Characterization of 3-D Metal−Organic Frameworks Formed through

Sep 22, 2004 - Metal−Organic Coordination Polymers Generated from Chiral Camphoric Acid and Flexible Ligands with Different Spacer Lengths: Synthese...
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Characterization of 3-D Metal-Organic Frameworks Formed through Hydrogen Bonding Interactions of 2-D Networks with Rectangular Voids by CoII- and NiII-Pyridine-2,6-dicarboxylate and 4,4′-Bipyridine or 1,2-Di(pyridyl)ethylene

CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 2 623-629

Sujit K. Ghosh,† Joan Ribas,*,‡ and Parimal K. Bharadwaj*,† Chemistry Department, Indian Institute of Technology, Kanpur, 208016, India, and Department de Quimica Inorganica, Universitat de Barcelona, Diagonal 647. 08028 Barcelona, Spain Received May 18, 2004;

Revised Manuscript Received August 9, 2004

ABSTRACT: Pyridine-2,6-dicarboxylic acid (pdcH2) and 4,4′-bipyridyl (4,4′-bpy) reacts under hydrothermal condition with Co(II)- or Ni(II)-nitrate to form very similar 2-D metal-organic framework (MOF) structures with the empirical formula [M(pdc)(4,4′-bpy)]‚1/2MeOH (M ) Co(II), 1 and Ni(II), 2). Compound 1 crystallizes in the monoclinic space group P21/n with a ) 11.419(3) Å, b ) 10.290(2) Å, c ) 15.695(5) Å, β ) 96.052(5)°, V ) 1833.9(13) Å3, Z ) 4, R1 ) 0.046, WR2 ) 0.125, and S ) 1.059. Compound 2 also crystallizes in the monoclinic space group P21/n with a ) 11.249(5) Å, b ) 10.277(2) Å, c ) 15.607(5) Å, β ) 97.916(5)°, V ) 1787.1(13) Å3, Z ) 4, R1 ) 0.058, WR2 ) 0.155, and S ) 0.99. Rectangular voids of approximate dimension 8.9 × 5.5 Å formed in these two structures remain empty. Each 2-D sheet thus formed is stacked on top of each other with an intricate array of hydrogen bonding, showing a perpendicular distance of ∼8.1 and 7.8 Å in 1 and 2, respectively. When 1,2-di(4-pyridyl)ethylene is used in place of 4,4′-bipyridyl to construct the MOFs, only Ni(II)-nitrate affords crystals whose structure differs greatly from that of 1 or 2. Here, 1,2-di(4-pyridyl)ethylene is not bonded to the metal but exists in the lattice in diprotonated form, and a hydrogen-bonded 3-D structure of discrete (dpeH2)[Ni(pdc)2]‚5H2O (3) units is formed. Compound 3 crystallizes in the monoclinic space group P21/n with a ) 8.915(2) Å, b ) 19.844(6) Å, c ) 15.799(5) Å, β ) 96.083(5)°, V ) 2779.2(19) Å3, Z ) 4, R1 ) 0.0391, WR2 ) 0.0998, and S ) 1.096. Interestingly, in 3, an acyclic trimeric water cluster is hydrogen bonded to the spacer and a free carboxylate O atom. Introduction Formation of metal-organic framework (MOF) structures is an active area of research, as these compounds can be potentially useful1-10 in gas storage, molecular sieves, size- and shape-selective catalysis, magnetism, optoelectronic devices, and so on. Success in producing such structures depends11,12 on understanding and controlling the topological and geometric relationships between molecular modules, along with the coordination characteristics of the metal ions. We have initiated a research program to assemble more than one type of organic ligands around metal ions, to form coordination polymers with designer structures with the ultimate goal(s) of having new materials with a range of applications. When two or more ligands are used, the design and choice of these components must fulfill criteria for spontaneously generating well-defined architectures. Much of the current efforts have been directed toward synthesis of open framework structures with rigid organic ligands. Aromatic polycarboxylates are chosen13-17 as rigid tectons, to bind metal centers for constructing supramolecular metal-organic framework structures. Rigid dicarboxylates are particularly attractive because as the metal carboxylate bonding is also rigid, use of appropriate spacers can lead to predeter* Corresponding author. (P.K.B.) E-mail: [email protected]; phone: +91(512)2597034; fax: +91(512)2597436. (J.R.) E-mail: joan.ribas@ qu.ub.es. † Indian Institute of Technology Kanpur. ‡ Universitat de Barcelona.

mined network structures. In the present studies, pyridine-2,6-dicarboxylic acid18 is used as a ligand for binding more than one metal ion through carboxylate bridging to form 1-D coordination polymeric chains. To connect these chains, either rigid 4,4′-bipyridine (4,4′bpy) or flexible 1,2-di(pyridyl)ethylene (dpe) has been used. With 4,4′-bpy spacer, stable 2-D coordination polymeric structures with rectangular voids are readily formed with Co(II) and Ni(II) ions. When a dpe spacer is used, it does not bind the metal ion and exists in the lattice in diprotonated form. In this case, a discrete complex (dpeH2)[Ni(pdc)2]‚5H2O is formed where the metal ion is octahedrally coordinated with two pdc2units. Three out of the five H2O molecules form an unprecedented acyclic water trimer. Herein, we describe the hydrothermal syntheses of three MOF structures and their characterization by X-ray crystallography, powder X-ray data, infrared, and variable temperature magnetic susceptibility measurements. Experimental Procedures Materials. The metal salts, 4,4′-bipyridine, 1,2-di(4-pyridyl)ethylene, and pyridine-2,6-dicarboxylic acid were acquired from Aldrich and used as received. All the syntheses were performed in Teflon-lined stainless steel autoclaves under autogenous pressure. Physical Measurements. Spectroscopic data were collected as follows: IR (KBr disk, 400-4000 cm-1) Perkin-Elmer model 1320; X-ray powder pattern (CuKR radiation at a scan rate of 3°/min, 293 K) Siefert ISODEBYEFLEX-2002 X-ray generator; and thermogravimetric analysis (heating rate of 5

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Table 1. Crystal Data and Structure Refinement for 1-3 empirical formula formula weight T (K) radiation wavelength (Å) crystal system space group a, Å b, Å c, Å β (deg) V, Å3 Z Fcalc Mg/m3 µ, mm-1 F(000) refl. collected independent refl. refinement method GOF final R indices [I > 2σ(I)] R indices all data

1

2

3

C17.5H13N3O4.5Co 396.24 293(2) MoKR 0.71073 monoclinic P21/n 11.419(3) 10.290(2) 15.695(5) 96.052(5) 1833.9(13) 4 1.435 0.965 808 2393 1806 full-matrix least-squares on F2 1.059 R1 ) 0.0460 wR2 ) 0.1253 R1 ) 0.0790 wR2 ) 0.1397

C17.5H13N3O4.5Ni 396.02 293(2) MoKR 0.71073 monoclinic P21/n 11.249(5) 10.277(2) 15.607(5) 97.916(5) 1787.1(13) 4 1.472 1.116 812 3148 1759 full-matrix least-squares on F2 0.99 R1 ) 0.0584 wR2 ) 0.1548 R1 ) 0.1322 wR2 ) 0.1916

C26H28N4O13Ni 663.23 293(2) MoKR 0.71073 monoclinic P21/n 8.915(2) 19.844(6) 15.799(5) 96.083(5) 2779.2(19) 4 1.585 0.775 1376 3639 2873 full-matrix least-squares on F2 1.096 R1 ) 0.0391 wR2 ) 0.0998 R1 ) 0.0599 wR2 ) 0.1170

°C/min) Mettler Toledo Star System. The magnetic studies were carried out in the Servei de Magnetoquimica (Universitat de Barcelona) on crystalline samples of 1-3 with a Quantum Design SQUID MPMS-XI susceptometer working in the temperature range of 2-300 K at an applied field of 0.1 T. EPR spectrum for complex 2 was recorded on powder samples at X-band frequency with a BRUKER 300E automatic spectrometer at 4 K. Microanalyses for the compounds were obtained from CDRI, Lucknow. Syntheses. [Co(pdc)(4,4′-bpy)]‚1/2MeOH, 1. This compound was synthesized solvothermally, by reacting Co(NO3)2‚ 6H2O, pdcH2, and 4,4′-bpy in the molar ratio of 1:1:1 in the presence of 3 mL of water and 1 mL of MeOH for 3 days at 170 °C and then cooled for 7 h. The final product obtained as long reddish needles was collected by filtration and washed with water, then MeOH, and finally with ether. Yield ∼40%. Anal. Calcd for C17.5H13N3O4.5Co: C, 53.05; H, 3.31; N, 10.61%. Found: C, 52.65; H, 3.43; N, 9.96%. [Ni(pdc)(4,4′-bpy)]‚1/2MeOH, 2. This complex was synthesized as stated previously, reacting Ni(NO3)2‚6H2O, pdcH2, and 4,4′-bpy in a 1:1:1 molar ratio in 3 mL of water and 1 mL of MeOH for 4 days at 180 °C followed by cooling for 8 h. The desired product was isolated as green needles in ∼35% yield. Anal. Calcd for C17.5H13N3O4.5Ni: C, 53.08; H, 3.308; N, 10.611%. Found: C, 52.74; H, 3.52; N, 9.87%. (dpeH2)[Ni(pdc)2]‚5H2O, 3. Solvothermal synthesis of 3 by reacting Ni(NO3)2‚6H2O, pdcH2, and dpe in equimolar amounts (following the procedure adopted for 2) afforded dark blue crystals in the form of rectangular parallelopipeds. Yield ∼42%. Anal. Calcd for C26H28N4O13Ni: C, 47.09; H, 4.255; N, 8.45%. Found: C, 46.74; H, 4.52; N, 8.23%. When 1-3 equiv of NaOH was added in the reaction mixture to inhibit protonation of dpe, no crystalline product could be isolated. X-ray Structural Studies. Single-crystal X-ray data on 1-3 were collected at room temperature on an Enraf-Nonius CAD4 Mach2 X-ray diffractometer using a graphite monochromated MoKR radiation (λ ) 0.71073 Å). The cell parameters in each case were determined by least-squares refinement of the diffractometer setting angles, from 25 centered reflections that were in the range 18° e 2θ e 23°. Three standard reflections were measured every hour to monitor instrument and crystal stability. The linear absorption coefficients, scattering factors for the atoms, and the anomalous dispersion corrections were taken from International Tables for X-ray Crystallography.19 The structures were solved by the direct method, using SIR9220, and were refined on F2 by the full-matrix least-squares technique using the SHELXL-9721

program package. All hydrogen atoms were located in successive difference Fourier maps, and they were treated as riding atoms using SHELXL default parameters. The crystal data for the structures are collected in Table 1.

Results and Discussion All three complexes are stable in air and insoluble in most organic solvents. High yields of the products indicate that the compounds are thermodynamically quite stable under the reaction conditions. Thermal gravimetric analysis of 1 and 2 show that the structures are stable and do not decompose up to at least 300 °C. The 3-D structure in 3 is largely a consequence of hydrogen-bonding interactions involving the water molecules, the spacer, and the free carboxylate O atoms of the pdc2- ligands. This association is quite strong as thermal gravimetric analysis of 3 with a 9.17 mg sample in air shows that weight loss due to water removal occurs in stages beginning at ∼50 °C and continuing till ∼180 °C. The complete decomposition of the compound is achieved above ∼200 °C. Powder X-ray diffraction patterns of the compound before and after water expulsion show major changes in peak positions as well as their intensities, suggesting breakdown of the host lattice due to the exclusion of water. The IR spectra of the three compounds show strong absorption bands between 1350 and 1550 cm-1 that are diagnostic of coordinated carboxylates.18a In 3, a broad band is observed around 3400 cm-1 attributable18a to the O-H stretching frequency of the water cluster. This broad band vanishes on heating the compounds under vacuum (0.1 mm) at 180 °C for 2 h, suggesting escape of the water clusters from the lattice. Deliberate exposure to water vapor for 3 days does not lead to reabsorption of water into the lattice as monitored by FTIR spectroscopy. Selective bond distances and bond angles involving the metal ion in 1-3 are given in Table 2, while hydrogen-bonding interactions present in the structures are collected in Table 3. The asymmetric unit of 1 consists of a Co(II), a pdc2- ligand, a 4,4′-bipy, and a

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Table 2. Selected Bond Distances (Å) and Bond Angles (deg) in 1-3 1 Co-O(1) Co-O(3) Co-N(3) O(1)-Co-N(1) N(1)-Co-O(3) N(1)-Co-N(2) O(1)-Co-N(3) O(3)-Co-N(3) O(1)-Co-O(2) O(3)-Co-O(2) N(3)-Co-O(2)

2.035(3) 2.128(3) 2.167(4) 170.86(13) 75.56(13) 90.55(14) 84.73(14) 89.11(14) 97.12(12) 149.26(12) 92.02(13)

Ni-N(1) Ni-N(3) Ni-O(3) N(1)-Ni-O(1) O(1)-Ni-N(3) O(1)-Ni-N(2) N(1)-Ni-O(3) N(3)-Ni-O(3) N(1)-Ni-O(2) N(3)-Ni-O(2) O(3)-Ni-O(2)

1.996(5) 2.109(5) 2.127(4) 173.0(2) 85.2(2) 87.7(2) 77.7(2) 89.0(2) 76.0(2) 92.2(2) 153.7(2)

Ni-N(1) Ni-O(3) Ni-O(7) N(1)-Ni-N(2) N(2)-Ni-O(3) N(2)-Ni-O(5) N(1)-Ni-O(7) O(3)-Ni-O(7) N(1)-Ni-O(1) O(3)-Ni-O(1) O(7)-Ni-O(1)

1.965(3) 2.097(3) 2.137(3) 177.9(1) 99.4(1) 77.9(1) 103.2(1) 93.7(1) 77.3(1) 156.2(1) 90.9(1)

Co-N(1) Co-N(2) Co-O(2) O(1)-Co-O(3) O(1)-Co-N(2) O(3)-Co-N(2) N(1)-Co-N(3) N(2)-Co-N(3) N(1)-Co-O(2 ) N(2)-Co-O(2 )

2.068(4) 2.165(4) 2.216(3) 113.56(13) 88.42(14) 93.10(13) 96.30(15) 173.13(14) 73.78(12) 89.41(13)

Ni-O(1) Ni-N(2) Ni-O(2) N(1)-Ni-Ν(3) N(1)-Ni-N(2) N(3)-Ni-N(2) O(1)-Ni-O(3) N(2)-Ni-O(3) O(1)-Ni-O(2) N(2)-Ni-O(2)

2.035(4) 2.110(5) 2.196(4) 95.7(2) 91.4(2) 172.9(2) 109.3(2) 93.3(2) 96.9(1) 88.7(2)

Ni-N(2) Ni-O(5) Ni-O(1) N(1)-Ni-O(3) N(1)-Ni-O(5) O(3)-Ni-O(5) N(2)-Ni-O(7) O(5)-Ni-O(7) N(2)-Ni-O(1) O(5)-Ni-O(1)

1.964(3) 2.132(3) 2.163(3) 78.9(1) 100.8(1) 90.3(1) 78.1(1) 156.0(1) 104.4(1) 94.8(1)

2

Figure 1. View illustrating the coordination geometry around the metal. For 1, M ) Co(II) and for 2, M ) Ni(II).

3

Table 3. Hydrogen Bonding Distances (Å) and Angles (deg) in 1-3 1 O1‚‚‚O5 O1‚‚‚C18 O2‚‚‚C18 O3‚‚‚C18 O3‚‚‚O5 C9‚‚‚O4 C11‚‚‚O4 C14‚‚‚O4

2.948(3) 3.095(2) 3.187(2) 2.862(2) 2.680(3) 3.264(2) 3.264(2) 3.321(3)

O1‚‚‚O5 O1‚‚‚C18 O2‚‚‚C18 O3‚‚‚C18 O3‚‚‚O5 C9‚‚‚O4 C11‚‚‚O4 C14‚‚‚O4

3.064(3) 2.867(2) 3.086(2) 2.763(2) 2.843(3) 3.556(2) 3.260(2) 3.325(3)

Figure 2. 2-D network structure present in 1 and 2 viewed in the ab plane.

C9-H‚‚‚O4 C11-H‚‚‚O4 C14-H‚‚‚O4

168.90(12) 134.69(16) 175.74(10)

2

Ow(2)‚‚‚N3 Ow(4)‚‚‚O8 Ow(3)‚‚‚Ow(4) Ow(3)‚‚‚O5 Ow(2)‚‚‚Ow(3) Ow(1)‚‚‚O7 Ow(1)‚‚‚O2 Ow(5)‚‚‚O4 Ow(5)‚‚‚O6 N2‚‚‚O2

2.664(3) 2.696(3) 2.728(2) 2.909(3) 2.789(2) 2.853(2) 2.805(2) 2.816(3) 2.781(2) 2.701(3)

C9-H‚‚‚O4 C11-H‚‚‚O4 C14-H‚‚‚O4 3 Ow(2)‚‚‚H-N3 Ow(4)‚‚‚H-O8 Ow(4)‚‚‚H-Ow(3) Ow(3)-H‚‚‚O5 Ow(3)‚‚‚H-Ow(2) Ow(1)-H‚‚‚O7 Ow(1)-H‚‚‚O2 Ow(5)-H‚‚‚O4 Ow(1)-H‚‚‚O6 N4-H‚‚‚O2

168.28(14) 134.65(15) 169.37(11) 161.71(10) 173.33(12) 137.00 152.84(19) 149.93(14) 178.58(12) 173.08(18) 162.11(17) 173.22(19) 166.17(12)

MeOH molecule. The structure converged when MeOH was given an occupancy of 0.5. The structure of 1 consists of octahedrally coordinated metal ions, with coordination from one pdc2- ligand and a carboxylate O from a neighbor in the equatorial plane (NO3 donor

set) and two 4,4′-bipy spacers in axial positions (Figure 1). The atom O1 of the bridging carboxylate propagates the 1-D polymeric chain along the crystallographic a axis while the 4,4′-bipy spacers join these chains to form an infinite 2-D network structure (Figure 2) with approximate rectangular voids of dimension, 7.73 × 4.05 Å. The axial Co-N2 and Co-N3 bond distances are 2.165(4) and 2.167(4) Å, respectively, both of which are slightly longer as compared to the equatorial Co-N1 distance of 2.068(4) Å. The Co-O2 and Co-O3 distances are 2.216(3) and 2.128(3) Å, while the Co-O1′ distance involving the neighboring pdc2- unit is 2.035(3) Å. However, these distances are comparable22 with reported Co-N and Co-O distances for octahedral complexes. The axial N2-Co-N3′ angle is 173.1(1)°, while the four equatorial angles, O2-Co-N1, N1-Co-O3, O3-Co-O1′, and O1′-Co-O2 are 73.78(12), 75.56(13), 113.56(13), and 97.12(12)°, respectively, leading to a significantly distorted octahedral coordination geometry. The structure of 2 is very similar to that of 1 (Figure 1). The Ni-N2 and Ni-N3 axial bond distances are 2.110(5) and 2.109(5) Å, while the equatorial Ni-N1 distance is 1.996(5) Å. The equatorial Ni-O1′, Ni-O2, and Ni-O3 distances are 2.035(4), 2.196(4), and 2.127(4) Å, respectively, which are shorter as compared to the distances in octahedral Ni(II) carboxylate complexes

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Figure 3. Packing of the layers viewed along the b crystallographic axis in 1 and 2.

known in the literature.23 The axial N2-Ni-N3′ angle is 172.9(2)°, while the four equatorial angles, O2-NiN1, N1-Ni-O3, O3-Ni-O1′, and O1′-Ni-O2, are 76.02(18), 77.74(19), 109.27(19), and 96.98(17)°, respectively. Thus, octahedral coordination geometry is less distorted here as compared to that of 1. The 2-D network that is approximately perpendicular to the crystallographic c axis shows larger rectangular voids of dimension 9.81 × 4.08 Å. The diagonal distances in 1 and 2 are calculated to be 12.17 and 12.88 Å. So, the 2-D structure is more flat in 2 as compared to that in 1. In both structures, the two pyridine rings of the 4,4′bipyridyl moiety are not coplanar. The dihedral angle between the two planar pyridine rings is about 30° in 1 and 35° in 2. The 2-D layers are stacked approximately along the crystallographic c axis with an interlayer separation of ∼8.1 Å in 1 and ∼7.8 Å in 2. The uncoordinated carboxylate O atoms of one layer form an intricate array of CsH‚‚‚O hydrogen bonding with neighboring H atoms of the 4,4′-bipy moiety of the next layer. A view of the packing of layers in the structures is illustrated in Figure 3. When the spacer is changed to 1,2-di(4-pyridyl)ethylene, the 2-D layer structure breaks down. While no crystalline product could be isolated with Co(II), a new discrete complex having the formula (dpeH2)[Ni(pdc)2]‚5H2O is formed. In 3, each Ni(II) ion is hexavalent with bonding from two pdc2- groups (Figure 4). The axial Ni-N1 and Ni-N2 distances are 1.967(3) and 1.966(3) Å and comparable to the Ni-N(pdc2-) distance present in 2. The equatorial Ni-O1, Ni-O3, Ni-O5, and Ni-O7 distances are 2.163(2), 2.098(2), 2.133(2), and 2.137(2) Å, respectively. These distances are also similar to those in 2. The axial angle, N1-Ni-N2, is 177.89(11)°, while the equatorial angles, O1-Ni-O5, O1-Ni-O7, O3-Ni-O5, and O3-Ni-O7 are, respectively, 94.82(10), 91.03(10), 90.27(10), and 93.70(10)°. The metal-ligand bond distances suggest that the geometry around Ni(II) is distorted through axial contraction and equatorial elongation. However, the differences in bond distances and angles involving the metal in 1-3 result from the bite angle provided by the pdc2- ligand and is found in complexes of other metal ions with the ligand.24,25 In 3, the spacer is not bonded to the metal but exists in the lattice in diprotonated form. All the free O atoms of the carboxylate groups in [Ni(pdc)2]2- units are hydrogen bonded either to the water molecules present in the lattice or to the proto-

Figure 4. View showing the coordination geometry around Ni(II) in 3.

Figure 5. (A) Close view of the hydrogen-bonded acyclic water trimer. (B) Hydrogen-bonded 3-D metal-organic framework structure in 3.

nated spacer forming a 3-D cavity (Figure 5A). To inhibit protonation and inducing the spacer to bind the metal ion, 1-3 equiv of NaOH was used during hydrothermal syntheses. However, in none of the cases could a crystalline product be isolated. We speculate that for an ensemble of two or more different ligands and a metal, the design and choice of the components must fulfill the criteria for spontaneously generating welldefined architectures. However, exhaustive studies must be carried out before the unsuitability of the dpe spacer in forming MOF structures with Ni(II) or Co(II) ion and pdcH2 ligand could be established. Unlike 1 and 2, the cavity is not empty in 3 but is occupied by five water molecules, three of which form

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Figure 7. X-band powder EPR spectrum of 1 at 4 K.

Figure 6. (A) Plot of the χMT vs T for complex 1. Inset: fit of the low-temperature region by the formula given in the text. Solid line represents the best fit with the parameters given in the text. (B) Plot of the reduced magnetization for 2 (M/Nβ) vs H at 2 K.

an acyclic water trimer. The water trimers are hydrogen bonded to dpeH2 at one end and to free carboxylate O at the other (Figure 5B). The other two water molecules are hydrogen bonded to different free carboxylate O atoms. Theoretical studies26 on water trimers predicted the existence of several cyclic as well as two acyclic structures. Here, the acyclic water trimers are formed through strong hydrogen bonding with the O‚‚‚O distances of 2.718 and 2.780 Å. For comparison, this distance in regular ice, in liquid water, and in the vapor phase are 2.74, 2.85, and 2.98 Å, respectively.27 The O‚‚‚O ‚‚‚O angle is found to be 116°, which is close to the theoretical value26 of ∼120° for the acyclic trimer in the gas phase. Magnetic Properties. The magnetic susceptibility measurements were carried out from 300 to 2 K. Diamagnetic corrections were estimated28 from Pascal’s table. The magnetic behavior of 1 is illustrated in Figure 6A by means of a plot of cmT versus the temperature. The value of cmT is 3.26 cm3 mol-1 K at 300 K, which is greater as compared to an isolated spin-only Co(II) ion. This feature is, however, normal in Co(II) complexes, where the cmT value at room temperature is normally larger than expected for isolated ions, indicating that

an important orbital contribution is involved. The cmT value of 1 continuously decreases, reaching 1.1 cm3 mol-1 K at 2 K. To complete and corroborate the susceptibility data, a magnetization experiment was performed at 2 K between 0 and 5 T (Figure 6B). The reduced molar magnetization (for the dimer) tends to 2.5 Nβ when the field increases to 5 T, which corresponds to an effective ground state of S ) 1/ 2 with g . 2.00 and very anisotropic. The degeneracy of the 4T1g ground state of the octahedral Co(II) ion prevents an exact fit of the data to any scheme, especially over the entire temperature range (300 to 2 K). In the lowtemperature range, the low-symmetry crystal-field components and spin-orbit coupling produce up to six Kramers doublets and result in a doublet ground state (S ) 1/2),28 leading to anisotropic exchange coupling. Assuming that the anisotropy is very strong, the system at low temperatures can be considered as an Ising chain, as has been the case in other Co(II) systems.29 The X-band EPR spectrum of a powdered sample is shown in Figure 7. It is not a typical spectrum for an octahedral CoII complex, likely because of the significant distortion in the environment of the CoII ion. In any case, even if the simulation has failed, the g values are much greater than 2.00, indicating strong anisotropy of the S ) 1/2 ground state at 2 K, which is in agreement to the M/Nβ measurements (Figure 6B). We have used the exact solutions of the uniform Ising S ) 1/2 chain expanded by Fisher30 and Katsura,31 as shown

χ|| ) Ng2β2/4kT exp(2x)

(i)

χ∧ ) Ng2β2/8kT (|x|-1 tanh|x| + sech2 x)

(ii)

with x ) J/4kT Using these relations at the temperature range of 20 -2 K, the best-fit parameters are J ) -5.35 cm-1, gav ) 4.93, and R ) 2.3 × 10-3 (R is the agreement factor defined as Σi[(χMT)obs - (χMT)calc]2/Σi[(χMT)obs]2 (Figure 6A, inset). Obviously, the calculated J value may be affected by the spin-orbit coupling and should be regarded only as the highest possible value for the antiferromagnetic coupling of 1. As indicated by Carlin,32 it is necessary to distinguish the spin anisotropy from dimensional anisotropy. Even if the system is structurally 2-D, existence of the long 4,4′-bpy bridge allows to assume that 1 is a 1-D complex from magnetic

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point of view (see structural part). In similar cases, almost all Co(II) systems have been fitted assuming Ising anisotropy. The best documented examples of chain compounds with an Ising behavior are certain Co(II) derivatives.28 In principle, the other alternative anisotropic model, XY system, can also be applied for Co(II) chains, with S ) 1/2 local spin, when gx,y . gz. Kahn reports28 that only one Co(II) complex, (N2H5)2Co(SO4)2, has been convincingly explained as a XY system. Considering that in 1, the g⊥ value seem to be large, this possibility (XY system) cannot be excluded33, although no mathematical expression can be used for deriving the J value by this model (contrary to the possibility with the Ising model). However, Reedijk et al. were able33 to find a simple formula to link the χM(max) and Tmax with J as

KTmax (χ⊥) |J| ) 0.64 In complex 1, there is no experimental maximum in χM (at least to 2 K). But making a simulation following the Ising model, this maximum should be very close to the value at 2 K. Thus, if we assume this value as the maximum and apply the formula of Reedijk, the approximate J value will be -1.62 cm-1. This value is different to that calculated from the Ising theory, which is expected because in the Ising formula uses explicit forms of χ⊥ and χ||, which is not the case of the expression given by Reedijk. However, it must be understood that both theories are only approximate and must be taken carefully. In most of Co(II) chains, the actual J value can be derived from a mixing of Ising and XY system.34 The χM versus T plot for 2 is shown in Figure 8. The χM curve starts at 0.00443 cm3 mol-1 at 300 K (χMT ) 1.32 cm3 mol-1 K, typical value for an isolated NiII ion with g > 2.00). As the temperature is lowered to 2 K, the value of χM increases gradually to 0.135 cm-1 mol-1 K at 2 K. Although no maximum is found in the χM curve (Figure 8A, inset), an inflection is observed close to 6-2 K. The general feature of these two curves (χM and χMT) is characteristic of weak intramolecular antiferromagnetic interactions and/or zero-field splitting of the single Ni(II) ion, which is also corroborated by the reduced magnetization experiment at 2 K (Figure 8, inset). The experimental value tends to be 1.2 Nβ at 2 K, while the saturation value for a mononuclear Ni(II) complex should be more than 2.00 Nβ at 2 K, indicating again the occurrence of zero-field splitting and/or small antiferromagnetic coupling. An empirical (Weng’s) formula has been reported in the literature to analyze 1-D systems with S ) 1, using numerical procedures.28,35

χΜ ) Ng2β2/kTA/B A ) 2.0 + 0.0194x + 0.777x2 B ) 3.0 + 4.346x + 3.232x2 + 5.834x3 with x ) |J|/kT Using this method, the best-fit parameters for 2 are found to be J ) -1.88 cm-1, g ) 2.28, and R ) 2.3 × 10-7 (R is the agreement factor). Here, the zero-field splitting (D) of Ni(II) ion is neglected. Owing to the small

Figure 8. Plot of the χM vs T for 2. Solid line represents the best with the parameters given in the text. Inset: open triangles show the reduced magnetization, M/Nβ vs H at 2 K. Solid line represents the Brillouin formula for isolated Ni(II) ions with the same g as obtained by the best fit.

calculated value of J, the D parameter could be important in the low-temperature region, where the χMT values tend to decrease. For this reason, experimental magnetic data up to 10, 5, and 2 K were fitted. However, no significant variation of the J parameter could be found. As found from the structure of 2, the Ni(II) ions are linked by carboxylato groups in syn-anti conformation. The versatility of carboxylato as a ligand is illustrated by the variety of its coordination modes when acting as a bridge, the most common being the so-called syn-syn, syn-anti, and anti-anti modes. Weak antiferromagnetic interactions are observed in the carboxylatobridged Ni(II) complexes in which the carboxylato adopts the syn-anti conformation.36 The small overlap between the magnetic orbitals of nickel through the syn-anti carboxylato bridge, for a Ni-O-C-O-Ni skeleton that is planar, accounts for the weak coupling observed in 2. For copper(II) ions, theoretical DFT calculations also corroborate this weak ferromagnetic coupling for the syn-anti conformation.37 Thus, the antiferromagnetic coupling of the syn-anti carboxylato configuration seems to be in agreement with the results

Characterization of 3-D Metal-Organic Frameworks

reported for copper(II) complexes, even if the magnetic pathway is more complicated in Ni(II) complexes due to the presence of two electrons in the dz2 and dx2-y2 orbitals. Complexes 1 and 2 are almost isostructural. In the 2-D network, Co(II) or Ni(II) ions are linked together by carboxylato bridging in the syn-anti conformation forming pseudo-1-D coordination polymers. These 1-D chains are bridged by 4,4′-bipyridine ligands that gives a very small antiferromagnetic coupling.38 In some cases, J values of very small magnitude, such as - 0.05 cm-1, have been reported.39 Thus, from a magnetic point of view, the two complexes can be considered as quasiisolated 1-D systems. Conclusion It is thus shown that pyridine-2,6-dicarboxylic acid in combination with a linear spacer such as 4,4′bipyridyl is an excellent ligand for constructing metalorganic framework structures with transition metal ions. The role of spacer is crucial as the spacer 1,2-(di(4-pyridyl)ethylene does not match to form the desired MOF. Instead, it exists separately in the lattice. This compound forms a 3-D structure through strong hydrogen bonding involving dpeH2, free carboxylate O atoms of the pdc2-, and five water molecules. Out of the five water molecules, three form a novel acyclic trimer. We are presently probing the capability of pyridine-2,6dicarboxylic acid to form MOF structures with different metal ions including lanthanides for possible applications. Acknowledgment. We gratefully acknowledge the financial support received from the Council of Scientific and Industrial Research, New Delhi, India (Grant 1638/ EMR II) and a SRF to S.G. J.R. acknowledges the financial support from the Spanish Government (Grant BQU2003/00539). Supporting Information Available: X-ray crystallographic files in CIF format for the structure determination of 1-3. IR and X-ray powder diffraction patterns. This material is available free of charge via the Internet at http://pubs. acs.org.

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