Qi-Qiang Wang

Halide Recognition by Tetraoxacalix[2]arene[2]triazine Receptors: Concurrent Noncovalent Halide–π and Lone-pair–π Interactions in Host–Halide–Water Ternary Complexes†

Anion recognition has attracted much attention because of its importance in biological and environmental sciences. A large number of synthetic host molecules have been synthesized for the purpose of anion complexation. Noticeably, almost all synthetic anion receptors were designed to exploit hydrogen bonding, electrostatic interactions, hydrophobic effects, and coordination to metal ions. In recent years, however, there has been a growing interest in anion–p interactions, the interaction between an anion and an electron-deficient p-arene species. A number of theoretical studies, for example, have reported the noncovalent interactions of anions with aromatic compounds such as perfluoro-, nitro-, and cyano-substituted benzene, pyridine, pyrazine, and triazine derivatives. Experimental evidence to support the purely noncovalent anion–p interaction with charge-neutral arenes, however, is very rare. A recent study indicated indeed that the noncovalent anion–p interaction contributed less significantly than hydrogen bonding to the formation of the complex chloride–p-C6FnH6 n (n= 0– 5). Heteroatom-bridged heteroaromatic calixarenes are an emerging type of novel macrocyclic molecules. Because of their electronic nature, some heteroatoms, such as nitrogen, can adopt different configurations to form varying degrees of conjugation with adjacent heteroaromatic rings, resulting in macrocyclic heteroatom-linked heteroaromatic calixarenes with conformations and sizes different to those of conventional calixarenes. They have recently been utilized as versatile host molecules in supramolecular chemistry. As a typical example of heteroatom-bridged heteroaromatic calixarenes, tetraoxacalix[2]arene[2]triazine 1 (Scheme 1) has been reported to adopt a pre-organized 1,3-alternate conformation, yielding a cleft formed by two p-electron deficient triazine rings. We envisioned that this pelectron-deficient cavity would act as a receptor to interact with anions through p–anion interactions. Herein, we report halide recognition by tetraoxacalix[2]arene[2]triazine host molecules, and considerable substituent effects of the triazine on the halide–p interaction. Most astonishingly, X-ray crystallography revealed the concurrent formation of noncovalent p–halide and p–lone-pair electron interactions between water, chloride, or bromide, and the dichlorosubstituted tetraoxacalix[2]arene[2]triazine host. By means of spectrophotometric measurements, we examined the interaction of halides with dichloro-substituted tetraoxacalix[2]arene[2]triazine 1, a macrocycle readily accessible in large scale and good yield from the coupling of resorcinol and cyanuric acid chloride, under very mild conditions. As illustrated in Figure 1 (top), a new absorption band formed at 302 nm in the UV/Vis spectrum of 1 upon titration with tetrabutylammonium fluoride. Interaction of 1 with tetrabutylammonium fluoride also led to quenching of the host molecule emission signal at 304 nm with the concomitant emergence of a new fluorescence emission at 452 nm (Figure 1 (bottom)). These spectral changes were not caused by the cation or by moisture in the sample, as the control experiments using tetrabutylammonium perchlorate or water as a titrant showed no influence on either the absorption or the emission spectrum of 1. We then investigated the spectrophotometric titration of 1 with tetrabutylammonium chloride and tetrabutylammonium bromide. Under identical conditions, the outcomes of titration with chloride and bromide were very different to that for the titration with fluoride. For example, addition of chloride or bromide did not affect the UV/Vis spectrum of the host Scheme 1. Dichloro-substituted tetraoxacalix[2]arene[2]triazine 1 and its transformations into 2 and 3. a) H2, Pd/C, NaOAc/HOAc, 50 8C, 12 h, 82%. b) Me2NH·HCl, K2CO3, THF, 70 8C, 9 h, 65%.

Self-assembled nanospheres with multiple endohedral binding sites pre-organize catalysts and substrates for highly efficient reactions

Tuning reagent and catalyst concentrations is crucial in the development of efficient catalytic transformations. In enzyme-catalysed reactions the substrate is bound—often by multiple non-covalent interactions—in a well-defined pocket close to the active site of the enzyme; this pre-organization facilitates highly efficient transformations. Here we report an artificial system that co-encapsulates multiple catalysts and substrates within the confined space defined by an M12L24 nanosphere that contains 24 endohedral guanidinium-binding sites. Cooperative binding means that sulfonate guests are bound much more strongly than carboxylates. This difference has been used to fix gold-based catalysts firmly, with the remaining binding sites left to pre-organize substrates. This strategy was applied to a Au(I)-catalysed cyclization of acetylenic acid to enol lactone in which the pre-organization resulted in much higher reaction rates. We also found that the encapsulated sulfonate-containing Au(I) catalysts did not convert neutral (acid) substrates, and so could have potential in the development of substrate-selective catalysis and base-triggered on/off switching of catalysis. Preorganization of catalysts and substrates can lead to significant rate enhancement—an effect often observed in enzyme catalysis. Now, a self-assembled nanosphere equipped with 24 guanidinium binding sites is demonstrated to strongly bind sulfonate-containing gold catalysts. Base-triggered co-encapsulation of carboxylate containing substrates leads to pronounced gating effects and dramatically enhanced reaction rates.

Synthesis and structure of oxacalix[2]arene[2]triazines of an expanded π-electron-deficient cavity and their interactions with anions.

Novel macrocyclic anion receptors based on the principle of anion-π interactions were reported. By means of both post-macrocyclization modification protocol and the stepwise fragment coupling approach, functionalized oxacalix[2]arene[2]triazines bearing two other electron-deficient (hetero)aromatic rings on the lower rim were efficiently synthesized. The resulting oxacalix[2]arene[2]triazine macrocyclesadopt 1,3-alternate conformation, yielding therefore an expanded electron-deficient cavity or space consisting of two triazine rings and two appending aromatic rings. Spectroscopic titration study showed the selective interaction of the pentafluorophenyl-substituted oxacalix[2]arene[2]triazine with azide and fluoride in solution with the binding constants (K(1:1)) ranging from 1.33 × 10(3) to 3.52 × 10(3) M(-1).

Molecular Thioamide ↔ Iminothiolate Switches for Sulfur Mustards

SNS platinum(II) pincer complexes reversibly bind and release the surrogate half sulfur mustard, 2-chloroethyl ethyl sulfide (CEES). The switch-like behavior of the pincers is attributed to a reversible transformation between the thioamide and iminothiolate forms of the pincer skeleton under slightly acidic and basic conditions, respectively. An amide-based palladium(II) pincer complex also binds CEES, as confirmed crystallographically and by NMR.

Tunable, shape-shifting capsule for dicarboxylates

A cylindrical amide-based tricycle provides a ditopic framework for encapsulating both aliphatic and aromatic dicarboxylate anions. Due to the flexible spacer between the two macrocyclic receptor sites, it can modulate its shape to conform to dicarboxylates of varying lengths. In the uncomplexed form, the host is elongated along its cylindrical axis, but when encapsulating the guest, it compresses to ensure the best structural fit.

Chemical Mustard Containment Using Simple Palladium Pincer Complexes: The Influence of Molecular Walls

Six amide-based NNN palladium(II) pincer complexes Pd(L)(CH3CN) were synthesized, characterized, and examined for binding the sulfur mustard surrogate, 2-chloroethyl ethyl sulfide (CEES). The complexes all bind readily with CEES as shown by (1)H NMR spectroscopy in CDCl3. The influence of para-substituents on the two amide phenyl appendages was explored as well as the effect of replacing the phenyl groups with larger aromatic rings, 1-naphthalene and 9-anthracene. While variations of the para-substituents had only a slight influence on the binding affinities, incorporation of larger aromatic rings resulted in a significant size-related increase in binding, possibly due to increasing steric and electronic interactions. In crystal structures of three CEES-bound complexes, the mustard binds through the sulfur atom and lies along the aromatic walls of the side appendages approximately perpendicular to the pincer plane, with increasingly better alignment progressing from phenyl to 1-naphthalene to 9-anthracene.

Designed self-assemblies based on cooperative noncovalent interactions including anion–π, lone-pair electron–π and hydrogen bonding

A dual functional tetraoxacalix[2]arene[2]triazine derivative as a building block was synthesized. Designed self-assemblies were obtained under the guidance of multiple noncovalent interactions including anion–π, lone-pair electron–π and hydrogen bonding.

Anionic Head Containing Oxacalix[2]arene[2]triazines: Synthesis and Anion−π-Directed Self-Assembly in Solution and Solid State

A series of oxacalix[2]arene[2]triazines bearing one anionic head such as carboxylate, sulfonate, sulfate, and phosphate were synthesized. With the anionic head and complementary V-shape electron-deficient cavity, these macrocycles can serve as dual building units, and their anion-π directed self-assembly was investigated. The formation of oligomeric aggregates in solution was revealed by nuclear magnetic resonance, dynamic light scattering, and mass spectroscopy. Crystal structures further confirmed chainlike assembly formation directed by anion-π interactions.

Molecular Barrel by a Hooping Strategy: Synthesis, Structure, and Selective CO2 Adsorption Facilitated by Lone Pair−π Interactions

A sophisticated molecular barrel 5 was efficiently constructed by hooping a 63-membered loop around a D3h-symmetric, shape-persistent bis(tetraoxacalix[2]arene[2]triazine) core. The hooping strategy involved 3-fold ring-closing metathesis (RCM) reactions of six branched olefin arms which were preanchored on the inner core. Through hooping, the loop tightens the cage structure and significantly enhances its stability toward nucleophilic decomposition. The X-ray crystal structure showed the molecular barrel bears three enclosed fan-shaped cavities as divided by the triazine rings and each of the cavities can hold a solvent CHCl3 or CH2Cl2 molecule. With the intrinsic porosity, the amorphous solids of 5 exhibit considerable CO2 uptake with an exceptionally large isosteric enthalpy. Lone pair (lp)-π interactions between the electron-deficient triazine rings and CO2 could contribute to the strong adsorption as supported by IR studies and DFT modeling.

Macrocyclic Influences in CO2 Uptake and Stabilization

Two 24-member diamine-tetraamido macrocycles (R = H and CH3), readily synthesized in one or two steps, were found to react with CO2 rapidly and efficiently (100% conversion within 1 min at rt). The resulting carbamate formation was demonstrated by (1)H, (13)C NMR, ESI-MS, and X-ray crystallography. The crystal structure clearly showed the carbamate group (N-CO2(-)) formed was tightly bound within the macrocyclic cavity, held by five internal hydrogen bonds, and stabilized by intramolecular carbamate-ammonium salt-bridge formation.