Axially chiral metal–organic frameworks produced from spontaneous resolution with an achiral pyridyl dicarboxylate ligand

Abstract

Three-dimensional chiral (10,3)-c MOFs are generated via symmetry breaking from achiral precursors, 5-(pyridine-3-yl)isophthalic acid and M²⁺ (M = Cd, Zn, Mn), and chiral transmission from molecular axially chiral conformations to framework chirality is established.

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Chirality plays an important role in chemistry and materials. Chiral metal–organic frameworks (MOFs) are of interest due to their potential applications in asymmetric catalysis, enantioselective separation, nonlinear optical materials, etc.¹ Compared with central chirality, axial chirality has been less exploited to design noncentrosymmetric MOFs. Axial chirality is most commonly observed in atropisomeric biaryl compounds wherein the rotation about the aryl–aryl bond is restricted, e.g., 1,1′-bi-2-naphthol and 2,2′-bis-(diphenylphosphino)-1,1′-binaphthyl.² Depending on the rotational barrier, axially chiral molecules may atropisomize upon temperature rise. If the barrier to atropisomerisation is high, enantiopure axially chiral compounds may be synthesised.

Chiral MOFs have been synthesised from enantiopure axially chiral precursors based on 1,1′-bi-2-naphthol by Lin and others,³ which show promising enantioselective activities. However enantioselective synthesis using enantiopure ligand is generally tedious and expensive, sometimes with additional complications of ligand racemisation.⁴ The use of achiral precursors is thus appealing to assemble MOFs via spontaneous resolution.⁵ Generation of chirality from achiral components and assembly into higher dimensional chiral MOFs without any chiral auxiliary are of great current interest.⁶ Spatial chiral organization has usually been used to form chiral MOFs from achiral building blocks by a topological framework construction. Such a framework may have a chiral topology due to the presence of helices, e.g. the (10,3)-a net.^7–9 Herein an achiral ligand, 5-(pyridine-3-yl)isophthalic acid (L), was chosen to construct axially chiral MOFs. For L, C–C single bonds between the benzene ring and pyridyl group can turn around discretionarily at RT and the rotational barrier to atropisomerisation is low.¹⁰ We wish to show that the rotation of L can be restricted by metal complexation and axially chiral conformational isomers of the pyridyl–aryl unit have been captured to result in three-dimensional (3D) frameworks.

Reactions of L with metal salt MCl₂ (ZnCl₂, CdCl₂·2.5H₂O or MnCl₂·4H₂O) in DMF–water at 80 °C for 1 d lead to formation of isostructural MOFs of Cd (M = Cd), Zn (M = Zn) and Mn (M = Mn).

The structures of Cd and Zn could be unambiguously established by single-crystal X-ray diffraction analysis.‡Cd and Zn are isostructural with infinite 3D structures. Cd contains two enantiomers, M-Cd and P-Cd, crystallising in the chiral hexagonal space groupsP6₅ and P6₁, respectively. The asymmetric unit of M-Cd or P-Cd consists of one Cd^II ion, one L and two coordinated H₂O molecules (Fig. 1). Each distorted octahedral Cd^II ion is six-coordinated by two chelating carboxylate oxygen atoms from one ligand, one monodentate carboxylate oxygen atom and one pyridyl nitrogen atom from another two ligands, and two coordinated water molecules. In turn, each ligand coordinates to three Cd atoms through two carboxylate groups and one pyridyl N donor. The coordination modes of two carboxylate groups are different for L. One carboxylate group connects with Cd ion through a monodentate O atom in a η¹-mode and the other carboxylate group binds to a second Cd ion through both carboxylate oxygens in a η²-mode. Single axially chiral conformation of L exists in M-Cd and P-Cd, i.e. Λ conformation in M-Cd and Δ conformation in P-Cd. The angles between the benzene ring and pyridyl ring are 18.8 and 18.9°, respectively (Fig. 1). The helical chain constructed by L and Cd^II atoms via Cd–N and Cd–O (η²-mode carboxylate) bonds is circumgyrated along the six-fold axial direction (c axis) with a pitch of 20.4 Å (c). Left-handed and right-handed helixes are formed for M-Cd and P-Cd, respectively (Fig. 2a–c). Each helical chain is further interconnected with six neighbouring chains with the same handedness to form 3D structure via Cd–O (η¹-mode carboxylate) coordination bond (Fig. 2d and e). Hence the chiral information of L is transmitted to form homochiral crystals in P6₅ or P6₁ space group. The Flack parameter with values close to zero implies high enantiopurity in single crystals.

Fig. 1 Asymmetric units (top) and two mirror images of the 3-connected chiral building blocks (bottom) of M-Cd and P-Cd.

Fig. 2 (a and b) Side views of left-handed and right-handed helical chains in M-Cd and P-Cd, respectively (a left-handed helix is denoted M and a right-handed helix is denoted P), (c) top view of the helix along the c-axis in P-Cd, (d) connections of two adjacent helical chains, (e) connections of one helical chain with six neighbouring helical chains to form a 3D structure, with coordinated water molecules omitted for clarity, and (f) a (10,3)-c topological diagram of P-Cd.

In the structures of Cd and Zn, each metal ion acts as a three connector to connect three L ligands, and each L ligand links to three metal ions. By treating the ligand as three-connecting nodes and the metal ion as three-connecting linkers, the overall net topology of Cd or Zn is a uninodal noninterpenetrating 3-connected net, and the net corresponds to bto, (10,3)-c chiral topology (Fig. 2f) as determined by TOPOS 4.0 software.¹¹ In contrast to a known hydrogen-bonded network with the same topology,⁸ the present networks are unprecedentedly built through coordinative bonding.⁹

As shown above, spontaneous resolution occurs during the course of crystallisation and enantiomerically pure crystals are obtained. Analysing the structure, the metal atoms are not stereogenic centers and not responsible for the chirality of the frameworks.¹² The origin of chirality is closely related to the ligand. The conformation of L plays a crucial role in the chirality generation. Two axially chiral conformations of Λ and Δ of L are fixed in the resultant frameworks. In contrast, a meso-state (equal right-handed and left-handed conformations) of L exists in a previously reported Cu^II framework.¹³ Therefore, a clear relationship between the molecular axial chirality and the framework chirality is established in the present frameworks.¹⁴

Structural determination and powder X-ray diffraction (PXRD) show that Zn, Cd and Mn are isostructural (Fig. 3). The phase purity was confirmed by X-ray powder diffraction analyses. The experimental XRD patterns agree well with the simulated ones generated on the basis of the single-crystal analyses.

Fig. 3 Simulated and measured powder XRD patterns of (a) Cd simulated, (b) Cd as-synthesised, (c) Zn simulated, (d) Zn as-synthesised, and (e) Mn as-synthesised.

The solid-state UV-vis absorption spectrum of Cd or Zn exhibits one broad absorption band centred around 300 nm which is attributed to n → π* transition (Fig. 4). Solid-state circular dichroism (CD) measurements of Cd indicate that randomly selected single crystals display dichroic signal with positive or negative Cotton effect. The results confirm each crystal is spontaneously resolved by crystallization. However, the bulk materials contain both enantiomorphs and thus are racemic.

Fig. 4 (a) UV-vis spectra of Cd, Zn and L and (b) single-crystal CD spectra of Cd in KCl pellets showing contrasting Cotton effects.

Thermogravimetric analyses (TGA) were performed to verify the thermal stabilities (Fig. 5). The weight losses of 9.8, 10.8 and 11.2% (calcd 9.2, 10.5 and 10.8%) from ca. 120 to 200 °C for Cd, Zn and Mn, respectively, correspond to the loss of two coordinated water molecules per formula unit. No further weight losses were observed below 340 °C for Cd, below 370 °C for Zn, and below 420 °C for Mn.

Fig. 5 TGA curves of (a) Cd, (b) Zn and (c) Mn.

The photoluminescent properties of Cd and Zn were studied in the solid state at room temperature (Fig. 6). Cd displays a blue emission band with a peak maximum at 408 nm (excitation at 344 nm), which is red-shifted in comparison to Zn with an emission maximum occurring around 378 nm (λ_ex = 336 nm). The emissions can be assigned to the intraligand n → π* transition of the ligand, as free ligand possesses a similar emission at ca. 390 nm upon excitation at 333 nm in the solid state.

Fig. 6 Solid-state photoluminescence spectra measured at room temperature. Excitation and emission spectra of (a) Cd (λ_em = 408 nm, λ_ex = 344 nm) and (b) Zn (λ_em = 378 nm, λ_ex = 336 nm).

In summary, isostructural 3D homochiral MOFs, Cd, Zn and Mn, have been successfully prepared by using achiral precursors. The axially chiral configurations of L are separated upon metal complexing to form 3D frameworks, although L has no axial chirality at RT due to free rotation of C–C single bond between the benzene ring and pyridyl group. The present approach may be promising in developing homochiral MOFs with axial chirality.

We gratefully acknowledge the NSFC (20903121), the NSF of Guangdong Province (S2011010001307), the RFDP of Higher Education of China, the Fundamental Research Funds for the Central Universities, and the SRF for ROCS of SEM for support.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Experimental section, IR spectra and structural data. CCDC reference numbers 836380–836383. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c1ce05995e

‡ Crystal data for M-Cd: C₁₃H₁₁CdNO₆, M = 389.63, hexagonal, space groupP6₅, a = 10.5423(17), b = 10.5423(17), c = 20.448(4) Å, V = 1968.1(6) ų, Z = 6, T = 298(2) K, ρ_calcd = 1.972 g cm⁻³, μ = 1.692 mm⁻¹, F(000) = 1152, 15 029 reflections were collected with 2984 unique for 3.73 < θ < 27.44, R_int = 0.0543, R₁ = 0.0242, wR₂ = 0.0507 (I > 2σ(I)), R₁ = 0.0260, wR₂ = 0.0516 (all data) for 190 parameters, GOF = 1.056, Flack parameter = 0.00(2). Crystal data for P-Cd: C₁₃H₁₁CdNO₆, M = 389.63, hexagonal, space group P6₁, a = 10.5544(2), b = 10.5544(2), c = 20.4446(6) Å, V = 1972.31(8) ų, Z = 6, T = 298(2) K, ρ_calcd = 1.968 g cm⁻³, μ = 1.689 mm⁻¹, F(000) = 1152, 7819 reflections were collected with 2933 unique for 2.99 < θ < 27.41, R_int = 0.0206, R₁ = 0.0177, wR₂ = 0.0337 (I > 2σ(I)), R₁ = 0.0207, wR₂ = 0.0346 (all data) for 190 parameters, GOF = 1.047, Flack parameter = −0.026(16). Crystal data for M-Zn: C₁₃H₁₁ZnNO₆, M = 342.60, hexagonal, space groupP6₅, a = 10.2531(2), b = 10.2531(2), c = 20.2456(6) Å, V = 1843.20(7) ų, Z = 6, T = 150(2) K, ρ_calcd = 1.852 g cm⁻³, μ = 3.080 mm⁻¹, F(000) = 1044, 3178 reflections were collected with 1313 unique for 4.98 < θ < 63.10, R_int = 0.0252, R₁ = 0.0235, wR₂ = 0.0595 (I > 2σ(I)), R₁ = 0.0240, wR₂ = 0.0597 (all data) for 190 parameters, GOF = 1.076, Flack parameter = 0.01(3). Crystal data for P-Zn: C₁₃H₁₁ZnNO₆, M = 342.60, hexagonal, space group P6₁, a = 10.2723(4), b = 10.2723(4), c = 20.4692(9) Å, V = 1870.54(13) ų, Z = 6, T = 298(2) K, ρ_calcd = 1.825 g cm⁻³, μ = 1.999 mm⁻¹, F(000) = 1044, 5821 reflections were collected with 2703 unique for 3.03 < θ < 27.46, R_int = 0.0293, R₁ = 0.0262, wR₂ = 0.0539 (I > 2σ(I)), R₁ = 0.0323, wR₂ = 0.0635 (all data) for 190 parameters, GOF = 1.123, Flack parameter = 0.013(12).

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