Homochiral crystallization of helical coordination chains bridged by achiral ligands: can it be controlled by the ligand structure?

Abstract

Homochiral/heterochiral crystallizations of helical chiral polymer chains bridged by achiral poly-pyridyl ligands dependent on the structures of the bridging ligands and independent on the solvent are described, implying a possible strategy to design achiral crystals of helical chains using chiral bridging ligands.

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Chiral coordination polymers with different networks,^1–3 which have potential functions, such as enantioselective separations⁴ and catalysis,⁵ could be obtained by using chiral ligands,^1a,6 and achiral ligands.⁷ The use of achiral ligands is more attractive since usually such ligands are easier to generate. Unfortunately, the construction of chiral coordination polymers using achiral ligands is currently a challenging research field, especially that of helical chains.⁸ So far our understanding of chiral coordination polymer chains constructed by achiral bridging ligands is very limited,⁹ only several asymmetric and achiral pyridyl-bridged polymers¹⁰ and chiral coordination chains constructed by some other asymmetric bridging ligands were recently reported.¹¹ In fact, the construction of non-centrosymmetrical solids based on 1D chains is a formidable task owing to the lack of control in the two other dimensions.⁸ On the other hand, it should be anticipated that the structures of asymmetric bridging ligands have significant effect on recognition and crystallization of the chiral chains into solids. Therefore, we synthesized two new poly-pyridyl ligands, 2-[5-(pyridin-3-yl)-1,3,4-oxadiazol-2-yl]pyridine (L1) and 2-[5-(pyridin-4-yl)-1,3,4-oxadiazol-2-yl]pyridine (L2) (Scheme 1).† Both L1 and L2 have two sites (chelate and monodentate) for metal ligation and may extend into polymeric structures with different bridging geometries.

Scheme 1 The asymmetrical bridging ligands and the synthesis of their Cd(II) complexes.

To generate polymer chains, L1 and L2 were used to react with CdI₂ (ratio 1 ∶ 1) in MeOH, furnishing [CdI₂(L1)]_∞ (1) and [CdI₂(L2)]_∞ (2), respectively (Scheme 1). Solvent effect on the crystallization was also evaluated by reactions of CdI₂ with L1 and L2 in N,N-dimethylformamide (DMF), which led to 1 and {[CdI₂(L2)(H₂O)]·2DMF}_∞ (3), respectively. Interestingly, 1, 2 and 3 crystallize in the space groups C2/c, P2₁, and P2₁2₁2₁, respectively.‡

Single-crystal X-ray analysis reveals that centrosymmetrical 1 consists of one Cd(II) atom and one L1 ligand in an asymmetric unit (Fig. 1a), and the metal atom is coordinated in square-pyramidal geometry [Cd–N 2.424(4)–2.434(4) Å, Cd–I 2.706(1)–2.738(1) Å, N–Cd–N 68.4(1)–151.8(1)°, N–Cd–I 92.5(1)–130.6(1)°, I–Cd–I 123.64(2)°]. Such chains helically extended along the 2₁ axis in the b direction have the both left- and right-handedness, having a pitch of 7.412(1) Å. A left-handed helix is represented in Fig. 2a. Adjacent right- and left-handed chains (1 ∶ 1) are packed in the lattice (see Fig. S1†), exhibiting significant π–π stacking interactions (offset face-to-face distance 3.64 Å) between 3-pyridyl rings from neighbour chains in a zipper-like fashion. It is notable that 1 could be isolated in both MeOH and DMF solutions, which implies 1 has the optimized molecular packing fashion.

Fig. 1 The coordination arrangements of the Cd(II) atoms in 1 (a), 2 (b) and 3 (c).

Fig. 2 Perspective views of the chiral chains in 1 (a), 2 (b) and 3 (c).

Each metal atom in 2 is coordinated in a square-pyramidal geometry [Cd–N 2.398(11)–2.469(11) Å, Cd–I 2.717(3)–2.745(2) Å, N–Cd–N 68.4(4)–143.5(4)°, N–Cd–I 90.6(3)–140.8(2)°, I–Cd–I 118.63(6)°] (Fig. 1b), very similar to that in 1. Interestingly, 2 crystallizes in the chiral space group with each (P) right-handed helical chain related by a 2₁-screw axis in the b direction, as shown in Fig. 2b. The pitch of the helix in 2 (14.116(1) Å) is almost twice than that of 1, attributable to the structural difference between L1 and L2. Consequently, the packing mode of 2 is different, neighbouring chains in 2 do not have significant π–π stacking interaction, and they are homochiral (Fig. S2†).

In 3, the metal atom exhibits a distorted octahedral geometry [Cd–N 2.360(5)–2.474(5) Å, Cd–I 2.790(1)–2.878(1) Å, Cd–O 2.463(5) Å, N–Cd–N 68.9(2)–155.5(2)°, N–Cd–I 86.4(1)–160.9(1)°, I–Cd–I 109.15(2)°, N–Cd–O 75.0(2)–83.1(2)°, O–Cd–I 88.7(1)–161.2(1)°], being additionally ligated by one aqua ligand (Fig. 1c). Although the coordination geometry has been drastically changed, similar to those in 2, homochiral, (P) right-handed helices exist in 3 (Fig. 2c) and are related by the 2₁-screw axes in the b direction. The pitch of 3 is only longer by 0.608 Å than that of 2, which is attributed to the increase of coordination number in 3. Different from those in 2, neighbouring chains in 3 have interchain π–π stacking interactions (offset face-to-face, distances between 2-pyridyl and 4-pyridyl rings 3.64 Å), being different from that in 1 (Fig. S3†). Also interestingly, two unusual C–H⋯I hydrogen bonds (C–H⋯I 4.03 and 4.08 Å, C–H⋯I 168 and 165°) (Fig. S4†) between chains are found in 3, which dominate the molecular packing of 3. Therefore, we may be able to conclude that the structure of the bridging ligand is critically important, compared to the solvent and coordination geometry. This observation represents a new strategy different from the literature reports that the existence of solvent, change of solvent or temperature can alter heterochiral/homochiral crystallization.^7d,10c,12,13

Quasi-Kurtz powder second harmonic generation (SHG) measurements¹⁴ show that 2 and 3 display modest powder SHG efficiencies approximately 0.4 and 0.5 times than that potassium dihydrogen phosphate (KDP), respectively. Unfortunately, we were unable to clarify whether the bulk materials were homochiral.

In summary, this work represents a successful example of ligand-controlled homochiral crystallization of coordination polymer chains via structural variation of achiral, asymmetric bridging ligands. This implies that upon appropriate design of analogous asymmetric bridging ligands, homochiral crystallizations could be achieved and may lead to potential applications. Further study on the related coordination polymers is in progress.

This work was supported by the National Natural Science Foundation of China (No. 20131020) and the Scientific and Technological Bureau of Guangdong Province (No. 04205405).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Synthesis for L1, L2, 1–3, and additional plots for 1–3. See http://www.rsc.org/suppdata/dt/b4/b416412a/

‡ Crystal data for 1 (C₁₂H₈CdI₂N₄O): monoclinic, space group C2/c, M = 590.42, a = 33.006(7), b = 7.412(3), c = 13.816(3) Å, β = 111.460(3)°, V = 3146(1) ų, Z = 8, µ = 5.314 mm⁻¹, R₁ = 0.0360, wR₂ = 0.0947. Crystal data for 2 (C₁₂H₈CdI₂N₄O): monoclinic, space group P2₁, M = 590.42, a = 7.057(7), b = 14.116(1), c = 8.743(9) Å, β = 112.36(2)°, V = 805.5(14) ų, Z = 2, µ = 5.188 mm⁻¹, R₁ = 0.0342, wR₂ = 0.0890, Flack x = −0.03(6). Crystal data for 3 (C₁₈H₂₄CdI₂N₆O₄): orthorhombic, space group P2₁2₁2₁, M = 754.63, a = 8.295(1), b = 14.724(2), c = 21.236(2) Å, V = 2593.8(5) ų, Z = 4, µ = 3.26 mm⁻¹, R₁ = 0.0453, wR₂ = 0.0983, Flack x = 0.02(3). CCDC reference numbers 253093–253095. See http://www.rsc.org/suppdata/dt/b4/b416412a/ for crystallographic data in CIF or other electronic format.

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