Calixarenes and resorcinarenes as scaffolds for supramolecular metallo-enzyme mimicry

Intra- and Intermolecular Cooperativity in the Catalytic Activity of Phosphodiester Cleavage by Self-Assembled Systems Based on Guanidinylated Calix[4]arenes

The calix[4]arene scaffold, blocked in the cone conformation through alkylation with long alkyl chains, and decorated at the upper rim with four guanidine or arginine units, effectively catalyzes the cleavage of the phosphodiester bond of DNA and RNA model compounds in water. An exhaustive kinetic investigation unequivocally points to the existence of spontaneous aggregation phenomena, driven by hydrophobic effect, occurring at different critical concentrations that depend on the identity of the compound. A pronounced superiority of the assembled structures compared with the monomers in solution was observed. Moreover, the catalytically active units, clustered on the macrocyclic tetrafunctional scaffold, were proved to efficiently cooperate in the catalytic mechanism and result in improved reaction rates compared to those of the monofunctional model compounds. The kinetic analysis is also integrated and corroborated with further experiments based on fluorescence spectroscopy and light scattering. The advantage of the supramolecular assemblies based on tetrafunctional calixarenes leads to believe that the active units can cooperate not only intramolecularly but also intermolecularly. The molecules in the aggregates can probably mold, flex and rearrange but, at the same time, keep an ordered structure that favors phosphodiester bond cleavage. This dynamic preorganization can allow the catalytic units to reach a better fitting with the substrates and perform a superior catalytic activity.

Selective Metal-ion Complexation of a Biomimetic Calix[6]arene Funnel Cavity Functionalized with Phenol or Quinone

In the biomimetic context, many studies have evidenced the importance of the 1^st and 2^nd coordination sphere of a metal ion for controlling its properties. Here, we propose to evaluate a yet poorly explored aspect, which is the nature of the cavity that surrounds the metal labile site. Three calix[6]arene-based aza-ligands are compared, that differ only by the nature of cavity walls, anisole, phenol or quinone (L^OMe , L^OH and L^Q ). Monitoring ligand exchange of their Zn^II complexes evidenced important differences in the metal ion relative affinities for nitriles, halides or carboxylates. It also showed a possible sharp kinetic control on both, metal ion binding and ligand exchange. Hence, this study supports the observations reported on biological systems, highlighting that the substitution of an amino-acid residue of the enzyme active site, at remote distance of the metal ion, can have strong impacts on metal ion lability, substrate/product exchange or selectivity.

Association Complexes of Calix[6]arenes with Amino Acids Explained by Energy-Partitioning Methods

Intermolecular complexes with calixarenes are intriguing because of multiple possibilities of noncovalent binding for both polar and nonpolar molecules, including docking in the calixarene cavity. In this contribution calix[6]arenes interacting with amino acids are studied with an additional aim to show that tools such as symmetry-adapted perturbation theory (SAPT), functional-group SAPT (F-SAPT), and systematic molecular fragmentation (SMF) methods may provide explanations for different numbers of noncovalent bonds and of their varying strength for various calixarene conformers and guest molecules. The partitioning of the interaction energy provides an easy way to identify hydrogen bonds, including those with unconventional hydrogen acceptors, as well as other noncovalent bonds, and to find repulsive destabilizing interactions between functional groups. Various other features can be explained by energy partitioning, such as the red shift of an IR stretching frequency for some hydroxy groups, which arises from their attraction to the phenyl ring of calixarene. Pairs of hydrogen bonds and other noncovalent bonds of similar magnitude found by F-SAPT explain an increase in the stability of both inclusion and outer complexes.

A calix[4]arene with acylguanidine units as an efficient catalyst for phosphodiester bond cleavage in RNA and DNA model compounds

Conjugated carbonyl units in a calixarene scaffold provide the right amount of flexibility for catalysis with a minimum entropic cost.

Single-crystal X-ray diffraction study, NMR and electrochemical analysis of a copper(i) 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrakis-[(6-methyl-2,2′-bipyridyl-6-yl)methoxy]calix[4]arene complex: an original M4L2“hand-to-hand” system

An original M₄L₂“hand-to-hand” copper(i)/tetra-bipyridyl-calix[4]arene complex was obtained and characterized in the solid state by X-ray diffraction. It changes in MeCN solution to its M₂L analogue.

Host-guest complexes of conformationally flexible C-hexyl-2-bromoresorcinarene and aromatic N-oxides: solid-state, solution and computational studies

Host-guest complexes of C-hexyl-2-bromoresorcinarene (BrC6) with twelve potential aromatic N-oxide guests were studied using single crystal X-ray diffraction analysis and 1H NMR spectroscopy. In the solid state, of the nine obtained X-ray crystal structures, eight were consistent with the formation of BrC6-N-oxide endo complexes. The lone exception was from the association between 4-phenylpyridine N-oxide and BrC6, in that case the host forms a self-inclusion complex. BrC6, as opposed to more rigid previously studied C-ethyl-2-bromoresorcinarene and C-propyl-2-bromoresorcinarene, undergoes remarkable cavity conformational changes to host different N-oxide guests through C-H···π(host) interactions. In solution phase CD3OD/CDCl3 (1:1 v/v), all twelve N-oxide guests form endo complexes according to 1H NMR; however, in more polar CD3OD/DMSO-d6 (9:1 v/v), only three N-oxides with electron-donating groups form solution-phase endo complexes with BrC6. In solid-state studies, 3-methylpyridine N-oxide+BrC6 crystallises with both the upper- and lower-rim BrC6 cavities occupied by N-oxide guests. Computational DFT-based studies support that lower-rim long hexyl chains provide the additional stability required for this ditopic behaviour. The lower-rim cavity, far from being a neutral hydrophobic environment, is a highly polarizable electrostatically positive surface, aiding in the binding of polar guests such as N-oxides.

The 3rd degree of biomimetism: associating the cavity effect, ZnII coordination and internal base assistance for guest binding and activation

The synthesis and characterization of a resorcinarene-based tetra(imidazole) ligand is reported. The properties of the corresponding ZnII complex are studied in depth, notably by NMR spectroscopy. In MeCN, acid-base titration reveals that one out of the four imidazole arms is hemi-labile and can be selectively protonated, thereby opening a coordination site in the exo position. Quite remarkably, the 4th imidazole arm promotes binding of an acidic molecule (a carboxylic acid, a β-diketone or acetamide), by acting as an internal base, which allows guest binding as an anion to the metal center in the endo position. Most importantly, the presence of this labile imidazole arm makes the ZnII complex active for the catalyzed hydration of acetonitrile. It is proposed that it acts as a general base for activating a water molecule in the vicinity of the metal center during its nucleophilic attack to the endo-bound MeCN substrate. This system presents a unique degree of biomimetism when considering zinc enzymes: a pocket for guest binding, a similar first coordination sphere, a coordination site available for water activation in the cis position relative to the substrate and finally an internal imidazole residue that plays the role of a general base.

Chemoselective guest-triggered shaping of a polynuclear CuII calix[6]complex into a molecular host

A new calix[6]arene scaffold bearing a tris-imidazole binding site at the small rim and three tetradentate aza ligands at the large rim was synthesized. The system binds three Cu^II ions at the large rim sites and is unable to bind a fourth one, which remains in solution. The charge repulsion between the complexes, together with the flexibility of the scaffold, disorganizes the small rim site for binding and prevents its use for host-guest studies. Although the presence of MeCN or DMF guests does not alter this state, the addition of a heptylamine guest, which further displays Brønsted basicity, restores its receptor ability by stabilizing the extra Cu^II ion at the tris-imidazole site with concomitant guest encapsulation and binding of an exo hydroxo ligand. This chemoselective nuclearity switch yields a tetranuclear complex in which the guest backbone is preorganized in front of three potentially reactive Cu(ii) complexes, reminiscent of polynuclear Cu^II enzyme active sites.

"Two-Story" Calix[6]arene-Based Zinc and Copper Complexes: Structure, Properties, and O2 Binding

A new "two-story" calix[6]arene-based ligand was synthesized, and its coordination chemistry was explored. It presents a tren cap connected to the calixarene small rim through three amido spacers. X-ray diffraction studies of its metal complexes revealed a six-coordinate Zn^II complex with all of the carbonyl groups of the amido arms bound and a five-coordinate Cu^II complex with only one amido arm bound. These dicationic complexes were poorly responsive toward exogenous neutral donors, but the amido arms were readily displaced by small anions or deprotonated with a base to give the corresponding monocationic complexes. Cyclic voltammetry in various solvents showed a reversible wave for the Cu^II/Cu^I couple at very negative potentials, denoting an electron-rich environment. The reversibility of the system was attributed to the amido arms, which can coordinate the metal center in both its +II and +I redox states. The reversibility was lost upon anion binding to Cu. Upon exposure of the Cu^I complex to O₂ at low temperature, a green species was obtained with a UV-vis signature typical of an end-on superoxide Cu^II complex. Such a species was proposed to be responsible for oxygen insertion reactions onto the ligand according to the unusual and selective four-electron oxidative pathway previously described with a "one-story" calix[6]tren ligand.

Mimicking the Regulation Step of Fe-Monooxygenases: Allosteric Modulation of FeIV -Oxo Formation by Guest Binding in a Dinuclear ZnII -FeII Calix[6]arene-Based Funnel Complex

A heteroditopic ligand associated with a calix[6]arene scaffold bearing a tris(imidazole) coordinating site at its small rim and an amine/pyridine ligand at its large rim has been prepared, and its regioselective coordination to Zn^II at the small rim and Fe^II in the amine/pyridine ligand has been achieved. The heterodinuclear complex obtained displays an overall cone conformation capped by the tris(imidazole)Zn^II moiety and bears a non-heme Fe^II complex at its base. Each of the metal centers exhibits one labile position, allowing the coordination inside the cavity of a guest alkylamine at Zn^II and the generation of reaction intermediates (Fe^III (OOH) and Fe^IV O) at the large rim. A dependence between the chain length of the encapsulated alkylamine and the distribution of Fe^III (OOH) intermediates and Fe^III (OMe) is observed. In addition, it is shown that the generation of the Fe^IV O intermediate is enhanced by addition of the alkylamine guest. Hence, this supramolecular system gathers the three levels of reactivity control encountered in oxidoreductases: i) control of the Fe^II redox properties through its first coordination sphere, allowing us to generate high valent reactive species; ii) control of guest binding through a hydrophobic funnel that drives its alkyl chain next to the reactive iron complex, thus mimicking the binding pocket of natural systems; iii) guest-modulated reactivity of the Fe^II center towards oxidants.

Triflate-functionalized calix[6]arenes as versatile building-blocks: application to the synthesis of an inherently chiral Zn(ii) complex

Cavity-based metal complexes can find many applications notably in the fields of catalysis and biomimicry. In this context, it was shown that metal complexes of calix[6]arenes bearing three aza-coordinating arms at the small rim provide excellent structural models of the poly-imidazole sites found in the active site of many metallo-enzymes. All these N-donor ligands were synthesized from the 1,3,5-tris-methoxy-p-tBu-calix[6]arene platform, which presents some limitations in terms of functionalization. Therefore, there is a need for the development of new calix[6]arene-based building-blocks selectively protected at the small rim. Herein we describe the regioselective one step synthesis of two calix[6]arenes decorated with triflate groups, i.e. X6H4Tf2 and X6H3Tf3, from the parent calix[6]arene X6H6. It is shown that the triflate groups can either act as protecting or deactivating groups, allowing the elaboration of sophisticated calixarene-based systems selectively functionalized at the large and/or at the small rim. In addition, X6H3Tf3 is functionalized on the A, B, and D rings and thus gives access to inherently chiral compounds, as demonstrated by the synthesis of a rare example of inherently chiral cavity-based metal complex.

Organic-inorganic supramolecular solid catalyst boosts organic reactions in water

Coordination polymers and metal-organic frameworks are appealing as synthetic hosts for mediating chemical reactions. Here we report the preparation of a mesoscopic metal-organic structure based on single-layer assembly of aluminium chains and organic alkylaryl spacers. The material markedly accelerates condensation reactions in water in the absence of acid or base catalyst, as well as organocatalytic Michael-type reactions that also show superior enantioselectivity when comparing with the host-free transformation. The mesoscopic phase of the solid allows for easy diffusion of products and the catalytic solid is recycled and reused. Saturation transfer difference and two-dimensional ¹H nuclear Overhauser effect NOESY NMR spectroscopy show that non-covalent interactions are operative in these host–guest systems that account for substrate activation. The mesoscopic character of the host, its hydrophobicity and chemical stability in water, launch this material as a highly attractive supramolecular catalyst to facilitate (asymmetric) transformations under more environmentally friendly conditions.

Coordination polymers and metal-organic frameworks are appealing synthetic hosts for mediating reactions. Here, the authors report a mesoscopic metal-organic structure that is shown to accelerate condensation reactions in water in the absence of acid or base, owing to the hydrophobic environment of the host.

Inherently chiral heterocyclic resorcinarenes using a Diels–Alder reaction

This paper presents a novel approach to highly diastereoselective synthesis of resorcinarenes having enlarged cavities.

The first water-soluble bowl complex: molecular recognition of acetate by the biomimetic tris(imidazole) Zn(ii) system at pH 7.4

The bowl-shaped cavity-ligand based on resorcinarene has been successfully transposed into a water-soluble version that readily binds Zn(ii) and acetate at physiological pH.

Primary amine recognition in water by a calix[6]aza-cryptand incorporated in dodecylphosphocholine micelles

A zinc calix[6]azacryptand complex was incorporated into dodecylphosphocholine micelles. This complex can strongly and selectively bind linear primary amines in an aqueous medium.

Synthesis of a cone-conformer bimodal calix[4]arene-crown-5 which forms a sensitive cesium ion sensing layer on gold-coated microcantilevers

A “bimodal” or upper- and lower-rim functionalized “calix-crown-5” reported herein was unexpectedly formed preferentially in a cone conformation and forms a SAM on Au.