LEI Xiao-Wu YUE Cheng-Yang ZHANG Hui-Ping

ZHAI Xiu-Rong FENG Li-Juan

(Key Laboratory of Inorganic Chemistry in Universities of Shandong, Department of Chemistry and Chemical Engineering, Jining University, Qufu, Shandong 273155, China)

1 INTRODUCTION

Luminescent d8and d10transition-metal complexes have drawn enormous interest in recent years because of their intriguing coordination architectures and potential applications in various fields including catalysis, photochemistry, fluorescence, biological properties, etc[1-8]. These complexes show a wide range of luminescence properties that depend strongly on the structural and electronic characteristics of their ligands, as well as on the temperature. Furthermore, in the research of d8and d10metal complexes, rich and diverse low-energy excited states, including IL (intraligand: π-π*), MLCT(metal-to-ligand charge transfer)[dσ*-pσ], and MMLCT (metal-metal-to-ligand charge transfer)[dσ*-π*]have been observed[9-10]. These enrich their luminescent research and practical applications.

In the research of luminescent complexes over decades, functional materials containing AgIion with d10closed-shell electronic configuration have attracted much attention considering their abundant resources and nontoxic properties compared to noble metal complexes, e. g. ReI, OsII, IrIII, RhI, PtII, and AuI[11-16]. Furthermore, AgIis a favorable and fashionable ion for the construction of coordination polymers because of its highly diversified and flexible coordination number and geometry as well as its positive coordination tendency with various donor atoms, such as O, S, N, P, I, and so on, which may lead to the discovery of interesting structural motifs[17-23]. These obvious advantages lead AgIcomplexes as excellent candidates of luminescent materials with the potential toward mass production and broad application.

Now, much effort has been focused on using organic nitrogen and carboxylic acid ligands as bridging ligands for adjacent AgIsites. Especially, the contribution of short Ag··Ag interactions, termed argentophilicity, plays an important role in determining the solid state structures as well as the luminescence properties. For example, a series of tetranuclear Ag4complexes with short Ag–Ag bonds have been reported based on different bridging ligands: tetrahedral Ag4cluster of[Ag4(S2C2H4(C6H4))3]2-and [Ag4(S2CdC(CN)2)4]4-(Ag–Ag: 2.953～3.169 ?)with dithiolate ligands[24];and [Ag4(μ2-L2)6(CH3CN)2](AsF6)·2H2O composed of triazole ligands exhibits blue emission in the solid state and green emission in solution at ambient temperature, respectively[25]. Furthermore,[Ag2(aip)(NH3)]nwas synthesized in solution by reacting AgNO3and 5-aminoisophthalic acid, and its structure features a 3D network containing tetranuclear silver(I)core and shows an visible intense purple luminescence in aqueous solution state[26].

Intrigued by the rich structural types and interesting physical properties of these AgIcomplexes, we have applied appropriate bridging ligands and obtained a series of 1D, two-dimensional (2D)and 3D complexes possessing interesting structural motifs and significant properties. For example, we have previously reported a silver organosulfur complex of Ag6(bmt)6·6THF (Hbmt = 2-benzimidazolethiol, and THF = tetrahydrofuran), which features hexanuclear silver(I)cores and shows intense red emission originating from a metal-centered triplet state modified by Ag–Ag interactions within the octahedral Ag6clusters[27]. Now we select 3-aminopyrazine-2-carboxylic acid (Hapca)as the ligand,which simultaneity contains heterocyclic nitrogen atoms and amidogen as well as carboxyl. Our exploratory study led to one new AgIcomplex, namely,[Ag(apca)(H2O)]n. Herein, we report its synthesis,single-crystal structure and luminescent property.

2 EXPERIMENTAL

2. 1 Materials and instrumentation

All chemicals and reagents were commercially available and used as received without further purification. Elemental analyses (C, H and N)were performed using a PE2400 II elemental analyzer.The UV/vis spectra were recorded at room temperature using a computer-controlled PE Lambda 900 UV/vis spectrometer equipped with an integrating sphere in the wavelength range of 200～800 nm.FT-IR spectra were recorded with a Nicolet Magna-IR 550 spectrometer in dry KBr disks in the 4000～400 cm-1range. Thermogravimetric analyses (TGA)were performed using a Mettler TGA/SDTA 851 thermal analyzer under N2atmosphere at a heating rate of 10 ℃ min-1in the temperature region of 30～800 ℃. Fluorescent spectra were measured on an Edinburgh Instruments analyzer model FLS920 with 450W xenon light.

2. 2 Synthesis of complex 1

The colorless solution of AgNO3(34 mg, 0.2 mmol)in 30 mL of water turned yellow after adding ammonia (2 mL)and 3-aminopyrazine-2-carboxylic acid (Hapac, 27.8 mg, 0.2 mmol)under stirring. The reaction mixture was kept stirring for one hour and filtered out, and the resulting filtrate was allowed to slowly evaporate at ambient atmosphere for one week. Yellow crystals suitable for single-crystal X-ray analysis were collected and air-dried. Yield: 52%(27 mg)based on Ag. Anal. Calcd. for C5N3O3H6Ag(%): H, 2.29; C, 22.78; N, 15.92. Found: H, 2.33; C,22.91; N, 15.81. IR (KBr pellet, cm-1): 3365 v(O-H,N-H); 1592, 1549, 1461, 1430 v(C=C, C=N);1386,1236, 1189, 1157 v(C–O); 910 v(H–Py); 813 v(Ag-N); 574 v(Ag–O).

2. 3 Crystal structure determination of complex 1

A yellow prism-shaped single crystal of complex 1 (0.08mm × 0.04mm × 0.04mm)was selected from the reaction products. Data collections for the compound were performed on a Bruker SMART CCD-based diffractometer (MoKα radiation,graphite monochromator)at 293(2)K in the range of 1.42＜θ＜27.13°. A total of 15628 reflections were collected and 3107 were independent (Rint= 0.0173),of which 2615 with I ＞ 2σ(I)were observed. The data set was corrected for Lorentz factor, polarization, air absorption and absorption because of variations in the path length through the detector faceplate. Absorption correction based on Multi-scan method was also applied.

The structure of the title compound was solved by direct methods (SHELXS)and refined by full-matrix least-square technique[28]. Based on systematic absences and E-value statistics, space group of P21/c was selected and gave satisfactory refinement.Non-hydrogen atoms were refined with anisotropic displacement parameters, and the hydrogen atoms bonded to C, N and O atoms were refined with isotropic displacement parameters. Site occupancy refinements for the title compound indicated that all sites were fully occupied. The final R = 0.0266, wR= 0.0788, (Δρ)max= 1.099, (Δρ)min= –1.156 e/?3,and S = 1.147 for 2615 observed reflections (I ＞2σ(I))with 265 parameters. The important bond distances and bond angles for complex 1 are listed in Table 1.

Table 1. Selected Bond Lengths (?)and Band Angles (°)

3 RESULTS AND DISCUSSION

3. 1 Crystal structure

Single-crystal X-ray crystallography reveals that complex 1 crystallizes in the monoclinic system P21/c space group and features a 3D network composed of 1D [Ag(apca)(H2O)]nribbons which are interconnected via weak hydrogen bonds and π-π interactions. The asymmetric unit of 1 contains two crystallographically independent AgIions, two apca ligands as well as two H2O molecules. The Ag(1)ion is bonded to one oxygen atom (O(1))and two nitrogen atoms (N(1)and N(5))from two different apca ligands to form a slightly distorted triangular coordination geometry. Differently, the Ag(2)ion is surrounded by two oxygen atoms (O(4)and O(6))and two nitrogen atoms (N(2)and N(4))from two different apca ligands and one H2O molecule,adopting a slightly distorted quadrangular coordination environment. Each apca molecule works as a μ3-bridging ligand to link three different AgIions:two nitrogen atoms of pyrazine ring bridge the Ag(1)and Ag(2)ions, respectively, and the carboxyl only affords one oxygen atom to connect one AgIion. The Ag–N and Ag–O bond distances are in the ranges of 2.174(3)～2.259(3)and 2.500(3)～2.571(3)?,respectively, which are comparable with those reported in other AgIcomplexes[7–10]. Here, it should be noted that all the N–Ag–O, O–Ag–O and N–Ag–N band angles around the Ag(1)and Ag(2)ions are very similar. Such result is related to the presence of weak Ag(1)–O(3)secondary bonding interaction (2.704 ?), approximately opposite to the Ag(1)–O(1)bonds. Hence, the coordination geometry of Ag(1)ion can also be considered as a distorted quadrangular coordination environment similar to that of Ag(2)ion.

The Ag(1)and Ag(2)ions are alternately bridged by the apca ligands as well as coordinated by H2O molecules via Ag–N and Ag–O bonds to form 1D[Ag(apca)(H2O)]nribbons along the a-axis (Fig. 1).The neighboring 1D ribbons are interlinked by O–H··O hydrogen bonds and π-π interactions among the neighboring pyrazine rings along the c-axis to form 2D layers, which are further interconnected by N–H··O hydrogen bonds to generate a 3D H-bonding network structure (Figs. 2 and 3). The N··O and O··O separations fall in the ranges of 2.692(5)～3.153(4)and 2.810(4)～3.126(5)?,respectively, which belong to the normal values(Table 2). The distance between two neighboring pyrazine rings is about 3.30 ?, and the dihedral angle between these two pyrazine rings is 0.42°.Among the neighboring 1D [Ag(apca)(H2O)]nribbons, the shortest Ag–Ag contact of 3.475 ? is evidently longer than the sum of vander Waals radii(3.44 ?), indicating the very weak argentophilicity.

Table 2. Hydrogen Bond Data for Complex 1

Fig. 1. General view of the 1D [Ag(apca)H2O]n ribbon along the c-axis

Fig. 2. View of the structure of 2D [Ag(apca)H2O]n layers as well as the O–H····O hydrogen bonds among the them

Fig. 3. View of the 3D network of complex 1 along the c-axis

3. 2 Thermal analysis

The thermal stability of complex 1 was examined by thermogravimetric analysis (TGA)in N2atmosphere from 30 to 700 ℃. As shown in Fig. 4, the thermogravimetric analysis (TGA)curve of complex 1 shows a slight weight loss below 180 ℃, corresponding to the release of crystalline water molecules. The observed weight loss of 6.7% is close to the theoretical value of 6.8%. The framework of the complex starts to collapse above 240 ℃.

Fig. 4. Thermogravimetric curve for complex 1

3. 3 Optical properties

The solid-state optical absorption spectra of complex 1 were measured at room temperature (Fig. 5).The optical band gap obtained by extrapolation of the linear portion of the absorption edge is estimated as 2.65 eV for 1, corresponding to its yellow color.The emission spectrum of complex 1 in solid state is investigated at room temperature with the results shown in Fig. 6. Upon excitation of the solid sample at the wavelength of 395 nm, complex 1 gives strong green emission with the main peak (λmax)appearing at 508 nm. It is reported that the free Hapca ligand displays strong blue emission with maximum intensity at 428 nm[27]. Considering the crystal structure of complex 1, the strong green emission of 508 nm may be assigned to the origination from the ligand-to-metal charge-transfer (LMCT)and/or metal-to-ligand charge-transfer (MLCT)characters.

4 CONCLUSION

In summary, one new AgI complex based on 3-aminopyrazine-2-carboxylic acid has been synthesized and structurally characterized. The complex features a 3D network built from 1D[Ag(apca)(H2O)]nribbons interconnected via weak hydrogen bonds and π-π interactions. In the solid state at room temperature, the title complex gives strong green emission originating from the ligandto-metal charge-transfer (LMCT)and/or metal-toligand charge-transfer (MLCT)characters. Further exploration in this field is in progress in our group.

Fig. 5. Solid-state optical absorption spectra of complex 1

Fig. 6. Solid emission spectrum of complex 1 at room temperature

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