From Helical Array to Porous Architecture: Exploring the Use of Side Chains of Amino Acids to Engineer 1D Infinite Coordination Polymeric Chain into Porous Frameworks Ben-Yong Lou,†,‡ Fei-Long Jiang,† Ben-Lai Wu,† Da-Qiang Yuan,† and Mao-Chun Hong*,†
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 4 989-993
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, The Chinese Academy of Sciences, Fuzhou, 350002, China, and Department of Chemistry and Chemical Engineering, Minjiang UniVersity, Fuzhou, 350108, China ReceiVed NoVember 7, 2005; ReVised Manuscript ReceiVed December 21, 2005
ABSTRACT: The self-assembly of D,L-amino acids and Cu(NO3)2‚3H2O with 4,4′-bipyridine in basic ethanol/aqueous solution gave rise to two distinct three-dimensional (3D) supramolecular architectures {[Cu2(D-ala)(L-ala)(4,4′-bipy)2](NO3)2‚5H2O}n (1) and {[Cu2(D-phe)(L-phe)(4,4′-bipy)2](NO3)(OH)‚5H2O}n (ala ) alanine anion, phe ) phenylalanine anion, 4,4′-bipy ) 4,4′-bipyridine) (2). In 1, Cu(II) centers chelated by deprotoned D,L-alanine direct linear 4,4′-bipy to form a one-dimensional (1D) helical chain, which is further assembled into 3D helical array through interchain hydrogen bonds and anion guest chains. In 2, Cu(II) centers chelated by deprotoned D,L-phenylalanine direct 4,4′-bipy to form a 1D twisted coordination polymeric chain that interacts with four adjacent chains through complementary coordination interactions and two chains through weak π-π interactions to give a 3D porous framework. Introduction Microporous metal-organic coordination frameworks proved to have a great advantage over the traditional zeolite in catalysis, separation, and gas sorption and storage.1-3 Such porous crystalline materials with different sizes and shapes have attracted a great deal of interest in recent years because the welldefined structures can be constructed under mild conditions and allow systematic design of pore size and chemical functionality.4 An effective strategy for obtaining such extended porous materials is to utilize discrete metal complexes and suitable linkers as building blocks.5 The coordination mode of organic ligands could transform the coordination environment of the metal center, which could direct the linker to connect discrete metal complexes into a specific extended structure. Onedimensional (1D) coordination polymeric chains as building blocks have been well exploited for the construction of porous architectures by the principle of crystal engineering.6 This represents another feasible path for the rational design of porous crystalline solids since the 1D metal-organic chains with different motifs could be further assembled into threedimensional (3D) open frameworks through various noncovalent interactions. Recently, amino acids with flexible coordination modes have been utilized to construct metal-organic supramolecular architectures.7 Generally, deprotoned amino acids prefer chelating metal centers through amino N atoms and one carboxylic O atom to give a five-membered ring unit (C-N-M-O-C), and the remaining coordination sites of the metal center could be occupied by second ligands.8 The five-membered ring could direct the bridging ligand such as 4,4′-bipyridine (4,4′-bipy) to form 1D helical chains, and the side chains of amino acids could induce different hydrogen-bonding interactions that further engineer the resulting 1D infinite chains.9 We are interested in exploring the use of Cu(II)-amino acid units as simple building blocks to construct porous crystalline materials based on 1D * Corresponding author. Tel: +86-059183792460. E-mail: hmc@ fjirsm.ac.cn. † The Chinese Academy of Sciences. ‡ Minjiang University.
coordination polymeric chains. In fact, amino acids have various side groups to offer various noncovalent interactions that play important roles in the structures and functions of proteins.10-12 So, it is feasible to construct 3D porous architectures based on 1D polymeric chains containing Cu(II)-amino acid units by selecting a suitable amino acid with specific residues that could efficiently control the packing arrangement of 1D coordination polymeric chains. In this paper, we selected two simple racemic amino acids, D,L-alanine and D,L-phenylalanine, which differ slightly in their side groups. In the presence of 4,4′-bipy, a 1D helical chain produced by D,L-alanine ligands and Cu(II) centers is further assembled into a 3D helical array by interchain hydrogen bonds and guest 1D chains. D,L-Phenylalanine has an extra phenyl ring and promises a subtle effect on the final supramolecular structure. Thus, the 1D twisted coordination polymeric chain resulting from D,L-phenylalanine ligands and Cu(II) centers interacts with four adjacent chains through complementary coordination interactions and two chains through weak π-π interactions to give a porous 3D framework. Experimental Section D,L-Alanine, D,L-phenylalanine, and 4,4′-bipyridine were purchased as analytical grade and used without further purification. The IR spectra as KBr disk were recorded on a Magna 750 FT-IR spectrophotometer. C, H, and N elemental analyses were determined on an Elementary Vario ELIII elemental analyzer. Thermogravimetric analyses were performed on a NETZSCH STA 449C instrument. X-ray powder diffractions were performed on Rigaku DMAX2500PC diffractometer. Synthesis of {[Cu2(D-ala)(L-ala)(4,4′-bipy)2](NO3)2‚5H2O}n (1). To an aqueous solution (10 mL) of D,L-alanine (0.089 g, 1 mmol) and NaOH (0.04 g, 1 mmol), Cu(NO3)2‚3H2O (0.24 g, 1 mmol) in water (10 mL) was added slowly. The reaction solution was stirred for half an hour, and then 4,4′-bipyridine (0.156 g, 1 mmol) in ethanol (5 mL) was added. The solution was kept in air, and after several days blue crystals were obtained in 80% yield. Elemental analysis for C26H38N8O15Cu2 (%), Calcd: C, 37.60; H, 4.62; N, 13.50; found: C, 37.69; H, 4.82; N, 13.48. IR (KBr): 3424b, 3256s, 1610s, 1388s, 1365s, 1220m, 1073m, 820m, 645m. Synthesis of {[Cu2(D-phe)(L-phe)(4,4′-bipy)2] (NO3)(OH)‚5H2O}n (2). Complex 2 was prepared as 1 except that D,L-phenylalanine was used instead of D,L-alanine. Yield: 70%. Elemental analysis for
10.1021/cg050589j CCC: $33.50 © 2006 American Chemical Society Published on Web 02/21/2006
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Figure 1. (a) The asymmetric unit of complex 1. (b) The 1D helix along the b-axis in 1. Table 1. Crystal Data and Structure Refinement for Complexes
formula fw crystal size crystal system space group a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 Z Dc, g/cm3 F000 2θmax, deg reflns collected unique reflns Rint parameters final GoF R1 (I > 2σ(I)) wR (all data)
1
2
C26H38Cu2N8O15 829.72 0.18 × 0.12 × 0.12 mm orthorhombic Pbca 18.051(4) 21.210(5) 17.979(4) 90 90 90 6883(3) 8 1.601 3424 50.0 42066 6062 0.0583 502 1.163 0.0587 0.1310
C38H47N7O13 Cu2 936.91 0.36 × 0.25 × 0.25 mm monoclinic C2/c 22.8992(13) 24.8190(11) 9.0518(5) 90 94.305(2) 90 5129.9(5) 4 1.213 1944 55.0 19697 5870 0.0272 256 1.141 0.0744 0.2368
C38H47N7O13Cu2 (%), Calcd: C 48.67, H 5.06, N 10.46; found: C 47.98, H 5.22, N 10.29. IR (KBr): 3420b, 3220s, 1610s, 1380s, 1368m, 1073m, 820m, 645m. X-ray Structure Determination. X-ray data were collected on a Rigaku Mercury-CCD diffractometer at 130 K for 1 and 293 K for 2, using graphite- monochromated Mo-KR radiation (λ ) 0.7107 Å). The empirical absorption corrections were applied by using the CrystalClear program.13 The structures were solved by direct methods and refined on F2 by full-matrix least-squares methods using the SHELXTL-97 program package.14,15 Non-hydrogen atoms were refined anisotropically, and organic hydrogen atoms were generated geometrically. In 1, the hydrogen atoms of water molecules were located by difference maps and constrained to ride on their parent O atoms. The C, O atoms of one alanine molecule were disordered. Solvent water molecules in 2 were disordered, and it failed to add hydrogen to O atoms of solvent water. Crystal data and structure refinement for 1 and 2 are given in Table 1.
Results and Discussion Crystal Structure of {[Cu2(D-ala)(L-ala)(4,4′-bipy)2](NO3)2‚ 5H2O}n (1). Complex 1 crystallizes in space group Pbca with a [Cu2(H2O)2(D-ala)(L-ala) (4,4′-bipy)2]2+ cation, two nitrate anions, and three lattice water molecules in an asymmetric unit. As shown in Figure 1, the Cu(II) center is in a distorted squarepyramidal coordination geometry in which deprotoned D,Lalanine chelates Cu(II) ion through an amino N atom and a carboxylic O atom. The remaining two sites in the basal plane
are occupied by two N atoms from different 4,4′-bipy ligands, and the apical position is occupied by O atoms of coordinated water. Two nitrate anions are alternately bonded to two solvent water molecules through hydrogen bonds [O8‚‚‚O14 ) 2.987(5) Å, O14‚‚‚O10 ) 2.755(5) Å, O12‚‚‚O13 ) 2.834(7) Å] to form the anion guest which interacts with the cation host through hydrogen bonds [O7‚‚‚N4 ) 3.043(6) Å, O13‚‚‚O15 ) 2.729(6) Å, O15‚‚‚O6 ) 2.914(5) Å]. The binuclear units self-assemble along the b axis through Cu(II)-pyridyl coordination bonds to form a 1D helical chain with the pitch of 21.210 Å containing channels of about 7 × 7 Å in dimensions. The helices exhibit both left- and right-handness in a 1:1 ratio. It is noteworthy that the helical chains are further assembled by two kinds of hydrogen bonds. Each helical chain connects adjacent four equal-handed chains in parallel through hydrogen bonds [O5‚‚‚O2A ) 2.666(8) Å, O6‚‚‚O4B ) 2.647(5) Å, A: 1/2 - x, -y, 1/2 + z; B: 1/2 + x,1/2 - y, -z] formed between coordinated water and the uncoordinated carboxylic oxygen atom of alanine to give the 1D tubular structure (Figure 2). The anion guest attached to the helical host is hydrogen bonded to each other to form a 1D zigzag guest chain along the c axis through hydrogen bonds [O13‚‚‚O9C ) 2.757(9) Å, C: x, 1/2 - y, z - 1/2]. Moreover, along the c axis guest chains link the host helices with opposite handness through hydrogen bonds [O6‚‚‚O14C ) 2.737(5) Å, C: x, 1/2 - y, z - 1/2] formed between coordinated water and solvent water in a left-rightleft-right way to form a stream of interlocked helices (Figure 3), which is further assembled into a highly ordered helical array by hydrogen bonds [O5‚‚‚O15D ) 2.776(5) Å, D: -x + 1/2, y - 1/2, z] between the stream of helices. Thus, at four corners each host chain connects four host helices with oppositehandness through guest chains. Consequently, the 1D channels resulting from left-handed (right-handed) helical array are fully overlapped by a right-handed (left-handed) array (Figure 4). Crystal Structure of {[Cu2(D-phe)(L-phe)(4,4′-bipy)2](NO3)(OH)‚5H2O}n (2). In 2, the Cu(II) center is also in a nearly square-pyramidal coordination geometry in which deprotoned phenylalanine chelates the Cu(II) center through amino N atoms and one carboxylic oxygen O to form a Cu(II)-phenylalanine five-membered ring cation unit, and the remaining sites in the basal plane are occupied by N atoms from two different 4,4′bipy molecules. However, the apical position is occupied by O2A (A ) x, -y, z - 1/2) of a carboxylic group instead of a water molecule at a distance of 2.218 Å (Figure 5). The Cu(II)phenylalanine units direct 4,4′-bipy to extend the cation to form a 1D twisted coordination polymeric chain with a distance of 25.247 Å running along the [1h 0 1] axis in which Cu(II)phenylalanine units are located at the four corners symmetrically
From Helical Array to Porous Architecture
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Figure 2. (a) The hydrogen-bonded modes of helical chains with equal chirality in 1. (b) Packing arrangement of helical chains with equal chirality in 1.
Figure 3. (a) Two guest chains (green) linking an array of host helices in 1. (b) 3D helical array in 1 (deleting methyl group of alanine for clarity).
Figure 4. (a) The helix surrounded by four helices with opposite handness. (b) The channel overlapped by four helices with opposite handness in 1.
in the top view (Figure 6). Four Cu(II)-phenylalanine units and two kinds of 4,4′-bipy ligands exist in each period of the novel twisted polymeric chain. One 4,4′-bipy bridges two adjacent Cu(II)-phenylalanine units with equal chirality to give a chiral binuclear unit which lies on the top and bottom in the top view. Another 4,4′-bipy bridges the resulting binuclear units with opposite chirality on the cross into a 1D twisted polymeric chain in which racemic D,L-phenylalanine molecules are arranged in the DDLL way. The location of Cu(II)-phenylalanine units in the chain is in a very favorable arrangement for an extended 3D open framework since carboxylic oxygen O2 as potential donors and Cu(II) centers as corresponding acceptors extend themselves outside in different directions. Thus, each Cu(II)-amino acid unit with both a potential donor and acceptor could interact with two similar five-membered units from their respective polymeric chains through coordination bonds Cu1O2. Interestingly, the chiral binuclear unit in each chain connects
Figure 5. The coordination environment of Cu(II) center in 2 with 30% thermal ellipsoids. Symmetry codes: A: x, -y, z - 1/2; B: 1/2 - x, 1/2 - y, 1 - z; C: -x, y, 3/2 - z.
two similar units with opposite chirality from two different chains by the complementary coordination modes, and two
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Figure 6. (a) 1D coordination polymeric chain viewed along c axis in 2 (deleting the side chain of phenylalanine for clarity). (b) The top view of the 1D twisted chain along the [1h 0 1] axis.
Figure 7. (a) The complementary coordination modes between binuclear units of two adjacent chains (deleting the side chain of phenylalanine for clarity). (b) The top view along the [1h 0 1] axis.
Figure 8. (a) The top view along the [1h 0 1] axis of the chain interacting with adjacent six chains. The dotted lines denote π-π interactions. (b) The 1D channels with the size of 0.8 × 2 nm along the c-axis.
adjacent chains connected by such interactions between binuclear units form 1D double chains based on a 62-membered macrocycle containing eight Cu(II)-phenylalanine units (Figure 7). Each chain connects four adjacent chains by the same interactions between binuclear units and the phenyl ring of phenylalanine is involved in weak π-π interactions16 with one pyridyl ring of 4,4′-bipy from the left (right) chain at a distance of 3.937 Å (centroid‚‚‚centroid). Thus, each 1D chain interacts with four chains above and below through coordination bonds and two chains on the left and right through weak π-π interactions to give rise to a 3D cation framework with large 1D channels (0.8 × 2 nm) along the c axis (Figure 8). The void space accounts for 40% of the crystal volume and is occupied by anions and guest water molecules.
TGA and XRD Studies. Thermogravimetry analysis curve of 1 revealed a weight loss from 60 to 160 °C (10.5%), which could be attributed to the loss of water (10.8%). When heated above 200 °C, the compound began to decompose slowly. TGA curve of complex 2 showed a weight loss in the range of 30120 °C (9.8%), corresponding to the total removal of the lattice water (9.6%). No weight loss occurred from 120-200 °C. Above 200 °C, complex 2 began to decompose rapidly. X-ray powder diffraction patterns of both compounds before and after removing water molecules were completely different, suggesting the host framework may be broken down due to the loss of water. It showed guest water molecules play an important role in stabilizing the supramolecular structures of complex 1 and 2.
From Helical Array to Porous Architecture
Zaworotko demonstrated two possible 1D coordination polymeric chains (zigzag or helical chain) that could result from self-assembly of either a cis-octahedral or a cis-square planar metal and a linear “spacer” ligand in a 1:1 ratio of metal to ligand.17 In 1, linear 4,4′-bipy ligands bridge Cu(II)-alanine units to form a 1D helical chain, which is further assembled into a highly ordered helical array by intriguing interchain hydrogen bonds. But in 2, Cu(II) centers chelated by racemic phenylalanine direct the 4,4′-bipy to form a 1D twisted coordination polymeric chain rather than a zigzag chain or helix. The novel chain connects four adjacent similar chains through coordination interactions and two chains through weak π-π interactions to give a porous 3D framework. The apical sites of Cu(II) centers in 1 are occupied by water molecules which afford three hydrogen bonds engineering the 1D helix into intriguing higher supramolecular helices. In 2, carboxylic oxygen atoms instead of water molecules occupy the apical sites of Cu(II) centers, and the resulting coordination bonds offer stronger interchain interactions than hydrogen bonds. Cu(II) centers in 2 are linked by 4,4′-bipy to form a long conjugated electron pathway and two pyridyl rings of one 4,4′-bipy (N2-N2B) are involved in weak π-π interactions with phenyl rings of phenylalanine from adjacent chains. This could decrease the electron density of Cu(II) centers since the conjugated electron pathway is broadened. This is helpful to the ligand-substitution reaction of the metal center,18 and so water molecules of the apical site of Cu(II) centers could be easily replaced by more aggressive ligands such as a carboxylic group. Moreover, the resulting coordination bonds adjust the shape and structure of the 4,4′-bipy-bridged coordination polymeric chain, which greatly facilitate the selfassembly of 1D chain further. Conclusion When exploring the use of side chains of amino acids to induce noncovalent interactions to construct porous solids, we discovered that D,L-alanine and D,L-phenylalanine gave rise to two completely different architectures based on similar 1D coordination polymeric chains. Alanine resulted in various hydrogen bonds connecting the 1D helical chains into intriguing helical arrays and the phenyl ring of phenylalanine induced interchain coordination bonds connecting 1D twisted coordination polymeric chains into 3D cation frameworks containing large 1D channels. The aromatic ring of phenylalanine plays a subtle role in determining the supramolecular structure. Acknowledgment. This work was supported by grants from the Nation Natural Science Foundation of China and the Natural Science Foundation of Fujian Province. Supporting Information Available: X-ray crystallographic file in CIF format for the structure determination of complex 1 and 2. This material is available free of charge via the Internet at http://pubs.acs.org.
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