A Series of Chiral MetalâOrganic Frameworks Based on Oxalyl Retro

Article pubs.acs.org/crystal

A Series of Chiral Metal−Organic Frameworks Based on Oxalyl RetroPeptides: Synthesis, Characterization, Dichroism Spectra, and Gas Adsorption Lang Lin,†,§ Rongmin Yu,*,†,‡ Wenbin Yang,†,‡ Xiao-Yuan Wu,†,‡ and Can-Zhong Lu*,†,‡ †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 35002, China ‡ Key Laboratory of Coal to Ethylene Glycol and Its Related Technology, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, 35002, China § Graduate University of Chinese Academy of Sciences, Beijing, 100049, China S Supporting Information *

ABSTRACT: By synergistic involvement of enantiopure oxalyl retro-peptide ligands and the conformationally flexible ligand 4,4′-bi-3,5-dimethylpyrazole (H2mbpz), four new threedimensional (3-D) homochiral metal−organic frameworks (MOFs), Cu2(L-OBAla)(H2mbpz)·2H2O (1), Cu2(L-OBVal)(H2mbpz) (2), Cu2(L-OBPhe)(H2mbpz) (3), and [Cu6(LOBLeu)3(H2mbpz)3 (H2O)2]·7H2O (4) (L-OBAla = N,N′oxalylbis(L-alanine); L-OBVal = N,N′-oxalylbis(L-valine); LOBPhe = N,N′-oxalylbis(L-phenylalanine); L-OBLeu = N,N′oxalylbis(L-leucine)), have been prepared under solothermal conditions. All of these structures contain binuclear subunits in which two copper atoms are coordinated by a single retropeptide to produce a unique tetracyclic planar rigid moiety. Compounds 1−3 are isostructural. The self-assembly of the binuclear subunits generates two-dimensional (2-D) networks which are further pillared by H2mbpz ligands to give three-dimensional (3-D) frameworks with one-dimensional (1-D) channels. In compound 4, the interaction of the binuclear copper subunits forms an unexpected U-shaped secondary building unit containing six copper atoms, and further linked by H2mbpz ligands to furnish a 3-D architecture. The chiral nature of all compounds was confirmed by solid-state circular dichroism spectroscopy. Gas adsorption experiments indicate the microporous property of compounds 1 and 2. Compound 1 exhibits the highest capability of gas adsorption among the four compounds due to its largest pore volume. Compound 2 reveals selectively adsorption of H2 over N2 as a result of the limited window size.

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INTRODUCTION Chiral metal−organic frameworks (MOFs), emerging as a promising subclass of coordination polymers, have received extensive attention, not only because of their diverse topologies and tailorable pores in structure, but also for their potential applications in asymmetry catalysis,1 enantioselective separation,2 and nonlinear optical materials.3 Although a few homochiral MOFs have been reported in the past decades, the challenge in design and preparation of the homochiral MOFs with permanent porosity remains. Recently, biomolecules have been used as building blocks in MOFs under mild reaction conditions,4 of which, α-amino acids, peptides, and their derivatives are especially attractive because they are naturally chiral and inexpensive, and also have rich coordination modes.5 However, because of the backbone flexibility of the amino acids, the self-assembly of amino acidbased ligands with transition metals prefers to form condensed packing structures without accessible pores or channels.6 In order to obtain robust and porous chiral MOF of amino acids, © 2012 American Chemical Society

rigid ligands, such as 4,4′-bipyridine, acting as a spacer are usually used.7 Oxalyl retro-peptides,8 also known as N,N′-oxalylbis(amino acid), contain a -NH−CO−CO−NH- (N,N′-disubstituted oxamide) core and two terminal carboxyl groups. Therefore, they are supposed to act as a multidentated ligand in various modes in the self-assembly of coordination polymers. However, only a few scattered examples of metal complexes using oxalyl retro-peptides as bridging ligands have appeared so far.9 Ranganathan reported the self-assembly of Cu(II) and an achiral oxalyl retro-peptide to give a two-dimensional (2-D) structure in which the nontotally saturated tetra-coordinated Cu(II) atoms provide chances for novel structures by incorporating suitable ligands.9b To the best of our knowledge, Received: April 2, 2012 Revised: April 25, 2012 Published: April 26, 2012 3304

dx.doi.org/10.1021/cg300440t | Cryst. Growth Des. 2012, 12, 3304−3311

Crystal Growth & Design

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Table 1. Summary of Crystal Data and Structure Refinements for Compounds 1−4, D-1

a

compound

1

2

3

4

D-1

formula formula weight crystal system space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z flack factor μ/mm−1 Dc/g·cm−3 F(000) collected reflections unique reflections R1a, wR2b [I > 2σ(I)] R1a, wR2b (all data) GOF (F2)

C18H26Cu2N6O9 577.50 orthorhombic P2(1)2(1)2 10.764(4) 26.969(9) 9.724(3) 90 90 90 2822.7(16) 4 0.12(3) 1.553 1.359 1176 21880 6421 0.0623, 0.1835 0.0809, 0.2037 1.000

C22H34Cu2N6O7 601.60 orthorhombic P2(1)2(1)2 11.821(5) 26.001(11) 9.892(4) 90 90 90 3041(2) 4 0.03(4) 1.440 1.314 1240 23411 6799 0.0634, 0.1489 0.0980, 0.1680 1.070


C72H116Cu6N18O27 2047.07 triclinic P1 12.7711(14) 14.229(2) 16.003(2) 106.640(2) 103.935(3) 106.606(3) 2499.8(5) 1 0.009(18) 1.319 1.276 994 28996 20861 0.0833, 0.2292 0.1154, 0.2676 0.980


R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = [Σw(Fo2 − Fc2)2/Σw(Fo2)2]1/2. 0.6 s/step were applied in a 2θ range of 5.00−65.00°. The solid-state circular dichroism (CD) spectra were recorded on a MOS-450 spectropolarimeter using KCl pellets. The gas sorption studies were performed on a Micromeritics ASAP 2020 surface area and pore size analyzer. Synthesis of Compounds 1−4, D-1. [Cu 2 ( L -OBAla)(H2mbpz)]·2H2O (1). Compound 1 was synthesized by stirring Cu(NO3)2·3H2O (24 mg, 0.1 mmol), L-OBAla methyl ester (26 mg, 0.1 mmol), and H2mbpz (40 mg, 0.2 mmol) in 10 mL of water/ ethanol (1:1) solvent. The resulting mixture was placed in a 23 mL Teflon reactor and kept under autogenous pressure at 100 °C for 2 days. After slowly being cooled to room temperature, the green prism crystals were filtered and washed with water (86% yield, based on Cu(II)). C18H26Cu2N6O9: Calc.: C 36.18, H 4.38, N 14.06. Found: C 36.19, H 4.66, N 14.10%. FTIR (KBr pellet, cm−1): 3199(s), 3078(s), 2924(s), 1568(m), 1544(m), 1416(s), 1371(w), 1308(m), 1256(m), 1171(m), 1061(w), 1015(s), 843(m), 785(s), 626(w), 519(w), 480(w). CCDC number: CCDC 867900. Cu2(L-OBVal)(H2mbpz) (2). Compound 2 was synthesized by stirring Cu(NO3)2·3H2O (48 mg, 0.2 mmol), L-OBVal methyl ester (32 mg, 0.1 mmol), and H2mbpz (20 mg, 0.1 mmol) in 10 mL of water/acetonitrile (1:1) solvent. The resulting mixture was placed in a 23 mL Teflon reactor and kept under autogenous pressure at 100 °C for 3 days. Upon cooling to room temperature, the green block-shaped crystals were filtered and washed with water (70% yield, based on Cu(II)). C22H34Cu2N6O7: Calc.: C 42.50, H 5.51, N 13.52. Found: C 42.55, H 4.86, N 13.80%. FTIR (KBr pellet, cm−1): 3203(m), 2960(m), 2929(m), 1655(s), 1624(s), 1465(w), 1367(m), 1323(w), 1071(w), 1037(w), 772(w). CCDC number: CCDC 867901. Cu2(L-OBPhe)(H2mbpz) (3). The light-green block crystals of compound 3 were obtained through the method similar to that of 1 except using L-OBPhe methyl ester (41 mg, 0.1 mmol) instead of LOBAla methyl ester (92% yield, based on Cu (II)). C30H30Cu2N6O6: Calc.: C 51.64, H 4.33, N 12.04. Found: C 51.54, H 4.54, N 11.86%. FTIR (KBr pellet, cm−1): 3162(m), 3061(m), 2928(m), 2825(m), 1655(s), 1617(s), 1497(w), 1383(s), 1357(m), 1279(w), 1070(w), 1033(w), 745(w), 701(w), 553(w). CCDC number: CCDC 867902. [Cu6(L-OBLeu)3(H2mbpz)3 (H2O)2]·7H2O (4). A similar solvothermal reaction to that for 1 with L-OBLeu (34 mg, 0.1 mmol) instead of OBV afforded deep green prism crystals of compound 4 in 54% yield (based on Cu(II). C72H116Cu6N18O27: Calc.: C 42.24, H 5.71, N 12.31.

there are no reports on three-dimensional (3-D) MOFs based on chiral oxalyl retro-peptides. Herein, we report the synthesis and characterization of a family of homochiral porous MOFs containing oxalyl retropeptide ligands by using the pillared-layer strategy which is one of the most successful methods for the rational design of MOFs.10 In our study, Cu(II) and readily available chiral oxalyl retro-peptides are used to form chiral 2-D layers similar to the 2-D structure reported by Ranganathan, which in turn, are connected together by conformationally flexible spacers 4,4′-bi3,5-dimethylpyrazole (H2mbpz)11 to provide chiral MOFs, in one-pot reactions under solothermal conditions. The solothermal reactions of Cu(NO3)2·3H2O, H2mbpz, and different oxalyl retro-peptides afforded four 3-D homochiral microporous MOFs, Cu2(L-OBAla)(H2mbpz)·2H2O (1), Cu2(LOBVal)(H2mbpz) (2); Cu2(L-OBPhe)(H2mbpz) (3); [Cu6(LOBLeu)3(H2mbpz)3 (H2O)2]·7H2O (4) (L-OBAla = N,N′oxalylbis(L-alanine); L-OBVal = N,N′-oxalylbis(L-valine); LOBPhe = N,N′-oxalylbis(L-phenylalanine); L-OBLeu = N,N′oxalylbis(L-leucine)). D-Configuration of compound 1 (D-1) was also synthesized for chiroptical study. All compounds are characterized by single crystal X-ray diffraction. Thermalgravimetric analysis, solid-state dichroism spectra and gas adsorption behaviors are also investigated.

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EXPERIMENTAL SECTION

General Methods. The ligands OBAla methyl ester, OBVal methyl ester, OBPhe methyl ester, OBLeu methyl ester, and H2mbpz were synthesized according to the literature methods.12 All other chemicals were purchased commercially and used without further purification. The elemental analysis was performed on an EA1110 CHNS-0 CE elemental analyzer. The IR spectra were recorded on a PECO (U.S.A.) SpectrumOne spectrophotometer with KBr pellets. Thermal stability studies were carried out on a NETSCHZ STA-449C thermoanalyzer with a heating rate of 10 °C/min in N2. The phase purity and crystallinity of each product were checked by powder X-ray diffraction (PXRD) using a Miniflex2 diffractometer with Cu−Kα radiation (λ = 1.54056 Å). A step size of 0.05° and counting time of 3305



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Found: C 42.29, H 5.88, N 12.36%. FTIR (KBr pellet, cm−1): 3315(w), 2955(m), 1655(s), 1381(m), 1283(m), 1169(w), 1037(w), 855(w), 756(w), 594(w). CCDC number: CCDC 867903. [Cu2(D-OBAla)(H2mbpz)]·2H2O (D-1). Compound D-1 was synthesized with the unnatural D-OBAla methyl ester under solvothermal conditions similar to that for preparing 1 (82% yield, based on Cu(II). It exhibits the same physical, chemical, and spectroscopic behavior as compound 1, except the optical rotation behavior that will be discussed later. CCDC number: CCDC 867904. Crystal Structure Determination. Single-crystal X-ray diffraction data of 1−4 and D-1 were collected on a Rigaku Mercury CCD diffractometer equipped with graphite-monochromated with Mo−Kα radiation (λ = 0.71073 Å) at room temperature. Structures were solved by direct methods and refined by full-matrix least-squares methods with SHELXL-97 program package.13 All non-hydrogen atoms were located in successive difference Fourier syntheses and were refined anisotropically. The hydrogen atoms were generated theoretically onto the specific atoms and refined isotropically with fixed thermal factors. Owing to a serious disorder problem at data collection, the solvent molecules and bulky alkyl chains in some structures could not be crystallographically defined successfully. Crystal data and refinements are summarized in Table 1. Selected bond lengths and angles are given in Tables S1−S4 in Supporting Information.

The asymmetric unit of compound 1 contains two cooper atoms, two halves of the H2mbpz ligand, and one retro-peptide OBAla ligand, as shown in Figure 1a. The copper atoms are five-coordinated and have a square pyramidal environment with three atoms from an OBAla ligand (including an oxygen atom and a nitrogen atom from the oxamide group, and an oxygen atom from the terminal carboxylate group), and a nitrogen from a H2mbpz ligand in the basal plane, and with oxygen atoms in the axial position from another OBAla ligand. The Cu atoms are displaced about 0.23 Å from the basal planes. Owing to the Jahn−Teller effect of the Cu(II) ion, the axial Cu−O distances (2.2276(5) Å for Cu(1)−O(2) and 2.2688(6) Å for Cu(2)−O(6)) are significantly longer than the equatorial Cu− O bonds (1.9668(5)−2.0275(4) Å). Furthermore, the two axial oxygen atoms for Cu(1) and Cu(2) are oppositely directed to each other, that is to say, the O(2) atom is displaced above the Cu2(OBAla) subunit plane, while O(6) is located below the same plane. The Cu−Cu separation within each binuclear unit is 5.2402(13) Å. The pyrazole rings in H2mbpz are twisted, in which the torsion angles are 68.00 and 65.88°, respectively. The OBAla ligand in 1 coordinates to the Cu(II) ions in the structure as a tetranion owing to the deprotonation in both the oxamide groups and the terminal carboxylic groups; therefore, the whole is charge neutral. Each OBAla coordinates to four copper atoms through two fashions: first, a bis-tridentate fashion to chelate two copper atoms, resulting in a planar binuclear Cu2(OBAla) unit that contains four five-member rings; second, a bridging bidentate fashion to link two copper atoms from two adjacent Cu2(OBAla) units. This arrangement leads to the formation of a corrugated 2-D network containing Cu2(OBAla) binuclear subunits, which is similar to that observed in the structure of (Aib-CO−CO-Aib)Cu.9b The 2D Cu2(OBAla) network can also be described as the connection of each Cu2(OBAla) subunit with four other Cu2(OBAla) subunits through the Cu−O bonding between Cu(II) centers and the terminal carboxyl groups of the retropeptide ligands. The 2-D layer paralleled to the ac plane is shown in Figure 1b. However, unlike in compound (Aib-CO− CO-Aib)Cu, in which the Cu(II) ions adopt uncommon square planar coordination geometry, the Cu(II) ions in the 2-D network of {Cu2(OBAla)}n are further furnished by nitrogen atoms from H2mbpz ligands to form their square pyramidal coordination geometries. Each H2mbpz connects two Cu atoms from the adjacent 2-D layers and extend the 2-D layer substructures into a pillared-layer porous 3-D framework with 1-D channels running along the c axis (Figure 1c,d). The distance between the two neighboring layers is 13.49 Å. The 1D channel is nearly linear, nonintersecting with a cross section of approximately 4.4 × 7.8 Å2, and are occupied by two water molecules, which is consistent with the results of elemental analysis and thermalgravimetric (TG) analysis. The chiral carbon atom of the retro-peptide imparts the chirallity to the internal surface of the channels. Methyl group of the retropeptide is projecting into the channel, which affects dramatically the void volume of the channel. The potential solvent accessible volume calculated by the PLATON program is 883.6 Å3 per unit cell, which equals 31.3% of the total cell volume. To get a better insight into the framework, we simplify the Cu2(OBAla) subunit as a six-connected node. Each binuclear copper subunit connects to six same subunits, thus generating a 3-D network pcu topology (Figure 2d). The structures of compounds 2 and 3 are essentially identical to compound 1 if the alkyl substituents at the α-position of the

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RESULTS AND DISCUSSION Synthesis and Structures. Four oxalyl retro-peptide ligands, namely, N,N′-oxalylbis(L-AlaOMe) (L-OBAla methyl ester), N,N′-oxalylbis(L-ValOMe) (L-OBVal methyl ester), N,N′-oxalylbis(L-PheOMe) (L-OBPhe methyl ester) and N,N′oxalylbis(L-LeuOMe) (OBLeu methyl ester) (see Scheme 1) Scheme 1. Structures of the Oxalyl Retro-Peptide Ligands

were prepared and used in our study. These ligands have the same gross structures except with different substituents at the α-positions. The solvothermal reactions of these ligands with Cu(NO3)2·3H2O and H2mbpz under similar conditions at 100 °C for 3 days afforded the chiral 3-D MOF products in high yield and high purity. It is noteworthy to point out that the reaction temperature is critical. Reaction temperature higher than 120 °C would cause partial decomposition of the oxalyl retro-peptides to oxalic acid and result in the formation of copper oxalate as a byproduct. Green block crystals of compounds 1, 3, and 4 can be obtained from water/ethanol solvent, while complex 2 can only be afforded from water/ acetonitrile solvent. In addition, compound 3 can be synthesized from an alternative method by refluxing the reactants in water/methanol solvent at 80 °C at ambient pressure. All the compounds are insoluble in common solvents. Single-crystal X-ray structure analysis revealed that compounds 1, 2, and 3 are isostuctural, adopt a three-dimensional non-interpenetrating framework structure, and crystallize in the chiral space group P21212. Herein, we present only the structure of compound 1 in detail as a representative. 3306



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Figure 1. (a) View of asymmetric unit for 1 (symmetry codes: A = 2 − x, −y, z; B = 0.5 + x, 0.5 − y, 1 − z, C = 2 − x, 1 − y, z, hydrogen atoms have been omitted for clarity). (b) 2-D layer of Cu with OBAla in the ac plane. (c) The square grid topological network of 2-D layer. The green atoms correspond to the Cu2(OBAla) subunit. (d) View of 3-D open framework of 1 overlaid by the topological edges (lime and green) (solvent molecules and hydrogen atoms have been omitted for clarity).

compounds 1−3, each OBLeu ligand is fully deprotonated and chelated to two Cu (II) centers in the bis-tridentate coordination fashion, forming binuclear Cu2(OBLeu) subunits. In contrast to the cases in compounds 1−3, in which the assembly of the chiral retro-peptide ligands with Cu(II) atoms generates a chiral 2-D network with Cu2(retro-peptide) subunits, the Cu2(OBLeu) subunits of 4 construct an unexpected U-shaped [Cu2(OBLeu)]3 secondary building unit (SBU). In each SBU, there are two types of the binuclear subunits, whose Cu (II) centers display different coordination geometries. The first type of binuclear Cu2(OBLeu) subunits are located on the wall of U-shaped SBUs and nearly parallel to each other, with a dihedral angle of 3.8°. In this type of binuclear subunits, one Cu(II) atom (Cu(1) or Cu(2)) resides in a square pyramidal environment with one carbonyl oxygen, one terminal carboxyl oxygen, one amide nitrogen from OBLeu ligand, one pyrazolyl nitrogen from H2mbpz ligand, in the basal plane, and one water molecule on the apical position. Meanwhile, the other Cu(II) atom (Cu(3) or Cu(4)) has a square coordination geometry similar to the five-coordinated Cu(II) atoms described above but without coordinated water molecule at the apical position. The second type of Cu2(OBLeu) subunits are located on the bottom of U-shaped SBU. Both Cu(II) atoms (Cu(5) and Cu(6)) adopt distorted square pyramidal coordination geometries with the apical positions occupied by carboxyl oxygens (Cu−O, 2.3688− 2.3795 Å) from other OBLeu ligands. As a result, these two

carboxyl groups are not considered (Figures S7 and S8 in the Supporting Information). However, the solvent accessible volumes of the compounds are dramatically different in 2 and 3 because the bulky alkyl substituents project into the open space of the 1-D channels. In compound 2, the narrow cross section of the 1-D channel is ca. 2.9 × 3.0 Å. The calculated solvent accessible volume is 738 Å3 per unit cell, which equals 24.3% of the total cell volume. In compound 3, the entrance of the 1-D channel is almost totally blocked by the large steric hindrance phenmethyl group. As a result, compound 3 has a rather small calculated solvent accessible volume of 125 Å3, which equals 4.1% of its crystal volume. It is noticeable that, in the case of compound 4, despite the presence of identical potential coordination sites on its OBLeu ligand to the retro-peptide ligands in compounds 1−3, this compound adopts a distinct self-assembly mode and has a chiral 3-D framework dramatically different from the above three compounds. The significant difference of 4 is possibly caused from the bulky isobutyl chain that not only prevents the selfassembly of the retro-peptide ligand and the Cu(II) ions from generating a 2-D {Cu2(OBAla)}n network, but also fails to fit in the similar 1-D channels in compounds 1−3, thus directing the self-assembly of this compound in a totally new manner. Compound 4 crystallizes in the chiral triclinic space group P1. As shown in Figure 3, the asymmetric unit consists of six Cu (II) ions, three OBLeu ligands, six halves of H2mbpz ligand, and two coordinated water molecules. Like the retro-peptide in 3307



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Figure 2. Ball-and-stick and space-filling views of open frameworks of compounds 1 (a), 2 (b), and 3 (c), along the c axis (hydrogen atoms have been omitted for clarity).

These U-shaped SBUs acting as a 6-connecting node are linked by the H2mbpz ligands to form a 3-D pcu-type framework with 1-D channels of ca. 4.6 × 2.9 Å along the a axis. Interestingly, all the isobutyl groups are oriented into the center of the 1-D channel (Figure 4). However, PLATON calculation showed that there is still 21.0% of the unit cell volume accessible for solvent molecules. Solid-State CD Spectra. To examine the chiroptical activities of the enantiopure MOFs and organic ligands they contain, compounds 1, D-1, L- and D-OBAla, and L- and Dalanine are selected for comparative experiment (see Figure 5a). The CD spectra of 1 and D-1 are mirror images of each other, thus demonstrating that 1 and D-1 are enantiomers. The spectra of L- and D-alanine and L- and D-OBAla exhibit single Cotton effects at 212 nm and 240 nm, respectively, whereas the spectra of 1 and D-1 have Cotton effect signals at 246 and 301 nm. It illustrates that the oxalyl retro-peptides are able to retain their chirality under hydrothermal reaction conditions, and

Figure 3. View of asymmetric unit for 4 (hydrogen atoms have been omitted for clarity).

types of binuclear Cu subunits are linked together to complete a U-shaped SBU.

Figure 4. (a) View of the 3-D open framework of 4 (solvent molecules and hydrogen atoms have been omitted for clarity); (b) topological representation of 4. 3308



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Figure 6. TGA curves for compounds 1−4.

curve of compound 3 remains nearly unchanged up to 265 °C, exhibiting higher thermal stability than 2. TG curves of 4 show a stepwise loss of weight upon heating. In the temperature range from 30 to 110 °C, the weight loss of 6.13% could be attributed to the loss of crystal water (calcd 6.15%). Further weight loss of 1.82% between 110 and 163 °C is in accordance with the loss of two coordinated water molecules (calcd 1.76%). Rapid weight loss occurred at 222 °C owing to the decomposition of organic ligands. With the lowest decomposition temperature, compound 4 has been proven to be the most thermally unstable compound among this family of chiral MOFs. PXRD confirms the phase purity of the compounds 1−4. To further investigate the thermal stability of these compounds, 1 and 2 were selected to examine the variable-temperature PXRD (VTPXRD) after calcination at elevated temperatures ranging from 30 to 280 °C. For compound 1, the reflection intensities decrease gradually as the temperature goes higher. However, even at 260 °C, the experimental PXRD pattern of 1 is still coincident with the simulated pattern, indicating that the desolvated framework of 1 remains unchanged up to 260 °C. With the temperature increasing to 270 °C, most of the diffraction peaks disappear, showing the decomposition of framework. For compound 2, the measured patterns closely match the calculated one until the temperature climbs to 240 °C, which is accordance with the TG result of 2. The TGA and VTPXRD results demonstrate that 1 and 2 are stable after desolvation, which is crucial for further gas sorption experiment. Gas Adsorption. The permanent porosity of compounds 1, 2, and 4 was further assessed by an sorption experiment with different probe gases, namely, N2 (77 K), H2 (77 K), CO2 (273 K) (see Figure 7). Compounds 1 and 2 were activated by heating at 120 °C under a vacuum for 12 h, while compound 4 was pretreated by soaking in CH2Cl2 solvent for 24 h and then heating at 120 °C under a vacuum for another 12 h. The N2 sorption of compound 1 reveals a type-I behavior, indicating the permanent porosity in the crystalline microporous materials. At 77 K, the uptake amount of N2 increases abruptly at low equilibrium pressures and then it reaches a maximum amount of 111 cm3 g−1 at 700 mmHg. The apparent Langmuir and Brunauer−Emmett−Teller (BET) surface areas of 1 are 523 m2 g−1 and 330 m2 g−1, respectively. The pore width estimated from N2 adsorption of 1 is ca. 8 Å, which is

Figure 5. (a) The solid-state CD spectra for compounds 1, D-1, L- and D-alanine, L- and D-OBAla. (b) The solid-state CD spectra for compounds 1−4.

transfer their chirality to the whole metal−organic skeleton. It is noteworthy that compounds 1 and D-1 show optical rotation that is opposite to L- and D-OBAla, but identical to L- and Dalanine. To further confirm the enantiomeric nature of the other MOFs in this series, the solid-state CD spectra of compounds 2−4 were also investigated. As shown in Figure 5b, the CD curves of the four compounds display similar dichroic signals: two positive peaks near 250 nm and 300 nm, and one negative peak near 379 nm. The proximity in dichroic signals of 1−4 might have been attributed to the similarity in structures of the four retro-peptide ligands. This result demonstrates conclusively that the chirality of MOFs is derived from the oxalyl retro-peptide that they are produced by. TG Analysis and PXRD. To examine the thermal stability of the porous framework, TG analysis and PXRD measurements were carried out (see Figure 6). For compound 1, the TG curve reveals a weight loss of 6.24% between 30 and 100 °C, which matches well the calculated value of 6.23% for removing two water guests per formula unit. Above 110 °C, the resultant guest-free framework shows almost no weight loss until 270 °C. Compound 2 has no obvious weight loss before 248 °C, which is consistent with the absence of solvent molecule in the crystal structure. Then the framework collapses with an abrupt weight loss as a result of ligand decomposition. A similar decomposition process is observed for 3. The TG 3309



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observed in the CO2 adsorption isotherm of 2 at 273 K, 760 mmHg. Compounds 3 and 4 fail to adsorb H2 owing to the absence of accessible aperture (Figure S12 in the Supporting Information). In short, according to the results of gas adsorption experiments, porosity trend of compounds 1−4 is consistent with the increasing steric bulk of methyl, isopropyl, and phenmethyl of the oxalyl retro-peptide ligands.

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CONCLUSIONS In summary, we have designed and synthesized four 3-D homochiral microporous MOFs, using enantiopure oxalyl retropeptide ligands and conformationally flexible achiral H2mbpz ligand. In compounds 1−4, each chiral oxalyl retro-peptide is chelated to two Cu(II) atoms to form a planar rigid chiral moiety which is further linked by H2mbpz ligands into a 3-D homochiral framework. Compounds 1−3 are isorecticular pillared-layer networks of pcu topology, while compound 4 possesses the same topology based on unique U-shaped SBUs. Compound 1 exhibits moderate capability of H2 and N2 adsorption but is the highest among the four compounds. Compound 2 reveals selectively adsorption of H2 over N2 owing to the narrow window size of apertures. The successful synthesis of 1−4 shows that oxalyl retropeptides are a class of effective chiral ligands for constructing homochiral MOFs due to their inherent chirality and multiple binding sites. Furthermore, by using different amino acids, versatile retro-peptide ligands can be synthesized to construct open frameworks with controllable pore volume and window size. This work opens a new route to rationally and systematically design porous homochiral MOFs based on biomolecules. Moreover, homochiral MOFs with permanent pores are promising materials for chiral recognition and catalysis. The enantioselectivity and catalytical properties of compound 1 are still in progress.

Figure 7. Gas adsorption (filled) and desorption (open) isotherms of 1 (a) and 2 (b) (black, N2 at 77 K; red, H2 at 77 K; blue, CO2 at 273 K).

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ASSOCIATED CONTENT

* Supporting Information S

X-ray crystallographic files in CIF format, tables of selected bond distances and angles, IR spectra, experimental and simulated X-ray powder diffraction patterns, additional data for gas sorption experiments, and additional crystallographic figures are presented. This material are available free of charge via the Internet at http://pubs.acs.org.

consistent with the crystal structure (Figure S9 in the Supporting Information). When the gas adsorbate is replaced by H2, 1 also shows a profile of type-I behavior and absorbs 116 cm3 g−1 of H2 at 760 mmHg, 77 K, which is comparable with some amino acid-extended MOFs,14 but less than that of highly porous MOFs.15 The adsorption and desorption processes are reversible. To estimate the hydrogen affinity of 1, H 2 adsorption isotherms at 87 K were also collected to calculate the heat of adsorption using the Virial method. The enthalpy of H2 adsorption of 1 is estimated to be 8.3 kJ/mol (Figures S10 and S11 in the Supporting Information). The relatively high adsorption enthalpy should be mainly related to the microporous nature of 1, and possibly to the alkyl groups dangling on the pores. In addition, compound 1 absorbs a moderate amount of CO2 (46 cm3 g−1) at 273 K, 760 mmHg. No apparent plateau emerging at higher partial pressures indicates slow uptake kinetic. Compound 2 scarcely adsorbs N2, suggesting that there is no sufficient space for N2 to diffuse into the open channels. The hydrogen adsorption isotherm of 2 exhibits a similar behavior as 1, and gives a moderate uptake of 70 cm3 g−1 at 77 K, 760 mmHg. Such adsorption selectivity of H2 over N2 could be attributed to the restricted size of the channels that distinguishing H2 from N2 by their kinetic diameters (H2: 2.89 Å, N2: 3.64 Å).16 As a result, compound 2 may have potential utility in H2/ N2 separation. Besides, a small amount of CO2 (15 cm3 g−1) is

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AUTHOR INFORMATION

Corresponding Author

*Fax: (+86) 591-83714946; tel: (+86) 591-83705794; e-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Chinese Academy of Sciences (KJCX2-YW-319, KJCX2-EW-H01), the 973 key program of the MOST (2010CB933501, 2012CB821705), and the National Natural Science Foundation of China and the Natural Science Foundation of Fujian Province (2007HZ0001-1, 2009HZ0004-1, 2009HZ0005-1, 2009HZ0006-1, 2006L2005).

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