Cyclic Hexamer Research Articles

Molecular simulation studies of water physisorption in zeolites

We report a series of Grand Canonical Monte Carlo simulations of water adsorption in NaY and NaX faujasite, as well as in silicalite-1. Computed adsorption isotherms and heats of adsorption were in good agreement with the available experiments. The existence of cyclic water hexamers in NaX located in the 12-ring windows, recently disclosed by neutron diffraction experiments (Hunger et al., J. Phys. Chem. B, 2006, 110, 342-353) was reproduced in our simulations. Interestingly enough, such cyclic hexamer clusters were also observed in the case of NaY, in which no stabilizing cation is present in the 12-ring window. We also report cation redistribution upon water adsorption for sodium faujasite with varying cation contents (Si ratio Al ratio in the range 1.53-3). A simple and transferable forcefield was used, that enabled to reproduce the different aspects of water physisorption in stable zeolites. The high pressure water condensation in hydrophobic silicalite-1 was reproduced without any parameter readjustment. The method and forcefield used here should be useful for engineering oriented applications such as the prediction of multi-component mixture adsorptive separations in various stable zeolites. It allows to address the issue of the effect of the small amounts of water that are almost inevitably present in zeolite-based separation processes.

The effect of P-cyclohexyl groups on the coordination chemistry of phosphaguanidinates

The anion of P-dicyclohexylphosphaguanidine, Cy2PC{NiPr}{NHiPr}, forms a P,N-chelate at lithium and cobalt(II); bridging through the Nimine atom forms cyclic hexamers in the former complex.

Hydrothermal synthesis and crystal structure of a novel three-dimensional supramolecular network containing cyclic water hexamers: [Co(en) 3] 4[P 2Mo 5O 23] 2·9H 2O(en=ethylenediamine)

A new compound, [Co(en) 3] 4[P 2Mo 5O 23] 2·9H 2O, has been prepared under mild hydrothermal conditions and structurally characterized by elemental analyses, IR spectrum, and single-crystal X-ray diffraction. Yellow crystals crystallized in the triclinic space group P 1 ¯ with a=10.303(2) Å, b=11.871(2) Å, c=20.288(4) Å, α=99.46(3)°, β=96.39(3)°, γ=115.32(3)°, V=2165.9(8) Å 3, Z=1 and R=0.0566. The compound consists of diphosphopentamolybdate clusters, [Co(en) 3] 3+ complexes, and water molecules, which are held together to generate a three-dimensional supramolecular network via hydrogen-bonding interaction. Interestingly, the three-dimensional supramolecular network contains cyclic water hexamers, which play an important role in the formation of the crystal.

Adsorption Structures of Water in NaX Studied by DRIFT Spectroscopy and Neutron Powder Diffraction

Diffuse reflectance infrared Fourier transform spectroscopic (DRIFTS) measurements (4000-1500 cm(-1)) and the results of neutron powder diffraction have been combined to study the structure of adsorption complexes of water in a NaX zeolite at different water loadings (25, 48, 72, and 120 water molecules per unit cell, respectively). Sharp bands corresponding to non-hydrogen-bonded OH groups of water molecules and broad associate bands due to hydrogen-bonded molecules are observed in the DRIFT spectra. We observe a remarkable downshift of the high-frequency associate band in a narrow temperature interval when the water amount decreases from 120 to 72 molecules per unit cell, which could signify some kind of "phase transition" for the water inside the zeolite cavities. Neutron powder diffraction results show that water molecules are predominantly localized in or near the 12-ring windows. Water molecules with hydrogen-bonded and non-hydrogen-bonded OH groups were found, in agreement with the observation of sharp and broad bands in the DRIFT spectra. We find strong evidence for the formation of cyclic hexamers of water molecules localized in the 12-ring windows, which are further stabilized by hydrogen bonds to framework oxygen atoms.

Infinite Chains of Quasi‐Planar Hexameric Water Clusters Stabilized in a Metal‐Organic Framework Built from CoII and Pyrazine‐ 2,3,5,6‐tetracarboxylic Acid

AbstractPyrazine‐2,3,5,6‐tetracarboxylic acid (ptcH4) reacts with Co(NO3)2·6H2O in 1:2 molar ratio in aqueous pyridine (py) at room temperature to form a coordination polymer with the empirical formula, {[Co2(ptc)·(py)2·4H2O]·4H2O}n, (1). The structure consists of 2D sheets of metal‐organic frameworks and infinite chains of cyclic water hexamers that join these sheets to form an overall 3D structure. Thermogravimetry, X‐ray powder diffraction and X‐ray structural analysis have been used in addition to infrared spectroscopy and elemental analysis to characterize 1. Crystal data for 1: monoclinic space group P21, a = 7.153(4), b = 17.003(2), c = 11.060(5) Å, β = 107.207(5)°, V = 1284.9(11) Å3, Z = 2, R1 = 0.0340, wR2 = 0.0875, S = 1.028. This structure demonstrates a new mode of association of water molecules. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Vertex‐Sharing Water Tape Consisting of Cyclic Hexamers

AbstractA 1D water tape consisting of vertex‐sharing cyclic water hexamers has been observed, which represents a subunit of ice structure. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

A Second Polymorphic Form of Trimethylindium: Topology of Supramolecular Architectures of Group 13 Trimethyls

Crystallization of Me3In from hexane solution at ambient temperature allows for the isolation of a new phase that cannot be obtained by classical sublimation routes. The crystal structure analysis revealed that this new polymorph of Me3In crystallizes in the rhombohedral space group R3 h. The primary structural effect of the In-CH3‚‚‚In intermolecular interactions results in the formation of cyclic hexamers with 12-membered (InC)6 rings of an extended-chair conformation. The shortest intermolecular In‚‚‚C contact is 3.028(4) A with In‚‚‚H distances between 2.74 and 2.79 A. The hexamers are further linked by longer In-CH3‚‚‚In bridges into the 3D network. In addition, the tetragonal form of Me3In was redetermined at 100 K. The presented results demonstrate that the impact of both crystallization conditions and the character of intermolecular forces on clustering events is evident. Finally, the network topology of a family of the group 13 trimethyls is analyzed and correlated.

Environment-controlled switching between cyclic hexamer and helical conformations of a water chloride cluster: An old compound viewed in a new perspective

On extrusion of a water molecule from the nano-channels created by the packing of [ cis-α-(trien)CoCl 2]Cl · 3H 2O, an unprecedented dehydration transition occurs that cause the guest water–chloride hexameric clusters to rearrange into a helical modification. The reverse process is achieved by the introduction of an additional water molecule.

Reaction Pathways Leading to Arylene Ethynylene Macrocycles via Alkyne Metathesis

Mechanistic studies on the direct formation of arylene ethynylene macrocycles via alkyne metathesis catalyzed by a molybdenum complex are reported. Gel permeation chromatography (GPC) and matrix-assisted laser desorption ionization (MALDI) mass spectrometry on the products from metathesis of monomer 1 show the initial formation of linear oligomers and large macrocycles (n > 6), followed by their transformation into the thermodynamically most stable product distribution-mainly the cyclic hexamer. Variable temperature and scrambling experiments reveal the reversibility of macrocycle formation. Nearly identical product distributions are observed from the cross metathesis of hexacycle 2 with diphenylacetylene and from the metathesis of bis(phenylethynyl) substituted monomer 4, demonstrating that macrocycle formation is thermodynamically rather than kinetically controlled. The metathesis byproduct, 3-hexyne, is shown to inhibit the catalyst. It is suggested that the relative metathesis rates of dialkylalkynes versus diarylalkynes trap the catalyst in a nonproductive manifold, rendering it unavailable for the productive metathesis of aryl alkylalkyne substrates. This finding indicates that dialkyl-substituted alkyne byproducts should be avoided (or efficiently removed) if the metatheses of aryl substrates, especially those with electron-withdrawing groups, are to proceed to high conversion.

A New Conformation of a Water Hexamer Chain and its Reversible Formation/Removal in the Host Channel by Gas–Solid Reaction

A new type of cyclic water hexamer chain was found to occupy the host channel of a supramolecule, formulated as [Zn(bdc)en·3(H2O)] n (1) [H2bdc=1,3-benzenedicarboxylic acid; en=ethylenediamine] and characterized by X-ray crystallography. As each water molecule was only involved in hydrogen bonding, facile removal of the lattice water was anticipated. The hydrogen-bonded water chain was completely removed by heating 1 at 96°C, forming the dehydrated solid [Zn(bdc)en] n (2). Interestingly, complex 1 can be regenerated in water vapor by a gas–solid reaction.

Extended Water Tapes of Cyclic Hexamers Encapsulated in the Channels of a Metal Phosphonocarboxylate Network

Water tapes consisting of fused cyclic water hexamers were formed in channels of a three-dimensional supramolecular framework. The geometrical parameters of water tapes are quite similar to those in ice Ih. The dehydration and rehydration of the title complex are partly reversible.

Unprecedented self‐assembled cyclic hexamer of ferrocenyldimethylsilanol, [FcSiMe2OH]6 (Fc = (η5‐C5H5)Fe(η5‐C5H4))

AbstractFerrocenyldimethylsilanol, FcSiMe2OH, Fc = (η5‐C5H5)Fe(η5‐C5H4), features a self‐assembled (through intermolecular hydrogen bonding) cyclohexameric supermolecule with a chair conformation. Copyright © 2005 John Wiley & Sons, Ltd.

A Cyclic Hexamer of Silver Trifluoroacetate Supported by Four Triphenylphosphine Sulfide Template Molecules

Crystallization of silver trifluoroacetate from chloroform solutions containing triphenylphosphine sulfide affords a trigonal and a monoclinic form of a 6:4 complex {[CF3C(O)OAg]6(Ph3PS)4} of C2 symmetry with different amounts of chloroform in the crystals. With the Ph3PS components as template molecules, the CF3C(O)OAg units are assembled to form a 6-membered metallacycle codetermined by metallophilic bonding and enclosed by a 24-membered ring [AgOCO]6. A complex of the type [LAgOC(O)CF3]2, with L representing the isocyanide ligand pTolSO2CH2NC, has been shown to have a conventional bicyclic structure with three-coordinate silver atoms engaged in transannular metallophilic interactions.

Formation of One-Dimensional Metal−Water Chain Containing Cyclic Water Hexamers

Cyclic water hexamers containing 1-D metal--water chains have been observed in the complexes of [M(H(2)biim)(2)(OH(2))(2)](ina)(2) x 4H(2)O [M = Zn (1) and Co (2), H(2)biim = 2,2'-biimidazole, ina = isoniconate]. The water clusters are trapped by the cooperative association of coordination interactions as well as hydrogen bonds.

A Probe for Capture and Fe3+-Induced Conformational Change of Lactoferrin Selected from Phage Displayed Peptide Libraries

Linear pentadecamer and cyclic hexamer peptide phage libraries were used to isolate phage clones with binding affinity toward lactoferrins purified from human and bovine milk. Phage clones with high specificity toward lactoferrin were selected with different binding strengths depending on the sequence of the peptide displayed by the phage. Phages coated to a microtiterplate were able to capture lactoferrin from crude milk samples without prior treatment. One of the selected sequences, EGKQRR, failed to bind to lactoferrin. In contrast, a branched tree-peptide bearing 4 EGKQRR sequences did bind to lactoferrin (Kd∼29μM) and was also capable of inhibiting the binding of the phage to lactoferrin (IC50∼17μM), indicating that avidity was important. Unexpectedly, the affinity of the phage for lactoferrin was influenced by the amount of bound Fe3+, with a much lower affinity when lactoferrin was saturated with Fe3+ as compared with the iron-depleted or partially saturated (natural) lactoferrin. As the phage does not bind to the Fe3+-binding site, the difference in binding affinity is due to differences in conformation of lactoferrin induced by Fe3+. These results demonstrate that avidity or multipoint attachment and Fe3+-induced conformational changes play an important role in the binding of the selected phage to lactoferrin. Thus, we could demonstrate that, by the use of selected phage clones, we are able not only to detect lactoferrin, but also to capture lactoferrin from crude milk samples. Furthermore, the extent of phage binding provides additional information about the iron content and the concomitant conformation of lactoferrin.

Synthesis, Structure, and Lanthanide Derivatives of an Unusual Hexameric Alcohol: [2,4,6‐(CF3)3C6H2CH2OH]6

AbstractHitherto unknown 2,4,6‐tris(trifluoromethyl)benzyl alcohol (3) was synthesized in 41 % yield by treatment of freshly prepared RFLi (2) with paraformaldehyde (RF = 2,4,6‐tris(trifluoromethyl)phenyl). According to an X‐ray diffraction study the crystal structure of 3 consists of S6 symmetric cyclic hexamers [2,4,6‐(CF3)3C6H2CH2OH]6. Deprotonation of 3 with NaN(SiMe3)2 in toluene afforded the unsolvated sodium alkoxide derivative RFCH2ONa (4). Homoleptic lanthanide alkoxides of the type Ln(OCH2RF)3 (Ln = Nd (5), Sm (6), Yb (7)) were made by treatment of Ln(C5H5)3 with three equivalents of 3. Similar reactions in a 1:1 molar ratio afforded the bis(cyclopentadienyl)lanthanide alkoxide derivatives (C5H5)2Ln(OCH2RF) (Ln = Nd (8), Sm (9), Yb (10)).

Phototriggered self-assembly of hydrogen-bonded rosette.

Azobenzene-appended melamine M2 and barbiturate B2, both possessing bulky tridodecyloxyphenyl (TDP) wedge(s), were designed and synthesized to establish a photoresponsive hydrogen-bonded supramolecular assembly. The geometrical isomer EE-M2 bearing two E-azobenzene moieties easily complexed with B2, affording a remarkably stable cyclic hexamer EE-M2(3).B2(3) (rosette) in chloroform, toluene, and methylcyclohexane, as confirmed by size exclusion chromatography, dynamic light scattering, (1)H NMR, and UV-vis studies. The E --> Z photoisomerization of the azobenzene moieties upon irradiation with UV light was significantly suppressed in the rosette because of the steric crowding of the TDP wedges (total of nine TDP wedges in a rosette), whereas irradiation of the monomeric EE-M2 resulted in facile transformation into ZZ-M2 bearing two Z-azobenzene moieties. (1)H NMR studies of the complexation of the initially photogenerated ZZ-M2 with B2 revealed that it is hard for ZZ-M2 to form a rosette with B2 because of the intermolecular steric interaction between the TDP wedges. The photoregulatable complexation efficiency of M2 allowed us to accomplish the phototriggered formation of the rosette by irradiation of a monomeric mixture of ZZ-M2 and B2 using visible light.

Highly efficient, one-step macrocyclizations assisted by the folding and preorganization of precursor oligomers.

Highly efficient, one-step macrocyclizations leading to the formation of macrocyclic hexa(aramides) in high yields (69-82%) are described. The one-step macrocyclizations were facilitated by the preorganization or folding of the backbones of uncyclized precursors in the course of macrocyclization. The preorganization of backbones was achieved by the presence of localized three-centered hydrogen bonds that were adopted in the design of a class of closely related, backbone-rigidified foldamers. The macrocyclization involved reactions between diacid chloride 1 and diamine 2. The crude reaction mixtures and products were conveniently examined by mass spectrometric method (MALDI-TOF). Compared to most traditional one-step macrocyclizations that usually require high dilution conditions and often lead to very low overall yields of the desired products, cyclic hexamers 3 were obtained as the overwhelmingly major product under a variety of reaction conditions, suggesting the generality of this approach.

Imaging water on Ag(111): field induced reorientation and contrast inversion.

Water adsorbed on Ag(111) at 70 K forms circular clusters that consist of six molecules. In scanning tunneling microscopy, this cyclic hexamer is imaged as a protrusion for voltages below V(SS)=-93 meV and as a depression for voltages above V(SS). The electronic density of states, however, increases around V(SS). We explain this counterintuitive result with the aid of calculated images by a change from constructive to destructive interference between different tunneling channels due to a field induced reorientation of the molecule under the tunneling tip.

Ozonolysis of tetramethylethylene: characterization of cyclic and open-chain oligoperoxidic products.

The ozonolysis of tetramethylethylene (TME) in solution to high conversion in nonparticipating solvents at -60 degrees C yields predominantly oligoperoxides. For the first time, these products have been characterized using electrospray ionization mass spectrometry (ESI-MS) under soft ionization conditions. The predominant structure formed in reactions carried out in pentane (up to 2.0 M TME) is shown to be the cyclic hexamer of acetone carbonyl oxide (oligocarbonyl oxide with degree of polymerization, n = 6), but cyclic structures with n up to 19 are observed. A small proportion of the oligoperoxides formed are open-chain compounds with end groups that suggest that chain termination of oligocarbonyl oxides can occur through reaction with either water or hydrogen peroxide. Ozonolysis in dried butyl acetate similarly produces mainly cyclic oligoperoxides. However, ozonolyses carried out in undried butyl acetate yield mainly open-chain oligoperoxides, confirming that propagating carbonyl oxide chains are readily terminated by water. Relative amounts of the open-chain oligomers so-formed suggest that undried butyl acetate contains ca. 0.1% w/w water. The ozonolysis of TME in the participating solvent, methanol, at -60 degrees C yields 2-methoxyprop-2-yl hydroperoxide via reaction of acetone carbonyl oxide with methanol; no oligoperoxidic products are formed in this case.