Synthesis of “Two-Story” Calix[6]aza-Cryptands

The first four members of a new family of C3v-symmetrical “two-story” calix[6]aza-cryptands have been synthesized. These large funnel shaped aza-ligands are formed through introduction of three aromatic arms as spacers onto the small rim of a calix[6]arene and subsequently capped with the tripodal aza caps tacn [1,3,5-triazacyclononane] or tren [tris(aminoethyl)amine]. A key feature for an efficient final 1:1 macrocyclization appears to be an adequate geometrical fit between the extended calixarene scaffold and the aza caps

Turning on anion and betaine hosting by a small structural change of a biomimetic cavity: a case study

This study explores the impact of a small structural modification on a biomimetic receptor. The hosting structure is a calix[6]arene capped by a tetraaza core. Two receptors are compared: one (1) presents three anisole units while the other (2) has three phenols. The latter was obtained with an excellent yield by selective demethylation thanks to a supramolecular strategy. Complexation studies showed that [2.H]+ displays strong affinities for Cl−, NO3−, HSO4−, whereas [1.H]+ is non-responsive. The anions are embedded at the level of the small rim, hydrogen-bonded to the protonated cap and the phenol groups. Ternary complexes are obtained in the presence of ammoniums. Finally, [2.H]+ shows high affinity for small zwitterions presenting a carboxylate and an ammonium groups separated by one or two carbon atom(s), not three, due to multi-point recognition. These results open routes to the design of new receptors for a variety of anionic and zwitterionic guests.</p

Coordination of Lead(II) in the Supramolecular Environment Provided by a “Two-Story” Calix[6]arene-based N₆ Ligand

First insights into the coordination properties and host–guest behavior of a “two story” calix[6]­aza-cryptand (<b>1</b>) are described. The ligand is constituted of a triazacyclononane (TACN) cap and three pyridine (PY) spacers connected to the calix[6]­arene small rim. The resulting N₆ donor site coordinates Pb^II ions to give complexes that are highly stable. X-ray diffraction structures reveal a hemidirected environment for Pb^II with strong coordination to the TACN cap and weaker bonds with the three PY residues. A guest molecule, either water or EtOH, sitting in the calixarene macrocycle and hydrogen-bonded to the phenoxyl units at the level of the small rim further stabilizes the complexes through electrostatic interactions with the metal center and the calixarene core. In-depth ¹H NMR studies confirm the host–guest behavior of the complexes in solution, with evidence of embedment of neutral guest molecules such as EtOH, BuOH, and <i>N</i>-Me-formamide. Hence, in spite of the presence of a N₆ donor, the calixarene macrocycle can be open to guest interaction, giving rise to seven-coordinate dicationic complexes. Noteworthy also is the flexibility of the macrocycle that allows Pb^II to adopt its preferred hemidirected environment in spite of the three covalent links connecting the calixarene core to the three PY groups. The flexibility of the system is further illustrated by the possible coordination of an exogenous anionic ligand in the exo position. Hence, compared to the previously described “one story” calix[6]­aza-cryptands, ligand <b>1</b> displays several similar but also new features that are discussed

Coordination of Lead(II) in the Supramolecular Environment Provided by a “Two-Story” Calix[6]arene-based N₆ Ligand

First insights into the coordination properties and host–guest behavior of a “two story” calix[6]­aza-cryptand (<b>1</b>) are described. The ligand is constituted of a triazacyclononane (TACN) cap and three pyridine (PY) spacers connected to the calix[6]­arene small rim. The resulting N₆ donor site coordinates Pb^II ions to give complexes that are highly stable. X-ray diffraction structures reveal a hemidirected environment for Pb^II with strong coordination to the TACN cap and weaker bonds with the three PY residues. A guest molecule, either water or EtOH, sitting in the calixarene macrocycle and hydrogen-bonded to the phenoxyl units at the level of the small rim further stabilizes the complexes through electrostatic interactions with the metal center and the calixarene core. In-depth ¹H NMR studies confirm the host–guest behavior of the complexes in solution, with evidence of embedment of neutral guest molecules such as EtOH, BuOH, and <i>N</i>-Me-formamide. Hence, in spite of the presence of a N₆ donor, the calixarene macrocycle can be open to guest interaction, giving rise to seven-coordinate dicationic complexes. Noteworthy also is the flexibility of the macrocycle that allows Pb^II to adopt its preferred hemidirected environment in spite of the three covalent links connecting the calixarene core to the three PY groups. The flexibility of the system is further illustrated by the possible coordination of an exogenous anionic ligand in the exo position. Hence, compared to the previously described “one story” calix[6]­aza-cryptands, ligand <b>1</b> displays several similar but also new features that are discussed

Replacement of a Nitrogen by a Phosphorus Donor in Biomimetic Copper Complexes: a Surprising and Informative Case Study with Calix[6]arene-Based Cryptands

The aim of the paper is to characterize Cu complexes in the PArN3 environment provided by ligands derived from triphenylphosphine P(C6H4CH2NHR)3 and compare their coordination behavior and reactivity with those obtained with all-nitrogen ligands such as tren. It is shown that coordination of the PN3 ligand (R = iPr) to Cu(I) and Cu(II) leads to complexes whose coordination sphere is hardly controlled as they readily undergo decoordination of either one N or the P donor together with oxidation of the latter. In strong contrast, when grafted on the small rim of a calix[6]arene, the PArN3 is geometrically constrained into a tripod that enforces the metal center to remain in the same environment with a P−Cu bond for both oxidation states. These calix[6]PN3-based Cu(I) and Cu(II) complexes react readily with exogenous ligands, making a comparison with calix[6]tren-based copper complexes possible. Indeed, reactivity studies in solution highlight very different behaviors. The complex [Cu(calix[6]PN3)]2+ shows an unusual affinity for weak σ-donors (e.g., MeCN > EtOH), while the analogous cuprous complex, [Cu(calix[6]PN3)]+, displays a surprising affinity for hard O-donor ligands (EtOH, DMF), which has never been observed for the tren analogues. Even more surprising is the lack of reactivity of [Cu(calix[6]PN3)]+ toward dioxygen, which contrasts strongly with the high reactivity of the [Cu(calix[6]tren)]+ complex. In an attempt to explain the observed differences in binding properties and reactivity, Density Functional Theory calculations and electronic spectra simulations were undertaken. They suggest that coordination of the soft P(Ar)3 center allows to tune the metal ion properties, either by absorbing excess electron density from Cu(I), or by increasing the electronic density of Cu(II). This is due to the simultaneous presence of the phosphorus atom (σ-donor) in apical position and the aromatic groups (π-acceptors) bound to the P-atom

Immobilization of Monolayers Incorporating Cu Funnel Complexes onto Gold Electrodes. Application to the Selective Electrochemical Recognition of Primary Alkylamines in Water

The immobilization of a copper calix[6]­aza­cryptand funnel complex on gold-modified electrodes is reported. Two different methodologies are described. One is based on alkyne-terminated thiol self-assembled monolayers. The other relies on the electrografting of a calix[4]­arene platform bearing diazonium functionalities at its large rim and carboxylic functions at its small rim, which is post-functionalized with alkyne moieties. In both cases, the CuAAC electroclick methodology proved to be the method of choice for grafting the calix[6]­aza­cryptand onto the monolayers. The surface-immobilized complex was fully characterized by surface spectroscopies and electrochemistry in organic and aqueous solvents. The Cu complex displays a well-defined quasi-reversible system in cyclic voltammetry associated with the Cu­(II)/Cu­(I) redox process. Remarkably, this redox process triggers a powerful selective detection of primary alkylamines in water at a micromolar level, based on a cavitary recognition process