Molecular tectonics: from 1-D interwoven racemic chains to quadruple-stranded helices

Abstract

The combination of two positional isomers tectons 1 and 2, based on a racemic 1,1′-spirobi(indane) scaffold bearing two pyridine units, with HgCl₂ affords doubly interwoven and quadruple-stranded helical architectures, respectively.

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The generation of helical assemblies is a topic of interest. These architectures, discrete or infinite in dimension, may either be purely organic or hybrid in nature engaging both organic parts and metallic centres.¹ For the design of infinite helical coordination strands,² which can be described as 1-D networks,³ one may apply the tectonic⁴ approach in designing specific helix inducing tectons. For example, the use of 1,1′-binaphthyl,⁵isomannide⁶ or calixarene⁷ based tectons has been reported. Another possibility consisting of introducing secondary interacting sites, such as oligoethylenglycol fragments, within the spacer connecting the primary coordination sites has also been explored.⁸ Another feature associated with the formation of helical architectures is the number of strands composing the assembly. Although single, double, and triple helices are relatively well known,^2b there are very few examples of higher stranded assemblies.⁹

Here we report the design and synthesis of tectons 1 and 2 (Scheme 1) based on a 1,1′-spirobi(indane) scaffold bearing two pyridine units, as well as the formation of helical architectures by their combination with HgCl₂.

Scheme 1

For the construction of helical architectures, the rigid 1,1′-spirobi(indane) is an interesting backbone.¹⁰ This unit is chiral and, due to the imposed angle between the two aromatic rings by the spiro centre, may be regarded as an analogue of binaphthol. We and others have already reported helical structures based on such a unit.⁵

The 1,1′-spirobi(indane) scaffold was functionalised with two pyridines as monodentate coordinating sites affording tectons 1 and 2. In both cases, the junction between the backbone and the pyridyl moiety was ensured by an ester group. Tectons 1 and 2 only differ by the position of the attachment of the pyridyl group to the spirobi(indane) moiety (position 4 for 1 and 3 for 2). For the synthesis of 1 and 2 as racemic mixtures, the starting material was bisphenol A, 4, which upon heating at 135 °C in the presence of methane sulfonic acid affords the diol3 in ca. 20% yield.¹⁰ In dry THF, the condensation of 3 with either the hydrochloride salt of isonicotinoyl chloride or nicotinoyl chloride in the presence of Et₃N afforded the desired compounds 1 and 2 in 57 and 49% yields, respectively as racemic mixtures.‡

Since both 1 and 2 are neutral tectons, HgCl₂ was used as the inorganic tecton for the generation of neutral helical strands.

At room temperature, a solution of tectons 1 or 2 in CHCl₃ was layered with DMSO and then a solution of HgCl₂ in EtOH was carefully added. Upon slow diffusion, colourless prism-type crystals, insoluble in common solvents, were obtained after several days. In both cases, crystals were analysed by X-ray diffraction on single crystals (XRD)†§ and by X-ray diffraction on powder (PXRD).

For 1·HgCl₂, the structural study revealed that the crystal (Triclinic, P1̄) was composed of both enantiomers of 1 (Fig. 1a) and HgCl₂. The interconnection of consecutive tectons 1 with the same chirality by HgCl₂ leads to the formation of both P and M helical strands (Fig. 1b and c). For the 1,1′-spirobi(indane) backbone, an angle of 70.7° between the two phenyl groups is observed. The Hg²⁺ cation adopts a silghtly distorted square pyramidal geometry and its coordination sphere is composed of 2 Cl⁻ anions (d_Hg–Cl = 2.34–2.35 Å, ClHgCl angle of 167.5°) and 3 N atoms (d_Hg–N = 2.59–2.72 Å, NHgN angles of 83.4° and 171.4°) belonging to three different tectons 1 (Fig. 1e). Among the 3 N atoms present in the coordination sphere of the Hg²⁺ cation, two belong to two consecutive tectons 1 of the same chiralty generating the helical strand (Fig. 1b–c) and the third one belongs to a second type of tecton 1 (angle between the two phenyl groups of the 1,1′-spirobi(indane) moiety of 78.8°) possessing the opposite handedness (Fig. 1d–f). The latter type behaves as an appended ligand and only one out of the two pyridine units are engaged in the binding of Hg²⁺ cation belonging to the helical strand (Fig. 1d). This is probably due to the rigidity of the backbone and to the junction between the scaffold and the pyridyl moiety in position 4. The overall structure is a 1-D interwoven architecture composed of two enantiomorphous helical strands (Fig. 1g).

Fig. 1 Schematic representation of the P (blue) and M (pink) helical configuration adopted by the tecton 1 (a), a portion of the helical strand generated upon bridging of consecutive tectons 1 by HgCl₂ (b) and its schematic representation (c), schematic representation of helical strand bearing appended tectons 1 (d: perpendicular view and f: view along the helix axis), a portion of the structure (e) and a portion of the interwoven architecture (g). H atoms are omitted for clarity.

For the second combination, i.e. tecton 2 and HgCl₂, the structural analysis revealed that the crystal (trigonal, space group R3̄c) is composed of 2, HgCl₂, 2H₂O and 2CHCl₃solvent molecules. The organic and inorganic tectons 2 and HgCl₂ form helical strands (Fig. 2a) with opposite handedness. This is not surprising, since for 2, a racemic mixture of both enantiomers was used and consequently both P and M helical strands are found in the crystal. The metric analysis presented here concerns the strand with P helicity. As expected, except for the handedness, the same analysis holds for both strands. Within the helical strand composed of the same enantiomer, a torsion angle of 87.5° between the two phenyls is observed for the central 1,1′-spirobi(indane) scaffold. The two 3-pyridyl units connected to the backbone through ester junctions (d_C–O = 1.36 Å and d_C=O = 1.20 Å) are divergently oriented towards the concave face of the scaffold and tilted by 18.7° (Fig. 2a).

Fig. 2 Portions of the structure of 2·HgCl₂ showing the chiral tecton 2, the helical strand generated upon interconnection consecutive tectons 2 by HgCl₂ (a), the quadruple-stranded helical architecture (b) and its schematic representation (c). Although both P and M helices are present in the crystal, arbitrary only the P handedness is presented. For clarity, different strands are differentiated by colour. H atoms and solvent molecules are omitted.

The Hg²⁺ cation adopts a distorted tetrahedral geometry and its coordination sphere is composed of two Cl⁻ anions (d_Hg–Cl = 2.34 Å and ClHgCl angle of 154.5°) and the two N atoms (d_Hg–N = 2.45 Å, NHgN angle of 89.5° and ClHgN angle varying between 97.6° and 100.4°). Owing to the nearly perpendicular disposition of the spiro-annulated rings along the helical axis, the minimum translation unit (one tecton and one HgCl₂) generates a helical rotation angle of 120° and thus the helical pitch of the helix, composed of three translation units, is ca. 43.47 Å (Fig. 2a). Probably, as a result of the long pitch, four strands of the same handedness form a quadruple stranded helical arrangement (Fig. 2b–c) with weak interstrand hydrogen-bonds between nicotinoyl units of different strands (distance between O and C atoms of ca. 3.14 Å). The quadruple stranded helices are not cylindrical but triangular in shape and are packed in a hexagonal fashion with alternated P and M chirality without any specific interactions between them (Fig. 3a–b).

Fig. 3 The cross section of the packing of 2·HgCl₂ trigonal quadruple-stranded helices (a) and its schematic representation (b). For clarity, the P (blue) and M (pink) quadruple stranded helices are differentiated by colour. The Cl atoms of CHCl₃ located at the centre of the hexagons are coloured in green.

A view of the cross section parallel to the helical axis of the hexagonal packing of the triangular quadruple helical strands with opposite handedness (3P and 3M in alternate disposition), shows that each triangular column of a given chirality is in contact with three columns of the opposite chirality (Fig. 3b).

The packing of quadruple helical strands generates two types of channels. The inner triangular channels, with a side length of 8.8 Å, are empty, whereas the hexagonal channels with a side length of ca. 6.2 Å are found to be filled with two chloroform and two water molecules per mercury cation without any specific interaction with the helical walls.

The thermal stability of the crystalline materials was investigated by TGA which revealed that whereas 1·HgCl₂ is stable up to ca. 250 °C, 2·HgCl₂ starts to decompose at ca. 200 °C (Fig. 4).

Fig. 4 TGA traces for 1·HgCl₂ and 2·HgCl₂. For the latter case, the sample was dried in air for ca. 1 h.

For the two crystalline materials, the purity was checked by PXRD which revealed a single set of diffraction peaks matching the simulated patterns (Fig. 5).

Fig. 5 Comparison of the simulated (a and c) and recorded (b and d) PXRD patterns for 1·HgCl₂ and 2·HgCl₂, respectively.

In conclusion, the 1,1′-spirobi(indane) backbone is a helix directing unit. The combination of racemic pyridine based tectons 1 and 2 with HgCl₂, depending on the position of attachment of the coordinating sites to the scaffold, leads to the formation of either a double stranded interwoven helical arrangement or to a quadruple-stranded metallaorganic helix. The synthesis and propensity to generate helical architectures by other 1,1′-spirobi(indane) derivatives as well as combinations of 1 and 2 with other metallatectons are currently under investigation.

We thank the Université de Strasbourg, the Institut Universitaire de France, the Ministry of Education and Research, the CNRS and Marie Curie Est Actions FUMASSEC Network (Contrat N° MEST-CT-2005-020992) for financial support.

Notes and references

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Footnotes

† CCDC 743764 and 743765. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b916249f

‡ General method for the synthesis of organic tectons1 and 2: Under argon and at rt, to a degassed solution of 3¹⁰ (1.54 g, 5 mmol) in dry THF (20 mL), the hydrochloride salt of isonicotinoyl chloride or nicotinoyl chloride (10 mmol) was added and the mixture was stirred at rt for 15 min before Et₃N (5 mL) was added and stirring was further continued for two days. After evaporation to dryness, a saturated aqueous solution of Na₂CO₃ (40 mL) was added to the residue and the mixture extracted with CHCl₃ (3 × 30 mL). The organic solvent was removed and the residue purified by column chromatography [SiO₂, CHCl₃], affording the pure compounds 1 and 2, respectively, as white powders. Tecton 1 (1.48 g, 57%): ¹H-NMR (300 MHz, 298 K, CDCl₃): δ [ppm] = 1.27–1.41 (d, 12H, J = 12), 2.30–2.45 (dd, 4H, J = 19.5, 13.2), 6.68 (d, 2H, J = 2.4), 7.1 (dd, 2H, J = 6, 2.1), 7.24 (m, 2H), 8.0 (dd, 4H, J = 3.0, 1.5), 8.8 (dd, 2H, J = 2.7, 1.5); ¹³C-NMR (75 MHz, 298 K, CDCl₃): δ [ppm] = 30.2, 43.3, 57.6, 59.4, 116.9, 120.4, 122.9, 123.1, 127.9, 149.9, 150.3, 150.8, 151.7, 163.9; Tecton 2 (1.27 g, 49%): ¹H-NMR (300 MHz, 298 K, CDCl₃): δ [ppm] = 1.38–1.42 (d, 12H, J = 12), 2.31–2.46 (dd, 4H, J = 16.8, 13.2), 6.69 (d, 2H, J = 2.1), 7.07–7.10 (dd, 2H, J = 6, 2.1), 7.21–7.24 (d, 2H, J = 8.4), 7.40–7.44 (m, 2H), 8.37–8.41 (m, 2H), 8.81–8.83 (m, 2H), 9.33–9.34 (m, 2H); ¹³C-NMR (75 MHz, 298 K, CDCl₃): δ [ppm] = 30.3, 43.3, 57.6, 59.4, 117.1, 120.6, 122.9, 123.4, 125.6, 137.5, 150.0, 150. 1, 151.7, 153.9, 164.0.

§ Data were collected at 173(2) K on a Bruker APEX8 CCD Diffractometer equipped with an Oxford Cryosystem liquid N₂ device, using graphite-monochromated Mo-Kα (λ = 0.71073) radiation. For both structures, diffraction data were corrected for absorption. The structures were solved using SHELXS-97 and refined by full matrix least-squares on F² using SHELXL-97.¹¹ The hydrogen atoms were introduced at calculated positions and not refined (riding model). Data for 1·HgCl₂: C₆₆H₆₀Cl₂HgN₄O₈, M = 1308.67, Triclinic, space groupP1̄, a = 13.1808(12) Å, b = 14.0766(12) Å, c = 17.3666(15) Å, α = 67.326(2)°, β = 89.185(2)°, γ = 76.700(3)°, V = 2883.5(4) ų, T = 173(2) K, Z = 2, Dc = 1.507 g cm⁻³, μ = 2.822 mm⁻¹, 20 466 collected reflections, 12 910 independent (R_int = 0.0379), GOF = 1.062, R₁ = 0.0622, wR₂ = 0.1613 for I > 2σ(I) and R₁ = 0.0888, wR₂ = 0.1738 for all data. 2·HgCl₂: 6(C₃₃H₃₀Cl₂HgN₂O₄)·2CHCl₃·2H₂O, M = 4983.25, Hexagonal, space group R3̄c, a = b = 39.9988(10) Å, c = 10.8667(3) Å, α = β = 90°, γ = 120°, V = 15056.4(7) ų, T = 173(2) K, Z = 3, Dc = 1.649 g cm⁻³, μ = 4.878 mm⁻¹, 120 760 collected reflections, 3864 independent (R_int = 0.0305), GOF = 1.088, R₁ = 0.0285, wR₂ = 0.0750 for I > 2σ(I) and R₁ = 0.0414, wR₂ = 0.0881 for all data.

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