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DOI: 10.1039/D4DT01903B (Paper) Dalton Trans., 2024, 53, 15258-15263

De novo fabrication of higher arene ring incorporated contorted calix[2]phyrin(2.2.1.1.1) and its F− bound complex†

Sourav Ranjan Pradhan a, Chetan Kumar Prasad a, Mainak Das *b and A. Srinivasan *a
aNational Institute of Science Education and Research (NISER), An OCC of Homi Bhabha National Institute, Bhubaneswar 752050, Odisha, India. E-mail: srini@niser.ac.in
bDepartment of Chemistry, University of York, Heslington, York YO10 5DD, UK. E-mail: mainak.das@york.ac.uk

Received 1st July 2024 , Accepted 19th August 2024

First published on 20th August 2024

Abstract

The rational design and synthesis of a novel contorted calix[2]phyrin(2.2.1.1.1) structure has been achieved, utilizing a terphenyl unit as a key building component. This terphenyl unit serves as a segment of the armchair periphery in a π-extended two-dimensional architecture. The resulting molecule exhibits remarkable properties, including the ability to self-assemble into solid-state supramolecular nanotubes. Additionally, it has demonstrated an affinity for complexation with fluoride anions, highlighting its potential for applications in molecular recognition and sensor technology. The incorporation of the terphenyl unit not only enhances the structural rigidity but also contributes to the unique electronic characteristics of the calix[2]phyrin.

Introduction

Polycyclic aromatic hydrocarbons (PAHs) have garnered significant attention owing to their potential applications and their role as a platform for fundamental investigations into unsaturated and conjugated derivatives.1–3 The arrangement of sp2 hybridized carbons facilitates a seamless connection of benzene and/or arene-like structures in the form of either an acene (one-dimensional)4 or graphene (two-dimensional)5 unit wherein both global6 and local conjugation7 effects influence the observed behaviours. In particular, graphene and/or graphene-resembling structures manifest specific reactivity patterns at the edges depending on the type or topology i.e. armchair vs. zigzag (Fig. 1, left).8 The quantum confinement effects observed in carbon armchair graphene nanoribbons result in the emergence of significant electronic bandgaps, which are intricately tied to their specific structural boundary configurations,9a,b whereas, nanoarchitectures featuring zigzag edges are anticipated to harbor spin-polarized electronic states localized along the edges, thus found suitable in graphene-based spintronics.9c The characteristics of graphene alike materials can be finely tuned through the edge-functionalization with suitable molecular entities10 through covalent11 or non-covalent means,12 which enhances the desirable properties of graphene. However, surprisingly edge functionalization of graphene-based structures in general and specifically for the armchair topology within macrocyclic constraints is nearly unknown. Thus, following those observations, an archetypal motif of one of the higher arene units which is the terphenyl moiety as a minuscule fragment of the armchair edge of the π-extended 2D structure is explored as a building block, incorporated into macrocyclic skeletons and linked with the dipyrromethane (DPM) moiety (Fig. 1, middle). The integration of the polyphenyl unit in a relatively small association leads to the generation of internal core steric strain and to mitigate that (a) one of the pyrrole rings of the DPM unit is inverted outside and (b) the entire macrocyclic structure is distorted with respect to the molecular plane. In consequence, a contorted terphenyl-macrocycle hybrid (1) has been reported, which can be designated as a carba analogue of calixphyrin (Fig. 1, right). Calixphyrins are hybrid macrocycles composed of both sp2- and sp3-hybridized carbon atoms, exhibiting structural resemblances to porphyrins and calixpyrroles.13 A diverse array of calixphyrins has been developed, including normal,14 N-confused,15 core-modified,16 arene ring-embedded,17 three-dimensional (3D),18 and expanded variants.19 However, calixphyrins incorporating higher arene units are rarely reported. Recently, we synthesized an (o–p–o)-terphenyl-embedded calix[2]phyrin(2.2.1.1.1) and utilized its macrocyclic core to stabilize the Cu(II) metal ion.20 The anion-binding properties of calixphyrins with higher arene units remain unexplored. Compared to our previously reported calix[2]phyrin(2.2.1.1.1),201 offers greater conformational flexibility due to the increased presence of sp3-hybridized meso carbons in the macrocyclic core. As such, compound 1 holds promise as a macrocyclic host, potentially serving as both an anion-binding agent and an ion-pair container.
image file: d4dt01903b-f1.tif
Fig. 1 Representation of a π-extended graphene-based structure on the left, fabrication strategy (middle), and structure of macrocycle 1 on the right.

Conversely, the detection of fluoride ions among biologically important anions has received considerable research attention due to their unique attributes, such as high charge density and hard Lewis basic nature.21 Herein, utilizing the tailored core of the calix[2]phyrin(2.2.1.1.1) (1) we wish to report the recognition and structural evidence of an F− bound complex (1.F).

Results and discussion

The synthesis of the proposed calix[2]phyrin(2.2.1.1.1) (1) macrocycle is delineated in Scheme 1. The p-toluenesulfonic acid (p-TSA)-catalyzed condensation of 2[thin space (1/6-em)]20 with 3-pentanone in dichloromethane as a solvent at room temperature under an inert atmosphere followed by chromatographic separation afforded 1 in 20% yield. The high resolution-electrospray ionization (HR-ESI) mass spectrometric analysis of 1 displayed the molecular ion signal at m/z = 609.3300 [M + 1] (Fig. S1†), and ascertained the exact composition.
image file: d4dt01903b-s1.tif
Scheme 1 Synthetic approach for the construction of 1. Conditions: (i) 3-pentanone, p-TSA, CH2Cl2, RT, and 2 h.

The 1H NMR spectrum of 1 in CD2Cl2 at 298 K is displayed in Fig. 2a. Owing to C2V symmetry, signals for only half of its structure are observed. The pyrrolic β-CHs [H(11,15)] and [H(10,16)] appeared as doublets at 5.31 and 5.79 ppm. The NH appeared as a broad signal at 7.01 ppm (Fig. S3†). The terminal ortho-phenylene protons of the terphenyl building block, resonated as a multiplet at 7.23 to 7.15 ppm [H(4,6,20,22)], a doublet at 7.11 ppm [H(3,23)] and a triplet at 6.81 ppm [H(5,21)], while the inner [H(26,27)] and outer protons [H(28,29)] of the p-phenylene unit appeared as a singlet at 7.45 ppm. The protons stem from the macrocyclic core concomitant (a) phenylene ring (Ph) resonated as a multiplet at 7.32 to 7.25 ppm and (b) H(8,18) appeared as a singlet at 6.18 ppm. The protons of the diethyl moiety from the pentanone group resonated at 1.61 (H38) and 1.90 (H37) as a quartet and H(36,39) at 0.61 ppm appearing as a triplet. All these signals were deduced using 1H–1H correlation spectroscopy (COSY) analysis (Fig. S4†).


image file: d4dt01903b-f2.tif
Fig. 2 1H NMR spectra of (a) 1 and (b) 1.F in CD2Cl2 at 298 K.

The 13C NMR analysis suggests that the connecting carbon atoms (i) between the terphenyl motif and dipyrromethane moiety [C(8,18)] and (ii) two amide pyrrole rings (C13) resonated at 45.82 and 42.47 ppm, respectively, indicating sp3 hybridization (Fig. S5†). Among them, [C(8,18)] showed a correlation with [H(8,18)] obtained from the 1H–13C HSQC NMR experiment (Fig. S7†). The presence of sp3 hybridized linking carbon atoms [C(8,13,18)] hindered the global macrocyclic π-conjugation and thus, attained the nonaromatic character.

To gain more insight into the structural details of 1, single crystal X-ray diffraction (SC-XRD) analysis was performed (Fig. 3). The crystal was grown in a solvent mixture of CH2Cl2/n-pentane and crystallizing in a monoclinic crystal lattice with the P21/c space group (Table S1†). As anticipated from spectral analysis, the macrocycle comprises a terphenyl unit adjoined with pyrrole rings [N1 and N2] by sp3 carbon atoms [C8 and C18] with bond distances of 1.531(2) Å (C7–C8) & 1.499(2) Å (C8–C9), and 1.497(2) Å (C17–C18) & 1.521(2) Å (C18–C19). Interestingly, the N1 pyrrole ring protruded outside the macrocyclic cavity while the N2 pyrrole unit was oriented in the reverse direction. These are connected by another sp3 hybridized carbon atom C13 through bond distances of 1.518(2) Å (C12–C13) & 1.510(2) Å (C13–C14). To ease core steric repulsion, 1 embraces a gable type of structure, resulting (a) in opposite orientations of both joining carbons C8 and C18, and C7–C8–C9 & C17–C18–C19 exhibit twist angles of 110.40(1)° & 109.80(1)°, respectively; (b) the central p-phenylene unit of the terphenyl building block is tilted by 82.90(3)° compared to peripheral o-phenylene moieties [for 37.50(3)° and 46.10(3)°] and (c) pyrrole units deviate by 63.09(4)° (N1) and 76.45(4)° (N2) from the mean molecular plane encompassing 17 inner core atoms.


image file: d4dt01903b-f3.tif
Fig. 3 SC-XRD structure of 1. Color code: C, grey; N, blue; H, white. (a) Top view and (b) side view. Peripheral hydrogen atoms in (a), (b) and meso-phenyl groups in (b) are omitted for clarity.

Hence, due to the presence of twisted heterocyclic and phenyl rings, the entire molecular structure attains a contorted shape, and each unit maintains its individual aromatic character (Fig. S16†). Furthermore, the analysis of the solid-state superstructure reveals the formation of the tubular architecture along the c-axis (Fig. S14†), directed by C–H⋯π interactions (C32–H32⋯π: 2.935 Å and 167.53° & C2–H21⋯π: 2.644 Å and 161.30°) (Fig. S15†). The individual supramolecular nanotubes22 are well-separated with a distance of 3.870 Å [Cmiddle-terphenyl(–H4)⋯Cmiddle-terphenyl(–H4)] signifying a negligible interaction between them which is akin to an iodo derivative of a molecular triangle.23

Using a custom-designed 1 core, preliminary anion binding studies were conducted via UV-vis spectroscopic titrations in acetonitrile solution with several anions, including F−, Cl−, Br−, I−, OH−, SCN−, and ClO4− as their tetrabutylammonium (TBA) salts (Fig. S18 & S19†). These studies revealed anion binding affinities of the macrocycle 1, in particular towards fluoride ions. However, the response was non-colorimetric, as 1 exhibited absorption bands at 252 and 264 nm, and upon the addition of TBAF, a broad band appeared near 363 nm, highlighting complete absorption in the UV region for the host–guest complex (Fig. 4). Subsequently, titration experiments were performed by gradually adding fluoride ions to assess their binding constants (Fig. S20†), which revealed that 1 bound fluoride ions in a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 ratio and the binding constant was Ka = 7.53 ± 7.09 × 104 M−1 (Fig. S21 & S22†).


image file: d4dt01903b-f4.tif
Fig. 4 UV/Vis spectra of 1 (navy) and 1.F (wine) (acetonitrile, 298 K).

The 1H NMR titration studies were conducted in CD2Cl2 at 298 K (Fig. S8 and S9†). Upon addition of 20 equivalents (ca. excess) of tetrabutylammonium fluoride salt, 1.F exhibited a significant downfield shift of the NH signal, resonating at 12.62 ppm (Fig. 2b), indicating strong hydrogen bonding interactions between F− and pyrrolic NH. Additionally, minor upfield shifts were observed for adjoining protons [H(8,18)] and the β-CHs of the pyrrole rings [H(10,16)] and [H(11,15)], appearing at 6.15, 5.73, and 5.25 ppm, respectively. This suggests a potential conformational change of 1 upon anion binding. However, the relatively rigid terphenyl moiety of the macrocyclic structure did not undergo significant positional changes, and thus, the related proton signals retained their original peak positions. The assignment of hydrogen atoms for 1.F was further confirmed by 1H–1H correlation spectroscopy (COSY) analysis (Fig. S10†).

Structural evidence of the binding between F− and 1 was obtained through single-crystal X-ray diffraction (Fig. 5a). The crystal was grown in a mixture of CH2Cl2 and n-hexane solvent, with an excess of TBAF. 1.F crystallizes in a monoclinic crystal lattice with a space group of P21/n (Table S1†). Analysis of the crystal unit cell of 1.F was performed using the Hirshfeld surface (HS) (Fig. 5b).24 By categorization of all atom⋯atom distances closer than the sum of van der Waals radii in a histogram (Hirshfeld surface fingerprint), it was found that the fluoride counterion of tetrabutylammonium salts formed a host–guest complex with 1, along with an entrapped water molecule, through intermolecular hydrogen bonding interactions (Table S2†). The major interactions occurred between the F− anion and pyrrolic NH (H1 and H2) [1.793(3) Å and 1.803(3) Å & 166.26(3)° and 161.56(3)°], H53B and H58A of the butyl chain [2.294(3) Å and 2.293(3) Å & 147.50(4)° and 139.55(4)°], and H1B from water via a bond distance and an angle of 1.744(2) Å and 178.29(3)°, respectively. The F− ion was located 1.42 Å above the mean plane containing 17 inner core atoms. Upon complexation with one fluoride ion, host 1 undergoes several changes, such as (i) assembling both pyrrole moieties (N1 and N2) in the same direction; (ii) conformational alteration of the macrocyclic structure to attain a cone shape, and (iii) disappearance of the solid-state supramolecular columnar organization of 1.


image file: d4dt01903b-f5.tif
Fig. 5 SC-XRD structure of 1.F. (a) Top view (peripheral hydrogens are omitted for clarity); color code: C, grey; N, blue; H, white; F, yellow; and O, red; (b) Hirshfeld surface of 1.F with the property dnorm color-coded onto it. Red, white, and blue areas indicate distances lower, commensurate, or longer than the sum of van der Waals radii.

Conclusion

In conclusion, we have successfully synthesized a contorted calix[2]phyrin(2.2.1.1.1) (1) macrocycle by combining a higher arene unit and a substituted dipyrromethane moiety, which propagates a nanotubular arrangement through C–H⋯π interactions. The straightforward structural design lays the foundation for stabilizing fluoride ions through intermolecular hydrogen bonding interactions. Our current efforts are directed toward examining the interaction with other anions and biologically important cations.

Experimental section

General considerations

The reagents and materials for the synthesis were used as obtained from Sigma Aldrich chemical suppliers. All solvents were purified and dried by standard methods prior to use. The NMR solvents were used as received and the spectra were recorded in a Bruker 400 MHz spectrometer with TMS as an internal standard. The ESI (HR-MS) mass spectra were recorded in a Waters, Xevo G2-XS QToF mass spectrometer. The electronic absorption spectra were recorded in a PerkinElmer-Lambda 750 UV-Visible spectrophotometer. The X-ray quality crystals of 1 and 1.F were grown by slow diffusion of n-pentane over CH2Cl2 solution. Single crystal X-ray diffraction data of 1 and 1.F were collected in a Rigaku Oxford Diffraction single crystal X-ray diffractometer with CrysAlisPro and CuKα (λ = 1.54184). The crystals have been deposited in the Cambridge Crystallographic Data Centre with reference no. CCDC 2350180 (1) and CCDC 2350181 (1.F).†

In order to evaluate the overall binding strength, the titration results were fitted to the nonlinear form of the Hill equation.25,26

image file: d4dt01903b-t1.tif
where ΔA (= Aobs − A0) is the change in absorbance, ΔAmax is the maximum change of absorbance, [L]0 is initial guest concentration, n is the Hill coefficient, and Ka is the association constant. A plot of ΔA against [L]0 can be used to estimate ΔAmax and Ka. The titration data were fit to this model using the nonlinear regression method within the Origin 9 software. Stock solution of 1 was prepared in CH3CN at a concentration of ∼131 × 10−6 M. The initial concentration of TBAF used for titration and binding experiments is 3.4 × 10−3 M.

Syntheses and spectral characterization

Synthesis of 1. Terphenyl dipyrromethene (2)20 (150 mg, 0.29 mmol) was dissolved in 100 ml CH2Cl2 solution and stirred under a N2 atmosphere covered with aluminium foil. Pentan-3-one (0.034 ml, 0.33 mmol) was added to it under the same conditions and stirred for 10 min at room temperature. After 10 min, p-toluenesulfonic acid (24 mg, 0.14 mmol) was added and stirred for 2 h. The formation of the product was monitored by TLC. After 2 h, column chromatography was performed to purify the compound using neutral alumina. The yellow band was eluted with 20% CH2Cl2/n-hexane and identified as 1. The compound was further recrystallized from CH2Cl2/n-pentane to afford a yellow crystalline solid of 1 in 20% yield (34 mg, 0.056 mmol). 1H NMR (400 MHz, CD2Cl2) δ 7.45 (s, 4H), 7.32–7.25 (m, 10H), 7.23–7.15 (m, 4H), 7.11 (d, J = 7.3 Hz, 2H), 7.01 (br s, 2H), 6.81 (t, J = 7.30 Hz, 2H), 6.18 (s, 2H), 5.79 (d, J = 4.6 Hz, 2H), 5.31 (d, J = 4.6 Hz, 2H), 1.90 (q, J = 14.1, 7.2 Hz, 2H), 1.61 (q, J = 14.0, 7.4 Hz, 2H), 0.61 (t, J = 7.4 Hz, 6H). 13C NMR (101 MHz, CD2Cl2) δ 143.55, 141.22, 139.74, 138.08, 136.07, 132.09, 131.75, 130.48, 130.33, 129.01, 128.29, 127.29, 126.62, 126.40, 126.31, 106.54, 104.57, 102.58, 45.82, 42.47, 27.58, 26.60, 7.56. HRMS (ESI-MS) m/z: calculated for C45H40N2 = 608.3191; found = 609.3300 [M + H]+. UV-Vis (CH2Cl2): λmax(nm) (ε[M−1 cm−1] × 104) = 252 (4.47), 264 (3.89). M.p.: 190 °C (decomposition).

1.F

19F NMR (376 MHz, CD2Cl2) δ 127.98 (s, 1F). UV-Vis (CH2Cl2): λmax(nm) (ε[M−1 cm−1] × 104) = 252 (7.37), 266 (10.19), 363 (1.13).

Author contributions

S. R. P. carried out synthesis and compound characterization. C. K. P. performed theoretical calculations. M. D. solved X-ray crystal structures. S. R. P., M. D. and A. S. wrote the manuscript and S. R. P. prepared the ESI.† All authors analyzed the data and commented on manuscript drafts. M. D. and A. S. conceived and directed the research.

Data availability

The data published in this contribution are available as the ESI,† submitted with the manuscript.

Crystallographic data have been deposited with the Cambridge Crystal Structure Database (CCDC).

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

Prof A. S. thanks NISER, Department of Atomic Energy (DAE), India, and CSIR New Delhi, India (No. 01(3070)/21/EMR-II) for financial support. Mr S. R. P. thanks CSIR, India, for his fellowship.

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedure, characterization of all new compounds and crystallographic data. CCDC 2350180 (1) and 2350181 (1.F). For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4dt01903b
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