Interaction of 1, 3-Adamantanediacetic Acid (H2ADA) and Ditopic

Nov 24, 2009 - Wei-Hong Zhang,† Yao-Yu Wang,*,† Elmira Kh. Lermontova,†,‡ ... Shaanxi Key Laboratory of Physico-Inorganic Chemistry, College o...
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DOI: 10.1021/cg900285b

Interaction of 1,3-Adamantanediacetic Acid (H2ADA) and Ditopic Pyridyl Subunits with Cobalt Nitrate under Hydrothermal Conditions: pH Influence, Crystal Structures, and Their Properties

2010, Vol. 10 76–84

Wei-Hong Zhang,† Yao-Yu Wang,*,† Elmira Kh. Lermontova,†,‡ Guo-Ping Yang,† Bin Liu,† Jun-Cheng Jin,† Zhe Dong,† and Qi-Zhen Shi† †

Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, Shaanxi Key Laboratory of Physico-Inorganic Chemistry, College of Chemistry & Materials Science, Northwest University, Xi’an 710069, P. R. China and ‡N. S. Kurnakov Institute of General and Inorganic Chemistry, RAS, Leninskii pr. 31, Moscow, 119991, Russia Received March 10, 2009; Revised Manuscript Received October 20, 2009

ABSTRACT: A systematic investigation of the pH influence on the interaction between 1,3-adamantanediacetic acid (H2ADA), 1,3-bis(4-pyridyl)propane (bpp)/4,40 -dipyridine (4,40 -bipy), and cobalt nitrate under hydrothermal conditions has resulted in a series of coordination polymers with different composition and dimensionality, namely [Co(HADA)2(bpp)]n (1), {[Co(ADA)(bpp)(CH3OH)] 3 H2O}n (2), [Co2(ADA)2(bpp)]n (3), [Co(HADA)2(4,40 -bipy)(H2O)2]n (4), and [Co(ADA)(4,40 -bipy)0.5]n (5). These neutral polymeric complexes exhibit structural and dimensional diversity due to various coordination modes of the flexible H2ADA ligand. X-ray structural analysis reveals that 1 and 4 are one-dimensional (1-D) chains. The chain of complex 1 is directed by bpp, in which HADA located at both sides of the chain. Compared with the case of 1, the 1-D linear chain is composed of 4,40 -bipy and Co(II) ions in 4. Complexes 2, 3, and 5 show different 2-D networks. In 2, the final structure is formed via the linkage of Co-bpp chains by ADA. Complex 3 is composed of 1-D looped chains in which there are two kinds of [Co2ADA2] metallomacrocycles, and the chains are further connected by bpp to form the 2-D structure. The building block represents a one-dimensional chain consisting of [Co2ADA2] units linked by 4,40 -bipy in 5. Five new coordination modes of H2ADA are observed; among them, the bridging modes have proven that H2ADA can be used as an effective bridging ligand for metal-organic complexes, especially for metal organic layered structures (MOLS). It has also been found that the structure formation of the complexes is mainly governed by the pH value of the solution. The other effect is probably the flexibility of the pyridyl-containing ligands optimizing their position within the complexes. The TG, XRPD data of synthesized complexes and the magnetic properties for 3 and 5 have also been investigated.

Introduction During the past two decades, the design and synthesis of metal-organic frameworks (MOFs) has achieved considerable progress in the fields of supramolecular chemistry and crystal engineering. The high interest in these new complexes arises not only from their intriguing ability to form a variety of architectures and topologies but also from their potential applications in industry as zeolite-like materials, electrical conductors and insulators, sensors elements, and membranes for separation processes and ion exchange, and also for luminescence and so on.1,2 It is well-known that the nature of organic ligands plays an important role in the design and construction of new metal-organic frameworks. Among numerous organic ligands, the versatile ligands with two or more carboxylic functional groups can exhibit a great variety of diverse coordination modes. One of the other important characteristics of an organic compound used as a ligand is its rigidity, because this can determine the orientation of the binding sites (coordination numbers and coordination geometries). So they can be used as linkers to design and construct porous coordination polymers and therefore greatly increase the chances of their potential industrial applications.3 Both of the properties are characteristic for substituted benzene di- or multicarboxylic acids, such as *To whom correspondence should be addressed. pubs.acs.org/crystal

Published on Web 11/24/2009

1,4-benzenedicarboxylic acid, 1,3,5-benzenetricarboxylic acid, and 1,2,4,5-benzenetetracarboxylic acid, which possess dior multicarboxyl groups that can rotate against their attached benzyl ring and exhibit abundant coordination modes, and have been widely used for the preparation of various MOFs.4-6 However, ligands with partially flexible carboxyl groups are also under intensive study, because their flexibility and conformational freedom may provide more possibilities for the construction of frameworks with new structures. Investigation of their properties can lead to a deeper understanding of the supramolecular assembly processes.7 In our previous research we have mostly focused on combining various rigid aromatic carboxylates, mainly pyridine-2,6dicarboxylic acid N-oxide and 3,30 ,4,40 -biphenyltetracarboxylic acid, with pyridyl containing pillars to construct novel MOFs8 and less attention was paid to flexible dicarboxylates. For the present study we have chosen 1,3-adamantanediacetic acid (H2ADA) for systematic investigation of its coordination ability. H2ADA contains two free independent carboxyl groups that can be completely or partially deprotonated which can be found in various coordination modes and conformations. Due to that, the carboxylic groups of H2ADA have both hydrogen bond donor and acceptor properties, which is important for the formation of complexes with higher dimensionalities. To our knowledge, there are few examples related to the metal-organic frameworks based on H2ADA present in the scientific literature.9 r 2009 American Chemical Society

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Scheme 1. Versatile Coordinate Modes of 1,3-Adamantanediacetic Acid (H2ADA) in Complexes 1-5

On the other hand, the meaningful use of the characteristic ligands for rational controlling of the overall framework structure still remains a great challenge.10 The desired structures of coordination complexes mostly depend on the geometrical and electronic properties of the metal ions and the multitopic organic spacer ligands; however, during the synthesis, their final structures are often unpredictable due to a lot of external factors, such as temperature, the nature of the metal counteranion, the solvent system, the metal-to-ligand ratio, and the pH value of the solution.11,18a Bidentate rodlike N,N0 -donor ligands, such as 4,40 -bipy, can have a large influence on the assembly processes and have been extensively used with multicarboxylate ligands to synthesize combined multiligand metal-organic frameworks.12 The use of a flexible analogue of 4,40 -bipy, 1,3-bis(4-pyridyl)propane (bpp), in which N-donor pyridine rings are separated by alkyl (CH2)3 spacers, can lead to fascinating architectures with new properties.13 The application of these N-donor complexes in MOFs synthesis often results in new structures with unique motifs and useful functional properties. In this paper, we have used the combination of H2ADA with bpp/4,40 -bipy and cobalt nitrate as precursors for new MOFs. The effect of pH influence was also investigated due to possible pH-induced changes in the coordination diversity of H2ADA under hydro(solvo)thermal synthesis conditions.11f,14 A variety of possible pH dependent coordinate modes and configurations of H2ADA ligand which can be found in these complexes are presented in Scheme 1. Five new coordination polymers [Co(HADA)2(bpp)]n (1), {[Co(ADA)(bpp)(CH3OH)] 3 H2O}n (2), [Co2(ADA)2(bpp)]n (3), [Co(HADA)2(4,40 -bipy)(H2O)2]n (4), and [Co(ADA)(4,40 -bipy)0.5]n (5) originated from similar systems (H2ADA/ bpp, H2ADA/4,40 -bipy) are synthesized and characterized by single crystal X-ray diffraction, X-ray powder diffraction (XRPD), and thermogravimetric analysis (TG). The magnetic properties for 3 and 5 have also been studied. Experimental Section Reagents and Physical Measurement. All used chemicals were commercially available and were purchased from different sources. The high purity grade allowed their use without further purification. Elemental analysis (C, H, and N) was done on a Perkin-Elmer 240C. Infrared spectra were obtained using KBr pellets on a Nicolet 170SX FT-IR spectrophotometer in the range 4000-400 cm-1. TGA curves were recorded on a Netzsch STA 449C microanalyzer under N2 atmosphere at a heating rate of 10 °C 3 min-1. The X-ray powder diffraction (XRPD) data were recorded on a Rigaku RU200 diffractometer at 60 KV, at 300 mA, and with Cu KR radiation (λ = 1.5406 A˚), with a scan speed of 2°/min and a step size of 0.02° in 2θ. The magnetic susceptibility of microcrystalline samples restrained in parafilm was measured on MPMS XL-5

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(Magnetic Property Measurement System XL-5) with an applied field of 1 kOe. Synthesis of [Co(HADA)2(bpp)]n (1). A mixture of Co(NO3)2 3 6H2O (0.20 mmol, 0.058 g), H2ADA (0.40 mmol, 0.101 g), bpp (0.20 mmol, 0.040 g), H2O (10 mL), CH3OH (2 mL), and a few drops of N,N0 -dimethylformamide (DMF) was stirred under air atmosphere until complete dissolution of all components. Then the pH value of the resulting solution was adjusted to 5 with a 0.5 M NaOH solution. The solution was placed in a Teflon-lined stainless steel vessel (25 mL), the vessel was sealed, and the mixture was heated up to 160 °C. This temperature was maintained for 3 days. By slow cooling of the reaction mixture to room temperature, pale pink needle crystals were obtained. Yield: 53%. Anal. C41H52CoN2O8 (759.78) Calcd: C, 70.26; H, 7.48; N, 4.00. Found: C, 70.35; H, 7.41; N, 4.11. IR (KBr, cm-1) for 1: 2906(s), 2850(s), 1696(s), 1592(s), 1448(s), 1410(m), 1324(s), 1277(s), 1175(w), 1151(m), 1132(w), 1107(w), 1151(w), 1021(w), 938(m), 803(m), 741(w), 696(w), 639(m). Complex 1 was also obtained using other cobalt salts, such as CoSO4 3 7H2O (0.20 mmol, 0.056 g) or CoCl2 3 6H2O (0.20 mmol, 0.048 g) instead of Co(NO3)2 3 6H2O. Synthesis of {[Co(ADA)(bpp)(CH3OH)] 3 H2O}n (2). The same synthetic procedure was used to construct 2 as that for 1 except that the pH value was adjusted to 6. Red prism crystals were obtained. Yield: 52%. Anal. C28H38CoN2O6 (557.53) Calcd: C, 60.32; H, 6.87; N, 5.02. Found: C, 60.23; H, 6.71; N, 5.09. IR (KBr, cm-1) for 2: 3669(w), 3477(s), 3065(w), 2894(s), 2842(s), 2131(w), 1957(w), 1615(s), 1544(s), 1450(s), 1401(s), 1341(m), 1312(m), 1218(m), 1186(w), 1151(w), 1067(w), 1022(m), 846(m), 806(m), 748(w), 712(m), 664(m), 645(w), 614(w). Synthesis of [Co2(ADA)2(bpp)]n (3). Similar to the preparation of 1, but with pH = 7: purple pyramid crystals were obtained. Yield: 26%. Anal. C41H50Co2N2O8 (816.69) Calcd: C, 60.30; H, 6.17; N, 3.43. Found: C, 60.14; H, 6.09; N, 3.47. IR (KBr, cm-1) for 3: 3669(w), 3423(m), 2894(s), 2842(s), 2136(w), 1958(w), 1616(s), 1544(s), 1450(s), 1403(s), 1341(s), 1312(m), 1219(m), 1187(w), 1151(m), 1067(w), 1021(m), 846(m), 806(m), 713(m), 664(m), 615(w). Synthesis of [Co(HADA)2(4,40 -bipy)(H2O)2]n (4). A mixture of Co(NO3)2 3 6H2O (0.20 mmol, 0.058 g), H2ADA (0.40 mmol, 0.101 g), 4,40 -bipy (0.20 mmol, 0.032 g), H2O (10 mL), CH3OH (2 mL), and several drops of DMF was stirred under air conditions until complete dissolution. Then the pH of the solution was adjusted to 5 with a 0.5 M NaOH solution. The solution was placed in the Teflon-lined stainless steel vessel (25 mL), the vessel was sealed, and the solution was heated to 160 °C for 3 days. Slow cooling of the reaction mixture to room temperature resulted in rose block crystals. Yield: 56%. Anal. C38H50CoN2O10 (753.73) Calcd: C, 65.69; H, 7.25; N, 4.03. Found: C, 65.59; H, 7.34; N, 4.11. IR (KBr, cm-1) for 4: 3751(w), 3378(s), 2923(s), 2845(s), 2505(w), 1680(s), 1603(s), 1543(s), 1488(w), 1436(s), 1403(s), 1359(m), 1312(s), 1269(s), 1149(m), 1077(w), 1049(w), 1006(m), 880(w), 828(m), 782(w), 739(m), 700(w), 675(m), 630(m). Complex 4 was also obtained using other cobalt salts, such as CoSO4 3 7H2O (0.20 mmol, 0.056 g) or CoCl2 3 6H2O (0.20 mmol, 0.048 g) instead of Co(NO3)2 3 6H2O. Synthesis of [Co(ADA)(4,40 -bipy)0.5]n (5). The synthesis of 5 was similar to that of complex 4, except the pH = 6. Red pyramid crystals were obtained. Yield: 36%. Anal. C19H22CoNO4 (387.31) Calcd: C, 58.92; H, 5.73; N, 3.62. Found: C, 58.99; H, 5.65; N, 3.73. IR (KBr, cm-1) for 5: 3445(m), 3103(w), 3054(w), 2958(m), 2912(s), 2847(s), 1618(s), 1554(s), 1426(s), 1390(s), 1340(m), 1306(s), 1243(w), 1220(m), 1193(w), 1134(w), 1106(m), 1079(m), 1044(w), 1013(w), 990(w), 829(m), 755(m), 728(m), 703(m), 672(m), 644(s). Crystallographic Data Collection and Refinement. Single-crystal diffraction data 1-5 were collected on a Bruker SMART APEXII CCD diffractometer with graphite-monochromated Mo KR radiation (λ = 0.71073 A˚) at room temperature. The data integration and reduction were processed with SAINT software. Absorption correction based on multiscan was performed using the SADABS program. The structures were solved by the direct method using SHELXTL and refined by a full-matrix least-squares method on F2 with the SHELXL-97 program.15 The structures of 2 and 3 contain the disordered alkyl chain (C36, C37, C6 for 2 and C6, C7 for 3) of bpp ligands, which adopt two positions and lie on inversion centers

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Table 1. Crystallographic Data and Structure Refinement Details for Complexes 1-5 3 C41H50Co2N2O8 816.69 triclinic P1 11.856(5) 12.173(6) 14.037(6) 101.821(7) 105.594(8) 103.570(7) 1817.6(14) 2 1.492 0.971 856 1.57-25.50 8938 0.0538 0.0795, 0.1909 0.1223, 0.2037

4 C38H50CoN2O10 753.73 monoclinic C2/c 23.67(2) 11.474(11) 13.229(13) 90.00 93.599(17) 90.00 3586(6) 4 1.396 0.540 1596 1.97-25.09 8610 0.0665 0.0487, 0.0911 0.0924, 0.0964

5 C19H22CoNO4 387.31 monoclinic C2/c 22.666(8) 13.946(5) 11.277(4) 90.00 113.441(6) 90.00 3270(2) 8 1.573 1.074 1616 1.76-26.00 8679 0.0425 0.0436, 0.1179 0.0610, 0.1259

)

)

complex 1 2 C28H38CoN2O6 empirical formula C41H52CoN2O8 M 759.78 557.53 crystal system monoclinic monoclinic space group C2/c P21/c a (A˚) 32.993(3) 11.5542(17) b (A˚) 6.3394(5) 10.2363(15) c (A˚) 23.0457(17) 23.376(3) R (deg) 90.00 90.00 β (deg) 130.831(1) 100.211(2) γ (deg) 90.00 90.00 3 3647.1(5) 2720.9(7) V (A˚ ) Z 4 4 1.384 1.361 dcalcd (g 3 cm-3) 0.528 0.675 μ (mm-1) F(000) 1612 1180 θ range (deg) 1.63-28.00 1.77-26.00 reflections collected 15835 23453 0.0451 0.0317 R(int) 0.0401, 0.0883 0.0402, 0.1014 R1,a wR2b [I > 2σ(I)] a b 0.0643, 0.0968 0.0588, 0.1082 R1, wR2 (all data) P P P P a R1 = Fo| - |Fc / |Fo|. b wR2 = [ w(Fo2 - Fc2)2/ w(Fo2)2]1/2.

Table 2. Selected Bond Lengths (A˚) of Complexes 1-5a 1 Co(1)-O(4) Co(1)-O(3) 3 Co(1)-O(5) Co(1)-O(7)#3 Co(1)-O(2) Co(1)-N(1) Co(1)-O(4)#2 5 Co(1)-O(1) Co(1)-N(1) a

2.117(2) 2.146(3)

Co(1)-N(1)

2.135(5)

2.005(5) 2.018(5) 2.023(5) 2.115(6) 2.331(5)

Co(1)-O(6) Co(2)-O(8)#3 Co(2)-O(1) Co(2)-O(4)#2 Co(2)-N(2)#4

2.408(6) 1.912(5) 1.945(5) 1.954(5) 2.014(5)

2.020(2) 2.090(3)

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

2.710(1) 2.001(2)

2 Co(1)-O(6) Co(1)-N(2) Co(1)-N(1) 4 Co(1)-O(2) Co(1)-O(1W)

2.037(6) 2.103(2) 2.142(1)

Co(1)-O(1) Co(1)-O(2) Co(1)-O(5)

2.143(1) 2.148(1) 2.155(1)

2.058(3) 2.135(3)

Co(1)-N(1)#2 Co(1)-N(2)

2.187(4) 2.199(4)

Co(1)-O(4)#2 Co(2)-N(2)#4

2.101(2) 2.070(3)

Co(2)-O(2)

2.056(2)

Symmetry codes: 3: (#2) -x, -y, -z; (#3) -x þ 1, -y þ 1, -z þ 1; (#4) x, y - 1, z - 1. 4: (#2) x, y þ 1, z. 5: (#2) x - 1/2, -y þ 1/2, z - 1/2; (#4) x, y - 1, z.

with equal occupancy. All non-hydrogen atoms were refined anisotropically. Some hydrogen atoms of water were located in successive difference Fourier maps; others were placed in calculated positions and refined using the riding model. Crystallographic data and selected bond lengths and angles with their estimated standard deviations of the complexes are shown in Tables 1, 2, and S1 (Supporting Information). Possible hydrogen bond geometries of partial complexes are listed in Table S2.

Scheme 2. Hydrothermal Synthesis of 1-5

Results and Discussion Synthesis. Complexes 1-5 were obtained as polymeric substances in methanol/distilled water/N,N0 -dimenthylformamide (DMF) mixed solvent solution by combination of H2ADA/bpp (H2ADA/4,40 -bipy) with Co(NO3)2 3 6H2O under different pH value conditions via a solvothermal approach. The pH conditions and corresponding reaction products are presented in Scheme 2. In general, there are three different products in the H2ADA/bpp system while in the H2ADA/4,40 -bipy system only two MOFs were obtained. The rise of the pH value in the H2ADA/4,40 -bipy system leads to formation of a fine black powder. Varying the reaction parameters such as temperature, the ligand-to-metal ratio, the cooling speed, and the solvent system has not succeeded in formation of product suitable for single crystal X-ray diffraction analysis. Probably, the introduction of the rigid N-containing auxiliary ligand leads to less stable product, which quickly decomposes at reaction conditions leading to blackish decomposition product formation. Contrarily, the flexible bpp ligand contains two dipyridyl groups which can

freely rotate to meet the requirements of metal ions coordination geometries during the structure assembly process. It is worthwhile to point out that the use of CoSO4 3 7H2O or CoCl2 3 6H2O instead of Co(NO3)2 3 6H2O, employing the identical technique as for preparation of 1 and 4, also results in crystalline substance formation. Suitable single-crystals were selected for X-ray diffraction. Their crystallographic cell data (a, b, c, R, β, γ) are identical to that for complexes 1 and 4. The results indicate that the product structure weakly depends on the nature of the cobalt counteranion. Crystal Structures [Co(HADA)2(bpp)]n (1). X-ray diffraction reveals that complex 1 is a 1-D zigzag chain directed by bpp ligands with two HADA located at both sides of the cobalt ion along the crystallographic c-direction (Figure S1). As illustrated in

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Figure S2, the building unit contains one crystallographically independent Co(II) center. This six-coordinated Co(II) center exhibits a slightly elongated octahedral geometry Co{N2O4}, with four oxygen atoms from two separate H2ADA ligands in the equatorial plane and two nitrogen atoms of different bpp ligands occupying the apical positions. The bond lengths of Co-O and Co-N are in agreement with those reported for carboxylate and bipycontaining Co(II) complexes.7a,16 The distance between the Co 3 3 3 Co centers is 11.523 A˚, and detailed bond parameters are listed in Table 2 and Table S1. In this complex, bpp takes the GG0 conformation13a and acts as a bidentate bridge for two cobalt atoms forming the zigzag chain, which has alternating helical parts (alternating right and left turns of the strand looking down the “growth axis”). However, the whole chain is not an exact helical structure, since the strand contains centers of inversion and does not have a defined chirality, thus resulting in a so-called meso-helix chain (Figure S3).17 The carboxylic groups of the H2ADA ligand are not completely deprotonated: one group coordinates to Co(II) ion in a chelating bidentate mode and the second carboxylic group is protonated and hydrogen bonded to a neighboring carboxylate. The two HADA ligands which coordinate to the same cobalt ion take the same coordination mode and conformation (Scheme 1a). Between the adjacent chains, weak hydrogen bond interactions (O2-H 3 3 3 O4) are occurring (Figure 1a). The distance of O2 3 3 3 O4 is 2.636(2) A˚, and the angle of O2-H 3 3 3 O4 is 168.0°. Thus, a 2-D sheet is formed perpendicular to the b axis. Moreover, these sheets can be extended into a 3-D supramolecular structure by the same hydrogen bonds (Figure 1a). It is noteworthy that the HADA ligands which connect 2-D sheets to the 3-D supramolecular architecture also form chiral helical chains (Figure 1a). The two parallel helices exist concurrently, and they are encased in 1-D channels by hydrogen bonds. So far, there are a few examples of complexes containing two kinds of helices.8b,18 {[Co(ADA)(bpp)(CH3OH)] 3 H2O}n (2). As shown in Figure S4, the asymmetric unit of 2 contains one Co(II) cation, one bpp, one deprotonated ADA, and one coordinated methanol. (The lattice-water molecule is omitted for clarity.) The Co(II) ion is six-coordinated, representing a distorted octahedral geometry by coordination of two bpp nitrogen atoms, three carboxylate oxygen atoms from two different ADA ligands, and an oxygen atom from methanol. The bond lengths of Co-O are in the range of 2.037(6)2.155(1) A˚, and the Co-N lengths are 2.103(2) and 2.142(1) A˚, which are in good agreement with those found in other Co-containing coordination frameworks.7a,16 As opposed to complex 1, in 2 the ADA ligand is presented as a tridentate mode, with one carboxylate group adopting a chelating bidentate mode, while another shows a monodentate mode (Scheme 1b) and the bpp shows the TG conformation.13a Two Co(II) centers are connected by two identical ADA ligands, resulting in an isolated [Co2ADA2] metallacycle which represents a small closed cyclic structure with a 1:1 metal-to-ligand ratio. Within each metallacyclic motif, the approximate dimensions are 8.0  5.1 A˚2 and the distance of Co-Co is 6.558(1) A˚. The adjacent rings are twisted with a dihedral angle of 59.8° and are further linked to each other by two bpp to form a 2-D structure. Treating the Co(II) atoms as single nodes and connecting them by simplified representations of ADA and bpp ligands results in a distorted 2-D brick wall network visualization (Figure 2).

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Figure 1a. (a) 2-D supramolecular network formed through hydrogen bonding between interchains in 1. (b) 3-D supramolecular network constructed by hydrogen bonding of 1. (c) Chiral helical chains in the 3-D supramolecular structure of 1 and the side view of them.

Furthermore, when simplifing the ADA ligand as a rod, the structure represents a 63 topology. The 2-D layers are held

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Figure 3. (a) 1-D looped chain containing two kinds of [Co2ADA2] metallacycle (A and B) in 3. (b) 2-D structure formed by the connecting of bpp with the looped chains in 3. (c) View of the parallel offset 2-D layers in 3 by weak van der Waals interactions.

Figure 2. 2-D infinite layer extended in the bc plane based on bpp and a [Co2ADA2] metallacycle in 2 and the schematic representation of it (blue and yellow represent ADA and bpp, respectively). (b) Weak van der Waals interactions between the 2-D layers in 2.

together just by weak van der Waals interactions (the shortest distance between C4 and C5 atoms in adjacent layers is 3.753 A˚) (Figure 2).19 [Co2(ADA)2(bpp)]n (3). As shown in Figure S5, there are two independent Co(II) atoms, two ADA ligands, and one bpp ligand in the fundamental building unit. Co1 is coordinated by five ADA oxygen atoms and one bpp nitrogen atom, thus representing a Co{O5N} distorted octahedral geometry, while Co2 stays in the Co{O3N} tetrahedron geometry surrounded by three ADA oxygen atoms and one bpp nitrogen atom. The distance between Co1 3 3 3 Co2 is 3.154 A˚. The bond lengths of Co-N are 2.115(6) and 2.014(5) A˚, and the Co-O lengths are in the range of 1.912(5)-2.408(6) A˚, which are a little longer but reasonable compared to typical Co-O bond lengths.7a,16,20 Compared with the complexes 1 and 2, completely deprotonated ADA ligands in complex 3 take two new coordination modes. The first type is that one carboxylate group of an ADA ligand adopts a chelating bidentate mode while the other adopts a bridging bidentate mode. The second type is formed when ADA coordinates to four Co(II) atoms with one carboxylate

group in a bridging monodentate mode and another in a bridging bidentate mode, as illustrated in Sheme 1 (types c and d). It should be noted that each bpp ligand adopts a TG conformation, which shows the same as it in 2.13a The ADA ligands and Co(II) atoms construct a 1-D looped chain (Figure 3). There are two kinds of [Co2ADA2] metallomacrocycles within the chain with a dihedral angle of 73.8°; the approximate dimensions are 6.8  5.1 A˚2 (blue) and 5.1  7.2 A˚2 (yellow). The chains are further connected by bpp ligands to form a 2-D structure (Figure 3), and these 2-D structures are held together by van der Waals forces (the shortest distance between C6 and C37 atoms in adjacent layers is 3.625 A˚) (Figure 3c).18 [Co(HADA)2(4,40 -bipy)(H2O)2]n (4). Figure S6 shows the coordination environment of the Co(II) ion in complex 4. The metal center possesses a distorted octahedral geometry with a Co{N2O4} coordination environment, being linked to two oxygen atoms from two HADA ligands, two other oxygen atoms from coordinated water molecules, and two nitrogens from different 4,40 -bipy ligands. The bond lengths of Co-N are 2.187(4) and 2.199(4), while the Co-O lengths are in the range 2.058(3)-2.135(3) A˚. Analogous to the case of complex 1, HADA ligands are also incompletely deprotonated, with one carboxylate group adopting a monodentate mode while another is not coordinated. This coordination mode (Scheme 1e) is different from that found in complexes 1, 2, and 3. All of the ADA (HADA) coordination modes exhibited in complexes 1, 2, 3, and 4 have not been reported before. The 4,40 -bipy extends 4 to a 1-D chain, with two symmetrical HADA ligands located on the opposite sides of the chain (Figure S7), but their arrangements are different from those for complex 1. The two pyridine rings of each 4,40 -bipy

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Figure 4. 2-D supramolecular network formed through hydrogen bonding between the two uncoordinated carboxylate oxygen atoms in 4.

molecule are twisted with a dihedral angle of 50.5°. There are two kinds of hydrogen bonds between the adjacent chains. The first is connecting oxygen atoms of coordinated water molecules with uncoordinated carboxylate oxygens (O1WH1WB 3 3 3 O4), which directs the 1-D chain to form a 2-D structure (Figure S8). The second is between two uncoordinated carboxylate oxygens (O3-H3 3 3 3 O1), allowing adjacent chains to arrange in the opposite directions and to form a new 2-D hydrogen-bonded sheet (Figure 4). Furthermore, the two kinds of hydrogen bonds are involved in the construction of the final 3-D supramolecular network (Figure S9). [Co(ADA)(4,40 -bipy)0.5]n (5). There are two kinds of crystallographically independent Co(II) ions in this complex, as shown in Figure S10. Both Co1 and Co2 are five coordinated, and their coordination environments are similar. Each Co(II) ion is coordinated by four oxygen atoms from four different ADA ligands and one nitrogen atom from one 4,40 -bipy molecule. The bond lengths and bond angles around the two Co(II) ions are different (Table 2), and the distance between the two Co(II) ions is 2.710(1) A˚, which is longer than that in Co metal (2.5 A˚) but still well below the sum of the van der Waals radii of two cobalts (4.0 A˚). The average bond lengths of Co-O and Co-N are 2.089(2) and 2.080(3) A˚, respectively, which are in good agreement with those earlier reported.7a,16 Two pyridine rings of each 4,40 -bipy molecule are twisted, with a dihedral angle of 42.3°. Different from the complexes discussed above, ADA adopts a bis(bridging-bidentate) mode and links four Co(II) ions (Scheme 1f), forming a 1-D [Co(ADA)]n double-chain structure (Figure S11). The chains are pillared by 4,40 -bipy to generate 2-D square-grids (Figure 5); simplifing the ADA ligand as a rod, the structure shows a 44 topology. These 2-D structures are stacking in the ABAB fashion along the ac-plane with some displacement between A and B. Finally, the existence of weak van der Waals interactions (the shortest distance between C37 and C37 atoms in adjacent layers is 3.650 A˚) holds these layers together (Figure S12).19 Comparison of the Structures of Complexes 1-5. The overall structure is predominantly controlled by the coordination preferences of the assistant building blocks, whose deprotonation ability can be controlled by the acidity of the solution. Thus, the pH value could have a significant influence on the final structure formation. As demonstrated

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Figure 5. Double chains pillared by 4,40 -bipy generating 2-D square-grids in 5.

by the comparison of complexes 1 with 2 and 3, the changes in the pH values of the initial solution lead to different kinds of deprotonation and therefore linking modes of the H2ADA ligand (HADA, ADA). Using the rigid auxiliary ligand 4,40 -bipy instead of bpp under the conditions similar to 1-3 synthesis, complexes 4 and 5 were obtained. These results confirm a crucial role of solution pH. Another aspect is that the flexible nitrogen-containing ligands can lead to a more complicated complex than the rigid one. The flexible dipyridyl-propane ligand bpp can freely rotate or bend to meet the requirements of coordination geometries of metal ions during the assembly process, while bending of the topically rigid assistant ligand 4,40 -bipy seems to be impossible. Furthermore, the basicity of the 4,40 -bipy is slightly different from that of bpp. These can rationally explain why the 4,40 -bipy could not meet the requirements of the coordination geometries of the cobalt ions at pH = 7. And the fact is in accordance with the different structures of the complexes and the failure of the complex 6 synthesis. A comparison of the backbones of H2ADA with the H2ADC (1,3-adamantanedicarboxylate) suggests that the flexible methylene of -CH2-COOH in H2ADA allows it to adjust easily, resulting in complicated network structures upon metal complexation, whereas the H2ADC group has only limited rotation freedom.19,21 Thermal Gravimetric Analysis and XRPD Measurement. To confirm the phase purity of the obtained complexes, the original samples were characterized by XRPD at room temperature. The patterns that were simulated from the single-crystal X-ray data of the complexes are in agreement with those that were observed (Figures S13-S17). In order to characterize the thermal stability of the complexes, thermal gravimetric analyses (TGA) were carried out. The results are presented in Figure S18. For complex 1, a significant weight loss has been observed in the temperature range 210-600 °C, corresponding to consecutive loss of bpp and HADA ligands. The remaining weight of 9.95% is in accordance with the mass of CoO residue (calcd 9.8%). The residual of all other complexes also results in CoO formation at temperatures close to 600 °C. The TGA curve of complex 2 shows the first weight loss of 9.69% over the temperature

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interdimer antiferromagnetic interaction and/or the zerofield splitting effect. In order to estimate the strength of the ferromagnetic exchange interaction between Co1 and Co2, the magnetic data of 3 were fitted using the following simple phenomenological equation:23 χM T ¼ A expð -E1 =kTÞ þ B expð -E2 =kTÞ

Figure 6. χMT and χM vs T plots for 3.

Figure 7. χMT and χM vs T plots for 5.

range 67-168 °C, corresponding to the loss of a free water and a coordinated methanol of cell unit (calcd 8.98%). When the temperature was increased to ca. 260 °C, the product began to lose the bpp and ADA ligands. Complex 3 has followed two consecutive steps of weight losses, losing bpp and ADA ligands between 220 and 600 °C. The slight differences in the TGA curves of complexes 1-3 are arising from different modes of ADA coordination, which can influence their stability at high temperatures. The TGA trace of 4 is much like that observed of 2; it loses two coordinated aqua molecules between 58 and 170 °C (obsd 2.45%, calcd 4.78%). Then at about 240 °C the main framework starts to collapse and the mass losses correspond to the losing of the organic ligands. Complex 5 is stable up to 198 °C; then it loses organic ligands and only CoO residue remains at 580 °C (obsd 19.67%, calcd 19.37%). Magnetic Properties. Magnetic studies have been performed on powdered samples for 3 and 5 in the range 2-300 K. The χMT and χM versus T plots of 3 and 5 are shown in Figures 6 and 7, respectively. For 3, the value of χMT at 300 K is 5.857 cm3 mol-1 K, which is larger than the calculated spin-only value (3.875 cm3 mol-1 K) for two Co(II) (S = 3/2) ions, indicating the important orbital contribution arising from the high-spin octahedral Co(II).22 Upon cooling, χMT first decreases smoothly to reach a minimum value of 3.992 cm3 mol-1 K at 22 K, and then it increases drastically to a maximum of 4.454 cm3 mol-1 K around 9.5 K, suggesting an appreciable ferromagnetic exchange between Co1 and Co2, connected to each other through two μ2-O and one O-C-O bridges. The sudden decrease in χMT below 9.5 K may be attributed to the

The least-squares analysis (R = 1  10-3), shown as solid lines in Figure 6, led to the parameters A = 2.5 cm3 mol-1 K, E1/k = 82 K, B = 3.9 cm3 mol-1 K, and E2/k = -0.25 K. Here, A þ B equals the Curie constant, and E1 and E2 represent the ‘‘activation energies’’, corresponding to the spin-orbit coupling and the magnetic exchange interaction. The Curie constant value found for A þ B = 6.4 cm3 mol-1 K agrees with that obtained from the Curie-Weiss law in the high temperature range (Figure S19), and the value for E1/k is consistent with those given in the literature for both the effects of spin-orbit coupling and site distortion (E1/k of the order of 100 K). The value E2/k = -0.25 K corresponding to spin coupling between two Co(II) ions shows the very weak ferromagnetic exchange mediated between Co1 and Co2 through two μ2-O and one O-C-O bridges. In addition, magnetization M versus H shown in Figure S20, saturating at MS = 3.70 Nβ, also indicates ferromagnetic exchange mediated between Co1 and Co2. The FCM and ZFCM data for complex 3 diverge very weakly below 10 K (Figure S21), and the field dependence of the magnetizations of 3 shows no hysteresis loop at 2 K. Complex 5, which is essentially a Co2 dimer with four carboxyl ligand chelates and bridges the two Co(II) ions, forming a typical paddlewheel coordination motif, is further connected via the axial site by 4,40 -bipy to generate a chain structure. The value of χMT at room temperature is 4.162 cm3 mol-1 K, which is close to the calculated spin-only value (3.875 cm3 mol-1 K) and much lower than that of complex 3, indicating that the orbital contribution arises from the highspin octahedral Co(II) and that antiferromagnetic exchange in the Co2 paddlewheel core cannot be excluded as well. The magnetic behavior of complex 5 can also be treated in a similar way to that of 3. The least-squares analysis (R = 4  10-4), shown as solid lines in Figure 7, led to the parameters A = 2.2 cm3 mol-1 K, E1/k = 101 K, B = 2.6 cm3 mol-1 K, and E2/k = 0.90 K. The positive value E2/k corresponding to spin coupling J < 0 shows the antiferromagnetic exchange between Co magnetic centers in such a typical paddlewheel coordination motif. Conclusion Our investigation of the Co(NO3)2 3 6H2O, H2ADA, and bpp(4,40 -bipy) system under different pH conditions resulted in a series of complexes ranging from one- to two-dimensional coordination polymers. Comparing these complexes, we have found that differences in H2ADA coordination modes have a significant effect on the formation and dimensionality of the resulting structures. A variety of new coordination modes of H2ADA have been observed in the complexes. The use of flexible dipyridyl ligands (bpp) as secondary building blocks leads to more complicated corresponding structures than in the case of rigid ones (4,40 -bipy). The results also indicate that the various coordination modes and deprotonation degrees of the H2ADA ligand can be influenced by pH values, and they represent a nice example of the acidity-controlled polymeric architecture construction. The layered structures of the

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2, 3, and 5 complexes are bound together only by weak van der Waals forces; therefore, there is no strong bonding between the layers and they can be good candidates for the synthesis of new materials via further delamination. Finally, our present research further enriches the crystal engineering strategy and offers new possibilities for controlling the formation of the desired framework structures.

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Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 20771090) and TRAPOYT and by the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20050697005). Supporting Information Available: Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, 715995-715999 for 1-5. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Tables of bond angles and hydrogen bond geometries of complexes and additional figures and XRPD and TG graphs of the complexes are available free of charge via the Internet at http://pubs.acs.org.

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