An S₁₀-Symmetric 5-Fold Interlocked [2]Catenane

Abstract

The reaction of sym-pentakis(4-aminothiophenyl)corannulene with 2-formyl-6-methylpyridine and Cu^I or 2-formyl-1,10-phenanthroline and M^II (M = Co, Zn) yields an S₁₀-symmetric 5-fold interlocked [2]catenane of two interpenetrating [Cu^I₅L₂]⁵⁺ cages or D₅-symmetric [M^II₅L₂]¹⁰⁺ cages, respectively. The new structures were characterized by X-ray crystallography, NMR spectroscopy, and mass spectrometry. Density functional theory computations point to dispersive energies on par with traditional covalent bond energies. Subcomponent exchange reactions favored formation of the [Co^II₅L₂]¹⁰⁺ cage over the [Cu^I₁₀L₄]¹⁰⁺ catenane. The single cage and catenane each cocrystallized with a corannulene guest to form a bowl-in-bowl substructure.

Despite the intricate and challenging nature of interlocked molecules, knots,¹ ravels,² and catenanes³ of manifold forms have been structurally characterized, including interlocked cages of higher order.⁴ A rational synthetic approach for these systems is not yet available, but studies have begun to assess structure–energy relationships for these complexes.⁵ In favorable situations, host–guest studies⁶ have provided insight into structure–energy correlations of molecular aggregation phenomena more generally.⁷ Dispersive interactions become more important with increasing size and can rival covalent bonding energies.⁸

The assembly of subcomponents around metal ion templates⁹ yields thermodynamically stable aggregates that may have complex topologies. Judicious choice of subcomponent geometries results in high-symmetry Platonic or Archimedean polyhedra^9a or even entwined higher-order topological architectures such as links and knots.¹⁰ In this context, corannulene, with 5-fold symmetry, has been the focus of dodecahedral “capsid” construction.¹¹ Herein, pentafold-substituted corannulene subcomponents capable of generating ligands for tetrahedral Cu^I assembled into a surprisingly highly entangled 5-fold interlocked [2]catenane of exceedingly rare S₁₀ symmetry.¹² These interlocked [2]catenanes expand the class of interlocked cages from 3-fold¹³ and 4-fold¹⁴ to 5-fold structures. A related assembly binding octahedral Zn^II or Co^II provides a noninterlocked cognate of D₅ symmetry.

Corannulene was chosen as a suitable 5-fold-symmetric scaffold^11,15 because of its potential for functionalization via a variety of synthetic routes¹⁶ and its curved aromatic surface, which may enhance the guest binding properties¹⁷ of assemblies via aromatic stacking interactions.¹⁸sym-Pentakis(4-aminothiophenyl)corannulene (subcomponent A, Figure 1) was synthesized by nucleophilic aromatic substitution from sym-pentachlorocorannulene and 4-aminothiophenol in 1,2-dimethylimidazolone in the presence of sodium hydride (Scheme S1).

Figure 1.

(a) Synthesis of 1–3 and the conversion of 1 to 2 through subcomponent exchange. Side-on views of the crystal structures of 1 and 2 are shown. (b) View down the S₁₀ axis of the crystal structure of 1. (c) View down the C₅ axis of the crystal structure of 2. In the images of 1, the carbon atoms of the two interlocked [Cu^I₅L₂]⁵⁺ cages are colored differently, and only one of the two crystallographically unique assemblies is shown; in all cases hydrogen atoms, counterions, solvent molecules, and disorder have been omitted for clarity.

The reaction of pentaaniline A (2 equiv) with 2-formyl-6-methylpyridine (12 equiv) and tetrakis(acetonitrile)copper(I) tetrafluoroborate (Cu^I(MeCN)₄BF₄) (6 equiv)¹⁹ in CD₃CN at room temperature gave product 1 (Figure 1a). The ¹H NMR spectrum of 1 was well-resolved but complex, consisting of two magnetically distinct environments of equal intensity per ligand proton (Figure 2a). All of the peaks between 2.26 and 9.31 ppm displayed a single diffusion constant in the diffusion-ordered ¹H NMR (DOSY) spectrum, suggesting that they belonged to a single species.²⁰

Figure 2.

(a) ¹H NMR spectra (500 MHz, CD₃CN, 298 K) of 1. Peak assignments for the interior and exterior ligands are marked with bold blue and italic red labels, respectively. (b) ESI-MS spectrum of 1. The inset shows the theoretical and observed isotope patterns for the +5 peak.

Crystals of 1 were grown through vapor diffusion of diethyl ether into an acetonitrile solution, and the solid-state structure of 1 was elucidated by single-crystal X-ray diffraction using synchrotron radiation.²¹ The crystal structure revealed a [Cu^I₁₀L₄]¹⁰⁺ assembly consisting of a pair of 5-fold interlocked [Cu^I₅L₂]⁵⁺ cages (Figure 1). The distances of 11.3–11.9 Å between neighboring metal centers create windows of sufficient size to allow two [Cu^I₅L₂]⁵⁺ cages to interlock to form a [2]catenane. The two ligands within each [Cu^I₅L₂]⁵⁺ cage display the same handedness, resulting in idealized D₅ cage symmetry. The interlocking of each cage with its enantiomer lends the complete [Cu^I₁₀L₄]¹⁰⁺ assembly S₁₀ point-group symmetry.

The two [Cu^I₅L₂]⁵⁺ cages of the [2]catenane interlock tightly, forming two bowl-in-bowl substructures with stacked corannulenes¹⁸ separated by a distance of 3.69(1) Å. Within a single cage, the distance of 6.93(1) Å between the mean planes of the central pentagons of the corannulenes creates a cavity that matches the form of the intercalated corannulene from the other cage.

The solution NMR data for 1 in CD₃CN are consistent with the solid-state structure, wherein the exo and endo ligands give rise to distinct magnetic environments (Figure 2a). The imine, corannulene, and phenylene signals for the endo ligand are shielded relative to those of the exo ligand. NOE cross-peaks are observed between NMR signals of the two ligands (Figure S8) in a manner consistent with the interlocked structure observed in the solid state. ESI-MS results are also consistent with the [Cu^I₁₀L₄]¹⁰⁺ composition (Figure 2b).

Density functional theory (DFT) computations of the [Cu^I₁₀L₄]¹⁰⁺ catenane and the hypothetical [Cu^I₅L₂]⁵⁺ cage allowed an assessment of dispersion effects on their relative energetics based on the crystal structure of 1 vs those on the optimized geometry and of the effects of solvation. Neglecting dispersion effects, the energetics of the catenation process in the acetonitrile environment was calculated to be +42.0 kcal/mol (B3LYP/6-31G(d,p)//B3LYP/6-31G(d,p):acetonitrile), indicating that the complex is unbound. With fixed geometry, the energetics including dispersion in the acetonitrile environment is −212 kcal/mol (B3LYP-D3/6-31G(d,p)//B3LYP/6-31G(d,p):acetonitrile). Estimation of the effects of dispersion on the geometry carried out by single-point analysis resulted in a contribution of ∼52 kcal/mol. This gives in total ∼264 kcal/mol for the catenation process including the effects of dispersion in an acetonitrile environment. Full optimization of the complex including dispersion in the acetonitrile environment (B3LYP-D3/6-31G(d,p):acetonitrile) resulted in a complexation energy of 272.2 kcal/mol, a difference of <8 kcal/mol from the estimated value. This substantial energy contribution for the catenation process is consistent with the large surface area of the corannulene moieties giving rise to substantial van der Waals interactions. Tight packing and the large contact area between ligands within 1 play a central role in driving the selective assembly of this structure.^18,32

Tridentate donor sites formed from imine condensation with 2-formyl-1,10-phenanthroline, suitable for the octahedral coordination of Co^II and Zn^II,²² enabled investigation of whether octahedral metal centers also give catenanes incorporating A. The reaction of A (2 equiv) with 2-formyl-1,10-phenanthroline (10 equiv) and cobalt(II) bis(trifluoromethanesulfonyl)imide (Co(NTf₂)₂·5H₂O) (5 equiv) in CD₃CN at 353 K yielded 2. The ¹H NMR spectrum reveals a single resonance for each symmetry-equivalent proton, with the signals spread over the range −65.9 to +259.4 ppm as a result of the paramagnetism of Co^II.²³ The ESI-MS spectra are consistent with the formula [Co^II₅L₂]¹⁰⁺ (Figures S14 and S17).

Crystals of 2 (cocrystallized with coronene) were obtained through vapor diffusion of diisopropyl ether into an acetonitrile solution of the BF₄^– salt containing excess coronene.²⁴ Single-crystal X-ray measurements revealed non-interlocked [Co^II₅L₂]¹⁰⁺ cages composed of two 5-fold-symmetric ligands of the same handedness bridging five octahedral Co^II centers (Figure 1). Both enantiomers of the D₅-symmetric cage are present in the unit cell, related by inversion. The hub pentagon mean planes of the two corannulene moieties sit at a distance of 3.18(1) Å from each other, closer than the 3.2–3.3 Å of graphite planes. The corannulenes in 2 are also flattened compared with those in 1, with an average bowl depth²⁵ of 0.84(3) Å versus 0.95(3) and 0.91(1) Å for the exo and endo corannulenes in 1. Significant distortion from regular octahedral geometry around the metal centers is also observed, with angles of 81–84° between the Co^IIN₃ chelate planes, compared with angles of 84–90° between the Cu^IN₂ chelate planes in 1, which displayed a more regular tetrahedral geometry.

Similarly, the reaction of A (2 equiv) with 2-formyl-1,10-phenanthroline (10 equiv) and zinc(II) bis(trifluoromethanesulfonyl)imide (Zn(NTf₂)₂) (5 equiv) in CD₃CN at 353 K yielded [Zn^II₅L₂]¹⁰⁺ assembly 3, as confirmed by ESI-MS. The ¹H NMR spectrum also displays a single set of resonances for each ligand proton environment. Crystals were obtained by diffusion of diethyl ether into an acetonitrile solution of 3 containing CsCB₁₁H₁₂. Single-crystal X-ray analysis of 3 confirmed it to be isostructural with 2, although the weakly diffracting nature of the crystals precluded detailed analysis of the structural parameters (Figures S52 and S53).

The relative stabilities of 1 and 2 were probed via subcomponent exchange reactions.²⁶ A mixture of Co(BF₄)₂·6H₂O (10.2 equiv per assembly) and 2-formyl-1,10-phenanthroline (20 equiv per assembly) was added to a solution of 1 in CD₃CN, and the mixture was stirred at 333 K for 24 h and then at 353 K for 72 h. ¹H NMR spectra of the reaction mixture showed the disappearance of the diamagnetic signals of 1 followed by the appearance of the paramagnetically shifted signals of 2. ESI-MS analysis of the resulting mixture indicated the formation of 2 as the major product in solution, indicating its greater stability relative to 1. We infer that the formation of 2 is enthalpically favored as a consequence of the stronger coordination bonds of the tridentate ligand arms with Co^II in 2 relative to the bidentate ligand arms with Cu^I in 1 and also entropically favored because one molecule of 1 is converted into two molecules of 2.

The potential for guests to intercalate²⁷ within diamagnetic assemblies 1 and 3 or for guests to induce an assembly to rearrange into a suitable host²⁸ was investigated through the addition of the prospective guests shown in Figure S33. The assemblies were initially investigated as hosts for corannulene, inspired both by the interlocked cage structure of 1 and the observation of corannulene encapsulation inside other polyaromatic hosts²⁹ and electron-deficient macrocycles.³⁰ Addition of corannulene (10 equiv) to CD₃CN solutions of 1 or 3 led to shifts in the signals of the ¹H NMR spectra of both the host and guest, consistent with fast-exchange complexation on the ¹H NMR time scale (Figures S34 and S35).

The host–guest complexes of 1 and 2 with corannulene were also characterized in the solid state through single-crystal X-ray diffraction. Crystals of (corannulene)₂·1 were obtained from vapor diffusion of diethyl ether into an acetonitrile solution of 1 saturated with corannulene. The structure reveals two corannulene molecules stacked on the externally facing corannulene moieties of 1 at a distance of 3.71(2) Å (Figure 3a).

Figure 3.

Side views of the cationic parts of the crystal structures of (a) (corannulene)₂·1 and (b) corannulene·2. Counterions, solvent molecules, and disorder have been omitted for clarity, and the carbon atoms of the stacked corannulenes are colored red.

Crystals of corannulene·2 were obtained from vapor diffusion of diethyl ether into an acetonitrile solution of the triflimide salt of 2 saturated with corannulene and containing excess tetrabutylammonium perrhenate to aid crystallization. In this case only a single corannulene was observed to stack with one of the corannulene moieties of 2, with a refined occupancy of ca. 0.6 and a distance of 3.57(4) Å between the stacked rings (Figure 3b). The close packing of corannulene on the exterior of 2 in the structure of corannulene·2 contrasts with that observed in the cocrystal of 2 with coronene (Figures S46 and S47), where the cocrystallized coronene molecules intercalate between ligand arms but do not show any specific stacking interactions with the [Co^II₅L₂]¹⁰⁺ cation.

The binding mode observed in the solid state is consistent with the solution NMR data, with the largest shifts in host proton signals observed for the exo-corannulene protons of 1, consistent with corannulene undergoing stacking interactions with the exterior of the cage.²² No interactions were observed between 1 or 3 and planar polycyclic aromatic hydrocarbons,³¹ CB₁₁H₁₂^– anions,³² or spherical C₆₀, despite the known tendency for fullerenes to interact with corannulenes¹⁷ and corannulene-based hosts.^15a,33

In summary, a 5-fold interlocked [Cu^I₁₀L₄]¹⁰⁺ [2]catenane, representing a new structure type, has been prepared from Cu^I and corannulene-based subcomponent A. A DFT study revealed the dominant role of aromatic stacking interactions in driving the formation of the interlocked structure, which is a common feature^13a,13d of other interlocked cage systems. This study demonstrates the power of van der Waals interactions together with coordination-driven assembly to generate new types of highly complex structures.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.0c03349.

• Detailed descriptions of synthetic procedures, characterization of new compounds, and spectroscopic data (PDF)

• X-ray data for 1·10BF₄ (CCDC 1913638) (CIF)

• X-ray data for (corannulene)₂·1·10BF₄ (CCDC 1913639) (CIF)

• X-ray data for 2·10BF₄ (CCDC 1913641) (CIF)

• X-ray data for 2·coronene·10BF₄ (CCDC 1913642) (CIF)

• X-ray data for 2·10ClO₄ (CCDC 1913643) (CIF)

• X-ray data for (corannulene)_0.6·2·4ReO₄·6NTf₂ (CCDC 1913646) (CIF)

• X-ray data for 3·2CB₁₁H₁₂·8NTf₂ (CCDC 1913640) (CIF)

The authors declare no competing financial interest.