Imidazoledicarboxylate Depositing Protons or Alkali Metal Ions

CRYSTAL GROWTH & DESIGN

Cubic Metal-Organic Polyhedrons of Nickel(II) Imidazoledicarboxylate Depositing Protons or Alkali Metal Ions

2008 VOL. 8, NO. 7 2458–2463

Qiang Xu,*,†,‡ Ru-Qiang Zou,†,‡ Rui-Qin Zhong,†,‡ Chihiro Kachi-Terajima,§ and Satoshi Takamizawa§ National Institute of AdVanced Industrial Science and Technology, Ikeda, Osaka 563-8577, Graduate School of Engineering, Kobe UniVersity, Nada Ku, Kobe, Hyogo 657-8501, and International Graduate School of Arts and Sciences, Yokohama City UniVersity, 22-2 Seto, Kanazawa-ku, Yokohama, Kanagawa 236-0027, Japan ReceiVed January 21, 2008; ReVised Manuscript ReceiVed March 1, 2008

ABSTRACT: Reactions of nickel(II) nitrate with 4,5-imidazoledicarboxylic acid (H3L) in neutral or basic environment result in a series of novel nickel coordination polymers with a uniform Ni8 cubic building block, [(Ni8(H2L)8(HL)4)] · 8(C2H5OH) · 18(H2O) 1, K20[Ni8L12] · 74(H2O) 2a, K20[Ni8L12] · 50(H2O) 2b, and K20[Ni8L12] · 29(H2O) 2c. The result shows that complex 1 assembles into a discrete neutral metal-organic polyhedron with 20 protons located in the carboxyl oxygen atoms, which are linked into a threedimensional supramolecular architecture by intermolecular hydrogen bonds between guest ethanol and host carboxyl oxygen atoms. Complex 2a exhibits a water ocean, in which there are 20 potassium ions and 74 water molecules around each Ni8 cubic building block. During the course of gradual loss of the guest water molecules, the adjacent cubic building blocks in 2a rearrange to form its dehydrated products 2b and 2c, the crystallinity remaining. The magnetic properties have been investigated. Introduction Design and synthesis of metal coordination polymers (MCPs) via deliberate selection of metal and organic ligand have been developed into one of the most attractive topics because of the fascinating structural diversity1 and potential applications in catalysis,2 separation,3 and gas storage.4 In particular, flexible MCPs5 have caught enormous attention lately, which generally lies in the utilization of intermolecular weak interactions, such as hydrogen bonding, π-stacking, and hydrophobic interaction, as well as strong covalent and coordinative bonding. Recently, a new breakthrough of flexible microporous MCP (MMCP), known as dynamic micropore approach, has emerged as a flexible interpenetrating coordination framework with a bimodal porous functionality,6 in which the pore size can be controlled through framework dynamics to achieve selectivity and increased binding of ions and gases.7 This strategy has been proved to be very effective for the design and syntheses of materials with specific structures and functions. On the other hand, to transition metal ions, alkali cations are intriguing because of their various coordination modes, low polarizability, and unique affinity for basic molecules ranging from a strong base to a weak one.8 For instance, alkali cations in zeolites are well-known to be catalytic sites for hydrocarbon transformation and also serve as guest recognition sites in biological systems.9 We recently presented a new route toward the construction of the flexible MCPs by employing a discrete metal-organic polyhedron, [Ni8L12]20- (L ) 4,5-imidazoledicarboxylate),10 which can not only be present in a multiacid form but also deposit alkali metal ions. It is worthy to note that the molecular polyhedron has a highly negative charge with a rich store of peripheral carboxylate donors and hydrogen-bond acceptors. Therefore, the molecular polyhedron may act as a metalloligand to interact with metal ions and hydrogen-bonded donors. It is most interesting that the discrete metal-organic polyhedrons exhibit flexible dynamic process present in different environments. * To whom correspondence should be addressed. E-mail: [email protected]. † National Institute of Advanced Industrial Science and Technology. ‡ Kobe University. § Yokohama City University.

For further detailed investigation of the properties of such molecular polyhedrons based on our former study,10 in this work we explore the influences of various alkalescence environments on the products and report four new MMCPs, [(Ni8(H2L)8(HL)4)] · 8(C2H5OH) · 18(H2O) 1, K20[Ni8L12] · 74(H2O) 2a, K20[Ni8L12] · 50(H2O) 2b, and K20[Ni8L12] · 29(H2O) 2c. Experimental Section Materials and General Methods. All the solvents and reagents for synthesis were obtained from commercial vendors and used as purchased. Thermogravimetric analysis (TGA) was carried out on a Shimadzu DTG-50 thermal analyzer from room temperature to 600 °C at a ramp rate of 5 °C/min in a flowing 150 mL/min helium atmosphere. FT-IR spectra (KBr pellets) were recorded on a BIORAD FTS-6000e spectrometer at 0.5 cm-1 resolution using a liquid-nitrogencooled HgCdTe (MCT) detector for the 5000-400 cm-1 spectral range. Powder X-ray diffraction (PXRD) patterns were recorded on a Rigaku X-ray diffractometer at 40 kV, 150 mA for Cu KR radiation (λ ) 1.5406 Å). Elemental analyses were performed on a Perkin-Elmer 2400 Series II analyzer. Magnetic measurements were made on a Quantum Design MPMS-XL SQUID magnetometer in the temperature range of 5-300 K with an applied field of 1 T. Data collections were performed with microcrystals of the fresh sample. The experimental data were corrected for the sample folder contribution and for the diamagnetic contribution estimated from Pascal constants.11 Syntheses of Complexes. [(Ni8(H2L)8(HL)4)] · 8(C2H5OH) · 18(H2O) (1). A mixture of Ni(NO3)2 · 6H2O (0.058 g, 0.2 mmol), H3L (0.047 g, 0.3 mmol) and C2H5OH/H2O (10:1, 10 mL) was sealed and heated for four days in vial at 140 °C. The reaction mixture was then allowed to cool to room temperature naturally. Several block-shape crystals of 1 formed on the wall and bottom, and were separated by hand and collected for single crystal X-ray structure determination. IR (KBr, cm-1): 3388b, 1571vs, 1472vs, 1393vs, 1297w, 1256m, 1108m, 844w, 799w, 666w. K20[Ni8L12] · 74(H2O) (2a). A mixture of Ni(NO3)2 · 6H2O (0.058 g, 0.2 mmol) and H3L (0.047 g, 3 mmol) was added to a 20 mL aqueous solution to form a suspension. To the suspension was added 0.1 M KOH to adjust pH to 8.0. The resulting solution was filtered, and allowed to stand at room temperature. Single crystals suitable for X-ray analysis were obtained after two weeks. Yield: ∼80% (based on H3L). K20[Ni8L12] · 50(H2O) (2b) and K20[Ni8L12] · 29(H2O) (2c). Complex 2a loses gradually the crystalline water molecules to result in the dehydrated 2b and 2c by exposing 2a in air for several days. Anal.

10.1021/cg8000762 CCC: $40.75  2008 American Chemical Society Published on Web 05/28/2008

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Figure 2. Views of (a) the [Ni8L12]20- cubic building block depositing KI ions, (b) the linked mode between the adjacent cubic building blocks, and (c) the three-dimensional network bridged by K-O bonds of 2a.

Figure 1. Views of (a) the neutral H20[Ni8L12] cubic building block, (b) 1D open channels through cubic center, and (c) three-dimensional network constructed by hydrogen-bonding interactions of 1. Calcd for 2c, C60H70K20N24O77Ni8: C, 19.96; H, 1.95; N, 9.31. Found: C, 20.04; H, 1.78; N, 9.11. IR (KBr, cm-1): 3410b, 1607s, 1464m, 1372vs, 1293w, 1247w, 1095m, 845m, 808m, 672w. X-ray Crystallography. Single-crystal X-ray diffraction data for complexes 1, 2a and 2b were collected on a Bruker Smart 1000 CCD diffractometer with Mo KR radiation (λ ) 0.71073 Å), and the crystal data of complex 2c were collected on Rigaku RAXIS-RAPID diffractometer with Mo KR radiation (λ ) 0.71073 Å). All the structures were solved by direct methods using the SHELXS program of the SHELXTL package and refined by full-matrix least-squares methods with SHELXL.12 Metal atoms in each complex were located from the E-maps, and other non-hydrogen atoms were located in successive difference Fourier syntheses, which were refined with anisotropic thermal parameters on F2. The hydrogen atoms of the ligands were generated theoretically onto the specific atoms and refined isotropically

with fixed thermal factors. It is worthy to be noted that complexes 1, 2a and 2b are all air-sensitive and it is very easy to volatilize crystalline water or ethanol molecules while exposing in air, which results in the disordered guest molecules from the difference map.

Results and Discussion Preparation and Characterization. Imidazole-4,5-dicarboxylic acid has six potential donor atoms, and can release one to three protons forming H2L1-, HL2- and L3- anions, which make the deprotonated ligands exhibit flexible coordination modes. Thus in different reaction conditions, the reaction of H3L with metal ions created variational species.13 Through the careful control of reaction condition, a series of Ni(II) coordination polymers with cubic metal-organic polyhedrons are created.10 In former research, we reported Li- or Na-containing coordination polymers with such cubic metal-organic polyhedrons by hydro/solvothermal reactions.10b,c However, the two

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Figure 3. Views of (a) the [Ni8L12]20- cubic building block depositing KI ions, (b) the linked mode between the adjacent cubic building blocks, (c) the two-dimensional layer bridged by K-O bonds, and (d) the packing figure of the adjacent two-dimensional layers of 2b .

complexes are insoluble in water or general organic solvent, so it is difficult to study the properties of such cubic metal-organic polyhedrons. In this work, we present the structures of the neutral entity of such cubic metal-organic polyhedrons by solvothermal reaction as well as the K-containing species by the reaction at room temperature. Notedly, the K-containing species is well soluble in water. To obtain these pure crystals, a slight stir to the mixture is necessary before it is transferred to the sealed vial for solvothermal reaction or stands at room temperature. It should be noted that only several block-shape crystals of 1 were separated by hand and collected for singlecrystal X-ray structure determination. The PXRD pattern indicates that the bulk faint green powder on the bottom of the reactor is a different phase from 1. Furthermore, due to the inclusion of many crystalline water or ethanol molecules, complexes 1, 2a and 2b are all air-sensitive and it is very easy to volatilize crystalline water or organic guest molecules while exposing in air. Complex 2c exhibits good stability to air, and the compositions were validated by elemental analyses and IR spectra. On the other hand, since the K-containing complex is

soluble in water with the structural integrity retained, it is feasible that this complex can serve as a metalloligand to interact with metal ions and hydrogen-bonded donors. Structural Description of 1-3. Complex 1 is synthesized by the direct reaction of Ni(II) ions with H3L ligand in EtOH/ H2O without any alkali reagents, and the ligands adopt monodeprotonated and dideprotonated forms to coordinate to Ni(II) ions (Figure 1a). The cubic structural unit is the same as the reported one.10 It should be noted that the structure of 1 exhibits high symmetry with the space group of Fmmm. Despite lacking the strong coordination bonds between the adjacent cubic building blocks, complex 1 still self-assembles 1D open channels through intermolecular hydrogen-bonding interactions (Figure 1b). The weak C-H · · · O H-bonding (Table S1, see Supporting Information) serves as a spring to link the adjacent cubic building blocks to form a three-dimensional network (Figure 1c). The shortest centroid-centroid separation of neighbor cubic building blocks is 16.14 Å. It is most important that the structural determination of 1 completely indicates that such cubic building

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Figure 5. TG curve of 2c.

Figure 4. Views of (a) the [Ni8L12]20- cubic building block depositing KI ions, (b) the linked mode between the adjacent cubic building blocks, and (c) the three-dimensional network bridged by K-O bonds of 2c.

blocks can exist stably in both acidic and alkaline conditions, which offers a wide application perspective for such cubic building blocks. Complex 2a crystallizes in the monoclinic system C2/c space group and consists of [Ni8L12]20- cubic building blocks bridged by KI ions. Figure 2a illustrates the cubic building block of 2a surrounded by KI ions through K-O bonds. Notably, there are 74 water molecules surrounding each [Ni8L12]20- cubic building block. The shortest centroid-to-centroid separation of neighbor cubic building blocks is 16.73 Å (Figure 2b). It is different from complex 1 as well as the reported Li- or Na-containing complexes10b,c that complex 2a adopts a compact and compli-

cated molecular packing mode without the open channels in the three-dimensional framework. All potassium ions assemble into a 3D network with normal K-O and K-K interactions. Furthermore, all of the carboxyl oxygen atoms of L ligands in 2a are bonded by potassium atom to form a three-dimensional compact network, which further strengthens the framework rigidity of 2a (Figure 2c). Complexes 2b and 2c are the dehydration products of 2a with the same [Ni8L12]20- cubic building blocks (Figures 3 and 4). There are 50 and 29 water molecules surrounding each cubic building block of 2b and 2c, respectively. The shortest centroidto-centroid separations of neighboring cubic building blocks in 2b and 2c are 15.87 and 15.75 Å, respectively, which are shorter than that of 2a (Figures 3b and 4b). Complex 2b forms a twodimensional layer structure (Figure 3c) lacking strong coordination bonding interactions between adjacent layers (Figure 3d), while complex 2c forms a three-dimensional compact network (Figure 4c). Interestingly, reactions of Ni(II) nitrate with imidazole-4,5dicarboxylic acid at different conditions create seven novel coordination polymers with a uniform cubic metal-organic polyhedron depositing 20 protons or alkali metal ions (Scheme 1), in which these cubic building blocks adopt different spacearrangement modes in different environments. As shown in Table 1, the shortest centroid-to-centroid distances of the neighboring cubic building blocks in the seven complexes fall into the range of 15.7-16.8 Å, among which the one of complex 2c shows the closest separation of 15.75 Å due to loss of plenty of crystalline water molecules. Complexes 1, Li-1, Na-1 and Na-2 form open channels by either intermolecular hydrogen bonding or alkali-O (Li-O and Na-O) coordination bonding interactions, while K-containing complexes 2a, 2b and 2c adopt compact packing modes without the open channels. Furthermore, complexes 2a, 2b and 2c exhibit unique water-solubility (Scheme 2), which makes it feasible to study the properties of the separated cubic building blocks. Thermal Stability of 2c. Thermogravimetric analysis (TGA) experiment was conducted to determine the thermal stability of 2c, which is an important aspect for metal coordination polymers. As shown in Figure 5, complex 2c loses the crystalline water molecules from room temperature to 300 °C, which is probably attributable to the strong K-Owater interactions. The cubic building block can be stable up to 400 °C. The powder X-ray diffraction (PXRD) patterns indicate that these cubic building blocks of 2c rearrange after the loss of the crystalline

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Figure 6. χT vs T plots for (a) Li-1, (b) Na-2, (c) 2c and (d) the merged ones of Li-1, Na-2 and 2c for comparison. The solid line represents the best fit of the data above 30 K.

Scheme 1

water molecules at different temperatures (see Supporting Information), which are also proved by the former cases of the reported Li-1 and Na-1 compounds.10b,c Magnetic Properties. The magnetic susceptibility on polycrystalline samples of complexes Li-1, Na-2, and 2c was

measured in the temperature range of 5-300 K at an applied field of 1 T. The temperature dependence of 1/χ for the three complexes above 150 K obeys a Curie-Weiss law with C ) 8.8 cm3 · K · mol-1, θ ) -69 K for Li-1, C ) 8.8 cm3 · K · mol-1, θ ) -61 K for Na-2, C ) 8.8 cm3 · K · mol-1, θ ) -61 K for

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Table 1. The shortest Centroid-to-Centroid Distances (dc-c, Å) between the Adjacent Cubic Building Blocks

dc-c (Å)

1

2a

2b

2c

Li-1

Na-1

Na-2

16.14

16.73

15.87

15.75

16.58

16.36

16.44

Scheme 2

2c. Curie constants are compatible with the expected value for highspin 8 NiII ions with a g value of 2.10. The negative Weiss constants indicate that the antiferromagnetic exchange interactions between the magnetic centers are dominant within the Ni8 cluster. It should be noted that the magnetic behavior of this series has been found to be almost identical. The plots of χT vs T for Li-1, Na-2 and 2c are shown in Figure 6. At 300 K, the χT product is 6.97 cm3 · K · mol-1 for Li-1 (Figure 6a), 7.26 cm3 · K · mol-1 for Na-2 (Figure 6b) and 7.15 cm3 · K · mol-1 for 2c (Figure 6c), respectively. These values are slightly below the expected spin-only value for high-spin 8 NiII ions considering g value of 2.00 (8.08 cm3 · K · mol-1). Upon lowering temperature, the χT decreases continuously to 5 K (0.60 cm3 · K · mol-1 for Li-1, 0.28 cm3 · K · mol-1 for Na-2 and 0.08 cm3 · K · mol-1 for 2c. This feature is also observed in the Ni8 cluster possessing the same cubic skeleton,10a and typical in antiferromagnetically coupled systems, resulting in an S ) 0 diamagnetic ground state. The simulation was performed with the MAGMUN-4.1 program14 in the temperature range of 30-300 K using the following Hamiltonian: H ) -2JNi-Ni[S1(S6 + S7 + S8) + S2(S5 + S7 + S8) + S3(S5 + S6 + S8) + S4(S5 + S6 + S7)], where all JNi-Ni within the cluster are considered to be equal in order to simplify the model. We did not use the data below 30 K for fitting in order to avoid the possible effects of the local ZFS of NiII ions and intercluster interactions. The obtained fitting parameters are g ) 2.04, JNi-Ni ) -12.0 K for Li-1, g ) 2.06, JNi-Ni ) -11.5 K for Na-2 and g ) 2.07, JNi-Ni ) -12.7 K for 2c. The antiferromagnetic coupling between NiII ions can be compared with that previously studied in an imidazolate-bridged complex (the magnitude of JNi-Ni varies from -9.1 to -43.8 K).15 These results indicate that the local exchange within a Ni8 cluster is essentially the same for the three samples in spite of the different intercluster environment. Conclusions and Perspectives In conclusion, a series of novel metal-organic coordination polymers of nickel(II) imidazoledicarboxylate depositing protons or alkali metal ions are presented. The result shows that these complexes embody a uniform cubic Ni8 building block, which can exist stably in both acidic and alkaline conditions. Notably, in different environments, the cubic building block exhibits different bonding abilities with alkali metal ions resulting in various metal-organic framework structures. The K-containing complex is soluble in water, implying that this complex can serve as a potential metalloligand to interact with metal ions and hydrogen-bonded donors. The corresponding studies are in progress in our group. Acknowledgment. We thank AIST and Kobe University for financial support. R.-Q. Zou thanks JSPS for a fellowship (DC). Supporting Information Available: Crystallographic information in CIF format, table, and PXRD patterns. This material is available free of charge via the Internet at http://pubs.acs.org.

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