Eric S. Larsen

The bent, polycyclic aromatic hydrocarbon provides a surface that topologically matches C60 and enables the primary and secondary binding events with positive cooperativity (α = 2.56) and high affinity (K1 × K M-2). A series of DFT calculations (M06-2X-D3/6-31G(d,p)) show that the first binding event results in charge transfer from azacorannulene to C60 and causes the fullerene to experience an allosteric change. Curvature, π-orbital axis vector and other structural analyzes indicate that the resulting compressed spheroid has an open site with increased shape complementarity to 1 and thus facilitates the secondary binding event.

A Perspective on Buckybowls and their Complexes with Fullerenes

Introduction
Historical Perspective
Conclusions and Outlook
References

4] However, exploration of this molecule has been severely limited by insufficient access resulting from the low yield (<1%) of the reported 17-step synthesis. Currently, production of corannulene on a kilogram scale now enables commercial retail sales of 1.[7] At the same time, and perhaps not surprisingly, the availability of larger quantities of buckminsterfullerene in the 1990s stimulated scientific interest in investigating the supramolecular chemistry of new carbon allotropes (e.g., C60, C70, and higher fullerenes). Although support for this interaction has been confirmed in the solid state.[9] Thus, the lack of solution-phase evidence for this connection led to the conclusion that "the attractive force of concave-convex interactions is not that important, if at all" for these systems [2].

Corannulene derivatives that showed improved binding to C60 by increasing the surface area and/or electronic properties of the parent scaffold 1. Ultimately, the synthesis of a pentakis (1,4-benzodithiino) derivative (2) showed the largest binding affinity for C60 (Ka = 1400 M-1) of every corannulene derivative to date.[11] In 2016, Siegel and Baldridge reported that a simple fivefold benzoidal extension of corannulene could be used to effect complexation with C60 in solution. Importantly, pentaindenocorannulene (3) demonstrated that electronic doping was not necessary for successful assembly of fullerene. This class of hosts, commonly referred to as molecular tweezers or clips, benefits from pre-organization and can consequently avoid some of the unfavorable entropic costs associated with complexation.

Importantly, the resulting solid structure of the inclusion complex showed unambiguous concave-convex π-π interactions and thus indicated the importance of complementary concave-convex π-π interactions in supramolecular chemistry. As will be detailed in Chapter 2, our work describes the judicious selection of the truncated derivative 5 to induce allosteric changes at C60, allowing the assembly of a termolecular complex that exhibits positive cooperativity.

Figure 1. A selection of polycyclic aromatic hydrocarbons and buckminsterfullerene demonstrating the curvature-effect of incorporating five-membered rings into a circulene scaffold has on curvature

Stereoelectronically-Induced Allosteric Binding: Shape Complementarity Promotes Positive

Introduction

Enhancement of binding has also been demonstrated by binding multiple buckybowls together in such a way that they become preorganized to chelate C60. [16a] For example, the "buckycatcher" consists of two corannulene units stretched over a tetrabenzocyclooctatetraene linker and acts as a kind of molecular tweezers. for C60 (Ka = 2700 M-1, toluene-d8).[22] Azacorannulene-based tweezers have also been synthesized and shown to exhibit large association constants with C60 (up to 3.0 x 108 M-1 in toluene), depending on the linker used. Despite the growing diversity, the majority composed of buckybowl-fullerene complexes can be classified as 1:1 host-guest systems. Multicomponent assemblies consisting of multiple fullerene hosts and/or multiple buckybowls are relatively rare in solid [10, 23] or solution states, [16b,. Although buckybowls generally exhibit smaller curvatures compared to those of C60, the binding affinity between two adjacent surfaces can be expected to increase as their curvatures become more closely aligned.

Furthermore, complexation of a buckybowl with C60 should result in charge transfer that effectively increases the curvature of the uncomplexed region of the fullerene. Such a process increases the topological complementarity between the complexed C60 intermediate and a free buckybowl, and thus can promote the formation of a superstoichiometric complex. To test this hypothesis, an azacorannulene (1) was selected with a surface that is relatively curved, rich in electrons, and monosubstituted to facilitate binding to C60 while maintaining a reasonably high degree of solubility.

As will be described below, it was discovered that the azacorannulene not only forms a 2:1 complex with C60 in solution and in the solid state, but also binds to C60 with positive cooperativity as predicted by the hypothesis. A number of calculations and models are also presented to gain a deeper understanding of the complex formed and to clarify the unique binding phenomena.

Results and Discussion

To determine the stoichiometry of the complex, a working graph was generated from the spectroscopic data. As shown in Figure 6c, the binding isotherm exhibited an inflection point when the stoichiometry of the two components was 2:1 and then began to saturate. and comparable to the values reported for the aforementioned molecular tweezers.[21b] Together, these data indicate that a tight 2:1 complex was formed between 1 and C60 and that the formation process occurred with positive cooperativity. A working outline was designed to determine the stoichiometry of the complex formed as obtained by measuring the response, defined as ⏐Δδ⏐ × 106, between 1 and C60 in toluene-d8 at 25 °C.

As shown in Figure 7, the solid-state structure has a concave-convex relationship analogous to those observed in other types of buckybowl•fullerene complexes reported in the literature.[18a, 19a, 20a] The bowl-to-ball (BtB) distance measured for ( 1)2•C60 (6.88 Å) was between the values reported for the solid-state structures of tetra-tert-butyl-6b2-azapentabenzo[bc,ef,hi,kl,no]corannulene)•C60 . To obtain a deeper understanding of the supramolecular assembly process and the structures of the complexes formed, a series of DFT calculations (M06-2X-D3/6-31G(d,p)) were performed on C60, 1, 1•C60 and ( 1)2•C60. For example, the calculated assembly angle of the azacorannulen units (179.9°) and the BtB distance (6.73 Å) were consistent with the X-ray data.

Key intermolecular charge transfer interactions were also identified upon inspection of the calculated structures for (1)2•C60 and 1•C60. Calculations showed that the first binding event caused the opposite (open) region of the host to experience compression as the semi-minor axis of C60 in 1•C60 (7.073 Å) shrank when compared to that of virgin C60 (7.079 Å) . ).

Figure 6. (a) 1 H NMR spectra that were recorded as 1 (240 µM) was titrated with up to 52 equivalents of C 60 (2.5 mM) in toluene-d 8 at 25 °C

Conclusion

14 . 2.70 eV), which may explain why the second binding event is less favored than the first (Figure 9). The exposed surface of 1•C60 is relatively electron-rich due to charge transfer and may therefore be unfavorable for binding to another unit of (electron-rich) 1. However, the electronic offset caused by the first binding event appears to be compensated by steric changes that effectively increase the topological complementarity between the host and the guest and therefore underline the importance of the stereoelectronically induced geometrical changes in fuller complexation phenomena. Energy orbital diagrams illustrating the changes in the LUMOs (top) and HOMOs (bottom) calculated for the C60, 1•C60 and (1)2•C60 structures.

Experimental

The combined organic layers were dried over anhydrous magnesium sulfate and filtered, and the remaining solvent was removed under reduced pressure to give a white, crystalline solid (9.46 g, 97% yield). The reaction mixture was then removed from the glovebox, filtered and the precipitate washed with triethylamine (3 x 40 mL). The remaining solvent was removed under reduced pressure and the crude material was allowed to stand overnight.

The reaction was cooled to room temperature and the organic layer was separated and the aqueous layer was extracted with diethyl ether ml). The flask and syringe were removed from the glove box and the flask was placed in an oil bath heated to 100 °C. The combined organic layers were dried with anhydrous sodium sulfate and the remaining solvent was removed under reduced pressure.

This mixture was extracted once with CH2Cl2 (20 mL) and the collected organic layer was discarded. This solution was filtered and the material was washed with hexane and methanol (1st batch of material).

Acknowledgements

However, this pursuit would not have been possible without the support of some of the people around me. The titrant solution was used to prepare a titrant solution of C60 (2.5 mM), which was also stirred for 30 min. The titration was then performed by injecting aliquots of C60 via syringe into a 4 mL septum-sealed NMR tube filled with 0.5 mL of the titrant solution.

Three different binding patterns (eg or 2:1) were used to determine the relative stoichiometry of the complex formed via the MatLab software package (BindFit). The data shown were obtained by averaging the change in chemical shift of the hydrogen atoms determined by peaks 10, 11 and 12 of 1 (see Figure 6). 1H NMR spectrum of 1 (toluene-d8 at 25 °C) and corresponding peak assignments that were used in titration experiments and nonlinear regression analyses. right).

Illustrative guide to the overlap between 1 and the corresponding underlying C60 surface as identified by the crystal structure of (1)2·C60. Illustrative guide of the rings on 1, which are defined by the black circles in the respective rings. Geometric data (distances and angles) used to assess the ring-to-ring interactions of ring Cg1 with C60, as identified in the crystal structure of (1)2·C60.

ORTEP diagram of (1)2•C60 showing interactions of Cg2-Cg6 rings (inner rings) with C60 between the rings. Geometrical data (distances and angles) for the interactions of the Cg7-Cg11 rings with C60, which were identified in the (1)2•C60 crystal structure. Packing arrangement of (1)2•C60 showing connections to Cg18 & Cg19 via intermolecular C–H···π interactions (dashed lines).

Packing arrangement of (1)2·C60 showing the coupling of Cg6 and Cg10 via intermolecular C–H···π interactions (dotted lines). Geometric data (distances and angles) for the hydrogen-ring (“C–H···π”) interactions identified in the crystal structure of (1)2•C60. Illustrative guide to the defined faces on 1 and their corresponding centroids used to determine bowl depths and bowl-to-ball (BtB) distances.

Summary of cup depths and cup-to-ball (BtB) distances calculated for complex (1)2•C60. Graphical representation of the mutually orthogonal axes used on C60 and its associated complexes to calculate eccentricity for a given spheroid (eg, oblate or prolate). Summary of the metrics used to calculate the eccentricity values for 1 or subunits of 1 and derived from the DFT calculations or X-ray diffraction data (indicated).

All quantum chemical calculations were performed using the Gaussian 09 software package.[12] The initial geometries for all structures were taken from the crystal structure data.