Synthesis of homochiral helical metal–organic frameworks based on lactate derivatives

Abstarct A pair of homochiral metal–organic frameworks (HMOFs) based on semi-rigid 5-(1-carboxyethoxy)isophthalic acid (H3CIA), [Zn5((R)-CIA)3(OH)2(H2O)4]·NH2(CH3)2·4.5H2O (1-D) and [Zn5((S)-CIA)3(OH)2(H2O)4]·NH2(CH3)2·4.5H2O (1-L), were synthesized and structurally characterized. They are enantiomers and exhibit 3-D framework structures. In each structure, the CIA ligands link the Zn centers into a homochiral open framework with tetranuclear and trinuclear Zn-clusters. An interesting double helix built from the connectivity between CIA ligands and Zn centers is presented. The chiral nature of 1-D and 1-L was further confirmed by CD spectra. Photoluminescence of 1-D was also investigated in the solid state at room temperature.

been attracting attention in coordination chemistry and material chemistry [15][16][17][18]. Although great progress has been made in recent years, the rational synthesis of the HHMOFs is still a huge challenge. research has shown that self-assembly of metal ions with enantiopure organic ligands should be the most effective synthesis to construct HHMOFs. Therefore, rational design and selection of enantiopure ligands is a key factor to get HHMOFs [19,23].
In order to construct HHMOFs, we have synthesized a pair of chiral ligands ((S)-H 3 CIA and (R)-H 3 CIA) with up to three advantages from natural lactic acid and dimethyl 5-hydroxyisophthalate (scheme 1) [24]. The first advantage comes from two rigid carboxylate groups that can efficiently construct interesting structures. The second is the enantiopure lactic acid unit which provides the chiral source for construction of HMOFs. The third is that, multiple coordination modes of three carboxyl moieties have superiority in the synthesis of fascinating coordination polymers. For the above advantages, the (S)-H 3 CIA and (R)-H 3 CIA should be the ideal enantiopure linkers and have provided a new approach to design and construct HHMOFs.

General information
All reagents and solvents used in the reactions were purchased commercially and used without purification. The enantiopure linkers (S)-H 3 CIA and (R)-H 3 CIA were synthesized from l-(-)-lactic acid methyl ester, D-(-)-lactic acid methyl ester and dimethyl 5-hydroxyisophthalate. elemental analyses and photoluminescent properties were carried out by the analysis center of our institute. FT-Ir spectra were measured as KBr pellets on a Nicolet Magna 750 FT-Ir spectrometer from 400-4000 cm −1 . Powder X-ray diffraction (PXrD) analyses were recorded on a rigaku Dmax2500 diffractometer with Cu K α radiation (λ = 1.54056 Å). Thermal stability studies were performed on a NeTsCHZ sTA-449C thermo analyzer with a heating rate of 10 °C min −1 under an N 2 atmosphere.

Measurements of solid CD spectra
The mixture of sample and 50 mg of dry KCl powder was well grounded and then pressed into a disk for CD measurement with a MOs-450 spectropolarimeter.

X-ray determination
A summary of crystal data and refinement details is provided in Table 1.
In the tetramer core, Zn (3) shows distorted octahedral ZnO 6 coordination from two carboxylate O of two (S)-CIA 3− ligands, two O from two μ 3 -OH groups, and two O from two coordinated waters. The Zn(4) has a tetrahedral ZnO 4 coordination from three carboxylates and one oxygen of μ 3 -OH (Figure 1(b)). In the trimeric unit, Zn(2) and Zn(5) also exhibit distorted octahedral geometries: Zn(2) is coordinated by five oxygens from four carboxylates and one of μ 3 -OH; Zn(5) is coordinated by three oxygens from three bridging carboxylates, two waters, and one of μ 3 -OH. unlike Zn(2) and Zn(5), Zn(4) is coordinated to three carboxylate oxygens and a μ 3 -OH group, which shows a tetrahedral geometry (Figure 1(c)).
The second outstanding structural feature of 1-D (or 1-L) is the presence of a double helix ( Figure  2(a) and (b)). As depicted in Figure 2(a), the Zn2 ion of the tri-nuclear Zn-cluster is coordinated by the isophthalate unit from (R)-CIA 3− ligand to form an infinite left-handed helical chain running along the b-axis. In the same way, an infinite left-handed helical chain is also constructed by the Zn2 ion and CIA fragment with lactic acid unit and a half isophthalate unit in 1-D (Figure 2(a)). The opposite phenomenon (double helix with right-handed helical chains) exists in 1-L (Figure 2(b)). A 2D homochiral helical layer is formed by the connection of tetranuclear Zn-clusters and trinuclear Zn-clusters through CIA 3− ligands (Figure 2(c)). From the viewpoint of structural topology, the two kinds of Zn-clusters and the CIA 3− ligands can be regarded as 6-and 3-connected nodes, respectively. Thus, the 2D framework of 1-D (or 1-L) can be described as a (3,6)-connected net with point (schläfli) symbol of (4 3 ·6 12 )(4 3 ) 2 ( Figure  2(d)). In 1-D, the distinct C-H⋯O hydrogen between adjacent layers can be found (Figure 3(b)). Taking all these extensive interlayer interactions into account, the overall crystal packing can be regarded as a 3D supramolecular framework containing a 1D channel along the crystallographic b-axis (Figure 3(a)).
At room temperature, the solid-state circular dichroism (CD) spectra of 1-D and 1-L were tested to further demonstrate their homochirality (Figure 4). The CD curve of 1-D shows an obvious positive cotton effect (Ce) peak at 360 nm, revealing its homochiral nature. Furthermore, a mirror image is also observed for 1-D, confirming that 1-D and 1-L are enantiomers.
To characterize thermal behaviors of the complexes, thermal gravimetric analyses (TGA) of 1-D and 1-L were carried out from 30 ℃ to 800 ℃ under nitrogen (Figure s4). The TG curves of 1-D and 1-L show the first weight loss of 10.7% from 30 to 150 °C, corresponding to the loss of guest molecules. A gradual weight loss from 150 °C is attributed to release of coordinated water; the anhydrous component began to decompose at 350 °C. luminescent compounds have attracted attention for their applications in chemical sensors, photochemistry, and electroluminescent display [25]. The photoluminescence properties of 1-D were studied in the solid state at room temperature. excitation of microcrystalline samples of 1-D at 327 nm produces intense luminescence with the peak maximum at 378 nm. To understand the nature of the emission band, the photoluminescence of (S)-H 3 CIA is also shown in Figure 5, upon excitation at 336 nm, which has similar emissions at 404 nm. In comparison to (R)-H 3 CIA, the emission maxima of 1-D have a blue shift, which may be attributed to a charge transfer transition between ligand and metal center, and a change in the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of deprotonated (R)-CIA 3− to metal centers [26,27].

Conclusion
By the employment of two predesigned lactate derivative ligands ((R)-CIA and (S)-CIA) to assemble with Zn 2+ ions, a pair of HMOFs has been synthesized. Compound 1-D (or 1-L) exhibits a 3D supramolecular structure with double helix and two types of Zn-clusters. The chiral nature of 1-D and 1-L was confirmed by CD spectra. Moreover, the photoluminescence of 1-D was also tested and may be a candidate -as a photoactive material.

Supplementary material
CCDC 1420104 and 1420105 contain the supplementary crystallographic data for 1-D and 1-L. These data can be obtained free of charge from the Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/data_request/cif. Ir spectra, PXrD patterns, and TGA curves associated with this article can be found in the online version.

Disclosure statement
No potential conflict of interest was reported by the authors.