Crystal Structure and Nonlinear Optical Effect of a Pyroelectric Crystal Composed of a Cyano-Bridged Cu-Mo Assembly Toshiya Hozumi,†,‡ Tomohiro Nuida,†,‡ Kazuhito Hashimoto,‡ and Shin-ichi Ohkoshi*,†,‡ Department of Chemistry, School of Science, and Department of Applied Chemistry, School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, Japan
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 8 1736-1737
ReceiVed February 21, 2006; ReVised Manuscript ReceiVed June 29, 2006
ABSTRACT: A pyroelectric crystal [Cu(NH3)2]2[Mo(CN)8] (Fdd2), which is thermally stable up to 150 °C, is synthesized, and this compound displays a strong second harmonic generation of 1.8 × 10-10 esu. The magnetic properties and functionalities of cyano-bridged metal assemblies have been extensively studied over the past decade.1-3 Recently, octacyanometalate-based compounds have received special attention due to their varied dimensional crystal structures such as zero-dimensional (0-D), 1-D, 2-D, and 3-D.4 It is considered that octacyanometalate-based magnets are useful for preparing a noncentrosymmetric crystal structure. Materials with noncentrosymmetric structures may exhibit nonlinear optical effects such as second harmonic generation (SHG). In this work, a pyroelectric-paramagnetic crystal composed of a cyano-bridged Cu(II)-Mo(IV) complex is synthesized. Herein, the crystal structure, thermogravimetric analysis (TGA), magnetic properties, and SHG of this compound are reported. The compound, [Cu(NH3)2]2[Mo(CN)8], was prepared by reacting K4[Mo(CN)8]‚2H2O and Cu(NH3)4SO4‚H2O using an H-shaped tube.5 X-ray single-crystal structural analysis indicated that [Cu(NH3)2]2[Mo(CN)8] consists of a noncentrosymmetric 3-D cyanobridged Cu-Mo bimetallic assembly with an orthorhombic structure of Fdd2 space group.6 Figure 1a shows the coordination environments around the Cu and Mo ions. The Mo ion has a square antiprism geometry where the Mo ion is coordinated to six Cu ions through cyano groups (C(1)-N(1), C(1a)-N(1a), C(2)-N(2), C(2a)-N(2a), C(4)-N(4), and C(4a)-N(4a)). The remaining two cyano groups (C(3)-N(3) and C(3a)-N(3a)) are terminal and are oriented in the same direction at a 30° angle from the c axis (Figure 1b). Hence, [Cu(NH3)2]2[Mo(CN)8] has an electric polarization along the c axis and is a pyroelectric material. The Cu ion is coordinated to three cyanonitrogens (N(1), N(2), and N(4)) and two nitrogen atoms from NH3 (N(5) and N(6)). The coordination geometry is square pyramidal. The bond angle of the apical ligand (Cu-N(2c)-C(2c)) deviates from 180° and is 146.2°, whereas the bond angles of the equatorial ligands Cu-N(1)-C(1) and CuN(4b)-C(4b) are 171.3° and 172.3°, respectively. [Cu(NH3)2]2[Mo(CN)8] is a dark green crystal, which has an absorption band around 470 nm and a broad band in the 5001200 nm region (Figure 2a). According to reports of other cyanobridged Cu(II)-Mo(IV) complexes,7 the observed absorption bands are explained by the sum of the intervalence transfer band between CuII-NC-MoIV and CuI-NC-MoV and the d-d transition of square-pyramidal CuII (2B1 f 2A1, 2B1 f 2B2, and 2B1 f 2E). The infrared spectrum (IR) shows sharp CN stretching frequencies at 2179, 2168, 2163, 2138, 2132, 2114, and 2112 cm-1 (Figure 2b). The TGA curve does not exhibit weight loss up to 423 K (150 °C) and has a 36% loss between 423 and 700 K. This weight loss is due to the loss of ammonia molecules and decomposition (Figure 2c). The IR spectrum, after being heated to 150 °C and cooled to room temperature, is consistent with the original one, even if this heating and cooling cycle is repeated three times. Most cyano* To whom correspondence should be addressed. Phone +81-(3)-58414331. Fax +81-(3)-3812-1896. E-mail:
[email protected]. † Department of Chemistry, School of Science. ‡ Department of Applied Chemistry, School of Engineering.
Figure 1. (a) ORTEP drawing of the coordination environments around Cu and Mo with an atom labeling scheme. Displacement ellipsoids are drawn at a 50% probability level. Hydrogen atoms are omitted for clarity. [Symmetry codes: (a) 1/2 - x, 1/2 - y, z; (b) 3/4 - x, -1/4 + y, 1/4 + z; (c) -1/4 + x, 1/4 - y, -1/4 + z.] (b) 3-D structure of the compound. Blue, red, light blue, and gray balls represent Mo, Cu, N, and C, respectively. Large light blue and gray balls represent terminal cyanide ligands. Hydrogen atoms are omitted for clarity. P represents electric polarization.
Figure 2. (a) Diffuse reflectance spectrum, (b) IR spectrum, (c) thermogravimetric analysis, (d) χMT vs T plots and χM-1 vs T plots. The solid line represents the fitted line using Curie-Weiss law.
bridged metal assemblies show weight loss near 100 °C due to dehydration or desorption of organic solvents. However, [Cu(NH3)2]2[Mo(CN)8] is thermally stable up to 150 °C since water
10.1021/cg060093h CCC: $33.50 © 2006 American Chemical Society Published on Web 07/19/2006
Communications
Crystal Growth & Design, Vol. 6, No. 8, 2006 1737 Supporting Information Available: X-ray crystallographic file in CIF format, experimental details, and the setup of SHG measurement. This material is available free of charge via the Internet at http://pubs.acs.org.
References
Figure 3. SH intensity vs incident light intensity at 295 K (incident light ) 1064 nm). The solid line represents the fitted curve using a quadratic function. The inset shows the image of SH light.
molecules and organic solvents are not present in the crystal. Figure 2d shows the magnetic susceptibility under an applied field of 5000 G. The χMT value at 300 K is equal to 0.87 cm3 K mol-1, which corresponds to the calculated spin-only moment value of 0.84 cm3 K mol-1 for SCu ) 1/2 and the obtained g-value of 2.12 from the ESR measurement. As the temperature decreases, the χMT value is nearly constant until 20 K and then it increases. The observed χM values obey the Curie-Weiss law, i.e., χM ) C/(T - θ), with a Weiss constant (θ) of +0.29(4) K and a Curie constant (C) of 0.87(1) cm3 mol-1 K. The positive sign in the θ value suggests that the magnetic interaction between the CuII through the diamagnetic -NC-MoIV(S ) 0)-CN- bridge is weak ferromagnetic coupling. The Fdd2 structure is a noncentrosymmetric structure with electric polarization. Hence, the present system is SHG active. A SHG measurement of a powder sample was conducted in the reflection mode using the integrating sphere method (Supporting Information). The incident light was provided by a Q-switched Nd: YAG laser (HOYA Continuum, Minilite II, wavelength of 1064 nm, pulse duration of 10 ns, repetition rate of 15 Hz). When the compound is irradiated by 1064 nm light at 295 K, 532 nm light is observed as shown in the inset of Figure 3. Since the intensity of the 532 nm light increases with the square of the incident light intensity (Figure 3), the observed 532 nm light is clearly SH light. The Fdd2 space group has SHG tensor components of χijk(2), i.e., χaca(2), χbbc(2), χcaa(2), χcbb(2), and χccc(2). The observed SH intensity is strong, and the nonlinear optical susceptibility at the fundamental light of 1064 nm is 1.8 × 10-10 esu, which is 12% of susceptibility for KH2PO4 (KDP).8 In summary, a pyroelectric-paramagnetic crystal, which is composed of a cyano-bridged Cu-Mo assembly, has been synthesized. This compound displays SHG at room temperature and also is thermally stable since the crystal does not contain water molecules. Since similar mixed-valence Cu(II)-Mo(IV) compounds are reported to show photoinduced magnetization,7 this material should exhibit optical control of the magnetization-induced SHG (MSHG). Work along this line is currently underway. Acknowledgment. The present research is supported in part by a Grant for 21st Century COE Program “Human-Friendly Materials based on Chemistry” and a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.
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