Antoni Costa

Squaramido-based receptors: Molecular recognition of carboxylate anions in highly competitive media

Abstract A new series of artificial receptors for binding carboxylates has been developed. The receptors are based on the hydrogen bonding abilities of secondary squaramides as binding subunits. The data reveal the formation of 1:1 complexes in highly competitive media. Using convenient spacers, receptors for di- and tri-carboxylates that incorporate two and three squaramide subunits have been prepared. The complexes are characterized by nmr and, when possible, by FAB spectral analysis.

Squaramide-based reagent for selective chromogenic sensing of Cu(II) through a zwitterion radical.

A minimalist squaramide-based chemodosimeter for Cu(2+) is described. Upon selective chelation to 2, Cu(2+) induces the formation of a highly colored zwitterionic radical, which is kinetically stable for hours. The presence of a radical is confirmed by EPR and ESI-MS. It is then possible to use reagent 2 for visual and selective sensing of Cu(2+) at neutral pH.

Lithium diffusion in single-walled carbon nanotubes: a theoretical study

Abstract Theoretical investigations on single-walled carbon nanotubes (SWCNT) have been carried out using ab initio and molecular interaction potential with and without polarization (MIPp and MIP, respectively) methodologies to explore the possibilities of Li+ ion insertion through the side-wall of the nanotube. Li-nanotube systems can improve the capacity of lithium batteries by using both nanotube exteriors and interiors. Ab inito calculations of the fully optimized nanotubes were used to examine the topological defects depending on the ring size. The movement of the lithium cation through 8, 9 and 10-membered rings is studied and discussed.

Squaramide as a binding unit in molecular recognition

Abstract Urea and squaramide are used as building blocks of powerful hosts which are involved in the molecular recognition of quaternary ammonium guests. Complexes of urea and squaramide with ammonium cations were studied using both high level MP2/6-311+G** ab initio and molecular interaction potential with polarization and MEP calculations, in order to explore the physical nature of the interactions. The same procedure was applied to ammonium complexes with water and benzene as prototype models of crown ethers and calixarenes. Our findings reveal that either urea or squaramide are better binding units for the recognition of ammonium cations than water or benzene.

Anion-pi interactions in four-membered rings.

In this study, the importance of anion-pi interactions in four-membered rings has been evidenced. Two kinds of rings have been found to be suitable for participation in anion-pi interactions: first, in different salts of cyclobuten-1,2-dione derivatives and, second, in eta(4)-cyclobutadiene complexes with transition metals.

MP2 study of cooperative effects between cation??, anion?? and ??? interactions

In this manuscript we report high level ab initio calculations (RI-MP2(full)/6-31++G**) and experimental evidence that demonstrate that important non-additivity effects between three noteworthy non-covalent interactions that involve aromatic rings (benzene and hexafluorobenzene), i.e. cation–π, anion–π and π–π interactions, occur when the interactions coexist in the same complex. In some cases we have observed very high non-additivity energies (up to −20.3 kcal mol−1) that indicate that the cation–π and/or the anion–π have a strong influence on the π–π interaction and vice versa.

Thermodynamic characterization of the squaramide–carboxylate interaction in squaramide receptors

Abstract The thermodynamic characterization of squaramides is described. This fundamental information shows that the association in chloroform or DMSO is mainly exothermic. In contrast, in MeOH the equilibrium is endothermic and is entropically driven. The data shows the influence of a squaramide ring alone, modified or combined with tetraalkylammonium groups as the binding subunit for molecular recognition.

AN EFFECTIVE FLUORESCENT SENSOR FOR CHOLINE-CONTAINING PHOSPHOLIPIDS

A set of weak CH small middle dot small middle dot small middle dotO interactions is responsible for the recognition of tetraalkylammonium ions (ball-and-stick structure) by the tripodal receptors depicted in the diagram, in which the binding pocket is lined with squaramide units. The association constants of the fluorescent receptors are remarkably high at about 10(5) M(-1) and can be used for signaling the presence of selected choline phospholipids. Z=RNH, where R=alkyl, arylamino; X(-)=BH(4)(-), Cl(-), Br(-), I(-).

Janus-like squaramide-based hosts: dual mode of binding and conformational transitions driven by ion-pair recognition.

New tripodal squaramide-based hosts have been synthesised and structurally characterised by spectroscopic methods. In 2.5 % (v/v) [D(6)]DMSO in CDCl(3), compound 4 formed dimeric assemblies [log K(dim)=3.68(8)] as demonstrated by (1)H NMR spectroscopy and UV dilution experiments. AFM and SEM analyses revealed the formation of a network of bundled fibres, which indicates a preferential mechanism for aggregation. These C(3)-symmetric tripodal hosts exhibited two different and mutually exclusive modes of binding, each one easily accessible by simultaneous reorientation of the squaramide groups. In the first, a convergent disposition of the NH squaramide protons allowed the formation of an array of N-H⋅⋅⋅X(-) hydrogen bonds with anions. In the second mode, reorientation of carbonyl squaramide groups allowed multiple C=O⋅⋅⋅H interactions with ammonium cations. The titration of 4 with different tetraalkylammonium iodides persistently showed the formation of 1:1 complexes, as well as 1:2 and 1:3 complexes. The corresponding stoichiometries and binding affinities of the complexes were evaluated by multi-regression analysis. The formation of high-order complexes, supported by ROESY, NOESY and mass spectrometry experiments, has been attributed to the insertion of NR(4)I ion pairs between the carbonyl and NH protons of the squaramide groups located in adjacent arms of 4. The observed effects reflect the induction of significant conformational changes in the hosts, mainly in relation to the relative orientation of the squaramide groups adapting their geometries to incoming ion-pair complementary substrates. The results presented herein identify and fully describe two different modes of ion-pair recognition aimed at directing conformational transitions in the host, therefore establishing a base for controlling more elaborate movements of molecular devices through ion-pair recognition.

Selective sensing of competitive anions by non-selective hosts: the case of sulfate and phosphate in water

Two sensing ensembles based on the common squaramide host 2 and the acid–base indicators Cresol Red (CR) and Bromocresol Green (BG) are described. Both couples were characterized in EtOH–H2O mixtures by UV-Vis spectroscopy. Analysis of the binding curves and the corresponding Job plots indicate 1 : 1 binding between the indicators CR or BG and host 2. The sensing solutions are responsive to sulfate and/or phosphate anions in water, changing the coloration of the solution. As in other indicator displacement assays (IDA), host 2 translates the chemical event into a colorimetric response that is analyzed by spectrophotometry. However, since other anions present in natural waters do not result in color changes, this method allows the quantification of sulfate and phosphate in water.