Chiral 1-D coordination polymer chains featuring 1,1′-binaphthyl

Introduction

Chirality has an integral role in nature, with many important biomolecules including enzymes and sugars consisting of chiral building blocks. The ability to detect or separate chiral molecules is particularly important in the pharmaceutical, food and agriculture industries as the presence of the incorrect enantiomer may lead to detrimental side effects. Carefully designed chiral molecular systems are therefore necessary to enable their detection and separation by the specific interactions of chiral molecules with other chiral systems.

1,1′-Binaphthyl has attracted significant attention as a source of chirality due to its modularity, robustness and versatility.¹ In contrast to the point chirality featured in most chiral molecules including amino acids, 1,1′-binaphthyl possesses axial chirality which is caused by the hindered rotation of two naphthyl rings around a C–C bond, making the system particularly robust to racemisation from changes in pH or chemical environment.² Racemisation of 1,1′-binaphthyl typically does not occur until high temperatures are reached.³

The use of 1,1′-binaphthyl in chiroptical and enantioseparation applications has been widely explored in smaller molecular systems,⁴ however, there exists significant advantages for the incorporation of 1,1′-binaphthyl as the source of chirality in larger supramolecular structures including coordination polymers (CPs), metal–organic frameworks (MOFs) and covalent organic frameworks (COFs).^2,5–8 These advantages include increased stability, the ability to operate in the solid state and opportunities for greater selectivity. A relatively small number of studies have involved MOFs and CPs containing 1,1′-binaphthyl systems and include 1-D,^9–14 2-D and 3-D systems.^15–24 Despite the limited reports of MOFs and CPs containing 1,1′-binaphthyl, they have been used in myriad applications including asymmetric catalysis,^15,16,23 chemical sensing,¹⁷ chiroptical fluorescence sensing^18,20 and hydrogen storage.¹⁹

The formation of 1-D chains with 1,1′-binaphthyl-containing ligands most commonly occurs with 6,6′-substituted species due to the bent orientation of the ligand.^9–14 The 1-D chains formed are most commonly formed as loops where a pair of 1,1′-binaphthyl ligands bind to the metal centre forming a parallelogram-shaped void. 1-D looping chains containing 1,1′-binaphthyl have been used in a number of stereoselective applications including circularly polarised luminescence and two-photon excited fluorescence in a Cd^II 1-D chain CP,¹¹ spin-crossover in an interlocked 1-D chain containing poly[n]catenane with Fe^II 12 and as a chiral stationary phase with high-performance liquid chromatography (HPLC) as a MOF–silica composite with Cu^II.¹³ Of particular importance in all applications is the ability of the framework to interact with chiral guests, which is facilitated by diffusion of the guests into the voids. Guest interaction with the framework structures can be promoted by engineering the 1-D channels or facilitating larger accessible voids through altering the packing of supramolecular components.

In this work we report the synthesis of four 1-D chains with Ni^II, Cu^II and Ag^I containing two bent 1,1′-binaphthyl ligands, (R)-4,4′-(2,2′-diethoxy-[1,1′-binaphthalene]-6,6′)dipyridine ((R)-L₁) and (S)-6,6′-dicarboxyl-2,2′-diethoxy-1,1′-binaphthalene ((S)-H₂L₂) (Fig. 1). The 1-D chains synthesised using Ni^II and Cu^II are structurally similar, featuring a 1-D looping chain with pairs of 1,1′-binaphthyl ligands coordinating to the metal centres. The 1-D chain obtained with Ag^I contains an open structure where the 1-D chains stack with each other to form a helix. The stacking of the 1-D chains is investigated in each case to rationalise the formation of large voids and channels.

Fig. 1.

The two 1,1′-binaphthyl ligands used in this study where L₁ = 4,4′-(2,2′-diethoxy-[1,1′-binaphthalene]-6,6′)dipyridine and H₂L₂ = 6,6′-dicarboxyl-2,2′-diethoxy-1,1′-binaphthalene.

Results and discussion

L₁ was synthesised using a Suzuki–Miyaura cross coupling reaction between (R)-6,6′-dibromo-2,2′-diethoxy-1,1′-binaphthalene and 4-pyridyl boronic acid to yield the product as a white solid in 97% yield. H₂L₂ was synthesised according to literature methods,^25,26 with additional ligand synthesis details provided in the Supplementary material. (R)-4,4′-(2,2′-Diethoxy-[1,1′-binaphthalene]-6,6′)dipyridine ((R)-L₁) was dissolved in DMF and combined with either Ni(NO₃)₂∙6H₂O or Cu(NO₃)₂∙3H₂O in diffusion reactions at room temperature to yield the formation of [Ni((R)-L₁)₂(H₂O)₂] (1) and [Cu((R)-L₁)₂(H₂O)₂] (2) respectively. Frameworks 1 and 2 are isostructural and crystallised in the orthorhombic P2₁2₁2 space group (Fig. 2 and Supplementary Fig. S1; Supplementary Table S1). The bulk purity of 1 and 2 were verified by powder X-ray diffraction (PXRD) (Supplementary Fig. S2 and S3). The coordination geometry around both the Ni^II and Cu^II centres is octahedral, featuring four N-donors from four (R)-L₁ ligands and two O-donors from coordinated water molecules in the axial positions (Fig. 2a; Supplementary Table S2). Typical Ni–N (2.096(3), 2.111(3) Å) and Ni–O bond lengths (2.093(3) Å) are observed in 1 whereas Jahn–Teller distortion occurs in 2 with elongated Cu–O bond lengths of 2.437(5) Å and shorter Cu–N bond lengths of 2.015(5) and 2.026(5) Å. The effect of the Jahn–Teller distortion of Cu^II is reflected in the cell parameters for 1 and 2; a = 12.954(3), b = 16.633(3), c = 18.387(4) Å for 1 and a = 12.662(3), b = 16.357(3) and c = 18.472(4) Å for 2 where the shortening of the a and b axes may be attributed to the contraction in the Cu–N bonds and the lengthening of the c axis to the longer Cu–O bonds. The (R)-L₁ ligands adopt a bent conformation with the two naphthalene rings lying at 73.6° for 1 and 72.1° for 2 relative to each other, clearly displaying the origin of 1,1′-binaphthyl’s axial chirality (Fig. 2b). Each (R)-L₁ ligand connects two M^II centres through the pyridyl groups.

Fig. 2.

Crystal structure of [Ni((R)-L₁)₂(H₂O)₂] (1) showing (a) the coordination environment around Ni^II, (b) the looping 1-D chain and stacking of the 1-D chain as viewed down the (c) c axis and (d) b axis.

Adjacent M^II centres are linked by pairs of (R)-L₁ ligands to form a looping 1-D chain containing parallelogram-shaped voids (Fig. 2b) with dimensions 18.4 × 12.2 Å for 1 and 18.5 × 11.8 Å for 2. The steric bulk provided by the ethoxy substituent on the 1,1′-binaphthyl ring prevents close packing of the 1-D chains. The 1-D chains pack in an AABB manner with each ‘layer’ of 1-D chains orientated in a perpendicular arrangement relative to its neighbours above and below (Fig. 2c, d). No catenation occurs through the looping chains. Intermolecular interactions are observed between the stacked 1-D chains with CH–π interactions between the ethoxy substituent and an adjacent naphthyl ring from a neighbouring 1-D chain dominating (d = 3.854 Å for 1, d = 3.773 Å for 2). The preclusion of close packing results in relatively large accessible void spaces of 40.2% for 1 and 35.3% for 2, which is reflected in the thermal gravimetric analysis (TGA) with a mass loss below 150°C of 25% for 1 and 22.5% for 2 (Supplementary Fig. S8 and S9). These mass losses below 150°C are attributed to the liberation of the DMF and H₂O molecules present in the voids of the crystal structure. A gradual mass loss above 300 and 250°C for 1 and 2 respectively represents decomposition of the framework structure. FT-IR spectra of 1 and 2 are similar, being dominated by the features of the ligand (Supplementary Fig. S5–S6).

A previous example of a CP exists containing an analogous dipyridyl 1,1′-binaphthyl ligand with substitution at the 6 and 6′ positions but which contains a OCH₂OCH₃ group in place of the ethoxy substituent, (R)-2,2′-bis(methoxymethoxy)6,6′-bis(4-pyridyl)-1,10-binaphthyl) ((R)-L₃).¹¹ In this report, (R)-L₃ was combined with Cd^II to yield [Cd(NO₃)₂(R)-L₃] (CSD refcodes: ZIJKIP and XITQUP).¹¹ [Cd(NO₃)₂(R)-L₃] also features a 1-D looping chain, but with a 7-c coordination geometry around the Cd^II centre from four N-donors on the equatorial plane and from four L₁ ligands and three O-donors from two NO₃⁻ in the axial positions. One of the NO₃⁻ ligands is bound in a bidentate manner, whereas the other NO₃⁻ is bound in a monodentate manner. A torsion angle of 72.8° exists between the naphthyl rings in (R)-L₃ in this structure, and it has voids with dimensions of 18.8 × 12.4 Å (for THF solvate) which is similar to 1 and 2. Modest accessible void spaces of 21.1% are present in [Cd(NO₃)₂(R)-L₃] that are smaller than those observed for 1 and 2. The differences in the accessible void spaces can be attributed to both the stacking of the 1-D chains and the lower steric bulk from the smaller OEt group on (R)-L₁ when compared to the OCH₂OCH₃ group in (R)-L₃.

(R)-2,2′-Bis(methoxymethoxy)-6,6′-bis(4-pyridylethynyl)-1,10-binaphthyl ((R)-L₄) containing an additional alkynyl group between the pyridyl and the 1,1′-binaphthyl has also been incorporated into a 1-D chain.¹² [Fe(NCBH₃)₂((R)-L₄)₂]∙6CH₃CN (CSD refcode: VUCXID with MeCN, VUCYAW with MeOH) was synthesised through the reaction of (R)-L₄ with [Fe(NCBH₃)₂] to yield looping 1-D chains in an ABAC manner. The longer (R)-L₄ enabled interlocking of the 1-D chains in a catenane-type motif and displayed spin crossover behaviour of the iron centre.¹²

Changing the dipyridyl coordinating groups in (R)-L₁ to dicarboxylate groups, (S)-H₂L₂, resulted in the formation of [Cu₂((S)-L₂)₂(DMF)₂]‧2H₂O (3) (Fig. 3). Framework 3 was synthesised using a solvothermal reaction of (S)-H₂L₂ with Cu₂(NO₃)₂‧3H₂O in an acidified DMF/H₂O mixture (3:1) at 85°C to yield green needles. The bulk purity of 3 was confirmed using PXRD (Supplementary Fig. S4) and characterised with FT-IR (Supplementary Fig. S7). Framework 3 displays a number of similarities to 1 and 2 with the largest difference being that of the Cu^II coordination environment due to the carboxylate coordinating groups. The naphthyl rings of 1,1′-binaphthyl in 3 are orientated at ~105° relative to each other, reflecting the axial chirality of the ligand. The structure of 3 includes dinuclear Cu^II paddlewheel units, with the carboxylate groups of (S)-L₂ ligands bridging two Cu^II centres in a bidentate manner (Fig. 3a). The apical sites of each Cu^II paddlewheel unit are occupied by DMF molecules with each Cu^II centre adopting a square pyramidal coordination geometry (Fig. 3b; Supplementary Table S3). The octahedral coordination geometry of 3 contrasts with 1 and 2, where water molecules are located in the axial positions. The Cu^II–O bond lengths in 3 range from 1.934(4) Å to 2.116(4) Å, typical for 5-coordinate Cu^II centres. The Cu^II paddlewheel units are linked by pairs of (S)-L₂ ligands to form looping 1-D chains (Fig. 3c).

Fig. 3.

Crystal structure of [Cu₂((S)-L₂)₂(DMF)₂]‧2H₂O (3) showing (a) the dinuclear Cu^II paddlewheel unit, (b) the (S)-L₁ ligand coordinated to Cu^II ions, (c) the 1-D chain where the DMF coordinated in the axial positions has been omitted for clarity and the packing of the 1-D chains down the (d) a axis and (e) c axis. Atoms have been denoted as follows: carbon (black), copper (light blue), oxygen (red). Hydrogens have been omitted for clarity.

The 1-D chains pack in an ABAB fashion, with each ‘layer’ of 1-D chains orientated in the same direction. In a similar manner to 1 and 2, the adjacent layers of 1-D chains are positioned perpendicular to each other such that it appears the 1-D chains are running in the opposite direction. The steric bulk provided by the ethoxy substituents on the 1,1′-binaphthyl prevents close packing of the 1-D chains, resulting in an accessible void space of 33.8% (Fig. 3d, e). A gradual mass loss of ~16% is observed in the TGA (Supplementary Fig. S10) under 200°C which can be assigned to the loss of DMF molecules within the voids as well as the coordinated DMF molecule. The higher temperature required above the boiling point for removal of the coordinated DMF molecule reflects the additional energy required to break the coordination bond. A sharp mass loss at 300°C may be attributed to decarboxylation of (S)-L₁ ligands, leading to thermal decomposition of 3.

The structure of 3 is similar to [Cu₂(BDA)₂(H₂O)₂]·2MeOH·4H₂O (BDA = (S)-2,2′-dihydroxy-1,1′-binaphthalene-6,6′-dicarboxylic acid, CSD refcode: LABCOH)⁹ and [Cu₂[(R)-L₅]₄(H₂O)₂]∙DMA (R)-L₅ = (R)-2,2′-dimethoxy-1,10-binaphthalene-6,6′-dicarboxylic acid, CSD refcode: WAFPEB)¹³ in that all three CPs are 1-D looping chains containing a Cu^II paddlewheel motif. [Cu₂(BDA)₂(H₂O)₂]·2MeOH·4H₂O and [Cu₂[(R)-L₅]₄(H₂O)₂]∙DMA both feature a coordinated water molecule in the apical position of the Cu^II paddlewheel which contrasts with the coordinated DMF molecule in 3. The largest difference between the frameworks is in the extended packing of the 1-D chains within the crystal structure. The OH group from the 1,1′-binapthyl ligand in [Cu₂(BDA)₂(H₂O)₂]·2MeOH·4H₂O is located on the edges of the 1-D chain pointing outwards, enabling the formation of an extensive hydrogen bonding network between adjacent chains yielding relatively tightly packed 1-D chains. The introduction of the methoxy group in [Cu₂[(R)-L₅]₄(H₂O)₂]∙DMA still yields stacking of the 1-D chains, however, the Cu^II paddlewheels in the 1-D chains are now tilted at 69.2° in contrast with [Cu₂(BDA)₂(H₂O)₂]·2MeOH·4H₂O where the Cu^II paddlewheels are orientated in the plane of the chain. The tilting of the 1-D chains and consequently the Cu^II paddlewheels is likely a result of accommodating the additional steric bulk of the methyl group and lack of hydrogen bonding ability of the methoxy group. The stacking of the 1-D chains in 3 can be attributed to the additional steric bulk afforded by the EtO substituent which consequently yields greater accessible void spaces of 33.8% when compared to 18% for [Cu₂(BDA)₂(H₂O)₂]·2MeOH·4H₂O and 14.6% for Cu₂[(R)-L₅]₄(H₂O)₂]∙DMA. The cylindrical voids present in [Cu₂(BDA)₂(H₂O)₂]·2MeOH·4H₂O and Cu₂[(R)-L₅]₄(H₂O)₂]∙DMA are retained in 3 where 1-D channels can be observed parallel to the a axis.

When a solution of (R)-L₁ and LiOAc·2H₂O in DMF/MeCN was allowed to slowly diffuse into an aqueous solution of AgNO₃ at room temperature, colourless needles formed amid a brown gel after 3 days. Unfortunately, the formation of the crystals within the gel precluded the isolation of a bulk powder, and thus PXRD on the powder could not be performed. A solution was obtained in the orthorhombic space group I2₁2₁2₁, which revealed a 1-D CP of composition [Ag((R)-L₁)] (4) (Fig. 4). The structure contains Ag^I centres bound to the pyridyl nitrogen atoms of two (R)-L₁ ligands. Each (R)-L₁ adopts a bent conformation and links two Ag^I centres to form a chain (Fig. 4a). In contrast to 1–3, no looping 1-D chain structure is obtained due to the 2-c linear coordination of the Ag^I centre.

Fig. 4.

Crystal structure of [Ag((R)-L₁)] (4) depicting (a) the 1-D zig-zag where both the Ag^I and (R)-L₁ ligands act as two-connecting linkers, (b) the extended strips formed by half of the chains where chains belonging to adjacent layers are shown in red, green and blue and (c) the triple helix formed by half of the chains in C9, with the three chains shown in red, green and blue and the bent Ag^I centres represented in grey. (d) A view of the helix down the a axis. (e) A view along the a axis of the crystal packing. The extended strips are shown in blue and the triple helices in red.

The structure contains two crystallographically distinct types of 1-D chains, which differ markedly in their packing arrangement. The first type consists of (R)-L₁ ligands linking linear Ag^I centres to form extended strips that run parallel to the c axis. Adjacent chains pack along the a axis to form layers, which stack in an ABAB manner along the b axis (Fig. 4b). In the other half of the chains, the Ag^I surprisingly adopts a bent coordination geometry, resulting in the formation of a helical structure. Three separate chains are packed to form a triple helix, with the helical axis running parallel to the a axis (Fig. 4c, d). Extensive π–π interactions are present along the triple helix with the pyridyl rings stacked on top of naphthyl rings from an adjacent 1-D chain in a slightly offset manner. Cation–π interactions between the Ag⁺ ion and the pyridyl rings are present, where the Ag⁺ is sandwiched between two pyridyl rings. The helices partially occupy the spaces between adjacent layers, with the helical axis perpendicular to the direction of the zig-zag chains in each layer. This results in a structure with channels running along the a axis, which make up 23.6% of the structure (Fig. 4e).

Conclusion

Four 1-D chains containing bent 1,1′-binaphthyl ligands with dipyridyl and dicarboxylate coordinating groups were synthesised. Three Ni^II and Cu^II 1-D looping chains 1–3 were obtained containing 1,1′-binaphthyl ligands with both the dipyridyl, 1 and 2, and dicarboxylate in framework 3. The packing of the 1-D chains differed between the use of the dipyridyl and dicarboxylate ligands, where 1 and 2 pack in an AABB manner whereas 3 yields an ABAB packing. Ag^I yielded two 1-D chains in 4 – one a zig-zag chain and the other a bent helical chain. Relatively large accessible void volumes were present in frameworks 1–4 enabled by the introduction of an ethoxy group on the 1,1′-binaphthyl, highlighting how minimising opportunities for hydrogen bonding and increasing the steric bulk can help to prevent close packing of 1-D chains. Studies are ongoing with luminescent CPs containing 1,1′-binaphthyl for chiroptical fluorescence sensing.

Experimental

All chemicals and solvents were used as obtained and without further purification. 4,4′-Dibromo-2,2′-binaphthol,²⁷ (S)-6,6′-dicarboxyl-2,2′-diethoxy-1,1′-binaphthalene (S)-L₂^25,26 and Pd(PPh₃)₄²⁸ were synthesised according to literature procedures. (R)-4,4′-(2,2′-diethoxy-[1,1′-binaphthalene]-6,6′)dipyridine (R)-L₁ was synthesised following a literature method²⁹ with modification from (R)-6,6′-dibromo-2,2′-diethoxy-1,1′-binaphthalene. FT-IR spectra were measured on a Bruker Alpha spectrometer between 4000 and 400 cm⁻¹ with 4 cm⁻¹ resolution, using 32 scans and normalised as absorbance spectra. TGA was conducted on either a Mettler Toledo TGA/SDTA851 instrument using aluminium crucibles as sample holders or a PerkinElmer TGA 8000 instrument using ceramic sample holders under high-purity nitrogen. A typical analysis involved heating the sample to 450°C with a temperature increment of 4°C min⁻¹. Microanalysis was performed at the Chemical Analysis Facility – Elemental Analysis Service in the Department of Chemistry and Biomolecular Science at Macquarie University, Australia.

Single crystal X-ray diffraction data were collected on either the MX1 or MX2 beamline at the Australian Synchrotron.^30,31 In general, single crystals were transferred directly from the mother liquor into immersion oil and placed under a stream of nitrogen at 100 K. Crystal structures were solved by direct methods using the program SHELXT³² (ver. 2019/3, see https://shelx.uni-ac.gwdg.de/SHELX/) and refined using a full matrix least-squares procedure based on F² (SHEXL),³³ within the Olex2³⁴ GUI program (ver. 1.5, see https://www.olexsys.org/olex2/). In structures containing disordered solvent molecules that could not be satisfactorily modelled, the solvent mask routine within the Olex2 GUI was used.³⁴

Supplementary material

Crystallographic and structural details, continuous shape analysis, PXRD, TGA and ATR-FTIR spectra are available in the Supplementary material. The crystallographic information files can be obtained from the Cambridge Structural Database: CCDC 2340981–2340984. Supplementary material is available online.