Interactions at the outside faces of calix

Halogen-bonded architectures of multivalent calix[4]arenes

A 2D halogen bonded network of calixarene macrocycles was obtained by co-crystallization of a 1,3-alternate calix[4]arene as a tetravalent XB donor with a bidentate XB acceptor.

Halogen versus Hydrogen Bonding in Binary Cocrystals: Novel Conformation a Coformer with [2+2] Photoreactivity of Criss-Crossed C=C Bonds

A series of cocrystals involving the hydrogen- and halogen-bond donor coformers catechol (cat) and 1,2-diiodotetrafluorobenzene (1,2-di-I-tFb), respectively, is reported. Each coformer forms a cocrystal with each of the three symmetric bipyridines trans-1,2-bis(n-pyridyl)ethylene (n,n'-bpe, where: n=n'=2, 3, 4). Four novel cocrystals (cat) ⋅ (3,3'-bpe), 2(1,2-di-I-tFb) ⋅ (2,2'-bpe), 2(1,2-di-I-tFb) ⋅ (3,3'-bpe), and (1,2-di-I-tFb) ⋅ (4,4'-bpe) comprise components that assemble by either O-H⋅⋅⋅N hydrogen bonds (cat) or N⋅⋅⋅I halogen bonds (1,2-di-I-tFb). In (cat) ⋅ (3,3'-bpe), cat acts as a template to support an intermolecular [2+2] photocycloaddition of 3,3'-bpe. The reactivity occurs via a one-dimensional (1D) hydrogen-bonded tape with stacked and criss-crossed olefins that react stereoselectively and quantitatively to form rctt-tetrakis(3-pyridyl)cyclobutane (3,3'-tpcb). The reactivity of the criss-crossed olefins is facilitated by a hitherto not reported cis-gauche conformation adopted by cat. The stereochemistry of 3,3'-tpcb is confirmed in the cocrystal 2(cat) ⋅ (3,3'-tpcb).

Comparison of isomericmeta- andpara-diiodotetrafluorobenzene as halogen bond donors in crystal engineering

The ability ofmeta- andpara-diiodotetrafluorobenzene to act as halogen bond donors in crystal engineering has been compared by the synthesis and crystal structure analysis of a family of 20 novel halogen-bonded cocrystals with simple monotopic and ditopic nitrogen-based acceptors.

Limitations and extensions of the lock-and-key principle: differences between gas state, solution and solid state structures

The lock-and-key concept is discussed with respect to necessary extensions. Formation of supramolecular complexes depends not only, and often not even primarily on an optimal geometric fit between host and guest. Induced fit and allosteric interactions have long been known as important modifications. Different binding mechanisms, the medium used and pH effects can exert a major influence on the affinity. Stereoelectronic effects due to lone pair orientation can lead to variation of binding constants by orders of magnitude. Hydrophobic interactions due to high-energy water inside cavities modify the mechanical lock-and-key picture. That optimal affinities are observed if the cavity is only partially filled by the ligand can be in conflict with the lock-and-key principle. In crystals other forces than those between host and guest often dominate, leading to differences between solid state and solution structures. This is exemplified in particular with calixarene complexes, which by X-ray analysis more often than other hosts show guest molecules outside their cavity. In view of this the particular problems with the identification of weak interactions in crystals is discussed.

Solid state halogen bonded networks vs. dynamic assemblies in solution: explaining N⋯X interactions of multivalent building blocks

In the solid state, pyridine functionalized resorcinarenes formed multidimensional networks with aryl halides. The solution behavior of these XB systems was analyzed by combining NMR spectroscopy and DFT computations.

[5,11,17,23-Tetra-tert-butyl-25,27-(3,6-dioxaoctan-1,8-di-oxy)-26,28-bis-(pyridin-2-ylmeth-oxy)calix[4]arene]sodium iodide-1,2,4,5-tetra-fluoro-3,6-diiodo-benzene-methanol (2/3/4)

The title compound, [Na(C62H76N2O6)]I·1.5C6F4I2·2CH3OH, is composed of five components: a calix[4]arene derivative (hereinafter C4), a sodium cation, an iodide anion, a 1,2,4,5-tetra-fluoro-3,6-diiodo-benzene (tFdIB) mol-ecule and a methanol mol-ecule in a 1:1:1:1.5:2 ratio. The complex shows several inter-esting features: (i) the polyoxygenated loop of C4 effectively chelates a sodium cation in the form of a distorted octahedron and separates it from the iodide counter-ion, the shortest Na(+)⋯I(-) distance being greater than 6.5 Å; (ii) the cavity of C4 is filled by a methanol mol-ecule; (iii) a second methanol mol-ecule is hydrogen-bonded to the N atom of a pyridinyl substituent pendant of C4 and halogen-bonded to the I atom of a tFdIB mol-ecule; (iv) the two I atoms of another tFdIB mol-ecule are halogen-bonded to two iodide anions, which act as monodentate halogen-bond acceptorss; (v) one of the two tFdIB molecules is located about a centre of inversion.

Interactions in supramolecular complexes involving arenes: experimental studies

The process of learning by doing has fueled supramolecular chemistry and, more specifically, the understanding of noncovalent aromatic interactions in synthetic and natural systems. The preparation of new host molecules and the investigation of their complexations have produced many insights into significant noncovalent binding mechanisms. In this Account, we attempt to discuss significant binding contributions involving aromatic units and their practical applications. We use typical examples from our group and the literature, but this Account is not a comprehensive view of the field. Other than systems with saturated frameworks, host compounds based on arenes offer better controlled conformations and active interactions with many guest molecules. Because of their fluorescent properties, larger aryl systems are particularly suitable for sensors. The noncovalent interactions observed with different supramolecular complexes can be compared and exploited for interactions with biopolymers such as nucleic acids. Complexes formed with cyclophanes have been a constant source of inspiration for understanding noncovalent forces and their use for the design of functional supramolecular systems. Other than cyclodextrins or ionophores, which occur in nature, arene-based macrocycles are synthetic and provide more opportunities for structural variations than other macrocycles. These derivatives allow researchers to study and to exploit an unusually broad variety of binding mechanisms in both aqueous and organic media. Systematic analyses of complexes with different substituents and structures in solution, based also on flat aromatic systems such as porphyrins, can lead to a consistent picture of the noncovalent forces that dominate in these systems. These studies have elucidated attractive interactions between many heteroatoms and π systems including cyclopropanes . Through systematic analysis of the equilibrium measurements one can derive binding free energy increments for different interactions. The increments are usually additive and provide predictive tools for the design of new supramolecular systems, benchmarks for computational approaches, and an aid for drug design. In aqueous media, the major noncovalent forces between different aryl systems or between arenes and heteroatoms of larger polarizibility are dispersive, and hydrophobic forces play a minor role. In several examples, we show that electrostatic forces also contribute significantly if donor and acceptor groups show complimentarity. In early investigations, researchers found cation-π and, to a lesser degree, anion-π interactions with several cyclophanes in systems where the host or the guest molecules bear charges in an orientation that facilitates contact between charged and aryl portions of the molecules. In supramolecular complexes, hydrogen bonding effects are usually only visible in apolar media, but very strong acceptors such as phenolate anions can also work in water. To facilitate potential applications, researchers have primarily developed water-soluble, arene-containing receptors through the implementation of permanent charges. Supramolecular complexes that mimic enzymes can also rely on aryl interactions. Examples in this Account illustrate that the conformation of host-guest complexes may differ significantly between the solid and solution state, and suitable spectroscopic methods are needed to observe and control these conformations.

Binding mechanisms in supramolecular complexes

Supramolecular chemistry has expanded dramatically in recent years both in terms of potential applications and in its relevance to analogous biological systems. The formation and function of supramolecular complexes occur through a multiplicity of often difficult to differentiate noncovalent forces. The aim of this Review is to describe the crucial interaction mechanisms in context, and thus classify the entire subject. In most cases, organic host-guest complexes have been selected as examples, but biologically relevant problems are also considered. An understanding and quantification of intermolecular interactions is of importance both for the rational planning of new supramolecular systems, including intelligent materials, as well as for developing new biologically active agents.

Highly Interpenetrated Supramolecular Networks Supported by N⋅⋅⋅I Halogen Bonding

Halogen bonding (XB) has been used to assemble tetrakis(4-pyridyl)pentaerythritol (tetradentate XB acceptor) with different alpha,omega-diiodoperfluoroalkanes (bidentate XB donors) or tetrakis(4-iodotetrafluorophenyl)pentaerythritol (tetradentate XB donor). The remarkable linearity of the XB formed, the rodlike character of alpha,omega-diiodoperfluoroalkanes and the mutual complementarities of pentaerythritol partners, translate the three-dimensional character of the XB acceptor into open primary networks, which interpenetrate to avoid the presence of voids and to ensure segregation of the modules. Two-dimensional (2D) square 4(4) layers (sql) with fourfold and fivefold interpenetration, as well as an eightfold diamondoid network (dia) of class Ia and a remarkable tenfold dia network of class IIIa, have been obtained.

Tunable N-substitution in Zwitterionic Benzoquinonemonoimine Derivatives: Metal Coordination, Tandemlike Synthesis of Zwitterionic Metal Complexes, and Supramolecular Structures

Full details on a very efficient transamination reaction for the synthesis of zwitterionic N,N-dialkyl-2-amino-5-alcoholate-1,4-benzoquinonemonoiminium derivatives [C6H2(=NHR)2(=O)2] 5-16 are reported. The molecular structures of zwitterions 5 (R=CH3) in 5.H2O, 13 (R=CH2CH2OMe), 15 (R=CH2CH2NMe2), and of the parent, unsubstituted system [C6H2(=NH2)2(=O)2] 4 in 4.H2O have been established by single-crystal X-ray diffraction. This one-pot preparation can be carried out in water, MeOH, or EtOH and allows access to new zwitterions with N-substituents bearing functionalities such as -OMe (13), -OH (9-12), NR1R2 with R1 = or not equal R2 (14-16) or an alkene (8), leading to a rich coordination chemistry and allowing fine-tuning of the supramolecular arrangements in the solid state. As previously described for 15, which reacted with Zn(acac)2 to afford the octahedral Zn(II) complex [Zn[C6H2(NCH2CH2NMe2)O(O)(NHCH2CH2NMe2)]2] (20), ligands 13 and 16 with coordinating "arms" afforded with Zn(acac)2 the 2:1 adducts [Zn[C6H2(NCH2CH2X)O(=O)(NHCH2CH2NX)]2] 19 (X=OMe) and 21 (X=NHEt), with N2O4 and N4O2 donor sets around the octahedral Zn(II) center, respectively. Furthermore, zwitterions 15 and 16 reacted with ZnCl2 to give the stable, crystallographically characterized Zn(II) zwitterionic complexes [ZnCl2[C6H2(NCH2CH2NR1R2)O(=O)(NHCH2CH2NHR1R2)]] 22 (R1=R2=Me) and 23 (R1=Et, R2=H) by means of an unprecedented, tandemlike synthesis in which 1) the two pendant amino groups of the organic benzoquinonemonoimine zwitterionic precursor favor metal coordination and proton transfer and 2) the saturated linker prevents pi-conjugation between the charges. The nature of the structural arrangements in the solid state for both inorganic (20, 22, 23) and organic (5, 9, 13, and 15) molecules is determined by subtle variations in the nature of the N-substituent on the zwitterion precursor.

Exhaustively Methylated Azacalix[4]arene: Preparation, Conformation, and Crystal Structure with Exclusively CH/π-Controlled Crystal Architecture

[structure: see text]. Described are the preparation, conformation, and crystal structure of exhaustively methylated azacalix[4]arene involving nitrogen atoms as bridging units. NMR and X-ray crystallographic analysis have demonstrated that this novel azacalix[4]arene adopts a 1,3-alternate conformation both in solution and in the solid state. The crystal structure has been characterized solely by intermolecular CH/pi interactions, by which the azacalix[4]arenes mutually interact with each other outside the cavity to furnish a two-dimensional network structure.

Halogen Bonding and π···π Stacking Control Reactivity in the Solid State

The halogen bonding and the pi...pi stacking interactions induce the noncovalent self-assembly of modules into photoreactive supramolecular architecture. The pi...pi interaction pre-organizes the template, and the halogen bonding aligns the olefins to conform to the topochemical principle for photoreaction. The UV irradiation of the crystal resulted in a cyclization product with quantitative yield and stereospecificity.

Synthesis and ESI-MS Alkali Metal Ion Binding Selectivities of Cone, Partial Cone, and 1,3-Alternate 1,3-Bis(α-picolyloxy)-p-tert-butylcalix[4]arene Crown-6 and 1,1'-Binaphthalene-2,2'-diyl Crown-6 Conformers

The syntheses of 1,3-bis(α-picolyloxy)-p-tert-butylcalix[4]arene crown-6 and 1,1'-binaphthalene-2,2'-diyl crown-6 title conformers have been achieved by two complementary synthetic strategies, which differ in the order in which the polyether loop and the pendant picolyl groups are introduced. The structure and conformation of all new compounds have been firmly established by NMR spectroscopy, and further proven by X-ray analysis for the intermediate p-tert-butyl-25,27-(1,1'-binaphthalene-2,2'-diyl-crown-6)-26,28-dihydroxycalix[4]arene. Within each set of conformers, the nature of the polyether chain has little or no influence on the overall conformation of the calixarene platform. The alkali metal ion binding selectivities of the two series of calixarenes have been evaluated in competitive complexation experiments by electrospray ionization mass spectrometry. In the p-tert-butylcalix[4]arene crown-6 series, partial cone and 1,3-alternate conformers show a peak selectivity for the larger Cs+ ions, while the cone one preferentially binds the smaller Na+ ions. On the other hand, the cone and 1,3-alternate binaphthyl-containing analogues show a preference for Na+ ions, the partial cone being quite unselective.

Through-space interactions between face-to-face, center-to-edge oriented arenes: importance of polar-pi effects

Two series of conformationally restricted polycyclic compounds (1-3 and 4-7) have been synthesized as model systems for studying the through-space interactions between face-to-face, center-to-edge (parallel-offset) oriented arenes. These compounds feature different X substituents on one of the interacting rings. By monitoring the variation of the delta Gz for the rotation around the aryl-aryl bond in 1-7 as a function of X by 2D [1H,1H] EXSY NMR spectroscopy, it was found that the barriers increase on passing from electron-donating to electron-withdrawing substituted derivatives. Quantum mechanical calculations [MP2/DVZ (2d,p)//B3LYP/DVZ(2d,p)] gave barrier values and variations in agreement with the experimental data. The results are consistent with a repulsive arene-arene interaction dominated by electrostatic effects.