å and 7.5 Å can be clearly identified, suggesting the high crystallinity of Ag-BHT nanocrystals, which is verified by the powder X-ray diffraction (PXRD) result shown in Figure 2a. However, the dimension of Ag-BHT nanocrystals are too small to be directly used for single crystal X-ray diffraction analysis. To solve this problem, the rotation electron diffraction (RED) technique was employed to collect and process 3D electron diffraction data for structure solving. First, the 3D reciprocal lattice is reconstructed through RED analysis (as shown in Figure S4). It clearly shows that Ag-BHT crystallizes in the monoclinic space group I2/m with the unit cell of a=14.32 Å, b=9.32 Å, c=4.34 Å, and β=94.5º. Based on these unit cell parameters and the PXRD data, the charge flipping algorithm 18 implemented in the computer program, Superflip, was applied for the further structure solving. After the structure model of Ag-BHT had been derived, final refinement was 19 made by the Rietveld method using the program Jana2006. The final refinement converged with Rp=3.45%, Rwp=4.98% and the details of the Rietveld refinement are listed in Table S1, and the refinement plots are shown in Figure 2a. The crystal structure determined offer strong evidence that the formula of Ag-BHT should be [Ag5(C6S6)]n. The crystal structure of [Ag5(C6S6)]n is shown in Figure 2b. There are two different types of silver, sulfur, and carbon atoms (denoted as Ag1, Ag2, S1, S2, C1, C2), which can explain the existence of the asymmetry XPS signals of silver atoms in [Ag5(C6S6)]n. Ag1 has a square-planar coordination mode and links BHT molecules to form a one-dimensional metal-dithiolene polymer chain (Figure S5a). Ag2 is six-coordinated with distorted octahedron geometry, connecting with three other Ag2, two S2 and one S1 (Figure S5b). As a result, Ag2 atoms form a graphene-like layer structure (Figure S6). This Ag6 hexagon is distorted with three different adjacent Ag-Ag bond lengths of 2.88, 3.03, and 3.13 Å, which is confirmed by Extended X-ray absorption fine structure (EXAFS) analysis (Figure S7). Furthermore, the characteristic symmetric vibration signal of Ag-Ag bond can be observed in Raman spectrum (Figure S8). The metal atoms in the Ag layer further connect with S atoms from the

Figure 2. a) Rietveld refinement against PXRD data for Ag-BHT. The curves from top to bottom are experiment (red), simulation (green), and difference profiles (black), respectively; the bars below curves indicate the positions of Bragg peaks. The inert is coordination environment of atoms in Ag-BHT. b) Projections of Ag-BHT. Gray, C; yellow, S; blue, Ag. c) High-resolution STEM image of Ag-BHT nanocrystal, and d) amplified imaged focused on the marked area in (c).

Figure 3. Schematic illustration of structure of Ag-BHT. Ag, S and C are depicted in blue, yellow and gray respectively. The Ag-centered tetrahedra is shown in pale blue.

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Journal of the American Chemical Society packed nanocrystals, the hopping process between neighboring Ag-BHT nanocrystals should be the critical step for the electrical conducting process. In other words, this behavior reflects the electronic property of Ag-BHT film rather than intrinsic properties of Ag-BHT crystal.

Figure 4. a) Electrical conductivity (σ, S·cm-1) of Ag-BHT film as a function of temperature ranging from -0.25 12 to 400 K. The inert is plots of ln σ (T) versus T . b) UPS of Ag-BHT film acquired at 300 K. c) Calculated electronic structure of Ag-BHT and d) corresponding first Brillouin zone and high-symmetry K-points. up-side and down-side BHT molecules to form an infinite two-dimensional Ag-S networks which shares the S1 atoms with metal dithiolene polymer chain. The interlayer distance of this two-dimensional networks is 7.0 Å. According to the Scanning Transmission Electron Microscopy (STEM) image (Figure 2c, d) and crystal structure of Ag-BHT, Ag-BHT nanocrystal prefers to grow along the direction which is parallel to the π-π stacking direction of metal dithiolene polymer chain. In a whole, the crystal structure analysis on Ag-BHT reveals that it has a lamella structure with alternatively stacked 2D Ag-S networks and layers composed of 1D metal-dithiolene polymer (Figure 3), which can be viewed as a hybrid material composed of inorganic semiconductor and coordination polymer. To our knowledge, the coexistence of metal-dithiolene motif and 20 metal-sulfur networks has never been reported . Considering the flexible electronic state of metal-dithiolene complexes, the tunability of the electronic structure of this hybrid material can be expected. The room-temperature electrical conductivity (σRT) of -1 Ag-BHT film reaches up to 250 S·cm (four-electrode method). This is the highest value for the reported Ag-based 21-22 CPs. As shown in Figure 4a, the σ(T) of Ag-BHT film increases with temperature and shows a nonlinear -1 relationship between ln σ(T) and T (Figure S9). The activation energy of the electrical conductivity continuously rises with the increase of temperature and ranges from 1.03 meV at 40 K to 16.26 meV at 400 K. Furthermore, the plot of -0.25 ln σ(T) versus T over the temperature region of 100-300 K was satisfactorily fitted with three-dimensional Mott variable range hopping model (the inset of Figure 4a). This behavior 1 is very similar with what have been observed in Cu-BHT film and other granular conductors where inter-grain charge 23 hopping mediates charge transport. Taking consideration that Ag-BHT film is polycrystalline and composed by closed

The Ultraviolet Photoelectron Spectroscopy (UPS) of Ag-BHT shows a clear Fermi edge near the top of valence band, which is an inherent feature for metallic states (as shown in Figure 4b). The Pauli-like paramagnetism at least above 50 K (Figure S10), and small thermopower of Ag-BHT (Figure S11) also suggest that it is metallic. The electronic band structure of Ag-BHT was calculated based on its crystal structure and plotted along the high symmetry points in the first Brillion zone (Figure 4c, d). Although there is a band gap of about 0.8 eV between the conduction band and valence band, the Fermi level cut valence band, consistent with the Fermi edge found in UPS. As a result, Ag-BHT is more like a degenerate semiconductor rather than metal. From the view of charge balance, the as-prepared Ag-BHT is neutral and there should be an in-situ oxidation process caused by the overall oxidation environment generated by the excess silver ions and dilute nitric acid formed in side reaction. One electron per Ag5C6S6 unit was taken away during the oxidation process and the electron number of Ag-BHT became odd. Thus, its valance band is partially filled with the

Figure 5. UPS of pristine and reduced Ag-BHT. Fermi level shifted in. This band structure gives rise to a large free carrier concentration along with high mobility and thus leads to the high conductivity. Reduced Ag-BHT can be obtained by treating Ag-BHT with LiBHEt3. The reduced Ag-BHT has a much lower room -2 -1 conductivity of 7.3 ×10 S·cm compared to pristine Ag-BHT. Furthermore, variable temperature conductivity reveals that the reduced Ag-BHT is semiconducting (Figure S12) and the UPS result shows that there is a band gap in reduced Ag-BHT instead of a Fermi edge in pristine Ag-BHT (Figure 5), indicating that Ag-BHT transfers from degenerate semiconductor to pristine semiconductor through chemical reduction. Both the S 2p peak and Ag 3d peak in the XPS spectra of reduced Ag-BHT possessed a lower binding energy than pristine Ag-BHT(Figure S13), which indicate that the negative charges introduced by the reduction process have accumulated at the silver and sulfur atoms. The presence of Li ions was observed in the XPS of the reduced product (as shown in Figure S14), this part of the lithium ions should be embedded in the structure of Ag-BHT or on the surface of Ag-BHT. So the reduction product can be denoted as Lix

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[Ag5(C6S6)] . Notably, the PXRD data shown in Figure S15 suggest that the oxidation state and charge transport property of Ag-BHT can be easily modulated by chemical reduction without damaging the crystal structures.

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The projected density of state (DOS) near the fermi level (Figure S16) have been calculated which shows that only the contributions from orbitals of Ag and S atoms rather than C atoms are reflected, suggesting that the 2D Ag-S network is the preferred charge transport path. Similar Ag-S network has also been found in silver thiolate polymers, such as 17 [Ag(C5H4NS)]n (C5H4NS= pyridine-2-thiolate), but most of them are semiconductor or insulator with much poorer electrical conductivity compared to Ag-BHT. The reason is that Ag-BHT has both the Ag-S network which can support long-range charge transport and the metal-dithiolene motif, which make the electronic structure of Ag-BHT tunable for creating a partially filled valence band.

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In conclusion, Ag-BHT films composed of highly crystallized nanocrystals were synthesized and characterized. The well characterized crystal structure shows a novel lamella structure with alternatively stacked two-dimensional Ag-S networks and layers composed of one-dimensional metal-dithiolene polymers. Benefiting from the unique -1 structure, its electrical conductivity is up to 250 S·cm , which is a recorded high value for Ag-based CPs ever reported. This result demonstrates the great prospect of this highly symmetric and rigid BHT ligand for the construction of multifunctional materilas with variable structural topologies. The unique 2D Ag-S networks also makes its optical properties worth exploring. Further fundamental physical studies and advanced electronic applications of Ag-BHT is ongoing.

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Supporting Information Experimental details and characterization data as well as the Crystallographic data of Ag-BHT are in Supporting Information. This material is available free of charge on the ACS Publications website.

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AUTHOR INFORMATION

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Corresponding Author *[email protected] *[email protected] *[email protected]

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Notes The authors declare no competing financial interests.

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ACKNOWLEDGMENT We gratefully acknowledge the financial support from the National Key R&D Program of China (Grant No. 2017YFA0204701), the Chinese Academy of Sciences (Strategic Priority Research Program No. XDB12000000, QYZDY-SSW-SLH024) and National Natural Science Foundation of China (21333011, 21372227).We gratefully thank Prof. Jingguang Cheng, Prof. Genfu Chen and Dr. Dongwei Wang for collecting heat capacity data. We thank beamline BL14W1 (Shanghai Synchrotron Radiation Facility) and beamline 1W1B (Beijing Synchrotron Radiation Facility) for XAFS measurements.

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