Hydrothermal synthesis and characterization of a novel luminescent lead(II) framework extended by novel Pb-μ1,1-(N)CS-Pb bridges

Journal of Molecular Structure 920 (2009) 248–251

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Hydrothermal synthesis and characterization of a novel luminescent lead(II) framework extended by novel Pb-l1,1-(N)CS-Pb bridges Bin Ding *, Yuan-Yuan Liu, Xiao-Jun Zhao *, En-Cui Yang, Xiu Guang Wang College of Chemistry and Life Chemistry, Tianjin Normal University, Tianjin 300387, PR China

a r t i c l e

i n f o

Article history: Received 12 October 2008 Received in revised form 28 October 2008 Accepted 31 October 2008 Available online 17 November 2008 Keywords: 3-Carboxylic acid-4H-1,2,4-triazole l1,1-NCS Tetra-dentate Luminescent

a b s t r a c t Under hydrothermal conditions, using a triazole derivative ligand 3-carboxylic acid-4H-1,2,4-triazole (HL), lead(II) oxide and NH4NCS, a novel two-dimensional (2D) lead(II) framework {[Pb(L)(l1,1NCS)(H2O)]}n (1) can be isolated. For 1 NCS anions adopt novel l1,1-(N)CS bridges, which are further extended by tetra-dentate coordinated L ligands to form a 2D corrugated layered structure. Solid-state luminescent spectrum of 1 shows that the excited peak at 375 nm while the low-energy emission peak at 607 nm. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction During the last two decades metal–organic coordination polymers have aroused great interest for chemists due to their extremely versatile coordination motifs and potential applications in many areas such as light-emitting diodes (LEDs), catalytic, magnetic and so on [1–4]. Especially the lead(II) frameworks also have attracted great interest based on below points: (1) large ion radius, flexible variable coordination number from 2 to 10 and diverse coordination geometries for PbII ions make there is great potential in the construction of functional materials; (2) The intrinsic feature of PbII, the presence of a 6S2 outer electron, make PbII frameworks have unique applications in coordination chemistry, photo-physics and photo-chemistry; (3) Inorganic–organic hybrid materials containing infinite Pb–O–Pb linkage also have aroused great interest [5]. Pseudo-halide NCS ligand is a favorable building block in the construction of novel metal–organic frameworks because it can adopt versatile coordination modes such as terminal N-, S-, or l1,3-, l1,1-bridging modes. Of all the coordination modes, l1,1(N)CS bridging way is scarcely reported and is also particular interest due to their important applications in magnetic systems, crystal engineering and so on [6]. On the other hand, 1,2,4-triazole and its derivates are also good building blocks for constructing novel functional materials because they combine the coordination modes of

* Corresponding authors. E-mail addresses: [email protected] (B. Ding), xiaojun_zhao15@yahoo. com.cn (X.-J. Zhao). 0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2008.10.064

pyrazole and imidazole [7]. In particular, some Zn(II), Cd(II) and Ag(I) complexes with these ligands have been isolated exhibiting that they are good candidates for photo-luminescent materials [8–9]. Recently a novel 3D porous Cd(II) MOF built with 4-pyridine-3-1,2,4-triazole has been isolated by us, its photo-luminescent properties can be tuned by the number of guest molecules [10]. However, to date, quite few luminescent lead(II) frameworks with 1,2,4-triazole and its derivatives are reported. There is still great potential in exploring this area. 3-Carboxylic acid-4H-1,2,4-triazole (HL) contains 1,2,4-triazole and one carboxylic acid functional group, to date, only one example about HL has been reported [11]. In this work HL was synthesized, under hydrothermal conditions a novel two-dimensional (2D) lead(II) framework {[Pb(L)(l1,1-NCS)(H2O)]}n (1) can be isolated. For 1 novel Pb-l1,1-(N)CS-Pb bridges can be observed, which are further extended by tetra-dentate coordinated L ligands to form a 2D corrugated layered structure. Solid-state luminescent spectrum of 1 shows that the excited peak at 375 nm while the low-energy emission peak at 607 nm. 2. Experimental section 2.1. General remarks Deionized water was used as solvent in this work. The ligand HL was prepared by the method described in the literature [12]. Other reagents were purchased commercially and used without further purification. C, H and N microanalyses were carried out with a Perkin-Elmer 240 elemental analyzer. FT-IR spectrum was recorded

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B. Ding et al. / Journal of Molecular Structure 920 (2009) 248–251 Table 1 Crystal data and structure refinement information for compounds 1a

Table 2 Selected bond length (Å) and bond angle (°) for 1a.

1

1

Empirical formula Formula weight Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z q (g/cm3) l (mm 1) F(0 0 0) Crystal size (mm3) h (°) Limiting indices

Reflections/collected/unique Data/restraints/parameters Good of fit on F2 Final R indices (I > 2r (I)) R indices (all data) Largest diff. peak and hole (e
Å a

R1 = R||Fo|

3

)

|Fc||/|Fo|, wR2 = [Rw(F 2o

C4H3N4O3PbS 394.35 Monoclinic P2(1)/c 6.9672(9) 13.4568(16) 9.6163(12) 90 110.715(2) 90 843.30(18) 4 3.106 20.233 708 0.28  0.24  0.22 2.72 to 25.02 8h8 16  k  10 10  l  11 4158/1486 [R(int) = 0.0354] 1486/0/119 1.101 R1 = 0.0244, wR2 = 0.0618 R1 = 0.0256, wR2 = 0.0624 1.563 and 2.173 F 2c )2/Rw(F 2o )2]1/2.

from KBr pellets in the range 4000–400 cm 1 on a Bio-Rad FTS 135 spectrometezr. Fluorescence spectrum was taken on a Varian Cary Eclipse spectrometer. 2.2. Preparation of compound 1 A mixture of PbO (44.6 mg, 0.2 mmol), H2L (22.4 mg, 0.2 mmol), NH4NCS (15.2 mg, 0.2 mmol) and H2O (18 mL) were placed in a 25 mL Teflon-lined steel vessel and heated to 140 °C for 5 days, then cooled to room temperature. The resulting colorless blockshaped crystals of 1 were washed several times by water and diethyl ether. The yield is 45% based on Pb. Elemental analysis (%) calcd for 1: C 12.19%, H 0.77%, N 14.21%; found: C 12.01%, H 0.81%, N 14.29%.

Pb(1)–N(1) Pb(1)–N(2)#1 Pb(1)–N(4)#2 N(1)–Pb(1)–O(1) O(1)–Pb(1)–N(2)#1 O(1)–Pb(1)–N(4) N(1)–Pb(1)–N(4)#2 N(2)#1–Pb(1)–N(4)#2 N(1)–Pb(1)–O(2)#1 N(2)#1–Pb(1)–O(2)#1 N(4)#2–Pb(1)–O(2)#1

2.525(4) 2.677(5) 2.725(5) 64.69(12) 105.97(15) 135.20(13) 74.23(15) 141.42(14) 109.27(13) 61.79(12) 152.57(13)

Pb(1)–O(1) Pb(1)–N(4) Pb(1)–O(2)#1 N(1)–Pb(1)–N(2)#1 N(1)–Pb(1)–N(4) N(2)#1–Pb(1)–N(4) O(1)–Pb(1)–N(4)#2 N(4)–Pb(1)–N(4)#2 O(1)–Pb(1)–O(2)#1 N(4)–Pb(1)–O(2)#1

2.621(4) 2.699(5) 2.746(4) 75.22(15) 72.49(14) 73.89(13) 81.36(13) 74.87(16) 76.27(12) 132.52(13)

a Symmetry transformations used to generate equivalent atoms: for #1 x, z 1/2; #2 x+2, y, z+1.

y+1/2,

2.3. Crystal structure determination Diffraction intensities for 1 was collected on a computer-controlled Bruker SMART 1000 CCD diffractometer equipped with graphite-monochromated Mo-Ka radiation with radiation wavelength 0.71073 Å by using the x-u scan technique. Lorentz polarization and absorption corrections were applied. The structure was solved by direct methods and refined with the full-matrix leastsquares technique using the SHELXS-97 and SHELXL-97 programs [13,14]. Anisotropic thermal parameters were assigned to all non-hydrogen atoms. The organic hydrogen atoms were generated geometrically. Analytical expressions of neutral-atom scattering factors were employed, and anomalous dispersion corrections were incorporated. The crystallographic data and details of refinements for 1 are summarized in Table 1; selected bond lengths and angles are listed in Table 2. CCDC-692328 (1) contains the supplementary crystallographic data for this work. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. 3. Results and discussion For 1 single crystal X-ray analysis shows that the asymmetrical structure unit of 1 contains one crystallographic independent PbII ion (Pb1), one l1,1-(N)CS bridges, one de-protonated L and one

Fig. 1. The coordination environments of central PbII ions in 1.

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B. Ding et al. / Journal of Molecular Structure 920 (2009) 248–251

Fig. 2. Novel Pb-l1,1-(N)CS-Pb bridges extended by tetra-dentate coordinated L ligands into a 2D corrugated layered structure (H atoms are omitted for clarity).

terminal coordinated water molecule (O3). As shown in Fig. 1, Pb1 is in the hemi-directed geometry and seven-coordinated by one water molecule (O3), two bridged l1,1-(N)CS nitrogen atoms (N4 and N4A), two carboxylate oxygen atoms (O1 and O2) and two triazole atoms (N1 and N2) forming a quite distorted pentagonal bipyramid geometry. The Pb(II) coordination geometry has close relationship with the 6s electrons. The stereochemical active 6s electrons of Pb usually produce hemi-directed coordination geometry, however, the difference between hemi-directed and holo-directed geometry around Pb(II) is ambiguous sometimes [15]. With the extending of bonding limit, the potential ligands, which are a little far from Pb(II) center due to the repulsion of the 6s electrons, will be found and the hemi-directed coordination sphere will be completed to holo-directed. For 1 the hemi-directed geometries of Pb1 also can be

extended by weak Pb???S interactions (Pb(1)–S(1), 3.292(2) Å) into holo-directed geometries. The carboxylic acid group of HL is de-protonated, N2, N3, O1 and O2 of L are involved in the coordination of two central lead(II) ions (Pb1 and Pb1A) forming tetra-dentate coordination modes. The result is also different form previously reported bi-dentate chelate coordination modes [11]. All the bond distances and angles are listed in the Table 2, all the Pb–O and Pb–N bond distances are below 2.8 Å, such bond distances are reasonable and can find a nearly ideal value assumed for oxidation state II on the PbII ions. The five-membered triazole ring is almost coplanar with mean deviation of 0.0014(3) Å. As shown in Fig. 1, two l1,1-NCS ligands act as bridging ligands to join neighboring lead(II) centers (Pb1 and Pb1A) forming novel Pb-(l1,1-NCS)-Pb bridges, which further extend 1 into a 2D corrugated layered structure. For Pb-(l1,1-NCS)-Pb bridges the separation of Pb1 and Pb1A is 4.307(5) Å and the corresponding angle of Pb–N–Pb is 105.1°. Further these dimeric lead(II) building blocks are arranged in two different directions and are extended by bridging L ligands forming a 2D corrugated layer structure parallel to crystallographic ac plane (Fig. 2). For 1 neighboring 2D layers stacking in ABAB. . . way. The interlayer Pb


Pb separations in the two neighboring layers are 7.645(1) Å. Between these neighboring 2D layers coordinated water molecule (O3), carboxylate oxygen atom O2 generate strong O–H


O hydrogen bonds, further consolidating the overall framework structure (O3–H3B


O2, 2.849 Å) (Fig. 3). Though the lead(II) frameworks containing l1,3-NCS bridging ligands have been reported, the framework structures containing l1,1-NCS bridging ligands are still scarcely reported. FT-IR spectrum of compound 1 exhibits the typical characteristic band centered at 1986 cm 1 expected for 1,1-l-bridging thiocyanate ligands [16]. A strong and broad absorption band centered at 3413 cm 1, which can be ascribed to the presence of coordinated water molecules in the structure. The absence of the bands at ca. 1700 cm 1 indicates that the carboxylic acid group is de-protonated. The triazole out of plane ring absorption can be observed at 644 cm 1 [17]. The solid-state luminescent spectrum of 1 also has been recorded at room temperature indicating a low-energy luminescence emission peak at 607 nm upon excitation at 375 nm (Fig. 4). The low-energy emissions associated with large Stokes shifts can be observed for s2-metal complexes, which can be assigned to a metal-centered transition involving the s and p

Fig. 3. A 3D supra-molecular packing architecture indicating strong O–H
O hydrogen bonds (purple lines) consolidate the overall framework structure.

B. Ding et al. / Journal of Molecular Structure 920 (2009) 248–251

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mode, which also reveals that thiocyanate co-ligand has great potential in the construction of novel inorganic–organic frameworks. Acknowledgment This work was supported financially by Tianjin Normal University (5RL055 and 52LX25), Tianjin Civil Aviation University of China (08CAUC-S01). References [1] [2] [3] [4] [5] [6]

590

600

610

620

630

640

[7] [8]

Fig. 4. The solid luminescent spectrum of compound 1. [9]

metal orbital as proposed by Vogler. Thus the emission band of 1 can be assigned to a metal-centered s–p transition.

[10]

4. Conclusions

[11] [12] [13]

In summary, under hydrothermal conditions, using a triazole derivative ligand 3-carboxylic acid-4H-1,2,4-triazole (HL), lead(II) oxide and NH4NCS, a novel two-dimensional (2D) lead(II) framework {[Pb(L)(l1,1-NCS)(H2O)]}n (1) can be isolated. For 1 NCS anions adopt novel l1,1-(N) bridges, which are further extended by tetra-dentate coordinated L ligands to form a 2D corrugated layered structure. NCS also represents a novel l1,1-(N)CS bridging

[14] [15]

[16] [17]

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