Spontaneous Resolution of Racemic Salen-Type Ligand in the

Oct 23, 2014 - Apparently, the 4-connected La(III) cation could be regarded as a node, ... Although dia topological networks are common for transition...
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Spontaneous Resolution of Racemic Salen-Type Ligand in the Construction of 3D Homochiral Lanthanide Frameworks Jing-Wen Sun, Jing Zhu, Hui-Feng Song, Guang-Ming Li,* Xu Yao, and Peng-Fei Yan* Key Laboratory of Functional Inorganic Material Chemistry (MOE), School of Chemistry and Materials Science, Heilongjiang University, No. 74, Xuefu Road, Nangang District, Harbin 150080, People’s Republic of China S Supporting Information *

ABSTRACT: A series of three 3D homochiral lanthanide− organic frameworks {[Ln(H2LSS)(NO3)2Cl]·2CH2Cl2}n [Ln = La (1), Ce (2), and Nd (3)] with the unique diamond (dia) topology have been obtained by utilizing racemic salen-type ligand and mixed lanthanide salts. Complexes 1−3 are isostructural, crystallizing in the chiral tetragonal space group P43212 and adopting an individual dia network formed by lanthanide ions and bridging ligand trans-N,N′-bis(salicylidene)-(1S,2S)-cyclohexanediamine (H 2 L SS ). This “from racemate to homochiral” approach may point to a new avenue for the preparation of homochiral materials.

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properties have been obtained.34−37 Among the reported salen-type ligands, N,N′-bis(salicylidene)-1,2-cyclohexanediamine (H2L) is now known as one of the “privileged chiral ligand” derivatives, which is widely used to construct homochiral metal complexes.38−42 It contains four isomers, namely, trans-N,N′-bis(salicylidene)-(1S,2S)- or (1R,2R)-cyclohexanediamine (H 2 L SS or H 2 L RR ) and cis-N,N′-bis(salicylidene)-(1S,2R)- or (1R,2S)-cyclohexanediamine (H2LSR or H2LRS), which can be a probable candidate to realize the selfassembly of HMOFs from racemate. However, the phenolic hydroxyl groups from H2L can serve as a linear spacer to link lanthanide ions, which is suitable to construct three-dimensional (3D) MOFs (see Supporting Information, Scheme S2). Our previous work have suggested that the reactions of H2L with Ln(NO3)3·6H2O or LnCl3·6H2O could generate different 1D spiral polymers, repectively, which trigger us to further study the effect of mixed lanthanide counterions on the construction of coordination polymer.43,44 Upon racemic N,N′-bis(salicylidene)-1,2-cyclohexanediamine (H2L) reaction with mixed Ln(NO3)3·6H2O and LnCl3·6H2O (3:1) in CH3OH/CH2Cl2 at room temperature, a series of three salen-type lanthanide complexes {[LnH2LSS(NO3)2Cl]·2CH2Cl2}n [Ln = La (1), Ce (2) and Nd (3)] have been isolated. X-ray diffraction analysis reveals that complexes 1−3 are isomorphic homochiral with dia topology crystallizing in chiral space group P43212 with the absolute structure parameters (Flack parameters) being −0.01(2), 0.02(2), and 0.03(2) (see Supporting Information). In a typical structure of 1, the unit cell of 1 has one La(III)

here is currently an enormous demand for practical synthetic methods to allow the preparation of chiral compounds as single enantiomers.1,2 The design and selfassembly of homochiral metal−organic frameworks (HMOFs) have attracted extensive attention due to their potential applications in enantioselective separation, nonlinear optics, catalysis, and sensor technology, as well as their intriguing architectures and topologies.3−9 Typically, there are three prime methodologies (Scheme 1) to assemble HMOFs: (i) the use of an enantiopure organic ligand, which translates chirality to the resultant framework;10−13 (ii) the use of a chiral inducer such as an enantiopure solvent, catalyst, or template;14−17 and (iii) by means of spontaneous resolution during crystallization without using any enantiopure substance, which results in chiral spatial arrangements of the metal and achiral ligands.18−25 Notably, most of them are faced with some drawbacks, e.g., (a) the first methodology that is considered as a more direct and predictable is costly and highly relies on the well-documented synthesis of enantiopure organic ligand; (b) the second methodology remains elusive due to the unpredictability of a suitable chiral induction agent for a given set of precursors; (c) for the third methodology, the resulting bulk sample tends to be a conglomerate, an equal mixture of crystals with opposite handedness. Thus, developing a more direct, convenient, and predictable method for homochiral crystallization is still a challenge. Generally, the reactions of racemic ligands with metal salts prefer to generate racemic products.26−29 However, it will be advantageous to employ racemate organic ligand (Scheme 1) as chiral source to construct HMOFs. In fact, few HMOFs assembled directly from racemic organic ligands have been reported.30−33 We focus on the design and synthesis of salen-type lanthanide coordination polymer on the basis of their physical properties, and many interesting structures with unique © 2014 American Chemical Society

Received: August 15, 2014 Revised: September 25, 2014 Published: October 23, 2014 5356

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Scheme 1. Prime Methodologies for Assembling HMOFs

dia topological network have not been documented. Therefore, complex 1 may represent the first 3D chiral lanthanide MOF with a dia topology. It is well-known that spontaneous resolution generally yields a conglomerate (racemic mixture of chiral crystals). However, in the present case, the result chiral bulk product contains only one type of enantiopure ligand (H2LSS). The prime reasons are expatiated as follows: (i) the steric configuration of cis- and trans-L ligands existing in a larger difference, the later owning higher stability and lower steric hindrance, which dominate the trans-L ligands in the assembly of frameworks;49 (ii) the hydrogen-bond interaction plays an important role in the selection of H2LSS and H2LRR; more specifically, in complexes 1−3, the distance of C13−H···O3 is 2.457 Å and C13···Cl is 3.667 Å, if H2LSS is changed to H2LRR, the configuration of 1,2cyclohexanediamine is high likely reverse. On the basis of calculation, the distance of C13−H···Cl will be 2.13 Å, which is shorter than the reported distance (2.3−3.0 Å).50 Simultaneously, the hydrogen bond C13−H···O3 will disappear and a new hydrogen-bond C8−H···O3 will appear with an even shorter distance of 2.01 Å, which are both obviously unreasonable for stabilizing the structure (Figure 3). Thermogravimetric analysis for complexes 1, 2, and 3 all show a two-step weight loss of 13.2%, 17.9%, and 22% between 25 and 300 °C, corresponding to the loss of guest CH2Cl2 molecules (calcd 15.3%), respectively. The relative high values (17.9% for 2 and 22% for 3) may be induced by part of the CH2Cl2 molecules evaporated before TG test. On further heating, a loss of 66.8%, 62.1%, and 58% between 300 and 570 °C should correspond to the release of the organic H2L ligand and the collapse of the lattice structure, respectively (Figures S3, S4, and S5, Supporting Information). The emission spectra of complex 3 in solid state excited at 400 nm exhibit the characteristic emissions of Nd(III) ions (Figure S6, Supporting Information). The emission spectra of complex 3 present three bands at 894, 1060, and 1336, respectively, which can be assigned to 4F3/2 → 4I9/2, 4F3/2 → 4 I11/2, and 4F3/2 → 4I13/2 transitions of the Nd(III) ions.

cation, two H2LSS molecules, two coordinated nitrate ions, one coordinated chloride ion, and four lattice dichloromethane molecules (Figure 1a). The La(III) cation is nine-coordinated in a tricapped triangular prism geometry defined by four O atoms from four H2L molecules, four O atoms from two nitrate ions, and one Cl ion, with La−O bond distances of 2.465− 2.671 Å and La−Cl bond distance of 2.875 Å. The structure of 1 is only made up of one building unit, [La(III)H2L]3+ metal−organic subunit (MOS). Interestingly, the MOS looks like a tadpole, in which the La(III) cation acts as the head and the H2L molecule as the tail (Figure 1b). It is worth noting that all the tails of the tadpole-like model are right-handed in complex 1. First of all, in a head-to-tail ligation mode, tadpole-like models link each other and form two of the same 1D right-handed helical chains, which only occupy different directions in the crystal (one along the a axis and another along the b axis) (Figure 1c). Further, these two helixes interweave each other by sharing the La(III) cations, thus resulting in the formation of 3D metal−organic frameworks (Figure 1d). To fully understand the structure of 1, the topological approach is applied to simplify such a complicated 3D coordination framework. Apparently, the 4-connected La(III) cation could be regarded as a node, and the helical H2L ligand could be regarded as a bridging linker in the construction of this network. The 3D architecture of 1 can be simplified as the classic diamond (dia) topology (Figure 2). In the single dia cycle, there are three types of cavities: the first one is an open window (A-type); and the second one is a closed window that is packaged by hexatomic rings (B-type); and the last one is an inner cavity that is larger than the previous two cavities (Ctype) (Figure 2). Although dia topological networks are common for transition metal coordination complexes, such as Zn(inic)2 (inic = isonicotinate),45 Zn(4-pya)2 (4-pya = (E)-3(pyridin-4-yl)acrylate),46 Cd((E)-4-pyv-4-bza)2 ((E)-4-pyv-4bza = (E)-4-(2-(pyridin-4-yl)vinyl)benzoate),47 and Zn(ima)2 (ima = 2-(imidazol-1-yl)acetate).48 To the best of our knowledge, the lanthanide coordination polymers with a chiral 5357

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Figure 1. Crystal structure of 1: (a) the structural unit, #1, y, x, 2 − z; #2, 0.5 − x, −0.5 + y, 1.75 − z; #3, −0.5 + y, 0.5− x, 0.25 + z; (b) tadpole-like metal−organic subunit; (c) combined ball and stick and cartoon representation of the 1D right-handed helical chain (hydrogen atoms are omitted for clarity); (d) combined ball and stick and cartoon representation of the 3D open metal−organic framework.

Figure 2. Combined ball and stick representation of the dia topology (left); single dia cycle with different types of cavities in complex 1 (middle); and cavities filled by spheres (right). Open window (yellow green sphere), closed window (blue sphere), and inner cavity (purple sphere).

Although complex 3 exhibits the characteristic emissions of Nd(III) ion, their intensities are so weak that their lifetimes and efficiency could not be measured, which is similar to salen -type neodymium complexes [Nd(NO3)(H2L)2]·0.2CH3OH possessing ladder-like double-chain structure.41

According to the method proposed by Kurtz, the SHG efficiency can be measured by using a powder technique.51 The SHG efficiencies of 1−3 are measured by using pure microcrystalline samples, and the results are in contrast with KDP. The SHG is performed with a pulsed laser at a wavelength of 1064 nm. The intensity of the green light 5358

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Figure 3. Schematic representation of hydrogen-bond interactions in complex 1 (left); and hydrogen-bond interactions simulate model after ligand replacement (right).

(frequency-doubled output: λ = 532 nm) produced by the powder sample of complexes 1, 2, and 3 is about 0.6, 0.56, and 0.56 times, respectively, which is consistent with each homochiral frameworks. Although lots of diamond MOFs synthesized by carboxylate and/or azole ligands have been reported, the NLO properties of salen-type based MOFs have never been researched. Table S6, Supporting Information, exhibited all SHG activity of 3D diamond MOFs involving the carboxylate and/or azole ligands. To the best of our knowledge, complexes 1−3 are the first three examples of NLO-active, homochiral lanthanide-salen frameworks. In summary, isolation of a series of three 3D homochiral light lanthanide−organic frameworks with unique dia topology by reactions of racemic salen-type ligand with mixed lanthanide counterions demonstrates that the mixed lanthanide counterions may play the key role on the spontaneous formation of the homochiral lanthanide−organic frameworks in the progress of self-assembly. The homochirity of the lanthanide−organic frameworks results in the SHG effects. It represents the first lanthanide MOF with a chiral dia topology. This approach may open up new opportunities in the preparation of homochiral materials.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (Nos. 51272069, 21272061, and 21072049).

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ASSOCIATED CONTENT

S Supporting Information *

Complete synthetic procedure, characterization of MOFs 1−3, additional figures, PXRD, TG-DSC analysis, and NIR luminescence. CCDC 927765, 927766, and 927767. Crystal data for 1: Tetragonal, space group P43212, a = b = 18.1798(2) Å, c = 16.4174(3) Å, α = β = γ = 90.00°, V = 5426.04(13), Z = 4, R1 = 0.0488, Rw = 0.1211. For 2: Tetragonal, space group P43212, a = b = 18.216(3) Å, c = 16.483(3) Å, α = β = γ = 90.00°, V = 5469.6(16), Z = 8, R1 = 0.0549, Rw = 0.1414. For 3: Tetragonal, space group P43212, a = b = 18.1041(2) Å, c = 16.4374(5) Å, α = β = γ = 90.00°, V = 5387.50(18), Z = 4, R1 = 0.0473, Rw = 0.1314. This material is available free of charge via the Internet at http://pubs.acs.org.

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*E-mail: [email protected] (G-M.L). *E-mail: [email protected] (P-F.Y). 5359

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