CRYSTAL GROWTH & DESIGN
{[Cd(bpo)(SCN)2]‚CH3CN}n: A Novel Three-Dimensional (3D) Noninterpenetrated Channel-Like Open Framework with Porous Properties
2002 VOL. 2, NO. 6 625-629
Miao Du, Shen-Tan Chen, and Xian-He Bu* Department of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China Received July 8, 2002;
Revised Manuscript Received August 28, 2002
ABSTRACT: Reaction of CdCl2‚0.5H2O with 2,5-bis(4-pyridyl)-1,3,4-oxodiazole (bpo) and NH4SCN in a 1:1:2 molar ratio under CH3CN-H2O medium afforded a novel neutral three-dimensional (3D) coordination polymer {[Cd(bpo)(SCN)2]‚CH3CN}n (1) [monoclinic, space group P21/c, a ) 7.680(2) Å, b ) 10.029(3) Å, c ) 25.786(7) Å, β ) 93.779(5)°, Z ) 4]. The crystal structure reveals that 1 has a 3D noninterpenetrating open framework with guest CH3CN molecules in the cavities, in which the CdII centers (with CdN4S2 octahedral geometry) are bridged by the bpo molecules and SCN- groups. It is interesting that the two-dimensional sheet in this 3D network consists of 16membered [Cd4(µ-SCN-N,S)4] rings, and the bpo molecules extend these layers to a 3D structure, which is further stabilized by C-H‚‚‚N hydrogen-bonding and strong π-π-stacking interactions. Thermogravimetric analysis and X-ray powder diffraction technique measurements illustrate that the framework of 1 is retained upon removal of the uncoordinated guest acetonitrile molecules, indicating that 1 might be used to generate microporous material. Introduction The rapid development in the area of polymeric metalorganic frameworks (MOFs) continues to produce various structures with interesting compositions and topologies,1-5 along with potential applications as functional solid materials for molecular selection, ion exchange, and catalysis.6-10 Especially, the host frameworks of such complexes that can facilitate the removal/adsorption of guest molecules have been extensively investigated.9,10 By the careful selection and design of organic ligands, such as linear 4,4′-bipyridyl-based building blocks, a wide variety of solid state supramolecular architectures have been constructed.1-5 In the meantime, from the reported studies, it is known that the assembly processes can also be significantly affected by selection of the metal ions, counteranions, pH conditions, and even solvents.11-14 Furthermore, in some cases, two or more different species can be obtained by altering the metal/ligand ratio.15,16 These results imply that subtle factors can vary the nature of the resultant assembled products. As a part of our efforts to investigate the design and control of the self-assembly of organic/inorganic supramolecular motifs with flexible17-19 or rigid20-22 bridging ligands, we have chosen an angular dipyridyl ligand, 2,5-bis(4-pyridyl)-1,3,4-oxadiazole (bpo, see Chart 1), which could potentially provide both discrete and divergent topologies upon metal complexation under appropriate conditions.22 It is especially interesting that the molecular structures of its CuII complexes are profoundly influenced by the anions (from achiral interpenetrating diamondoid networks (for ClO4- and PF6-) to chiral noninterpenetrating three-dimensional (3D) open framework (for SO42- and N3-)22,23), although it has been known that the anions have remarkable influence on the interpenetrating structures.24-27 We have also studied the MII-bpo-NCS- system (MII ) CoII, MnII, and CdII),28,29 which has the general formula * To whom correspondence should be addressed. Tel: +86-2223502809. Fax: +86-22-23502458. E-mail:
[email protected].
Chart 1
[M(bpo)2(NCS)2(H2O)2] and unexpected monomeric structures (the bpo molecule and NCS- anion act only as the monodendate terminal ligands) exhibiting a hydrogenbonded 3D network. Thus, we anticipated that this structure can be extended into multidimensional coordination architecture bridged by bpo and the NCSanion under different self-assembly processes, such as varying the metal/ligand ratio. For CoII and MnII complexes, it failed, and for the CdII complex, a novel covalent instead of hydrogen-bonded 3D network {[Cd(bpo)(SCN)2]‚CH3CN}n (1) was obtained successfully. Here, we report the synthesis and X-ray single-crystal structure of 1 and the generation of a microporous coordination network via the removal of guest acetonitrile molecules from 1. Experimental Section Materials and General Methods. All of the starting materials and solvents for syntheses were obtained commercially and used as received. The bridging ligand bpo was synthesized according to the literature method.30 Fourier transform (FT)-IR spectra (KBr pellets) were taken on a FTIR 170SX (Nicolet) spectrometer. Carbon, hydrogen, and nitrogen analyses were performed on a Perkin-Elmer 240C analyzer. A thermal stability (thermogravimetric analysis, TGA) experiment was carried out on a Dupont thermal analyzer from room temperature to 800 °C under nitrogen atmosphere at a heating rate of 10 °C/min. X-ray powder diffraction technique (XRPD) data were recorded on a Rigaku RU200 diffractometer at 60 kV, 300 mA for Cu KR radiation (λ ) 1.5406 Å), with a scan speed of 2 deg/min and a step size of 0.02° in 2θ. The calculated XRPD pattern was produced using the SHELXTL-XPOW program and single-crystal reflection data.
10.1021/cg025551a CCC: $22.00 © 2002 American Chemical Society Published on Web 09/18/2002
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Crystal Growth & Design, Vol. 2, No. 6, 2002 Table 1. Crystallographic Data and Structure Refinement Summary for Complex 1
formula Mr crystal size (mm) crystal system space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z Dcalcd (g cm-3) µ (mm-1) F(000) range of h, k, l total reflections independent reflections parameters R indices (I >2 σ (I)) goodness of fit on F2 residual electron densities (e Å-3)
C16H11CdN7OS2 493.84 0.15 × 0.20 × 0.30 monoclinic P21/c 7.680(2) 10.029(3) 25.786(7) 93.779(5) 1981.8(9) 4 1.655 1.333 976 -7/9, -11/11, -26/30 7841 3440 230 0.0387, 0.0888 1.003 0.881 to -0.458
Synthesis of Complex {[Cd(bpo)(SCN)2]‚CH3CN}n 1. To a solution of CdCl2‚0.5H2O (0.25 g, 1.0 mmol) in H2O (40 mL) was added a solution of bpo (0.22 g, 1.0 mmol) in CH3CN solution (30 mL) with vigorous mixing for 10 min, and then, an excess of NH4SCN (0.17 g, 2.0 mmol) in aqueous solution (30 mL) was slowly added to the above solution under reflux condition. The reaction mixture was filtered and left to stand at room temperature. Well-shaped prismatic light-yellow single crystals suitable for X-ray analysis were obtained after several days by slow evaporation of the solvent. Yield: 0.42 g (85%). Anal. Calcd for C16H11CdN7OS2: C, 38.91; H, 2.25; N, 19.86%. Found: C, 38.79; H, 2.31; N, 19.66%. IR (KBr, cm-1): 3087m, 2878w, 2251m, 2104vs, 1616s, 1570s, 1540s, 1485s, 1424vs, 1334m, 1321w, 1278m, 1234m, 1217s, 1158w, 1120m, 1062s, 1011s, 968m, 841s, 745s, 727s, 712s, 698m, 506m. X-ray Crystallographic Data Collection and Structural Determination. Single-crystal X-ray diffraction measurement of complex 1 was carried out with a Bruker Smart 1000 CCD diffractometer equipped with a graphite crystal monochromator situated in the incident beam for data collection at 293(2) K. The lattice parameters were obtained by leastsquares refinement of the diffraction data of 7828 reflections, and data collections were performed with Mo KR radiation (λ ) 0.71073 Å) by ω scan mode in the range of 1.58 < θ < 25.03°. All of the measured independent reflections were used in the structural analysis, and semiempirical absorption corrections were applied using the SADABS program. The maximum and minimum transmission factors were 0.8251 and 0.6905. The program SAINT31 was used for integration of the diffraction profiles. The structure was solved by direct methods using the SHELXS program of the SHELXTL package and refined with SHELXL.32 The CdII atom was located from the E-maps, and other nonhydrogen atoms were located in successive difference Fourier syntheses. The final refinement was performed by fullmatrix least-squares methods with anisotropic thermal parameters for all of the nonhydrogen atoms on F2. All of the hydrogen atoms were generated theoretically onto the specific atoms and refined isotropically with fixed thermal factors. A summary of the crystallographic data and structure refinement is listed in Table 1.
Results and Discussion Synthesis and IR Spectra. {[Cd(bpo)(SCN)2]‚CH3CN}n 1 was obtained as light-yellow rectangular plate crystals in high yield (85%) by directly assembling it between CdII, NCS-, and bpo in CH3CN-H2O medium at very dilute conditions to avoid producing a precipitate during this process. As stated above, a hydrogen-bonded 3D network [Cd(bpo)2(NCS)2(H2O)2] can be obtained29
Du et al. Table 2. Selective Bond Lengths (Å) and Angles (deg) for Complex 1a Cd(1)-N(6) Cd(1)-N(4a)i Cd(1)-S(2)ii S(1)-C(13) S(2)-C(14)
Bond Lengths 2.294(4) Cd(1)-N(5) 2.358(4) Cd(1)-N(1) 2.744(2) Cd(1)-S(1)iii 1.633(5) N(5)-C(13) 1.649(5) N(6)-C(14)
2.307(4) 2.360(4) 2.777(2) 1.141(6) 1.156(6)
Bond Angles N(6)-Cd(1)-N(5) 171.97(17) N(6)-Cd(1)-N(4)i 89.19(14) i N(5)-Cd(1)-N(4) 90.13(15) N(6)-Cd(1)-N(1) 90.10(15) N(5)-Cd(1)-N(1) 90.84(15) N(1)-Cd(1)-N(4)i 177.95(15) N(6)-Cd(1)-S(2)ii 96.25(13) N(5)-Cd(1)-S(2)ii 91.75(13) S(2)ii-Cd(1)-N(4)i 89.58(12) N(1)-Cd(1)-S(2)ii 88.59(11) iii iii S(1) -Cd(1)-N(6) 87.04(13) S(1) -Cd(1)-N(5) 85.03(13) S(1)iii-Cd(1)-N(4)i 94.33(12) S(1)iii-Cd(1)-N(1) 87.55(11) S(1)iii-Cd(1)-S(2)ii 174.94(4) N(5)-C(13)-S(1) 178.8(5) N(6)-C(14)-S(2) 178.0(5) a Symmetry codes: (i) x - 1, -y + 3/2, z - 1/2. (ii) -x + 1, y 1/2, -z + 1/2. (iii) -x, y + 1/2, - z + 1/2.
under similar conditions. In both cases, the ligand (bpo), metal ion (CdII), solvent (CH3CN-H2O), and anion (NCS-) were kept constant, and the only varied factor during the two processes was the metal/ligand ratio. Indeed, for both complexes, we found that the final products were independent to the quantity of NCS-, and the key factor governing the structural topologies of them is the CdII/bpo molar ratio. It is also worthy to mention that when other metal ions, such as CoII and MnII, were used, the resulting structures (hydrogenbonded 3D network [M(bpo)2(NCS)2(H2O)2]) were independent to this molar ratio, which was confirmed by X-ray diffraction, IR spectra, and elemental analyses. The IR spectrum of 1 showed the CtN stretching vibrations of thiocyanate at 2104 cm-1 with very strong intensity, suggesting a bridging binding mode of NCS-.33 The characteristic absorption band with medium intensity of guest CH3CN molecules (νCtN) appeared at 2251 cm-1. In addition, the absorption bands resulting from the skeletal vibrations of the aromatic rings were displayed in the 1400-1600 cm-1 region. Description of the Structure of {[Cd(bpo)(SCN)2]‚ CH3CN}n 1. The polymeric structure of 1 was revealed by an X-ray single-crystal structure determination. Each independent unit contains one CdII atom, two coordinated NCS- anions, one bpo molecule, and one included CH3CN solvent molecule. All of the atoms lie on general positions. The local coordination geometry around the Cd(1) center can be best described as a slightly distorted elongated octahedral geometry as shown in Figure 1. In the equatorial plane, the Cd(1) center coordinates to two pyridyl nitrogen atoms of bpo molecules in a trans arrangement (average Cd-N length: 2.301 Å) and to two nitrogen atoms of NCS- ligands (average Cd-N distance: 2.359 Å) and deviates from this plane by ca. 0.0611 Å. Four N-Cd-N bond angles within this plane are very close to 90°, and the sum of these angles is 359.84° (see Table 2). The mean atomic displacement from the least-squares plane of bpo is equal to 0.0576 Å. The two pyridine rings in the same bpo molecule form the dihedral angles of 3.4 and 2.5°, respectively, with the central oxadiazole plane and a dihedral angle of 5.9° with each other. It is interesting that the NCS- ligands interlink the CdII centers to form two-dimensional (2D) sheets in the xy plane in the crystal structure 1, featuring hourglass-
Open Framework with Porous Properties
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Figure 1. Coordination environment of the Cd(1) center in complex 1 (displacement ellipsoids drawn at 50% probability).
Figure 2. Perspective view of the 2D sheet containing [Cd4(µ-SCN-N,S)4] subunit along xy plane (the bpo molecules are omitted for clarity). Green, Cd; yellow, S; black, C; blue, N.
shaped, 16-membered [Cd4(µ-SCN-N,S)4] macrocycles as subunits, as depicted in Figure 2. To our knowledge, such a novel sheet structural topology in Cd-thiocyanate systems is only known for two 2D coordination polymers [Cd(SCN)2(L2)2]‚H2O and [Cd(SCN)2(L3)2] (L2 and L3 refer to nicotinamide and isonicotinamide),34 and several other similar structures have been reported for Mn-thiocyanate or Mn-azido systems.35 The intralayer Cd‚‚‚Cd separations through the µ-SCN-S,N bridge in a [Cd4(µ-SCN-N,S)4] unit are 6.182 and 6.531 Å, comparable to those of related compounds, and the corresponding Cd‚‚‚Cd diagonal distances are 10.028 and 7.680 Å. The bridging thiocyanate anions are almost linear with the S-C-N angles of 178.8(5) and 178.0(5)°. The torsion angles of Cd(1)-N(5)-C(13)-S(1)-Cd(1A) and Cd(1)-N(6)-C(14)-S(2)-Cd(1B), defined as the dihedral angle between two SCN conjoint Cd-N-C-S planes, are 142.0 and 121.2°, respectively. As shown in Figure 3, the 2D [Cd4(µ-SCN-N,S)4] layers are further linked by bridging bpo molecules to form a 3D open framework with channels that are occupied by the included CH3CN molecules. The interlayer Cd‚‚‚Cd distance through bridging bpo, which is also the nearest interlayer Cd‚‚‚Cd separation, is 14.567 Å. If the guest CH3CN molecules are omitted, the channels possessing approximate dimensions of 14.5 Å × 6.5 Å are clearly visible along the crystallographic a-axis (Figure 4). Although an analysis of the voids36 shows that only ca. 2.4% of the space is empty if calculated with the CH3CN molecules, after the removal of these guest solvents, the empty space adds up to 24.2%. One hydrogen atom from the pyridine ring of bpo has been activated by the positive charge due to the
Figure 3. Packing structure of 1 showing the channels along the x-axis (irrelevant hydrogen atoms are omitted for clarity).
Figure 4. Space-filling model of the open framework of 1 exhibiting the voids (cavities are occupied by the included CH3CN molecules, which are omitted for clarity).
coordination of a nitrogen donor to the CdII center, and each CH3CN guest molecule in the channel forms the acceptor of the intramolecular C(4)-H(4A)‚‚‚N(7) hydrogen bond, as depicted in Figure 3. The C‚‚‚N separation is 3.473 Å with a H‚‚‚N length of 2.569 Å, and the C-H‚‚‚N bond angle is 164.1°. In addition, the separation of the interlaced aromatic rings along the a-axis is only 3.25 Å, indicating very strong π-π-stacking interactions. The coeffects of hydrogen-bonding and π-πstacking interactions further stabilize this 3D network. Removal of Guest Molecules: TGA and XRPD Studies. The X-ray single-crystal structure clearly
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weight loss of 9.8 mg, equivalent to the loss of one CH3CN molecule per formula unit (calculated, 10.0 mg). Interestingly, an XRPD pattern for the sample after removal of the included CH3CN molecules (C14H8CdN6OS2: C, 37.13; H, 1.78; N, 18.55%. Found: C, 36.84; H, 2.01; N, 18.51%) remains essentially identical to those of the pristine and calculated compound 1, as shown in Figure 5. The slight shift and splitting of some peaks may be attributed to the subtle change of the relative positions of some atoms in the crystal lattice.37 This phenomenon is commonly observed in zeolites, which indicates the distortion of micropores but does not preclude porosity and maintenance of the framework of 1.37,38 Moreover, the guest CH3CN molecules can be reintroduced into the evacuated sample of 1 by exposure to CH3CN at room temperature for 12 h, which is confirmed by the XRPD pattern, the weight gain of the sample, and the elemental analyses (Found: C, 38.71; H, 2.43; N, 19.79%). This result conclusively demonstrates that the guest CH3CN molecules in 1 have been successfully removed to result in a microporous material with the same network structure as that of 1. In summary, the two main points of the present work are as follows: (i) the first 3D coordination network {[Cd(bpo)(SCN)2]‚CH3CN}n 1 containing a µ-SCN-S,Nbridged [Cd4(µ-SCN-N,S)4]n sheet has been synthesized and structurally characterized, and topochemical conversion, controlled by the metal/ligand ratio, of this covalently bonded framework and a hydrogen-bonded 3D network [Cd(bpo)2(NCS)2(H2O)2] has been found; (ii) the present work also demonstrates that the framework of 1 is retained on removal of the guest CH3CN molecules, indicating that this complex may be used to generate porous materials. Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 29971019). Figure 5. XRPD patterns for 1: (top) calculated; (a) blue, taken at room temperature; (b) green, after removal of the guest CH3CN molecules; (c) red, after reintroduction of the guest CH3CN molecules.
indicates that open channels occupied by guest CH3CN molecules along the a-direction in 1 exist. We were intrigued by the possibility of generating a microporous framework by removing the guest molecules. Thus, TGA for 1 has been investigated first. The TGA curve shows the first weight loss of 8.13% from 75 to 115 °C, corresponding to the loss of one guest CH3CN molecule (calculated: 8.30%). Then, the obtained [Cd(bpo)(SCN)2]n framework does not lose weight upon further heating to 263 °C: two consecutive weight losses in the 260-360 °C region correspond to stepwise removal of the bpo molecules (observed, 45.35%; calculated, 45.40%). Subsequent weight loss in the range of 380-480 °C may correspond to decomposition of the thiocyanate groups. The final product at 800 °C should be a CdS solid according to the residual weight (observed, 28.76%; calculated, 29.25%). According to the TGA results, removal and reintroduction of guest molecules experiments were carried out and monitored by XRPD. A freshly ground sample of 1 (120 mg, 0.24 mmol) was placed inside an oven at 120 °C under vacuum. After 4 h, the sample exhibited a
Supporting Information Available: X-ray crystallographic file in CIF format for 1. This material is available free of charge via the Internet at http://pubs.acs.org.
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CG025551A
