Halide-Controlled ExtendingâShrinking Motion of a Covalent Cage

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Halide-controlled extending–shrinking motion of a covalent cage Anna B. Aletti, Ana Miljkovic, Lucio Toma, Rosaria Bruno, Donatella Armentano, Thorfinnur Gunnlaugsson, Greta Bergamaschi, and Valeria Amendola J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b00219 • Publication Date (Web): 18 Mar 2019 Downloaded from http://pubs.acs.org on March 18, 2019

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The Journal of Organic Chemistry

Halide-controlled extending–shrinking motion of a covalent cage Anna B. Aletti,§ Ana Miljkovic,† Lucio Toma,† Rosaria Bruno,⁋ Donatella Armentano,⁋ Thorfinnur Gunnlaugsson,§ Greta Bergamaschi‡* and Valeria Amendola†* †

School of Chemistry, Trinity College Dublin, The University of Dublin, Dublin 2, Ireland; Department of Chemistry, v.le Taramelli 12, Pavia, Italy; ⁋Department of Chemistry & Chemical Technologies, via Pietro Bucci, ‡ Arcavacata di Rende, Cosenza, Italy; Consiglio Nazionale Delle Ricerche, Istituto di Chimica del Riconoscimento Molecolare (ICRM), via M. Bianco 9, Milano, Italy. §

KEYWORDS Molecular cages. Halide binding. Molecular recognition ABSTRACT: Herein, we present an example of covalent cages, whose flexible framework undergoes extending– shrinking motion under halide control. In absence of halide anions, the free cage assumes a flattened conformation: the cavity is compressed along the C3 axis passing through the tertiary amines, and the two tribenzylamine platforms are eclipsed. Halide encapsulation promotes a large conformational rearrangement of the cage, involving an extension of the cavity along the C3 axis and shrinkage along the equatorial plane. Interestingly, the rearrangement is accompanied by the pyramidal inversion of the tertiary amines, and by the rotation of the tribenzylamine-based platforms, which become staggered. The imidazolium containing arms wrap around the spherical anion, leading to a racemic mixture of the M and P−helical complexes. As expected from the flexible structure of the cage, the switch between the two limit conformations can be repeated for several cycles under alternating chemical stimuli (AgNO3/TBACl). This result is consistent with the low activation barriers determined by computational investigations. These also allowed us to quantify the energy difference between the shrunk and expanded cage conformations, and to hypothesize an energetic pathway along which the conformational rearrangement can occur.

Introduction Organic molecular cages, as the systems described in this work, are organic molecules possessing a threedimensional (3D) cavity and a backbone, consisting of carbon–carbon and/or carbon-heteroatom bonds.1-3 Compared to coordination cages, in which metal ions are pivotal structural elements,4-5 organic cages are generally made more stable by the covalent bonds.2,6 In the recent years, organic cages are gaining attention for potential applications as nano-reaction vessels,7-8 in catalysis,9,10 sensing,11 stabilization of reactive compounds,12 and as materials for separation processes.13 When applied as molecular hosts,1-3 guest selectivity is mainly driven by the complementarity of shape, size and interaction sites between the cavity of the organic cage and the guest. Molecular cages with a rigid cavity, hardly adapting to different guests, generally lead to a higher degree of selectivity. This is also important in the solid state, where shapepersistent cages (i.e. whose rigid structure prevents collapse) have emerged as materials for gas storage, separation and heterogeneous catalysis.9-10,13 In the recent years, organic cages have been also utilized as building blocks for molecular machines and stimuliresponsive systems.16-18 Within this frame, molecules with a flexible framework are generally privileged. In

particular, cages of special interest are those with a cavity of controllable shape and size under an external chemical stimulus, e.g. pH variation, addition of guests.19-20 The input may induce large conformational changes and even modify the binding tendencies of the cage-like host. Among 3D hosts for molecular recognition in solution, a special place is occupied by anion receptors.21-23 These organic molecules generally display a well-defined cavity, towards which a series of multiple binding sites (e.g. hydrogen and/or halogen donor groups, coordinatively unsaturated metal ions, etc.) are oriented. Imidazolium-based receptors are particularly effective due to the bolstering contribution of the positive charge on the (C−H)+ donors.24-25 When immobilized within cage-like or bowl-shaped systems, the (C−H)+ groups converge towards the center of the cavity, where the anion is included, leading to strong binding even in competing media. In this work, we present a novel type of imidazoliumbased covalent cages, whose flexible structure can undergo a notable conformational rearrangement (extending–shrinking and twisting) under halide encapsulation. The affinity for halide anions was tested in pure organic solvent and in presence of water.

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Figure 1. Synthesis of the imidazolium-based covalent cage 1(PF6)3. In 2(PF6)3, the imidazolium units are replaced by benzimidazolium groups (see Scheme S1).

Results and discussion Molecular cage 13+ (Figure 1) was synthesized as the PF6− salt, through a multi-step procedure (see the Experimental Section). The final step, shown in Figure 1, consists in the reaction of tris(3-((1H-imidazol-1yl)methyl)benzyl)amine (see the Supplementary) with tris(3-(bromomethyl)benzyl)amine in acetonitrile:acetone mixture. The cage was then isolated as 1(PF6)3 by anion metathesis. Analogous procedure was followed in the synthesis of 2(PF6)3. The reversible extending–shrinking motion of the cage structure was investigated experimentally with spherical anions (chloride, in particular) in both solid state and solution, and through computational studies. Binding studies were also performed by 1H-NMR experiments on 13+, while UV-vis absorption titrations were carried out on the benzimidazolium analogue system 23+. Conformations of 13+ in the solid state Crystals suitable for X-ray diffraction studies were obtained from diffusion of diethyl ether in acetonitrile solutions of 1(PF6)3 and of its 1:1 chloride complex. The structure of the free cage crystallizes in the P͞3 space group of the trigonal system (Table S1) and is rather flattened (see Figure 2) with the shorter axis passing through the tertiary amines [dNtert∙∙∙Ntert = 5.65(1) Å, Figure 2a] and the imidazolium arms extending apart symmetrically due to electrostatic repulsion [dCHim∙∙∙C-Him = 11.02(1) Å].

Figure 2. Different views of the crystal structure of 1(PF6)3. (T = 90 K). Nitrogen and Carbon atoms are depicted by sky blue and grey sticks, respectively. Hydrogens and PF6− anions are omitted for clarity. Structural parameters are shown in Å.

Figure 3. Different views of the crystal structure of [1(Cl)](PF6)2∙CH3CN. Nitrogen and carbon atoms are depicted by sky blue and grey sticks, respectively. The chlorine atoms are represented as green spheres. Hydrogen atoms, PF6− anions and CH3CN are omitted for clarity. Main structural parameters are shown in Å.

Figure 4. Crystal structure of 1(PF6)3 (a and c) and [1(Cl)](PF6)2 (b and d) as stick and space filling (Van der Waals radii) models. Gold spheres underlines the spherical potential accessible voids with diameters ranging from 0.18 to 0.50 nm.


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1

3+

+

Figure 5. a) H-NMR spectra (400MHz) of 1 alone and after the addition of excess (i.e. ~5 eqv.) halide anions, as TBA 1 salts, in CD3CN; b) H-NMR titration profiles (variation of the chemical shift proton Hα vs. equivalents of the added anion) 3+ − − − for the titration of 1 with Cl (purple triangles), Br (white triangles), I (purple circles) in CD3CN. In the case of chloride, the signal of proton Hα disappears after the first additions, and reappears around 1 eqv. of the added anion. Further information is available in the Supplementary Section.

In this conformation, the cage is divided in two parts by a symmetry plane, and the two tribenzylamine-based platforms are completely eclipsed. PF6− anions reside outside the cages interacting with them by very weak F···H−C supramolecular interactions (Figure S6). Upon chloride encapsulation, a notable conformational rearrangement followed by a crystalline phase transition clearly occurred. The [1(Cl)]2+ structure crystallizes in the P͞1 space group of the triclinic system (Table S1) with two cages per unit cell. In the chloride complex, the receptor cavity looks more elongated than in the free cage, the two tribenzylamine-based platforms are staggered with dihedral angles approaching 60° (Figure 3; Figures S7-S11). This result indicates that the anion encapsulation promoted the separation of the tribenzylamine groups, and induced a twist in the receptor structure with either counter or clockwise rotation of the upper platform. The two cages of the unit cell are actually enantiomers of opposite helicity (Figure S11). Figure 3 shows the P enantiomer. The conformational change upon chloride binding is also accompanied by the pyramidal inversion of the tertiary amines; the distance dNtert∙∙∙Ntert is now increased to 9.542(2) Å (see Figure 3a). The imidazolium groups are close to each other (dCHim∙∙∙C-Him = 5.30 − 6.00 Å) and point towards the center of cage, i.e. towards the Cl− anion. The chloride anion is perfectly recognized by the cage, while PF6− anion reside outside. The distances between the positively charged imidazolium (C−H)+ groups and chloride (i.e. dCH∙∙∙Cl = 3.15 − 3.29 Å, see Figure S7a) suggest that the latter is bound through three H-bonding interactions. Taking van der Waals radii into account, the diameter of the spherical void at the center of the cage cavity ranges from a very low value of 0.18 nm, for the flattened structure of 1(PF6)3 (Figures 4a and 4c), to 0.50 nm in [1(Cl)](PF6)2 (Figures 4b and 4d). In the

latter compound, the void is filled by the chloride anion. Indeed, PLATON26 analysis on this crystal structure after removal of the Cl− guests revealed freeaccessible voids of 51.3 A3 which constitute 1.7% of the unit cell’s volume [i.e. 3041.5 Å3 see Table S1]. This value corresponds to the void volume for the two cages residing in the unit cell, thus the calculated void volume per cage is ca. 25 A3 (~ the volume of a chloride anion). In the crystal packing of [1(Cl)](PF6)2 distinct cages are aligned by π-π stacking interactions (Figure S9) generating interstitial voids where PF6− anions and acetonitrile solvent molecules reside (Figure S10).

UV-vis. binding studies on a model compound To gain insights into the binding capabilities of the cage, UV-vis absorption titrations were performed with spherical anions in pure organic solvent and water mixture. The imidazolium system 13+, absorbing below 200 nm (as the majority of the investigated anions), was unsuitable for this kind of study. Therefore, the benzimidazolium-based receptor 23+ was synthesized, and UV-vis absorption titrations were carried out on the analogue system with Br− and Cl− (as TBA salts) in both CH3CN and CH3CN:H2O (98:2, v:v) mixture. Upon addition of the anion, the absorption bands of 23+ in pure acetonitrile solution underwent a slight blue-shift, similar to that observed in other benzimidazolium based systems.24-25 In the case of Br− and Cl−, a plateau was reached upon the addition of 1 eqv. of anion, indicating the formation of stable 1:1 host-guest stoichiometry (see Supporting Information). Treatment of the titration data gave a binding constant only for Br−, calculated as 5.8(1) log units (details are reported in the Supplementary material). For Cl− the titration profile was too steep for a reliable fitting (logK11 > 6). However, for both Cl− and Br− the association constants could be determined in CH3CN:H2O (98:2 v:v) mixture [logK11 = 6.1(1) for Cl−; 5.1(1) for Br−, see Figures S14-S15].



Notably, anion affinity followed the same trend found for other (benz)imidazolium-based receptors, with the binding constants decreasing with the decrease of the charge density of the spherical anion. These UVvis absorption studies on 23+, while providing useful information on the anion binding tendencies of the novel cages, did not give any indication on the conformational changes before and after the anion encapsulation.

NMR investigations on 13+ with anions In order to demonstrate that the large conformational changes observed for 13+ in the solid state also occurred in solution, NMR studies were performed on receptor 13+ with Cl− and other halides in CD3CN solution. Under 1H-NMR titrations (see Figure 5 and Supporting Information), significant shifts of the receptor’s signals were found for all the investigated anions, consistent with the formation of 1:1 receptor:anion adducts. In particular, the peaks of protons Hα and H1 underwent an initial broadening, alongside deshielding, and a final sharpening. In particular, the proton Hα resonance shifted up to 11.9 and 11.0 ppm with Cl− and Br−, respectively [∆δ, ppm = +3.2, Cl−; + 2.3, Br−, see Figure 5b]. In the case of fluoride, an even stronger downfield shift of Hα was observed (∆δ, ppm = +5.3; see Supporting Information); importantly, the attribution of the signal at 14.0 ppm to Hα was confirmed by the 1H−13C HSQC NMR spectrum (Figure S20), which showed a cross-peak between this resonance on the 1H NMR spectrum and a carbon resonance at 141.5 ppm, indicating that the resonance at 14.0 ppm could not be assigned to the formation of a HF2− species.27 The downfield shifts of Hα and H1 are consistent with the polarization effect exerted by the anion towards the C−Hα and C−H1 bonds. The crystal structure [1(Cl)]2+ actually evidenced that both the C−Hα and C−H1 groups point towards the center of cavity, where the anion is included; a similar structure can be expected in solution. In contrast to these results, a smaller shift, up to 9.8 ppm [∆δ = +1.1 ppm], was observed for iodide (see Figure 5b). This result, in line with the increasing radius of the spherical anions descending the halogen group, is indicative of weaker H-bonding interactions between the receptor and iodide, compared to the other halides. However, for I−, a 1:1 binding constant of 4.5(1) log units could be determined, while for other investigated guests the titration profiles were too steep for a reliable fitting (see Supporting Information). Conformational changes of 13+ in solution The conformational change upon anion binding in solution was explored by performing DOSY NMR experiments on 13+ and [1(Cl)]2+ in CD3CN (Figure S22). These studies allowed for the calculation of the hydrodynamic radius of the cage from the diffusion coefficients obtained for 13+ and [1(Cl)]2+, which were determined to be D = 8.6×10-10 m2/s and D = 9.4×10-10

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m2/s, respectively.28 According to the Stokes-Einstein equation, the diffusion coefficient is inversely proportional to the hydrodynamic radius of the molecule in solution. Hence, the hydrodynamic radius of the cage was calculated to be 7.5Å and 6.8Å in 13+ and [1(Cl)]2+, respectively. This result indicates that the binding event caused a change in the hydrodynamic radius of the receptor, which got smaller upon chloride encapsulation. However, this result does not necessarily entail a decrease of the cage size upon chloride complexation. The hydrodynamic radius is actually dependent both on the size of a molecule and on its electrostatic charge. Therefore, it must be considered that the receptor charge varies from +3 to +2 upon chloride binding, i.e. from 13+ and [1(Cl)]2+, and this certainly had an impact on the obtained values. To gain greater insight into the conformational changes of the cage under chloride inclusion, and to understand how this compared to the crystal structure of [1(Cl)]2+, NOE spectroscopy studies were performed on both 13+ and the chloride adduct in CD3CN solution. As demonstrated by X-ray diffraction studies, in the crystals of 13+ the tertiary amines are separated by a short distance (5.650 Å) and point their free electron pair away from the cavity. The cage assumes a flattened conformation, in which the aliphatic protons Ha are close to the protons H1 of the aryl fragment. Notably, interactions between protons Ha and H1 are also clearly visible in the NOE spectrum of 13+, suggesting that the collapsed shape found in the crystals is maintained in solution (see Figure S23). However, the presence of additional through-space interactions between aliphatic and aromatic protons indicates a certain flexibility of the molecule. The cross peak Hb−H1, not consistent with the X-ray study, is instead supported by our computational investigations (see later). These actually suggested several conformations for the cage, some of which presenting the Hb−H1 distance below 3Å. In the crystal structure of [1(Cl)]2+, the tertiary amines inverted their configuration, pointing the free electron pair towards the center of the cavity. In this new arrangement, the two Ntert are at longer distance (9.54 Å) and the cavity shape is more elongated than in the free cage. In addition, the protons Ha are turned to the outside, and are closer to H2 than to H1. Interestingly, the NOESY spectrum of the chloride complex points out that the interactions between protons Ha and H2 are also present in solution, while the cross-peaks between protons Ha and H1 are no longer visible (see Figure S24). The ensemble of these NMR investigations demonstrated that the free cage 13+ in solution assumes a collapsed conformation, similar to that found in crystals, which undergoes a significant rearrangement upon chloride binding. This rearrangement entails the elongation of the cage


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along the axis passing through the tertiary amines, and is accompanied by the convergence of the (C−H)+ donors towards the chloride guest.

Figure 6. Chemical shifts of protons Hα and H1 under alternating chemical stimuli (AgNO3/TBACl, CD3CN). 3+ Over the first cycle, 1 (i.e. free) was converted into 2+ [1(Cl)] (i.e. Cl) by treatment with 1.1 eqv. TBACl, and then recovered as free cage by addition of 1.1 eqv. AgNO3. The cycle was repeated at least four times (see NMR spectra in the S.I.).

Reversibility of the anion-induced motion The reversibility of the anion-induced motion was verified by studying the binding–unbinding properties of cage 13+ with the chloride anion. To remove the chloride guest from the cavity, slight excess of AgNO3 was added to a solution of [1(Cl)]2+ in CD3CN. Upon AgNO3 addition, the chloride anion was fully displaced, as was evidenced by the shifts observed for the protons Hα and H1, and the 1H NMR spectrum of the free cage was completely restored (see Figures 6 and S25). This result suggests that: i) the interaction of nitrate with 13+ is significantly weaker than that found for chloride; and that ii) in the absence of a coordinating anion, the collapsed conformation of the cage is favored compared to the elongated one. The first cycle could be closed, thus restoring the inflated conformation, by adding excess Cl− (as TBA salt) to the solution into the NMR tube. The extending–shrinking motion of the cage cavity could be repeated for several cycles (see Figure 6), just alternating the chemical stimuli (AgNO3/TBACl). The full reversibility of the molecular motion was attributed to the flexibility of the cage framework. Computational study Insight into the energetics of the conformational rearrangement of the cage 13+ upon chloride binding– unbinding was obtained through computational studies. The crystallographic coordinates of both the free cage and the chloride adduct were used as starting geometries for a first optimization at the B3LYP/6-31G(d) level. Optimized 13+, named conformer 1A (Figure S26), maintains the symmetry elements (a C3 axis and a mirror plane) already found

in the solid state with only small variations in the geometrical features, as indicated by the slightly elongated dNtert∙∙∙Ntert (5.98 Å) or shortened dCHim∙∙∙C-Him (10.51 Å) distances. However, the search in the conformational space of 13+ allowed to locate a more stable conformer, 1B (Figure S27) favored by 1.83 kcal/mol over 1A. Though 1B is as flattened as 1A (dNtert∙∙∙Ntert = 5.85 Å and dC-Him∙∙∙C-Him = 10.24 Å), it lacks the symmetry plane but maintains the C3 symmetry element. Notably, the unsymmetrical solid state geometry of [1(Cl)]2+ converged, after optimization, to a C3 symmetrical structure characterized by dNtert∙∙∙Ntert = 10.10 Å and dC-Him∙∙∙C-Him = 5.82 Å (Figure S28). In this optimized chloride complex, the receptor displays a twisted conformation, as found in the solid state; thus, upon anion binding, a racemic mixture of the helical M and P complexes can be obtained. Only the structure of the M−enantiomer is shown in Figure S28.Calling ϕ the torsion angle C−Ntert∙∙∙Ntert−C, we found that ϕ varies from 0° in the free cage, 13+, to -43° in [1(Cl)]2+ (see Figures S26 and S28).

Figure 7. Calculated structure of the intermediate int3 (M-enantiomer) obtained from the corresponding conformer 1C by inversion of one of the tertiary amines. The distance (in Å) between the tertiary amines is shown … in green. ϕ (in degree) = torsion angle C−Ntert Ntert−C. See Table S2 for more details.

When the chloride anion was removed from [1(Cl)]2+ and the cage optimized again, a new conformer of 13+, named 1C (Figure S29), was achieved. Notably, 1C is less stable than 1A by 10.61 kcal/mol; it maintains almost the same geometrical features and the helicity of [1(Cl)]2+ besides a slightly shorter dNtert∙∙∙Ntert distance (9.43 Å) and a larger cavity (dC-Him∙∙∙C-Him = 7.59 Å). On the other hand, a significant decrease of the torsion angle ϕ can be found, from -43° in [1(Cl)]2+ to -17° in 1C. The obtained result indicates that the crystal structure of [1(Cl)]2+ corresponds to an energy local minimum of the free cage shrunk by three hydrogen bonds with the chloride anion. The shrinkage along the equatorial plane, as well as the screwing along the C3 axis, found in [1(Cl)]2+ are mainly induced by the anion encapsulation. Interestingly, the calculated molecular volumes of 1A and 1C are very similar (1066 and 1069 Å3, respectively); while a slightly larger volume was found for [1(Cl)]2+ (1101 Å3), due to the contribution of the encapsulated chloride.



This result confirms that the smaller hydrodynamic radius of [1(Cl)]2+ vs. 13+, obtained in the DOSY experiment, depended on a lower electrostatic charge and not on size. The notable conformational differences between 1A and 1C suggest that they cannot directly interconvert as two nitrogen inversions and refolding of the three arms are difficult to occur simultaneously. So, we asked whether a pathway from 1C to 1A could be hypothesized. Inversion of one tertiary nitrogen of 1C might give rise to an intermediate species which, after inversion of the second tertiary nitrogen, might yield 1A. When we looked for a structure characterized by a tertiary amine resembling 1A and the other one resembling 1C, we found a local minimum (int3, Figures 7 and S30) characterized by a C3 symmetry and dNtert∙∙∙Ntert = 7.76 Å. This value is intermediate with respect to the corresponding values found in 1A and 1C as well as its relative energy (+8.37 kcal/mol with respect to 1A, see Figure 7 and Table S2). Notably, int3 displays the same helicity as 1C (i.e. M, in the structure shown in Figure 7) and a higher torsion angle (ϕ ~ 23°). We hypothesized that the interconversion of 1C with int3, as well that of int3 with 1A, could take place more easily if the C3 symmetry was initially lost through a refolding of only one of the three arms; then, nitrogen inversion and a back refolding of the arms follow to give a symmetrical geometry again. Probably, several trajectories for such interconversion do exist. One of them is reported in Figure S30: it connects 1C to int3 through three transition states and two new local minima (see TS1-TS3 and int1-int2 in Figure S30 and Table S2). The highest energy barrier was found to be only about 5.5 kcal/mol higher than 1C (see TS1 in Figure S30). Similar paths can be active for the interconversion of int3 with 1A. We looked for them and found conformational transition states characterized by extremely low energy barriers, in some cases within the computational error. This makes meaningless any suggestion about the real paths, only confirming the high flexibility of the cage.

Conclusions In this work, we have presented a novel type of (benz)imidazolium cages, whose flexible framework undergoes extending-shrinking motion under alternating chemical input (i.e. binding/unbinding of halide anions). When the cavity is empty, the cage assumes a flattened conformation, which allows the imidazolium units to stay far from each other, thus minimizing electrostatic repulsions. In this situation, the cavity is compressed along the C3 axis passing through the tertiary amines, and the two tribenzylamine platforms are eclipsed. Halide complexation is accompanied by a large rearrangement of the cage conformation. For instance, upon chloride binding, both an expansion of

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the cage cavity (∆d ~ 4Å) along the C3 axis and a shrinkage along the equatorial plane occur. Interestingly, the motion is accompanied by the pyramidal inversion of the tertiary amines, and by the rotation of the tribenzylamine-based platforms, which become staggered. The imidazolium containing arms wrap around the chloride anion, leading to a racemic mixture of the M and P−helical complexes. The cage motions are completely reversible: the switch between the two limit conformations can be actually repeated for several cycles under alternating chemical stimuli (AgNO3/TBACl). Computational investigations allowed us to quantify the energy difference between the shrunk and expanded cage conformations, and to hypothesize an energetic pathway along which the conformational rearrangement can occur. According to our computational model, the rearrangement passes through several transition states characterized by low energy barriers and separated by a series of intermediate structures. A proper sequence of events, corresponding to folding/refolding of the arms and pyramidal inversion at the tertiary amines, makes very easy the interconversion of the two structures. Besides shedding light on the halide-induced conformational rearrangements of an interesting class of anion receptors, the obtained insights can be of great interest for the important applications in which covalent organic cages are involved (as e.g. molecular hosts, nano-reaction vessels and catalysts)3, 6-10,12-15, 17-20.

Experimental Section All reagents for syntheses were purchased from Sigma Aldrich or VWR, and used without further purification. MALDI Q-ToF mass spectra were recorded on a MALDI Q-TOF Premier (Waters Corporation, Micromass MS Technologies, Manchester, UK) and high-resolution mass spectrometer was performed using Glu-Fib as an internal reference (m/z = 1570.677). The high resolution mass spectrum of 2(PF6)3 was recorded on a TripleTOF 6600 System (AB SCIEX) implemented with ESI. Melting points were determined using an Electrothermal IA9100 digital melting point apparatus. NMR spectra were recorded on a Bruker ADVANCE 400 spectrometers (operating at 9.37 T, 400 MHz). Deuterated solvents used for NMR analysis (CDCl3, CD3CN) were purchased and used as received. Chemical shifts are reported in ppm with the residual solvent as internal reference, while 2D spectra were graphically referenced. All NMR spectra were carried out at 25.0 °C. UV-vis. spectra were run on a Varian Cary 50 SCAN spectrophotometer, with quartz cuvettes of the appropriate path length (1 or 0.1 cm at 25.0 ± 0.1 °C). The synthesis of tris(329 (bromomethyl)benzyl)amine is reported elsewhere. Tris(3-((1H-imidazol-1-yl)methyl)benzyl)amine and tris(3-((1H-benzo[d]imidazol-1-yl)methyl)benzyl)amine were prepared by reacting tris(3(bromomethyl)benzyl)amine with excess imidazole and benzimidazole, respectively, using a modified procedure


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25c,e

already reported by our group. The syntheses yielded 88 mg of tris(3-((1H-imidazol-1-yl)methyl)benzyl)amine (Y = 83%) and 72 mg of tris(3-((1H-benzo[d]imidazol-1yl)methyl)benzyl)amine (Y = 59%), starting from 103 mg and 100 mg of tris(3-(bromomethyl)benzyl)amine, respectively. The obtained platforms were applied in the syntheses of the new receptors, 1(PF6)3 and 2(PF6)3. The experimental details regarding X-ray diffraction, UV-vis., NMR and computational studies are reported in the Supplementary Section.

ORCID

Synthesis of 1(PF6)3 Tris(3-((1H-imidazol-1-yl)methyl)benzyl)amine (0.095 mmol, 50 mg) was dissolved in 30 mL of 1:1 (v:v) chloroform:acetone mixture. The obtained solution was added dropwise to a solution of tris(3(bromomethyl)benzyl)amine (0.095 mmol, 53 mg) in 60 mL acetone. The reaction mixture was then refluxed for 72 hrs. The product, precipitated as a white solid, was filtered and dissolved in hot water. The final receptor was recovered as hexafluorophosphate salt, using NH4PF6 for the anion metathesis (0.046 mmol, 59 mg, 1 48% yield). Melting point range: 144-145°C. H-NMR (400 MHz, CD3CN): δ 8.69 (s, 3H, Hα), 7.49 (s, 6H, Hβ), 7.31 (d, 6H, H4), 7.30 – 7.20 (m, 12H, H3+H2), 7.11 (s, 6H, H1), 13 5.35 (s, 12H, Hb), 3.40 (s, 12H, Ha). C-NMR (100 MHz, CD3CN) δ 140.4 (Cα), 136.0 (q), 134.4 (q), 129.6 (C3), 129.5 (C2), 127.9 (C1), 127.5 (C4), 123.6 (Cβ), 57.2(Ca), 53.1(Cb). 3+ + + HRMS-MALDI (m/z): [M -2H ] calculated for C57H55N8, 851.4550; found 851.4552.

The authors declare no competing financial interest.

Synthesis of 2(PF6)3 A similar procedure was followed for the synthesis of using tris(3-((1H-benzo[d]imidazol-12(PF6)3, yl)methyl)benzyl)amine (0.097 mmol, 66 mg) and tris(3-(bromomethyl)benzyl)amine (0.097 mmol, 55 mg) as the starting compounds (see Scheme S1 in the Supplementary). The receptor was then precipitated as hexafluorophosphate salt (white solid) by anion metathesis (0.024 mmol, 35 mg, 25% yield). Melting 1 point range: 206-207°C. H-NMR (400 MHz, CD3CN) δ 9.07 (s, 3H, Hα), 7.69 – 7.50 (m, 12H, Hβ+ Hβ’), 7.46 (bs, 6H, H4), 7.31 (m, 12H, H3+H2), 6.82 (s, 6H, H1), 5.65 (s, 13 12H, Hb), 3.28 (s, 12H, Ha). C-NMR (100 MHz, CD3CN) δ 141.82, 140.97, 133.59, 131.87, 130.11, 129.47, 127.73, 114.27, 3 -1 58.11, 51.01. UV-vis. (MeCN): λ(ε) 278 nm (20 × 10 M cm 1 3 -1 -1 ), 271 nm (23× 10 M cm ). ESI-TOF (m/z, MeOH): 2+ [M(PF6)] calculated for C69H63N8PF6, 574.6282; found 574.5734.

ASSOCIATED CONTENT Supporting Information. Supplementary material: 1 details on X-ray diffraction, H-NMR, UV-vis. studies. Crystallographic data for 1(PF6)3 (both at low and room temperature) and 1(PF6)2·Cl and computational details; mp4 file showing the conformational conversion of 1C into int3. Supplementary Figures and Tables are also available (Figures S1–S32 and Tables S1–S2). CCDC reference number: CCDC 1867679-1867681. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION *[email protected]; *[email protected]

Anna B. Aletti: 0000-0002-2251-1122 Valeria Amendola: 0000-0001-5219-6074 Donatella Armentano: 0000-0002-8502-8074 Greta Bergamaschi: 0000-0002-4501-4057 Thorfinnur Gunnlaugsson: 0000-0003-4814-6853 Ana Miljkovic: 0000-0002-8875-8902 Lucio Toma: 0000-0001-8916-7445

NOTES

ACKNOWLEDGMENT MIUR, INSTM and the University of Pavia are acknowledged for funding. RB thanks the MIUR (Project PON R&I FSE-FESR 2014-2020) for predoctoral grant. We thank School of Chemistry TCD for the funding of a postgraduate studentship (to ABA) and Science Foundation Ireland (SFI) for a PI Award (13/IA/1865 to TG). Dr. Samuele Scurati and Dr. Stefano Fiorina is gratefully thanked by VA the for HRMS on 2(PF6)3.

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