Coordination polymers have been intensely investigated due to their intriguing topologies and potential applications as functional materials[1-8]. Both the ligands and metal centrals play an important role in the construction of coordination polymers with fascinating topology and physicochemical properties. In the last few years, the MOFs based on the tripodal ligands bearing imidazole or triazole nitrogen atoms have gained great attention because of their diverse topologies and interesting properties[9-12]. The tripodal ligand 1, 3, 5-tris(1, 2, 4-triazol-1-ylmethyl)benzene (ttmb) has been selected for the construction of new coordination polymers based on the following structural features: (a) The flexible nature of -CH2-spacers allows the triazole group to bend and rotate freely as bridging ligand when it coordinates to metal centers; (b) It is a tripodal ligand with three triazole rings which may provide different coordination nodes to construct new coordination polymers with novel structures and properties[13, 14]. The dca anion is also a good ligand with multiple coordination modes in the synthesis of coordination polymers[15, 16].
The Zn(II) coordination polymers were widely synthesized because it can construct interesting topologies and good luminescence due to its d10 electron configuration[17-19]. In the present work, a new zinc coordination polymer {[Zn2(ttmb)2(dca)4(H2O)]·H2O}n (1·H2O) was synthesized by the reaction of 1, 3, 5-tris(1, 2, 4-triazol-1-ylmethyl)benzene (ttmb), Zn(II) and sodium dicyanamide (dca). The syntheses, crystal structure, luminescent property and thermal stability properties were studied.
1, 3, 5-Tris(1, 2, 4-triazol-1-ylmethyl) benzene (ttmb) was synthesized according to a literature method[14]. All other reagents were of analytical grade and used without purification. Elemental analyses for C, N and H were performed on a Perkin-Elmer 240C analyzer. IR spectra were obtained for KBr pellets on a Nicolet 170SX FT-IR spectrophotometer from 4000 to 400 cm–1. The luminescence measurement was carried out in the solid state at room temperature with a Perkin-Elmer LS50B spectrofluorimeter. TGA analyses were measured on a Thermal Analyst 2100 TA Instrument and SDT 2960 Simultaneous TGA-DTA Instrument in flowing dinitrogen at a heating rate 10 ℃·min–1.
A CH3OH (10 mL) solution of ttmb (0.161 g, 0.50 mmol) was slowly added to an aqueous solution (10 mL) of Zn(NO3)2·6H2O (0.159 g, 0.50 mmol) and Na(dca) (0.089 g, 1.00 mmol). The resultant solution was filtered and the filtrate was stood on the desk at room temperature for one month to give colorless block crystals 1·H2O in a 46% yield based on ttmb (0.0124 g). Anal. Calcd. for C38H34N30O2Zn2 (%) (1·H2O): C, 42.51; H, 3.19; N, 39.15%. Found: C, 42.38; H, 3.14; N, 39.08%. IR data (cm–1): 3416s, 2149s, 2133s, 1617w, 1526m, 1437w, 1384m, 1282m, 1131m, 1022m, 992w, 748m, 676m, 652m.
A suitable single crystal was carefully selected under an optical microscope and glued to thin glass fibers. The diffraction data were collected on a Rigaku Mercury CCD diffractometer at 293(2) K with graphite-monochromated MoKα radiation (λ = 0.71070 Å). Intensity data were collected by the ω scan technique. The structure was solved by direct methods using SHELXS-97 and refined with full-matrix least-squares technique using SHELXL-2015 program[20, 21]. The disordered lattice water molecule was deleted with the SQUEEZE method. The final R = 0.0529, wR = 0.1212 (w = 1/[σ2(Fo2) + (0.0518P)2 + 4.0684P], where P = (Fo2 + 2Fc2)/3), (Δ/σ)max = 0.000, S = 1.119, (Δρ)max = 0.440 and (Δρ)min = –0.370 e·Å–3.
1 exhibits a (3, 4)-connected 2D double-layer network with the point symbol of {63}{(66}. The coordination structure of 1 is shown in Fig. 1. The selected bond lengths and bond angles are listed in Table 1. X-ray single-crystal structural analysis shows that 1 crystallizes in the monoclinic space group of P21/c. The asymmetric unit of 1 consists of two Zn(II) cations (Zn(1) and Zn(2)), two ttmb and four dca anion ligands and one coordination water molecule. There are two voids (ca. 117 Å3) in the cell, occupied by the disordered lattice water molecule. Zn(1) is coordinated by three 4-position triazole nitrogen atoms from three distinct ttmb ligands (N(3), N(6A), N(9B)) and three nitrogen atoms from three dca anion ligands (N(20), N(23), N(25)) in the distorted octahedral geometry. Zn(2) is in the distorted octahedral coordination geometry and coordinated by three 4-position triazole nitrogen atoms from three different ttmb ligands (N(12), N(15C), N(18D)), two nitrogen atoms from two dca anion ligands (N(21), N(29)) and one oxygen atom from water molecule (O(1)). The Zn(2)–O(1) bond length (2.406(3) Å) is relatively longer. There are three kinds of dca anion ligands. The first one has the bridging coordination mode and links Zn(1) and Zn(2) atoms with its two cyano-nitrogen atoms (N(20), N(21)) with the Zn(1)···Zn(2) distance of 8.6466(8) Å. The second one has the terminal coordination mode and links one Zn atom with its one cyano-nitrogen atom (N(23), N(29)) with the Zn(1)–N(23) bond in 2.122(3) Å and Zn(2)–N(29) in 2.121(3) Å. The third one has the monodentate coordination mode and coordinates Zn(1) with its amide nitrogen atom (N(25)) with relatively longer Zn(1)–N(25) bond length (2.383(3) Å).
Symmetry cods: A: x, 3/2 – y, –1/2 + z; B: x, 1/2 – y, –1/2 + z; C: x, 1 + y, z; D: x, 1/2 – y, 1/2 + z
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| Bond | Dist. | Bond | Dist. | Bond | Dist. | ||
| Zn(1)–N(3) | 2.128(3) | Zn(1)–N(23) | 2.122(3) | Zn(2)–N(15C) | 2.115(3) | ||
| Zn(1)–N(6A) | 2.091(3) | Zn(1)–N(25) | 2.383(3) | Zn(2)–N(18D) | 2.126(3) | ||
| Zn(1)–N(9B) | 2.126(3) | Zn(2)–O(1) | 2.406(3) | Zn(2)–N(21) | 2.116(3) | ||
| Zn(1)–N(20) | 2.169(3) | Zn(2)–N(12) | 2.102(3) | Zn(2)–N(29) | 2.121(3) | ||
| Angle | (°) | Angle | (°) | Angle | (°) | ||
| N(3)–Zn(1)–N(6A) | 175.57(10) | N(9B)–Zn(1)–N(23) | 94.24(13) | N(12)–Zn(2)–N(15C) | 93.09(11) | ||
| N(3)–Zn(1)–N(9B) | 89.44(11) | N(9B)–Zn(1)–N(25) | 176.55(11) | N(12)–Zn(2)–N(18D) | 91.21(10) | ||
| N(3)–Zn(1)–N(20) | 88.37(11) | N(20)–Zn(1)–N(23) | 170.69(13) | N(12)–Zn(2)–N(21) | 96.32(12) | ||
| N(3)–Zn(1)–N(23) | 90.11(12) | N(20)–Zn(1)–N(25) | 83.23(12) | N(12)–Zn(2)–N(29) | 96.80(12) | ||
| N(3)–Zn(1)–N(25) | 87.59(10) | N(23)–Zn(1)–N(25) | 87.53(13) | N(15C)–Zn(2)–N(18D) | 175.29(10) | ||
| N(6A)–Zn(1)–N(9B) | 93.81(11) | O(1)–Zn(2)–N(12) | 175.90(11) | N(15C)–Zn(2)–N(21) | 90.90(11) | ||
| N(6A)–Zn(1)–N(20) | 88.36(11) | O(1)–Zn(2)–N(15C) | 89.81(11) | N(15C)–Zn(2)–N(29) | 89.09(11) | ||
| N(6A)–Zn(1)–N(23) | 92.64(11) | O(1)–Zn(2)–N(18D) | 85.99(11) | N(18D)–Zn(2)–N(21) | 90.53(11) | ||
| N(6A)–Zn(1)–N(25) | 89.06(11) | O(1)–Zn(2)–N(21) | 80.72(12) | N(18D)–Zn(2)–N(29) | 88.50(11) | ||
| N(9B)–Zn(1)–N(20) | 94.93(12) | O(1)–Zn(2)–N(29) | 86.14(12) | N(21)–Zn(2)–N(29) | 166.86(13) | ||
| Symmetry transformations used to generate the equivalent atoms: A: x, 3/2 – y, –1/2 + z; B: x, 1/2 – y, –1/2 + z; C: x, 1 + y, z; D: x, 1/2 – y, 1/2 + z | |||||||
Each ttmb ligand serves as a tridentate ligand to connect three Zn atoms to form the [Zn(ttmb)]n two-dimensional network (Fig. 2) simplified as a 3-connected 63-hcb net according to the topological analysis[22]. Adjacent two [Zn(ttmb)]n 2D networks are linked by the bridging dca anion ligands to construct the [Zn2(ttmb)2(dca)]n two-layer network (Fig. 3). There are hydrogen bonding interactions between the hydrogen atoms of coordination water and cyano-nitrogen atom of dca ligands (O(1)–H(1B)···N(24) (–1 + x, y, z) 2.922(6) Å).
The [Zn2(ttmb)2(dca)]n two-layer network can be simplified as a (3, 4)-connected 2-nodal 2D topology with the point symbol of {63}{66} (Fig. 4)[22], which is different from of those of (3, 4)-connected coordination polymers[23-26]. Some (3, 4)-connected coordination polymers were presented for comparison. For example, [Mn3(O2N-btb)2(bpp)2(H2O)4] (O2N-H3btb = 5-nitro-1, 2, 3-benzenetricarboxylic acid, bpp = 1, 3-di(4-pyridyl)propane) and {[Zn(bismip)(bimb)]2·2H2O·DMF}n (H2bismip = 5-(1H-benzoimidazol-2-ylsulfanylmethyl)-isophthalic acid, bimb = 4, 4-bis(1-imidazolyl)bibenzene) have 2D (3, 4)-connected (42·6)(42·63·8) framework[23, 24]. {Zn2(bismip)2(phen)}n (phen = 1, 10-phenanthroline) exhibits a 3D (3, 4)-connected network with the point symbol of (83)(86)[24]. [Cu3(L)3(BTC)2]3n (L = N, N΄-bis(4-pyridinecarboxamide)-1, 4-benzene, BTC = 1, 3, 5-benzenetricarboxylate) presents a (3, 4)-connected 3D topology with the point symbol of (63)(64·102)[25]. {[Co4(OH)2(adc)6(H2O)5][Co2(OH)(btrb)]2· 8H2O}n (H2adc = 1, 3-adamantanedicarboxylic acid, btrb = 1, 4-bis(1, 2, 4-triazol-4-ylmethyl)benzene) shows a (3, 4)-connected 2D network with a point symbol of (4·62)2(42·62·82) based on the [Co4(μ3-OH)2] and [Co2(μ3-OH)] units[26].
In the IR spectra of 1·H2O, the strong absorption band at 3416 cm–1 is assigned to the stretching vibration of water molecules. The absorption peaks at 1526 and 1282 cm–1 are ascribed to the triazole rings of ttmb[13, 14]. The strong absorptions at 2149 and 2133 cm–1 result from the stretching bands of dca[15, 16].
The solid-state luminescence of 1·H2O and ttmb ligand was investigated at room temperature and its emission spectra are shown in Fig. 5. The ttmb ligand shows the emission maxima around 491 nm upon excitation at 396 nm. 1·H2O displays the strong luminescence emission at 485 nm upon excitation at 384 nm, which can be attributed to the intraligand (π-π*) charge transition for free ttmb ligand[17-19].
TG experiment was carried out to explore thermal stability of the title compound (Fig. S1). The water molecules of 1·H2O were completely lost at 152 ℃ (calcd.: 3.36%, found: 3.27%). The anhydrous framework was stable up to 274 ℃. Then the weight loss continuously occurred and did not end until 800 ℃. The residue at 800 ℃ (found: 16.26%) should be ZnO (calcd.: 15.16%) and a small amount of carbon particles.
A new zinc coordination polymer with (3, 4)-connected 2D double-layer network was synthesized by the reaction of 1, 3, 5-tris(1, 2, 4-triazol-1-ylmethyl)benzene, Zn(II) salt and sodium dicyanamide, which shows the topology with the point symbol of {63}{(66}. This Zn coordination polymer gives a strong luminescence emission band at 485 nm.
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Figure 1 Coordination environment of the zinc atoms in 1, with displacement ellipsoids drawn at the 30% probability level.
Symmetry cods: A: x, 3/2 – y, –1/2 + z; B: x, 1/2 – y, –1/2 + z; C: x, 1 + y, z; D: x, 1/2 – y, 1/2 + z
Figure 3 [Zn2(tmb)2(dca)]n two-layer network in 1 (The non-bridging dca ligands and coordination water molecule were omitted for clarity)
Figure 4 (3, 4)-Connected 2D network in 1. The bright green balls show 3-connected ttpe ligands. Red sticks exhibit the bridging dca ligands
Table 1. Selected Bond Lengths (Å) and Bond Angles (°) of 1
| Bond | Dist. | Bond | Dist. | Bond | Dist. | ||
| Zn(1)–N(3) | 2.128(3) | Zn(1)–N(23) | 2.122(3) | Zn(2)–N(15C) | 2.115(3) | ||
| Zn(1)–N(6A) | 2.091(3) | Zn(1)–N(25) | 2.383(3) | Zn(2)–N(18D) | 2.126(3) | ||
| Zn(1)–N(9B) | 2.126(3) | Zn(2)–O(1) | 2.406(3) | Zn(2)–N(21) | 2.116(3) | ||
| Zn(1)–N(20) | 2.169(3) | Zn(2)–N(12) | 2.102(3) | Zn(2)–N(29) | 2.121(3) | ||
| Angle | (°) | Angle | (°) | Angle | (°) | ||
| N(3)–Zn(1)–N(6A) | 175.57(10) | N(9B)–Zn(1)–N(23) | 94.24(13) | N(12)–Zn(2)–N(15C) | 93.09(11) | ||
| N(3)–Zn(1)–N(9B) | 89.44(11) | N(9B)–Zn(1)–N(25) | 176.55(11) | N(12)–Zn(2)–N(18D) | 91.21(10) | ||
| N(3)–Zn(1)–N(20) | 88.37(11) | N(20)–Zn(1)–N(23) | 170.69(13) | N(12)–Zn(2)–N(21) | 96.32(12) | ||
| N(3)–Zn(1)–N(23) | 90.11(12) | N(20)–Zn(1)–N(25) | 83.23(12) | N(12)–Zn(2)–N(29) | 96.80(12) | ||
| N(3)–Zn(1)–N(25) | 87.59(10) | N(23)–Zn(1)–N(25) | 87.53(13) | N(15C)–Zn(2)–N(18D) | 175.29(10) | ||
| N(6A)–Zn(1)–N(9B) | 93.81(11) | O(1)–Zn(2)–N(12) | 175.90(11) | N(15C)–Zn(2)–N(21) | 90.90(11) | ||
| N(6A)–Zn(1)–N(20) | 88.36(11) | O(1)–Zn(2)–N(15C) | 89.81(11) | N(15C)–Zn(2)–N(29) | 89.09(11) | ||
| N(6A)–Zn(1)–N(23) | 92.64(11) | O(1)–Zn(2)–N(18D) | 85.99(11) | N(18D)–Zn(2)–N(21) | 90.53(11) | ||
| N(6A)–Zn(1)–N(25) | 89.06(11) | O(1)–Zn(2)–N(21) | 80.72(12) | N(18D)–Zn(2)–N(29) | 88.50(11) | ||
| N(9B)–Zn(1)–N(20) | 94.93(12) | O(1)–Zn(2)–N(29) | 86.14(12) | N(21)–Zn(2)–N(29) | 166.86(13) | ||
| Symmetry transformations used to generate the equivalent atoms: A: x, 3/2 – y, –1/2 + z; B: x, 1/2 – y, –1/2 + z; C: x, 1 + y, z; D: x, 1/2 – y, 1/2 + z | |||||||
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