Synthesis and crystal structures of three novel coordination polymers generated from AgCN and AgSCN with flexible N-donor ligands

Polyhedron 26 (2007) 107–114 www.elsevier.com/locate/poly

Synthesis and crystal structures of three novel coordination polymers generated from AgCN and AgSCN with flexible N-donor ligands Jian-Di Lin a

a,b

, Zhi-Hua Li a, Jian-Rong Li a, Shao-Wu Du

a,*

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR China b Graduate School of Chinese Academy of Sciences, Beijing 100039, PR China Received 21 April 2006; accepted 28 July 2006 Available online 11 August 2006

Abstract The solvothermal reactions of AgX (X = CN, SCN) with mbix [mbix = 1,3-bis(imidazole-l-yl-methyl)benzene] or bix [bix = 1,4bis(imidazole-l-yl-methyl)benzene] afforded the polymers [AgCN(mbix)]n (1), [(AgCN)4(bix)2]n (2) and [(AgSCN)2(mbix)]n (3). They were all characterized by infrared spectroscopy, elemental analysis and X-ray single-crystal analysis. The structure of 1 contains a 3-fold-interpenetrated 2D network, while that of 2 exhibits 2-fold parallel interpenetration. There is no interpenetration observed in 3. Compounds 2 and 3 show 3D supramolecular structures built from 2D networks through weak p    p interactions. The photoluminescent properties of the present compounds were also investigated.  2006 Elsevier Ltd. All rights reserved. Keywords: Coordination polymers; Cyanide-bridging; Triply-bridged SCN ligand; Photoluminescent property; Interpenetration; Flexible N-donor ligands

1. Introduction The design and synthesis of inorganic–organic hybrid coordination polymers are currently receiving much attention due to their fascinating network topologies and their potential application as functional materials [1–14]. The hydro(solvo)thermal technique is a powerful method for the construction of supramolecular polymers through their self-assembly. Self-assembly is a complicated process, highly influenced by lots of factors, such as the coordination nature of metal ions, the structural characterization of organic ligands, solvent system, temperature, pH value of the solution, the ratio of metal to ligand, the templates and the counter ions. Besides these aspects, other forces such as hydrogen-bonding, p–p interactions, metal–metal interactions can also greatly influ*

Corresponding author. Tel.: +86 591 83709470; fax: +86 591 83709470. E-mail address: [email protected] (S.-W. Du). 0277-5387/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2006.07.034

ence the supramolecular topology and its dimensionality [15–17]. Therefore, much work should be done to extend the area of the relevant structural types and to strive to find out proper synthetic strategies leading to the anticipated compounds. The CN and SCN anions, with their ambidexterous character, show considerable flexible coordination modes in the construction of supramolecular architectures. Silver (I) is a good candidate as a soft acid tending to coordinate to soft bases such as ligands containing sulfur and nitrogen atoms. On the other hand, Ag(I) ions are apt to form silver– silver interactions which are propitious to control supramolecular architectures and dimensionality. Although cyanate and thiocyanate are well-known bridging ligands, their silver (I) coordination polymers are rare. To our knowledge, most of the AgCN and AgSCN polymers are those involving pyridine derivatives, organic amines and chelating ligands [18–20]. There are only two examples of AgCN and AgSCN polymers constructed from bridging dipodal N-donor ligands [21,22].

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The ligands mbix and bix, which have been considered to be highly flexible N-containing ligands, have been employed in recent years for the construction of numerous inorganic–organic hybrid supramolecular networks [23,24]. Enlightened from our previous work [25], we introduced mbix and bix into the silver cyanate or silver thiocyanate systems and isolated three polymeric compounds, namely, [AgCN(mbix)]n (1), [(AgCN)4(bix)2]n (2) and [(AgSCN)2(mbix)]n (3). 2. Experimental 2.1. Materials The ligands bix and mbix were synthesized according to the literature [24a]. Other chemicals were purchased from commercial sources and were used as received without further purification. 2.2. Physical measurements The infrared spectra were recorded on a Perkin–Elmer FT-IR spectrophotometer in KBr matrices. Elemental analyses were carried out using a Perkin–Elmer 2400 II elemental analyser. Photoluminescence analyses were performed on an Edinburgh FLS920 fluorescence spectrometer. 2.3. Synthesis of the compounds 2.3.1. [AgCN(mbix)]n (1) AgCN (33.47 mg, 0.25 mmol), mbix (30.00 mg, 0.13 mmol) and 2 mL of CH3CN were placed in a 15 cm Pyrex tube. The tube was evacuated and sealed. The glass tube was heated at 120 C for 24 h. After cooling to room temperature over 48 h, colorless crystals suitable for X-ray diffraction were formed, yielded: 13 mg, about 28% based on mbix. Anal. Calc. for C15H14AgN5: C, 48.41; H, 3.79; N, 18.82. Found: C, 48.22; H, 3.98; N, 18.65%. FT-IR (cm1) (KBr disk): 3107m, 2102m, 2093m, 1611m, 1516vs, 1495s, 1450s, 1439s, 1392m, 1368m, 1274m, 1231m, 1242vs, 1112s, 1107vs, 1087m, 1076vs, 1030 m, 929m, 744s. 2.3.2. [(AgCN)4(bix)2]n (2) Compound 2 was prepared in a similar manner as for 1 except that bix was used instead of mbix and the reaction was carried out at 160 C. Yield: 21 mg, about 33% based on AgCN. Anal. Calc. for C32H28Ag4N12: C, 37.97; H, 2.79; N, 16.61. Found: C, 37.98; H, 3.09; N, 16.65%. FTIR (cm1) (KBr disk): 3445m, 3134m, 3106m, 2928m, 2135s, 2113m, 2085m, 1512vs, 1437m, 1232vs, 1107vs, 1077vs, 757m, 720s, 656s, 618m. 2.3.3. [(AgSCN)2(mbix)]n (3) Pale yellow crystals of 3 were obtained in a similar way as for 1 except that AgSCN was employed instead of

AgCN and the reaction temperature was 160 C. Yield: 33 mg, about 46% based on AgSCN. Anal. Calc. for C16H14Ag2N6S2: C, 33.70; H, 2.47; N, 14.74. Found: C, 34.03; H, 2.77; N, 14.73%. FT-IR (cm1) (KBr disk): 3435vs, 2104s, 2085s, 1631s, 1514s, 1441w, 1383w, 1368w, 1275w, 1236m 1103m, 1084m, 1078m, 830m, 770w, 743m, 657m. 2.4. X-ray crystallographic studies Crystallographic data of 1–3 were collected on a Rigaku Mercury CCD diffractometer equipped with graphite ˚ ). Crysmonochromated Mo Ka radiation (k = 0.71073 A tals of 1–3 suitable for X-ray crystallography were mounted on the top of a glass fiber with epoxy cement. The structure factors were obtained after Lorentz and polarization corrections. All the non-hydrogen atoms were located by the direct method and refined by full-matrix least-squares techniques. Crystallographic parameters, details of data collection and structure refinement for the crystals of 1–3 are summarized in Table 1. 3. Results and discussion 3.1. Syntheses and spectroscopic studies The compounds [AgCN(mbix)]n (1), [(AgCN)4(bix)2]n (2) and [(AgSCN)2(mbix)]n (3), were solvothermally prepared by mixing AgCN or AgSCN with corresponding ligands in vacuo. The reaction between AgSCN and bix failed to give any identified products. The IR spectra of 1–3 show characteristic CN groups around 2110 cm1. Other complicated peaks show the characteristic C–N and C–C vibrational frequencies for the bix and mbix ligands. 3.2. Structure of [AgCN(mbix)]n (1) Fig. 1a shows the asymmetric unit of compound 1, which consists of one Ag atom, one mbix ligand and one CN group. The Ag atom is in a deviated trigonal environment [the bond angles C1–Ag–N5A = 126.1(1), C1–Ag– N2 = 123.4(1) and N5A–Ag–N2 = 103.46(9)], coordinated by two different mbix ligands [Ag–N2 = 2.285(2), ˚ ], and one terminally bonded Ag–N5A = 2.284(2) A ˚ , Ag–C1–N1 = 166.7(3)] CN ligand [Ag–C1 = 2.113(3) A ˚ , which is (Fig. 1b). The Ag    AgB distance is 2.888(1) A much shorter than twice the van der Waals radius of silver ˚ ), indicating significant silver–silver interactions [26– (3.4 A ˚ , suggesting a non28]. The AgB–C1 distance is 2.87(3) A negligible interaction compared to the Ag–C bond length ˚ ] found in [Me4N][Ag2(CN)Cl] [Ag2–C2B = 2.757(5) A [26]. On the other hand, the direction of distortion from the terminal Ag–C1–N1 angle also indicates the existence of a weak interaction between AgB and C1. Thus, the cyanide group C1N1 can be described as having a semi-bridging coordination mode. Along the a-axis, the adjacent Ag

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Table 1 Crystallographic data for compounds 1–3 Compound

1

2

3

Formula Formula weight Crystal system Space group Unit cell dimensions ˚) a (A ˚) b (A ˚) c (A a () b () c () ˚ 3) V (A Z Dcalc (g/cm3) F(0 0 0) l(Mo Ka) (mm1) Range (h) for data collection () Temperature (K) Limiting indices

C15H14AgN5 372.18 monoclinic P21/n

C32H28Ag4N12 1012.14 triclinic P 1

C16H14Ag2N6S2 570.19 triclinic P 1

9.635(3) 7.550(2) 20.940(7) 90 100.135(7) 90 1499.4(8) 4 1.649 744 1.344 2.87 6 h 6 27.48 293(2) 12 6 h 6 12, 6 6 k 6 9, 27 6 l 6 26 9953 3388 (0.0177) 238/0/2881 1.066 R1 = 0.0353, wR2 = 0.0851 R1 = 0.0434, wR2 = 0.0916

9.6244(3) 12.7147(1) 14.6305(4) 73.042(4) 89.280(6) 82.927(6) 1698.96(7) 2 1.979 984 2.314 2.53 6 h 6 7.48 293(2) 12 6 h 6 11, 16 6 k 6 15, 18 6 l 6 18 13 184 7667 (0.0125) 537/0/6442 1.046 R1 = 0.0290, wR2 = 0.0720 R1 = 0.0368, wR2 = 0.0764

8.0205(2) 10.7058(4) 11.7723(3) 83.61(1) 71.540(9) 80.64(1) 944.13(5) 2 2.006 556 2.308 2.70 6 h 6 7.48 293(2) 8 6 h 6 10, 13 6 k 6 13, 15 6 l 6 15 7308 4257 (0.0146) 291/0/3586 1.019 R1 = 0.0292, wR2 = 0.0711 R1 = 0.0369, wR2 = 0.0760

Reflections collected Independent reflections (Rint) Parameter/restraints/data Goodness-of-fit Final R indices [I > 2r(I)]a,b R indices (all data) a b

P jjF o j  jF c jj= jF o j. hP i0:5 P wR2 ¼ wðF 2o  F 2c Þ2 = wðF 2o Þ2 . R1 ¼

P

atoms are linked to each other through imidazole N atoms to form a zigzag chain with Ag    Ag separations along ˚ (Fig. 1b). Two such zigAg    mbix    Ag being 14.93 A zag chains are consolidated by silver–silver interactions to form a 2D layer structure with 50-membered rings, possess˚ . It is worth mentioning ing an effective size of ca. 15 · 17 A that each layer network with (4, 4) topology is interlocked by two adjacent identical networks (above and below) as illustrated in Fig. 3c, forming a 3-fold-interpenetrated 2D framework. The nearest Ag    Ag separation between the ˚ . There is no significant p    p stacking 2D layers is 4.777 A among these 3-fold-interpenetrated 2D networks (the centroid-centroid distance between benzene and imidazole ˚ ). rings being 4.802 A 3.3. Structure of [(AgCN)4(bix)2]n (2) The asymmetric unit of compound 2 contains four crystallographically independent Ag atoms, four CN groups and two bix ligands. All the Ag atoms are in an approximately trigonal environment. The Ag1 atom is coordinated to N3 from the bix ligand [Ag1–N3 = ˚ ], C2 from the bridging cyanide [Ag1–C2 2.343(3) A ˚ ] and C1 from the l-C,C-cyanide ligand = 2.112(3) A ˚ ]. The Ag2 atom is surrounded by [Ag1–C1 = 2.177(3) A

˚ ], N5 from the other bix ligand [Ag2–N5 = 2.371(3) A ˚] N2 from the bridging cyanide [Ag2–N2 = 2.218(3) A and C1 from the l-C,C-cyanide ligand [Ag2–C1 = ˚ ]. The distance of Ag1    Ag2 [2.8462(4) A ˚ ] is 2.319(3) A a little shorter than that in 1, but a little longer than that in the structure of [Me4N][Ag2(CN)Cl] [26] and the compounds reported by Mak [29]. As shown in Fig. 2a, the bridging cyanide ligands and two pairs of Ag1    Ag2 bonds form a 10-membered ring with the bond angles Ag2–C1–Ag1 78.5(1) and Ag1–C2–N2A 171.1(3). The Ag3 atom is coordinated to C20 and N9 from the bridg˚; ing cyanides [Ag3–C20 = 2.091(3), Ag3–N9 = 2.341(3) A Ag3–C20–N10D = 176.5(3), Ag3–N9–C21 = 176.8(3)] ˚ ]. The and N8 from the bix ligand [Ag3–N8 = 2.215(3) A coordination environment of Ag4 is similar to that of Ag3. It is coordinated to C21 and N10 from the bridging cyanides [Ag4–C21 = 2.067(3), Ag4–N10 = ˚ ; Ag4–C21–N9 = 175.3(3), Ag4–N10–C20C = 2.424(3) A 175.1(3)] and N11 from the bix ligand [Ag4– ˚ ]. As illustrated in Fig. 2b, the Ag3 atom N11 = 2.195(2) A set and Ag4 atom sets are bridged alternately by C20N10 and C21N9 group sets to make up an [AgCN]n infinite zigzag chain with the bond angles N10–Ag4–C21 116.41(1) and N9–Ag3–C20 123.57(1). These zigzag chains connect the 10-membered rings through two bix

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Fig. 1. (a) Representation of the asymmetric unit of 1. (b) Representation of 50-membered rings in 1. (c) View of the 3-fold interpenetrating 2D network, the CN groups are omitted for clarity. Hydrogen atoms are omitted for clarity. Symmetry codes: (A) x  1/2, y + 1/2, z + 1/2; (B) x + 2, y + 2, z + 1.

ligands to generate a 2D framework with cavities (ca. ˚ ) along the a-axis. 16 · 13 A It is interesting and worth mentioning that the structure contains two independent layers that interpenetrate in a parallel manner. As shown in Fig. 2c, the two undulating layers pass through each other to generate an interpenetrating double-layer structure. The corrugations in the layers, which make the interpenetration possible, arise from the flexible bix ligands that adopt asyn-conformation coordination mode. Upon interpenetration, the total effective pore sizes have been drastically decreased for those 10-

membered rings that are located in the cavities along the a-axis. These 2-fold interpenetrating 2D networks are stacked into a 3D supramolecular structure through two types of weak p    p interactions (the centroid–centroid distance of imidazole rings between the adjacent layers ˚ , respectively) [30–32]. being 4.1 and 3.992 A 3.4. Structure of [(AgSCN)2(mbix)]n (3) The structure of 3 crystallizes in the triclinic space group P 1. Each asymmetric unit of 3 includes two Ag atoms, two

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Fig. 2. (a) A portion of the structure of 2. (b) View of the 2D framework extending along the a-axis, the bix ligand has been simplified as a wavelike line. (c) View of the 2-fold interpenetrating 2D network, the spacer stands for the bix ligand. Hydrogen atoms are omitted for clarity. Symmetry codes: (A) x + 1, y, z  1; (B) x + 1, y, z + 1; (C) x + 1, y, z; (D) x  1, y, z.

SCN groups and one mbix ligand. The Ag1 atom is coordinated to N3 from the mbix ligand [Ag1–N3 = 2.301 ˚ ], three different thiocyanate sulfur atoms [Ag1– (2) A S1 = 2.6133(8), Ag1–S2 = 2.6457(9) and Ag1–S2A = ˚ ], resulting in a highly distorted tetrahedral 2.6338(8) A coordination geometry for Ag1, with bond angles ranging from 92.09(3) to 134.18(3). The environment of the Ag2 atom is also badly deviated from a tetrahedral coordination geometry, surrounded by two N and one S atoms from SCN groups [Ag2–N1C = 2.440(3), Ag2–N2B = 2.358(3) ˚ ] and one N atom from the mbix and Ag2–S1 = 2.5469(9) A ˚ ], with bond angles in the ligand [Ag2–N5 = 2.211(2) A range 87.38(1)–125.40(7). The SCN groups act as triply-bridged ligands in the architecture of 3. The Ag–S bond lengths varying from ˚ are in the normal range [18,33–35]. 2.5469(9)–2.6547(9) A ˚ , which is shorter The Ag1    Ag2 distance is 3.1882(4) A ˚ ), sugthan twice the van der Waals radius of silver (3.4 A gesting the existence of an argentophilic interaction in 3 [18,26–28]. As shown in Fig. 3a, a pair of thiocyanate groups bridge a pair of Ag(I) atoms to form an eightmembered Ag2(SCN)2 macrocycle similar to the chair conformation of cyclohexane [Ag2–S1–C1 = 99.2(1), N1C–Ag2–S1 = 102.03(8) and S1–C1–N1 = 177.4(3)].

Since the sulfur atoms of the SCN groups act in a bridging fashion, four Ag–S bonds make up a four-membered centrosymmetric Ag2S2 ring [Ag1–S2–Ag1A = 87.91(3), S2– ˚ ]. The Ag1–S2A = 92.086(3), Ag1    Ag1A = 3.6708(5) A eight-membered macrocycle is connected to the fourmembered ring through Ag1–S1 bonds and Ag2C–N2A– C2A–S2A edges to form a one-dimensional [Ag2(SCN)2]n chain, which is unprecedented in the literature. The chains are further interlinked by mbix ligands that bridge the Ag atoms to produce a two-dimensional [(AgSCN)2(mbix)]n layer (Fig. 3c). Weak p    p interactions occur between adjacent layers to produce a 3D supramolecular structure (the centroid–centroid distance of the imidazole rings is ˚ , Fig. 3d) [30–32]. ca. 4.048 A 3.5. Luminescent properties As illustrated in Fig. 4, 1 displays a sharp purple emission band in the solid state at 424 nm (kex = 350 nm), while 2 exhibits a strong and broad emission band in the solid state at 440 nm (kex = 376 nm). Compound 3 emits a very sharp and intense emission band in the solid state at 461 nm upon excitation at 380 nm. The origin of the emission bands of 1 and 2 is neither metal-to-ligand charge

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Fig. 3. (a) A portion of the structure of 3. (b) Representation of a [AgSCN]n chain in 3. (c) Representation of the 2D network of 3. (d) Representation of the p    p interactions between adjacent layers. Hydrogen atoms are omitted for clarity. Symmetry codes: (A) x + 1, y + 2, z + 1; (B) x + 1, y, z; (C) x + 2, y + 2, z + 1; (D) x  1, y, z; (E) x, y + 1, z + 2; (F) x, 2  y, 1  z; (G) 1  x, 1  y, 2  z.

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Research on the Structure of Matter, Chinese Academy of Sciences (CAS), the Ministry of Science and Technology of China (001CB108906) and the National Science Foundation of China (20333070). Appendix A. Supporting material Supplementary data for the structures of 1–3 are available from the CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK on request; fax: +44 1223 336 033; e-mail: deposit@ccdc. cam.ac.uk or http://www.ccdc.cam.ac.uk, quoting the deposition numbers 604652–604654. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.poly.2006.07.034. Fig. 4. Solid-state emission spectra of 1 (kex = 350 nm), 2 (kex = 376 nm) and 3 (kex = 380 nm) at room temperature.

transfer (MLCT) nor ligand-to-metal charge transfer (LMCT) in nature and can be tentatively attributed to an intraligand emission because similar emissions are observed at 395 nm (kex = 334 nm) for free mbix and 392 nm (kex = 340 nm) for free bix ligands [24i]. The origin of the emission band of 3 may be attributed to the intraligand fluorescent emission where the electron is transferred from the p orbital of the thiocyanate groups to an unoccupied p* orbital of the mbix ligands [36]. The clear red shift of the emission bands may be associated with aromatic p    p stacking interactions in the solid state. 4. Conclusion The synthesis and crystal structures of 1–3 have been successfully synthesized via the solvothermal technique. In 1, the 3-fold interpenetration of adjacent (4, 4) layers produces a 2D network structure in which the CN groups act as terminal ligands. In 2, two undulating sheets interpenetrate each other to form a 2D network with the coexistence of linear-bridged CN groups and l-C,C-cyanide bridging groups. In the structure of 3, the SCN groups function as triply-bridging ligands and the structure of the [AgSCN]n subunit has not been previously observed for the silver thiocyanate system. The p    p interactions and silver–silver interactions play a significant role in the formation of the three polymers. Compounds 1–3 all exhibit strong photoluminescence at room temperature. The structures presented here show that the flexibility of the N-donor ligands can have dramatic effects on the structures of the silver–cyanide subunits. Further studies on the synthesis of related metal–cyanide polymers and their physical properties by using other imidazole ligands are already in progress. Acknowledgements This work was supported by grants from the State Key Laboratory of Structural Chemistry, Fujian Institute of

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