Concentration dependent supramolecular interconversions of triptycene-based cubic, prismatic, and tetrahedral structures

Sourav Chakraborty a, Kevin J. Endres a, Ranajit Bera b, Lukasz Wojtas c, Charles N. Moorefield d, Mary Jane Saunders e, Neeladri Das b, Chrys Wesdemiotis *af and George R. Newkome *afg
aDepartments of Polymer Science, University of Akron, Akron, Ohio 44256, USA. E-mail:;
bDepartment of Chemistry, Indian Institute of Technology Patna, Patna 801106, Bihar, India
cDepartment of Chemistry, University of South Florida, 4202 East Fowler Avenue, Tampa, Florida 33620, USA
dDendronex, LLC, 109 Runway Drive, Lubbock, Texas 79416, USA
eDepartment of Biological Sciences, Florida Atlantic University, Boca Raton, Florida 33431, USA
fDepartments of Chemistry, University of Akron, Akron, Ohio 44256, USA
gCenter for Molecular Biology and Biotechnology, Florida Atlantic University, Jupiter, Florida 33458, USA

Received 5th December 2017 , Accepted 12th January 2018

First published on 12th January 2018

The quantitative, single step, self-assembly of a shape-persistent, three-dimensional C3v-symmetric, triptycene-based tris-terpyridinyl ligand initially gives a platonic-based cubic architecture, which was unequivocally characterized by 1D and 2D NMR spectroscopy, mass spectrometry, and single crystal X-ray structural analysis. The unique metal–ligand binding properties of the Cd2+ analogue of this construct give rise to a concentration-dependent dynamic equilibrium between cube, prism, and tetrahedron-shaped architectures. Dilution transforms this cube into two identical tetrahedra through a stable prism-shaped intermediate; increasing the concentration reverses the process.


Biological assemblies that undergo dynamic structural changes play key roles in molecular recognition, replication, and catalysis.1–3 Probing the precise control over the interconversion between different supramolecular assemblies is challenging yet offers critical insight into geometric, kinetic, and thermodynamic control. Considering these facts, the morphological dynamics4 of several supramolecular grippers,5 grids/helicates,6,7 macrocycles,8,9 and cages10–12 have been reported. These supramolecular entities undergo reversible shape changes triggered by ionic and environmental stimuli, which are efficient to control the supramolecular equilibria, and rely on an assembly–disassembly–reassembly process, while maintaining the overall topology of the original constructs.13–18

Coordination-driven, supramolecular self-assembly paved the way for the design and construction of highly symmetric polyhedral supramolecules as well as other 3D molecular architectures.19–31 These assemblies rely on the preciseness-of-fit of the building blocks, the intrinsic shape information instilled in the subcomponents, and the directivity of metal–ligand coordination bonding. Utilizing the linearly coordinated, pseudo-octahedral tpy–M2+–tpy connectivity32,33 provides desirable synthetic characteristics by facilitating metal coordination sites that can act as sides in the contemplated shape. We have reported the fabrication of several metallosupramolecules,34,35 such as triangles,36 hexagons,37 various spoked wheels,38,39 nanospheres,40 Sierpiński triangles,41,42 and a molecular triangulane.43 Recently, the concentration-dependent interconversions between multiple discrete architectures in solution have been demonstrated.44–46 Concentration can also be considered as another critical parameter for an in-depth understanding of the fundamental aspects in morphological dynamics involving supramolecular assemblies through the coordination connectivity.

Herein, we expand on the dynamic supramolecular structural interconversion by providing a new insight into this molecular fission–fusion process.45 Thus, the construction of the novel, rigid triptycene-based tris-terpyridinyl monomer 3 is reported and shown to self-assemble with Zn2+ to generate a corresponding cube-shaped architecture 4. Whereas, the Cd2+ analog of this cage undergoes a dynamic equilibrium between a cube 5 and tetrahedron cage 7 through a prismatic cage structure 6, in which the formation can be controlled by the concentration to give exclusively either both the end products or a mixture of each.

Results and discussion

The triptycene-based monomer 3 was obtained, by a three-fold Suzuki cross-coupling between 2,7,14-tribromotriptycene47 and ([2,2′:6′,2′′]terpyridin-4′-yl)boronic acid,48 in 75% yield (Scheme 1). The 1H NMR spectrum of 3 exhibits one set of signals that were attributed to the terpyridine moieties and three aromatic peaks assigned to the aryl protons of the triptycene core supporting the overall three-fold symmetry. Two singlets appearing at 5.85 and 5.67 ppm were assigned to the bridgehead protons of triptycene motifs (Fig. 1a). A consistent MALDI-MS signal at m/z = 948.29 amu for [3 + H]+ also confirmed the formation of the desired monomer 3.
image file: c7dt04571a-s1.tif
Scheme 1 Synthesis of the triptycene-based tris(terpyridine) monomer 3 and the Zn2+-based cube 4. Concentration dependent interconversion between the Cd2+-based cube 5, prism 6 and tetrahedron 7.

image file: c7dt04571a-f1.tif
Fig. 1 1H NMR spectra (500 MHz, 300 K) of monomer 3 (a) in CDCl3 and complexes 4 (b) and 5 (c) in CD3CN.

The one step, self-assembly of 3 with Zn(NO3)2 in a precise 2[thin space (1/6-em)]:[thin space (1/6-em)]3 ratio in a stirred mixture of CHCl3 and MeOH (1[thin space (1/6-em)]:[thin space (1/6-em)]1, v/v) at 25 °C for 1 h led to a colorless solution. Counterion exchange by subsequent treatment with excess aqueous NH4PF6 solution (to exchange NO3 for PF6) gave the desired complex 4, as a white solid in quantitative yield. The structure of 4 was completely characterized by 1D and 2D NMR spectroscopy, ESI, and traveling-wave ion mobility (TWIM) mass spectrometry, along with single-crystal X-ray crystallography.

The 1H NMR spectrum of 4 in CD3CN exhibits characteristic sharp peaks with a simple pattern indicative of the formation of a single discrete species with a high degree of inherent structural symmetry (Fig. 1b). The 2D COSY and NOESY NMR spectra were used to verify the assignments of peaks in the 1H NMR spectrum. The imbedded bridgehead markers of triptycene units appear as two singlets in a precise 1[thin space (1/6-em)]:[thin space (1/6-em)]1 ratio at 6.29 and 6.28 ppm, supporting the formation of a highly symmetric species. The 6,6′′ doublet of the terpyridine units was significantly shifted upfield (7.92 ppm) due to the shielding effect of opposing perpendicular ring currents, and is indicative of the pseudo-octahedral bis(terpyridine) complex. All other aromatic peaks show downfield shifts following complex formation.

The ESI mass spectrum of 4 further supports the cubic structure by revealing a series of dominant peaks at m/z = 1829.50, 1447.65, 1336.08, 1171.52, 1039.76, 932.06, 842.47, 766.36, 701.27, 644.91, and 595.61 amu corresponding to the charge states 6+ to 16+, respectively, due to the successive loss of PF6 ions (Fig. S10). The observed m/z values of each charge state are consistent with the corresponding theoretically calculated values, confirming that 4 was composed of exactly 8 ligands, 12 Zn2+ metal ions, and 24 PF6 counter ions. ESI-MS coupled with travelling wave ion mobility (TWIM),49–51 a variant of ion mobility spectrometry, resolves the isomeric ions by their charge and shape/size and further supports the formation of the cube 4. The TWIM mass spectrum (Fig. S11) exhibits bands for charge states ranging from 7+ to 15+ with a narrow drift time distribution for signals collected for each band, indicative of a single structural conformer ruling out the possibility of other species.

Further evidence supporting a cubic structure was provided by the collision cross section (CCS) data of ions deduced from their drift times measured by TWIM-MS. CCS can be viewed as the rotationally averaged forward-moving surface area of the ions. The nominal difference between the CCS values of charge states 5+ to 9+ (Table S1) suggests that 4 possesses a rigid, shape-persistent geometry. The average theoretical CCS of the complex (1424.6 Å2) for 50 energy-minimized structures without counter anions, as obtained by the trajectory method,52–54 correlates well with the average experimental values (1320.0 Å2), supporting the proposed architecture.

The 1H DOSY NMR of 4 in CD3CN clearly shows one single diffusion band, with a diffusion coefficient D = 3.21 × 10−10 m2 s−1, indicative of a single species in solution and rules out the possibility of any other macrocyclic or oligomeric products (Fig. S12). The experimental hydrodynamic diameter (3.9 nm) derived therefrom is fully consistent with the value obtained from the energy minimized structure.

Colorless, cubic single crystals, suitable for X-ray crystallography, were obtained after two weeks by vapor diffusion of EtOAc into a MeCN solution of cube 4. Owing to the large internal volume of this cubic complex and the solvent molecules and numerous counterions, diffraction spots, not unexpectedly, were observed only to a resolution of ca. 1.2–1.3 Å. Despite the low resolution it was, however, sufficient to model the relative position of the eight ligands and twelve Zn2+ ions, thereby leading to the cubic structure of 4 (Fig. 2). As expected, a highly symmetrical shape-persistent structure showing the 6 faces with the requisite 12 edges was revealed in which the greatest distance between two Zn2+ ions is 3.1 nm, the average distance across the interior is ca. 2.3 nm, and an overall volume is about 6469 Å3.

image file: c7dt04571a-f2.tif
Fig. 2 The single crystal structure of nanosphere 4 viewed (a) from the front and (c) along the diagonal. (b, d) The packing of eight molecules of 4 viewed as described in (a) and (c), respectively. The counterions and solvent molecules were highly disordered; thus, these groups have not been included in the structural model that is based on X-ray diffraction.

Substitution of Zn2+ with Cd2+ also forms a structurally identical cube 5. The preparation and characterization of the more labile55 Cd2+ analog (5) mirrored that of 4. The ESI-MS and NMR experimental (10 mg mL−1; MeCN or CD3CN, respectively) results along with DOSY NMR data unequivocally confirm the formation of the molecular cube 5 (Fig. 1c and 3a). Most recently, we have explored the molecular fission–fusion process45 to transform one motif into another by simply changing the concentration. We have observed that upon dilution of the larger molecular construct it underwent molecular fission to generate exactly two identical smaller molecules possessing a single discrete architecture, each characterized by precisely one half of the original molecular weight and vice versa. But unlike our previous observation, herein the subsequent dilution of Cd2+-based cube 5 results in the formation of two equivalents of the tetrahedral cage 7via a prismatic intermediate 6 possessing three-fourth the molecular weight of 5, as shown in both ESI-MS and NMR studies.

image file: c7dt04571a-f3.tif
Fig. 3 Left: ESI-MS spectra of the cube 5 (a, 10 mg mL−1), the cube-prism-tetrahedron mixture obtained upon dilution of the cube (b, 0.4 mg mL−1), and the tetrahedron 7 architecture ultimately obtained upon further dilution (c, 0.05 mg mL−1). Right: The dynamic equilibrium between cube 5, prism 6, and tetrahedron 7. Pertinent 1H NMR spectra of the concentration dependent dynamic equilibrium of 5, 6, and 7 at 300 K (d, in CD3CN). The cubic complex 5 (10 mg mL−1) is completely transformed to the pure tetrahedron complex 7 (0.05 mg mL−1) via prism 6 (always in equilibrium with one or both of 5 and 7).

Thus, dilution of a solution of 5 from 10 to 4 mg mL−1 with MeCN led to an ESI-MS spectrum showing the expected signals for the cube along with a new series of charge states at m/z = 2182.44, 1717.00, 1406.66, 1184.99, 1018.75, 889.45, 786.00, and 701.37 amu corresponding to charge states 4+ to 11+, respectively (Fig. S27). This confirmed that this structural transformation gave a new complex 6 composed of 6 ligands, 9 Cd2+, and 18 PF6 counterions, which supports a prismatic cage. The experimental isotopic distribution of 10+ charge state agrees well with the simulated isotope pattern (Fig. S28). ESI-MS of a ten times dilute solution (0.4 mg mL−1) suggests a mixture of cages 5–7 by revealing specific charge states directly assigned to the cube 5 and prism 6; peaks calculated for tetrahedron 7 overlapped with 5 and 6, as expected (Fig. 3b). This overlap gives a higher intensity of peaks due to an increased amount of identical charged species originating from the different cages. Still further dilution of this solution to 0.05 mg mL−1 gave the pure tetrahedron 7 with the appearance of charge states from 3+ to 9+, thereby providing strong evidence for the combination of 4 ligands and 6 Cd2+ (Fig. 3c). The 8+ charged species of 7 was isotopically resolved and found in agreement with the theoretically simulated pattern (Fig. S22). The ESI-TWIM-MS further supports the formation of complexes 5 and 7 by the presence of charge states ranging from 5+ to 11+ for complex 5 and 3+ to 8+ for complex 7 (Fig. S18 and S23). Both complexes exhibit narrow bands for each charge state with a narrow drift time distribution for the signal collected for each band suggesting a single species in solution. Unlike complexes 5 and 7, the prismatic cage 6 never exists in solution as a pure component as it is always in a concentration dependent equilibrium with either both or one of the cages.

The 1H NMR spectra also showed the structural changes upon dilution of complex 5, which exhibits (10 mg mL−1; CD3CN) one set of sharp signals for the aromatic protons along with two obvious singlets at 6.32 and 6.23 ppm for the bridgehead protons. When the concentration was reduced from 10 to 4 mg mL−1, a new distinct series of peaks was observed with the appearance of a new bridgehead proton at 6.28 ppm corresponding to prism 6 (Fig. 3d). Upon dilution to 0.4 mg mL−1, another new series of peaks belonging to tetrahedron 7 was observed. At a concentration of 0.2 mg mL−1, the peaks corresponding to cube 5 completely disappear leaving a mixture of 6 and 7 in solution as confirmed by the disappearance of the bridgehead signal at 6.32 ppm for cube 5 (Fig. S31). This NMR spectrum clearly revealed two distinct singlets at 8.9 and 8.89 ppm in a 1[thin space (1/6-em)]:[thin space (1/6-em)]2 ratio for the two different 3′,5′-protons of arms A and B of prism 6, respectively, which further confirms its formation (Fig. S24). Another distinct singlet at 8.87 ppm came from the 3′,5′-proton of the tetrahedron cage 7. All peaks are assigned with the aid of COSY and NOESY spectroscopy (Fig. S25 and S26). Upon further dilution, the peaks from 6 completely disappeared, leaving only a pristine spectrum for the newly formed structure 7 (Fig. 3d). When compared with cube 5, all of the peaks for 7 show an upfield shift suggesting the formation of a smaller structure with similar components.

In diffusion-ordered NMR spectroscopy (DOSY) all the signals of complex 5 exhibit a single narrow diffusion trace with a diffusion coefficient D = 3.12 × 10−10 m2 s−1, suggesting a single species in CD3CN (Fig. S29). The calculated hydrodynamic diameter of this complex, based on the viscosity of CD3CN at 298 K, is found to be 1.98 nm. A DOSY experiment using a solution of prism 6 and tetrahedron 7 was performed and the diameters were found to be 1.71 and 1.59 nm, respectively (Fig. S30). These results agree with the values obtained from the energy minimized structures as well as with the TEM results.

Further evidence for the proof-of-structure for complexes 5, 6, and 7 includes the collision cross-section (CCS) data obtained from the drift times measured in TWIM experiments (Tables S2–S4). CCS is a physical property representing the size of the corresponding ion and gives insight into the basic architecture. The difference between the average experimental CCSs of cages 5–7 matches the sizes of their respective cubic, prismatic, and tetrahedral structures. The theoretical CCSs were calculated based on 50 counterion-free energy minimized structures for each complex using the trajectory (TJ)/projection approximation (PA) method. The average theoretical CCSs of cages 5–7 (1424.4 Å2 for 5, 1125.0 Å2 for 6, and 757.7 Å2 for 7) correlate well with the corresponding average experimental CCSs, hence supporting the proposed structures (Tables S2–S4). The minimal difference of CCSs between the charge states of these complexes suggests the structural rigidity and shape-persistent architecture.

Transmission Electron Microscopy (TEM) facilitated the visualization of cube 5, prism 6, and tetrahedron 7, directly revealing both the shape and size of individual molecules. TEM images were obtained upon deposition of a very dilute MeCN solution of complexes on a copper coated grid (Cu, 400 mesh), which concentrates the complexes upon evaporation of the solvent before inserting the grid into the TEM under vacuum. Evaporation and subsequent increasing concentration of the complexes facilitated the production of all three molecules on one grid. Low magnification showed uniform dispersion of individual molecules with clear edges (Fig. 4), which at higher magnification resolved into a mixture of single molecules whose shape and size correlate with the optimized molecular models of complexes 5–7.

image file: c7dt04571a-f4.tif
Fig. 4 TEM images of a mixture of cages 5–7. Lower magnification shows a uniform field of particles. Higher magnification shows a mixture of particle sizes and shapes matching predicted structures.

This phenomenon demonstrated the importance of entropic forces in such complexation reactions where smaller assembly is expected preferentially. As the number of molecules in unit volume decreases with dilution, entropic force14,18,56 dictates the self-assembled complex to switch into stable smaller tetrahedra. Entropic factors play a critical role in this fission–fusion process to reform a new equilibrium along with the supramolecular transformation from unfavored species to those that are more favored by the change in concentration, according to Le Chatelier's principle.


In summary, the generation of a rigid, platonic cube-shaped molecular cage 4, using a single step, self-assembly of eight novel triptycene-based tris-terpyridinyl monomers and 12 Zn2+ ions, is detailed. Along with unequivocal conventional characterization with NMR and ESI-MS, its single crystal X-ray structure was determined. The corresponding Cd2+ complex of the same ligand shows a concentration-dependent dynamic equilibrium between the cubic 5 and tetrahedron-shaped 7, via a prismatic intermediate 6. Unlike our previously studied systems, where one molecular construct directly converts to another motif characterized precisely one half or double the original molecular weight (both depend on the concentration); herein, the structural interconversion takes place via a stable species that only exists in solution as a mixture with either or both end conformers. This result provides new insight into the molecular fission–fusion process and this phenomenon provides access to other large platonic or Archimedean multicomponent architectures, which can mimic naturally occurring biological molecules/systems.

Conflicts of interest

There are no conflicts to declare.


The single crystal diffraction data for 4 were collected at Argonne National Laboratory, Advanced Photon Source, Beamline 15-ID-B of ChemMatCARS, which is principally supported by the National Science Foundation Divisions of Chemistry (CHE) and Materials Research (DMR) under grant number NSF/CHE-1346572. The use of the PILATUS3 X CdTe 1 M detector is supported by the National Science Foundation under the grant number NSF/DMR-1531283. The use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE Contract No. DE-AC02-06CH11357. We gratefully thank the National Science Foundation (CHE-1151991 to G. R. N. and CHE-1308307 to C. W.) for financial support and Jessi A. Baughman (UA) for valuable expertise and assistance with the DOSY NMR experiment.

Notes and references

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Electronic supplementary information (ESI) available: Details of the synthesis, spectroscopic data, and computation details for 1–7 (PDF). CCDC 1582582. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7dt04571a

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