Selective Host–Guest Interactions of a Transformable Coordination Capsule/Tube with Fullerenes

Abstract

An M 2 L 4 coordination capsule or an M 2 L 2 coordination tube was selectively formed by the combination of Hg II hinges and bent bispyridine ligands. The two structures reversibly interconvert at room temperature in response to modulation of the metal-to-ligand ratio and exhibit different host-guest interaction behavior. The capsule alone encapsulates large spherical molecules, fullerenes C 60 and C 70 , and the bound guests are released upon capsule-to-tube transformation by the simple addition of metal ions.Covalently bonded assemblies and nearly all noncovalent assemblies formed by hydrophobic, hydrogen-bonding, and aromatic-aromatic interactions typically adopt a single molecular structure. Coordination-driven supramolecular structures, on the other hand, can change their conformation, geometry, and even molecular structure, often in a reversible fashion, depending on the solvent, the counterion, the template molecule, and the metal-to-ligand ratio.[ ions, have been reported. However, with the exception of guest-templated assemblies, reports on the host-guest behavior of interconvertible coordination cages are scarce.[4b] A combination of dynamic structural changes and host behavior of supramolecular nanostructures is essential for the development of multistimuli-responsive functional molecules and materials.[5] Herein we describe the preparation of M 2 L 4 coordination capsule 2 and M 2 L 2 coordination tube 3, which reversibly interconvert at room temperature in response to changes in the metal-to-ligand ratio of Hg II ion and bent bidentate ligand 1 (Figure 1).[6] The two molecular assemblies both possess well-defined cavities with diameters of approximately 1 nm, but show different host-guest behavior. The capsule was found to effectively bind large spherical molecules, such as fullerenes C 60 and C 70 , whereas the tube displayed no binding affinity for these molecules. [7,8] ).[9] An M 2 L 2 coordination tube was also obtained from the same ligand and Ag I ions only in the presence of fullerene C 60 .[10]These reported capsular and tubular structures were the exclusive products, and their formation was insensitive to the ratio of the metal ion to the ligand. In sharp contrast, we can selectively generate the M 2 L 4 capsule 2 or M 2 L 2 tube 3 by varying the M/L ratio of the Hg II ion [11] and ligand 1 (Figure 1).Mercury(II)-linked molecular capsule 2 and tube 3 were first prepared individually, and the structures were confirmed by NMR spectroscopic and ESI-TOF MS analysis (Figure 2).[12] When a mixture of Hg(OTf) 2 (6.2 mmol) and ligand 1 (12.3 mmol) in CD 3 CN (0.5 mL) was stirred at room temperature for 5 min, capsule 2 was formed quantitatively. The aromatic region of the 1 H NMR spectrum of 2 showed nine signals derived from the anthracene (H b,c,d,e ), pyridine (H f,g,h,i ), and meta-phenylene (H a ) moieties (Figure 2 c). The large upfield shifts of the two inner hydrogen atoms H a and H f (Dd = À1.01 and À0.84 ppm, respectively) are indicative aromatic shielding effects by the anthr...

A polyaromatic molecular tube that binds long hydrocarbons with high selectivity

“…2a) with two embedded anthracene panels to design molecular tube 1. The rigidly bent conformation of 2b facilitates the formation of supramolecular assemblies such as (i) capsular structures through noncovalent, coordinative transition-metal linkages [18][19][20][21] and (ii) bowlshaped structures through covalent methylene linkages 22,23 . For the preparation of the tubular structure, we employed Zincke reaction 24,25 to form two phenyl-pyridyl bonds on 1 without using a transition-metal catalyst.…”

Programmed guest confinement via hierarchical cage to cage transformations

“…[10][11][12][13] The possibility to regulate guest uptake and release can open in principle novel properties in storage, 14,15 sensing, [16][17][18] and catalysis. 19,20 Common strategies for release of a guest from cages are based on: (i) the presence of a stronger binder in solution which substitutes the guest, 21 (ii) disassembly of the host, 22,23 modication of the (iii) guest (viz. protonation/deprotonation, oxidation/reduction, light) [24][25][26][27][28] or the (iv) host by external factors (viz.…”

Guest‐Induced Transformations in Metal‐Organic Cages

“…[3] Today, the directional and geometrical principles underpinning the construction of MOCs with specific structural qualities are well-established; [4] however, cages and related coordination assemblies are inherently dynamic and can interconvert upon the introduction of external stimuli due to the reversibility of their ligand!metal dative connections. [5] Besides responding to solvent, [6][7][8] the concentration of reactants, [9][10][11] action of light, [12,13] the introduction of new components, [14][15][16][17] and stoichiometry variations, [18][19][20] the addition of templating guests may also trigger cage rearrangements.…”