Two-Dimensional Noninterpenetrating Transition Metal Coordination

Two-Dimensional Noninterpenetrating Transition Metal Coordination Polymers with Large Honeycomb-like Hexagonal Cavities Constructed from a Carboxybenzyl Viologen Ligand

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Yan-Qiong Sun, Jie Zhang,* Zhan-Feng Ju, and Guo-Yu Yang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China Received June 2, 2005;

Revised Manuscript Received June 30, 2005

ABSTRACT: A viologen derivative, 1,1′-bis(4-carboxybenzyl)-4,4′-bipyridinium dichloride (H2BpybcCl2) as a versatile ligand, has been designed and synthesized by nucleophilic substitution reaction. A series of transitional metal coordination polymers of {(Bpybc)3[M2(H2O)6](OH)4‚15H2O}n (M ) Mn 1, Ni 2, Co 3) were synthesized by addition of MCl2 into aqueous solution of H2BpybcCl2. X-ray single crystal diffraction studies confirm that the complexes are isostructural to each other and crystallize in space group R3 h (no. 148) with a ) b ) 17.525(3) Å, c ) 26.230(5) Å, and Z ) 3 for exemplified Mn complex. Each metal ion coordinates to three Bpybc ligands, and each Bpybc ligand bridges two metal ions, thereby creating an infinite two-dimensional (6, 3) network with large honeycomb-like hexagonal cavities. The whole structure is a noninterpenetrating framework sustained by hydrogen bonding interactions. Introduction Crystal engineering research on metal-organic framework coordination polymers is continuing to undergo a rapid expansion, not only because of their intriguing structural motifs but also because of their promising technological applications in the areas of catalysis, gas storage, magnetism, nonlinear optics, electrical conductivity, and molecular recognition.1,2 A convenient path to obtain coordination polymers is to use a multifunctional ligand to link metal ions to form an infinite configuration.3 The aromatic multicarboxylate ligands have been extensively employed in the preparations of metal-organic coordination polymers. Such a ligand has a rigid conformation and would therefore facilitate the construction of the resulting coordination networks with regular shapes that can be reasonably predicted. For example, multidentate aromatic polycarboxyl systems, including 1,3,5-benzenetricarboxylic acid, 1,4-bezenedicarboxylic acid, 1,2,4,5-benzentetracarboxylic acid, and biphenyldicarboxylic acid, have been widely used by Yaghi4 and other groups5-9 in the construction of open frameworks with various cavities or channels. In contrast with these rigid ring carboxylate ligands, flexible or modified carboxylate ligands as functional building blocks are less developed in the construction of open framework materials. Despite a challenge in the predictability of the polymeric network topology, the unique conformation and coordination versatility of flexible ligands offer the great possibility for the construction of unprecedented frameworks. Especially, a flexible coordination network can be induced fit by guest molecules, having more advantages in the encapsulation of a large variety of guest molecules than the rigid host frameworks.10 With an aim of understanding the coordination chemistry of flexible ligands and developing new open * To whom correspondence should be addressed. Fax: 86-59183710051. E-mail: [email protected].

framework materials, we designed and synthesized a versatile carboxylate ligand, 1,1′-bis(4-carboxybenzyl)4,4′-bipyridinium dichloride (H2BpybcCl2), as functional building blocks based on the following considerations: (i) The introduction of a benzyl group can increase the flexibility of the molecular backbone. (ii) The carboxylate ligands may provide various coordination modes and longer link spacing than N-bound organic linkers with similar structures by reason of carboxylate fragments. The present ligand exhibits a lineal distance of ca. 19 Å between the two carboxylate functional groups, which would be ideal for building nanoscopic metallamacrocycles and extended coordination networks. (iii) H2BpybcCl2 is an interesting member of the viologen family that has been well-known for a long time in both electrochemistry and photochemistry due to their potential applications as redox mediators and display materials.11,12 They can undergo one-electron reduction to produce the cation radicals and play an active role as electron acceptors in charge transfer complexes in the solid state for electrical and ionic conductors.13 So, H2BpybcCl2 is a multifunctional ligand that contains viologen’s specific functions and carboxylate coordination groups. Furthermore, the incorporation of viologen unit enables the molecular skeleton to be positively charged, which may be helpful to avoid the occurrence of interpenetration by electrostatic repulsion in the construction of porous materials. Herein, we reported the design and synthesis of a novel H2BpybcCl2 ligand and the self-assembly with MCl2 (M ) Mn, Ni, Co). Three coordination polymers, {(Bpybc)3[M2(H2O)6](OH)4‚ 15H2O}n (M ) Mn 1, Ni 2, Co 3), are isostructural and possess a two-dimensional noninterpenetrating network with large honeycomb-like hexagonal cavities. Experimental Section General Remarks. All chemicals were used as purchased without further purification. IR spectra in the range of 4004000 cm-1 were measured with a Bomem MB102 FT-IR

10.1021/cg050249y CCC: $30.25 © 2005 American Chemical Society Published on Web 08/12/2005

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Scheme 1.

Synthesis of H2BpybcCl2 Ligand

Sun et al. Table 1. Crystallographic Data for Complexes 1-3 1

spectrometer in KBr pellets. The elemental analyses were carried out on a Vario EL III CHNOS elemental analyzer. Thermogravimetric data were collected using a Mettler Toledo TGA/SDTA 851e analyzer under nitrogen flows at a heating rate of 10 °C/min. Synthesis of H2BpybcCl2 Ligand. The H2BpybcCl2 ligand was synthesized according to the route shown in Scheme 1. A mixture of 4,4′-bipyridine (3.12 g, 20 mmol) and 4-(chloromethyl)benzoic acid (6.824 g, 40 mmol) was dissolved in 15 mL of DMF and then stirred at 120 °C under nitrogen for 4 h. After the mixture was cooled to room temperature, the resulting precipitate was filtered off, washed with DMF, and dried in a vacuum to give H2BpybcCl2 as a white powder (yield 35.3%). IR (KBr pellet, cm-1): 3438 (s), 3122 (w), 3052 (s), 3007 (s), 2589 (w), 2478 (w), 2365 (w), 1710 (s), 1688 (s), 1636 (s), 1579 (w), 1559 (s), 1506 (m), 1449 (m), 1419 (s), 1376 (m), 1318 (w), 1224 (s), 1185 (m), 1106 (m), 1021 (m), 861 (w), 827 (w), 772 (w), 745 (s), 708 (w), 588 (w), 513 (w), 470 (w). Synthesis of {(Bpybc)3[M2(H2O)6](OH)4‚15H2O}n (M ) Mn 1, Ni 2, Co 3). The general procedures for preparations of the complexes were as follows: H2BpybcCl2 (0.099 g, 0.2 mmol) was dissolved in 30 mL of water, and the pH value of the solution was adjusted to 7 with 1.0 mol/L NaOH solution. An aqueous solution of MCl2 (0.3 mmol in 5 mL of water) was then added to the above reaction mixture. After the mixture was stirred for 30 min, the residue was filtered. The filtrate was allowed to stand for several days to give prism crystals. Yield, 19%. IR (KBr pellet, cm-1): 3410 (s), 3121 (w), 3047 (w), 1635 (s), 1594 (s), 1551 (s), 1437 (m), 1394 (s), 1160 (m), 1017 (w), 842 (w), 798 (w), 769 (s), 703 (w), 593 (w), 490 (w). Anal. calcd for C78H106Mn2N6O37 (1829.57): C, 51.21; H, 5.83; N, 4.59%. Found: C, 51.29; H, 5.71; N, 4.27%. Anal. calcd for C78H106Ni2N6O37 (1837.11): C, 51.00; H, 5.82; N, 4.57%. Found: C, 50.99; H, 5.61; N, 4.27%. Anal. calcd for C78H106Co2N6O37 (1837.55): C, 50.98; H, 5.81; N, 4.57%. Found: C, 50.89; H, 5.55; N, 4.20%. Crystallographic Data Collection and Refinement. Suitable single crystals of the as-aynthesized compounds with dimensions of 0.38 × 0.28 × 0.25 mm3 for 1, 0.20 × 0.20 × 0.18 mm3 for 2, and 0.50 × 0.50 × 0.40 mm3 for 3 were carefully selected under an optical microscope and glued to a thin glass fiber with epoxy resin. Data sets for 1-3 were collected using a Mercury-CCD diffractometer with graphitemonochromated Mo KR (λ ) 0.71073 Å) radiation in the ω scanning modes at 293 K. Empirical absorption corrections were applied using the CrystalClear program.14 The structures were solved by direct methods and refined by full-matrix least squares on F2 using the SHELXTL-97 program package.15,16 All nonhydrogen atoms were refined anisotropically. Hydrogen atoms bound to carbon were computed and refined isotropically using a riding model. No chloride expected as counteranion was found in the crystal structure, and there are four OHanions among the lattice water molecules. The crystallographic data, selected bond lengths, and angles for compound 1-3 are listed in Tables 1 and 2.

Results and Discussion Synthesis and Characterization. Complexes 1-3 were synthesized in solution. It seems that the formation of these complexes is greatly influenced by the pH value of the solution. When the solution is acidic, the

empirical formula formula weight crystal system space group T (K) λ (Å) a (Å) b (Å) c (Å) R (°) β (°) γ (°) V (Å3) Z Dc (g cm-3) µ (mm-1) F(000) goodness-of-fit on F2 Rint R1 [I > 2σ(I)] wR2 [I > 2σ(I)]

2

3

C78H106Mn2N6O37 C78H106Ni2N6O37 C78H106Co2N6O37 1829.57

1837.11

1837.55

trigonal

trigonal

trigonal

R3 h

R3h

R3h

293 0.71073 17.525(3) 17.525(3) 27.230(5) 90 90 120 7242(2) 3 1.258 0.345 2886 1.104

293 0.71073 17.562(3) 17.562(3) 26.710(5) 90 90 120 7134(2) 3 1.283 0.480 2904 1.089

293 0.71073 17.7161(8) 17.7161(8) 26.869(3) 90 90 120 7303.3(8) 3 1.253 0.423 2898 1.129

0.0245 0.0778 0.2179

0.0242 0.0769 0.2152

0.0244 0.0773 0.2062

Table 2. Selected Bond Lengths (Å) and Angles (°) for 1-3a 1 Mn(1)-O(2) 2.071(2) Mn(1)-O(1w) 2.121(3) O(2)-Mn(1)-O(2)#1 86.52(10) O(2)-Mn(1)-O(1w)#2 89.96(10) O(2)#2-Mn(1)-O(1w) 91.34(11) O(1w)#2-Mn(1)-O(1w) 92.06(11) O(2)-Mn(1)-O(1w) 175.99(10) 2 Ni(1)-O(2) 2.059(2) Ni(1)-O(1w) 2.115(3) O(2)#1-Ni(1)-O(2) 87.05(10) O(2)#1-Ni(1)-O(1w)#2 90.92(11) O(2)-Ni(1)-O(1w)#2 89.08(10) O(1W)#2-Ni(1)-O(1w) 92.82(11) O(2)-Ni(1)-O(1w) 175.71(10) 3 Co(1)-O(2) 2.072(2) Co(1)-O(1w) 2.133(3) O(2)#1-Co(1)-O(2) 87.20(10) O(2)#1-Co(1)-O(1w)#2 90.82(11) O(2)-Co(1)-O(1w)#2 89.00(10) O(1w)#2-Co(1)-O(1w) 92.85(11) O(2)-Co(1)-O(1w) 175.80(10) a Symmetry transformations used to generate equivalent atoms. #1: -y + 1, x - y + 1, z; #2: -x + y, -x + 1, z.

H2BpybcCl2 molecule will appear. If the solution is basic, natrium salt of H2BpybcCl2 may be produced. By adjusting the pH value of the solution to 7.0 with a NaOH aqueous solution, complexes 1-3 were successfully obtained. The FT-IR spectrum of H2BpybcCl2 exhibits broad bands around 3000-3450 cm-1 due to the stretching vibrations of the OH, CH2, and CH groups. The weak bands at 2589 and 2478 cm-1 can be assigned to overtones and combination bands (due to OH bend and C-O stretch) enhanced by Fermi resonance with the broad OH stretch band.17 The characteristic CdO stretching vibrations of carboxylic acid appear at 1688 and 1710 cm-1. The strong band at 1636 cm-1 can be attributed to the typical CdN and CdC stretching vibrations of bipyridinium. The C-OH in-plane bending and C-O stretching vibrations are observed at 1419 and 1318 cm-1, respectively. In the IR spectra of 1-3, the absence of any strong bands around 1700 cm-1 indicates that two carboxylic acid groups are deprotonated. Moreover, they show the asymmetric and symmetric stretching vibrations of the COO groups at 1594 and

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Figure 1. TG curve under a N2 atmosphere for complex 1.

Figure 3. View from c-axis showing 2D honeycomb-like grid; H atoms are omitted for clarity.

Figure 2. Coordination environment of Mn in complex 1 with atom labeling scheme (thermal ellipsoids at 30% probability level); H atoms are omitted for clarity.

1394 cm-1 separately. The difference in νas(COO) and νs(COO) (∆ ) 203 cm-1) suggests that the carboxylate groups coordinate to the metal ions only in monodentate bridging mode.18 The thermal stability of the compounds was evaluated by thermogravimetric analysis. As shown in a TGA curve of 1, for example (Figure 1), two-step major weight loss processes are observed. The first weight loss of 22.29% from 40 to 170 °C corresponds to the release of the 21 water molecules and dehydration of four hydroxide groups, in good agreement with the calculated value (22.64%). Then, these complexes remain stable up to 220 °C and begin to decompose upon further heating. Structural Description. X-ray diffraction studies reveal that all of the complexes are isostructural and the structure of {(Bpybc)3[Mn2(H2O)6](OH)4‚15H2O}n (1) is described here representatively. Complex 1 exhibits a two-dimensional network with honeycomb-like hexagonal cavities. There is a flowerlike macrocyclic ring with a crystallographic inversion center located at the center of the molecule. As shown in Figure 2, each Mn center located on the crystallographic 3-fold axis is coordinated by three water molecules and three oxygen atoms of three distinct Bpybc ligands. The Mn-O bond distance is 2.071(2) Å while the Mn-Ow bond length is 2.121(3) Å. The cis bond angles range from 86.5(1) to 92.1(1)° while the trans bond angle is 176.0(1)°. Therefore, all of the Mn centers display distorted octahedral coordination geometry. Because the methylene groups are used as knots to link together the pyridine rings and phenyl rings, the whole Bpybc ligand is not linear

and exhibits a zigzag conformation with an inversion center lying in the middle of the C(1)-C(1A) bond. The two pyridine rings of the 4,4′-bipyridyl group of Bpybc ligand are in the same plane, while the phenyl rings are twisted significantly from the neighboring pyridine ring with the dihedral angle of 76.0(1)°. The lineal distance between the two terminal carboxylate groups is ca. 18.8 Å. Each Bpybc ligand bridges two metal ions through two terminal carboxylate groups in monodentate coordination mode. The Mn centers serve as 3-connected nodes in the framework, thereby creating an infinite two-dimensional (6, 3) network, which contains large edge-sharing hexagons with a metal ion at each corner and a Bpybc ligand molecule at each edge (Figure 3), where (n, m) presents the topology of a given net, m is the number of connection to neighboring nodes, and n is the number of nodes in the shortest closed loop within the net. The hexagonal grids exhibit identical edge lengths [22.00(2) Å] and identical corner angles [47.48(1)°], and the separation of the opposite corner is 28.25 Å. Because of the length and flexibility of the Bpybc ligand, the metal ions of (6, 3) network do not lie in the same plane but represent a chair conformation. The two phenyl rings of each Bpybc ligand are parallel to each other and lie “above” and “below” the hexagonal grids alternately. The whole two-dimensional hexagonal layer extends parallel to the ab plane in a wavelike fashion with a layer thickness of ca. 20.14 Å. The packing views of the complex 1 are shown in Figure 4. The two-dimensional honeycomb-like layers are stacked in parallel in -ABCABC- alternations along the crystallographic c-axis, and each layer is shifted by (-a b+B b +b c)/3 with respect to the next one. It is interesting to note that 2D coordination networks of 1 do not interpenetrate each other, which is rare in such large macrometallacycle frameworks.19,20 Unlike [Ag(L)(PPh3)(Rh2(OAc4))1.5]n19 (23.9 × 23.8 × 23.9 Å3) and [Ag2L′3(ClO4)2]20 (8.42 × 10.88 × 8.69 Å3) complexes with large noninterpenetrating honeycomb-like 2D (6, 3) networks, in which the repeating units are sustained by the large PPh3 ligands or pheny groups situated in the cavities, there exist only water molecules and OHanions within the 2D layers of the present complexes. The water molecules and OH- counteranions connect

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(OH)4‚15H2O}n (M ) Mn 1, Ni 2, Co 3) have been obtained by assembling H2BpybcCl2 with MCl2. These complexes possess two-dimensional network with large honeycomb-like hexagonal cavities. The positively charged molecular skeleton provides a potential advantage in avoiding the occurrence of interpenetration. The resulting noninterpenetrated framework is sustained by hydrogen bonding interaction and steric hindrance of the ligand’s backbone between the frames. The successful fabrication of these complexes may provide valuable insights into the construction of open framework materials with specific structures and functions. Acknowledgment. We acknowledge the financial support of the Natural Science Foundation of China (20201010/50372069), Natural Science Foundation of Fujian Province of China (E0220003), and the Ministry of Personnel of China. Supporting Information Available: CIF files for compounds 1-3, IR spectra, and TG curves. This material is available free of charge via the Internet at http://pubs.acs.org.

References

Figure 4. Schematic representation of 1, showing the stacking of the noninterpenetrating (6, 3) networks. (a) Parallel stacking in -ABC- alternations along the crystallographic c-axis. (b) The corrugated 2D layer architectures. The nodes represent metal ions, and the lines represent ligand molecules.

with each other (O‚‚‚O 2.44-2.92 Å) and with the oxygen atoms of the Bpybc ligands (O‚‚‚O 2.76 Å) through hydrogen bonding interactions to form twodimensional supramolecular network. So, it can be considered that the repeating (6, 3) sheets of the complexes are sustained by hydrogen bonding interactions and the steric hindrance of the ligand’s backbone between the frames. When the solid of 1 was immersed in an aqueous solution of NaNO3 for 24 h, a characteristic vibration of NO3- at 1385 cm-1 appears in the IR spectrum, and the asymmetric stretching vibration of the COO groups at 1594 cm-1 tends to shift to 1618 cm-1, suggesting the OH- ions in 1 can be partially exchanged with NO3- ions and 1 undergoes a structural change upon the ion exchange process. A detailed investigation involving the effect of anions on the framework topology is in progress. Conclusions In this work, we have designed and synthesized a versatile viologen derivative ligand, 1,1′-bis(4-carboxybenzyl)-4,4′-bipyridinium dichloride (H2BpybcCl2). Three novel transition metal complexes {(Bpybc)3[M2(H2O)6]-

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