Fullerene-Functionalized Halogen-Bonding Heteroditopic Hosts for Ion-Pair Recognition

Despite their hydrophobic surfaces with localized π-holes and rigid well-defined architectures providing a scaffold for preorganizing binding motifs, fullerenes remain unexplored as potential supramolecular host platforms for the recognition of anions. Herein, we present the first example of the rational design, synthesis, and unique recognition properties of novel fullerene-functionalized halogen-bonding (XB) heteroditopic ion-pair receptors containing cation and anion binding domains spatially separated by C60. Fullerene spatial separation of the XB donors and the crown ether complexed potassium cation resulted in a rare example of an artificial receptor containing two anion binding sites with opposing preferences for hard and soft halides. Importantly, the incorporation of the C60 motif into the heteroditopic receptor structure has a significant effect on the halide binding selectivity, which is further amplified upon K+ cation binding. The potassium cation complexed fullerene-based receptors exhibit enhanced selectivity for the soft polarizable iodide ion which is assisted by the C60 scaffold preorganizing the potent XB-based binding domains, anion−π interactions, and the exceptional polarizability of the fullerene moiety, as evidenced from DFT calculations. These observations serve to highlight the unique properties of fullerene surfaces for proximal charged guest binding with potential applications in construction of selective molecular sensors and modulating the properties of solar cell devices.


■ INTRODUCTION
−26 Although molecular electrostatic potential (MEP) surfaces of simple fullerenes are positive, surprisingly, their interaction with anions has been largely overlooked. 6,27Only recently, Matile and co-workers demonstrated remarkable examples of the stabilization of anionic transition states in anion−π catalysis on a fullerene surface, 28−30 while Lei and co-workers reported facilitated charge transfer in solid-state aggregates of self-n-doped fullerene ammonium iodide, which is believed to be a result of iodide− C 60 interactions. 31However, thus far fullerene surfaces have not been exploited as potential supramolecular host platforms for the recognition of simple anions (e.g., halides) in molecular receptor structural design.Nevertheless, it is worth noting that an open-cage fullerene was demonstrated as a molecular container for F − , Cl − , Br − , and I − . 32he MEP surface of C 60 reveals highly localized areas of positive potential, π-holes (Figure 1), which can be presumably used in anion recognition.−36 Strong attraction between an anion and a π-system can be achieved by electron-withdrawing substituents that further polarize the molecule and lead to a positive quadrupole moment along the axis perpendicular to the π-system, increasing the depth of a π-hole. 33Interestingly, fullerenes are known for their remarkable polarizability 37 which, in principle, may enable a significant enhancement of anion−π interactions by exposure to an external electric field, produced for example by a proximate anion (so-called dynamic contribution) or cation.Moreover, the well-defined bulky architecture of fullerenes provides a potential scaffold for preorganization of binding motifs and a hydrophobic shield, which can create a microenvironment that excludes solvent molecules and enhances strength of noncovalent interactions. 38,39Due to these features, C 60 constitutes an exceptional and unexplored platform for the design and construction of heteroditopic ion-pair receptors with increased affinity and selectivity.The positive cooperativity associated with the simultaneous proximal binding of oppositely charged species has been crucial in augmenting the ion-pair binding properties of heteroditopic receptors relative to their monotopic receptor counterparts.As such, heteroditopic receptors have been increasingly employed in a myriad of applications including salt extraction and solubilization, 40,41 membrane transport, 42,43 and biological zwitterion binding. 44,45−52 Herein, we describe for the first time the rational design, synthesis, and unique recognition properties of   novel fullerene-functionalized halogen-bonding heteroditopic ion-pair receptors containing cation and anion binding domains spatially separated by C 60 (Figure 2).

■ RESULTS AND DISCUSSION
Synthesis of Fullerene Heteroditopic Ion-Pair Receptors.Interest in crown ether−fullerene adduct materials has been stimulated primarily by their photophysical, electrochemical, and superconducting properties. 53In particular, Echegoyen, Pretsch, Diederich, and co-workers obtained the C 60 -dibenzo-18-crown-6 (DB18C6) adduct in a highly regioselective double cyclopropanation (Bingel addition) taking place exclusively in the trans-1 positions on the opposite poles of C 60 (Figure 3a). 54,55Potassium cation crown ether binding in the proximity of the fullerene surface was shown to elicit significant perturbations of the fullerene host's reduction potentials, proving that alkali metal cation complexation can alter the physicochemical properties of C 60 .We hypothesized that a complexed cation could further polarize the fullerene surface, resulting in anion binding enhancement on the opposite size of the molecule (Figure 2).Therefore, we adapted the regioselective DB18C6 double Bingel fullerene addition for the synthesis of heteroditopic ion-pair receptors 1 and 2 containing neutral acyclic XB donors based on 1,3bis(iodotriazole)nitroaryl motifs in the anion binding domains, 56 spatially separated from the polarizable fullerene surface by linkers of different lengths (Figure 3b).
The separate appropriately functionalized crown ether− fullerene cation and halogen-bonding anion binding domain synthons were prepared according to Schemes 1 and 2. The synthesis of the bis-alkyne appended crown ether−fullerene synthon 9 was achieved via modification of the regioselective procedure reported by Diederich and co-workers (Scheme 1). 543,4-Dihydroxybenzaldehyde 3 was alkylated with an excess of bis(2-chloroethyl) ether to obtain 4 (21%), which could be readily separated from the other regioisomer.Macrocyclization of 4 in the presence of the K + template afforded the poorly soluble trans-dialdehyde of DB18C6 5 (31%), which upon reduction using NaBH 4 gave the diol 6  (62%).Monomalonate 7 was prepared either by treating 4pentyn-1-ol with Meldrum's acid (68%) or via an alternative approach involving selective monohydrolysis of a symmetric malonic ester (see the Supporting Information for details).Diol 6 was coupled with excess 7 using EDC to obtain bismalonate ester crown ether 8 (76%).Bingel reaction of 8 with C 60 in the presence of K + and I 2 led exclusively to doubly substituted fullerene adduct 9 (25%).The 1 H NMR and 13 C NMR spectra of 9 (Figures S5 and S6)were in agreement with the trans-1 addition pattern (C 2 symmetry), which was later unambiguously confirmed by single crystal X-ray diffraction structural analysis (Figure 4). 57Solid-state analysis also revealed that the DB18C6 ester groups of the cyclopropane rings are situated on the same side of the fullerene (out−out isomer).Interestingly, rotation of the crown ether moiety is significantly limited, causing planar chirality and splitting of the benzylic and ether CH 2 signals in the 1 H NMR spectrum (see Figure S5).
The synthesis of receptor 2 with a shorter linker between the anion binding domain and the fullerene surface was initially attempted in an alternative approach with a Bingel reaction conducted on precursor 16 (Scheme 3).Unfortunately, this resulted in a complex mixture of products, suggesting that the anion binding motif is not compatible with the conditions of the cyclopropanation reaction.An alternative route involving Cu(I)-catalyzed azide−alkyne cycloaddition (CuAAC) reaction between alkyne 12 and DB18C6-fullerene bis-azide 18 proved successful (Scheme 3).−60 However, a Bingel reaction of short duration time between 17 and C 60 , followed by rapid chromatographic purification afforded 18, which was used immediately in the next step to obtain final receptor 2 (14%).
Anion and Ion-Pair Binding Studies.The anion binding properties of fullerene containing heteroditopic receptors 1 and 2 were investigated by 1 H NMR titration experiments in 3:1 CDCl 3 :CD 3 CN.Addition of TBA halides (Cl − , Br − , I − ) to solutions of the free receptors 1 or 2 caused significant shifts of the XB anion binding domain proton signals, and no changes (Δδ < 0.01 ppm) of the crown ether cation binding protons were observed.Such a behavior strongly indicates that the ditopic binding domains of receptors 1 and 2 are electronically and spatially well-separated from each other.Notably, the respective receptor's internal nitroaryl proton (a) shifted downfield, which is indicative of halide binding in a cavity formed by the iodotriazole XB donors (Figure 5).Bindfit analysis of the titration isotherm data revealed that 1:1 and 1:2 stoichiometric host−guest complexes are formed (Table 1). 61,62In the 1:1 complex, the halide anion is most likely bound by both bidentate XB motifs, contributing up to four halogen bond donors.Upon addition of excess halide anion, each appended XB bidentate recognition site binds an individual anion with two halogen bond donors to form a 1:2 stoichiometric host−guest complex.Unsurprisingly then, K 1:1 association constant values are more than an order of magnitude larger than those of K 1:2 .Receptor 1 binds halides more strongly than 2 with a selectivity trend of Br − > I − > Cl − .The enhanced halide anion binding by 1 may be attributed to the longer, flexible linker providing the conformational freedom to facilitate the formation of stronger XB−halide anion interactions in the 1:1 complex.Control receptor 16, which does not contain fullerene, exhibits a halide selectivity trend mirroring that of 1, however with lower K 1:1 values.Interestingly, receptor 2 exhibits a unique, however modest, preference for I − over Br − and Cl − which may be a result of a shorter distance between the XB binding units and the fullerene surface.The combination of the C 60 fullerene scaffold's preorganization of the two XB anion binding arms proximal to the hydrophobic fullerene surface and possible additional anion−π interactions are most likely responsible for the enhanced binding of 1 and a unique iodide binding selectivity of 2.
Dibenzo-18-crown-6 is known for its high affinity for potassium cations.Therefore, the K + binding properties of the fullerene containing receptors 1 and 2 were also investigated by 1 H NMR titration experiments in 3:1 CDCl 3 :CD 3 CN.Addition of KBAr 4 F to solutions of free receptors 1 or 2 resulted in significant perturbations of the crown ether cation binding domain chemical shifts.With the first aliquots of KBAr 4 F , notable signal broadening was observed, and a new set of signals emerged due to slow exchange on the NMR time scale (Figure 6).In particular, the aromatic proton signals of the receptor's DB18C6 motif experienced notable downfield shifts (Δδ ≈ 0.10 ppm), concomitant with −OCH 2 − crown ether perturbations, which due to significant broadening were difficult to follow.After 1.4 equiv of K + , however, no further changes were observed, which is indicative of strong binding in a 1:1 stoichiometric host− guest complex.Importantly, no significant changes (Δδ < 0.01 ppm) were observed in the proton signals of the XB anion binding domains of both receptors.This observation further corroborates a good separation of the anion binding domain from the cation one.
To investigate the ion-pair binding properties of receptors 1 and 2, halide anion 1 H NMR titration experiments in the presence of 1 equiv of KBAr 4 F were undertaken.Addition of TBAI to a solution of K + complexed 1 or 2 caused perturbations of the XB anion binding sites, confirming that XB donors are involved in anion binding.In fact, these shift patterns were qualitatively similar to those observed during titrations of the free receptors.However, small changes were also observed in the −OCH 2 − proton signals of the crown ether cation binding domain.Notably, the downfield perturbations of the −OCH 2 − crown ether signals around 3.90 ppm were larger during titrations with bromide and even greater with chloride (Figure 7).This perturbation pattern cannot be explained by simple potassium cation decomplexation of the crown ether and precipitation of the potassium halide salt (compare with the spectra shown in Figure 6).
The aforementioned observations suggest that crown ether bound K + is directly involved in complexation of the anions.Interestingly, a qualitatively similar perturbation pattern of the crown ether signals was also observed during analogous titrations of K + complexed C 60 -DB18C6 adduct 9, which does not contain a XB anion binding domain (Figure 7).In this case, however, the overall signal shifts were more pronounced.In the presence of 1 equiv of KBAr 4 F receptor 9 forms 1:1 stoichiometric halide complexes with the preference for hard small anions: Cl − > Br − > I − in 3:1 CDCl 3 :CD 3 CN, as determined by the Bindfit analysis of binding isotherms (Table 2).This assembly is driven predominantly by electrostatic interactions with the crown ether complexed potassium cation resulting in the formation of a close contact ion-pair.
We suspect that such a binding mode is also present during titrations of receptors 1 and 2 with halides, particularly harder ones such as Cl − and Br − .However, due to spatial separation of the cation and anion binding domains in receptors 1 and 2, two distinct types of 1:1 stoichiometric complex A and B can be simultaneously formed, contributing to the experimentally determined overall 1:1 association constants (Figure 8).In binding mode A, the anion is exclusively bound by the XB binding site, while in mode B the anion associates solely with the crown ether bound potassium cation in a contact ion-pair recognition fashion. 63,64Similarly, upon excess addition of anion, two types of 1:2 stoichiometric complexes are possible: C, with two anions bound individually by the XB donor arms, and D, in which one anion is bound in a tetradentate XB fashion and the other anion is associating with the potassium cation (Figure 8).
This hypothesis was further corroborated by the 1 H NMR titration of 2•K + with NO 3 − , which exhibits strong preference for the close contact ion-pair formation (mode B).During the titration with TBANO 3 no changes were observed in the XB anion binding domain, while significant perturbation of the Quantitative analysis of the halide binding isotherms in the presence of 1 equiv of KBAr 4 F , using a 1:2 stoichiometric host−guest model, gave overall association constant values shown in Table 2, where K 1:1 represents the sum of 1:1 stoichiometric anion binding modes A and B and K 1:2 the sum of 1:2 stoichiometric anion binding modes C and D. Comparing Tables 1 and 2, the presence of the complexed    K + in the respective C 60 -DB18C6 cation binding domain of the XB receptors 1 and 2 results in a significant enhancement of halide association constants, particularly of K 1:1 .Importantly, it was possible to deconvolute and estimate the individual contributions of A and B halide binding modes to the overall K 1:1 association constant value through the analysis of the chemical shifts of the crown ether protons of receptors 1 and 2 and control receptor 9 during halide anion titrations in the presence of KBAr 4 F (see the Supporting Information for details).In the case of 1, the binding mode A accounts for approximately 63% of the overall 1:1 chloride association constant, 83% of 1:1 bromide association constant, and more than 95% of 1:1 iodide association constant, clearly showing the preference of the heavier softer halide anions toward fullerene-assisted XB binding.Further corroborating these estimates, the values of 1:1 association constants for mode B of receptors 1 and 2, obtained using this method, are in good agreement with the values obtained during titrations of control receptor 9, which is able to bind anions only via mode B.
Deconvolution of the A and B binding modes of 1:1 association enabled a direct comparison of K + coordination  effects on the fullerene-assisted halide binding in the XB domain.The K 1:1 association constant values for anion binding mode A (K 1:1 A ) of both heteroditopic receptors 1 and 2 are significantly increased in the presence of cobound K + (Table 3).Notably, the binding enhancement for iodide is particularly strong, resulting in a remarkably increased selectivity for this anion.In the case of receptor 2, K 1:1 A association constants increased by a factor α = 2.2 (α = K 1:1 A (K + )/K 1:1 ) and 2.1 for chloride and bromide, respectively, while for iodide α = 5.2.Even stronger I − enhancement was observed for receptor 1 (α = 7.4); however, overall selectivity of 1 for I − vs other halides is reduced in comparison with 2. In the presence of cobound K + , control heteroditopic receptor 16, without the fullerene scaffold, binds all the halides significantly more strongly (α = 12.7−13.5),however notably at the expense of much lower selectivity.This is most likely due to the formation of a close contact ion-pair, resulting in stronger electrostatic interactions.The remarkable properties and influence of C 60 for ion recognition are particularly evident in comparison of iodide binding affinity exhibited by 1 and 16.Impressively, the iodide K 1:1 association constant value of receptor 1, whose anion and cation binding domains are separated by the fullerene, almost matches the magnitude for receptor 16, which is capable of anion binding assisted by close contact with the crown ether cobound potassium cation.Importantly, this suggests that the polarizing C 60 surface can elicit particularly strong interactions with polarizable soft anion species such as iodide and can transfer electrostatic effects over significant distances within the fullerene heteroditopic host design.
Computational Analysis.Having demonstrated the unique ion-pair recognition properties of fullerene-containing heteroditopic receptors 1 and 2, DFT calculations were undertaken to gain insight into the electronic and structural aspects of ion-pair complexation.In the computational analysis, we focused on receptor 2, which manifested the highest selectivity, presumably due to the closer proximity of the fullerene surface.−73 This combination was employed to balance the accurate description of the noncovalent interactions and the structural features of the large receptors 1 and 2.
The MEP surface of C 60 (Figure 1) exhibits distinct electrophilic regions, with π-holes situated above its 12 five-Table 3. Anion Association Constants for Receptors 1 and 2 (K 1:1 A , M −1 , Anion Binding Mode A) and 16 (K 1:1 , M −1 ) in the Presence of 1 equiv of K + 1 2 16   membered (C 5 ) rings and 20 six-membered (C 6 ) rings.These π-holes have molecular surface electrostatic potential (V S ) values ranging between 7.2 and 7.9 kcal mol −1 .For comparison, 9 and its K + complex were also optimized by DFT (Figure 9).The DB18C6 motif in adduct 9 has a significant effect on the electrostatic potential map, resulting in a nearly negatively charged fullerene surface.The exposed surface of the fullerene displays several π-holes with V S values ranging between −5.  2).The starting geometries of 1 and 2 were generated via crude gas-phase MD simulations of KCl complexes, enforcing halogen bonds through geometric restraints, as detailed in the Supporting Information.Multiple conformations were selected and subsequently subjected to geometry optimizations using DFT. Figure 10 shows the optimized structures of chloride complexes of 2 with potassium hosted within the DB18C6 cation binding domain for the A−D anion binding scenarios, consistent with the binding modes hypothesized based on 1 H NMR titrations, while Figures S72 and S73 show equivalent optimized binding arrangements for bromide and iodide.
Potassium binding by the DB18C6 moiety can be characterized by K + •••C 6 distances summarized in Table S7.In scenarios A and C, the average K + •••C 6 distance is ca. 3 Å, whereas in binding mode B, the ca.3.5 Å average K + •••C 6 distance is significantly larger.Due to the close ion-pair contact in B, the anion pulls K + from the crown ether, weakening the potential interaction with the fullerene surface. 54Interestingly, in scenario D, the K + •••C 6 distances have intermediate values between those computed for B and A/C, showing that binding in one domain can influence the behavior on the opposite side.
In binding mode A, the four convergent halogen bonds with chloride are not equivalent (Table S8).S8).This asymmetry is not surprising considering the differences in the iodotriazole units of the anion binding domain.One is directly connected to a strong electron-withdrawing 3,5-bis(trifluoromethyl)phenyl group, while the other is connected to an electron-rich alkyl-substituted phenyl.Importantly, the halogen-bonding anions' recognition is assisted by anion−π interactions with short contacts between the fullerene surface and each anion, leading to the trend of C   After optimization of the putative binding arrangements for the recognition of halides in binding modes A−D, the MEP maps of 2 and 2•K + were evaluated through single-point DFT calculations.To achieve this, we used the optimized structure of the Cl − association in scenario D and removed the necessary ions (Figure 11).The most negative region of electrostatic potential on the electronic surface of 2 covers the oxygen atoms of the crown ether (including the V S,min of −45.8 kcal mol −1 ), while the most positive regions are found in front of the iodine binding clefts, with their σ-holes characterized by V S values of 43.6 and 44.3 kcal mol −1 and of 48.2 and 48.3 kcal mol −1 for the iodo-triazole unit activated by neighboring −CF 3 groups.Complexation of K + in the cation binding domain of 2 leads to a significant redistribution of the MEP surface.Naturally, the V S,max of 130.2 kcal mol −1 is located over the cation; however, the four XB units display augmented V S values between 75.3 and 79.7 kcal mol −1 .Notably, an V S point of 61.5 kcal mol −1 was found positioned over the C 6 ring in the vicinity of the preorganized XB binding units.
The strength of the XB interactions in different binding modes was further evaluated with the natural bond orbital (NBO) analysis using the second-order perturbation theory interaction energies (E 2 , see Table S9).The analysis of the E 2 values for the interactions between the C−I antibonding orbitals of 2 and the halides' lone pairs orbitals (n X → σ* C−I ) revealed that in binding mode A the total energies follow the trend Cl − (37.5 kcal mol −1 ) > Br − (36.3 kcal mol −1 ) > I − (33.1 kcal mol −1 ).A similar analysis was also performed for the interactions between the fullerene scaffold's π-holes and the three halides in binding modes A and D, revealing that E 2 values resulting from n X → σ* C−C are over an order of magnitude weaker (0.5−1.3 kcal mol −1 ) than the XB interactions.
Although the computational analysis suggests that in the gas phase binding mode B is preferred for 2 by 3.0 (Cl − ), 5.1 (Br − ), and 6.2 kcal mol −1 (I − ) , it is worth noting that in the case of 1•KCl, binding mode A is favored by 24.9 kcal mol −1 relative to B (Figure S74). 75In this complex, however, the halide does not form a contact with the fullerene surface, and the receptor adopts a conformation that maximizes the strength of the halogen bonding.
Altogether, the computational analysis reveals that the fullerene platform can play a dual role in anion binding by receptors 1 and 2. Indeed, it can be actively involved in the binding events by exploiting π-holes on its surface, but it can also serve as a bulky scaffold to preorganize the potent XBbased binding units into a tight binding cavity.

■ CONCLUSIONS
For the first time, the C 60 fullerene motif has been successfully integrated into a heteroditopic ion-pair host design.The combination of highly potent XB donors and a crown ether moiety separated by C 60 led to the rationally designed receptors with ion-pair binding properties influenced and modulated by the fullerene motif.Receptors 1 and 2, which differ in length of linkers separating anion binding motifs from the fullerene surface, are capable of strong and more selective binding of halide anions than their non-fullerene analogue.This is achieved by the preorganization of binding units, solvent shielding, and π-hole assistance provided by the bulky and highly polarizable C 60 architecture.Remarkable halide anion binding enhancements can be achieved by the complexation of a potassium cation by the rigid crown ether moiety located close to the fullerene surface.Notably, potassium binding by receptors 1 and 2 results in strong augmented iodide binding selectivity, with association constants matching the value of the non-fullerene heteroditopic receptor analogue which is capable of anion binding assisted by the close contact with the crown ether cobound potassium cation.
Fullerene spatial separation of XB donors and the crown ether complexed K + resulted in a rare example of an artificial receptor containing two anion binding sites with opposing preferences for hard and soft halides.Detailed analysis of 1 H NMR titration data allowed for deconvolution of the ion-pair binding modes contributing to the overall 1:1 stoichiometric halide anion binding by 1•K + and 2•K + .Soft polarizable iodide was bound almost exclusively (>90%) by the tetradentate XB binding domain in proximity to the fullerene surface.This binding mode also dominated in the case of smaller and harder chloride (ca.60%); however, a significant portion of chloride (ca.40%) was associated in a close contact ion-pair with the crown ether bound potassium cation without assistance of the XB binding domains.Altogether, the presented results demonstrate the unprecedented potential of fullerene surfaces in anion recognition host−guest chemistry.Importantly, polarizing the C 60 surface via proximal cation recognition can elicit particularly strong interactions with polarizable soft anion species such as iodide and transfer electrostatic effects over significant distances.Modulating the properties of fullerene-based compounds via reversible noncovalent charged guest recognition may find future applications in molecular sensors, solar cell devices, and photodynamic therapy.

Figure 1 .
Figure 1.Molecular electrostatic potential (MEP) surfaces of C 60 (left), C 60 functionalized with methylene (center left) or two cyano electronwithdrawing groups (center right), and C 60 in association with naphthalene diimide (right).The MEP surfaces are rendered at the 0.001 electrons Bohr −3 contour and the π-holes on the surface of C 60 are identified as black dots.

Figure 2 .
Figure 2. Cartoon representation of a tetradentate XB fullerene-functionalized heteroditopic host for ion-pair recognition.Purple spheres represent iodine halogen bond donors.

Scheme 2 .
Scheme 2. Synthesis of Anion Binding Domain and Receptor 1 a

Figure 4 .
Figure 4. Solid-state structure of C 60 -DB18C6 adduct 9 with a water molecule (green color) hydrogen bonded to a crown ether.

Scheme 3 .
Scheme 3. Receptor 16 and the Synthesis of Receptor 2 a

Figure 5 .
Figure 5. Truncated 1 H NMR spectra of receptor 2 in 3:1 CDCl 3 :CD 3 CN with an increasing amount of TBAI.Corresponding binding isotherms were used for determining values of binding constant.

aSolvent: 3 : 1
CDCl 3 :CD 3 CN at 298 K. Values reported as the mean and the standard error of the mean from independently repeated experiments.

Figure 6 .
Figure 6.Truncated 1 H NMR spectra of receptor 2 in 3:1 CDCl 3 :CD 3 CN with an increasing amount of KBAr 4 F .Broadening of the aliphatic crown ether signals hinders their assignment.

Figure 8 .
Figure 8. Proposed anion binding modes of fullerene-functionalized halogen-bonding heteroditopic hosts in the presence of crown ether bound potassium cation.

aSolvent: 3 : 1
CDCl 3 :CD 3 CN at 298 K. Values reported as the mean and the standard error of the mean from independently repeated experiments.b Binding enhancement factors in the presence of K + .c Contributions of anion binding mode A to overall 1:1 binding.

Figure 9 .
Figure 9. DFT-optimized structure of 9•K + (center, top), together with the MEP surfaces calculated on the adduct free of potassium (left) or on the complex (right), in lateral and top views.The MEP surfaces are rendered at the 0.001 electrons Bohr −3 contour and the π-holes on the surface of C 60 are identified as black dots.
5 and −0.2 kcal mol −1 .The lowest V S values of 9 were found between the oxygen atoms of the crown ether cavity, varying between −58.6 and −57.5 kcal mol −1 .The MEP surface of 9's C 60 moiety has an additional negative point of V S with −29.5 kcal mol −1 , perpendicular to the C 6 ring just below the crown ether.This electron-rich site is perfectly prepared for the coordination of K + , as evidenced by a computed K + •••C 6 distance of 2.93 Å in 9•K + .The potassium cation binding induces a significant redistribution of the electrostatic potential in 9, with the C 60 π-holes' V S values now ranging from 30.4 to 47.2 kcal mol −1 .For comparison, typical π-hole donors trifluoro-1,3,5-triazene or hexafluorobenzene,74 investigated at the M06-2X/Def2-SVP theory level, respectively display V S,max values of 40.9 and 21.5 kcal mol −1 .However, in complex 9•K + , the V S,max of 111.8 kcal mol −1 is found over the metal cation, which explains its strong tendency to form close putative ion-pair contacts with halides, as depicted in FigureS71with the DFT optimized structures of 9•K•X (X = Cl, Br, or I) complexes.The computed K + •••X − distances (Cl − : 2.85; Br − : 3.02; I − : 3.23 Å) mirror the anion's size, with the shortest contact corresponding to the highest association constant, found for 9•KCl (Table

Figure 10 .
Figure 10.DFT-optimized anion binding modes of fullerene-functionalized halogen-bonding heteroditopic host 2 in the presence of the crown ether complexed potassium cation.

Figure 11 .
Figure 11.MEP surfaces calculated on the DFT-optimized geometry of 2 in binding scenario D (Figure10D): free of ions (left) and in the presence of K + (right).The MEP surfaces are rendered at the 0.001 electrons Bohr −3 contour and the σ-holes in front of the XB binding units are identified with black dots, while a neighboring π-hole on the surface of the C 60 scaffold is identified with a white dot.