Andrea Pappalardo

Self-Assembly Dynamics of Modular Homoditopic Bis-calix[5]arenes and Long-Chain α,ω-Alkanediyldiammonium Components

Homoditopic building blocks 1, featuring two pi-rich cone-like calix[5]arene moieties connected at their narrow rims by a rigid o-, m-, or p-xylyl spacer in a centrosymmetric divergent arrangement, show a remarkable tendency to spontaneously and reversibly self-assemble with the complementary homoditopic alpha,omega-alkanediyldiammonium dipicrate guest salts C8-C12 x 2Pic through iterative intermolecular inclusion events, forming supramolecular assemblies whose composition and dynamics strongly depend upon the length of the connector, the geometry of the spacer, as well as the concentration and/or molar ratios between the two components. (1)H NMR spectroscopy and ESI-MS studies of 1/C(n) x 2Pic modular homoditopic pairs support the formation of discrete (bis)-endo-cavity assemblies with the shorter C8 and C9 connectors, and/or (poly)capsular assemblies with the longer C10-C12 components under appropriate concentrations and molar ratios (50 mM equimolar solutions). (1)H NMR titration experiments and diffusion NMR studies provide clear evidence for the self-assembly dynamics of the complementary pairs here investigated.

Anion-assisted supramolecular polymerization: from achiral AB-type monomers to chiral assemblies.

Control over the self-assembly process of monomeric species by functional group modulation is highly desirable in the context of supramolecular polymer design. These materials, unlike covalently linked polymers, consist of monomeric arrays held together by reversible and highly directional noncovalent bonds. Owing to the dynamic and reversible nature of noncovalent interactions, supramolecular polymers display unique topologies and unconventional properties (such as stimuli responsiveness and self-healing) and are thus becoming cutting-edge species in modern materials science. Multiple hydrogen bonds, metal–ligand coordination, and p–p stacking are, by far, the most common weak forces used for engineering supramolecular polymers. Recently, however, oligomeric and polymeric architectures based on host–guest inclusion complexes have started to become more and more popular. Within this research frame, we have recently described a pH-responsive aminododecyloxy-calix[5]arene derivative (C5-NH2) that, upon exposure to a variety of acids, selfassembles into linear oligomers. Protonation activates the two latent self-complementary binding sites of this heteroditopic monomer precursor (i.e. a preorganized cone-shaped p-rich calix[5]arene cavity and a linear alkylamine pendant chain) and, according to a well-established host–guest recognition pattern, which involves a concerted set of weak interactions (NH···O, CH–p, cation–p), supramolecular oligomer formation readily occurs. However, because of the intrinsically saline nature of the monomers used, the growth of these supramolecular assemblies was found to be aniondependent. More specifically, the looser the ion-pairing interactions between the ammonium monomer and its counterion, the higher the degree of polymerization observed. Although ion-pairing effects have been analyzed extensively in relation to simple one-to-one host–guest systems, to the best of our knowledge they have not yet been examined in the context of supramolecular polymers derived from charged monomers. Elegant examples of polymeric species derived from crown ethers, cryptands, cyclodextrins, cucurbiturils, calixarenes, 16] and resorcinarenes have been described, but in none of these instances—neither ABtype (self-complementary heteroditopic) 12a,b, 13a, 17a] nor AA/BB-type (complementary homoditopic) 15, 17b] systems—has the role of the counterion in the growth of the polymer or the tuning of the supramolecular properties been addressed. Drawing on our earlier investigations on the simultaneous complexation of cations and anions 18] and on the design of heteroditopic and heterotetratopic receptors in an attempt to override the drawback of ion-pairing effects in AB-type salt monomers, we have now incorporated an ancillary anion-binding site (namely a ureido moiety) into calix[5]arene C5-NH2 with the aim of facilitating salt dissociation and ultimately making polymer formation more efficient. In this communication we demonstrate that the addition of this anion-binding site to the monomer scaffold is beneficial to the supramolecular polymerization process and, most importantly, we show that modulation of the properties [*] Dr. C. Capici, Dr. G. Gattuso, Dr. A. Notti, Prof. M. F. Parisi Dipartimento di Chimica Organica e Biologica Universit di Messina Viale F. Stagno d’Alcontres 31, 98166 Messina (Italy) E-mail: ggattuso@unime.it mparisi@unime.it

Threading the Calix[5]arene Annulus

Among the fast-expanding collection of molecular systems available for nanoscale applications, mechanically interlocked molecules, such as rotaxanes and catenanes, have already moved towards becoming technological realities. Rotaxanes (and their pseudorotaxane precursors), in particular, have been constructed by using the most diverse macrocyclic receptors as wheel components. Crown ethers, cyclodextrins, cyclic amides, and more recently cucurbiturils have all been threaded onto suitable complementary linear axle components. Calix[n]arenes, on the other hand, have received much less attention as building blocks for the construction of interlocked supermolecules, despite their tunable size, their versatility of derivatization (both at the wide and narrow rims), and their ready availability. The only notable exceptions are the findings of Arduini, Pochini, and co-workers, who have extensively investigated pseudorotaxanes and rotaxanes based on heterotopic calix[6]arene receptors adorned with ureido groups and viologen-derived linear components. Leaving aside calix[4]arenes, the cavity of which is too small to be threaded by a linear guest, the slightly larger calix[5]arenes are the next potential candidates for pseudorotaxane formation. Although calix[5]arenes have previously been shown to efficiently perform a number of tasks, which range from the complexation of alkyl(di)ammonium ions and ion pairs to the self-assembly of supramolecular polymers, to the best of our knowledge, no studies have so far been carried out to evaluate whether or not a linear thread can interpenetrate the calix[5]arene annulus. Herein we describe the first examples of [2]pseudorotaxanes derived from a calix[5]arene and linear secondary alkylammonium ions and we show that the ease of formation of these species is predominantly determined by salt ion-pairing effects, whereas the time course of the threading/dethreading process depends on the length of the cation alkylammonium chains. Very recently we have reported the solid-state structure of a calix[5]arene/n-butylammonium endo-cavity complex. Inspection of this structure revealed that the spatial arrangement of the oxygen atoms around the nitrogen atom of the included guest is reminiscent of the oxygen array present in crown ether/secondary ammonium ion complexes. Even though the cavity size of a calix[5]arene (at its narrow rim) appears to be slightly smaller than that found in the solidstate structure of the dibenzo[24]crown-8 encircling the dibutylammonium ion (ca. 5 vs. 6 , respectively), calix[5]arenes were judged to be sufficiently flexible to allow for the inclusion of secondary alkylammonium cations. After a preliminary screening, penta-tert-butylpentakis(tert-butoxycarbonylmethoxy)calix[5]arene (1) was selected as the prototype wheel component, whereas di-n-butylammonium (2·H) and di-n-hexylammonium (3·H) were chosen as axle components. The axle components were all tested as chloride, picrate (Pic ), and hexafluorophosphate salts to evaluate the influence of ion pairing on the pseudorotaxane assembly process.

Recognition of Achiral and Chiral Ammonium Salts by Neutral Ditopic Receptors Based on Chiral Salen-UO2 Macrocycles

A mononuclear (M20) and a dinuclear (M40) uranyl chiral macrocyclic complex, incorporating both a salen unit containing two phenyl rings linked to a chiral diimine bridge and the (R)-BINOL unit, behaves as an efficient ditopic receptor for achiral and chiral quaternary ammonium salts. Binding affinities in chloroform solution have been measured for 1:1 complexes of many quaternary salts encompassing tetramethylammonium (TMA), tetraethylammonium (TEA), tetrabutylammonium (TBA), and acetylcholine (ACh), as well as trimethylanilinium (TriMAn), benzyltrimethylammonium (BnTriMA), (alpha-methylbenzyl)trimethylammonium and pyrrolidinium cations. The anion of the salt is bound by the hard Lewis acidic uranyl site, with an increasing binding efficiency on increasing the anion hardness (I(-) < Br(-) < Cl(-)), whereas CH-pi or pi-pi attractions by binapthyl moiety, or the salicylaldehyde unit, or the phenyl rings of diimine bridge ensure the recognition of the cation partner. Optimized structures of receptor-anion-cation ternary complexes obtained by MM calculations are supported by 2D-ROESY NMR measurements.

Pair of Diastereomeric Uranyl Salen Cavitands Displaying Opposite Enantiodiscrimination of α‑Amino Acid Ammonium Salts

A pair of diastereomeric salen cavitands and their uranyl complexes combine a chiral (R,R) salen bridge and an inherent chiral tris-bridged quinoxaline cup within the same molecule. Whereas the free ligands show a preference for the same enantiomer of an α-amino acid pair, the corresponding UO(2) complexes display opposite enantiodiscrimination and exceptionally high enantioselectivities (K(D)/K(L) = 26.4).

Applications of supramolecular capsules derived from resorcin[4]arenes, calix[n]arenes and metallo-ligands: from biology to catalysis

Supramolecular architectures developed after the initial studies of Cram, Lehn and Pedersen have become structurally complex but fascinating. In this context, supramolecular capsules based on resorcin[4]arenes, calix[n]arenes or metal–ligand structures are dynamic assemblies inspired by biological systems. The reversible formation of these assemblies, combined with the possibility to modify their dimensions and shapes in the presence of a guest (concepts of reversibility and adaptivity) make them similar to biological macromolecules, such as proteins and enzymes. The small space inside a supramolecular capsule is characterized by different properties compared to the bulk solution. This review describes concrete applications of capsular supramolecular self-assemblies in the biomedical field, in catalysis and in material science.

A Kinetic Study of the Thermal and Oxidative Degradations of a New Poly(arylene)ether Copolymer

The degradation of a new thermoplastic poly(arylene)ether copolymer was carried out in both dynamic and isothermal heating conditions, under nitrogen flow and in a static air atmosphere. The measurements showed that the copolymer degraded through two stages in both investigated environments with the formation of a stable residue in N2 and complete mass loss in air. The apparent activation energy values associated with the degradation processes were evaluated. The obtained results suggested different degradation mechanisms between N2and air. Results were discussed and compared with those obtained for several polymers previously investigated.

Sequence, Stoichiometry, and Dimensionality Control in Porphyrin/Bis‐calix[4]arene Self‐Assemblies in Aqueous Solution

The use of a water-soluble octacationic bis-calix[4]arene with divergent cavities (BC(4)) as a templating agent for the assembly of a tetraanionic porphyrin (CuTPPS) has allowed the noncovalent synthesis of 2D or 3D multiporphyrin assemblies. Self-assembly of CuTPPS and BC(4) molecules proceeded under hierarchical control in a stepwise fashion to yield discrete and isolable supramolecular nanostructures containing up to 33 molecular elements (i.e., the CuTPPS/BC(4) 17:16 assembly, obtained in less than three hours). The formation of these species could be conveniently monitored by means of UV/Vis spectroscopy by following the absorbance of the Soret band at 412 nm. In particular, the attainment of the pivotal CuTPPS/BC(4) 5:4 species with a cruciform structure, as the key fork-point intermediate for the subsequent formation of the higher 2D and 3D assemblies, has been demonstrated by light-scattering studies and by an unequivocal synthesis of mixed-porphyrin/calixarene 5:4 species involving the use of two different types of metallated porphyrins, namely CuTPPS and MnTPPS. The remarkable stability of these assemblies permits a stepwise synthesis that makes it possible to choose the desired porphyrin sequence.

Sensing of linear alkylammonium ions by a 5-pyrenoylamido-calix(5)arene solution and monolayer using luminescence measurements†

Covalent chemisorption of a p-chloromethylphenyltrichlorosilane monolayer on silica substrates was achieved. A cone 5-pyrenoylamido-calix[5]arene bearing a 12-aminododecyl moiety at the lower rim was further covalently bonded, producing a 5-pyrenoylamido-calix[5]arene-based monolayer on silica. The surface chemical characterization of this hybrid material was carried out by X-ray photoelectron spectroscopy. The sensing properties of this pyrenoylamido-calix[5]arene system were probed by both NMR and luminescence measurements in solution. The optical recognition properties of the functional monolayer were similarly studied at room temperature by emission measurements. This system demonstrates to have significant recognition properties for the linear alkylammonium ions. As a result, n-dodecylammonium ions can be detected at ppm levels. The adopted synthetic procedure provided evidence to be useful in transferring molecular properties to a solid state device.

Complexation of biologically active amines by a water-soluble calix[5]arene

Host–guest complexes of an amphiphilic water-soluble p-tert-butylcalix[5]arene bearing 4-sulphonatobutoxy groups at the narrow rim with trace amines (2-phenylethylamine and tyramine) and a neurotransmitter (dopamine), originally investigated via 1H NMR, have been re-examined via ITC in order to double check the reliability of the values obtained through a van’t Hoff analysis of the NMR data. The calorimetrically determined data confirm the existence of a 1:1 host–guest species; however, there are inconsistencies between the van’t Hoff derived values and the values determined via direct calorimetry. These discrepancies do not result from proton displacement due to inclusion, but rather result from the temperature dependence of the van’t Hoff enthalpies.