The exchangeable self-assembly behaviour of bis-pseudorotaxanes with metallo-bisviologens (CH₃PyTpyFeTpyPyCH₃⁴⁺) and crown ether (Tpy = 2,2′:6′,2′′-terpyridine)

Abstract

Based on two newly synthesized terpyridine-(1-methyl-pyridiniums) and their metallo-complexes, the exchangeable self-assembly behaviours of metallo-bisviologens with dibenzo-24-crown-8 ether (DB24C8) are comprehensively investigated.

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Non-covalent self-assembly is the natural organization of small or biological molecules into relatively stable, well-defined structures.¹ The concept of self-assembly mainly originates from natural processes such as the folding of polypeptide chains into functional proteins, the formation of the DNA double helix, and the assembly of phospholipids into cell membranes. Construction of biological architectures that mimic naturally occurring assemblies has received considerable attention since Lehn introduced the concept of supramolecular chemistry.² It has been widely studied with the aim of developing sophisticated self-assembled structures from small building blocks by molecular recognition, self-replication, and self-organization based on non-covalent interactions.³ Among the types of non-covalent interactions possible, coordination bonding is one of the most common.⁴ Specifically, the use of 2,2′:6′,2′′-terpyridine–metal complexes as building blocks has attracted wide interest since several transition metals can coordinate with the terpyridine ligand, providing a wide range of bonding situations, resulting in a host of utilitarian applications.⁵ In addition, molecular recognition has attracted much attention in recent years for its great potential for biological, analytical, and environmental applications. For example: crown ether–organic viologen salt host–guest binding,^6,7 cyclodextrin based hydrophilic–hydrophobic interactions,⁸ and CO₂ linking with calixarene capsules via hydrogen bonding.⁹

Herein we report an approach that employs two types of non-covalent interactions for bispseudo-rotaxane formation and host–guest exchange between the rotaxanes. The rotaxane assemblies include: (1) a metallo-bisviologen based on the [CH₃PyTpyFeTpyPyCH₃]⁴⁺ dimer and DB24C8, and (2) a CH₃PyTpy⁺/DB24C8 rotaxane derived from the organo-viologen, followed by the addition of a metal ion to obtain a double metallo-rotaxane through dimerization. The incorporation of more than one type of self-assembly provides a potentially useful route to obtain supramolecular architectures that have higher levels of structural complexity, more compositional combinations and greater functional configurations.¹⁰

Inspired from Hayami's work¹¹ on the arylpyridine substituted terpyridines, this research started first from Tpy–Py ligand 1 which was obtained (71%) using an alternative synthetic approach (Scheme 1, see ESI†); after methylating the pyridine group, a terpyridine based organo-viologen {[(2,2′:6′,2′′-terpyridin)-4′-yl]-[(1,1′-biphenyl)-4-yl]-1-methylpyridin-1-ium} 3 was generated (92%). Reacting 3 with iron dichloride in MeOH, a purple colored solution immediately appeared, indicating the formation of the TpyFeTpy dimer 5 in nearly quantitative yield. ¹H NMR chemical shifts of 3 and 5 presented several characteristic changes. Firstly, the sharp singlet H^3′,5′ had a shift from 8.83 to 9.60 ppm after complexation; and the doublet H^6,6′′ appeared to shift from 8.75 in 3 to 7.40 in 5. The structures had also been confirmed by 2D NMR and mass spectroscopy.

Scheme 1 Structures of metallo-bisviologens 5 and 6, and the synthetic routes of (a) MeI, CH₃CN; (b) FeCl₂, MeOH.

To investigate a possible host–guest exchange, a new metallo-bisviologen 6 containing an anthracene stopper was designed to compare the complexing capability with DB24C8. Complex 6 was synthesized by a Suzuki coupling of 9,10-dibromoanthracene with one equivalent 4-pyridinylboronic acid to first form a 9-pyridine-10-bromoanthracene; after the similar Suzuki coupling with TpyB(OH)₃, methylation of the pyridine moiety and complexation with iron dichloride, the multi-ionic metallo-bisviologen[(2,2′:6′,2′′-terpyridin)-4′-yl]phenyl(anthracen-9-yl)-1-methylpyridin-1-ium] 6 was obtained as a dark purple solid (see ESI†). Due to the multi-ionic properties, 5 and 6 proved to have excellent solubility in MeOH and EtOH.

Combination of metallo-bisviologen 5 with a macrocyclic host, DB24C8, via 1 : 2 stoichiometry in MeOH afforded a self-assemblied bis-pseudorotaxane 8. Using a similar procedure, organo-bisviologen 6 was converted to bis-pseudorotaxane 9 (Scheme 2 and Fig. 1).

Scheme 2 Illustrating formation of bis-pseudorotaxanes using guest 5 and 6 and two equivalents of host 7.

Fig. 1 (A) ¹H-NMR of viologen 5 and rotaxane 8. (B) 2D COSY NMR of rotaxane 8 in CD₃OD at 25 °C.

The resulting supermolecular bis-pseudorotaxane 8 was characterized by ¹H-NMR, 2D COSY (500 MHz, in CD₃OD, Fig. 1), 2D NOESY-NMR (Fig. S9†) and Q-TOF-MS (Fig. S12†). The ¹H-NMR spectra exhibited a number of interesting trends that resulted from the interaction of the two non-covalently linked components. Firstly, when comparing to its precursor 5, chemical shift of H¹ (N-methyl protons) in 8 was 4.59 ppm, about 0.11 ppm lower in frequency than in 5 (4.48 ppm) which is attributed to hydrogen bonding between the N-methyl protons and oxygens of the crown ether, the stronger binding affinity gave a evidence for the formation of rotaxane.¹⁰ Secondly, H^a in 8 had shifted to 8.90 from 8.99 ppm, and H^b to 8.11 from 8.56 ppm in 5, respectively. The cationic viologens passed through the central cavity of the DB24C8, which led to interactions between the crown ether and the phenylpyridinium salt. Thus, the shielding effect caused the chemical shift of H^a and H^b protons to move upfield. Thirdly, the presence of two separate multiple resonances for H^Ph_24C8 at 6.87 and 6.84 ppm compared to 6.93 ppm for the free DB24C8, are indicative of π-stacking between pairs of electron-poor phenylpyridiniums and electron-rich crown aromatic rings. Q-TOF-MS was carried out with the complex 8 which showing peaks at m/z 477.160 and m/z 487.128 corresponding to [7₂·5]⁴⁺ and [7₂·5 + CH₃CN]⁴⁺, respectively, to give a direct evidence for the formation of the 2 : 1 complex 8. Also another two fragments were found including 5 at m/z 252.525 and [DB24C8·Na]⁺ at m/z 471.146, respectively, (Fig. S12†).

An alternative method of assembling host–guest 8 was achieved first through association of organo-viologen 3 (guest) with DB24C8 in precisely a 1 : 1 stoichiometry to obtain organo-pseudorotaxane 10 (equilibrium constant K_av = 228 ± 9 M⁻¹, Table S3†).¹⁴ The ¹H-NMR spectra of 3 and 10 (Fig. 2, also see ESI† for 2D COSY) showed that chemical shifts of H^a, H^b and H^c in 10 were all moved to higher frequencies, respectively. However, protons of N-methyl at 4.35 ppm in 10 compared to 3 wasn't moved during the host–guest assembling, along with the lower K_av value, which may elucidate a taco-type association between organic viologen 3 and crown ether.¹⁰ The presence of two separate resonances for H^Ph_24C8 at 6.91 and 6.86 ppm in 10 compared to 6.93 ppm in its free DB24C8 confirmed the formation of an host–guest association. Upon addition of FeCl₂ in MeOH to 10 in 1 : 2 stoichiometry, a purple color instantly occurred, indicating the formation of the TpyFeTpy complex. ¹H NMR and other spectroscopes verified the formation of metal-containing pseudorotaxane 8.

Fig. 2 Conceptual diagram of formation of rotaxane 8 (up), and comparing ¹H NMR of 3 and 10 in DMSO-D6 at 25 °C (bottom).

Metallo-bisviologen 6 possessed an anthracene moiety along with the pyridinium salt which could be used as a stopper to adjust the complexation with the host. When DB24C8 and 6 were mixed in 2 : 1 stoichiometry, the double metallo-rotaxane 9 was created. ¹H NMR showed a lower frequency chemical shift for H^a compared to 8 (Fig. 3); however, H^b and H^c still moved to higher fileds after associating with DB24C8, respectively. Due to the steric hindrance of anthracene, H^d, H^e in addition to other protons had kept their original positions. The structures of bisviologen 6 and metallo-rotaxane 9 had been also verified by 2D NMR, mass spectroscopy. Q-TOF-MS experiment of 9 exhibited a peak at m/z 526.96 [7₂·6]⁴⁺ which confirmed the metallo-rotaxane 9 at 2 : 1 ratio of DB24C8 and 6. A peak at m/z 414.91 [7·6]⁴⁺ related to 1 : 1 stoichiometry was also found (Fig. S13†).

Fig. 3 (A) ¹H-NMR comparison of viologen 6 and rotaxane 9. (B) 2D COSY NMR of rotaxane 9 in CD₃OD at 25 °C.

When comparing the structures of viologens 5 and 6, 5 should have a higher complex constant than 6 owing to its smooth complex pathway along with the adjacent phenyls. The evidence can be achieved by designing a competitive exchange reaction. When adding the same equivalent amount of bisviologen 5 to a solution of metallo-bisrotaxane 9, after heating at 60 °C for 2 hours, the NMR showed an apparent proof of DB24C8 exchange completely from 9 to form rotaxane 8 and released the free viologen 6 (Fig. 4).

Fig. 4 Conceptual diagram of rotaxane 9 exchanging to 8 (up), and ¹H-NMR comparison of adding viologen 5 to a solution of rotaxane 9 in CD₃OD at 25 °C (bottom).

The continuous titration method¹² were employed to calculate the equilibrium constant K_av. According to this method, the continuous systematical addition of viologens 5 or 6 into a solution containing a mixed mole (1 μmol) of DB24C8 gave the corresponding time-averaged ¹H-NMR signals (see ESI†). On the basis of these NMR data, Δ₀, the difference in values for H^b in the unrotaxaned and fully rotaxaned species was determined by the Benesi–Hildebrand method.¹³ Then p = Δ/Δ₀; Δ = observed chemical shift change relative to uncomplexed species. The complexed fraction, p, of viologens units were determined (Table S1†) and a Scatchard plot¹² was made (Fig. 5). From the intercept and the slope of the plot, the average association constant for rotaxane 8 (K_av) is 8.74 (±3.45) × 10³ M⁻¹, which is about 4.1 times than K_av for rotaxane 9 (2.13 (±0.22) × 10³) (see Table S2†). Even in a higher polarity solvent (CD₃OD), the bis-rotaxane 8 and 9 still demonstrated high equilibrium constant K_av, which should be derived from the electron withdraw effect of MLCT (metal-to-ligand charge transfer) after metallo-complexation.

Fig. 5 Scatchard plots for the complexations of (left) DB24C8 with viologen 5 in CD₃OD; (right) DB24C8 with viologen 6 in CD₃OD. p = complexed fraction.

Conclusions

In conclusion, novel exchangeable bis-pseudorotaxanes, prepared as TpyFeTpy double quaternary ammonium salts with crown ethers, were successfully accomplished by means of facile, feasible and effective inclusion coordination and crown ether–organic salt host–guest binding. This method provides a new route to design more sophisticated metal-contained self-assembled supermolecular architectures.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (21274165) and the Distinguished Professor Research Fund from Central South University of China. The authors gratefully acknowledge Prof. Dr Carol D. Shreiner for her professional consultation. Authors acknowledge the NMR measurements from The Modern Analysis and Testing Center of CSU.

Notes and references

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14. Association of organo-viologen 4 (guest) with DB24C8 in precisely a 1 : 1 stoichiometry gave organo-host–guest 11 (K_av = 212 ± 16 M⁻¹, Table S4†). 10 (K_av = 228 ± 9 M⁻¹, Table S4†) and 11 had nearly equal equilibrium constant K_av, so we designed a competitive exchange reaction as well. When adding the same equivalent amount of 3 to a solution of organo-pseudorotaxane 11, after heating at 60 °C for 2 hours, the ¹H NMR showed that the solution is a mixture of partial complex 10 and 11 (Fig. S11†).

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Footnote

† Electronic supplementary information (ESI) available: Detailed synthetic procedures, NMR spectra and UV-vis. See DOI: 10.1039/c4ra12749h

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