Influence of H-bond competitors on the solvent-dependent structures of an octaurea-calix[4]tube.

A novel octaurea-calix[4]tube (UC4T) has been synthesized in three steps from the original Beer's p-tert-butylcalix[4]tube ionophore. In a polar solvent (DMSO-d6), UC4T rapidly interconverts between two identical conformations with C2v symmetry for the two calix[4]arene subunits. However, in a less polar solvent mixture (CDCl3/DMSO-d6, 98 : 2), UC4T adopts a highly distorted asymmetric structure, which hinders the formation of typical tetraurea calix[4]arene capsular assemblies. The complexation of potassium (or barium) cations inside the dioxyethylene ionophoric binding site of UC4T triggers a C2v to C4v symmetry rearrangement of the two calix[4]arene subunits. This rearrangement leads to the formation of a transient capsular dimeric species observed in solution upon the addition of KI or BaCl2·2H2O to a solution (CDCl3/DMSO-d6, 98 : 2) of the macrocycle. X-ray studies confirm UC4T's ability to adopt different asymmetric conformations, depending on its interactions with solvent molecules. Two distinct crystal forms (α and ß) of UC4T have been obtained, each displaying divergent calix[4]arene subunits with pinched-cone conformations. These conformations exhibit distinctive head-to-tail (α) or head-to-head/tail-to-tail (ß) orientations of the ureido groups, which are involved in hydrogen bonding with solvent molecules. Notably, the pseudo-capsular 1D supramolecular polymeric arrays observed in the ß form of UC4T assemble to create large parallel solvent channels.

Probing the determinants of porosity in protein frameworks: co-crystals of cytochrome c and an octa-anionic calix[4]arene.

Sulfonato-calix[n]arenes (sclxn) are promising tools to generate crystalline protein frameworks. We report, for the first time, a lower rim functionalised octa-anionic calix[4]arene (sclx4mc) in complex with proteins. Two crystal structures of sclx4mc bound to yeast or horse heart cytochrome c (cytc) are described. Highly porous honeycomb or tubular assemblies were obtained with yeast or horse cytc, respectively. Related frameworks were obtained previously with sclx8 and sclx6 but not with sclx4, suggesting that the ligand charge is a determining factor.

Probing the Structural Determinants of Amino Acid Recognition: X-Ray Studies of Crystalline Ditopic Host-Guest Complexes of the Positively Charged Amino Acids, Arg, Lys, and His with a Cavitand Molecule.

Crystallization of tetraphosphonate cavitand Tiiii[H, CH3, CH3] in the presence of positively charged amino acids, namely arginine, lysine, or histidine, afforded host-guest complex structures. The X-ray structure determination revealed that in all three structures, the fully protonated form of the amino acid is ditopically complexed by two tetraphosphonate cavitand molecules. Guanidinium, ammonium, and imidazolium cationic groups of the amino acid side chain are hosted in the cavity of a phosphonate receptor, and are held in place by specific hydrogen bonding interactions with the P=O groups of the cavitand molecule. In all three structures, the positively charged α-ammonium groups form H-bonds with the P=O groups, and with a water molecule hosted in the cavity of a second tetraphosphonate molecule. Furthermore, water-assisted dimerization was observed for the cavitand/histidine ditopic complex. In this 4:2 supramolecular complex, a bridged water molecule is held by two carboxylic acid groups of the dimerized amino acid. The structural information obtained on the geometrical constrains necessary for the possible encapsulation of the amino acids are important for the rational design of devices for analytical and medical applications.

Recognition and optical sensing of amines by a quartz-bound 7-chloro-4-quinolylazopillar[5]arene monolayer.

Covalent bonding of 7-chloro-4-quinolylazo-octamethoxypillar[5]arene molecules to silylated quartz substrates readily produced a new chromogenic reusable pillararene-coated quartz slide, for the direct UV detection of "transparent" analytes in solution. This device provides an analyte-selective optical response towards linear (di)amines with a highly reproducible optical read-out.

Selective Binding of Spherical and Linear Anions by Tetraphenyl(thio)urea-Based Dihomooxacalix[4]arene Receptors.

Three novel tetra(thio)ureido dihomooxacalix[4]arene anion receptors (phenylurea 4a, phenylthiourea 4b, and tert-butylurea 4c) were synthesized and obtained in the cone conformation in solution, as shown by NMR studies. The X-ray crystal structure of 4c is reported. The host-guest properties of these receptors toward several anions were investigated by 1H NMR titrations. Phenylurea 4a displayed a very efficient binding toward the spherical F- and Cl- anions, and the linear CN- (log Kass = 3.46, 3.50, and 4.02, respectively). In comparison to related bidentate phenylurea dihomooxacalix[4]arenes, tetraphenylurea 4a is more preorganized and the higher number of hydrogen bond donor sites provides a remarkable enhancement of its binding efficiency.

Three-Dimensional Network Structures Based on Pyridyl-Calix[4]Arene Metal Complexes.

A new series of supramolecular assemblies is obtained through the interaction of 3-pyridylmethyl-calixarenes and copper or zinc ions. The complexes formed are characterized both in solution and in the solid state. The results offer appealing insights into how the steric crowding of the lower rim and different coordination modes of the metal centers have a great impact on the overall architecture of the complexes with the formation of monomeric, dimeric, or oligomeric/polymeric species.

The Origin of Selectivity in the Complexation of N-Methyl Amino Acids by Tetraphosphonate Cavitands.

We report on the eligibility of tetraphosphonate resorcinarene cavitands for the molecular recognition of amino acids. We determined the crystal structure of 13 complexes of the tetraphosphonate cavitand Tiiii[H, CH3, CH3] with amino acids. (1)H NMR and (31)P NMR experiments and ITC analysis were performed to probe the binding between cavitand Tiiii[C3H7, CH3, C2H5] or the water-soluble counterpart Tiiii[C3H6Py(+)Cl(-), CH3, C2H5] and a selection of representative amino acids. The reported studies and results allowed us (i) to highlight the noncovalent interactions involved in the binding event in each case; (ii) to investigate the ability of tetraphosphonate cavitand receptors to discriminate between the different amino acids; (iii) to calculate the Ka values of the different complexes formed and evaluate the thermodynamic parameters of the complexation process, dissecting the entropic and enthalpic contributions; and (iv) to determine the solvent influence on the complexation selectivity. By moving from methanol to water, the complexation changed from entropy driven to entropy opposed, leading to a drop of almost three orders in the magnitude of the Ka. However, this reduction in binding affinity is associated with a dramatic increase in selectivity, since in aqueous solutions only N-methylated amino acids are effectively recognized. The thermodynamic profile of the binding does not change in PBS solution. The pivotal role played by cation-π interactions is demonstrated by the linear correlation found between the log Ka in methanol solution and the depth of (+)N-CH3 cavity inclusion in the molecular structures. These findings are relevant for the potential use of phosphonate cavitands as synthetic receptors for the detection of epigenetic modifications of histones in physiological media.

Improved Synthesis of Larger Resorcinarenes.

The synthesis of the larger resorcin[5 and 6]arene macrocycles [5](OMe) and [6](OMe) has been realized by a Lewis acid-catalyzed condensation of 1,3-dimethoxy-2-methylbenzene with paraformaldehyde in o-dichlorobenzene as the solvent. The methoxy-resorcin[5 and 6]arenes were quantitatively demethylated by treatment with BBr3 to obtain the corresponding macrocycles with free OH groups. X-ray studies showed that in the solid state both the conformation and the packing of [6](OMe) and [5](OMe) are driven by C-H···O, C-H···π, and π···π interactions.

Triptycene-Roofed Quinoxaline Cavitands for the Supramolecular Detection of BTEX in Air.

Two novel triptycene quinoxaline cavitands (DiTriptyQxCav and MonoTriptyQxCav) have been designed, synthesized, and applied in the supramolecular detection of benzene, toluene, ethylbenzene, and xylenes (BTEX) in air. The complexation properties of the two cavitands towards aromatics in the solid state are strengthened by the presence of the triptycene moieties at the upper rim of the tetraquinoxaline walls, promoting the confinement of the aromatic hydrocarbons within the cavity. The two cavitands were used as fiber coatings for solid-phase microextraction (SPME) BTEX monitoring in air. The best performances in terms of enrichment factors, selectivity, and LOD (limit of detection) values were obtained by using the DiTriptyQxCav coating. The corresponding SPME fiber was successfully tested under real urban monitoring conditions, outperforming the commercial divinylbenzene-Carboxen-polydimethylsiloxane (DVB-CAR-PDMS) fiber in BTEX adsorption.

Polyoxomolybdate-Calix[4]arene Hybrid: A Catalyst for Sulfoxidation Reactions with Hydrogen Peroxide.

An easily accessible polyoxomolybdate-calix[4]arene hybrid 1 has been synthesized and applied as a heterogeneous catalyst in the sulfoxidation of thioethers to sulfoxides and to sulfones under strictly stoichiometric amounts of 30% H2O2 in CH3CN as the solvent. This study represents the first promising example of successful employment of calixarenes-polyoxometalate (POM) hybrid materials in the area of catalytic oxidations.

A general exit strategy of monoheme cytochromes c and c2 in electron transfer complexes?

Using our previously reported maps of the electrostatic surface of horse heart ferri- and ferro-cyt c, comparisons were made between the complementary electrostatic surfaces of three cyt c peroxidase-cyt c complexes and the photosynthetic reaction center-cyt c complex, considering both iron oxidation states. The results obtained were consistent with a sliding mechanism for the electron shuttle on the surface of the protein complexes, promoted by the change in iron oxidation state. This mechanism was found to be in agreement with theoretical and NMR studies reported in the literature. Importantly, the analysis also provided a rationale for recognition of nonproductive associations. As we have previously reported the same conclusion on examination of redox partners of cyt c in the mitochondrial respiratory pathway, our hypothesis is that the proposed mechanism could represent a general exit strategy of monoheme cyts c and c2 in electron transfer complexes.

Probing the inner space of salt-bridged calix[5]arene capsules.

A combined DOSY and XRD study indicates that a carboxylcalix[5]arene receptor is able to encapsulate α,ω-diamines of appropriate length by means of a proton-transfer-mediated recognition process followed by salt-bridge-assisted bis-endo-complexation.

Novel chiral (salen)Mn(III) complexes containing a calix[4]arene unit in 1,3-alternate conformation as catalysts for enantioselective epoxidation reactions of (Z)-aryl alkenes.

Two new chiral calix[4]arene-salen ligands 1a,b, based on calix[4]arene platforms in 1,3-alternate conformation, have been prepared by a new general synthetic pathway. Their Mn(III) complexes, 3a,b have shown fairly good efficiency in the asymmetric epoxidation of styrene and substituted styrenes, whereas excellent catalytic activity and selectivity were observed with rigid bicyclic alkenes, namely 1,2-dihydro-naphthalene and substituted 2,2'-dimethyl-chromene. The higher catalytic properties of 3a may be ascribed to the more rigid and inherently chiral structure as proved by molecular modelling, NMR spectroscopy and X-ray data of the similarly structured UO2 complexes 2a,b.

Trans and cis effects of axial fluoroalkyl ligands in vitamin B12 analogues: relationship between alkyl- and fluoroalkyl-cobalamins.

CF2HCbl, CF3Cbl , and CF3CH2Cbl have been synthesized and characterized in solution by (1)H NMR and UV-vis spectroscopy, and their X-ray crystal structures have been determined using synchrotron radiation. The structure of CF3CH2Cbl is reported for the first time, whereas those of CF2HCbl and CF3Cbl are re-examined to obtain more precise structural data. Comparison of the structural data obtained with the alkylcobalamin analogues, MeCbl and EtCbl, indicates that the Co-C and Co-NB3 bond lengths are shorter in the fluoroalkylcobalamins. The structural data of the fluoroalkylcobalamins previously reported in the literature had been conflicting in this regard. Thus, a much less dramatic shortening of the two axial bonds was found for CF3Cbl, whereas in the case of CF2HCbl, the Co-NB3 bond length is shorter than in MeCbl. Direct comparison of the structures of CF3CH2Cbl and EtCbl indicates a large distortion of the axial fragment in the former case that can be attributed to steric effects. A number of previously reported correlations of the effect of the ß-ligand on the structure and properties of cobalamins are re-examined in light of the present results. Particular emphasis is placed on the axial fragment. This analysis substantially confirms and, with the new data reported here, adjusts and expands the data set for correlations between trans and cis influences of the ß-ligand of cobalamins and their structure (Co-X and Co-NB3 distances and corrin fold angle) and properties (UV-vis spectra, NMR spectra, and pK(base-off)).

Selective amine recognition driven by host-guest proton transfer and salt bridge formation.

The stepwise synthesis of ionizable p-tert-butylcalix[5]arenes 1a·H and 1b·H, featuring a fixed cone cavity endowed with a carboxyl moiety at the narrow rim, is described. Single-crystal X-ray analyses have shown that in the solid state 1a·H and 1b·H adopt a cone-out conformation with the carboxylic OH group pointing in, toward the bottom of the aromatic cavity, as a result of a three- or two-center hydrogen-bonding pattern between the carboxyl group and the phenolic oxygen atom(s). The affinity of amines for calix[5]arene derivatives 1a·H and 1b·H was probed by (1)H NMR spectroscopy and single-crystal X-ray diffraction studies. These carboxylcalix[5]arenes are shown to selectively recognize linear primary amines--over branched, secondary, and tertiary amines--by a two-step process involving a proton transfer from the carboxyl to the amino group to provide the corresponding alkylammonium ion, followed by binding of the latter inside the cavity of the ionized calixarene. Proton transfer occurs only with linear primary amines, that is, when the best size and shape fit between host and substrate is achieved, while the other amines remain in their noncompeting unprotonated form. The role of the solvent in the ionization/complexation process is discussed. Structural studies on the n-BuNH(2) complexes with 1a·H and 1b·H provide evidence that binding of the in situ formed n-BuNH(3)(+) substrate to the cavity of the ionized macrocycle is ultimately secured, in the case of 1a·H, by the formation of an unprecedented salt-bridge interaction.

3-[2-(1H-1,3-Benzodiazol-2-yl)eth-yl]-1,3-oxazolidin-2-one.

In the title compound, C(12)H(13)N(3)O(2), the dihedral angle between the oxazolone ring and the benzimidazole unit is 45.0 (5)°, exhibiting a staggered conformation at the Cα-Cß bond. In the crystal, a strong N-H⋯N hydrogen bond links the mol-ecules into a C(4) chain along the c axis while a C-H⋯O hydrogen-bonding inter-action generates a C(5) chain along the a axis, i.e. perpendicular to the other chain.

(S)-2-[(2-Hy-droxy-benz-yl)aza-nium-yl]-4-(methyl-sulfan-yl)butano-ate.

The zwitterionic title compound, C(12)H(17)NO(3)S, is a reduced Schiff base derived from (S)-N-(2-hy-droxy-benzyl-idene)methio-nine. An intra-molecular inter-action between the N-H and carboxyl-ate groups forms a roughly planar (r.m.s. deviation = 0.1405 Å) five-membered ring containing the H(N), N, Cα, C(carboxyl-ate) and O atoms in a penta-gonal conformation. In the crystal, a supra-molecular triangle-shaped motif is generated by mol-ecules held together by O-H⋯O and N-H⋯O hydrogen bonds.

Chlorido(η-cyclo-octa-1,5-diene)(N,N'-diethyl-thio-urea-κS)rhodium(I).

In the title rhodium(I) complex, [RhCl(C(8)H(12))(C(5)H(12)N(2)S)], N,N'-diethyl-thio-urea acts as a monodenate S-donor ligand. The rhodium(I) coordination sphere is completed by the Cl atom and the COD [= 1,5-cyclo-octa-diene] ligand inter-acting through the π-electrons of the double bonds. If the midpoints of these two bonds are taken into account, the Rh atom exhibits a distorted square-planar coordination. The syn conformation of the N,N'-diethyl-thio-urea ligand with respect to the Cl atom is stabilized by an intra-molecular N-H⋯Cl hydrogen bond. A weak inter-molecular N-H⋯Cl inter-action links mol-ecules along the a axis.

2,5-Dicarboxyanilinium chloride monohydrate.

The title compound, C8H8NO4+.Cl-.H2O, is the chlorohydrated form of 2-aminobenzene-1,4-dicarboxylic acid, the basic crystal structure of which is still not known. Molecules are linked by classical N-H...O, O-H...O, N-H...Cl and O-H...Cl hydrogen bonds, mainly along the molecular plane, into sheets built by unusual R6(4)(26), R6(4)(22) and R4(3)(22) rings. The stacking between layers is stabilized by another N-H...Cl hydrogen bond and by pi-pi interactions between aromatic rings facing each other.