Coordination Chemistry of Cyclohexane-1, 2, 4, 5-tetracarboxylate (H4

Coordination Chemistry of Cyclohexane-1,2,4,5-tetracarboxylate (H4L). Synthesis, Structure, and Magnetic Properties of Metal-Organic Frameworks with Conformation-Flexible H4L Ligand Jing Wang,† Yong-Cong Ou,† Yong Shen, Lei Yun, Ji-Dong Leng, Zhuojia Lin,* and Ming-Liang Tong*

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 5 2442–2450

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-Sen UniVersity, Guangzhou 510275, P. R. China, and School of Mechenical and Elecric Engineering, Guangzhou UniVersity, Guangzhou 510006, P. R. China ReceiVed December 12, 2008; ReVised Manuscript ReceiVed February 20, 2009

ABSTRACT: To study the conformations and coordination chemistry of cyclohexane-1,2,4,5-tetracarboxylic acid (H4L), we have obtained four new coordination polymers, [Mn2(µ4-LI)(H2O)6] (1), [Mn2(µ8-LI)(H2O)2] (2), [Mn2(µ7-LII)(H2O)3] (3), and [Ag4(LI)(4,4′bpy)3] · 10H2O (4), from the reactions of Mn(II) or Ag(I) salts with cyclohexane-1e,2a,4a,5e-tetracarboxylic acid (2e+2a, H4LI). A new conformation, cyclohexane-1e,2a,4e,5e-tetracarboxylate (3e+1a, LII), was derived from conformational conversions of LI and trapped in 3 upon coordination by controlling the hydrothermal reaction conditions. The stabilization and separation of different conformations of the flexible organic ligands in coordination structures may help us understand and utilize various interesting conformational transformations. Magnetic studies show that antiferromagnetic interaction occurs in both 1 and 2. Introduction The synthesis of metal coordination polymers has attracted more and more attention in recent years not only due to their intriguing structural topologies and novel properties for potential applications, but also because they build up a new bridge between the fields of coordination and organic chemistry.1-3 Unusual organic ligands that are not easily accessible via conventional methods, or some interesting mesostable conformations of organic compounds, can be obtained via solvothermal in situ metal/ligand reactions.3c Although various rigid polycarboxylate ligands, such as benzenepolycarboxylates and pyridinepolycarboxylates, have been used widely to construct stable metal organic frameworks, only a few cases of carboxylic acid type ligands with flexible backbones or adaptable conformations, e.g., cycloalkane-polycarboxylates and tetrahydrofurantetracarboxylate, have been reported so far.4,5 Since the simplest cyclohexane-carboxylic acid was first used in the area of coordination chemistry,6 many metal coordination compounds containing cyclohexane-polycarboxylate ligands have been prepared. Among them, cyclohexane-1,4-dicarboxylic acid,7 cyclohexane-1,2-dicarboxylic acid,8 and cyclohexane-1,3dicarboxylic acid9 have been successfully utilized as flexible linkers for various coordination networks. Three conformations of the two carboxylate groups on the cyclohexane ring, a,a-, a,e-, and e,e-forms, have been observed in compounds from diverse room temperature and hydrothermal synthetic conditions. Cyclohexane-1,3,5-tricarboxylic acid is another interesting flexible ligand that is usually used to compare with the rigid benzene-1,3,5-tricarboxylic acid. The three carboxylate groups in this ligand usually adapt e,e,e-conformation upon coordination.10 Apparently, when there are more carboxylic groups on the cyclohexane ring, there will be more possible conformations; consequently, it is more difficult to control their orientations in the final coordination structures. Our recent work systematically investigated the coordination chemistry of conformation-flexible * Corresponding author. E-mail: [email protected]. Phone: 86 20 8411-0966. Fax: 86 20 8411-2245. † These authors contributed equally to this work.

cyclohexane-1,2,3,4,5,6-hexacarboxylic acid.11 According to theoretical calculations, the free ligand has six possible stable conformations, one 3e+3a, one 6e, three 4e+2a, and one 5e+a. By carefully controlling the reaction conditions, we successfully trapped two predominant conformations, 3e+3a and 6e, and two less-common conformations, 4e+2a (e,e,e,e,a,a) and 5e+1a, in different coordination polymers. The observed in situ decarboxylation and aromatization of cyclohexane-1,2,3,4,5,6,hexacarboxylate to benzene-1,3,5-tricarboxylate proposed the possible conformational transformation mechanism involving removal and recovery of the R-protons.11d Inspired by the previous work, we extended our study to the unexplored cyclohexane-1,2,4,5-tetracarboxylic acid (H4L), hoping to trap different conformations (Scheme 1) of the ligand via coordination under different reaction conditions. Three manganese(II) coordination polymers, [Mn2(µ4-LI)(H2O)6] (1), [Mn2(µ8LI)(H2O)2] (2), and [Mn2(µ7-LII)(H2O)3] (3) were yielded at successively high temperatures from hydrothermal reactions. Two conformations of L, 1e,2a,4a,5e-form (2e+2a, LI) and 1e,2a,4e,5eform (3e+1a, LII), were frozen in these compounds. Furthermore, a silver(I) coordination polymer [Ag4(LI)(4,4′bpy)3] · 10H2O (4) was prepared from room temperature reaction, suggesting that LI is the most stable conformation upon coordination at relatively low temperatures. Experimental Section Materials and Physical Measurements. The reagents and solvents used were commercially available and used as received without further purification. Cyclohexane-1,2,4,5-tetracarboxylic acid (H4L) used in all the reactions was in LI (e,a,a,e) conformation (see the Supporting Information). C, H, and N microanalyses were carried out with Elementar Vario-EL CHN elemental analyzer. FT-IR spectra were recorded in KBr tablets in the range 4000-400 cm-1 on Bio-Rad FTS-7 spectrometer. X-ray powder diffraction (XPRD) intensities for polycrystalline samples of 1-3 (see Figure S1 in the Supporting Information) were measured at 293 K on Bruker D8 Advance Diffratometer (CuKR, λ ) 1.54056 Å) by scanning over the range of 5-60° with step of 0.2°/s. Simulated XPRD patterns of 1-3 were generated with PowderCell. Variable-temperature magnetic susceptibility measurements were carried out using SQUID magnetometer MPMS XL-7 (Quantum

10.1021/cg801348n CCC: $40.75  2009 American Chemical Society Published on Web 03/26/2009

Coordination Chemistry of Cyclohexane-1,2,4,5-tetracarboxylate

Crystal Growth & Design, Vol. 9, No. 5, 2009 2443

Scheme 1. Four Possible Conformations of H4L

Table 1. Crystal Data and Structure Refinements for 1-4

empirical formula M wavelength (Å) cryst syst space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Fcalcd (g cm-3) µ (mm-1) no. of reflns collected no. of unique reflns S R1a, wR2b (I > 2σ(I)) R1a, wR2b (all data) a

1 (150 K)

2 (150 K)

3 (293 K)

4 (150 K)

C10H20O14Mn2 474.14 0.71073 triclinic P1j 7.0769(6) 9.5083(8) 12.3146(7) 102.361(6) 95.785(5) 94.597(7) 800.89(11) 2 1.966 1.654 8128 3113 (0.0490) 0.849 0.0341, 0.0615 0.0614, 0.0659

C10H12O10Mn2 402.08 1.54178 orthorhombic Pnma 7.6071(2) 14.6486(3) 11.1213(2) 90 90 90 1239.28(5) 4 2.155 17.154 2108 949 (0.0236) 1.071 0.1302, 0.3195 0.1540, 0.3386

C10H14O11Mn2 420.09 0.71073 monoclinic P21/n 6.4745(9) 25.905(3) 8.1890(11) 90 90.422(3) 90 1373.5(3) 4 2.032 1.899 7297 2900 (0.0454) 1.165 0.0734, 0.1375 0.0917, 0.1443

C40H52O18N6Ag4 1336.36 1.54178 triclinic P1j 12.348(7) 13.308(8) 15.115(7) 70.90(5) 87.04(4) 78.51(5) 2300(2) 2 1.930 14.170 12911 6542 (0.0310) 0.985 0.0465, 0.1150 0.0616, 0.1208

R1 ) ∑|Fo| - |Fc|/∑|Fo|. b wR2 ) [∑w(Fo2 - Fc2)2/∑w(Fo2)2]1/2.

Design) at 1.0 kOe for 1 and 2. Diamagnetic correction was applied from Pascal’s constants. Synthesis of [Mn2(µ4-LI)(H2O)6] (1). A mixture of NaOH (0.040 g, 1.0 mmol) in H2O (5.0 mL) and H4LI (0.065 g, 0.25 mmol) in H2O (5.0 mL) was added to the aqueous solution (5.0 mL) of MnCl2 · 4H2O (0.198 g, 1.0 mmol) with stirring. The resultant solution was heated in a stainless steel reactor with Teflon liner (25 mL) at 140 °C for 72 h and then was cooled at a rate of ca. 5 °C h-1 to give colorless block crystals of 1 (yield: 20%, based on H4L). Elemental anal. Calcd (%) for C10H20O14Mn2: C, 25.33; H, 4.25. Found: C, 25.16; H, 4.31. IR (KBr) (400-4000 cm-1): 3402vs, 3259m, 2940m, 2885m, 1604s, 1409vs, 1325m, 1280m, 1240m, 1153w, 1066w, 1036w, 965m, 859m, 795m, 688w, 579w, 532w. Synthesis of [Mn2(µ8-LI)(H2O)2] (2). The preparation of 2 is similar to that of 1 except that the reactant mixture was heated at 160 °C for 48 h. Colorless lamellar crystals of 2 were produced upon cooling (yield: 68%, based on H4L). Elemental anal. Calcd (%) for C5H6O5Mn2: C, 29.87; H, 3.01. Found: C, 29.05; H, 2.93. IR (KBr) (400-4000 cm-1): 3392m, 3234m, 2955m, 2892m, 1584vs, 1411vs, 1320s, 1284m, 1262m, 1165m, 1072w, 1045w, 977m, 850m, 780m, 678w, 588w, 546w. Synthesis of [Mn2(µ7-LII)(H2O)3] (3). The preparation of 3 is similar to that of 1 except that the reactant mixture was heated at 160 °C for 96 h. Colorless sheet crystals of 3 were found upon cooling (yield: 37%, based on H4L) along with the lamellar crystals of 2. Elemental anal. Calcd (%) for C10H14O11Mn2: C, 28.59; H, 3.36. Found: C, 28.99; H, 3.25. IR (KBr) (400-4000 cm-1): 3425vs, 3119s, 2942m, 2871m, 1654s, 1543s, 1420vs, 1313m, 1283m, 1200m, 1127w, 1061w, 1006w, 974w, 941w, 870w, 846w, 821m, 760m, 698m, 599m, 510w, 480m, 426w. Synthesis of [Ag4(LI)(4,4′-bpy)3] · 10H2O (4). Excess aqueous NH3 solution was dropped slowly into a suspension of Ag2O (0.087 g, 0.375 mmol) in MeCN/H2O (30 mL, V/V ) 2:1) with magnetic stirring for 15 min until a homogeneous clear solution was formed at room temperature. The solids of H4LI (0.065 g, 0.25 mmol) and 4,4′bipyridine (0.078 g, 0.50 mmol) were added slowly into the solution in turn. The mixture was kept stirring for 30 min after all the reactants dissolved. The filtrated solution evaporated slowly in darkness in 2 weeks to give colorless needlelike crystals (yield: 35%, based on Ag). Elemental anal. Calcd (%) C40H52O18N6Ag4: C, 35.95; H, 3.92; N, 6.29.

Found: C, 35.99; H, 3.84; N, 6.35. IR (KBr) (400-4000 cm-1): 3418s, 3044w, 1594s, 1405s, 1325m, 1219m, 1070m, 985w, 853w, 804s, 622m. X-ray Crystallography. Diffraction data for compounds 1, 2, and 4 were recorded on an Oxford Diffraction Gemini R CCD diffractometer with Mo KR radiation (λ ) 0.71073 Å) for 1 or Cu KR radiation (λ ) 1.54178 Å) for 2 and 4 at 150 K. The measurement temperature was controlled using an Oxford Cryosystems Cryostream cooling apparatus. Diffraction data for compound 3 were recorded on a Bruker Apex CCD area detector diffractometer with Mo KR radiation (λ ) 0.71073 Å) at 293(2) K. Processing data was accomplished with use of the program SAINT; an absorption correction based on symmetry equivalent reflections was applied using the SADABS program.12 The structures were solved by direct method, and all non-hydrogen atoms were refined anisotropically by full-matrix least-squares techniques using the SHELXTL program.13 All hydrogen atoms were included in calculated positions and refined with isotropic thermal parameters riding on those of the parent atoms. Further details for structural analysis for compounds 1-4 are summarized in Table 1.

Computational Method All of the structures were optimized using Becke’s three-parameter hybrid functional (B3LYP) method14-16 and the 6s31G(d,p) basis set.17 The PCM solvent model was used during the optimization to simulate the effect of water solvent. The stable configurations of the compounds were confirmed by means of frequency analysis, whereby no imaginary frequency was found for any of the configurations at the energy minima. The sum of the electronic (include solvent contribution) and thermal free energies was used to compare stability. All of the calculations were performed with the Gaussian 03 program package.18

Results and Discussion Theoretical Calculations on the Free H4L Ligand in Different Conformations. Theoretical calculation shows that cyclohexane-1,2,4,5-tetracarboxylic acid (H4L) has four possible stable conformations, I(e,a,a,e), II(e,a,e,e), III(e,a,e,a), and IV(e,e,e,e), in gas phase or as a solvate in water (Figure 1).

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Wang et al.

Figure 1. Structures of conformation I(e,a,a,e), II(e,a,e,e), III(e,a,e,a), and IV(e,e,e,e) optimized by the B3LYP method and 6-21G (d,p) in water. Table 2. Free Energies of the Conformations and ∆G Values Compared with Conformation I gas phase a.u. I II III IV

-989.964446 -989.987394 -989.983688 -989.976385

kcal mol

Scheme 2. Temperature- and Time-Dependent Synthesis of Compounds 1-3

solvate in water -1

(∆G)

0 -14.4001 -12.0745 -7.49184

a.u.

kcal mol-1 (∆G)

-990.007046 -990.026199 -990.025561 -990.016613

0 -12.0187 -11.6183 -6.0034

Table 2 shows II is the most stable among the four conformations both in gas phase and in water. Therefore, the conformational change from I, III, or IV to II is thermodynamically permitted, which is in agreement with our experiments that II can be obtained from I at higher temperature and for longer reaction time. Although conformations III and IV are more stable than I, they have not been trapped in our current study and the corresponding compounds have not been found yet. The possible reason that I can more easily transform to II rather than III or IV may lie in, besides the energy factor, the fact that there is only one COOH group that is needed to turn from a- to e-position when changing from I to II, whereas for the transformation from I to III or IV, two COOH groups are required to change their positions. Conformation Transformation and Synthesis Conditions. It is interesting that although II is the most stable conformation in the gas phase and in aqueous solution, cyclohexane-1,2,4,5-tetracarboxylic acid (H4L) itself crystallizes in conformation I,19 for which energy disfavor may be compensated by hydrogen-bonding interaction between the carboxylic groups and the lattice energy of packing. Our previous studies on cyclohexane-1,2,3,4,5,6-hexacarboxylic acid showed that the size and versatile coordination environments of the metal ions may play an important role in controlling the conformation of carboxylate groups on the cyclohexane ring.11 In the present H4L system, the two metal ions we used, Ag+ and Mn2+, seemingly have little influence on the conformation of the ligand. The coordination bonds, similar to the hydrogen-bonding interaction, help to stabilize the energetically less favorable conformation and it is still LI that exists in the ambient-temperature product compound 4 and low-temperature hydrothermal product compound 1. Conformation II can be found only in compound 3, which was prepared at higher temperature (160 °C) for longer time (4 days). Such conformational transformation is in agreement with the theoretical calculation result that II is thermodynamically more stable. Notably, compound 2 with LI and 3 with LII could coexist in the same reaction and the amount of 3 would increase with longer reaction duration. By carefully tuning the time, pure phase of 2 or 3 could be found (Scheme 2). Furthermore, similar to that observed in conformational transformation of cyclohexane-1,2,3,4,5,6-hexacarboxylic acid,11b different alkali metal ions that were used to adjust the pH value of the solution, may have some effect on the yield of different phases of the products. For the same reaction time at 150 °C, different compounds, 1, 2, or 3, were

favorable in the presence of LiOH, NaOH, or KOH, respectively. Crystal Structures Structure of [Mn2(µ4-LI)(H2O)6] (1). Single-crystal X-ray diffraction study reveals that the structure of 1 is composed of 1D LI coordination chains with dinuclear manganese(II) units. The asymmetric unit contains two Mn(II) atoms, one LI ligand, and six coordinated water molecules (Figure 2a, Table 3). Mn1 adopts a heavily distorted octahedral geometry coordinated by four oxygen atoms from three LI ligands and two water molecules (Mn1-O ) 2.122(2)-2.325(2) Å, O-Mn1-O ) 56.85(7)-153.04(9)°). Mn2 is in a slightly distorted octahedral geometry surrounded by one LI ligand and five bonded aqua molecules (Mn2-O ) 2.152(2)-2.224(2) Å, O-Mn2-O ) 78.74(8)-175.18(9)°). The a-carboxylate groups on LI adopt a monodentate binding manner, whereas one of the e-carboxylate connects two Mn(II) atoms in the µ2:η1,η2 mode. One µ-aqua molecule and the bridging carboxylate link Mn1 and Mn2 into a dinuclear manganese unit, which is further joined by LI into an infinite coordination chain along the b axis (Figure 2b). 3D supramolecular network is formed through extensive hydrogen bonds among the unbonded LI e-carboxylates and coordinated water molecules (Figure 2c). Structure of [Mn2(µ8-LI)(H2O)2] (2). 2 is a 3D coordination network that crystallizes in the space group of Pnma. There are one Mn(II) atom, half of LI, and one coordinated water molecule in the asymmetry unit (Figure 3a). The metal atom adopts a slightly distorted octahedral geometry coordinated by five oxygen atoms from four LI ligands and one water molecule (Mn1-O ) 1.988(15)-2.403(19) Å, O-Mn1-O ) 51.4(6)167.4(6)°). LI is situated in a special position with a mirror plane running along two unsubstituted carbon atoms on the cyclo-

Coordination Chemistry of Cyclohexane-1,2,4,5-tetracarboxylate

Crystal Growth & Design, Vol. 9, No. 5, 2009 2445 Table 3. Selected Bond Lengths (Å) and Angles (deg) for 1-4a Compound 1 Mn(1)-O(3a) Mn(1)-O(5b) Mn(1)-O(1) Mn(1)-O(2w) Mn(1)-O(2) O(1w)-Mn(1)-O(3a) O(1w)-Mn(1)-O(5b) O(3a)-Mn(1)-O(5b) O(1w)-Mn(1)-O(1) O(3a)-Mn(1)-O(1) O(5b)-Mn(1)-O(1) O(1w)-Mn(1)-O(2w) O(3a)-Mn(1)-O(2w) O(5b)-Mn(1)-O(2w) O(1)-Mn(1)-O(2w) O(1w)-Mn(1)-O(2) O(3a)-Mn(1)-O(2) O(5b)-Mn(1)-O(2) O(1)-Mn(1)-O(2) O(2w)-Mn(1)-O(2) Mn(2)-O(1)-Mn(1)

2.122(2) 2.144(2) 2.247(2) 2.309(2) 2.326(2) 82.68(8) 87.96(8) 153.03(9) 148.13(8) 105.22(8) 96.06(8) 135.98(8) 83.38(7) 86.00(7) 75.89(7) 91.29(8) 105.62(8) 99.80(8) 56.86(7) 132.70(7) 103.10(8)

Mn(1)-O(3a) Mn(1)-O(4b) Mn(1)-O(1w) Mn(1)-O(1) O(3a)-Mn(1)-O(4b) O(3a)-Mn(1)-O(1w) O(4b)-Mn(1)-O(1w) O(3a)-Mn(1)-O(1) O(4b)-Mn(1)-O(1) O(1w)-Mn(1)-O(1) O(3a)-Mn(1)-O(2c) O(4b)-Mn(1)-O(2c) O(1w)-Mn(1)-O(2c)

1.983(7) 2.069(7) 2.150(6) 2.165(6) 113.8(3) 88.1(3) 93.0(3) 109.5(3) 133.0(3) 106.2(2) 82.2(3) 83.5(3) 167.3(3)

Mn(1)-O(8a) Mn(1)-O(2w) Mn(1)-O(1w) Mn(1)-O(3w) Mn(1)-O(1) Mn(1)-O(2) O(1w) · · · O(7) O(1w) · · · O(8j) O(2w) · · · O(6e) O(8a)-Mn(1)-O(2w) O(8a)-Mn(1)-O(1w) O(2w)-Mn(1)-O(1w) O(8a)-Mn(1)-O(3w) O(2w)-Mn(1)-O(3w) O(1w)-Mn(1)-O(3w) O(8a)-Mn(1)-O(1) O(2w)-Mn(1)-O(1) O(1w)-Mn(1)-O(1) O(3w)-Mn(1)-O(1) O(8a)-Mn(1)-O(2) O(2w)-Mn(1)-O(2) O(1w)-Mn(1)-O(2) O(1w)-H(1wa) · · · O(7) O(1w)-H(1wb) · · · O(8j) O(2w)-H(2wa) · · · O(6e)

2.071(5) 2.132(5) 2.161(5) 2.199(5) 2.248(4) 2.477(4) 2.940(8) 2.626(7) 2.702(7) 118.4(2) 99.2(2) 97.8(2) 88.50(19) 86.2(2) 168.2(2) 98.27(18) 142.78(19) 81.26(19) 88.86(18) 151.76(17) 88.14(17) 85.43(19) 161.1 162.7 168.3

Ag(1)-N(6a) Ag(1)-N(4) Ag(1)-O(1w) Ag(2)-O(1) Ag(2)-O(7b) Ag(2)-N(1) Ag(2) · · · Ag(2b) Ag(2) · · · Ag(3b) N(6a)-Ag(1)-N(4) N(6a)-Ag(1)-O(1w) N(4)-Ag(1)-O(1w) O(1)-Ag(2)-O(7b) O(1)-Ag(2)-N(1)

2.158(3) 2.162(3) 2.555(3) 2.146(3) 2.153(3) 2.389(3) 3.2809(16) 3.2909(19) 172.87(12) 96.14(12) 90.66(12) 173.46(10) 92.87(11)

Mn(2)-O(6w) Mn(2)-O(4w) Mn(2)-O(3w) Mn(2)-O(2w) Mn(2)-O(1) O(5w)-Mn(2)-O(6w) O(5w)-Mn(2)-O(4w) O(6w)-Mn(2)-O(4w) O(5w)-Mn(2)-O(3w) O(6w)-Mn(2)-O(3w) O(4w)-Mn(2)-O(3w) O(5w)-Mn(2)-O(2w) O(6w)-Mn(2)-O(2w) O(4w)-Mn(2)-O(2w) O(3w)-Mn(2)-O(2w) O(5w)-Mn(2)-O(1) O(6w)-Mn(2)-O(1) O(4w)-Mn(2)-O(1) O(3w)-Mn(2)-O(1) O(2w)-Mn(2)-O(1) Mn(2)-O(2w)-Mn(1)

2.150(2) 2.164(2) 2.186(2) 2.192(2) 2.224(2) 83.87(8) 93.44(8) 98.33(8) 94.90(9) 92.51(8) 166.97(8) 175.21(8) 91.42(8) 86.37(8) 86.14(8) 106.04(8) 168.07(8) 87.86(8) 80.22(8) 78.74(7) 102.13(8)

Compound 2 Mn(1)-O(2c) Mn(1)-O(2) O(1w) · · · O(1a) O(1w) · · · O(4g) O(1)-Mn(1)-O(2c) O(3a)-Mn(1)-O(2) O(4b)-Mn(1)-O(2) O(1w)-Mn(1)-O(2) O(1)-Mn(1)-O(2) O(2c)-Mn(1)-O(2) Mn(1d)-O(2)-Mn(1) O(1w)-H(1wa) · · · O(1a) O(1w)-H(1wb · · · O(3g)

2.237(8) 2.394(8) 3.021(10) 2.748(8) 84.8(3) 149.8(3) 93.8(3) 77.8(2) 51.4(2) 114.5(3) 119.3(3) 151.9 165.1

Compound 3

Figure 2. (a) ORTEP drawing of the coordination environments of the Mn and the bridging mode of the LI ligand with ellipsoids at the 50% probability level, (b) perspective views of 1D chain with dinuclear manganese units, and (c) the 3D supramolecular architecture formed via interchain O-H · · · O contacts.

hexane ring. Each cystallographically different carboxylate on the a- and e-positions bridges two Mn2+ ions in the µ2:η1,η1 and µ2:η1,η2 modes, respectively, and links the metal atoms into a 1D Mn-COO- chain (Mn-O-Mn ) 119.2(8)°) along the a axis (Figure 3b). Accordingly, although the conformation of the ligand is the same as in 1, LI in 2 now binds eight Mn(II) atoms as a µ8-bridging node and a 3D condensed coordination network, rather than low-dimensional coordination chains as in 1, is thus formed (Figure 3c). Structure of [Mn2(µ7-LII)(H2O)3] (3). As the reaction duration was extended, the conformation of LI (e,a,a,e) transformed to thermodynamically more stable conformation LII (3e+1a) and a new phase compound 3 was yielded. 3 is a 3D coordination framework composed of two cystallographically distinct manganese(II) atoms, one LII ligand, and three coordinated water molecules in the asymmetric unit (Figure 4a). Mn1 adopts a distorted octahedral geometry coordinated by three oxygen atoms from two LII ligands and three water molecules (Mn1-O ) 2.071(5)-2.477(4) Å, O-Mn1-O ) 54.63(14)151.73(17)°). Mn2 is in a square pyramidal geometry (τ ) 0.0055) surrounded with five LII ligands (Mn2-O ) 2.072(4)-

Mn(2)-O(2) Mn(2)-O(3b) Mn(2)-O(5c) Mn(2)-O(4d) Mn(2)-O(6e) Mn(2) · · · Mn(2f) O(2w) · · · O(1j) O(3w) · · · O(7d) O(3w) · · · O(4d) O(3w)-Mn(1)-O(2) O(1)-Mn(1)-O(2) O(2)-Mn(2)-O(3b) O(2)-Mn(2)-O(5c) O(3b)-Mn(2)-O(5c) O(2)-Mn(2)-O(4d) O(3b)-Mn(2)-O(4d) O(5c)-Mn(2)-O(4d) O(2)-Mn(2)-O(6e) O(3b)-Mn(2)-O(6e) O(5c)-Mn(2)-O(6e) O(4d)-Mn(2)-O(6e) Mn(2)-O(2)-Mn(1) O(2w)-H(2wb) · · · O(1j) O(3w)-H(3wa) · · · O(7d) O(3w)-H(3wb) · · · O(4d)

2.072(4) 2.095(4) 2.117(4) 2.146(4) 2.147(4) 3.1109(17) 2.835(7) 2.843(7) 2.861(7) 83.62(16) 54.63(14) 111.22(17) 108.11(17) 88.98(18) 93.49(17) 154.80(18) 87.86(18) 95.83(17) 88.52(18) 155.13(18) 83.99(18) 126.32(18) 157.4 159.2 174.5

Compound 4 Ag(3)-O(8b) Ag(3)-O(2) Ag(3)-N(3) Ag(4)-N(2c) Ag(4)-N(5) Ag(2) · · · Ag(3) Ag(2) · · · Ag(4) Ag(3) · · · Ag(4) O(7b)-Ag(2)-N(1) O(8b)-Ag(3)-O(2) O(8b)-Ag(3)-N(3) O(2)-Ag(3)-N(3) N(2c)-Ag(4)-N(5)

2.327(3) 2.327(3) 2.377(3) 2.154(3) 2.166(3) 2.8012(19) 3.2377(19) 3.1999(15) 91.47(11) 155.61(9) 112.96(11) 91.35(11) 168.53(14)

a Symmetry codes for 1: a -x + 1, -y + 1, -z + 1; b x, y - 1, z. For 2: a -x, -y + 1, -z + 1; b -x - 1/2, -y + 1, z - 1/2; c x + 1/2, y, -z + 1/2; d x - 1/2, y, -z + 1/2 g -x - 1, -y + 1, -z + 1. For 3: a x + 1/ 2, -y + 1/2, z + 1/2; b -x + 1, -y, -z + 1; c -x, -y, -z + 1; d x, y, z + 1; e x + 1, y, z + 1; f -x + 1, -y, -z + 2; j x + 1, y, z. For 4: a -x + 2, -y - 1, -z + 1; b -x + 1, -y + 1, -z; c -x + 1, -y + 2, -z.

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Figure 3. (a) ORTEP drawing of the coordination environments of the Mn atoms and the bridging mode of the LI ligand with ellipsoids at the 50% probability level, (b) perspective views of Mn-COO- chain along the a axis, and (c) the 3D coordination network along the a axis in 2.

2.147(4) Å, O-Mn2-O ) 83.99(18)-155.13(18)°), which is joined by four LII on the basal positions to the adjacent Mn2 ion and thus form a paddle wheel Mn2(COO)4 dinuclear unit with a Mn2 · · · Mn2 distance of 3.111 Å (Figure 4b). The carboxylate groups on the a- and e-positions on LII have quite distinct binding modes compared with LI in 2. Only one of the thee e-carboxylates retains the µ2:η1,η2 binding fashion to link the Mn1 and Mn2 ions, whereas the others, on the contrary, bridge two Mn2 centers in a µ2:η1,η1 mode. The remaining a-carboxylate in LII is coordinated to Mn1 in a monodentate manner. The dramatic coordination behaviors of a- and ecarboxylates for LI and LII suggest that the conformation transformation of the ligand should occur in solution and phase conversion from 2 to 3 might involve a coordination disassembly and reassembly process. The bridging carboxylates link the manganese paddle wheels into an infinite layer spread in the ac plane, which is connected by the Mn1 ions into a 3D coordination network (structures c and d in Figure 4). Extensive hydrogen bonds are found among the aqua molecules and carboxylate groups, which help to stabilize the conformation of II.

Figure 4. (a) ORTEP drawing of the coordination environments of the Mn atoms and the bridging mode of the LII ligand with ellipsoids at the 50% probability level, (b) perspective views of the Mn4(carboxylate)6 unit, (c) the linking between Mn4 units, and (d) the 3D polyhedral network along the c axis in 3.

Structures of [Ag4(LI)(4,4′-bpy)3] · 10H2O (4). 4 is a 2D coordination compound prepared from a room-temperature solution reaction. The acid retains the conformation of LI and the change of metal ions or the addition of ancillary amine seems to have little influence on the conformation transformation. The asymmetric unit of 4 contains four silver(I) atoms, one LI ligand, three 4,4′-bpy, and ten water molecules (Figure 5a). Both Ag2 and Ag3 are surrounded

Coordination Chemistry of Cyclohexane-1,2,4,5-tetracarboxylate

Crystal Growth & Design, Vol. 9, No. 5, 2009 2447 Table 4. Hydrogen-Bond Parameters for Compound 4a 4 D-H · · · A

d(D-H)

d(H · · · A)

d(D · · · A)

∠(DHA)

O(1w)-H(1wa) · · · O(2w) O(1w)-H(1wb) · · · O(4wd) O(2w)-H(2wa) · · · O(3w) O(2w)-H(2wb) · · · O(3e) O(3w)-H(3wa) · · · O(4w) O(3w)-H(3wb) · · · O(3f) O(4w)-H(4wa) · · · O(5w) O(4w)-H(4wb) · · · O(7w) O(5w)-H(5wa) · · · O(6w) O(5w)-H(5wb) · · · O(5) O(6w)-H(6wa) · · · O(4) O(6w)-H(6wb) · · · O(4f) O(7w)-H(7wa) · · · O(8w) O(7w)-H(7wb) · · · O(10g) O(8w)-H(8wa) · · · O(5) O(8w)-H(8wb) · · · O(10w) O(9w)-H(9wa) · · · O(6) O(9w)-H(9wb) · · · O(6h) O(10w)-H(10a) · · · O(8h) O(10w)-H(10b) · · · O(6)

0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85

1.95 2.12 1.95 1.90 2.02 1.89 1.98 2.04 1.90 1.94 1.85 2.05 2.03 2.18 1.96 2.22 1.93 2.03 1.97 2.02

2.787(4) 2.973(4) 2.794(5) 2.744(5) 2.867(5) 2.676(5) 2.710(5) 2.868(5) 2.727(5) 2.788(4) 2.698(5) 2.767(4) 2.834(5) 2.924(5) 2.810(5) 2.890(4) 2.776(4) 2.848(4) 2.799(4) 2.868(5)

166.2 178.8 175.2 169.3 174.9 153.7 143.3 162.9 165.1 179.1 176.6 141.2 156.6 146.6 179.4 135.7 178.0 162.8 164.4 175.1

a Symmetry codes for 4: d -x + 1, -y - 1, -z + 1; e x, y - 1, z; f -x + 1, -y, -z + 1; g -x, -y, -z; h -x, -y + 1, -z.

Figure 5. ORTEP drawings of the coordination environments of (a) Ag atoms and (b) Ag · · · Ag interaction in the Ag4(LI)2 core in the rhombic Ag6(LI)2(4,4′-bpy)6 cluster with ellipsoids at the 50% probability level. (c) Perspective views of the 2D coordination layer, (d) 1D water aggregates between the layers, and (e) the 3D supramolecular architecture in 4. The guest water molecules in the channels are highlighted in sky blue spheres.

with two LI and one 4,4′-bpy in a distorted T-shape geometry (Ag-O ) 2.146(3)-2.327(3) Å, O-Ag-O ) 173.5(1)° and 155.61(9)°, Ag-N ) 2.389(6) and 2.376(7)Å, O-Ag-N ) 91.35(11)-112.96(11)°). The a-carboxylate groups on LI

adopt a µ2:η1,η1 mode while the e-carboxylates are uncoordinated. The bridging carboxylates from two distinct ligands link Ag2 and Ag3 in the Ag · · · Ag distance of 2.801(2) Å. Neighboring Ag2 · · · Ag3 units are connected by two acarboxylates on the same LI to construct a Ag4(LI)2 core (Ag · · · Ag ) 3.281(2)-3.291(2) Å). Ag4, which is connected to two 4,4′-bpy in a linear geometry (Ag4-N ) 2.154(3) and 2.166(3) Å, N-Ag4-N ) 168.53(14)°), has unsupported Ag · · · Ag interaction20-23 with Ag2 and Ag3 (Ag4 · · · Ag ) 3.1999(15) and 3.2377(19) Å, shorter than the sum of van der Waals radii for two silver atoms (3.40 Å)24) and consequently forms a rhombic Ag6(LI)2(4,4′-bpy)6 cluster (Figure 5b). The remaining metal atom Ag1 is linked to two 4,4′-bpy and one aqua molecule in a T-shape geometry (Ag1-N ) 2.158(3) and 2.162(3) Å, Ag1-O ) 2.555(3) Å, N-Ag1-N ) 172.87(12)°, O-Ag1-N ) 90.66(12) and 96.14(12)°), connecting the hexanuclear silver clusters into an infinite layer on the crystallographic (1,0,-1) plane (Figure 5c). The guest water molecules occupy the void space between the silver-4,4′-bpy layers and form 1D hydrogenbonded water aggregates with cyclic R68(16) (H2O)8 and R46(12) (H2O)4 units (Figure 5d) with the coordinated water molecules (Table 4), resulting in a 3D supramolecular architecture (Figure 5e). Thermogravimetric Analysis. Thermogravimetric (TG) analyses were carried out to examine thermal stabilities of the compounds 1-4. Phase purity of the bulk materials was characterized by comparison of the experimental powder diffraction (XRPD) patterns with simulated ones (see Figure S1 in the Supporting Information). Samples of 1, 2, 3, and 4 were heated in nitrogen to 600 °C. Compounds 1-3 have similar coordination bonds and therefore similar thermal stabilities (Figure 6a-c). Only compound 1 is discussed in detail here. A rapid weight loss of 22.1% between 145 and 160 °C is in accordance with the release of six coordinated water molecules per formula unit (calcd, 22.8%). The compound is stable until 460 °C. After that, a big weight loss is observed corresponding to the decomposition of the organic ligands (found, 24.9%; calcd, 23.2%). Notably, no plateaus between 180 and 460 °C can be found for 3. The removal of four coordinated aqua molecules on the equatorial coordination plane of one metal atom (Mn1) may result in

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Figure 6. Thermogravimetric analysis curves of 1-4.

Figure 7. Temperature dependence of χMT (•) and χ

-1

(O) for (a) 1 and (b) 2. The solid line represents the best fitting.

the fragility of the framework. Compound 4 is less stable than 1-3, which may be due to the weak interaction between the silver ions (Figure 6d). The first weight loss of 13.5% between 60 and 140 °C corresponds to the escape of 10 lattice and bonded water molecules (calculated: 13.5%). The framework began to decompose at 180 °C with a weight loss of 52.3% (calcd, 54.2%). Magnetic Properties. The temperature dependence of the magnetic susceptibility of 1 (χM is the magnetic susceptibility per Mn2 unit) was measured ranged from 2 to 310 K (Figure 7a). The observed χMT at room temperature is 8.585 cm3 K mol-1, which is close to the value (8.7 cm3 K mol-1) of two independent high-spin Mn(II) (g ) 2) centers. The χMT value decreases slowly until 50 K. After that, it drops rapidly to 4.927 cm3 K mol-1 at 2 K. The χ(T) data in the range of 50-300 K were fitted to Curie-Weiss law χ(T) ) C/(T-θ) with a Weiss constant of θ ) -2.30 K and Curie constant of C ) 8.670 cm3

K mol-1, indicating a weak antiferromagnetic coupling between two Mn(II) atoms through the µ2:η1,η2-carboxylate, and µ2-H2O bridges. The χmT vs T plot for magnetic behavior of 2 is depicted in Figure 7b. The room-temperature χMT value per Mn(II) unit is 4.470 cm3 K mol-1, corresponding to the spin-only state per Mn(II) ion (4.375 cm3 K mol-1). Similar to the curve of 1, the χMT value reduces slowly from ambient temperature to 50 K and then decreases sharply to 0.316 cm3 K mol-1 at 2 K. The χ(T) data in the range 50-320 K were fitted to Curie-Weiss law χ(T) ) C/(T-θ) with Weiss constant of θ ) -22.30 K and Curie constant of C ) 4.788 cm3 K mol-1. The magnetization value (see Figure S3 in the Supporting Information) increases linearly up to 7 T, suggesting a stronger antiferromagnetic coupling than that in 1. A simulation for the experimental magnetism behavior of 1, similar to that for other dinuclear high-spin Mn(II) (S ) 5/2) compounds, was carried out on the basis of its crystal structure.

Coordination Chemistry of Cyclohexane-1,2,4,5-tetracarboxylate

Equation 1 was deduced from the spin Hamiltonian H ) -JS1S2.25 χM ) 5exp(-24J/kT) + exp(-28J/kT) 2Ng2β2 kT 11 + 9exp(-10J/kT) + 7exp(-18J/kT) + 5exp(-24J/kT) + 3exp(-28J/kT) + exp(-30J/kT) (1) The least-squares analysis of the magnetic susceptibility data results in J ) -0.12 cm-1, g ) 1.97, and R ) 1.48 × 10-4, where R is calculated from Σ[(χM)obs s (χM)calcd]2/Σ[(χM)obs]2. The small and negative J value, similar to the other reported J values for materials with mere carboxylate as bridging groups, indicates the existing of weak antiferromagnetic coupling interaction.26 In the crystal structure of 2, the shortest distance between adjacent 1D manganese(II) chains is ca. 4.76 Å, indicating that the magnetic exchange pathway propagated via hydrogen bonds can not be ignored. Magnetic susceptibility data were analyzed through Fisher’s nearest-neighbor classical Heisenberg coupling model for the infinite linear manganese(II) chains27 by eq 2,

Ng2β2 1+u S(S + 1) 3kT 1-u

Acknowledgment. This work was supported by the NSFC (Grants 20525102, 20821001, and J0730420) and the National Basic Research Program of China (2007CB815305). Supporting Information Available: X-ray crystallographic files of 1-4 (CIF); simulated and experimental XRPD data of 1-3 and the magnetization at 2 K for 1 and 2 (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

55 + 30exp(-10J/kT) + 14exp(-18J/kT) +

χc )

Crystal Growth & Design, Vol. 9, No. 5, 2009 2449

(2)

where u ) coth[JS(S + 1)/kT] - kT/[JS(S + 1)]. A good fitting result with the interchain perturbation (zJ′)28 was obtained to be J ) -0.16 cm-1, g ) 2.07, and zJ′ ) -1.97 cm-1 (R ) 4.71 × 10-4). A second fitting result with J ) 1.17 cm-1, g ) 2.06, and zJ′ ) -3.14 cm-1 (R ) 3.24 × 10-4) was discarded. On the basis of the structure of 2 and previously documented related Mn(II) compounds, it was tentatively proposed that there exists a weak intrachain antiferromagnetic coupling interaction between Mn(II) atoms through single-oxygen (carboxylate) bridges and a much stronger antiferromagnetic interaction between adjacent manganese-carboxylate chains. Although ferromagnetic exchange interaction has been observed in a few manganese(II)-carboxylate compounds, in that case, the ferromagnetic interactions were propagated by single oxygen bridges such as phenoxide/hydroxide O atoms with Mn-O-Mn angles in the range of 92.1-103.2°.29 Conclusion We have successfully prepared four new Mn(II) and Ag(I) coordination polymers from cyclohexane-1,2,4,5-tetracarboxylic acid (H4L). Though theoretical calculation shows that the ligand has four possible conformations and LII (e,a,e,e) is energetically favorable in the gas phase and as a solvate in aqueous solution, LI (e,a,a,e) is more stable in the crystalline state and in coordination compounds, as we found in 1, 2 and 4. Conformational transformation of the ligand was achieved upon coordination under hydrothermal conditions and thermodynamically more stable LII (e,a,e,e) was obtained from the starting LI (e,a,a,e) by extending the reaction time and raising the reaction temperature. Variable conformations of the flexible ligands undoubtedly play an important role in both the construction and properties of metal-organic frameworks. Further work on the relationship between the conformational transformation of the organic ligands and the physical/ chemical properties of the resulted coordination compounds30 are currently under study.

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CG801348N