A Novel Chiral Cd(II) Coordination Polymer Based on Achiral

Aug 10, 2006 - Synopsis. A unique chiral three-dimensional coordination polymer of [Cd(AmTAZ)Cl]n (AmTAZH = 3-amino-1,2,4-triazole) with two types of ...
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CRYSTAL GROWTH & DESIGN

A Novel Chiral Cd(II) Coordination Polymer Based on Achiral Unsymmetrical 3-Amino-1,2,4-triazole with an Unprecedented µ4-Bridging Mode

2006 VOL. 6, NO. 9 2136-2140

Wei Li, Hong-Peng Jia, Zhan-Feng Ju, and Jie Zhang* State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China ReceiVed June 16, 2006

ABSTRACT: A novel chiral cadmium coordination polymer, Cd(AmTAZ)Cl (AmTAZH ) 3-amino-1,2,4-triazole), has been solvothermally synthesized and structurally characterized by IR, elemental analysis, thermogravimetric analysis, and singlecrystal X-ray diffraction. This compound crystallizes in the chiral space group P212121 with a ) 6.4126(5), b ) 7.8307(9), c ) 10.4658(10) Å, V ) 525.54(9) Å3, and Z ) 4. Interestingly, the amino group of the AmTAZ ligand is also coordinated to a cadmium ion besides three other N atoms of the triazolate ring, which “locks” the asymmetry of the AmTAZ and may transfer this asymmetry throughout the crystal structure to “direct” the formation of a chiral network. The structure possesses a unique three-dimensional open-framework and contains two different types of helical channels with left- and right-handed character presented alternately along the c-axis. The compound exhibits second-order nonlinear optical (NLO) activity, intense blue fluorescent emission, and high thermal stability. Introduction

Experimental Section

The designed construction of chiral coordination polymers are of great current interest because of their unique applications, such as asymmetric heterogeneous catalysis, enantioselective separations, biomimetic chemistry, and optical materials.1-4 The chiral coordination polymers can be synthesized by using either chiral organic linkers or chiral metal complexes as structuredirecting agents5 or by using achiral ligands under a spontaneous resolution without any chiral auxiliary.6 Recently, excellent work performed by Lin et al. has demonstrated that combining achiral unsymmetrical bridging ligands and metal centers with welldefined coordination geometries or employing in situ generation of unsymmetrical organic ligands under hydrothermal conditions are all effective strategies.2c,4d,7 It seems that the asymmetry of the organic ligands can be transferred to certain building blocks of these compounds, which can further connect with each other through organic ligands to produce chiral networks and keep the initial asymmetry.7j Nevertheless, to our knowledge, few three-dimensional (3D) chiral coordination polymers based on achiral unsymmetrical ligands have been reported except for some carboxylic and pyridyl bridged ligands, etc.2c,4d,7,8 Along these lines, we focused our attention on the metal/unsymmetrical bridging 3-amino-1,2,4-triazolate system with the idea that the structures of achiral unsymmetrical bridging ligands play a significant role in the recognition and crystallization of metal centers into chiral networks.7 As a result, we present a unique 3D chiral network Cd(AmTAZ)Cl (1) based on the linkages between cadmium ions and achiral unsymmetrical 3-amino-1,2,4 triazolate anions (AmTAZH ) 3-amino-1,2,4-triazole) in an unprecedented µ4-bridging mode, in which the asymmetry of the AmTAZ was “locked” by the coordination of the amino group with a cadmium cation and might be transferred throughout the crystal structure to “direct” the formation of a chiral network. The compound exhibits the second-order nonlinear optical (NLO) activity, intense blue fluorescent emission, and high thermal stability.

Materials and General Methods. All chemicals are of analyticalreagent grade and were used as received. IR spectra were recorded with a Perkin-Elmer Spectrum One FT-IR spectrophotometer on KBr pellets in the range 4000-400 cm-1. Elemental analysis was determined with an Elementar Vario EL III elemental analyzer. Fluorescence spectra were recorded on a Perkin-Elmer LS 55 luminescence spectrometer. The thermal analysis was performed under air with a Netzsch STA449C thermal analyzer. Synthesis of Cd(AmTAZ)Cl (1). Yellow crystals of 1 were obtained by the solvothermal reaction of CdCl2, AmTAZH in an ethanol-water system. In a typical synthesis of 1, CdCl2 (0.114 g, 0.5 mmol) was added to 10 mL of ethanol-water (4/6, vol/vol) solution with stirring. After a few minutes, AmTAZH (0.137 g, 1 mmol) was added to the above mixture, and stirring was continued for 4 h. The final mixture with CdCl2/AmTAZH/C2H5OH/H2O in a molar ratio of 1:2:136:666 was heated in a Teflon-lined stainless steel autoclave at 150 °C for 3 days under autogenous pressure and then cooled to room temperature. The pure yellow prism-like crystals of 1 suitable for X-ray analysis were obtained in 20% yield based on Cd. If 0.04 g of NaOH (1 mmol) was further added to the above reaction mixture under stirring to deprotonate the AmTAZH, the compound 1 in polycrystalline form was obtained under the same solvothermal conditions with a much higher yield (60% based on Cd, the XPRD pattern; see Figure 1), no highquality single-crystal could be obtained. Anal. Calcd for C2H3N4ClCd (M ) 230.93): C 10.40; H 1.31; N 24.27%. Found: C 10.29; H 1.47; N 23.46%. IR (KBr pellet, cm-1): 3436 (m), 3306(m), 3231(m), 1634(w), 1579(s), 1509(s), 1483(s), 1454 (m), 1396(s), 1288(m), 1211(m), 1194(m), 1070(s), 977(s), 893(w), 713(m), 670(m), 457(m) (Figure S1, Supporting Information). X-ray Crystallography Study. Crystal structure determination of 1 with dimensions of 0.30 × 0.10 × 0.10 mm was performed on a Rigaku Mercury CCD diffractometer with graphite-monochromated Mo KR radiation (λ ) 0.71073 Å) at 293 K. An empirical absorption correction for 1 was performed using the CrystalClear program.9 The structure was solved by direct methods and successive Fourier difference syntheses (SHELXS-97) and refined by full-matrix least-squares procedure on F2 with anisotropic thermal parameters for all nonhydrogen atoms (SHELXL-97).10 All non-hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms were located by geometric calculations. Crystal 1 was found to be an inversion twin with Flack parameters of 0.51(9). Crystal data and details on refinements for 1 are summarized in Table 1. Selected bond distances and angles are listed in Table 2.

* To whom correspondence should be addressed. Fax: 86-591-83710051. E-mail: [email protected].

10.1021/cg060363w CCC: $33.50 © 2006 American Chemical Society Published on Web 08/10/2006

Novel Chiral Cd(II) Coordination Polymer

Crystal Growth & Design, Vol. 6, No. 9, 2006 2137

Figure 1. XPRD patterns for 1: (a) Calculated pattern from singlecrystal X-ray data; (b) polycrystalline diffraction pattern of the as-synthesized compound in case of alkalinization. Table 1. Crystal Data and Structure Refinement for 1 empirical formula formula weight crystal system space group a/Å b/Å c/Å V/Å3 Z Dc/g cm-3 µ (mm-1) λ/Å (Mo KR) F(000) θ range (deg) Flack parameter GOF on F2 final R1, wR2 [I > 2σ(I)]a R1, wR2 (all data) a

C2H3N4ClCd 230.93 orthorhombic P212121 6.4126(5) 7.8307(9) 10.4658(10) 525.54(9) 4 2.919 4.537 0.71073 432 3.25-27.48 0.51(9) 1.085 0.0272, 0.0565 0.0294, 0.0584

Figure 2. The structure of 1, showing the local coordination geometry. Atom labels having “A”, “B”, and “C” refer to symmetry-generated atoms.

R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) {∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]}1/2. Table 2. Select Bond Lengths (Å) and Angles (deg) for 1a

Cd1-N2 Cd1-N3B Cd1-Cl1 N2-Cd1-N1A N1A-Cd1-N3B N1A-Cd1-N4C N2-Cd1-Cl1 N3B-Cd1-Cl1 N2-Cd1-Cl1D N3B-Cd1-Cl1D Cl1-Cd1-Cl1D

Bond Lengths 2.277(5) Cd1-N1A 2.311(5) Cd1-N4C 2.6339(17) Cd1-Cl1D Bond Angles 172.48(19) N2-Cd1-N3B 86.91(19) N2-Cd1-N4C 82.72(17) N3B-Cd1-N4C 92.40(14) N1A-Cd1-Cl1 90.80(15) N4C-Cd1-Cl1 89.81(14) N1A-Cd1-Cl1D 88.06(15) N4C-Cd1-Cl1D 177.66(3)

2.300(5) 2.526(5) 2.6799(17) 99.92(17) 90.06(17) 166.93(18) 90.63(14) 97.23(12) 87.27(14) 83.53(12)

a Symmetry transformations used to generate equivalent atoms were as follows: (A) -1 - x, 0.5 + y, -0.5 - z; (B) -1.5 - x, -1 - y, 0.5 + z; (C) 0.5 + x, -0.5 - y, -1 - z; (D) -1 - x, -0.5 + y, -0.5 - z.

Results and Discussion Description of Structure. X-ray crystallographic analysis reveals that the title compound possesses a 3D coordination framework with a chiral space group P212121. The asymmetric unit of 1 contains one unique Cd2+, one AmTAZ, and one Clanion (Figure S2, Supporting Information). As depicted in Figure 2, each Cd2+ center has a six-coordinate, distorted octahedral geometry with four N atoms from four different AmTAZ moieties lying in the equatorial plane and two Cl- anions in the axial positions. The Cd-N and Cd-Cl distances are in the range of 2.277(5)-2.526(5) Å and 2.634(2)-2.680(2) Å, and the angles of N-Cd-N, Cd-Cl-Cd, and Cl-Cd-Cl are about 86.9(2)°-172.5(2)°, 94.98(5)°, and 177.66(3)°, respectively. All four nitrogen atoms in the AmTAZ ring are coordinated to four separate Cd2+ centers; the alternate connectivity between the

Figure 3. View of 3D framework based on the linkages of Cd2+ ions and AmTAZ groups along the approximate [010] and [001] directions, showing the 16- (a) and 14-MRs (b) channels, respectively. H atoms and Cl- anions are omitted for clarity, respectively.

Cd2+ ions and the AmTAZ ligands give rise to a 3D macrocationic [Cd(AmTAZ)]nn+ framework with 7-/8- (ca. 3.2 × 3.9 Å and 3.7 × 5.1 Å), 16- (ca. 6.1 × 6.3 Å), and 14-membered (ca. 3.9 × 6.1 Å) rings (MRs) channels running along the a-, b-, and c-axis directions, respectively (Figures 3, 4c, and S3, Supporting Information). The charge of the macrocationic [Cd(AmTAZ)]nn+ framework is balanced by Cl- anions, which are located in the center of the 7-MR channels and coordinated to two adjacent Cd2+ centers as a µ2-bridging ligand to form one-dimensional (1D) {Cd-Cl}n zigzag chainlike units extending along the b-axis (Figure 4c). Interestingly, the structure of 1 possesses two types of helical channels made from unclosed 8-MRs with opposite chirality in the [100] direction (Figure 4a). The left-handed and right-handed helical channels are comprised of the linkages of Cd1, N1, N3, C2 atoms and Cd1, N2, N4, and C1 atoms, respectively. The unclosed -N3-Cd1-N1-C2-N3-Cd1N1-C2- linkage (type I) gives rise to the left-handed helices, which is different from another helical channel consisting of the unclosed linkage of -N2-Cd1-N4-C1-N2-Cd1N4-C1- (type II) not only in its chirality but also in its shape and size. The left- and right-handed helices couple with each other along the c-axis through Cd2+ ions to form twodimensional (2D) layered networks with alternating left- and right-handed helical channels (Figure 4b). The layered networks are stacked in parallel with -ABAB- alternations along the b-axis

2138 Crystal Growth & Design, Vol. 6, No. 9, 2006

Li et al.

Figure 5. Excitation and emission spectra of 1 in the solid state at room temperature.

Figure 4. (a, b) View of the two fashions of alternating left- and righthanded 2D helical channels along the c-axis. (c) View of the linkages of the two fashions of 2D layered networks, which are stacked in parallel with -ABAB- alternations along the b-axis. L (left-handed) and R (righthanded). H atoms are omitted for clarity.

and further linked by triazole rings, resulting in a novel 3D framework containing additional 7-MR channels where the Clanions are located (Figure 4c). The most fascinating aspect of 1 is that all four N atoms in AmTAZ are coordinated to four separate Cd2+ centers, including the amino N atom (Scheme 1a). To our knowledge, it is the first observation of the amino N atom, in 3-amino or 3,5diaminotriazole coordination complexes, coordinating to the metal ion.11,12 Although in some N-substituted triazole derivatives amino N atoms can bond to a metal center, most of them are involved in the chelating coordination mode together with a thionate group or other coordination atoms.13 In addition, it is worth mentioning that all the uncoordinated amino groups in other reported AmTAZ coordination complexes, which all crystallized in achiral space groups, are disordered.11 This kind of disorder enables the AmTAZ ligand to have a 2-fold rotational axis and destroys its original asymmetry (Scheme 1b). Scheme 1.

While in compound 1, the coordination of the Cd2+ cation with the amino N atom “locks” the amino group and eliminates its possible disorder. Thus, the original asymmetry of AmTAZ was retained. This locked asymmetry of AmTAZ assisted by a coordination bond provides a favorable approach to build a chiral molecular framework by controlling the freedom of the ligands to facilitate the transfer of unsymmetry in the molecular assemblies. Thermal Properties. Thermogravimetric analysis (TGA) reveals that 1 possesses high thermal stability. There is no weight loss from 40 to 380 °C in the TGA curve. The compound starts decomposing above 380 °C, and the mass loss in the first stage (-7.1%) corresponds to the release of the amino group (-6.94%). Then the compound suffers complete decomposition until it reaches a temperature of 830 °C; the total mass loss of 61.31% corresponds to the removal of the residual organic species (calc. 29.0%), Cl- (calc. 15.4%), and the slow evaporation of ca. 0.30 CdO per formula (calc. 16.9%).14 Nonlinear Optical Properties. To confirm its acentricity as well as to evaluate its potential application as second-order nonlinear optical material, we studied 1 using powder secondharmonic generation (SHG) measurements15,16 at room temperature. The intensity of the green light (frequency-doubled output: λ ) 532 nm) produced by the powder sample of 1 is much lower than that of potassium dihydrogen phosphate (KDP) powder, indicating that the compound only has a weaker SHG effect than KDP. This may be a result of racemic twinning17 with the Flack parameter of 0.51. However, the successful observation of second-order NLO behavior of the twinned crystals is inspiring, which indicates that perfect single crystals of 1 may have much better second-order NLO behavior. Further research is still ongoing in our laboratory. Fluorescent Properties. Compound 1 displays intense blue luminescence at 435 nm in the solid state at room temperature with excitation at 295 nm (Figure 5). In the same excitation, the emission maximum of the AmTAZH ligand can be observed at 421 nm. Such fluorescence behaviors suggest that the broad

Coordination Mode of the AmTAZ in 1 (a) and the Disorder of AmTAZ about the Two-fold Axis in Other Coordination Polymers (b).

Novel Chiral Cd(II) Coordination Polymer

emission band of 1 is mainly due to an intraligand emission state as reported for Cd(II) or other d10 metal complexes with N-donor ligands.18 The slight red-shift of the emission energy from the free ligand to the complex may be related to the deprotonation of the AmTAZH ligand as well as the coordination of AmTAZ to the Cd2+ ions, which could result in a decrease in HOMO-LUMO energy gap of the complex.19 Along with the fact that the compound has high thermal stability and is virtually insoluble in most common solvents such as acetone, methanol, chloroform, benzene, water, etc., the emission property makes 1 a potential blue fluorescent material. Conclusions In summary, we have successfully obtained the first chiral 3D metal-organic framework with two types of helical channels based on the linkages between cadmium ions and AmTAZ ligands in an unprecedented µ4-bridging mode, in which the asymmetry of the AmTAZ was “locked” and might be transferred throughout the crystal structure to “direct” the formation of a chiral network. The compound exhibits the second-order NLO activity, intense blue fluorescent emission, and high thermal stability. Our results together with Lin et al. shed light on the significance of the structures of achiral unsymmetrical bridging ligands in the process of recognition and crystallization of metal centers into chiral networks. We will pursue our research by replacing cadmium and 3-amino-1,2,4-triazole with other metals and achiral unsymmetrical triazole ligands to synthesize novel chiral and noncentrosymmetric coordination polymers and to study their fascinating properties. Acknowledgment. The authors acknowledge the financial support of the Natural Science Foundation of China (No. 50372069/20201010), the Natural Science Foundation of Fujian Province of China (No. E0220003//E0310030). Supporting Information Available: Crystallographic file in CIF format; IR spectrum, TGA curve, and additional structural plots. This material is available free of charge via the Internet at http://pubs.acs.org.

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