Article One-Dimensional Helical Homochiral Metal-Organic Framework Built from 2,2′-Dihydroxy-1,1′-binaphthyl-3,3′dicarboxylic Acid

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

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 C 62 H 74 Mn 2 N 6 O 22 , Formula weight 1365.17

Crystal Structure of (R)-MOF-2
X-ray diffraction measurement revealed that (R)-MOF-2 crystallizes in a chiral space group of P2 1 .An asymmetric unit of (R)-MOF-2 contains two Mn 2+ ions, two (R)-2 2− groups, four DMF molecules, four water molecules, and two DMF guest molecules, as shown in Figure 3.The Mn 2+ ion is coordinated by two (R)-2 2− 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.

Conclusions
We have synthesized a one-dimensional helical homochiral MOF (MOF-2) using MnCl 2 and C 2 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.

Figure 3 .
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.

Figure 4 .
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.