New supramolecular organization for a glycoluril: chiral hydrogen-bonded ribbons

A series of substituted glycoluril molecules exhibits a substantial twist of the fused five-membered rings and assembles exclusively chiral hydrogen-bonded ribbons in the solid-state.     

Organic/inorganic supramolecular channel frameworks containing a photosensitive azobenzene molecule as an included guest

Two independent channel frameworks are occupied by photoactive E-azobenzene in an unprecedented one-dimensional zigzag motif and thus form virtual three-dimensional supramolecular host-guest networks.     

Scope and mechanism of the C-H bond activation reactivity within a supramolecular host by an iridium guest: a stepwise ion pair guest dissociation mechanism

A chiral self-assembled supramolecular M(4)L(6) assembly has been shown to be a suitable host for a series of reactive monocationic half-sandwich iridium guests 1, 3, and 4 that are capable of activating C-H bonds. Upon encapsulation, selective C-H bond activation of organic substrates occurs. Precise size and shape selectivity are observed in the C-H bond activation of aldehydes and ether substrates. The reactions exhibit significant kinetic diastereoselectivities. Thermodynamic studies have shown that the iridium starting materials and products are bound strongly by the host assembly. The encapsulation process is largely entropy-driven. Kinetic investigations with water-soluble phosphine traps and added salts have provided evidence for a unique stepwise mechanism of guest dissociation for [4 subset Ga(4)L(6)]. Iridium guest 4 first dissociates from the host cavity to form an ion pair with the host exterior. This species then fully dissociates from the host exterior into the bulk solution. Model ion pair intermediates were characterized directly with (1)H NMR NOESY techniques. The rate of iridium guest dissociation is slower than the rate observed for the C-H bond activation processes, indicating that the selective C-H bond activation reactivity occurs within the cavity of the supramolecular host.     

Symmetric 3,5-pyrazole and isoxazole heterocycles comprising a bent core unit: synthesis and mesomorphic characterisation

The synthesis and characterisation of some new liquid crystalline (LC) heteroaromatic compounds containing the five-membered pyrazole/isoxazole rings is reported. Some of the compounds exhibited enantiotropic LC properties. The transition temperatures and LC textures of the mesophases were determined using polarised optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray studies. The five-membered heterocyclic compounds with the pyrazole core exhibit smectic C (SmC) phase, whereas isoxazoles show SmC and nematic (N) phases.     

New bisphosphonium salt containing a 1,4-dihydroxynaphthalene moiety: molecular and supramolecular structure

Russian Chemical Bulletin - Bisphosphonium salt containing a 1,4-dihydroxynaphthyl-substituted moiety was synthesized in high yield by the reaction of 2-methyl-1,4-naphthoquinone with three...     

Dinuclear zinc(II) dithiocarbamate macrocycles: ditopic receptors for a variety of guest molecules

The synthesis of a series of dinuclear zinc(II) dithiocarbamate (dtc) macrocyclic receptors containing aryl spacer groups of different sizes is reported. As evidenced from 1H NMR titration investigations, these receptors have the ability to bind various neutral and anionic bidentate guests species, including 1,4-diazabicyclo[2.2.2]octane (DABCO), isonicotinate and terephthalate in a cooperative 1 : 1 intramolecular inclusion complex. Stability constant determinations reveal a correlation between the strength of complexation and complementary receptor cavity : guest molecule size. In particular, the X-ray structure of a 1 : 1 host-guest complex between a dinuclear zinc(II) dtc receptor and DABCO illustrates the cooperative nature in which the dinuclear receptor associates with the bidentate guest.     

Square‐grid coordination networks of (5,10,15,20‐tetra‐4‐pyridylporphyrinato)zinc(II) in its clathrate with two guest molecules of 1,2‐dichlorobenzene: supramolecular isomerism of the porphyrin self‐assembly

The title compound, (5,10,15,20‐tetra‐4‐pyridylporphyrinato)zinc(II) 1,2‐dichlorobenzene disolvate, [Zn(C₄₀H₂₄N₈)]·2C₆H₄Cl₂, contains a clathrate‐type structure. It is composed of two‐dimensional square‐grid coordination networks of the self‐assembled porphyrin moiety, which are stacked one on top of the other in a parallel manner. The interporphyrin cavities of the overlapping networks combine into channel voids accommodated by the dichlorobenzene solvent. Molecules of the porphyrin complex are located on crystallographic inversion centres. The observed two‐dimensional assembly mode of the porphyrin units represents a supramolecular isomer of the unique three‐dimensional coordination frameworks of the same porphyrin building block observed earlier. The significance of this study lies in the discovery of an additional supramolecular isomer of the rarely observed structures of metalloporphyrins self‐assembled directly into extended coordination polymers without the use of external ligand or metal ion auxiliaries.     

Templation of the excited-state chemistry of alpha-(n-alkyl) dibenzyl ketones: how guest packing within a nanoscale supramolecular capsule influences photochemistry

Excited-state behavior of eight alpha-alkyl dibenzyl ketones (alkyl = CH3 through n-C8H17) that are capable of undergoing type II and/or type I photoreactions has been explored in isotropic solution and within a water-soluble capsule. The study consisted of two parts: photochemistry that explored the excited-state chemistry and an NMR analysis that revealed the packing of each guest within the capsule. The NMR data (COSY, NOESY, and TOCSY experiments) revealed that ternary complexes between alpha-alkyl dibenzyl ketones and the capsule formed by two cavitands are kinetically stable, and the guests fall into three packing motifs modulated by the length of the alpha-alkyl chain. In essence, the host is acting as an external template to promote the formation of distinct guest conformers. The major products from all eight guests upon irradiation either in hexane or in buffer solution resulted from the well-known Norrish type I reaction. However, within the capsule the excited-state chemistry of the eight ketones was dependent on the alkyl chain length. The first group consisted of alpha-hexyl, alpha-heptyl, and alpha-octyl dibenzyl ketones that yielded large amounts of Norrish type II products within the host, while in solution the major products were from Norrish type I reaction. The second group consists of alpha-butyl and alpha-pentyl dibenzyl ketones that yield equimolar amounts of two rearranged starting ketones within the capsule (combined yield of ca 60%), while in solution no such products were formed. The third group consisted of alpha-methyl, alpha-ethyl, and alpha-propyl dibenzyl ketones that within the capsule yielded only one (not two) rearranged starting ketone in larger amounts (21-35%) while in solution no rearrangement product was obtained. Variation in the photochemistry of the guest within the capsule, with respect to the alpha-alkyl chain length of the guest, highlights the importance of how a small variation in supramolecular structure can influence the selectivity within a confined nanoscale reactor.     

Anion-anion assembly: a new class of anionic supramolecular polymer containing 3,4-dichloro-2,5-diamido-substituted pyrrole anion dimers

Two molecules containing two 2,5-diamido,3,4-dichloropyrrole units linked via 1,3- or 1,4-substituted benzene rings have been synthesized. The pyrrole groups in these compounds deprotonate in the presence of tetrabutylammonium fluoride and form pyrrole anion dimers via NH...N- hydrogen bonds. This dimerization process results in the formation of polyanionic chains linked via an unusual "orthogonal" hydrogen-bonding array.     

Non adiabatic vibrational resonances in molecules containing low barrier moieties: a classical dynamics study

 We discuss the classical dynamics of a CH-stretching and an OH-stretching vibration coupled to a hindered rotation around a CC (OC) bond of a CH₃ group or an OH group. Our model is based on a two-dimensional system, in which zero angular momentum is assumed. The model is further simplified by considering only kinetic coupling between the CH (OH) stretching and the hindered rotation. Through numerical calculations, a new set of states is found, which originates from n:1 resonances between the internal rotation frequency and the stretching frequency, n being associated to the order of symmetry (n = 3 and 6 for the cases investigated). We also present a perturbative approach based on the Lie series method, which provides insight into these nonadiabatic states. Received: 28 April 1999 / Accepted: 21 September 1999 / Published online: 15 December 1999     

A hydrogen bond between a calixarene (as host and its guest: Crystal and molecular structure of thep-t-butyldihomooxacalix[4]arene : Triethylamine (1 : 2) complex/clathrate

The structure of thep-t-butyldihomooxacalix[4]arene triethylamine (1 : 2 complex has been determined by X-ray crystallography. Crystals are monoclinic, space groupCc,a = 22.821(3),b = 15.257(3),c = 16.365(2) Å, = 97.44(1)°,V = 5650(1) ų,Z = 4,D _calc = 1.03 g cm^–3. One triethylamine (Et₃N) molecule has one of its ethyl groups pointing inside the cavity of a calixarene molecule; its nitrogen atom is directed exo to this calixarene molecule and is involved in a hydrogen bond with one of the hydroxy groups of a neighbouring calixarene molecule. This scheme leads to columns of alternating host and guest molecules. The other Et₃N molecule is in the interhost space. This feature allows us to define the title compound as a complex/clathrate hybrid.Supplementary data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82144 (8 pages).     

1-butyl-3-methylimidazolium 3,5-dinitro-1,2,4-triazolate: a novel ionic liquid containing a rigid, planar energetic anion

The novel ionic liquid, 1-butyl-3-methylimidazolium 3,5-dinitro-1,2,4-triazolate has been synthesized and exhibits an unexpectedly low melting point (35 degrees C) considering the size and shape of the rigid, planar anion; analogous tetraalkylammonium salts (methyl, ethyl and n-butyl) have also been prepared and the tetraethylammonium example was characterized by single crystal X-ray diffraction.     

Crystal and molecular structure of the (1 : 1) clathrate between a calix[4]arene containing onep-nitrophenol unit and toluene

p-(Methyl,tert-butyl, nitro,tert-butyl) calix[4]arene: toluene, C₃₇H₄₁NO₆. C₇H₈,M _r = 687.87, triclinic, ,a = 13.668(2),b = 12.187(2),c = 13.231(1) Å, = 106.78(8), = 77.88(1), = 114.00(1)°,V = 1916.8(8) ų,Z = 2,D _x = 1.19 g cm^–3, (CuK ) = 1.54178 Å, = 5.90 cm^–1,F(000) = 736,T = 293 K, finalR = 0.068 for 6309 observed reflections. This macrocycle, having different substituents at the positionspara to the hydroxyl groups, is the first one of its type to be studied. The general conformation of this calix[4]arene is compared to similar symmetrical ones. Thetert-butyl groups are not disordered as is usual and toluene is retained between the macrocycles. Two calixarene molecules are positioned to permit atert-butyl group of one to be inside the cavity of the second to establish CH₃- interactions.     

Conformational dimorphism of 1,1,3,3,5,5-hexachloro-1,3,5-trigermacyclohexane: solvent-induced crystallization of a metastable polymorph containing boat-shaped molecules

Two crystalline modifications of 1,1,3,3,5,5-hexachloro-1,3,5-trigermacyclohexane have been experimentally obtained as phase pure products and studied by single-crystal X-ray diffraction. The six-membered heterocycles adopt a chair conformation in the alpha-phase; this polymorph is accessible by crystallisation from solution and from the melt. In contrast, the beta-form is built up from boat-shaped molecules; it can exclusively be crystallised from n-hexane. At the molecular level, formation energies of the 1,1,3,3,5,5-hexachloro-1,3,5-trigermacyclohexane conformers have been compared by using molecular mechanics, semiempirical and ab-initio quantum mechanical calculations. Possible reasons for the selective formation of the alpha- or beta-phase in specific solvents have been considered. Formation of the metastable phase is suggested to occur via a hypothetical intermediate of composition [(GeCl2CH2)3].0.5C6H14. For such an in-silico solvate, a crystal structure of favourable lattice energy, closely related to the experimentally observed beta-modification, has been found through global energy minimisation. Elimination of the n-hexane molecules from this computer-generated solid and subsequent simulated annealing resulted in a crystal structure that corresponds to the experimentally observed beta-phase within the limits of the force field calculations. This scenario implies solvent directed crystallisation of a metastable polymorphic molecular crystal.