Guest Exchange Mechanisms in Mono‐Metallic Pd II /Pt II ‐Cages Based on a Tetra‐Pyridyl Calix[4]pyrrole Ligand

Enhanced binding of methyl alkylammonium cations through preorganization of a water-soluble calix[4]pyrrole

We describe the synthesis of two tetra-α aryl-extended calix[4]pyrroles (C[4]Ps) 4a-b bearing four terminal carboxylic groups in their meso-propyl chains defining the lower rims. The synthesized C[4]Ps became soluble (1-3 mM) in water at pD = 10. We probed the interaction of 4a towards tetra-methylammonium (G1) chloride in water using 1H NMR spectroscopy. The C[4]P 4a includes G1 in the shallow aromatic cavity defined by the pyrrole rings in cone conformation forming a 1 : 1 complex G1⊂4a. Pyridine-N-oxide (PNO) binding in the larger polar aromatic cavity of 4a results in the quantitative self-assembly of the supramolecular receptor PNO@4a featuring the pyrrole rings preorganized in cone conformation. The PNO@4a receptor displays improved binding properties towards G1 than the parent C[4]P 4a. We thermodynamically characterized (1H NMR titrations and ITC experiments) the 1 : 1 complexes of PNO@4a with a series of tetra-alkylammonium salts, including biologically relevant examples. The PNO@4a supramolecular receptor displays significant affinity (log K = 3-4) but lacks selectivity in water binding of methyl trialkyl ammonium cations. Cation-π and coulombic interactions are the main intermolecular forces stabilizing the complexes. We also performed DFT calculations to gain some insights into the complexes' structures.

Coordination cages integrated into swelling poly(ionic liquid)s for guest encapsulation and separation

Coordination cages have been widely reported to bind a variety of guests, which are useful for chemical separation. Although the use of cages in the solid state benefits the recycling, the flexibility, dynamicity, and metal-ligand bond reversibility of solid-state cages are poor, preventing efficient guest encapsulation. Here we report a type of coordination cage-integrated solid materials that can be swelled into gel in water. The material is prepared through incorporation of an anionic FeII4L6 cage as the counterion of a cationic poly(ionic liquid) (MOC@PIL). The immobilized cages within MOC@PILs have been found to greatly affect the swelling ability of MOC@PILs and thus the mechanical properties. Importantly, upon swelling, the uptake of water provides an ideal microenvironment within the gels for the immobilized cages to dynamically move and flex that leads to excellent solution-level guest binding performances. This concept has enabled the use of MOC@PILs as efficient adsorbents for the removal of pollutants from water and for the purification of toluene and cyclohexane. Importantly, MOC@PILs can be regenerated through a deswelling strategy along with the recycling of the extracted guests.

Ditopic Covalent Cage for Ion-Pair Binding: Influence of Anion Complexation on the Cation Exchange Rate

A new hemicryptophane host with a ditopic molecular cavity combining a cyclotriveratrylene (CTV) unit with a tris-urea moiety was synthesized. The complexation of halides, tetramethylammonium (TMA+) cation, and ion pairs was investigated. A positive cooperativity was observed, since halides display a higher binding constant when a TMA+ cation is already present inside the cage. When TMA+ was complexed alone, a decrease of temperature from 298 K to 230 K was required to switch from a fast to a slow exchange regime on the NMR time scale. Nevertheless, the prior complexation of a halide guest in the lower part of the host resulted in significant decrease of the exchange rate of the subsequent complexation of the TMA+ cation. Under these conditions, the 1H NMR signals characteristic of a slow exchange regime were observed at 298 K. Addition of an excess of salts, increases the ionic strength of the solution, restoring the fast exchange dynamics. This result provides insight on how the exchange rate of a cation guest can be modulated by the complexation of a co-guest anion.

Gated, Selective Anion Exchange in Functionalized Self-Assembled Cage Complexes

Appending functional groups to the exterior of Zn4 L4 self-assembled cages allows gated control of anion binding. While the unfunctionalized cages contain aryl groups in the ligand that can freely rotate, attaching inert functional groups creates a "doorstop", preventing rotation and slowing the guest exchange rate, even though the interiors of the host cavities are identically structured. The effects on anion exchange are subtle and depend on multiple factors, including anion size, the nature of the leaving anion, and the electron-withdrawing ability and steric bulk of the pendant groups. Multiple exchange mechanisms occur, and the nature of the external groups controls associative and dissociative exchange processes: these bulky groups affect both anion egress and ingress, introducing an extra layer of selectivity to the exchange. Small changes can have large effects: affinities for anions as similar as PF6 - and SbF6 - can vary by as much as 400-fold between identically sized cavities.

Aryl-Extended and Super Aryl-Extended Calix[4]pyrroles: Design, Synthesis, and Applications

ConspectusProteins exhibit high-binding affinity and selectivity, as well as remarkable catalytic performance. Their binding pockets are hydrophobic but also contain polar and charged groups to contribute to the binding of polar organic molecules in aqueous solution. In the past decades, the synthesis of biomimetic receptors featuring sizable aromatic cavities equipped with converging polar groups has received considerable attention. "Temple" cages, naphthotubes, and aryl-extended calix[4]pyrroles are privileged examples of synthetic scaffolds displaying functionalized hydrophobic cavities capable of binding polar substrates. In particular, calix[4]pyrroles are macrocycles containing four pyrrole rings connected through their pyrrolic 2- and 5-positions by tetra-substituted sp³ carbon atoms (meso-substituents). In 1996, Sessler introduced the meso-octamethyl calix[4]pyrrole as an outstanding receptor for anion binding. Independently, Sessler and Floriani also showed that the introduction of aryl substituents in the meso-positions produced aryl-extended calix[4]pyrroles as a mixture of configurational isomers. In addition, aryl-extended calix[4]pyrroles bearing two and four meso-aryl substituents (walls) were reported. The cone conformation of "two-wall" αα-aryl-extended calix[4]pyrroles features an aromatic cleft with a polar binding site defined by four converging pyrrole NHs. On the other hand, "four-wall" αααα-calix[4]pyrrole isomers possess a deep polar aromatic cavity closed at one end by the converging pyrrole NHs. Because of their functionalized interior, aryl-extended calix[4]pyrroles are capable of binding anions, ion-pairs, and electron-rich neutral molecules in organic solvents. However, in water, they are restricted to the inclusion of neutral polar guests.Since the early 2000s, our research group has been involved in the design and synthesis of "two-wall" and "four-wall" aryl-extended calix[4]pyrroles and their derivatives, such as aryl-extended calix[4]pyrrole cavitands and super aryl-extended calix[4]pyrroles. In this Account, we mainly summarize our own results on the binding of charged and neutral polar guests with these macrocyclic receptors in organic solvents and in water. We also describe the applications of calix[4]pyrrole derivatives in the sensing of creatinine, the facilitated transmembrane transport of anions and amino acids, and the monofunctionalization of bis-isonitriles. Moreover, we explain the use of calix[4]pyrrole receptors as model systems for the quantification of anion-π interactions and the hydrophobic effect. Finally, we discuss the self-assembly of dimeric capsules and unimolecular metallo-cages based on calix[4]pyrrole scaffolds. We comment on their binding properties, as well as on those of bis-calix[4]pyrroles having a fully covalent structure.In molecular recognition, aryl-extended calix[4]pyrroles and their derivatives are considered valuable receptors owing to their ability to interact with a wide variety of electron-rich, neutral, and charged guests. Calix[4]pyrrole scaffolds have also been applied in the development of molecular sensors, ionophores, transmembrane carriers, supramolecular protecting groups and molecular containers modulating chemical reactivity, among others. We believe that the design of new calix[4]pyrrole receptors and the investigation of their binding properties may lead to promising applications in many research areas, such as supramolecular catalysis, chemical biology and materials science. We hope that this Account will serve to spread the knowledge of the supramolecular chemistry of calix[4]pyrroles among supramolecular and nonsupramolecular chemists alike.

Influence of the solvent in the self-assembly and binding properties of [1 + 1] tetra-imine bis-calix[4]pyrrole cages

We report the self-assembly of shape-persistent [1 + 1] tetra-imine cages 1 based on two different tetra-α aryl-extended calix[4]pyrrole scaffolds in chlorinated solvents and in a 9 : 1 CDCl₃ : CD₃CN solvent mixture. We show that the use of a bis-N-oxide 4 (4,4'-dipyridyl-N,N'-dioxide) as template is not mandatory to induce the emergence of the cages but has a positive effect on the reaction yield. We use ¹H NMR spectroscopy to investigate and characterize the binding properties (kinetic and thermodynamic) of the self-assembled tetra-imine cages 1 with pyridine N-oxide derivatives. The cages form kinetically and thermodynamically stable inclusion complexes with the N-oxides. For the bis-N-oxide 4, we observe the exclusive formation of 1 : 1 complexes independently of the solvent used. In contrast, the pyridine-N-oxide 5 (mono-topic guest) produces inclusion complexes displaying solvent dependent stoichiometry. The bis-N-oxide 4 is too short to bridge the gap between the two endohedral polar binding sites of 1 by establishing eight ideal hydrogen bonding interactions. Nevertheless, the bimolecular 4⊂1 complex results as energetically favored compared to the 5₂⊂1 ternary counterpart. The inclusion of the N-oxides, 4 and 5, in the tetra-imine cages 1 is significantly faster in chlorinated solvents (minutes) than in the 9 : 1 CDCl₃ : CD₃CN solvent mixture (hours). We provide an explanation for the similar energy barriers calculated for the formation of the 4⊂1 complex using the two different ternary counterparts 5₂⊂1 and (CD₃CN)₂⊂1 as precursors. We propose a mechanism for the in-out guest exchange processes experienced by the tetra-imine cages 1.

Pseudo-heterolepticity in Low-Symmetry Metal-Organic Cages

Heteroleptic metal-organic cages, formed through integrative self-assembly of ligand mixtures, are highly attractive as reduced symmetry supramolecular hosts. Ensuring high-fidelity, non-statistical self-assembly, however, presents a significant challenge in molecular engineering due to the inherent difficulty in predicting thermodynamic energy landscapes. In this work, two conceptual strategies are described that circumvent this issue, using ligand design strategies to access structurally sophisticated metal-organic hosts. Using these approaches, it was possible to realise cavity environments described by two inequivalent, unsymmetrical ligand frameworks, representing a significant step forward in the construction of highly anisotropic confined spaces.

Ternary host-guest complexes with rapid exchange kinetics and photoswitchable fluorescence

Confinement within molecular cages can dramatically modify the physicochemical properties of the encapsulated guest molecules, but such host-guest complexes have mainly been studied in a static context. Combining confinement effects with fast guest exchange kinetics could pave the way toward stimuli-responsive supramolecular systems-and ultimately materials-whose desired properties could be tailored "on demand" rapidly and reversibly. Here, we demonstrate rapid guest exchange between inclusion complexes of an open-window coordination cage that can simultaneously accommodate two guest molecules. Working with two types of guests, anthracene derivatives and BODIPY dyes, we show that the former can substantially modify the optical properties of the latter upon noncovalent heterodimer formation. We also studied the light-induced covalent dimerization of encapsulated anthracenes and found large effects of confinement on reaction rates. By coupling the photodimerization with the rapid guest exchange, we developed a new way to modulate fluorescence using external irradiation.

Self-assembly of a water-soluble endohedrally functionalized coordination cage including polar guests

Coordination cages containing endohedrally functionalized aromatic cavities are scarce in the literature. Herein, we report the self-assembly of a tetra-cationic super aryl-extended calix[4]pyrrole tetra-pyridyl ligand into a water-soluble Pd(ii)-cage featuring two endohedral polar binding sites. They are defined by the four pyrrole NHs of the calix[4]pyrrole unit and the four inwardly directed α-protons of the coordinated pyridyl groups. The efficient assembly of the Pd(ii)-cage requires the inclusion of mono- and ditopic pyridyl N-oxide and aliphatic formamide guests. The monotopic guests only partially fill the cage's cavity and require the co-inclusion of a water molecule that is likely hydrogen-bonded to the endohedral α-pyridyl protons. The ditopic guests are able to completely fill the cage's cavity and complement both binding sites. We observed high conformational selectivity in the inclusion of the isomers of α,ω-bis-formamides. We briefly investigate the uptake and release mechanism/kinetics of selected polar guests by the Pd(ii)-cage using pair-wise competition experiments.

A tetra-cationic calix[4]pyrrole tetra-pyridyl ligand self-assembles into a water-soluble Pd(ii)-cage featuring two endohedral polar binding sites. The Pd(ii)-cage encapsulates pyridyl N-oxide and aliphatic formamide guests in water.

Guest Recognition Control Accompanied by Stepwise Gate Closing and Opening of a Macrocyclic Metallohost

Host-guest binding behavior of macrocyclic hosts is significantly influenced by the shapes and sizes of the hosts. In particular, closing/opening the apertures of the hosts controls the guest uptake/release. A post-metalation modification method was used to achieve the open/close conversions. The starting open complex, [LCo₂ (pip)₄ ](OTf)₂ , was efficiently converted to the closed complex, [LCo₂ (hda)₂ ](OTf)₂ , which has a doubly bridged structure. The conversion of this closed complex to the open complex [LCo₂ (hda)₂ (OAc)]⁺ was too slow to be completed, but this gate-opening was dramatically accelerated by the addition of Na⁺ . The Na⁺ binding was also significantly enhanced by the gate-opening, that is, conversion of [LCo₂ (hda)₂ ]²⁺ to [LCo₂ (hda)₂ (OAc)]⁺ .

Host-Guest Exchange of Viologen Guests in Porphyrin Cage Compounds as Studied by Selective Exchange Spectroscopy (1D EXSY) NMR

Dynamics in complexes of porphyrin cage compounds and viologen-derived guest molecules are investigated by selective exchange NMR spectroscopy (1D EXSY). Exchange rates were found to be independent of excess guest concentration, revealing a dissociative exchange mechanism, which is accompanied by negative activation entropies, indicating significant reorganization of the host-guest complex during dissociation. Nonsymmetric viologen guests with bulky head groups had more unidirectional binding and slower exchange rates than guests with less-bulky head groups. Thermodynamic and kinetic studies revealed that the exchange process is primarily driven by the thermodynamics of binding and that guest binding can be influenced by introducing steric and electronic groups on the host . Exchange studies with guests bearing a polymer chain revealed that both slippage and full dissociation takes place and the rate constants for both processes were determined. The slippage rate constant revealed that for smaller guests exchange takes place nearly exclusively under thermodynamic control.

Emergent Self-Assembly of a Multicomponent Capsule via Iodine Capture

Described here is a three-component self-assembly system that displays emergent behavior that differs from that of its constituents. The system comprises an all-hydrocarbon octaaryl macrocycle cyclo[8](1,3-(4,6-dimethyl)benzene (D_4d-CDMB-8), corannulene (Cora), and I₂. No appreciable interaction is seen between any pair of these three-components, either in cyclohexane or under various crystallization conditions. On the other hand, when all three-components are mixed in cyclohexane and allowed to undergo crystallization, a supramolecular iodine-containing capsule, ((D_4d-CDMB-8)₃⊃(Cora)₂)⊃I₂, is obtained. This all-hydrocarbon capsule consists of three D_4d-CDMB-8 and two Cora subunits and contains a centrally bound I₂ molecule as inferred from single-crystal and powder X-ray diffraction studies as well as solid-state ¹³C NMR and Raman spectroscopy. These analyses were complemented by solution-phase ¹H NMR and UV-vis spectroscopic studies. No evidence of I₂ escape from the capsule is seen, even at high temperatures (e.g., up to 418 K). The bound I₂ is likewise protected from reaction with alkali or standard reductants in aqueous solution (e.g., saturated NaOH(aq) or aqueous Na₂S₂O₃). It was also found that a mixed powder containing D_4d-CDMB-8 and Cora in a 3:2 molar ratio could capture saturated I₂ vapor or iodine from aqueous sources (e.g., 1.0 mM I₂ in NaCl (35 wt %) or I₂ + NaI(aq) (1.0 mM each)). The present system displays structural and functional features that go beyond what would be expected on the basis of a simple sum-of-the-components analysis. As such, it illustrates a new approach to creating self-assembled ensembles with emergent features.

Cavitand-Based Pd-Pyridyl Coordination Capsules: Guest-Induced Homo- or Heterocapsule Selection and Applications of Homocapsules to the Protection of a Photosensitive Guest and Chiral Capsule Formation

A 2 : 4 mixture of tetrakis[4-(4-pyridyl)phenyl]cavitand (1) or tetrakis[4-(4-pyridyl)phenylethynyl]cavitand (2) and Pd(dppp)(OTf)₂ self-assembles into a homocapsule {1₂  ⋅ [Pd(dppp)]₄ }⁸⁺  ⋅ (TfO^- )₈ (C1) or {2₂  ⋅ [Pd(dppp)]₄ }⁸⁺  ⋅ (TfO^- )₈ (C2), respectively, through Pd-Npy coordination bonds. A 1 : 1 : 4 mixture of 1, 2, and Pd(dppp)(OTf)₂ produced a mixture of homocapsules C1, C2, and a heterocapsule {1 ⋅ 2 ⋅ [Pd(dppp)]₄ }⁸⁺  ⋅ (TfO^- )₈ (C3) in a 1 : 1 : 0.98 mole ratio. Selective formation (self-sorting) of homocapsules C1 and C2 or heterocapsule C3 was controlled by guest-induced encapsulation under thermodynamic control. Applications of Pd-Npy coordination capsules with the use of 1 were demonstrated. Capsule C1 serves as a guard nanocontainer for trans-4,4'-diacetoxyazobenzene to protect against the trans-to-cis photoisomerization by encapsulation. A chiral capsule {1₂  ⋅ [Pd((R)-BINAP)]₄ }⁸⁺  ⋅ (TfO^- )₈ (C5) was also constructed. Capsule C5 induces supramolecular chirality with respect to prochiral 2,2'-bis(alkoxycarbonyl)-4,4'-bis(1-propynyl)biphenyls by diastereomeric encapsulation through the asymmetric suppression of rotation around the axis of the prochiral biphenyl moiety.

A Regulable Internal Cavity inside a Resorcinarene-Based Hemicarcerand

Covalent organic capsules, such as carcerands and hemicarcerands, are an interesting class of molecular hosts. These container molecules have confined spaces capable of hosting small molecules, although the fact that the size of the inner cavities cannot be changed substantially limits the scope of their applications. The title covalently linked container was produced by metal-directed dimerization of a resorcinarene-based cavitand having four 2,2'-bipyridyl arms on the wide rim followed by olefin metathesis at the vertices of the resulting capsule with a second-generation Grubbs catalyst. The covalently linked bipyridyl arms permit expansion of the inner cavity by demetalation. This structural change influences the molecular recognition properties; the metal-coordinated capsule recognizes only 4,4'-diacetoxybiphenyl, whereas the metal-free counterpart can encapsulate not only 4,4'-diacetoxybiphenyl, but also 2,5-disubstituted-1,4-bis(4-acetoxyphenylethynyl)benzene, which is 9.4 Å longer than the former guest. Molecular mechanics calculations predict that the capsule expands the internal cavity to encapsulate the long guest by unfolding the folded conformation of the alkyl chains, which demonstrates the flexible and regulable nature of the cavity. Guest competition experiments show that the preferred guest can be switched by metalation and demetalation. This external-stimuli-responsive guest exchange can be utilized for the development of functional supramolecular systems controlling the uptake, transport, and release of chemicals.