A NaCl-like Metal-Organic Framework Constructed by Unprecedented Tetrahedral Cd4O6 and Cd5O6 Units Rui-Qing Fang, Xu-Hui Zhang, and Xian-Ming Zhang* School of Chemistry & Material Science, Shanxi Normal UniVersity, Linfen, Shanxi 041004, China
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 12 2637-2639
ReceiVed June 29, 2006; ReVised Manuscript ReceiVed October 24, 2006
ABSTRACT: A solvothermal reaction generated a face-centered cubic cadmium 4,5-imidazoledicarboxylate framework with threedimensional intersecting channels containing unprecedented tetrahedral Cd4O6 and Cd5O6 building units. There is considerable current interest in the crystal engineering of metal-organic frameworks (MOFs) due to their potential applications in catalysis, chirality, conductivity, luminescence, magnetism, sensors, nonlinear optics, and zeolite-like materials as well as in the intriguing architectures and topologies.1,2 The oxocentered metal clusters have played an important role in the development of MOFs because they can be reticulated with rigid organic linkers to form porous hybrid solids. The major goal is to control the design of the architectures to obtain materials with required properties. Interesting architectures have been obtained with Zn4O units,3 Zn8(SiO4) cores,4 and triangular oxocentered building units with vanadium,5 zinc,6 iron,7 and chromium.8 Recently, two three-dimensional (3D) metal-organic frameworks with ReO3 and fluorite topologies have been constructed by octahedral Co6O2 and Cd4O8 building units, respectively.9,10 Synthetically, these metal cluster based MOFs can be classified as “controlled” and “serendipitous” two types. For example, Zn4Obased MOFs can be controlled via addition of a small amount of hydrogen peroxide, and M3O-based MOFs are available from M3O(MeCO2)6 precursors. The MOFs containing other metaloxygen clusters such as Co6O2 and Cd4O8 are serendipitously synthesized by hydrothermal methods. We report herein a novel noncentric 3D MOFs [Cd27(dcbi)16](SO4)3‚22DMF (1) (H3dcbi ) 4,5-dicarboxyimidazole) that has a face-centered cubic structure with NaCl-like topology containing unprecedented tetrahedral Cd4O6 and Cd5O6 building units. We have recently found that the coordination architectures of cadmium 4,5-imidazoledicarboxylates could be tuned from mononuclear, one-dimension, two-dimension, to three-dimension.11 The deprotonation process of H3dcbi may give a suitable explanation to the diversity of architectures of complexes and coordination modes of ligand. The general deprotonation process of H3dcbi can be denoted as -H+
-H+
-H+
H3dcbi y\z H2dcbi- y\z Hdcbi2- y\z dcbi3(1) If Cd(II) ions are superfluous in the system, the chemical balance should right-shift with the coordination of Cd(II) ions and ligands, but the increase of proton concentration will counteract the right-shift of chemical balance. Thus, the neutralization of the generated proton is a key to triple deprotonation of H3dcbi to form dcbi. Solvent MeCN is preferable to water in the formation of complexes of triply deprotonated dcbi ligand, which is indicated by the fact that simple replacement of solvent water with H2O/ MeCN results in a structural change from chain-like [Cd(H2dcbi)2(H2O)2] to a 3D framework of [Cd5(Hdcbi)2(dcbi)2(H2O)]‚XH2O. (2) If Cd(II) ions are insufficient, the M/L molar ratio is a very * To whom correspondence should be addressed. E-mail: zhangxm@ dns.sxnu.edu.cn.
important factor to the product. In an aqueous system and at around 140 °C, a lower M/L molar ratio (2:3) gives rise to mononuclear [Cd(H2dcbi)2(H2O)3]‚H2O, [Cd(H2dcbi)2(H2O)2]‚2H2O, and [Cd(H2dcbi)2(H2O)2]‚2H2O, while a higher M/L molar ratio (3:2) generates one-dimensional (1D) chain-like [Cd(H2dcbi)2(H2O)2]. Similarly, in the mixture of MeCN and water and at 140 °C, M/L molar ratios of 1:2 and 5:3 give two-dimensional (2D) layered [Cd(Hdcbi)(H2O)] and 3D porous [Cd5(Hdcbi)2(dcbi)2(H2O)]‚XH2O, respectively. All these observations show that a large metal-to-ligand molar ratio and utilization of non-water organic solvent are two very important factors to obtain intriguing 3-D open frameworks. To further verify the claim, a solvothermal reaction of CdSO4‚8/3H2O, H3dcbi, and N,N-dimethylformamide (DMF) was performed at 160 °C for 5 days, and yellowish crystals in the form of truncated tetrahedra were obtained.12 EA, IR, and X-ray crystallography confirmed the formula of 1.13 Compound 1 crystallizes in the cubic noncentric space group F4h3m, and the asymmetric unit consists of crystallographically independent three Cd(II), one triply deprotonated dcbi, and 1/4 sulfate as shown in Figure S1 (Supporting Information). The C1, C2, C3, C4, N1, N2, O2, O3, and O4 atoms of dcbi are dually disordered, and the crystallographic mirror plane passes through the O1 and C5 atoms of dcbi. All three of the crystallographically independent Cd(II) sites have peculiar geometries as shown in Figure 1. The Cd(1) is coordinated by three O(1), three O(2), and three N(1) atoms that are arranged into a two-vertex truncated trigonal bipyramid. The Cd(1)-O(1), Cd(1)-O(2), and Cd(1)N(1) bond lengths are 2.312(3), 2.37(4), and 2.264(17) Å, respectively. The Cd(2) site has -43m point group symmetry, enclosed by 12 disordered O(3) atoms that show a truncated tetrahedron. The Cd(2)-O(3) distance is 2.21(2) Å. The Cd(3) site has mm3 point group symmetry, enclosed by three O(3), three O(4), and three N(2) atoms. The Cd(3)-N(2), Cd(3)-O(3), and Cd(3)-N(4) bond lengths are 2.152(18), 2.469(19), and 2.39(2) Å, respectively. The nine disordered coordination atoms around the Cd(3) site are a figure formed by juxtaposition of a trigonal prism and a one-vertex truncated tetrahedron. After consideration of half occupancy of N1, N2, O2, O3, and O4 sites caused by dual disorder of dcbi, the average coordination numbers are six for Cd(1) and Cd(2) but four and half for Cd(3). In detail, the Cd(1) is coordinated by 4.5 oxygen and 1.5 nitrogen atoms; the Cd(2) is coordinated by six oxygen atoms; the Cd(3) is coordinated by 3 oxygen and 1.5 nitrogen atoms. The triply deprotonated dcbi shows a new µ5 coordination mode (Figure S3, Supporting Information), which has not been documented in other complexes of dcbi.14,15 For a Cd(II) ion with d10 configuration, the coordination numbers of 4, 5, 6, 7, and 8 have been observed. However, to the best of our knowledge, coordination number of 4.5 for a Cd(II) ion has not been documented. It is clear that the peculiar geometries of Cd(II) sites should come from the crystallographically imposed symmetry and disorder of dcbi.
10.1021/cg060408t CCC: $33.50 © 2006 American Chemical Society Published on Web 11/09/2006
2638 Crystal Growth & Design, Vol. 6, No. 12, 2006
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Figure 1. View of the coordination geometries around Cd(II) sites. Note that the N1, N2, O2, O3, O4, and their equivalent sites are half occupied due to the dual disorder of dcbi.
Figure 2. View of the tetrahedral Cd4O6 (a) and Cd5O6 (b) building units along the [100], [111], and [110] directions, respectively. Note that 12 oxygen atoms appear to be present in the pentanuclear unit due to the dual disorder of O(3) atoms.
There are two types of secondary building units (SBUs) in 1 as shown in Figure 2. The smaller one is a Cd4O6 cluster, and the larger one is a Cd5O6 cluster, and both have -43m point group symmetry. The Cd4O6 cluster is formed by four Cd(1) and six O(1) atoms, while the Cd5O6 cluster is formed by one Cd(2), four Cd(3), and six O(3) atoms. To note, there are only six O(3) atoms in the Cd5O6 cluster, although it looks like 12 due to the dual disorder of the O(3) atoms. Four Cd(1) atoms in the Cd4O6 cluster are an ideal tetrahedron with a Cd(1)-Cd(1) length of 4.4 Å; the peripheral four Cd(3) atoms in Cd5O6 cluster are also an ideal tetrahedron with a Cd(3)-Cd(3) length of 6.1 Å. Each Cd4O6 SBU is connected to six Cd5O6 SBUs via 12 dcbi groups and vice versa. This linking mode results in a 3D NaCl-like cationic framework with intersecting 3D channels that are occupied by sulfate anions and solvent DMF molecules (Figure 3 and Scheme 1). After consideration of van der Waals radii, the window size of the channels is ca. 6.5 × 6.5 Å. A calculation by PLATON indicates that the free volume of the channels is 3751.9 Å3/unit cell volume of 8413.2 Å,3 namely, 44.6% of the crystal volume.16 Thermogravimetric analysis (TGA) in air and under 1 atm pressure at the heating rate of 10 °C min-1 was performed on polycrystalline samples, which shows a continuing weight loss in the temperature range of 130-620 °C. An attempt to exchange sulfate anions with phosphate anions resulted in the decomposition of 1. The initial weight loss of 21% in the range of 130-350 °C corresponds to the removal of solvent DMF molecules, and the final residue of 47% is consistent with the percentage of CdO in 1. Upon photoexcitation with ultraviolet light of 225 nm, complex 1 exhibits strong violet blue photoluminescent emission with maxima in the range of 382 nm. The similarity of emission spectra of 1 to other Cd(II) 4,5-imidazoledicarboxylates indicates that ligand-centered π f π* excitation is responsible for the emission in 1.
Figure 3. The ball-and-stick (a) and space-filling (b) view of the 3D framework with intersecting 3D channels.
In summary, a novel cadmium 4,5-imidazoledicarboxylate framework constructed from tetrahedral Cd4O6 and Cd5O6 cores has been prepared. The crystallographically imposed symmetry and disorder of dcbi result in the peculiar geometries of Cd(II) sites. Synthesis and structural characterization of 1 further confirm that the coordination architectures of cadmium 4,5-imidazoledicarboxylates could be tuned by change of synthetic conditions such as a metal-to-ligand molar ratio and solvent, which is important in crystal engineering chemistry whose goal is controllable synthesis of structures with special properties. The tetrahedral Cd4O6 and Cd5O6 may be used as building units for other functional coordination frameworks. The cubic noncentric space group and good transparency indicate that 1 may be good candidate for nonlinear optical material.
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Crystal Growth & Design, Vol. 6, No. 12, 2006 2639
Scheme 1. Schematic View of the NaCl-like Topology of 1 along the [100] (a) and [111] (b) Directions
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Acknowledgment. This work was financially supported by NSFC (20401011), Foundation for the Author of National Excellent Doctoral Dissertation of PR China (200422), A Program for New Century Excellent Talents in University (NCET-05-0270), and Youth Academic Leader of Shanxi and Education Bureau of Shanxi. Supporting Information Available: XRPD, TGA, other structural figures and CIF file are available free of charge via the Internet at http:// pubs.acs.org.
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