Expanded aromatic carboxylate anion induced molecular sandwich construction via a tetracationic imidazolium macrocycle conversion from molecular box to molecular tweezer

Tetrahedron 73 (2017) 3266e3270

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Expanded aromatic carboxylate anion induced molecular sandwich construction via a tetracationic imidazolium macrocycle conversion from molecular box to molecular tweezer Zhen-Hua Ma a, Huan-Rong Li a, **, Han-Yuan Gong b, * a b

Department of Chemistry, Renmin University of China, Beijing 100872, PR China College of Chemistry, Beijing Normal University, Beijing 100875, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 March 2017 Received in revised form 19 April 2017 Accepted 24 April 2017 Available online 27 April 2017

The molecular box, namely cyclo[2](2,6-di(1H-imidazol-1-yl)pyridine)-[2](1,4-dimethylene benzene) (14þ; as PF 6 salt), fold its conformation as molecular tweezer to clip the specific carboxylates with expanded aromatic plane. The binding modes between 14þ and carboxylate, namely pseudorotaxane, outside or clipping (i.e., sandwich like), also depend on the location of carboxylate on the large conjugated moiety. These finding develop the usability of 14þ and carboxylates as important building block pairs to create non-covalent self-assembly structures. © 2017 Published by Elsevier Ltd.

Keywords: Self-assemble Molecular tweezer Carboxylate anion with expanded aromatic plane Binding mode Sandwich complex

1. Introduction Molecular tweezers are characterized via a rigid tether holding two flat pincers in a syn-conformation/configuration.1e5 On the basis of non-covalent weak interactions (e.g., p-p donor-acceptor interaction), molecular tweezer half-open cavities tend to clip additional substrate for molecular recognition and functional selfassembly.6e17 In past decades, macrocyclic moieties have been introduced as flat pincer part in molecular tweezer. Reported macrocyclic princer to accommodate guests involved crown ethers,18 cyclotriveratrylenes (CTV),19 calix[n]arenes and their derivatives,20,21 dipyrene tweezer-like (DPT) molecules,22e24 incorporating zinc porphyrins,25 methylazacalix[4]pyridine (MACP-4),26 and other macrocycles.9,27e29 On the other hand, the strategy of molecular tweezer construction via macrocylic framework is under development. In this strategy, macrocycle tune its conformation as tweezer for additional guest clipping, and then achieve molecular

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H.-Y. Gong).

(H.-R.

http://dx.doi.org/10.1016/j.tet.2017.04.059 0040-4020/© 2017 Published by Elsevier Ltd.

Li),

[email protected]

recognition and supramolecular self-assembly formation. Recent examples of converting macrocyclic conformation as molecular tweezer included bis-tetrathiafulvalene-calix[2]pyrrole[2]thiophene,29 tetra-tetrathiafulvalene-calix[4]pyrrole30 and methylazacalix[4]- pyridine (MACP-4).26 Compared with the strategy using macrocyclic fragment as pincer, the example of the late approach is still lacking, which implied that the strategy to build molecular tweezer is still under development. Herein, we demonstrated that a tetracationic imidazolium macrocyclic ‘Texas-sized’ molecular box, namely cyclo[2](2,6di(1H-imidazol-1-yl)pyridine)[2](1,4-dimethylene benzene) (14þ; as PF 6 salt), can change its conformation as boat, so that act as molecular tweezer to selectively clip carboxylate containing expanded conjugated plane. Furthermore, we summarized that three factors determined the different binding modes between 14þ and carboxylate precursor with aromatic ring: (1) the width of aromatic plane perpendicular to the linking carboxylate; (2) the species and site(s) of substitute group(s) on the aromatic ring of the carboxylate; (3) the location of carboxylate on the aromatic plane. This work shows a new mode and related strategy to construct functional self-assembly materials using cationic macrocyclic precursor and anionic species.

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2. Results and discussion Our prior study presented that aromatic carboxylate (e.g., 2,6naphthalene dicarboxylate dianion) or sulfonate anion (e.g., 2,6naphthalene disulfonate dianion) species can easily thread the cavity of 14þ to form interpenetrated structures (i.e., pseudorotaxanes) and highly ordered interlocked complexes, including poly(pseudorotaxanes),31e38 rotaxanated supramolecular frameworks (RSOFs)39e43 and metal-organic rotaxane frameworks (MORFs).44e47 These results also implied that the width of aromatic plane, which is perpendicular to the linking carboxylate or sulfonate, determine the complexation mode between 14þ and aromatic anionic precursor. When the width of aromatic ring on anion precursor is less than 5.8 Å,50 14þ may adopt the “chair” or “box” like conformation and formed pseudorotaxanes48e50; In contrast, while the width of conjugated plane on anion substrate larger than 5.8 Å, 14þ with “chair” or “box” framework would only can bind aromatic anion outside of its cavity.49e51 But even the wide anion species (e.g. tri-1,3,5-benzenetricarboxylate anion) cannot thread through the core of 14þ, it still have the possibility to combine with metal cation to construct mechanically interlocked molecules (MIMs) with environmental responsivity.51 To further explore usability of 14þ in the multi-component supramolecular self-assembly, our investigation turn to obtain deeper insights into the expanded aromatic plane effect on carboxylate anion precursor, namely how the big aromatic structure of the anion guests might influence the nature of the complex built up from 14þ. Herein, we report the results of the interaction study between 14þ and a series of carboxylates containing large conjugated plane, including anthracene-1-carboxylate anion (2), anthracene-9-carboxylate anion (3), phenanthrene-3-carboxylate anion (4), pyrene-1-carboxylate anion (5) and pyrene-1-butyrate anion (6) (cf. Scheme 1). It is found that host 14þ bind anions 3 or 6 outside of its cavity both in solution and in solid state, since the width of guests larger than 5.8 Å, meanwhile 14þ adopt chair or box conformation. The result was consistent with the prior study.49e51 Intriguingly, new sandwich-like binding mode between 14þ and aromatic carboxylate anion (e.g., guest 2, 4 or 5) is observed. In these cases, 14þ with “boat-like” framework clip anionic precusor both in solution and solid states (cf. Scheme 2). These new complexes continue to attract attention not only as the examples of using building blocks to create non-covalent organic frameworks or self-assembly materials, but also aggregate one dimensional supramolecular species which are stabilized by a combination of apparent electrostatic and donor-acceptor-donor (DAD) interactions. Our initial investigation was to explore the binding between 14þ and anthracene-1-carboxylate anion (2) (studied as its tetramethylammonium (TMAþ) salt made in situ by mixing Hþ$2 with 1 M equiv. of TMAþ$OH$5H2O) in DMSO-d6 solution (for reasons of solubility, DMSO-d6 was used as the solvent in all NMR studies unless noted otherwise). A direct 1H NMR spectral titration of 14þ with 2 was carried out via maintaining the concentration of 14þ as 1 mM and changing the molar ratio of [2] to [14þ] from 0 to 4. It is

Scheme 1. Structures of macrocyclic host 14þ (as PF 6 salt) and anionic guests shown in this work.

Scheme 2. Schematic summary of the binding modes between 14þ and carboxylates with aromatic plane moiety show in prior and this work.

observed that changes in the chemical shift of the peaks associated with the proton resonance on 14þ depend on the concentration of 2, which are consistent with an interaction between 14þ and 2. Further analysis (Job plot) was carried out. A [G]/([G] þ [H]) ratio maximum at 0.6 suggested that the stoichiometry of 2:3 (H/G) best describes the interaction between 14þ and anion 2 in the solution, as had been seen in previous study of 14þ.52. It was supposed that the 2:3 complex form from separate 1:1 (H:G) and 1:2 (H:G) complexes. More evidence of the assumption came from electrospray ionization mass spectrometry (ESI-MS) analysis, with revealed peaks at m/z values as 283.7856 and 536.2101 that correspond to the calculated mass for [14þ$2]3þ and [14þ$22]2þ, respectively (cf. Supplementary data). From the 1H NMR titration data, association constants of lgKa1 ¼ 4.2(2), lgKa2 ¼ 3.0(2) and lgKa3 ¼ 3.7(4), corresponding to the formation of the initial 1:1 complex ([14þ$2]3þ), its subsequent conversion to 1:2 complex ([14þ$22]2þ), and the 2:3 complex [(14þ)2$23]5þ constructed between ([14þ$22]2þ) and ([14þ$2]3þ), could be calculated. Association constants and the thermodynamic parameters (enthalpy and entropy changes DHo and DSo) corresponding to the interaction between 14þ and 2 were also calculated base on the isothermal titration calorimetry (ITC) data (cf. Table 1).53 The association constants calculated via ITC investigation are well consistent with 1H NMR titration results. The width of 2 (7.3 Å) is larger than maximum cavity size of 14þ (5.8 Å),50 such finding is consistent with the anionic guest 2 being bound to the outside of the central cavity of 14þ, if 14þ prefer chair or box conformation in the complexation, rather than the formation of a pseudorotaxane or other MIM structure. Surprisingly, in the one-dimensional nuclear Overhauser enhancement spectroscopy (1D-NOE) of the mixture containing 14þ and 1 M equiv of 2, the distinct correlations between protons on 14þ and guest 2 were observed (cf. Supplementary data). It implied that 14þ still include anionic guest 2 in its cavity. Herein, it is suggested that 14þ adopt ‘boat-like’ conformation as a molecular tweezer to clip guest 2, so that a sandwich-like structure is finally formed (cf. Scheme 2), which is further supported via single crystal X-ray diffraction study (cf. Fig. 1). Furthermore, diffusion ordered 1H NMR spectroscopic (DOSY) analyses revealed that all proton signals, including those located on 14þ and 2, were characterized by similar diffusion times in the 1:1

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Table 1 Summary of the association constants determined via 1H NMR spectroscopic titrations in DMSO-d6 or ITC analyses corresponding to the interaction between 14þ and 2, 3, 4, 5 or 6 in DMSO. Also tabulated are the enthalpy change, DHo, the Gibbs free energy change, △Go, and the entropy change, △So as determined from the ITC analyses. The relevant equilibrium equations are:

Guest

Stoichiometry (H:G)

2:3

1:2

Complex

[14þ$2]3þ [14þ$22]2þ

Binding mode of [HG]

sandwich

lgKa (NMR)

nb (ITC)

lgKa (ITC)

△Go kJ$mol1

△Ho kJ$mol1

△So J$mol1 K1

4.2(2)a 3.0(2)

1.64(2)

4.1(2) 3.4(2)

23.1(1) 19.3(1)

12.6(6) 8.1 (5)

35.4(2) 37.7(2)

[(14þ)2$23]5þ

3.7(4)

3.5(3)

20.2(3)

1.05(6)

71.23(7)

[14þ$3]3þ

4.7(2)

4.6(2)

26.3(1)

12.1(6)

47.4(2)

3.4(2)

19.6(2)

5.9 (5)

46.1(4)

4.1(2) 3.2(2)

23.4(1) 18.5(1)

8.3(4) 3.8(3)

50.6(5) 49.5(3)

2.5(3)

2.6(3)

15.1(3)

6.4(8)

29.2(6)

5.1(2) 3.2(2)

4.9(3) 3.7(2)

27.8(1) 21.2(1)

8.8(4) 5.8(3)

63.8(3) 51.7(3)

[1

4þ

2þ

$32]

4þ

outside

3þ

[1 $4] [14þ$42]2þ 2:3

3.0(3) 4.3(2) 2.9(2)

sandwich [(14þ)2$43]5þ 4þ

3þ

[1 $5] [14þ$52]2þ 2:3

1.28(1)c

1.44(1)

sandwich

1.32(1)

[(14þ)2$53]5þ

3.2(3)

2.8(3)

16.0(2)

14.4(7)

102.1(8)

[14þ$6]3þ [14þ$62]2þ

4.3(2) 3.0(1)

4.4(2) 3.2(2)

25.0(1) 18.0(1)

17.3(8) 12.0(7)

25.8(1) 20.1(1)

3.1(3)

17.6(3)

8.7(2)

29.9(5)

2:3

outside [(14þ)2$63]5þ

1.50(1) 3.6(3)

a The standard errors both in NMR titration and ITC titration for a nonlinear fit parameter are less than 10% and in previously examined cases, the 10% rule of thumb has proved a reliable guideline for applicability.61 The standard errors in each binding parameter are calculated by the Bevington method.62 b n values detected from ITC method are well consistent with the results of 1H NMR Job plots in the case of 2, 4, 5 or 6.63 c When guest is 3, the low ratio of [14þ32] cause the value of n is near 1 (cf. Supplementary data).

(H:G) mixtures (cf. Supplementary data). These evidences suggested that 14þ and guest 2 formed stable 1:1 complex structure in solution. In an effort to probe the generality of these findings, we broadened our investigation to four constitutional anionic precursors, namely anthracene-9-carboxylate anion (3), phenanthrene-3-carboxylate anion (4), pyrene-1-carboxylate anion (5) and pyrene-1-butyrate anion (6). As above, direct 1H NMR spectroscopic titrations revealed chemical shift changes. corresponding to binding between 14þ and 3, 4, 5 or 6 in solution. Further investigation by Job plot analysis revealed that the binding stoichiometries is 1:2 (H:G, anion as 3) or 2:3 (H:G, in the

Fig. 1. The interaction between 14þ and 2 was observed in the single crystal X-ray structure of [14þ2(9,10-dioxo-9,10-dihydroanthracene-1-carboxylate)2PF 6 2DMF4H2O] shown in ellipsoid form (a). Also shown as a top view (b), side view (c), and front view (d) is the structure in stick form. All the other molecules and atoms have been omitted for clarity.

case of 4, 5 or 6), when the ratio of host to guest was not constrained to 1:1. As in the case of anion 2, 1D-NOE spectroscopic studies and ESIMS analyses provided support for the formation of 1:1 sandwich complex [14þ4]3þ and complex [14þ5]3þ (cf. Supplementary data). Differently, stable complexation with outside mode formed between 14þ and anionic precursor 3 or 6 are observed (cf. Supplementary data). Previously, we noted that exposing 14þ to anthracene-2carboxylate anion resulted in formation of a pseudorotaxane structure, as judged by Two-dimensional nuclear Overhauser effect spectroscopy analysis.52 Wherein macrocycle 14þ was suggested to adopt a chair-like conformation. In an effort to expand our understanding the binding mode of 14þ with the isomers of anthracene carboxylate anion, and explore whether these building blocks could be used to create different self-assembly species, we have studied the interactions of 14þ with two isomers of anthracene carboxylate anion, namely anion 2 or 3. Anion 2 induced a sandwich structure as shown above, wherein 14þ accommodate anion 2 via transferring its conformation as boat-like for clipping the anionic precusor. On the other hand, anion 3 was found to bind to the outer periphery of 14þ. All results shown above implied that through these simple anion precursors combine the same aromatic plane and different carboxylate locations, we can generate different supermolecule self-assembly species with 14þ. It provides self-assembly foundation for the later creation of organic frameworks or materials. In the case of 2 and 3, these results shown above were further supported via solid state study (cf. Supplementary data). The slow kinetic complexation between 14þ and carboxylate linking expanded aromatic plane (e.g., 2, 3, 4, 5 or 6) are also

Z.-H. Ma et al. / Tetrahedron 73 (2017) 3266e3270

Fig. 2. The interaction between 14þand 5 were observed in the single crystal X-ray structure of [14þ522OH9H2O] shown in ellipsoid form (a). Also shown as a top view (b), side view (c), and front view (d) is the structure in stick form. All the other molecules and atoms have been omitted for clarity.

Fig. 3. Extended structure of the 1D array (i.e., ([14þ5]3þ5)2nþ n ) shown as view in stick form as seen in the structure of [14þ522OH9H2O]. All the other molecules and atoms are omitted for clarity (Here, A and D refer to acceptor and donor, respectively).

distinguish with the fast equilibrium cases of carboxylate with benzene, naphthyl, or biphenyl group shown in prior work.49,51,52 Collecting 1H NMR spectra of the 1:1 ([H]/[G]) mixture containing 14þ and 2, 3, 4, 5 or 6 at 298 K, it is observed that all the signals corresponding with protons on 14þ or anion were broadening (i.e., the resolution was reduced), these outcome convincingly indicated that the relaxation time T1 of the complexes were extended.54e60 The findings led us suggest that slow equilibrium(s), appeared in the complexation and decomposition, and/or the conversion of the different binding modes of the complexation between 14þ and expanded aromatic anion. The assumption was further supported via temperature dependent 1H NMR spectroscopic study (cf. Supplementary data). Herein, it was seen that increasing temperature improves the resolution of the signals (characterized with narrower half-height width of the signal corresponding to H(1) and

Fig. 4. The interaction between 14þ and 3 were observed in the single crystal X-ray  structure of [(14þ)2334PF 6 OH 26H2O], shown in ellipsoid form (a). Also shown as a top view (b), side view (c), and front view (d) is the structure in stick form. All the other molecules and atoms have been omitted for clarity.

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H(7) on 14þ at higher temperature in the scale of 298 K - 368 K), which implies heating accelerate the slow equilibrium(s). Evidence of sandwich complex construction between 14þ and guest 2 or 5 came from single crystal X-ray diffraction analyses of [14þ2(9,10-dioxo-9,10-dihydroanthracene-1carboxylate)2PF (9,10-dioxo-9,106 2DMF4H2O] dihydroanthracene-1-carboxylate was suggested from the oxidation of anthracene ring on guest 2 during the crystal cultivation process) and [14þ522OH9H2O]. These structures revealed that macrocycle 14þ adopt “boat-like” conformation to accommodate guest 2 or 5 (cf. Figs. 1 and 2). Presumably, the wide nature of guest 2, 5 prevent them from successfully threading through the core of 14þ with chair or box like conformation, thus form molecular sandwich with 14þ adopting boat framework. The complex species appears stabilized in part via weak non-covalent interactions, including CH$$$p and p$$$p donor-acceptor interactions (cf. Supplementary data). The anionic 5 outside of macrocyclic cavity interacts with neighboring 14þ moieties on [14þ5]3þ sandwich units via inferred p-p donor-acceptor interactions, the result is a 1D DAD array and seen in [14þ522OH9H2O] (cf. Fig. 3). The study in solution suggested that 14þ adopt outside mode to combine with anions 3 or 6. This result, at least in the case of 3, was further supported via single-crystal X-ray diffraction analyses of  [(14þ)2334PF 6 OH 26H2O]. The structure revealed that macrocycle 14þ adopt “chair-like” conformation to bind guest 3 outside of its core (cf. Fig. 4). Weak non-covalent interactions (e.g., CH$$$p, p$$$p donor-acceptor interactions, etc.) appear to contribute to the overall stability of the structure. 3. Conclusion In this work, it is reported that tetracationic imidazolium macrocycle 14þ can change its flexible framework as boat conformation, so that play as a molecular tweezer to clip carboxylate with large conjugated planar moieties. Our study suggested that three factors resulted in the different complexation modes between 14þ and aromatic anionic precursor. First important issue is the width of aromatic plane, which is perpendicular to the linking carboxylate. The width of aromatic plane less than 5.8 Å, 14þ may adopt the “chair” or “box” like framework for pseudorotaxane formation. When the width of aromatic ring on anion is larger than 5.8 Å, 14þ with “chair” or “box” conformation would only can bind aromatic carboxylate anion outside of its cavity (e.g., the complexation between 14þ and anion 3 or 6). Meanwhile, 14þ has the potential to act as molecular tweezer to bind carboxylate with large conjugated planar via boat conformation, and then give out final sandwich complex (e.g., [14þ$2]3þ, [14þ$4]3þ and [14þ$5]3þ). Secondly, the species and site(s) of substitute group(s) on the aromatic ring of the carboxylate also determine the complex mode between 14þ and aromatic carboxylate. Finally, the location of carboxylate on the aromatic plane is one determined factor of their binding modes with 14þ, which was proved via the complexation modes, namely clipping, outside, or pseudorotaxane between 14þ and 2, 3, or anthracene-2-carboxylate anion, respectively. The findings in this work led us to propose that these anion-induced sandwich structures could have a guiding role to play in the post-design of molecular device and the construction of novel molecular tweezer self-assembled systems, which is in the present research further enriches the toolbox of supramolecular chemistry. Acknowledgements H.-Y. G. thanks the National Natural Science Foundation of China (21472014), National Basic Research Program of China (973

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Program 2015CB856502), the Young One-Thousand-Talents Scheme, the Fundamental Research Funds for the Central Universities, the Beijing Municipal Commission of Education, and the Beijing Normal University for financial support. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.tet.2017.04.059. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.

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