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Polymers 2011, 3(4), 1866-1874; doi:10.3390/polym3041866

Article
One-Dimensional Helical Homochiral Metal-Organic Framework Built from 2,2′-Dihydroxy-1,1′-binaphthyl-3,3′-dicarboxylic Acid
Koichi Tanaka 1,*, Yuki Kikumoto 1 and Motoo Shiro 2
1
Department of Chemistry and Materials Engineering, Faculty of Chemistry, Materials and Bioengineering, Kansai University, Suita, Osaka 564-8680, Japan
2
Rigaku X-ray Laboratory, 3-9-12 Matsubara-cho, Akishima, Tokyo 196-8666, Japan; E-Mail: shiro@rigaku.co.jp
*
Author to whom correspondence should be addressed; E-Mail: ktanaka@kansai-u.ac.jp; Tel.: +81-06-6368-0861; Fax: +81-06-6368-0861.
Received: 17 August 2011; in revised form: 19 September 2011 / Accepted: 31 October 2011 /
Published: 1 November 2011

Abstract

: A homochiral metal-organic framework (MOF) based on enantiopure (R)-2,2′-dihydroxy-1,1′-binaphthyl-3,3′-dicarboxylic acid was synthesized. X-ray crystal diffraction studies revealed that the MOF adopts a one-dimensional infinite right-handed helical tubular structure along the a-axis, which serves as a host for the inclusion of guest dimethylformamide (DMF) molecules.
Keywords:
homochiral metal-organic framework; chiral ligand; helix structure; one-dimensional network

1. Introduction

The field of metal-organic frameworks (MOFs) has grown explosively in recent years [1-8]; numerous studies have been reported owing to the potential applications of MOFs in gas storage [9-15], separation [16-23], luminescent materials [24-32], and heterogeneous catalysis [33-41]. While several MOFs have been discovered so far, only a few examples of chiral MOFs for enantiomer separations or heterogeneous asymmetric catalysis have been investigated [42]. We recently reported the synthesis of a novel two-dimensional homochiral MOF, (R)-MOF-1, from (R)-2,2′-dihydroxy-1,1′-binaphthyl-5,5′-dicarboxylic acid (1) (Scheme 1) and its application as an effective catalyst for the asymmetric ring-opening reaction of epoxide with amine [43] and the alcoholytic kinetic resolution of styrene oxide under heterogeneous conditions [44]. The helical structures of MOFs have also attracted considerable attention because of not only their intriguing structures, but also their potential applications in chiral recognition, nonlinear optical materials, and asymmetric catalysis. Over the past two decades, several MOFs containing single-, double-, and multi-stranded helices have been constructed and recently reviewed [45]. For example, one-dimensional helical metal-organic framework built from a chiral octahydrobinaphthalene-derived dicarboxylic acid showed the intense broad photoluminescence emission in the solid state [46]. Tridentate chiral Schiff base ligands has been found to form 1D helical framework which allow highly enantioselective separation of racemic secondary alcohols by inclusion crystallization [47]. Chiral binaphthylbisbipyridine-based copper (I) coordination polymer gels for use as catalysts in 1,3-dipolar Huisgen cycloaddition reactions are also reported [48]. Herein, we report the synthesis and X-ray crystal structure of the one-dimensional helical homochiral MOF, (R)-MOF-2, constructed from (R)-2,2′-dihydroxy-1,1′-binaphthyl-3,3′-dicarboxylic acid (2) (Scheme 2).

2. Experimental Section

General

1H-NMR spectra were recorded on a JEOL JNM-GSX 400 spectrometer with tetramethylsilane (TMS) as the internal standard. IR spectra were recorded with a JASCO FT-IR 4100 spectrometer. Thermogravimetric (TG) analyses were performed on a Rigaku TG8120 instrument. Solid-state circular dichroism (CD) spectra were recorded as KBr pellets on a JASCO J-820 CD system.

Synthesis of enantiopure 2,2′-dihydroxy-1,1′-binaphthyl-3,3′-dicarboxylic acid (2)

(R)- and (S)-2,2′-dihydroxy-1,1′-binaphthyl-3,3′-dicarboxylic acid (2) were synthesized according to the procedure previously reported by D. J. Cram et al. [49].

Synthesis of [Mn2((R)-1)2(DMF)4(H2O)4]·2DMF

A mixture of (R)-2,2′-dihydroxy-1,1′-binaphthyl-3,3′-dicarboxylic acid (2) (78 mg, 0.2 mmol) and MnCl2·4H2O (40 mg, 0.2 mmol) was dissolved in DMF (1 mL) and H2O (2 mL), and then pyridine (1 mL) was added to the solution. The solution was stirred for 30 min at room temperature and then left for 3 days. Pale yellow prisms were obtained, filtered, and dried at room temperature to give (R)-MOF-2 (132 mg). IR (KBr pellet, cm−1): 3,402, 2,931, 1,655, 1,559, 1,505, 1,457, 1,392, 1,336, 1,309, 1,242, 1,101, 932, 874, 810, 755, 702.

X-ray analysis

X-ray single-crystal diffraction data for (R)-MOF-2 were collected on a Rigaku RAXIS RAPID imaging plate diffractometer using Cu Kα radiation. Crystal data: Formula C62H74Mn2N6O22, Formula weight 1365.17, Space group P21(# = 10.9585(3), b = 25.2165(8), c = 11.8505(9) Å, β = 96.629(7)°, V = 3252.8(3) Å3, Z = 2, ρ = 1.394 g/cm3, 2θmax = 136.4°, R1 = 0.0514 (for 8372 reflections with I > 2σ(I)), wR2 = 0.1315 (for 11,652 reflections), GOF = 0.985, Flack parameter = 0.009(4) (calculated using 5,571 Friedel pairs). The structure was solved by SHELXS97 and refined by SHELXL97. The absolute structure was deduced from the Flack parameter.

CCDC

838075. See http://www.rsc.org/suppdata/cc/….….…./ for crystallographic data in cif or other electric formats.

3. Results and Discussion

3.1. Synthesis of Chiral MOF

Chiral ligand (R)-2,2′-dihydroxy-1,1′-binaphthyl-3,3′-dicarboxylic acid (2) was prepared in good yield by the diastereomeric complexation of rac-2 with L-(+)-leucine methyl ester (Scheme 3). The CD spectra of (R)-(+)- and (S)-(−)-2 in CHCl3 are shown in Figure 1.

New homochiral (R)-MOF-2 [Mn2((R)-1)2(DMF)4(H2O)4]·2DMF was synthesized by the reaction of (R)-(+)-2 and MnCl2·4H2O in the presence of pyridine in DMF at room temperature. The product was characterized by IR spectroscopy, CD spectroscopy, thermogravimetric analysis (TGA), and X-ray analysis. The IR spectra of (R)-MOF-2 exhibited peaks of νOH and νCO2 at 3,401 and 1,559 cm−1, respectively. TGA showed that (R)-MOF-2 loses 34.3% of its total weight in the range of 26–300 °C, which is ascribed to the loss of six DMF and four water molecules per formula unit (calculated at 37.4% of the total weight) (Figure 2).

3.2. Crystal Structure of (R)-MOF-2

X-ray diffraction measurement revealed that (R)-MOF-2 crystallizes in a chiral space group of P21. An asymmetric unit of (R)-MOF-2 contains two Mn2+ ions, two (R)-22−groups, four DMF molecules, four water molecules, and two DMF guest molecules, as shown in Figure 3. The Mn2+ ion is coordinated by two (R)-22-groups, two DMF molecules, and two water molecules. The sixth coordination site of Mn1, although vacant in Figure 3, is occupied by O11 of the (R)-2 group lying in the next unit cell in the direction of a-axis. A helical chain composed of –Mn–(R)-2–Mn–(R)-2– is thus formed in the right-handed form and extends along the a-axis as shown in Figure 4. The guest molecules are bound to the water molecules by the hydrogen bonds of O21–H…O18 and O22–H…O19.

We also prepared (S)-MOF-2 using (S)-2 as the chiral ligand. As shown in Figure 5, the solid-state CD spectra of (R)- and (S)-MOF-2 synthesized from (R)- and (S)-2, respectively, are mirror images of each other, thus indicating that the helices built from (R)- and (S)-2 are enantiomeric.

4. Conclusions

We have synthesized a one-dimensional helical homochiral MOF (MOF-2) using MnCl2 and C2 symmetric chiral ligands (R)- and (S)-2 as the building blocks. We are currently studying its potential applications in heterogeneous asymmetric catalysis and enantioselective separations.

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Figure 1. CD spectra of (R)- and (S)-2 in CHCl3.

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Figure 1. CD spectra of (R)- and (S)-2 in CHCl3.
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Figure 2. TG trace of (R)-MOF-2.

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Figure 2. TG trace of (R)-MOF-2.
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Figure 3. Structure of (R)-MOF-2 in an asymmetric unit and atomic numbering system. Hydrogen atoms, excluding those of water, are omitted for clarity.

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Figure 3. Structure of (R)-MOF-2 in an asymmetric unit and atomic numbering system. Hydrogen atoms, excluding those of water, are omitted for clarity.
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Figure 4. Right-handed helical structure in the crystal of (R)-MOF-2. The a-axis of the crystal is oriented vertically. All hydrogen atoms and guest molecules are omitted for clarity.

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Figure 4. Right-handed helical structure in the crystal of (R)-MOF-2. The a-axis of the crystal is oriented vertically. All hydrogen atoms and guest molecules are omitted for clarity.
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Figure 5. Solid-state CD spectra of (R)- and (S)-MOF-2 in KBr pellet.

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Figure 5. Solid-state CD spectra of (R)- and (S)-MOF-2 in KBr pellet.
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Scheme 1. Synthesis of (R)-MOF-1.

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Scheme 1. Synthesis of (R)-MOF-1.
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Scheme 2. Synthesis of (R)-MOF-2.

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Scheme 2. Synthesis of (R)-MOF-2.
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Scheme 3. Optical resolution of rac-2.

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Scheme 3. Optical resolution of rac-2.
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We thank JASCO Corporation Tokyo Japan for help with solid-state CD spectral data collection.

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