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. 2020 Mar 5;26(16):3609–3613. doi: 10.1002/chem.201905390

Supramolecular Metallacycles and Their Binding of Fullerenes

Christian R Göb 1, Andreas Ehnbom 2, Lisa Sturm 1, Yoshito Tobe 3, Iris M Oppel 1,✉
PMCID: PMC7155124  PMID: 31833098

Abstract

The synthesis of a new triaminoguanidinium‐based ligand with three tris‐chelating [NNO]‐binding pockets and C 3 symmetry is described. The reaction of tris‐(2‐pyridinylene‐N‐oxide)triaminoguanidinium salts with zinc(II) formate leads to the formation of cyclic supramolecular coordination compounds which in solution bind fullerenes in their spherical cavities. The rapid encapsulation of C60 can be observed by NMR spectroscopy and single‐crystal X‐ray diffraction and is verified using computation.

Keywords: cage compounds, density functional calculations, fullerenes, host–guest systems, self-assembly

Cage bound: Synthesis of a new triaminoguanidinium‐based ligand with three tris‐chelating [NNO] binding pockets and C 3‐symmetry is described. Reaction with zinc(II) formate leads to the formation of cyclic supramolecular coordination compounds that in solution are able to bind fullerenes in their spherical cavities. The rapid encapsulation of C60 is observed by NMR spectroscopy and X‐ray diffraction and is validated by DFT.

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Introduction

The development of supramolecular coordination compounds and their corresponding cages has attracted wide interest in recent years.1 They have the potential to transport chemicals from one location to another in a specific manner, for example, being used in drug delivery or as contrast agents.2 The structural design of these containers requires precise and complementary building blocks but they are not limited to the incorporation of small organic molecules.3 There are only a few examples of toroid coordination compounds and larger aggregates in the literature (Figure 1 A).4 Those compounds can be used as single molecular magnets or in the separation of fullerenes.5 Stang introduced the concept of using building blocks to form cage compounds,6 whereas the groups of Saalfrank, Fujita, and Nitschke used cages to stabilize reactive species like white phosphorous P4 and organometallic complexes.6b, 7

Figure 1.

Figure 1

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(a) Previously reported supramolecular metallacycles. Hydrogen atoms and solvent molecules were omitted for clarity. Different scales were applied and metal ions highlighted as spheres. (b) Synthesis of the ligands [H3(pyO)3L]X (5‐X), (X=Cl−, NCS−, BF4 −). (c) Supramolecular metallacycle [Zn24Cl24{(pyO)3L}12] (ZnCl2 in green), schematic drawing and inclusion complex C60⊂9.

Toroids can be useful in binding guest molecules. Covalently bonded systems are quite common, for example, cyclodextrins, cucurbiturils and cryptands which bind cations,8 anions,9 or hydrophobic molecules10 by variation of their peripheral decoration. Toroidal coordination cages are also able to bind guest molecules and separate fullerenes.11

These systems are challenging to model using density functional theory (DFT) due to their large size. Herein we demonstrate not only that a geometry‐optimized structure of a large empty metallacycle can be obtained, but also the C60 and C70 encapsulated supramolecular entity. We were able to compute spectroscopic properties and assign the C60 signals in both the free state and bound inside of the metallacycle (Figure 1 C). There are a large number of studies reporting the computations of C60 itself. However, publications focusing on the interaction of C60 with other macrocycles are limited. Most studies including these interactions involve smaller, and often purely organic macrocyclic systems.12 To the best of our knowledge, this is the first study to computationally investigate such large interacting systems (>4000 electrons) using non‐truncated models.

Results and Discussion

Our group demonstrated the synthesis of supramolecular structures by self‐assembly of C 3‐symmetric building blocks with three tris‐chelating binding pockets and suitable co‐ligands (analogous to Figure 1 B).13 Counter ions or solvent molecules typically serve as templates in the synthesis, so that discrete coordination cages like M12L4 (Figure 2, left), M18L6, or M24L8 are accessible.13d We did not observe any activation of small molecules with these assemblies, even though a high number of potential catalytically active metal centers are located proximal to each other.

Figure 2.

Figure 2

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M12L4 tetrahedron and M2L2 dimer (M=Cd, Sn, respectively). Hydrogen atoms and solvent molecules were omitted for clarity.[14, 13a]

The overall negative charge of these complexes might be the reason why substrates like lactide are not satisfyingly activated. It was thus necessary to increase the amount of positive charges of the resulting coordination compounds by using stronger Lewis acids, such as SnIV or ZrIV, instead of CdII or PdII.14 The resulting complexes typically form dimers or trimers and are able to oligomerize acetone in up to 15 repeating units (Figure 2, right).13d, 15

In this work we address the charge issue by increasing the number of positive charges in the ligand itself, while maintaining the isoelectronic structure of the previously reported ligands. The ligand is prepared from a condensation reaction between 2‐formylpyridine‐N‐oxide and the corresponding triaminoguanidinium salts TAG‐Cl (4‐Cl), TAG‐NCS (4‐NCS), or TAG‐BF4 (4‐BF4), respectively (Figure 1 B).

The synthesis of N‐oxide (3) requires standard protection and deprotection procedures for the aldehyde moiety. The oxidation of the pyridine nitrogen can be realized under mild conditions by using urea hydrogen peroxide with phthalic anhydride in acetonitrile.16 Triaminoguanidinium salts (4‐X) are obtained by the amination of guanidinium salts with hydrazine hydrate.17 The resulting compounds (5‐X) serve as an excellent ligands for ZnII. The reaction of [H3(pyO)3L]Cl (5‐Cl) with Zn(O2CH)2 in N,N‐dimethylformamide results in the formation of a supramolecular coordination compound [Zn24Cl24{(pyO)3L}12] (6, Figure 1 C) next to a coordination polymer of unknown composition. This torus‐shaped metallacycle exhibits an outer diameter of 31.7 Å and ZnII ions are octahedrally coordinated between two alternately oriented ligands holding together the assembly (Figure 3 and Figure 4, left). The ZnCl2 moieties occupy the remaining [NNO] binding pockets. A spherical cavity of 10.7 Å can be found inside the complex with a pore opening of 8.2 Å. Each value is corrected by the covalent radii of hydrogen or carbon atoms.

Figure 3.

Figure 3

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Molecular dimensions of metallacycles by X‐ray diffraction compared to spectroscopic data of 7. The void space is indicated by the blue sphere.

Figure 4.

Figure 4

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Schematic representation of metallacycles (X=Cl, Br, NCS, O2CH) and asymmetric unit of [Zn24(NCS)16(O2CH)8{(pyO)3L}12] 8. Disordered solvent molecules were removed by the Squeeze routine (Platon) and hydrogen atoms were omitted for clarity.18

An isostructural cyclic coordination oligomer [Zn24Br24{(pyO)3L}12] (7) can be crystallized from the reaction mixture containing [H3(pyO)3L]BF4, ZnBr2 and NaO2CH. DOSY‐NMR spectroscopy ([D6]DMSO) of 7 shows neither decomposition nor aggregation of the coordination complex in solution. Only one species is detected in addition to solvents and water. The diffusion coefficient of 7 is found to be D=(7.36±0.08)×107 cm2 s−1. The hydrodynamic diameter of 7 can be determined to be 29.9±1.2 Å using the Stokes–Einstein equation, which corresponds well with the observed diameter of the crystal structure (Supporting Information Figure S5 and S6).

Introduction of isothiocyanate leads to the formation of the analogous NCS‐metallacycle [Zn24(NCS)16(O2CH)8{(pyO)3L}12] (8) with a 59 % yield. The zinc(II) ions, which were formally occupied by the halides Cl− or Br−, share those sites with isothiocyanate and formate co‐ligands (Figure 4, right). The presence of this coordination compound, and the absence of smaller or larger aggregates in the DMSO solution, was confirmed by dynamic light scattering (Supporting Information). Since the co‐ligands point outwards and exhibit a slightly increased steric demand, the system crystallizes in the tetragonal space group P 4‾ 21 c with solvent filled channels along the crystallographic c‐axis (Figure 5 a). Host‐guest chemistry seems feasible since the cavities of the coordination oligomers should be accessible in the solid state. To our surprise, it was not possible to soak crystals of [Zn24(NCS)16(O2CH)8{(pyO)3L}12] (8) with a toluene solution of C60, as there was no observed color change.19

Figure 5.

Figure 5

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(a) Crystal packing of 8, π–π‐interactions are highlighted in turquoise. (b,c) C60⊂9, (d) C70⊂9. Disordered solvent molecules were removed by the Squeeze routine and hydrogen atoms were omitted for clarity.

From these experimental results we decided to encapsulate the fullerenes into the metallacycles in solution. A solution of empty metallacycle (8) was added to a toluene solution of C60 or C70. Single crystals of the respective inclusion compounds were collected after a few days (Figure 5 b–d). The C60 and C70 are fully incorporated into the cavity of the metallacycle [Zn24(NCS)20(O2CH)4{(pyO)3L}12] (9). The lattice parameters underwent a slight change compared to 8, whereas the space group was maintained. The periphery of the metallacycle was slightly perturbed, presumably due to the change in polarity of the solvent mixture.

The incorporation of C60 in solution is validated by NMR spectroscopy (Figure 6). Crystals of 8 and C60⊂9 were removed from the crystallization solution, washed with THF and dissolved in [D6]DMSO. The 1H NMR spectrum shows a shift and broadening of the signals due to the molecular tumbling of C60. The 13C NMR of C60⊂9 shows only one signal for C60 which is in agreement with all carbon atoms being chemically and magnetically equivalent. The encapsulated C60 can only be detected next to free C60 in a highly diluted solvent mixture, and therefore 13C‐enriched C60 was used. The 2 ppm signal shift from 142.18 ppm (free C60) to 140.40 ppm (C60⊂9) clearly indicates the incorporation of C60 in the cavity of the metallacycle.

Figure 6.

Figure 6

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(a,b) 1H NMR spectra in [D6]DMSO of 8 and C60⊂9 respectively. (c) 13C NMR spectrum of 8 (1 equiv) and C60 (2 equiv) in [D6]DMSO/1,2‐dichlorobenzene‐d 4 1:1 (v/v). (d) 13C NMR spectrum of C60⊂9 in [D6]DMSO.

Calculations predicted the encapsulation of C60 by the metallacycle (6) to be favorable (Figure 7 c A). However, there is an additional local minimum (C) where the C60 interacts with the periphery of the metallacycle. This has also been observed crystallographically. An analogous binding plot is observed for the C70 case (see Supporting Information for details). The use of dispersion corrections in these calculations is critical in order to correctly model the attractive interaction. In addition to the entropy loss, there is an electronic energy barrier (B) that C60 (and C70) must overcome caused by steric interactions between the C60/70 and pyridinyl groups located at the entrance of the metallacycle. Due to the nature of these calculations, stepwise single‐point calculations from the fully optimized state (A) were applied. The binding energetics are only qualitative. The differences in electrostatic potentials for 6–8 are shown in Figure 7 b. The chloride metallacycle has the largest positive inner core while the outside ZnCl2 moieties carry the negative potential. This is gradually attenuated in the series of 6>7>8 with 8 having the least positive inner core. These potentials might be useful in tuning the affinity and specificity for guest molecules.

Figure 7.

Figure 7

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(a) Computed 13C NMR of 8 and C60⊂9, (b) electrostatic potential maps (ESP) of 6, 7, and 8. (c) An electronic energy diagram showing the relative stability (kcal mol−1) as a function of the C60 distance from its position in the fully optimized state (A) in C60⊂9. The C60 is stepwise extruded.

In agreement with experimental data, spectroscopic assignments using computations (DFT, see Supporting Information for details) predict an approximately 2.0 ppm upfield shift for the encapsulated C60⊂9 species versus the free C60 (144.3 ppm vs. 142.2 ppm) (Figure 7 a). The IR signatures of an empty chloride metallacycle (6) also match the computed IR spectrum and the fingerprint region contains several characteristic absorptions (Figure S25 in the Supporting Information).

Summary

We report the synthesis of a new type of tris‐chelating pyridine‐N‐oxide based ligands [H3(pyO)3L]X. Coordination of zinc(II) ions leads to structurally interesting cyclic coordination oligomers which serve as hosts for fullerenes C60 and C70. The inclusion of fullerenes was observed by single‐crystal X‐ray diffractometry and NMR spectroscopy, and was validated using computations. These metallacycles are robust and show no sign of decomposition. Although computations on such large systems are challenging, we were able to model the C60/C70 encapsulated metallacycles and even predict spectroscopic data, which are in strong agreement with the experimental results. Electrostatic potential maps reveal that the positive charges of the cavity cores can be tuned by the peripheral halide co‐ligands. These computations help to provide a deeper understanding of host‐guest interaction in these metallacycles. Similar computations will no doubt be useful for the rational design of host molecules with other cargo. In the future, these complexes could serve as container molecules transporting important cargo.2f, 20

Experimental Section

Experimental methods, synthesis, computational details and results can all be found in the Supporting Information.

CCDC https://www.ccdc.cam.ac.uk/services/structures?id=doi:10.1002/chem.201905390 (3, 5‐Cl, 6, 7, 8, C60⊂9, C70⊂9, respectively), contain the supplementary crystallographic data for this paper. These data are provided free of charge by http://www.ccdc.cam.ac.uk/.

Conflict of interest

The authors declare no conflict of interest.

Supporting information

As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

Supplementary

Click here for additional data file. (29.1MB, pdf)

Acknowledgements

We gratefully acknowledge the International Research Training Group 1628 SeleCa “Selectivity in Chemo‐ and Bio‐catalysis” http://www.seleca.rwth-aachen.de (Deutsche Forschungs‐gemeinschaft) for funding this work. Furthermore, we like to thank Dr. Khai Nghi Truong from the group of Prof. Dr. Ulli Englert (RWTH Aachen University) and Manuela Winter from the group of Prof. Dr. Roland A. Fischer (Ruhr‐Universität Bochum) for collection single‐crystal X‐ray diffraction data, Rachida Bomahrat and Dr. Gerhard Fink for NMR spectroscopic measurements. We are grateful to the Alexander von Humboldt Foundation for support of A.E. We thank Prof. Dr. John A. Gladysz (Texas A&M University) and the U.S. National Science Foundation (CHE‐1566601, 1664866, 1900549) for financial support. We thank Prof. Dr. Michael B. Hall and Dr. Lisa M. Pérez (Texas A&M University) for fruitful discussions and the Laboratory for Molecular Simulation. We thank the Texas A&M High Performance Research Computing Facility for computational resources. We thank Prof. Dr. Steven E. Wheeler (University of Georgia) for providing scripts to generate ESP plots.

C. R. Göb, A. Ehnbom, L. Sturm, Y. Tobe, I. M. Oppel, Chem. Eur. J. 2020, 26, 3609.

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Supplementary Materials

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Supplementary

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