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Interlocked Supramolecular Polymers Created by Combination of Halogen- and Hydrogen-Bonding Interactions Through Anion-Template Self-Assembly Fabiola Zapata, Lidia Gonzalez, Antonio Caballero, Adolfo Bastida, Delia Bautista, and Pedro Molina J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12612 • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on January 29, 2018

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Interlocked Supramolecular Polymers Created by Combination of Halogen- and Hydrogen-Bonding Interactions Through Anion-Template Self-Assembly. Fabiola Zapata,† Lidia González,† Antonio Caballero,*† Adolfo Bastida,‡ Delia Bautista,§ and Pedro Molina.*† †

Departamento de Química Orgánica, Universidad de Murcia, Campus de Espinardo, 30100, Murcia, Spain

‡ §

Departamento de Química Física, Universidad de Murcia, Campus de Espinardo, 30100, Murcia, Spain

Servicio de Apoyo a la Investigación, Universidad de Murcia, Campus de Espinardo, E-30071, Murcia, Spain

KEYWORDS Anion • halogen bonding • hydrogen bonding • supramolecular chemistry • supramolecular polymer ABSTRACT: We present the synthesis and oxoanion-assembling properties of a monomer with a naphthalene ring as a central 2core decorated with two arms containing iodotriazolium rings as anion binding sites. Interactions with SO4 , H2PO4 and 3HP2O7 anions, via cooperative mechanism, afforded new supramolecular materials stabilized by a combination of halogen and 1 hydrogen bonding interactions. H-NMR experiments and solid state structure provided evidence for the initial formation of a supramolecular linear chain, nucleation step and then two different supramolecular chains are interpenetrated with each other, elongation steps, by the formation of hydrogen bonds between two oxygens of the anion from one of the chain with the naphthalene inner protons from the other chain. SEM studies revealed that the morphology of the crystals changed dramatically with the nature of the anion added.

Supramolecular polymers represent a novel class of macromolecules, in which the self-assembly serves as a powerful tool and keeps the monomer units together through reversible non-covalent bonds. The dynamic and reversible nature of non-covalent interactions provides an elegant and interdisciplinary combination of supramolecular chemistry and [1] polymer science. A variety of non-covalent interactions, [2] such as multiple hydrogen bonding, hydrophobic interac[3] [4] [5] tions, π-π stacking, metal-ligand coordination, inte[6] grated non-covalent interactions, and an elegant combina[7] tion of π-π and hydrogen bonding interactions have been widely served as the non-covalent driving forces for controlled supramolecular polymerization processes. The interaction of molecules by non-covalent forces generates supramolecular polymers that can exhibit properties (optical, [8] chiroptical, etc.) not presented by isolated block molecules. Among noncovalent interactions used to bring the building blocks together to prepare supramolecular polymers, halogen bonding and anion-templated self-assembly remain almost unexplored. Halogen bonding is a relevant example of a σhole directional interaction that plays an important role in [9] many chemical and biological environments. In fact, halogen bonding has emerged as an effective alternative to hydrogen bonding although in supramolecular chemistry is a developing and relatively immature field of research. It has become a promising driven force in the self-assembly of [10] extended structures, because of their linear directionality, plays a particularly critical role in the design and synthesis of supramolecular systems as it translates tectons geometry into [11] self-assembled architecture geometry. Although the anion[12] templated self-assembly is a well establised topic, its role

in the field of the supramolecular polymers is much less when compared to the impressive successes of the transition metal-templated self-assembly. However, despite significant challenges, notable successes have been reported, including [13] the anion-templated syntheses of interlocked structures, [14] stimuli-sensitive pseudorotaxanes, threaded architecture [15] and, very recently, cages. Here we describe a new class of supramolecular polymer, where the design is based on the hypothesis that polydentate halogen-bonding anion receptors could form a supramolecular polymer by forming consecutive intermolecular anion complexes through non-covalent host-guest halogen bonding interactions preferably to the anion recognition by a simple receptor. Thus, it would be possible to obtain selfassembled systems where each monomer is linked to another by an anion by halogen bonding interactions. The target monomer is composed by a naphthalene spacer group decorated with two arms containing iodotriazolium motifs as anion binding sites, end-capped with pnitrobenzene rings (Figure 1 top). Initial evidence for the formation of supramolecular polymers was detected by analyzing the titration profiles obtained by 1H-NMR (CD3CN/CD3OD (9:1 v/v)) following the signal of the naphthalene Hd protons. From all the anions [16] 32tested, only HP2O7 , H2PO4 and SO4 anions (Figure 1) 1 produced significant changes in the H-NMR spectrum of the 2+ -4 monomer 1 ·2BF4 (c = 1 x 10 M).

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Job plot analysis of the H-NMR titration data in the elongation region revealed 1:1 receptor to anion binding stoichiome2− 3try for SO4 and HP2O7 and 1:2 for H2PO4 (see Supporting Information). Further confirmation that supramolecular polymers are 3formed in solution after addition of HP2O7 , H2PO4 and 2SO4 anions came from DOSY-NMR and Dynamic light scattering (DLS) studies. In order to distinguish between the formation of the supramolecular polymer by consecutive intermolecular complexes or whether the recognition process is taking place by a simple receptor in solution phase, diffusion NMR experiments were carried out in CD3CN/CD3OD.

1

2+

-

Figure 1. H-NMR spectral changes observed in 1 ·2BF4 in CD3CN/CD3OD (9/1, v/v) during the addition of up to 6 2equiv of SO4 . 2-

The titration profile obtained with SO4 (Figure 2 blue 3points) and HP2O7 (Figure 2 red points) showed a critical transition when 1 equivalent of the anion was added while the critical transition for H2PO4 anion was found after the addition of 4 equivalents (Figure 2 green points). The addi23tion of up to 1 equivalent of SO4 or HP2O7 or 4 equivalents 1 of H2PO4 did not promote any changes in the H-NMR spectrum of the monomer, but subsequent additions of the respective anions promoted a remarkable downfield shift of the naphthalene Hd protons.

Formation of supramolecular polymers should cause an important decrease in the diffusion coefficients D values. The obtained results clearly indicate the formation of supramole23cular structures when H2PO4 , SO4 and HP2O7 anions were 2+ -3 added to a solution of the monomer 1 ·2BF4 (c = 1.25 x 10 M) in CD3CN/CD3OD (9:1 v/v). The presence of H2PO4 promoted an important decrease of the diffusion coefficient of -9 2 -1 -9 the monomer from D = 1.069 x 10 m s to D = 0.696 x 10 2 -1 m s with a decrease of ∆D = -35%. The decreased observed 23in the presence of SO4 and HP2O7 were similar and lower than the obtained for H2PO4 anions (∆D ~ -15%). Additional studies by DOSY-NMR also demonstrate the dependence of 2+ - 2+ 3the diffusion coefficients of the 1 ·2H2PO4 , 1 ·HP2O7 and 22+ 1 ·SO4 species with the concentration in which a gradual decrease in D was observed when the concentration of the 2+ 1 ·2BF4 was increased, which is consistent with the formation of self-assembled supramolecular structures. On contrary, the diffusion coefficients D of the monomer alone remained practically unperturbed (see Supporting Information), indicating no aggregation of the monomer in absence of anions.

DLS measurements at different concentrations (c = 0.1, 0.05 and 0.01 mM) were performed to determinate the size of the supramolecular polymers in CH3CN. In the case of the 2+ 1 ·2H2PO4 species, the Z-average provided support for the formation of a large supramolecular polymer in a very dilutes solutions, the values of the hydrodynamic diameter: were dH = 3538 nm, dH = 1781 nm and dH = 178 nm at c = 0.1, 0.05 and 0.01 mM respectively. As expected for this kind of supramolecular structures, a clear decrease of the hydrodynamic diameter with decreasing concentration was observed (Figure 3). Figure 2. Changes in the chemical shift of the naphthalene 2+ Hd proton of the monomer 1 ·2BF4 upon addition of increas32ing amounts of HP2O7 (red), SO4 (blue) and H2PO4 (green) anions. Points represent experimental data, continuous lines represent fitted curves. This behavior suggests rather a cooperative polymerization 1 process than a simple recognition event. Interestingly, HNMR titrations of the proto-triazolium analogous with the same anions did not showed any evidences of the forma[17]

tion of supramolecular polymers, which shows the key role played by the halogen bonding in the supramolecular system formation (see Supporting Information).

Figure 3. Distribution of the hydrodynamic diameter of 2+ 1 ·2H2PO4 as measured though DLS at c = 0.1 mM (red) 0.05 mM (green) and 0.01 mM (blue) in CH3CN at 25ºC.

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2+

-

2-

Figure 4. SEM images of the self-assembled compounds 1 ·2H2PO4 a)-d) and 1·SO4 e)-f). g) X-ray crystal structure of the su22+ pramolecular polymer 1 ·SO4 in which one chain is represented in ball and stick and the other in space-filling and h) in which 2one chain has been omitted for clarity. i) Schematic representation of the supramolecular polymer 1·SO4 showing the halogen (blue) and hydrogen (red) bonding interactions. Distances are in angstroms, in brackets the ΣrvdW. 2+

2-

The hydrodynamic diameter obtained for 1 ·SO4 (dH = 327 nm and dH = 214 nm at c = 0.1 and 0.05 mM respectively) and 2+ 31 ·HP2O7 (dH = 192 nm, dH = 179 nm and dH = 106 nm at c = 0.1, 0.05 and 0.01 mM respectively) were smaller than the 2+ obtained for 1 ·2H2PO4 . These results are in good agreement with the DOSY-NMR results discussed previously. In order to quantitatively describe the thermodynamics of the supramolecular polymerization, and take into account 1 experimental H-NMR titration profiles showed in Figure 2, we proposed a cooperative polymerization model, which is characterized by the formation of a thermodynamically unfavorable nucleus, with a very low association constant KN, followed by energetically favored elongations steps, with another association constant KE, which describe the reversi[18] ble addition of the monomers to post-nucleus polymer.













where A, B and (AB)i denote the two monomers and the supramolecular polymer of length i, and KN and KE are the equilibrium constants for the nucleation and elongation steps. Typically all elongation steps are assumed to have the [19] same equilibrium constant under the conditions where only two nearest neighbor interactions are significant so that the reactivity of the functional groups remain constant, irrelevant of the lengths of the chains to which they are attached. This assumption allow us to derive (see Supporting Information) the following expressions relating the total

concentrations of A and B in the experiments (CA and CB) and their concentrations at equilibrium ([A] and [B]). 1

1

1

2

The [A] concentrations can be estimated from the measured NMR shifts assuming that they can be evaluated using the following weighted sum 3 where δA and δsp are the NMR shifts in the isolated monomer and the supramolecular polymer, respectively. The use of Equations (1)-(3) allowed us to fit the values of the KN, KE and δsp constants to reproduce the observed NMR shifts as shown in Figure 2. The results reveal very low association constants for the formation of the nucleus KN = 0.15, -1 230.06 and 18.86 M for HP2O7 , SO4 and H2PO4 respectively. On contrary, the calculated association constants for the elongation process were very high KE = 9.2 x 107, 1.0 x 108, and 7 -2 321.4 x 10 M for HP2O7 , SO4 and H2PO4 respectively. SEM studies revealed the formation of a highly directional 1D 2+ crystal growing process for 1 ·2H2PO4 . Due to the overlap of the fibers, it was difficult to estimate their average length (Figure 4a and 4b). Fortunately, it was possible to observe isolated fibers whose length were above 2 mm (Figure 4c and 4d). The width of all of them was very uniform around 2 µm. 2SEM images of the 1·SO4 supramolecular polymer show drastic changes in the morphology of the crystals regarding 2+ the observed in the supramolecular polymer 1 ·2H2PO4 . In

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this case, the formation of 2D fractals was observed (Figures 4e,f). Single crystals suitable for X-ray diffraction structural analy2+ 2sis were obtained for the supramolecular polymer 1 ·SO4 [20] . The solid state structure shown in Figures 4g,h provided unequivocal evidence for the formation of a supramolecular linear chain, the anion-assembled supramolecular polymer is 2+ built up from a sequence of alternating organic molecules 1 2and SO4 anions as the result of strong and high directional halogen bonds formed between the iodine atom of the iodo2triazolium rings and SO4 anions. Additionally, two different supramolecular chains are interpenetrated each other by the formation of hydrogen bonds between two oxygen of the 2SO4 anion from one of the chain with the naphthalene proton Hd from the other chain (Figure 4i). PXRD analysis was carried out, experimental pattern match with the simulated from single crystal analysis (see Supporting Information). 2+

-

[21]

Due to the high affinity of Zn cation for H2PO4 anion , the reversibility of the supramolecular polymerization2+ depolymerization process of 1 ·2H2PO4 was investigated by [22] 1 2+ and H-NMR spectroscopies by addition of Zn emission cations which is able to sequestering the H2PO4 from the 2+ supramolecular polymer. Thus, after addition of Zn , both 1 the emission and H-NMR spectra of the monomer were recovered. Several polymerization-depolymerization cycles 2+ were carried by the sequential addition of H2PO4 and Zn 2+ ions to a solution of the compound 1 ·2BF4 . The emission 2+ 2+ spectrum of the 1 ·2H2PO4 and the monomer 1 ·2BF4 were recovered after each step demonstrating the reversibility of the polymerization-depolymerization process (see Supporting Information). In conclusion, we have described oxoanion-mediated supramolecular polymers based on combined hydrogen- and halogen bonding interactions. The monomer interacted with sulfate, hydrogen phosphate and pyrophosphate, via cooperative mechanism, provided a new supramolecular materials stabilized by a combination of halogen and hydrogen bonding interactions. The confirmation that supramolecular systems have been formed in solution came from the DOSYNMR and DLS analysis. SEM studies revealed that the morphology of the crystals can be modulated by changing the nature of the anion added: from long needle-like fibers for 21 H2PO4 to fractals for SO4 . The combined H-NMR experiments and the solid state structure studies and the thermodynamic calculations provided strong evidence of an initial formation of a supramolecular linear chain, nucleation step with low association constant, which is generated from a sequence of alternating monomers and anions as a result of the high-direction halogen bonds and then interpenetrating two different supramolecular chains, elongation steps with high association constants, by the formation of hydrogen bonds between two oxygen of the anion of one of the chains with the internal protons of naphthalene from the other 1 chain. Emission and H-NMR studies demonstrated the reversibility of the polymerization-depolymerization process.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Experimental procedures, synthesis, NMR experiments, cooperative model analysis, X-ray data and fluorescence studies.

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AUTHOR INFORMATION Corresponding Author *[email protected],

*[email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Ministerio de Economía y Competitividad of Spain and FEDER projects CTQ201346096-P and CTQ2017-86775-P and Fundación Séneca Región de Murcia (CARM) Projects 18948/JLI/13 and 19337/PI/14. We also thank Prof. Gloria Villora for allowing us the use of the DLS.

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(12) a) Vilar, R. Angew. Chem. Int. Ed. 2003, 42, 1460-1477; b) Evans, N. H.; Beer, P. D. Angew. Chem. Int. Ed. 2014, 53, 11716-11754; c) Busschaert, N.; Caltagirone, C.; van Rossom,; Gale, P. A. Chem. Rev. 2015, 115, 8038-8155. (13) Barendt, T. A.; Robinson, S. W.; Beer, P. D. Chem. Sci. 2016, 7, 5171-5180. (14) Gong, H.-Y.; Rambo, B. M.; Karnas, E.; V Lynch,. M. J.; Sessler, L. Nat. Chem. 2010, 2, 406-409. (15) Pandurangan, K.; Kitchen, J. A.; Blasco, S.; Boyle, E. M.; Fitzpatrick, B.; Feeney, M.; Kruger, P. E.; Gunnlaugsson, T. Angew. Chem. Int. Ed. 2015, 54, 4566-4570. (16) The following set of anions: HP2O73-, H2PO4-, SO42-, HSO4-, NO3-, F-, Cl-, Br-, I-, AcO-, ClO4-, PF6-, and C6H5CO2-, were added as tetrabutylammonium salts in CD3CN/CD3OD (9:1 v/v). (17) Zapata, F.; Gonzalez, L.; Caballero, A.; Alkorta, I.; Elguero, J.; Molina, P. Chem. Eur. J. 
2015, 21, 9797-9808. (18) Sorrenti, A.; Leira-Iglesias, J.; Markvoort, A. J.; de Greef, T. F. A.; Hermans., T. M. Chem. Soc. Rev., 2017, 46, 5476-5490. (19) Zhao, D.; Moore, J. Org. Biomol. Chem. 2003, 1, 3471-3491. (20) CCDC 1577010 contains the supplementary crystallographic data for this paper. These data are provided free of charge by The Cambridge Crystallographic Data Centre. (21) Kim, S. K.; Lee, D. H.; Hong, J-I.; Yoon, J. Acc. Chem. Res. 2009, 42, 23-31. (22) The emission spectrum of the monomer exhibits three weak emission bands at λ = 342, 360 and 383 nm when excited at λ = 280 nm. The addition of H2PO4- showed an increase 20-fold of the intensity of its emission bands and 30-fold of the quantum yield (see Supporting Information).

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