CRYSTAL GROWTH & DESIGN
Two Novel Noncentrosymmetric Zinc Coordination Compounds with Second Harmonic Generation Response, and Potential Piezoelectric and Ferroelectric Properties
2009 VOL. 9, NO. 5 2026–2029
Tian Hang, Da-Wei Fu, Qiong Ye,* and Ren-Gen Xiong* Ordered Matter Science Research Center, Southeast UniVersity, Nanjing 211189, P. R. China ReceiVed June 12, 2008; ReVised Manuscript ReceiVed March 12, 2009
ABSTRACT: Hydrothermal treatments of ZnCl2 and organic ligands afforded two novel noncentrosymmetric coordination compounds 1 and 2, respectively. 1 crystallizes in the chiral P212121 space group because of the introduction of a bent ligand 3-(2-(2-pyridyl)ethenyl)benzoic acid to the complex, whereas 2 crystallizes in hexagonal crystal system (R3) with a 3-fold axis and bears two octahedral chelate zinc cation and two [ZnBr4]2-. Both 1 and 2 display a strong second harmonic generation (SHG) response and moderate piezoelectric properties. Because of the assignment of a polar point group of chiral space group for compound 2 (R3), its electric hysteresis loop was recorded; we found that it may be a potential ferroelectric with spontaneous polarization of 0.02 µC/cm2. Noncentrosymmetric bulk materials or polar crystals are found to possess technologically useful properties such as ferroelectricity, pyroelectricity, piezoelectricity, triboluminescence, and nonlinear optical (NLO) function (especially second harmonic generation, SHG),1 where SHG, piezoelectric, and ferroelectric properties are of particular importance for their practical importance in areas such as telecommunications, optical storage, and information processing as well as mechanical energy transfer. Because of our systematic investigations on how to assemble acentric metal-complexes through achiral building blocks by crystal engineering design, we have summarized some useful strategies to design acentric metal-organic solid materials through the protocols as follows: (a) using asymmetric, flexible, or racemic organic ligand to self-assemble with metal ion; (b) racemic organic ligand coordinating to metal ion to result in self-resolution to get chiral compounds; (c) asymmetric coordination compounds obtained through self-assembling of metal and homochiral organic ligands.2,3 However, the generation of acentric solids from achiral building blocks still depends upon Edisonial approaches and the construction of structurally ordered noncentrosymmetic and chiral metal-organic solids still remains a great challenge. We have been interested in hydrothermal or solvothermal reactions because without exerting synthetic control, a fascinating variety of acentric and novel metal-organic coordination compounds have been obtained from such a process.4 Herein, we report the solvothermal synthesis (see Scheme 1) and physical properties of two interesting acentric zinc coordination compounds 1 and 2 obtained through achiral organic ligand, both of which crystallize in chiral space groups (P212121 and R3). Moreover, because compound 2 (C3) belonging to one of ten polar point groups (C1, Cs, C2, C2V, C3, C3V, C4, C4V, C6, C6V), besides SHG and piezoelectric properties, it also displays a moderate ferroelectric behavior. Hydrothermal treatment of flexible organic ligand 3-(2-(2pyridyl)ethenyl)benzoic acid (H-peba) and ZnCl2 at 100 °C affords a one-dimensional chain zinc coordination polymer 1 [Zn(peba)2] as depicted in Figure 1. The local coordination environment around the zinc atom can be best described as a distorted tetrahedron defined as two nitrogen atoms and two oxygen atoms from different four ligands.5 Thus, every peba anion acting as a bidentate linker to connect two zinc centers through the nitrogen of the pyridine ring and one oxygen atom of carboxylate group results in the formation of the 1D chain as shown in Figures 2and 3. The bond distances of Zn-O (1.9717(15) and 1.9749(15) Å) are shorter than those found in {[Zn-2(bdaip)(mu-OH)(OH)]NO3 · H2O},6a (Hbdaip * Corresponding author.
[email protected].
E-mail:
[email protected] (R.-G.X.);
Scheme 1
) 6-bis(N-[2-(dimethylamino)ethyl]iminomethyl)-4-methylphenol) (1.975 and 1.979 Å) and [Zn(PEBA)2],7b {PEBA) 4-[2-(4pyridylethenyl)] benzenecarbonitrile (PEBC)} (2.297 and 2.088 Å), whereas the bond distances of Zn-N (2.1123(19) and 2.1052(18) Å) in compound 2 are comparable to those of the two compounds mentioned above (2.112, 2.139, and 2.085 Å). Compound 2 is achieved through the reaction of 2-pyridyl carboxylic acid, 2-bromo-acetic ester and ZnCl2 in the presence of alcohol as solution at 100 °C under solvothermal conditions. Unexpectedly, the 2-bromo-acetic ester ligand has not taken part in the coordination, whereas the bromide anion coordinated to zinc center to form a tetrahedron [ZnBr4]2-. At the same time, three 2-pyridyl carboxylate surrounded another zinc center come out to be an octahedron coordination environment to balance
Figure 1. Asymmetric unit of 1 in which the coordination environment of Zn can be best described as a distorted tetrahedron. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angels (deg): Zn1-O4 1.9717(15), Zn1-O(3) 1.9749(15), Zn1-N2A1 2.1123(19), Zn1-N1B 2.1052(18), O4-Zn1-O3 128.87(8), O4-Zn1-N2A1 115.27(7),O3-Zn1-N2A96.80(7),O4-Zn1-N1B98.54(7),O3-Zn1-N1B 111.69(7), N2A-Zn1-N1B 103.84(7). Symmetry transformations used to generate equivalent atoms: A -x, y + 1/2, -z + 1/2; B -x, y 1/2, -z + 1/2.
10.1021/cg800618v CCC: $40.75 2009 American Chemical Society Published on Web 03/30/2009
Communications
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Figure 2. 1D network representation of 1.
Figure 5. Local coordination geometries around Zn1 and Zn2. Left, oxygen atoms are on the top; right, oxygen atoms are on the bottom.
Figure 3. Arrangement of compound 1 in the same layer. The direction of crystal packing polarity is shown.
Figure 6. Projection of compound 2 through the C3 axis.
Figure 4. Asymmetric unit of 2. There is a 3-fold rotation axis through Zn1, Zn3, Zn4, Br1, and Br3 atoms. Some important bond distance and angles have been deposited in the Supporting Information.
the electric charge of compound 2. The single-crystal X-ray diffraction of compound 2 certified the crystal structure of compound 2 as shown in Figure 4.8 There are four unique zinc centers. Two of them are surrounded by four bromide anions; the other two can be described as the zinc being chelated by 2-pyridyl carboxylate, which coordinates to the zinc atom through the nitrogen atom of its pyridine ring and the oxygen atom of its carboxylic group. Moreover, compound 2 was found in the hexagonal crystal system and in a chiral space group R3, Zn1 is on a 3-fold axis, and the three chelate rings are in a propeller-shaped configuration twisted 98.2° from the coplanar arrangement. At the same time, Zn3, Zn4, Br1, and Br3 are also on the 3-fold axis. Zn2 is not on the axis, though it is surrounded by three propeller-shaped ligands. It is worth noting that the coordination configuration of Zn1 and Zn2 are identical to each other (∆-cis) as shown in Figure 5 but they arrange in the reverse direction. The packing view of compound 2 has been explained in Figure 6. Every Zn1 chelate is enclosed by three Zn2 chelates; meanwhile, three Zn1 chelates and three Zn2 chelates can form asymmetriccavity.Atlast,thelengthsofZn-Br(2.387(3)-2.4176(15)
Å) are slightly shorter than those found in [ZnBr2(C5H4NCOOMe)2]9 (2.443 and 2.465 Å). And the same thing happened to the bond distances of Zn-O (2.087(9), 2.094(10) Å) and Zn-N (2.211(11), 2.228(10) Å), all of which are shorter than those found in [ZnBr2(C5H4NCOOMe)2] (Zn-N, 2.131 and 2.124 Å; Zn-O, 2.482 and 2.394 Å), wherease other bond distances such as C-C, C-N, and C-O are normal. It is well-known that octahedral chelating compound has four diastereoisomers: ∆-cis, ∆-trans, Λ-cis, and Λ-trans. Although some of the diastereoisomers of neuter octahedral chelating compounds can be separated through column chromatography, the only protocol of separation for an ionic octahedral chelating compound is achieved through introducing some corresponding chiral anionic or cationic compound to cocrystallize with the octahedral chelating compound. However, it is lucky for compound 2 to crystallize in a chiral space group through self-resolution, and the octahedron chelating mode in compound 2 bears the same configuration. Because 1 belongs to crystal class 222 and point group (D2), its optical activity will occur as a specific physical effect. Also because of the ligand coordination enhancing the rigidity of ligand, a good donor-acceptor system is formed, which is essential for SHG effect. Furthermore, the polar direction of the compound is the same as one layer, that is, there is no counteraction to the large dipole moment. Compound 1 displays a second harmonic generation (SHG) response comparable to urea. As mentioned before, compound 1 is in the D2 point group, which is nonferroelectric active compound. Furthermore, given that compound 2 crystallizes in an acentric and polar space group (R3) its optical properties were investigated. Preliminary studies of powdered sample with diameters between ca. 80 - 150 µm indicate that 1 is SHG-active with a value approximately 2 times greater than that of KDP. The space group R3 is associated with the point group C3, one of the 10 polar point groups (C1, Cs, C2, C2V, C3, C3V, C4, C4V, C6, C6V) required for ferroelectric behavior. Experimental results shows that compound
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Communications measurements at different frequencies, additional crystallographic data, and diagram (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
References
Figure 7. Electric hysteresis loop of compound 2.
2 displays potential for ferroelectric behavior as depicted in Figure 7, showing that there is an electric hysteresis loop, which is indicative of ferroelectric feature with a remnant polarization (Pr) of ca. 0.003 µC cm-2 and a coercive field (Ec) of ca. 0.99 KV cm-1. The saturation spontaneous polarization (Ps) of 1 (ca. 0.02 µC cm-2) is much smaller than that of Ps of 0.25 µC cm-2, which was reported for ferroelectric Rochelle salt.10 However, its ferroelectric property still needs to be further confirmed by many experiments, such as dielectric measurements and DSC, to see whether the permittivity anomaly, phase transition, as well as real saturation hysteresis loop could be obtained or not.11 Normally, ferroelectric compounds should be piezoelectric-active, whereas piezoelectric compounds are less restricted and need to satisfy only the noncentrosymmetric requirement. One the other hand, ferroelectricity and piezoelectricity have found wide applications in modern electric devices such as memory elements, filtering devices, and high-performance insulators. Ferroelectric crystals, which have a spontaneous electric polarization arising from the coherent arrangement of electric dipoles (specifically, a polar displacement of anions and cations), and piezoelectric materials, which convert mechanical to electrical energy (and vice versa), are crucial in medical imaging, telecommunication, and ultrasonic devices. Thus, we are encouraged to investigate the piezoelectric property of compound 2. Our preliminary result indicates that compound 2, which was measured in PM200 (high-precision PiezoMeter system measuring d33 in four ranges with 0.01 pC/N resolution), displays a d33 value of ca. 2.1 pC/N. Though it is significantly smaller than that of practically useful BaTiO3 (300-2500 pC/N),12 to the best of our knowledge, 2 is the first molecule-based complex with possible ferroelectric and piezoelectric properties because most piezoelectric materials are pure inorganic salts, such as ceramic compositions, PbTiO3, etc. Moreover, our preliminary investigation of the piezoelectric property of compound 1 indicates that it is an active piezoelectric but without a d33 value. Though it displays a d15 value in principle, unfortunately, d15 measurement needs a large single crystal. We are still trying to get larger single crystals of compound 1 and this progress is under way. In summary, we have successfully synthesized two interesting zinc coordination compounds through crystal engineering strategies with solvothermal synthesis technique and both of them are lucky to crystallize in chiral space group and display interesting physical properties. Thus it opens a new avenue to explore functional molecule-based materials.
Acknowledgment. This work was supported by the National Natural Science Foundation of China B010303 as well as JiangSu province NSF BK2008029. Supporting Information Available: Crystallographic information files (CIF) for structures 1 and 2; variable-temperature dielectric
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CG800618V
