Cyclic Hexamer Research Articles

Cyclic tetranuclear and hexanuclear palladium(II) complexes and their host-guest chemistry.

Cyclic tetrameric complexes have been prepared by the reaction of Pd(en)Cl2 or Pd(dapol)Cl2, or their nitrato analogues, with Na2(5'GMP) in aqueous solution, where en=1,2-diaminoethane, dapol=1,3-diamino-2-propanol, and 5'GMP=guanosine 5'-monophosphate. Addition of certain small molecules containing hydrophobic groups resulted in the expansion of the tetramer to a cyclic hexamer with strong bonding of one guest per hexameric host. At pH 5-6, the guest molecule can be a cation, anion, or neutral, and those species containing trimetylsilyl and t-butyl groups bonded the most strongly. The size of the central cavity of the [Pd(en)(5'GMP)]6 host has been estimated to be 5.2 A. Formation of the host-guest complex caused a large upfield shift (Deltadelta) of 2.5-2.9 ppm in the 1H NMR spectrum of the most highly affected guest protons, which were those in closest proximity to the guanine nucleobases. NOESY spectra were used to determine the interaction sites between the host and the guest. Apparent association constants determined at 26 degrees C and pD 5.4 for the [Pd(en)(5'GMP)]6-DSS and [Pd(en)(5'GMP)]6-t-butanol systems, where DSS is 3-(trimethylsilyl)-1-propanesulfonate anion, were 1.36+/-0.11x10(4) and 2.74+/-0.95x10(4) M(-3/2), respectively. The Pd(dapol)-5'GMP system forms hexameric host-guest complexes, similar in nature to those of the Pd(en)-5'GMP system. The molecular and crystal structures of Pd(dapol)Cl2 are also reported.

Ice nanoclusters at hydrophobic metal surfaces

Studies of the structure of supported water clusters provide a means for obtaining a rigorous molecular-scale description of the initial stages of heterogeneous ice nucleation: a process of importance to fields as diverse as atmospheric chemistry, astrophysics and biology. Here, we report the observation and characterization of metal-supported water hexamers and a family of hydrated nanoclusters--heptamers, octamers and nonamers--through a combination of low-temperature scanning tunnelling microscopy experiments and first-principles electronic-structure calculations. Aside from achieving unprecedented resolution of the cyclic water hexamer--the so-called smallest piece of ice--we identify and explain a hitherto unknown competition between the ability of water molecules to simultaneously bond to a substrate and to accept hydrogen bonds. This competition also rationalizes previous structure predictions for water clusters on other substrates.

Extended Tapes of Cyclic Water Hexamers in an Unusual Oxalate-Bridged One-Dimensional Copper(II) Complex

Extended tapes of cyclic water hexamers are formed in a one-dimensional copper(II) complex bridged by oxalate with a rare mono, bidentate coordination mode. The water tapes are stabilized by the hydrogen bond interactions between uncoordinated oxalate oxygen atoms and water molecules. Magnetic analysis reveals weak ferromagnetic interactions mediated by the oxalate bridge with the exchange parameter J = 0.36 cm-1.

A planar cyclic hexamer of water molecules in dichlorido(2,4,6-tri-2-pyridyl-1,3,5-triazine)cadmium(II) trihydrate

In the title compound, [CdCl2(C(18)H(12)N(6))].3H2O, the Cd atom has a distorted square-pyramidal coordination geometry. The solvent water molecules are hydrogen bonded to each other to form planar cyclic water hexamers, which, together with other hydrogen bonds, interlink the Cd complex molecules to give one-dimensional supramolecular ribbons that extend along the [111] direction. The chains are assembled into two-dimensional layers parallel to (111) by pi-pi stacking interactions. Furthermore, interlayer pi-pi stacking interactions and weak C-H...Cl hydrogen bonds complete the formation of a three-dimensional framework.

Conformational study of Co(II), Ni(II), and Cr(III) complexes of the edta-type: Crystal structure of 1D polymeric trans(O 6)-Ba[Co(1,3-pddadp)] · 8H 2O complex stabilized by infinite water tapes

The trans(O 6) isomer of the Ba[Co(1,3-pddadp)] · 8H 2O complex (where 1,3-pddadp represents hexadentate 1,3-propanediamine- N, N′-diacetate- N, N′-di-3-propionate ion) has been prepared and characterized by X-ray crystallography. In the crystal structure the complex cations and anions are bridged by carboxylate oxygen atoms from the in-plane coordinated glycinate rings (G-rings) of [Co(1,3-pddadp)] 2− and by the barium-coordinated water molecules, thus forming 1D polymeric chains, separated by infinite water tapes hydrogen bonded to the [Co(1,3-pddadp)] 2− carboxylate oxygens from the out-of-plane β-alaninate rings (R-rings). Conformational analysis of the three possible geometrical isomers: trans(O 5), trans(O 5O 6), and trans(O 6) of the [Co(1,3-pddadp)] 2− complex, with ligand acting as hexadentate, as well as of the corresponding complexes of Ni(II) and Cr(III) has been performed using the consistent force field (CFF) method, with the parameters developed previously for edta-type complexes of chromium(III) and supplemented with new parameters for cobalt(II) and nickel(II). The energy-minimized structure of the trans(O 5O 6) isomer represents the global minimum for the [M(1,3-pddadp)] n− (M = Co(II), Ni(II), and Cr(III)) species. The occurrence of the least energetically favored trans(O 6) isomer in a crystal and the exceptional conformation of the axially oriented β-alaninate rings can be accounted for by the stabilizing role of the infinite tapes of planar cyclic water pentamers and hexamers which act as a “glue” to reinforce the coordination polymeric chains.

Novel sodium alginate composite membranes prepared by incorporating cobalt(III) complex particles used in pervaporation separation of water–acetic acid mixtures at different temperatures

The present paper is our continuing effort to develop a new type of sodium alginate (NaAlg) composite membrane by incorporating cobalt(III)(3-acetylpyridine-o-aminobenzoylhydrazone) (Co-APABZ) complex as filler particles in different ratios. Membranes were prepared by solution casting followed by solvent evaporation and crosslinked with glutaraldehyde. Pervaporation (PV) performance of the prepared composite membranes was assessed in terms of flux and selectivity and these data were compared with the pristine NaAlg membrane in PV dehydration of water–acetic acid mixtures. Pristine Co-APABZ particles in crystalline form were prepared and characterized by the solid state X-ray diffraction (XRD) technique, while the NaAlg/Co(III)composite membranes were characterized by thermogravimetry (TGA) and dynamic mechanical thermal analyzer (DMTA). X-ray crystal structure of Co-APABZ has shown that the complex formed was crystalline in nature with six lattice water molecules, which are interconnected by hydrogen bonds linking together to form cyclic hexamers that are analogous to cyclohexane, creating water channels for an easy transport of water molecules. TGA indicated no changes in thermal stability of the membranes due to the presence of Co-APABZ in the NaAlg matrix. DMTA confirmed NaAlg crosslinking with glutaraldehyde. Effects of Co-APABZ content, membrane thickness, temperature and feed water compositions on membrane performance were investigated to find an optimum PV performance of the membranes developed. NaAlg composite membranes in the presence of Co-APABZ particles preferentially absorbed water molecules to facilitate diffusion of water through the membranes and thus enhance the selectivity to water. However, the amount of Co-APABZ present in the NaAlg matrix and the degree of membrane swelling has an effect on membrane performance. Selectivity of 174 for water with a flux of 0.123 kg/m2 h was obtained for 5 wt.% Co-APABZ containing NaAlg matrix, when tested for the feed mixture containing 10 wt % water. The present results are superior to the previously published data based on NaAlg membranes.

Cyclic Peptoids

Foldamers are an intriguing family of biomimetic oligomers that exhibit a propensity to adopt stable secondary structures. N-Substituted glycine oligomers, or "peptoids", are a prototypical example of these foldamer systems and are known to form a helix resembling that of polyproline type I. Ongoing studies seek to improve the stability of peptoid folding and to discover new secondary structure motifs. Here, we report that peptoids undergo highly efficient head-to-tail macrocyclization reactions. A diverse array of peptoid sequences from pentamers to 20mers were converted to macrocyclic products within 5 min at room temperature. The introduction of the covalent constraint enhances conformational ordering, allowing for the crystallization of a cyclic peptoid hexamer and octamer. We present the first X-ray crystallographic structures of peptoid hetero-oligomers, revealing that peptoid macrocycles can form a reverse-turn conformation.

Crystallized Water: Internal and External Ice Fragments in Polycyclic Hosts

Two crystal structures of cyclic water hexamers are reported, one with a bicyclic cryptand and the other with a tricyclic host. In the bicycle structure, the water molecules lie between the cryptands. In the tricycle structure, the hexameric water cluster is almost totally encapsulated within the host framework.

Poisson's ratio of simple planar ‘isotropic’ solids in two dimensions

AbstractThe influence of molecular geometry on the Poisson's ratio has been investigated for a few two‐dimensional (2D) hard body systems (hard discs, hard dimers, hard cyclic trimers, and hard cyclic hexamers) forming elastically isotropic solid phases. The Poisson's ratio of the studied periodic and aperiodic solids was determined by Monte Carlo simulations using the analysis of the box fluctuations in the constant pressure ensemble (NpT) with variable box shape. The results obtained for all the studied systems showed that the Poisson's ratio grows when the density is decreased. It has been also found that the Poisson's ratio decreases with increasing number of discs in the molecule and its value for hard trimers is negative near close packing. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A well-resolved discrete dodecameric water cluster in a metal–organic complex

A well-resolved discrete water dodecameric cluster containing core cyclic chair-conformational hexamer has been observed within a metal–organic network built from the combination of 4,4′-bipyridine and 4,4′-dipyridine-N,N′-dioxide linkers.

Molecular assembly formation of cyclic hexa‐β‐peptide composed of acetylated glycosamino acids

A novel cyclic hexamer of acetylated beta-glycosamino acid was synthesized and its conformation and molecular assembly formation was investigated. Variable temperature NMR study indicated that the cyclic hexapeptide took a C(3) symmetric conformation at room temperature, but at elevated temperatures a C(6) symmetric one, which was not due to averaging of the C(3) symmetric conformation, appeared. Computational geometry optimization showed that the C(6) symmetric conformation was a highly planar structure with amide groups orienting perpendicular to the ring plane. The cyclic hexa-beta-peptide formed rod-shaped crystals from an N,N-dimethyl formamide solution at elevated temperature. The optical microscopy observation with a sensitive tint plate under cross-nicol configuration and electron diffraction analysis of the crystals revealed that the cyclic hexa-beta-peptides were stacked one after the other to form a regular nanotube structure.

A novel polypseudorotaxane composed of cyclic β-peptide as bead component

A polypseudorotaxane composed of cyclic hexamers of beta-glucosamino acid and poly(ethylene glycol) (PEG) was constructed, where the beads were stacked unidirectionally together via hydrogen bonds to form a peptide nanotube.

Organometallic molecular materials: self-assembly through hydrogen bonding of an organoplatinum network structure with zeolite-like topology

The complex [Pt(OH)(2)Me(2)(dpa)], dpa = di-2-pyridylamine, undergoes a remarkable form of self-assembly through hydrogen bonding in the solid state; there are cyclic tetramer units formed by head-to-head association between pairs of hydroxyl groups, and cyclic hexamer units formed by head-to-tail association between NH and OH groups; these building blocks then self-assemble to form a complex network structure containing large interconnected cavities, reminiscent of zeolites.

Synthesis of a Catechol-Based Poly(ether ether ketone) (“o-PEEK”) by Classical Step-Growth Polymerization and by Entropically Driven Ring-Opening Polymerization of Macrocyclic Oligomers

An amorphous, catechol-based analogue of PEEK (“o-PEEK”) has been prepared by a classical step-growth polymerization reaction between catechol and 4,4‘-difluorobenzophenone and shown to be readily soluble in a range of organic solvents. Copolymers with p-PEEK have been investigated, including an amorphous 50:50 composition and a semicrystalline though still organic-soluble material comprising 70% p-PEEK. o-PEEK has also been obtained by entropy-driven ring-opening polymerization of the macrocyclic oligomers (MCO's) formed by cyclo-condensation of catechol with 4,4‘-difluorobenzophenone under pseudo-high-dilution conditions. The principal products of this latter reaction were the cyclic dimer 3a (20 wt %), cyclic trimer 3b (16%) cyclic tetramer 3c (14%), cyclic pentamer 3d (13%) and cyclic hexamer 3e (12%). Macrocycles 3a−c were isolated as pure compounds by gradient column chromatography, and the structures of the cyclic dimer 3a and cyclic tetramer 3c were analyzed by single-crystal X-ray diffraction. A ...

Controllable assembly of a 3-D metal–organic supramolecular framework with the inclusion of a well-resolved 1-D water morph

Two mixed-ligand metal–organic complexes [Cd 2(adipato)(phen) 2(NO 3) 2(H 2O) 2](H 2O) 6 ( 1) and {[Cd(adipato)(phen)](H 2O) 3} n ( 2) have been synthesized upon the reaction of Cd(II) nitrate with 1,10-phenanthroline (phen) and adipate under similar conditions. For 1, the adipato-bridged dinuclear entities are extended into a 3-D supramolecular architecture via hydrogen bonds and π–π stacking. Interestingly, a well-resolved 1-D water morphology consisting of alternate cyclic water tetramer and hexamer is detected as the guests in each channel of this 3-D lattice. Whereas 2 represents a layered coordination polymer in which the Cd II centers are interlinked by adipate dianions via different coordination modes.

A Supramolecular Hexameric Ring from Alumazene and Methylsulfonate

A dealkylsilylation reaction of alumazene [2,6-(i-Pr)2C6H3NAlMe]3 (1) with trimethylsilyl methylsulfonate in a 1:2 molar ratio in toluene afforded a supramolecular cyclic hexamer {Me[2,6-(i-Pr)2C6H3NAl]3(O3SMe)2}6 (2) composed of six intact alumazene rings and possessing two types of bridging sulfonate groups. Changing the reagent molar ratio to 1:3 caused the third sulfonate to partially substitute the last alumazene methyl group in an eta2 fashion and introduced a disorder in the crystal lattice.

Hydrogen‐Bonded Self‐Assembly of Pyridinium and Anilinium Bisphenols Mediated by the Hydrogen Difluoride Ion: Towards Control of CH⋅⋅⋅π (Aromatic) Interactions

The role of C-H...pi and C-H...O interactions in HF(2)(-)-assisted self-assembly of nitrogen-containing bisphenols are investigated. Unusual control on the end-face and edge-face geometries of aromatic C-H...pi interactions by a water molecule in the 4-[(4-hydroxy-3,5-dimethylphenyl)methyl]pyridinium difluoride lattice is observed. The bis(4-hydroxy-3,5-dimethylphenyl)(4-ammoniophenyl)methane.HF(2)(-) self-assembles through the formation of hydrogen-bonded cyclic hexamers.

Kinetically and thermodynamically controlled synthesis of tetraoxa[1 4]metacyclophanes and hexaoxa[1 6]metacyclophanes

The nucleophilic aromatic substitution of 1,5-difluoro-2,4-dinitrobenzene with 2-propylresorcinol in Et 3N/CH 3CN produces a mixture of syn and anti conformers of the cyclic tetramer and the cyclic hexamer with a kinetically controlled product distribution. Moreover, the reaction in DMF was catalyzed by CsF to also produce a mixture of these cyclic oligomers. In this case, however, the C–O bond is cleaved by the fluoride ion and the cyclization reaction is reversible; therefore, in the presence of excess CsF, the thermodynamically favored product ( syn-isomer of cyclic tetramer) is obtained as the major product. The structures of the two conformational isomers of cyclic tetramers were determined by X-ray crystallography.

Supramolecular Aggregation of Hexameric Water Clusters into a 2D Water Polymer Containing (H2O)18 Holes

We report herein a novel infinite two-dimensional water morphology, found in a metallo-organic host, made up of discrete cyclic water hexamers and which contains large circular holes defined by 18 molecules of water.

Coexistence of Planar and Chair-Shaped Cyclic Water Hexamers in a Unique Cyclohexanehexacarboxylate-Bridged Metal−Organic Framework

The hydrothermal reaction of CoCl2·6H2O with cis,cis,cis,cis,cis-1,2,3,4,5,6-cyclohexane-hexacarboxylic acid (L) generates a unique three-dimensional metal−organic framework (MOF) of [Co3(L-6H)(H2O)6]n (1). X-ray crystallography reveals that each L-6H ligand changes its conformation to a trans,trans,trans,trans,trans form to connect nine Co(II) atoms, and each Co(II) atom is connected to three L-6H ligands, leading to a MOF structure with high symmetry of R3. The coexistence of planar and chair-shaped cyclic water hexamers is found in this MOF. Magnetic studies show the occurrence of the field-dependent magnetic transition.