CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 9 1568-1570
Communications Deuteration Effect of Ferroelectricity and Permittivity on Homochiral Zinc Coordination Compound Qiong Ye,† Yu-Mei Song,⊥ Da-Wei Fu,† Guo-Xi Wang,⊥ Ren-Gen Xiong,*,† Philip Wai Hong Chan,*,‡ and Songping D. Huang*,§ Ordered Matter Science Research Center, Southeast UniVersity, Nanjing 210096, P. R. China, DiVision of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological UniVersity, Nanyang AVenue, Singapore 639798, Chemistry Department, Kent State UniVersity, Kent, Ohio 44240, and Department of Chemistry, Nanjing UniVersity, Nanjing 210093, P. R. China ReceiVed February 24, 2007; ReVised Manuscript ReceiVed June 29, 2007
ABSTRACT: Hydrothermal [2+3] cycloaddition reactions of N-4′-caynobenzylcinchonidine bromide (CBCBr) with NaN3 in the presence of ZnBr2 offer a novel Zn dimer (CBC-N4)2ZnBr2(N3)2(X2O), X ) H for 1 and D for 2) in which the reactant Br- and N3- take part in the coordination to Zn. The measurements of ferroelectricity and permittivity of 1 and 2 show that there exists a large deuterated effect both in ferroelectric behavior (40% increase) and in dielectric constant (2.75 times increase), which is unprecedented in metal coordination compounds. The deuteration effect (DEF) by exchanging hydrogen with deuterium on the hydrogen bonds plays a very important role not only in the many physical property enhancements such as nonlinear optics (especially second harmonic generation SGH response), ferroelectricity, and permittivity but also in the basic theoretical point of views.1 Some of typical examples are found in pure organic and inorganic compounds, such as Phz-H2ca-Phz-D2ca (for huge dielectric response with an increase of 50.00 in the dielectric constant at 1 MHz as a function of temperature, Phz ) phenazine, H2ca ) bromanilic acid,2 whereas dielectric DEF in phenylsquaric acid increases significantly from 8 to 18)3 and KDP (KH2PO4)DKDP (KD2PO4) (for both, dielectric DEF increased by 80% and ferroelectric saturation spontaneous polarization (Ps) increased by 24%).4 DEF in new systems, especially in metal coordination compound (MCC) systems, have been rarely reported to the best of our knowledge, even if MCC bear the advantages of both pure inorganic and organic compounds. Recently, we have developed a powerful in situ synthesis homochiral MCC with tetrazoyl group as building block method through a [2+3] cycloaddition reaction between cyano and azide in the presence of Lewis acid catalytic action. In light of Nobel winner Sharpless’s tetrazole synthesis method,5 some MCCs with novel structures and physical properties such as high SHG response and good ferroelectric-dielectric behavior have been found.6 Keeping this method in mind, we have carried out the hydrothermal reactions of N-4′-caynobenzylcinchonidine bromide (CBCBr) with NaN3 in the presence of ZnBr2 to offer a novel Zn dimer (CBC-N4)2Zn2 (Br)2(N3)2(X2O), X ) H for 1 and D for 2) where the reactant Br- and N3- take part in the coordination to Zn (Scheme 1). Herein, we report their synthesis, crystal structure, and ferroelectric properties as well as DEF. A typical peak at ca. 2080 cm-1 indicates the presence of azide anions, whereas a medium broad peak at 3385-3409 cm-1 for 1 and weak peak at ca. 2445 cm-1 for 2 suggests there is H2O or deuterated water molecules in MCC 1 and 2, which display basically * Corresponding author. E-mail:
[email protected] (R.-G.X.); waihong@ ntu.edu.sg (P.W.H.C.);
[email protected] (S.D.H.). † Southeast University. ‡ Nanyang Technological University. § Kent State University. ⊥ Nanjing University.
Scheme 1
identical IR spectra. Interestingly, a peak at 1451 cm-1 in both MCC 1 and 2 surely suggests the formation of a tetrazoyl group,
Figure 1. (a) Perspective view of dimeric 1 with one crystallized water molecule in which the Zn center displays a tetrahedral geometry. (b) Perspective view of dimeric 2 with one crystallized deuterated water molecule in which the Zn center also displays a tetrahedral geometry. Typical bond distances (Å): MCC 1, Zn-N ) 1.955(7), 1.958(7), 1.953(6), 1.966(7), 1.889(8), 2.058(6); Zn-Br ) 2.910(11), 2.3077(13); MCC 2, Zn-N ) 1.858(7), 1.972(9), 2.018(9), 2.3462(15), 2.065(8), 2.066(7); Zn-Br ) 2.3179(15), 2.3462(15).
10.1021/cg070190q CCC: $37.00 © 2007 American Chemical Society Published on Web 08/18/2007
Communications
Figure 2. Three-dimensional frameworks through H-bonds in MCC 1 and MCC 2
indicating that [2+3] cycloaddition reaction must have taken place between the cyano group and azide anion to give an N-4′tetrazoylbenzyl-cinchonidine (TBC-N4), later confirmed by X-ray crystal structural determinations. X-ray crystal structural determinations of MCC 1 and 2 reveal that the local coordination geometry around each Zn center can be best described as a slightly distorted tetrahedron in which a tetrazoyl group links two Zn centers to result in the formation of a dimer composed of three N atoms (two from a tetrazoyl group and one from terminal azide) and one terminal Br atom (see Figure 1).7 In all cases of MCC 1 and 2, the pyridyl group of quinoline ring failed to bind to Zn atoms, and thus each TBC-N4 acts as bidentate chelating agent using 1,2-µ2-tetrazoyl groups. Both 1 and 2 are isostructural, but are different in that 1 contains one crystallized H2O, whereas 2 contains one deuterated D2O; both crystallize in the chiral space group (P1, No. 1, belonging to polar point group C1). Because of the many H-bonds between the O atom of the 9-hydroxy group of TBC-N4 and the N atom of azide or Br, and the O atom of water or deuterated water and the N atom of the azide or tetrazoyl group, a 3D framework is formed through H-bonds as shown in Figure 2. There have several different H-bonds in MCC1: the hydroxyl group of the organic ligand has a strong H-bond interaction with the bromide (2.8493, 3.151(6) Å) or nitrogen atom of azide (2.4031, 2.5931 Å). Although crystalline water connects with nitrogen atoms separately from azides and tetrazole rings of ligands through strong H-bonds (2.5199, 2.892(14) Å; 2.0366, 2.489(13) Å). Interestingly, there are similar H-bonds in MCC2 but they display relatively shorter H-bond distances, as shown in the Supporting Information. As a result, the difference in the hydrogen-bond distances in MCC1 and MCC2 will effect the polarity and interaction of molecules. Interestingly, TBC-N4 in 1 and 2 is a zwitterion with long charge separation, which is essential for strong SHG response. Thus, powdered samples 1 and 2 display strong SHG responses ∼20 times larger than that of KDP. However, because of the approximate estimation of SHG strength, the DEF on NLO is not evident in our experiments.8 Samples 1 and 2 theoretically display ferroelectric behavior because they fall in one of 10 polar point groups (C1, Cs, C2, C2V, C3, C3V, C4, C4V, C6, C6V). Figure 3 clearly shows there is an electric hysteresis loop in 1 (a typical ferroelectric feature) with a remnant polarization (Pr) of ca. 0. 25 µC cm-2 and coercive field (Ec) of ca. 12 kV cm-1. Saturation of the spontaneous polarization (Ps) of 1 occurs at ca. 0.84 µC cm-2, which is slightly higher than that for a typical ferroelectric compound (e.g., NaKC4H4O6‚4H2O, Rochelle salt; usually Ps ) 0.25 µC cm-2), but significantly smaller than that found in KDP (∼5.0 µC cm-2). Similarly, the Ec, Pr, and Ps of 2 (as shown in Figure 4) can be estimated to be 14 kV cm-1, 0.46 µC cm-2, and 1.18 µC cm-2, respectively. Thus, larger ferroelectric DEF (with an increase in Ps of 40%) ((1.18-0.84)/
Crystal Growth & Design, Vol. 7, No. 9, 2007 1569
Figure 3. Electric hysteresis loops of MCC 1 were observed by virtual ground mode in a powdered sample in the form of a pellet using an RT6000 ferroelectric tester at room temperature at different voltages.
Figure 4. Eectric hysteresis loops of MCC 2 were observed by virtual ground mode in a powdered sample in the form of a pellet using an RT6000 ferroelectric tester at room temperature at different voltages.
0.84) on ferroelectric behavior was detected between MCCs 1 and 2. To the best of our knowledge, a large DEF in MCC is unprecedented to date. The ferroelectric behavior may come from their H-bonds. This can be confirmed by the cubic moment value. Assuming one dipole in the unit cell (Z ) 1) containing one DA pair (the density of dipoles, N1 ) Z/Vcell ) 7.7057 × 1026 m-3 for the MCC 1 crystal), the cubic moment µs ) Ps/N (10.90. × 10-30 C m ≈ 3.27 Debye calculated from the saturated polarization Ps (0.84 µC cm-2 at 298 K, derived by extrapolation of the P-E curve as drawn in Figure 3). Similarly, N2 ) Z/Vcell ) 7.6937 × 1026 m-3 for the MCC 2 crystal, whereas µs is Ps/N (15.31× 10-30 C m ≈ 4.59 Debye), slightly larger than that of MCC 1. The value of µs is comparable to that found for typically displacive type ferroelectrics in pure organic compound systems such as Phz-H2ca-Phz-D2ca.9,10 Finally, the static dielectric constants MCC 1 and MCC 2 at room temperature can be estimated from the extrapolation (Figures 5 and 6) to be 25.1 and 69.1, respectively. Thus, the dielectric constant of MCC 2 is approximately 2.75 times than that of 1. The large DEF probably comes from the facts that the cubic moment of MCC 2 is significantly larger than that of 1, whereas the permittivity is strongly dependent with molecular polarity. The larger the permittivity, the larger the molecular polarity, generally speaking.11 In conclusion, in the present work, the large ferroelectric and dielectric DEF in the MCC system will open a new avenue to explore functional compounds with fascinating properties.
1570 Crystal Growth & Design, Vol. 7, No. 9, 2007
Figure 5. Frequency dependence of permittivity [ ) 1(ω)+i2(ω)] of 1 at ca. room temperature in which 1 and 2 are the respective real and imaginary parts of permittivity. Dielectric loss ) 1/2.
Figure 6. Frequency dependence of permittivity [ ) 1(ω)+i2(ω)] of 2 at ca. room temperature in which 1 and 2 are the respective real and imaginary parts of permittivity. Dielectric loss ) 1/2.
Acknowledgment. This work was supported by Project 973 (Grant 2006CB806104), the National Natural Science Foundation of China and EYTP of MOE (P. R. China). R.-G.X. thanks Prof. Y.-Z. Li for his assistance in solving the X-ray crystal structure reported in this work. P.W.H.C. thanks Nanyang Technological University for funding. Supporting Information Available: Detailed experimental procedures, IR spectroscopic data, and additional ORTEP views (PDF); X-ray crystallographic information files (CIF). This material is available free of charge via the Internet at http://pubs.acs.org.
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