CRYSTAL GROWTH & DESIGN
Typical Ferroelectric Olefin-copper(I) Organometallic Oligmer with Flexible Organic Ligand
2008 VOL. 8, NO. 10 3501–3503
Qiong Ye,* Tian Hang, Da-Wei Fu, Guang-Hai Xu, and Ren-Gen Xiong Ordered Matter Science Research Center, Southeast UniVersity, Nanjing 211189, P. R. China ReceiVed April 22, 2008; ReVised Manuscript ReceiVed August 5, 2008
ABSTRACT: Solvothermal treatment of a flexible olefin organic ligand and CuBr offers a novel olefin-copper(I) coordination compound. The induction of the flexible ligand leads the olefin-copper(I) organometallic compound to crystallize in a chiral or polar point group P41. The investigation of its ferroelectric property shows that the compound is a typical ferroelectric and its electric hysteresis loop shows a remanent polarization (Pr) of ca. 0.2 µC/cm2 and coercive field (Ec) of 800 V/cm. Ferroelectric materials can be switched rapidly between different states by means of an external electric field. Thus such materials are widely applied to electric-optical devices, information storages, switchable NLO (nonlinear optical) devices and light modulators, etc.1 Before the first non-hydrogen-bonded ferroelectrics, BaTiO3, was discovered,2 it was assumed that hydrogen bonding was indispensable to inspire ferroelectricity. However, the ferroelectric behavior requires the adoption of a space group associated with one of 10 polar point groups (C1, Cs, C2, C2v, C3, C3v, C4, C4v, C6, C6v). Consequently, all ferroelectrics are piezoelectric (when a voltage is applied across a material, the material can undergo a mechanical distortion in response) and pyroelectric (materials with spontaneous polarization whose amplitude changes according to temperature gradients). At present, much attention in ferroelectric material field is focused on developing ferroelectric pure organic or inorganic compounds,3 whereas studies toward developing ferroelectric materials based on metal-organic coordination compounds remain relatively sparse.4 It is surprising given that the first ferroelectric Rochelle salt was a real metal-organic coordination compound.5 On the other hand, acentric or homochiral metal-complexes crystallizing in one of the ten polar point groups are attractive targets because those metal-complexes can potentially display ferroelectric properties. A complex can be considered as a bridge between the organic ligand and inorganic metal ion through coordinating bond. As a result, the complex bears the advantages of both organic compound (tailorability, π-conjugation, and chirality) and inorganic compound (d and f orbits combined into coordination bonds and electronic spinning properties). This work has succeeded in generating a host of new homochiral or noncentrosymmetric metal complexes with fascinating molecular architectures and potential ferroelectric applications. Recent progresses in the synthesis (or self-assembly) and design of novel materials based upon metalorganic coordination compounds give us some ideas that acentric and homochiral metal-organic solid materials can be achieved through the protocols as follows: (a) use asymmetric, flexural, or racemic organic ligand to self-assemble with metal; (b) by coordinating to metals, racemic organic ligands self-resolute and get chiral coordination compounds; (c) asymmetric coordination compounds are afforded through self-assembly of metals and optical chiral organic ligands.6,7 We are interested in hydrothermal or solvothermal reactions and have obtained a fascinating variety of acentric and novel metal-organic coordination compounds from such process. In particular, unstable copper(I) salt exists in a sealed tube under vacuum, and a stable copper(I) coordination compound can then be obtained.8 Herein, we report the solvothermal synthesis and ferroelectric property of an interesting asymmetric olefincopper(I) coordination compound with achiral organic ligand. Polar * Corresponding author. E-mail:
[email protected].
Scheme 1
coordination compound was achieved by introducing a flexible organic ligand, which induces the compound to crystallize in an acentric space group or to crystallize in a chiral space group P41 with only one C4 axis, even if there is no chiral or asymmetric element in the compound. This is a successful example for crystal engineering strategy to get acentric complex through kink or flexible building block. Solvothermal treatment of flexible olefin organic ligand N,N′,N′′(2,4,6-trimethyl-benzene-1,3,5-triyl)tris(methylene)tris (N-allylprop2-en-1-amine)(TTT) and CuBr in the presence of hydrobromide acid and methanol as solvent at 75 °C affords a novel olefincopper(I) organometallic compound 1 (Scheme 1). The crystal structure of compound 1 reveals that there are two crystallized water molecules; whereas nitrogen atoms in olefin organic ligand are protonated and fail to coordinate to copper(I) center (Figure 1). Three allyl arms of organic ligand chelate to a [Cu4Br7]3- cluster and form a bowl-like discrete structure. From the asymmetric structure of Olifen-copper(I) organmetallic compound 1, it can be found that there are four crystallography different copper(I) centers, one of which shows a trigonal plane coordination environment, whereas the other three sit in the centers of distorted tetrahedron. Meanwhile, bromide atoms have three types of coordination modes, terminal, bidentate bridge, and tridentate bridge, to connect copper(I) atoms together and result in the formation of [Cu4Br7]- aggregation (Figure 1). Figure 2 shows clearly the local coordination geometry of compound 1, and Cu1 coordinates to three η2 (Br4, Br5, and Br7) and a η3 Br(Br7) to bear an almost perfect tetrahedron coordination environment with Br-Cu-Br angles range from 105.12 to 112.56°. The coordination geometries of both Cu3 and Cu4 can be best described as distorted tetrahedrons, which are fulfilled by olefin-copper interaction, terminal Br (Br1 or Br3), bidentate bridging Br (Br5 or Br6), and
10.1021/cg8004129 CCC: $40.75 2008 American Chemical Society Published on Web 09/05/2008
3502 Crystal Growth & Design, Vol. 8, No. 10, 2008
Figure 1. Asymmetric unit of olefin-copper(I) coordination compound 1 in which three allyl groups of one olefin organic ligand coordinate to three independent copper(I) centers, whereas the remaining CdC bonds and protonated nitrogen atoms fail to link to copper(I) center.
Figure 2. Local coordination core representation of 1 shows that there are four Cu centers connecting with each other through four µ2-Cl atoms. Some bond distances (Å) are shown in the picture, and some key angles (°) are list below: Cu1-Br7-Cu4 70.46(7), Cu1-Br7-Cu3 69.49(8),Cu4-Br7-Cu3107.54(8),Cu3-Br6-Cu178.50(9),C11-Cu4-C28 39.8(6), C(11)-Cu(4)-Br(1) 141.0(5), C28-Cu4-Br1 101.2(4), C11-Cu4-Br5 102.2(5), C28-Cu4-Br5 135.5(4), Br1-Cu4-Br5 111.31(14),C11-Cu4-Br794.2(6),C28-Cu4-Br7105.2(4),Br1-Cu4-Br7 99.85(10), Br5-Cu4-Br7 98.42(9), C9-Cu3-C5 36.5(7), C9-Cu3-Br3 103.6(5), C5-Cu3-Br3 139.8(5), C9-Cu3-Br6 138.2(5), C5-Cu3-Br6 105.2(5),Br3-Cu3-Br6109.51(11),C9-Cu3-Br7101.1(5),C5-Cu3-Br7 95.7(5),Br3-Cu3-Br798.67(11),Br6-Cu3-Br798.51(8),Cu4-Br5-Cu1 78.90(10), Cu2-Br4-Cu1 84.44(13), C1-Cu2-C20 36.2(7), C1-Cu2Br2142.3(6), C20-Cu2-Br2 106.1(4), C1-Cu2-Br4 106.5(6), C20-Cu2-Br4 141.5(4), Br2-Cu2-Br4 110.81(15), Br5-Cu1-Br4 113.56(12), Br5-Cu1-Br6 109.87(13), Br4-Cu1-Br(6) 112.56(12), Br5-Cu1- Br7 105.25(10), Br4-Cu1-Br7 107.65(13), Br6-Cu1-Br7 107.48(10).
tridentate bridging Br (Br7, long bond distance and weak interaction). The tetrahedral coordination geometries of Cu1 and Cu3 share one side, which is similar to those of Cu1 and Cu4. Differently, Cu2 bonds to olefinic moiety, bridging Br4n and terminal Br2 to complete its trigonal plane coordination. Generally, half-part of ally groups connects with copper(I) centers, whereas the remaining double bonds fail to coordinate to copper(I) centers. As shown in Figure 3, there are strong hydrogen bond interactions among protonated nitrogen atoms, Br atoms and crystallized water, which result in the formation of three-dimensional framework. Some H-bond interaction distances and angles are listed in the caption of Figure 3. Oxygen atoms of water and nitrogen atoms act as donors
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Figure 3. Packing view and hydrogen bond interactions of compound 1 along a axis. Hydrogen bond distances (Å) and angles (°): N1-H1C · · · Br2, 3.231(13), 150.69; O1-H1D · · · Br5, 3.620(16), 145.11; N2-H2C · · · Br3, 3.325(15), 154.27; O2-H2D · · · Br4, 3.58(2), 159.54; O2-H2E · · · Br1, 3.564(19), 125.36; N3-H3C · · · Br1, 3.282(14), 155.49.
Figure 4. Electric hysteresis loop of compound 1 was observed by virtual ground mode in a powdered sample in the form of a pellet at 30 Hz using a Precision Premier II-radiant technology Ins. Ferroelectric tester was made at room temperature.
while Br atoms act as acceptors. It is necessary to note that the flexible olefin organic ligand and H-bond interactions lead the olefin-copper(I) coordination compound 1 to crystallize in a chiral space group P41, although compound 1 has no chiral or asymmetric element presented. As a result, to the best of our knowledge, 1 represents the first example of a polar olefin-copper(I) coordination compound constructed by flexible olefin ligand through crystal engineering design. Interestingly, 1 crystallizes in a chiral and polar point group (space group P41, which belongs to the C4 point group), in which its ferroelectric properties will occur. This idea prompts us to investigate its ferroelectric properties. The experimental results (Figure 4) clearly indicate that the electric hysteresis loop of 1 holds typical characteristics of ferroelectric materials with a remanent polarization (Pr) of ca. 0.2 µC/cm2 and coercive field (Ec) of 0.8 kV/cm, whereas the spontaneous polarization values (Ps) of ferroelectric KH2PO4 (KDP) and triglycine sulfate (TGS) are 5.0 and 3.0 µC/cm2, respectively. The ferroelectric property (Ps ≈ 0.3 µC cm-2) of 1 is smaller than those found in KDP and TGS. However, it is important to know that its Ec value is much lower. Thus, it is able to enhance the Pr value by increasing the applied electric field. As shown in Figure 5, the investigation of ferroelectric properties relative to frequencies shows that Ps and Pr decrease
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Crystal Growth & Design, Vol. 8, No. 10, 2008 3503
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Figure 5. Electric hysteresis loop of compound 1 was observed by virtual ground mode in a powdered sample in the form of a pellet at the applied electric field of 1.5 KV/cm under different frequencies.
when the frequency of ferroelectric measurement increases at the applied electric field of 1.5 KV/cm. In conclusion, the solvothermal synthesis technique provides a powerful synthetic method to get organometallic compounds. Further, some acentric and polar coordination compounds with interesting ferroelectric properties can be obtained through inducing flexible organic ligand.
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Acknowledgment. This work was supported by the National Natural Science Foundation of China (20701007 and 50673039), Jiangsu Province Natural Science Fund (to Q.Y.) and Start-up Grant from SEU. Supporting Information Available: X-ray crystallographic file (CIF). This material is available free of charge via the Internet at http:// pubs.acs.org.
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CG8004129
