Development of redox-switchable resorcin[4]arene cavitands

CONSPECTUS: Within the framework of miniaturization of electromechanical devices, the development of a redox-switchable molecular gripper as a tool for nanorobotics is appealing both from an academic and from a practical perspective. Such a tool should be able to controllably grab a molecular cargo, translocate it over considerable distances and time scales, and release it. Resorcin[4]arene cavitands seem to be an ideal platform for the development of molecular grippers due to their ability to adopt two spatially well-defined conformations: an expanded kite and a contracted vase. Furthermore, they possess "legs" for functionalization and attachment to metal surfaces. While changes in temperature, pH, and metal-ion concentration were known to induce conformational switching, redox-switchable cavitands remained a challenge. In this Account, we describe our efforts toward the development of a new class of resorcin[4]arene cavitands that are redox-switchable. First, we introduced the naphthoquinone moiety as a redox-active wall component and showed that cavitands containing four quinone walls strongly prefer the open kite conformation in both the quinone and hydroquinone redox states, while cavitands that contain two quinone and two quinoxaline walls can adopt both the vase and the kite conformations depending on solvent but not on redox state. Next, in order to introduce a driving force for the conformational switching process in diquinone cavitands, we designed cavitands with hydrogen bond acceptor groups on the quinoxaline walls. These acceptors were sought to establish hydrogen bonds with the hydroquinone groups in the reduced redox state, thereby stabilizing the vase form. Oxidation to the quinone state would remove these interactions, switching the cavitand back to the kite form. Cavitands equipped with different hydrogen bond acceptor groups were synthesized and evaluated. We found that carboxamide moieties are best suited to assist redox-induced switching of conformational and binding properties. With the goal of further increasing association constants and reducing guest-exchange rates via steric congestion, we exchanged the naphthoquinone with the triptycene-quinone moiety. The congesting influence of the triptycene-quinone moiety on the binding properties was quantified both in the presence and in the absence of additional hydrogen bond interactions that stabilize the vase form. X-ray crystallographic studies provided insights into the solid-state structures of the cavitands in different solvents and redox states. A significant enhancement of association constants and reduction in guest release rates was observed in the reduced redox state compared with the top-open system, yielding redox-switchable cavitand baskets. These studies represent a step towards the development of redox-switchable molecular grippers on metal surfaces. Future challenges will consist in the development of cavitands that will no longer rely on an external proton source for the switching process, allowing redox-switching to be performed in purely aprotic media. Finally, suitable leg functionalization would enable the grippers to be interfaced with metal surfaces.

Building on Cram's legacy: stimulated gating in hemicarcerands

Donald Cram’s pioneering Nobel Prize-winning work on host–guest molecules led eventually to his creation of the field of container molecules. Cram defined two types of container molecules: carcerands and hemicarcerands. Host–guest complexes of carcerands, called carceplexes, are formed during their synthesis; once a carceplex is formed, the trapped guest cannot exit without breaking covalent bonds. Cram defined a quantity called constrictive binding, arising from the mechanical force that prevents guest escape. The constrictive binding in carceplexes is high. In contrast, hemicarcerands have low constrictive binding and are able to release the incarcerated guests at elevated temperatures without breaking covalent bonds. We have designed molecules that can switch from carcerand to hemicarcerand through a change in structure that we call gating.

The original discovery of gating in container molecules involved our computational studies of a Cram hemicarceplex that was observed to release a guest upon heating. We found that the side portals of this hemicarceplex have multiple thermally accessible conformations. An eight-membered ring that is part of a portal changes from a “chair” to a “boat” structure, leading to the enlargement of the side portal and the release of the guest. This type of gating is analogous to phenomena often observed with peptide loops in enzymes. We refer to this phenomenon as thermally controlled gating.

We have also designed and synthesized redox and photochemically controlled gated hemicarceplexes. Gates are built onto host molecules so that the opening or closing of such gates is stimulated by reducing or oxidizing conditions, or by ultraviolet irradiation. In both cases, the appropriate stimuli can produce a carceplex (closed gates) or hemicarceplex (open gates). A hemicarceplex with closed gates behaves like a carceplex, due to its very high constrictive binding energy. When the gates are opened, constrictive binding is dramatically lowered, and guest entrance and exit become facile. This stimulated switching between open and closed states controls access of the guest to the binding site. The experimental and computational investigations of gated hemicarcerands and several potential applications of gated hemicarceplexes are described in this Account.

Probing the inner space of salt-bridged calix[5]arene capsules

A combined DOSY and XRD study indicates that a carboxylcalix[5]arene receptor is able to encapsulate α,ω-diamines of appropriate length by means of a proton-transfer-mediated recognition process followed by salt-bridge-assisted bis-endo-complexation.

Giant electroactive M4L6 tetrahedral host self-assembled with Fe(II) vertices and perylene bisimide dye edges

Self-assembly of octahedral Fe(II) ions and linear perylene bisimide (PBI) dyes with 2,2'-bipyridine groups covalently attached at the imide positions quantitatively yields an Fe4(PBI)6 tetrahedron by the directional bonding approach. With an edge length of 3.9 nm and estimated internal volume >950 Å(3), tetrahedron T is one of the largest M4L6 tetrahedra ever reported. Importantly, many of the desirable photo- and electroactive properties of the PBI ligands are transferred to the nanoscale metallosupramolecule. Tetrahedron T absorbs strongly across the visible spectrum out to 650 nm and exhibits a total of 7 highly reversible electrochemical oxidation and reduction waves spanning a 3.0 V range. This facile cycling of 34 electrons between +18 and -16 charged species is likely enabled due to the porous nature of the tetrahedron that allows the necessary counterions to freely flow in and out of the host. Host-guest encapsulation of C60 by T in acetonitrile was studied by (13)C NMR spectroscopy, UV-vis spectroscopy, and ESI-MS, confirming that the tetrahedron is a suitable host for large, functional guest molecules.

More chemistry in small spaces

This Account is about coaxing molecules into spaces barely big enough to contain them: encapsulation complexes. In capsules, synthetic modules assemble to fold around their molecular targets, isolate them from the medium for relatively long times, place them in a hydrophobic environment, and present them with functional groups. These arrangements also exist in the interior spaces of biology, and the consequences include the familiar features of enzymes: rapid reactions, stabilization of reactive intermediates, and catalysis. But inside capsules there are phenomena unknown to biology or historical chemistry, including new structures, new stereochemical relationships, and new reaction pathways. In encapsulation complexes, as in architecture, the space that is created by a structure determines what goes on inside. There are constant interactions between the container and contained molecules: encounters are not left to chance; they are prearranged, prolonged, and intense. Unlike architecture, these reversibly formed containers emerge only when a suitable guest is present. The components exist, but they cannot assemble without anything inside. Modifications of the capsule components give rise to the results of the present Account. The focus will be on how seemingly small changes in the encapsulation complexes, exchanging a C═S for a C═O, reducing an angle here and there, or replacing a hydrogen with a methyl, can lead to unexpectedly large differences in behavior.

Ligand Binding Pathway Elucidation for Cryptophane Host-Guest Complexes

Modeling binding pathways can provide insight into molecular recognition, including kinetic mechanisms, barriers to binding, and gating effects. This work represents a novel computational approach, Hopping Minima, for the determination of conformational transitions of single molecules as well as binding pathways for molecular complexes. The method begins by thoroughly sampling a set of conformational minima for a molecular system. The natural motions of the system are modeled using the normal modes of the sampled minima. The natural motions are utilized to connect conformational minima and are finally combined to form association/binding pathways in the case of molecular complexes. We provide an implementation and example application of the method using alanine dipeptide and a set of chemical host-guest systems: two cryptophane hosts with two guest cations, trimethylammonium and tetramethylammonium. Our results demonstrate that conformational transitions can be modeled and extended to find binding pathways as well as energetic information relevant to the minimum conformations involved. This approach has advantages over simulation-based methods for studying systems with slow binding processes and can help design molecules with preferred binding kinetics.

The entrapment of chiral guests with gated baskets: can a kinetic discrimination of enantiomers be governed through gating?

The capacity of gated hosts for controlling a kinetic discrimination between stereoisomers is yet to be understood. To conduct corresponding studies, however, one needs to develop chiral, but modular and gated hosts. Accordingly, we used computational (RI-BP86/TZVP//RI-BP86/SV(P)) and experimental (NMR/CD/UV/Vis spectroscopy) methods to examine the transfer of chirality in gated baskets. We found that placing stereocenters of the same kind at the rim (R(1) =CH3, so-called bottom) and/or top amide positions (R(2) =sec-butyl) would direct the helical arrangement of the gates into a P or M propeller-like orientation. With the assistance of (1)H NMR spectroscopy, we quantified the intrinsic (thermodynamic) and constrictive (kinetic) binding affinities of (R)- and (S)-1,2-dibromopropane 5 toward baskets (S3b/P)-2, (S3t/M)-3, and (S3bt/P)-4. Interestingly, each basket has a low ( ≤1.3 kcal mol(-1)), but comparable (de<10%) affinity for entrapping enantiomeric (R/S)-5. In terms of the kinetics, basket (S3b/P)-2, with a set of S stereocenters at the bottom and P arrangement of the gates, would capture (R)-5 at a faster rate (kin(R)/kin(S) =2.0±0.2). Basket (S3t/M)-3, with a set of S centers at the top and M arrangement of the gates, however, trapped (S)-5 at a faster rate (kin(R)/kin(S) =0.30±0.05). In light of these findings, basket (S3bt/P)-4, with a set of S stereocenters installed at both top and bottom sites along with a P disposition of the gates, was found to have a lower ability to differentiate between enantiomeric (R/S)-5 (kin(R)/kin(S) =0.8). Evidently, the two sets of stereocenters in this "hybrid" host acted concurrently, each with the opposite effect on the entrapment kinetics. Gated baskets are hereby established to be a prototype for quantifying the kinetic discrimination of enantiomers through gating and elucidating the electronic/steric effects on the process.

A Dynamic Route to Structure and Function: Recent Advances in Imine-Based Organic Nanostructured Materials

The chemistry of imine bond formation from simple aldehyde and amine precursors is among the most powerful dynamic covalent chemistries employed for the construction of discrete molecular objects and extended molecular frameworks. The reversible nature of the C=N bond confers error-checking and proof-reading capabilities in the self-assembly process within a multi-component reaction system. This review highlights recent progress in the self-assembly of complex organic molecular architectures that are enabled by dynamic imine chemistry, including molecular containers with defined geometry and size, mechanically interlocked molecules, and extended frameworks and polymers, from building blocks with preprogrammed steric and electronic information. The functional aspects associated with the nanometer-scale features not only place these dynamically constructed nanostructures at the frontier of materials sciences, but also bring unprecedented opportunities for the discovery of new functional materials.

Size-selective phase-transfer catalysis with interfacially cross-linked reverse micelles

Cross-linking of the reverse micelles (RMs) of a triallylammonium surfactant afforded organic nanoparticles with introverted cationic groups. The cross-linked reverse micelles catalyzed size-selective biphasic reaction between sodium azide and alkyl bromides. Size selectivity of up to 9:1 was obtained for alkyl bromides with similar structures. The selectivity was influenced strongly by the size of the water pool and proposed to happen as a result of the "sieving" effect of the alkyl corona.

On the mechanism of action of gated molecular baskets: The synchronicity of the revolving motion of gates and in/out trafficking of guests

We used dynamic (1)H NMR spectroscopic methods to examine the kinetics and thermodynamics of CH(3)CCl(3) (2) entering and leaving the gated molecular basket 1. We found that the encapsulation is first-order in basket 1 and guest 2, while the decomplexation is zeroth-order in the guest. Importantly, the interchange mechanism in which a molecule of CH(3)CCl(3) directly displaces the entrapped CH(3)CCl(3) was not observed. Furthermore, the examination of the additivity of free energies characterizing the encapsulation process led to us to deduce that the revolving motion of the gates and in/out trafficking of guests is synchronized, yet still a function of the affinity of the guest for occupying the basket: Specifically, the greater the affinity of the guest for occupying the basket, the less effective the gates are in "sweeping" the guest as the gates undergo their revolving motion.