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Five-membered Ring (five-membered + ring)
Selected AbstractsCyclopenta[a]quinolizine: A Novel Pseudoazulene with a Bridgehead Nitrogen AtomEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 28 2010Pavel V. Gormay Abstract Cyclopenta[a]quinolizine, a novel tricyclic pseudoazulene, was prepared by reaction of cyclopentadienyllithium with oxazolo[3,2- a]pyridinium or substituted pyridinium salts. Structural analysis (X-ray analysis, NMR and UV spectroscopy) and DFT calculations confirm its highly dipolar nature (due to delocalization of the negative charge around the five-membered ring) combined with partial localization of bonds in both six-membered rings. [source] Inhibition of recombinant human maltase glucoamylase by salacinol and derivativesFEBS JOURNAL, Issue 12 2006Elena J. Rossi Inhibitors targeting pancreatic ,-amylase and intestinal ,-glucosidases delay glucose production following digestion and are currently used in the treatment of Type II diabetes. Maltase-glucoamylase (MGA), a family 31 glycoside hydrolase, is an ,-glucosidase anchored in the membrane of small intestinal epithelial cells responsible for the final step of mammalian starch digestion leading to the release of glucose. This paper reports the production and purification of active human recombinant MGA amino terminal catalytic domain (MGAnt) from two different eukaryotic cell culture systems. MGAnt overexpressed in Drosophila cells was of quality and quantity suitable for kinetic and inhibition studies as well as future structural studies. Inhibition of MGAnt was tested with a group of prospective ,-glucosidase inhibitors modeled after salacinol, a naturally occurring ,-glucosidase inhibitor, and acarbose, a currently prescribed antidiabetic agent. Four synthetic inhibitors that bind and inhibit MGAnt activity better than acarbose, and at comparable levels to salacinol, were found. The inhibitors are derivatives of salacinol that contain either a selenium atom in place of sulfur in the five-membered ring, or a longer polyhydroxylated, sulfated chain than salacinol. Six-membered ring derivatives of salacinol and compounds modeled after miglitol were much less effective as MGAnt inhibitors. These results provide information on the inhibitory profile of MGAnt that will guide the development of new compounds having antidiabetic activity. [source] 5-Benzyl-3-methylimidazolidin-4-one-Derived Reactive Intermediates of Organocatalysis , A Comforting Resemblance of X-Ray, NMR, and DFT Solid-Phase, Liquid-Phase, and Gas-Phase StructuresHELVETICA CHIMICA ACTA, Issue 1 2009Abstract The X-ray crystal structures of three (E)-1-cinnamoylidene iminium PF6 salts of 5-benzyl-3-methylimidazolidin-4-ones (2,2-dimethyl-, cis -2-(tert -butyl)-, and cis -2-styryl-substituted; 2,4, resp.) are reported (Figs.,3,5). In the 2,2-dimethyl and in the cis -2-styryl derivative, 2 and 4, respectively, a CH bond of the cis -substituent in 2-position points to the center of the benzene ring of the benzyl group above the five-membered ring (Fig.,6,,a and b). NMR Measurements (Fig.,8) provide evidence that the same structure is present in solution, and that a fourth derivative of this type, 5 (Scheme), has (Z)- instead of (E)-configuration around the CN bond. In the cis -2-(tert -butyl) derivative 3, the benzyl group is located over the iminium , -system (Figs.,4 and 6,,c). Overlays with DFT-calculated crotonylidene analogs, A and B, show that the theoretical and experimental structures are almost superimposable (Fig.,9 and Table). The structures are discussed in view of their role as reactive intermediates in organocatalysis and in view of the help synthetic organic chemists may experience from theory. [source] Anionic ring-opening polymerization of small phosphorus heterocycles and their borane adducts: An ab initio investigationHETEROATOM CHEMISTRY, Issue 2 2007Michelle L. Coote The kinetics and thermodynamics of anionic ring-opening reactions of phosphiranes, phosphetanes, and phospholanes and their borane adducts have been studied by high-level ab initio procedures. For the free heterocycles, model propagation reactions involving nucleophilic attack by Me2P, at the ring ,-carbon were found to be feasible for the three- and four-membered rings, but not for the five-membered ring. For the borane adducts, nucleophilic attack by Me2(BH3)P, was only facile for the three-membered ring, despite an increased thermodynamic tendency toward ring opening for the borane adducts of both the three- and four-membered rings. The formation constants of the borane adducts of methylphosphirane and methylphosphetane were K = 2.6 × 1013 L mol,1 and K = 1.2 × 1020 L mol,1, respectively. A Marcus analysis of the ring-opening reactions of methylphosphirane, methylphosphetane, and their borane adducts showed that the release of ring strain and an "additional factor" contribute to rate enhancement compared with their strain-free analogues. The additional factor was larger for the three-membered rings than for the four-membered rings and was larger in the free heterocycles than in their borane adducts. The additional factor is complex in origin and appears to reflect an increase in the separation between the bonding and antibonding orbitals of the breaking bond on going from the three-membered rings to the four-membered rings, and on going from the free heterocycles to the borane adducts. © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:118,128, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20323 [source] The First Anionic Thia-Fries Rearrangements at Ferrocene: Ready Access to Trifluoromethylsulfonyl-Substituted Hydroxyferrocenes and an Extremely High Interannular Stereoinduction between Cyclopentadienyl LigandsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 8 2010Georg Werner Abstract Attempts originally directed towards the generation of ferrocyne (1,2-dehydroferrocene, 4) and ferrocenediyne (1,2,1,,2,-tetradehydroferrocene, 5) by triflate elimination from the respective ferrocenyl triflates led to the discovery of the first anionic thia-Fries rearrangements at a five-membered ring. These reactions take place with remarkable efficiency under very mild reaction conditions and yield the respective trifluoromethylsulfonyl-substituted ferrocenols. Most remarkably, the reaction starting from 1,1,-ferrocenediyl ditriflate (9) adopts an extremely high interannular stereoinduction in that exclusively the meso rearrangement product, meso -2,2,-bis(trifluoromethylsulfonyl)-1,1,-ferrocenediol (16), is formed, the corresponding racemic product 17 is not observed. It is shown that the second anionic thia-Fries rearrangement proceeds at a much larger rate than the first one. The stereochemistry and the high rate of this reaction are explained on the basis of electronic as well as steric considerations. The redox behavior of some of the unprecedented ferrocene derivatives has been characterized by cyclovoltammetry. [source] A hybrid density functional theory study of the low-temperature dimethyl ether combustion pathways.ISRAEL JOURNAL OF CHEMISTRY, Issue 2-3 2002I: Chain-propagation Dimethyl ether (DME) has been proposed to be a promising alternative to conventional diesel fuel because of its favorable compression ignition property (high cetane number) and its soot-free combustion. A radical chain mechanism for hydrocarbon autoignition has been proposed for DME at low temperatures. In this mechanism, the chain initiation step consists of DME undergoing hydrogen abstraction by a highly reactive species (typically ·OH). The CH3O·H2 created in the initiation step then combines with O2; the subsequent CH3OCH2OO· radical is involved in a Lindemann-type mechanism, which can lead to the production of formaldehyde (CH2 = O) and ·OH. This concludes the chain-propagating step: the one ·OH produced then sustains the chain-reaction by creating another CH3O·H2. A relatively stable intermediate (·CH2OCH2OOH), formed via isomerization of CH3OCH2OO· in the chain-propagation step, can combine with a second O2 to produce a radical (·OOCH2OCH2OOH) that can potentially decompose into two ·OH radical (and other products). This path leads to chain-branching and an exponential increase in the rate of DME oxidation. We have used spin-polarized density functional theory with the Becke-3-parameter Lee,Parr,Yang exchange-correlation functional to calculate the structures and energies of key reactants, intermediates, and products involved in (and competing with) the chain-propagating and chain-branching steps of low-temperature DME oxidation. In this article, Part I, we consider only the chain-propagation mechanism and its competing mechanisms for DME combustion. Here, we show that only certain conformers can undergo the isomerization to ·CH2OCH2OOH. A new transition state has been discovered for the disproportionation reaction ·CH2OCH2OOH , 2CH2O + ·OH in the chain-propagating step of DME autoignition that is much lower than previous barriers. The key to making this decomposition pathway facile is initial cleavage of the O,O rather than the C,O bond. This renders all transition states along the chain-propagation potential energy surface below the CH3O·H2 + O2 reactants. In contrast with the more well-studied CH3·H2 (ethyl radical) + O2 system, the H-transfer isomerization of CH3OCH2OO· to ·CH2OCH2OOH in low-temperature DME oxidation has a much lower activation energy. This is most likely due to the larger ring strain of the analogous transition state in ethane oxidation, which is a five-membered ring opposed to a six-membered ring in dimethyl ether oxidation. Thus low-temperature ethane oxidation is much less likely to form the ·ROOH (where R is a generic group) radicals necessary for chain-branching, which leads to autoignition. Three competing reactions are considered: CH3O·H2 , CH2O + ·CH3; ·CH2OCH2OOH , 1,3-dioxetane + ·OH; and ·CH2OCH2OOH , ethylene oxide + HOO·. The reaction barriers of all these competing paths are much higher in energy (7,10 kcal/mol) than the reactants CH3O·H2 + O2 and, therefore, are unlikely low-temperature paths. Interestingly, an analysis of the highest occupied molecular orbital along the CH3O·H2 decomposition path shows that electronically excited (1A2 or 3A2) CH2O can form; this can also be shown for ·CH2OCH2OOH, which forms two formaldehyde molecules. This may explain the luminosity of DME's low-temperature flames. [source] Theoretical study of protonated xylenes: ethene elimination and H,C-scrambling reactionsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 11 2004Bjørnar Arstad Abstract Quantum chemical calculations have been carried out to investigate various unimolecular rearrangements that can take place in protonated gas-phase xylenes. Hydrogen and methyl group ring migrations were investigated. The barriers for hydrogen migrations are lower than the barriers for methyl group migrations. Mechanisms for ring expansion to seven-membered rings, and for contraction to five-membered rings were studied. Both of these mechanisms can eventually lead to ethene elimination. The most favourable ring expansion step goes through a 1,3-hydrogen shift from a methyl group onto the arenium ring, forming a protonated methylcycloheptatriene. Interconversions between various ring-expanded forms have been investigated. Re-contraction can lead to an ethylbenzenium ion that could subsequently split off ethene. Alternatively, the xylenium ion can contract to a five-membered ring. The immediate product is a bicyclic ion (bicyclo[3.1.0]hexane skeleton) that can rearrange further to give an ethylbenzenium ion, or the five-ring system can split off ethene, and be converted into a cyclopentadienyl ion that can isomerize into a benzenium ion. Stable structures and transition states are calculated both at the B3LYP/cc-pVTZ//B3LYP/6-311G(d,p) and at the MP2/cc-pVTZ//MP2/6-31G(d) levels. The energies needed for ring expansion or ring contraction are not very different, and the calculations suggest that both reaction paths are possible, but the energy needed for actually splitting off an ethene molecule is lower along the expansion path. Copyright © 2004 John Wiley & Sons, Ltd. [source] Studies of ethylene,styrene copolymerization with dinuclear constrained geometry complexes with methyl substitution at the five-membered ring in indenyl of [Ti(,5:,1 -C9H5SiMe2NCMe3)]2 [CH2]nJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 7 2004Seok Kyun Noh Abstract The new dinuclear half-sandwich CGC (constrained geometry catalyst) with methyl substitution in indenyl, [Ti(,5:,1 -2-methylindenyl)SiMe2NCMe3]2 [(CH2)n] [n = 6 (10), n = 9 (11), n = 12 (12)], have been synthesized, and structure of these complexes has been characterized by 1H and 13C NMR. The most important feature is that two protons of methylene directly bonded to the indenyl ring become inequivalent to be shown as two separated resonances at 2.9 and 3.0 ppm, probably due to the formation of planar chirality caused by a titanium complex formation. It has been found that the dinuclear CGCs with methyl substitution at an indenyl ring were very active catalysts for ethylene and styrene copolymerization. The activity increases in the order of 10 < 11 < 12, which indicates that the presence of a longer bridge between two active sites contributes to facilitate the polymerization activity of the dinuclear CGC more effectively. This result might be understood by the implication that the steric factor rather than the electronic factor may play a major role to direct the polymerization behavior of the dinuclear CGC. It is found that the dinuclear catalysts are very efficient to incorporate styrene in the polyethylene backbone. The styrene contents in the formed copolymers ranged from 5 to 40% according to the polymerization conditions. One can observe strong signals at 29.7 ppm of the polyethylene sequences, and, in addition, peaks at 27.5, 36.9, and 46. 2ppm (S,,, S,,, and T,,, respectively) of sequences of EESEE. Weak peak at 25.3 ppm are attributed to S,,, which represents SES sequence. The absence of a signal for T,, at 41.3 ppm and for S,, at 43.6 ppm shows there is no styrene,styrene sequences in copolymers. This result indicates that the dinuclear CGC are very effective to generate well-distributed poly(ethylene- co -styrene)s. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1712,1723, 2004 [source] A study of the elimination of water from lithium-cationized tripeptide methyl esters by means of tandem mass spectrometry and isotope labelingRAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 20 2006Erach R. Talaty Extensive isotope labeling (2H, 13C and 15N), collision-induced dissociation (CID) and multiple-stage tandem mass spectrometry were used to investigate the elimination of H2O from a series of model, metal-cationized tripeptide methyl esters. The present results corroborate our earlier suggestion that loss of water from lithiated peptides is initiated by a nucleophilic attack from the N-terminal side upon an amide carbonyl carbon atom to form a five-membered ring as an intermediate followed by 1,2-elimination of water. We show that the nucleophilic atom is the oxygen atom of the N-terminal amide group in the fragmentation of [AcGGGOMe+Li]+ as well as [GGGOMe+Li]+. However, the subsequent fragmentation is markedly different in the two cases as a result of the absence and presence of a free amino group. In particular, extensive scrambling of protons in the , -positions of GGGOMe is observed, presumably as a consequence of intervention of the basic amino group. Copyright © 2006 John Wiley & Sons, Ltd. [source] A pentanuclear bimetallic complex of manganese(II) and aluminium(III) ions: tetra-,2 -iodido-iodidobis(,3 -2-methoxyethanolato)bis(,2 -2-methoxyethanolato)dimethyl(tetrahydrofuran-,O)aluminium(III)tetramanganese(II)ACTA CRYSTALLOGRAPHICA SECTION C, Issue 11 2007Lucjan B. Jerzykiewicz The molecule of the title compound, [Mn4Al(CH3)2(C3H7O2)4I5(C4H8O)], contains one AlIII and four MnII ions. Two Mn atoms are five-coordinate in the form of a trigonal bipyramid or a square pyramid. The two other Mn atoms are six-coordinate with an octahedral geometry. The fourcoordinate Al atom is linked to the manganese core by ,-Oalkoxo bridges, forming an almost planar five-membered ring. [source] 3,,4,-Bis(4-chlorophenyl)spiro[chroman-3,5,(4,H)-isoxazol]-4-oneACTA CRYSTALLOGRAPHICA SECTION C, Issue 4 2001A. Abdul Ajees The title compound, C23H15Cl2NO3, crystallizes with two independent molecules in the asymmetric unit. The chromanone moiety consists of a benzene ring fused with a six-membered heterocyclic ring which adopts a sofa conformation. The five-membered spiroisoxazoline ring is in an envelope conformation. The p -chlorophenyl rings bridged by the five-membered ring are nearly perpendicular to each other. The chromanone moiety of one molecule packs into the cavity formed by the p -chlorophenyl rings of a second molecule through the formation of C,H,, interactions. The structure is stabilized by weak C,H,O, C,H,Cl and C,H,, interactions. [source] N -Carboxy- l -phenylalanine anhydrideACTA CRYSTALLOGRAPHICA SECTION C, Issue 4 2000Hitoshi Kanazawa The molecules of the title compound, 4-benzyl-1,3-oxazolidene-2,5-dione, C10H9NO3, are linked by intermolecular hydrogen bonds between the imino group of the five-membered ring and an adjacent carbonyl O-atom, along the c axis. The benzyl groups are stacked in a layer and the five-membered rings are arranged in another layer sandwiched by the benzyl group layer. This sandwich structure should explain the high polymerizability of the title compound in the solid state. [source] Crystallization and preliminary X-ray analysis of a DNA dodecamer containing 2,-deoxy-5-formyluridine; what is the role of magnesium cation in crystallization of Dickerson-type DNA dodecamers?ACTA CRYSTALLOGRAPHICA SECTION D, Issue 2 2001Masaru Tsunoda To investigate the role of divalent cations in crystal packing, four different crystals of a Dickerson-type dodecamer with the sequence d(CGCGAATXCGCG), containing 2,-deoxy-5-formyluridine at X, were obtained under several conditions with and without divalent cations. The crystal structures are all isomorphous. The octahedrally hydrated magnesium cations found in the major groove cement the two neighbouring duplexes along the b axis. In the Mg2+ -free crystals, a five-membered ring of water molecules occupies the same position as the magnesium site and connects the two duplexes similarly to the hydrated Mg2+ ion. It has been concluded that water molecules can take the place of the hydrated magnesium cation in crystallization, but the magnesium cation is more effective and gives X-ray diffraction at slightly higher resolution. In all four crystals, the 5-formyluracil residues form the canonical Watson,Crick pair with adenine residues. [source] Bisamides Derived from Azulene-1,3- and -5,7-dicarboxylic Acids as New Building Blocks for Anion ReceptorsCHEMISTRY - A EUROPEAN JOURNAL, Issue 3 2008Tomasz Zieli, ski Dr. Abstract Bisamides based on the azulene moiety were investigated as building blocks for anion receptors. In the course of these studies, derivatives of azulene-1,3- and -5,7-dicarboxylic acid were synthesized and thoroughly characterized. The anion affinities of the derivatives based on functionalization in the five-membered ring and in the seven-membered ring were determined by 1H,NMR titration. The structural analysis of these building blocks was performed by X-ray diffractometry, molecular modelling and 2D NMR spectroscopy. The five-membered ring derivatives are easy to obtain, offer a binding site preorganized in the syn,syn conformation and bind anions with a strength similar to those of pyrrole-based analogues. There is also strong evidence for aromatic CH,,,anion interactions. The ligands substituted at the 5- and 7-positions offer a binding cleft with an uncommon geometry that originates from the seven-membered ring and seems to be complementary to the chloride anion. [source] Dual Supermesityl Stabilization: A Room-Temperature-Stable 1,2,4-Triphosphole Radical, Sigmatropic Hydrogen Rearrangements, and Tetraphospholide Anion,EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 15 2008Alex S. Ionkin Abstract Cesium 3,5-bis(2,4,6-tri- tert -butylphenyl)-1,2,4-triphospholide (12) and cesium 5-(2,4,6-tri- tert -butylphenyl)tetraphospholide (13) were synthesized and isolated with flat five-membered rings, which are an indication of the aromaticity in these anions. Compound 13 is the first example of a stable tetraphospholide anion, which is structurally characterized. Kinetic stabilization of the 1,2,4-triphospholide system by two supermesityl groups resulted in the detection of the room-temperature-stable radical 17 and the observation of a series of successive sigmatropic hydrogen shifts in the first stable 1H -1,2,4-triphosphole 14 with a P,H bond.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Synthesis and Characterization of a Gold Complex Containing [SbPh]2, and [SbPh2], Anions as Bridging LigandsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 22 2007Dieter Fenske Abstract The synthesis and structural characterization of the gold antimony complex [Au8(SbPh)2(SbPh2)4(PEt3)6] (1) is reported. Complex 1 can be obtained by the reaction of [AuCl(PEt3)] with a mixture of PhSb(SiMe3)2 and Ph2(SbSiMe3) in presence of the bidentate phosphane ligand dppm [dppm = bis(diphenylphosphanyl)methane]. Complex 1 consists of a distorted heterocubic central [Au6Sb2] unit connected to two annulated five-membered rings on two opposite edges. The AuI ions show weak aurophilic interactions and are bridged by [SbPh]2, and [SbPh2], anions. Despite the high oxidation potential of AuI, the reaction mixtures and crystals are surprisingly stable. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] Zinc Hydrazide and Zinc Alkoxide Hydrazide Cages with Zn4N8 and Zn4N6O Cores , Cluster Isomerism as a Result of Subtle Changes in Ligand SizeEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 19 2006Surajit Jana Abstract The hydrazide cluster [(iPrZn)4(NHNMe2)4] (1) was synthesised by the reaction of diisopropylzinc with N,N -dimethylhydrazine and was characterised by 1H- and 13C NMR and IR spectroscopy, mass spectrometry, elemental analysis and X-ray crystallography. This compound forms asymmetric aggregates containing Zn4N8 cores. The Zn atoms in these aggregates are arranged in topological tetrahedra in which the triangular faces are bridged by NHNMe2 substituents. Each NH group is connected to two Zn atoms and each NMe2 group to one Zn atom. Alkoxide clusters were prepared in one-pot syntheses by treating diisopropylzinc solutions with mixtures of N,N -dimethylhydrazine and ROH (R = Et, iPr). The resulting compounds have the formula [(iPrZn)4(NHNMe2)3(OR)] [R = Et (3), iPr(4)] and contain Zn4N6O cages, such that one NHNMe2 in 1 is replaced by one alkoxide group. Two different aggregation modes were found for these Zn4N6O cages. In compound 3, one Zn atom is bound to two NMe2 groups and one NH group. The other three Zn atoms each have three bonds to NH groups and one dative bond to an NMe2 group. The cage consists of one four-membered and one six-membered ring as well as four five-membered rings. In compound 4, the fourth zinc atom is exclusively bonded to three anionic NH functions in such a way that the rings in the cage are all five-membered. Compounds 3 and 4 were characterised by NMR spectroscopy and single-crystal X-ray diffraction. Hydrazide hydroxide clusters were also obtained through the reaction of a diisopropylzinc solution with N,N -dimethylhydrazine and a small amount of water. The structure of the resulting cocrystalline material, [(iPrZn)4(NHNMe2)4]·[(iPrZn)4(NHNMe2)3(OH)] (2), was also confirmed by X-ray diffraction. The hydroxide cluster in 2 contains a Zn4N6O cage, with a similar aggregation mode to that of 4. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Fused Tetracyclic Heterocycles by Thermally Initiated Intramolecular Criss-Cross Cycloaddition of 3-Substituted HomoallenylaldazinesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 12 2005Hana Zachová Abstract The thermally initiated intramolecular criss-cross cycloaddition of 3-substituted homoallenylaldazines 5 has been explored. Their cyclization led to interesting new fused heterocyclic systems 6 consisting of four five-membered rings with two nitrogen heteroatoms. The azines were prepared by the reaction of homoallenyl aldehydes 4 with hydrazine. The homoallenyl aldehydes 4 were synthesized by the Claisen rearrangement of new N,N -disubstituted 4-[(2-methylprop-1-en-1-yl)oxy]but-2-yn-1-amines 3a,f prepared by Mannich reaction of 2-methylprop-1-en-1-yl prop-2-yn-1-yl ether (2). The success of the reaction was based on the improved solvent-free synthesis of 1-chloro-2-methylpropyl prop-2-yn-1-yl ether (1) and its conversion to isolable 2-methylprop-1-en-1-yl prop-2-yn-1-yl ether (2) in high yield. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Novel organometallic fullerene complexes for vehicular hydrogen storagePHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 11 2007A. C. Dillon Abstract Theoretical studies have predicted that scandium can bind to the twelve five-membered rings in C60. It is then possible to stabilize four dihydrogen ligands (H2) on each Sc atom with a binding energy of ,30 kJ/mol, ideal for vehicular hydrogen storage. The resulting C60[ScH2(H2)4]12 complex is predicted to be a minimum energy structure with ,7.0 wt% reversible hydrogen capacity. However, wet chemical synthesis of the calculated ,5 -coordinated fullerene complex is unprecedented. The chemistry of C60 is generally olefinic (i.e., ,2 -coordination, in which the metal is coordinated to two carbon atoms contributing two electrons to the bonding). Furthermore, stabilization of multiple dihydrogen ligands on a single transition metal has not been demonstrated. Recently we have probed new synthesis techniques in order to coordinate C60 with either Fe, Sc, Cr, Co or Li. The new compounds were characterized with solid-state nuclear magnetic resonance, and structures have been proposed. All of the structures were found to have unique binding sites for hydrogen employing the technique of temperature programmed desorption. Furthermore, some of the structures were shown to have significant hydrogen capacities with volumetric measurements. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Supramolecular structure of 1H -pyrazoles in the solid state: a crystallographic and ab initio studyACTA CRYSTALLOGRAPHICA SECTION B, Issue 6 2000Concepción Foces-Foces The secondary structure of 1H -unsubstituted pyrazole derivatives bearing only one hydrogen donor group and one or more acceptor groups has been analyzed in terms of some descriptors representing the substituents at C3 and C5. The substituent at C4 appears to affect mainly the tertiary or quaternary structure of these compounds. The proposed semi-quantitative model, which explains most hydrogen-bonded motifs as a combination of the effects of substituents at C3 and C5, has also been examined as a function of the steric and polarizability effects of these substituents represented by molar refractivity. The model also applies to other five-membered rings (1,2,4-triazoles, 1,2,4-diazaphospholes and 1,2,4-diazaarsoles). Furthermore, ab initio calculations at RHF/6-31G* have been performed to discover the relative stability of three of the four hydrogen-bond patterns displayed by several symmetrical pyrazoles (dimers, trimers, tetramers). The fourth motif, catemers, has only been discussed geometrically. [source] Conformations of three heterocyclic perhydropyrrolobenzofurans and polymeric assembly via co-operative intermolecular C,H,O hydrogen bondsACTA CRYSTALLOGRAPHICA SECTION C, Issue 5 2006H. S. Yathirajan In 1-cyclohexyl-6,6,8a-trimethyl-3a,6,7,8a-tetrahydro-1H -1-benzofuro[2,3- b]pyrrole-2,4(3H,5H)-dione, C19H27NO3, (I), and the isomorphous compounds 6,6,8a-trimethyl-1-phenyl-3a,6,7,8a-tetrahydro-1H -1-benzofuro[2,3- b]pyrrole-2,4(3H,5H)-dione, C19H21NO3, (II), and 6,6,8a-trimethyl-1-(3-pyridyl)-3a,6,7,8a-tetrahydro-1H -1-benzofuro[2,3- b]pyrrole-2,4(3H,5H)-dione, C18H20N2O3, (III), the tetrahydrobenzo,dihydrofuro,pyrrolidine ring systems are folded at the cis junction of the five-membered rings, giving rise to a non-planar shape of the tricyclic cores. The dihydrofuran and pyrrolidine rings in (I) are puckered and adopt an envelope conformation. The cyclohexene rings adopt a half-chair conformation in all the molecules, while the substituent N -cyclohexyl ring in (I) assumes a chair form. Short intramolecular C,H,O contacts form S(5) and S(6) motifs. The isomorphous compounds (II) and (III) are effectively isostructural, and aggregate into chains via intermolecular C,H,O hydrogen bonds. [source] N -Carboxy- l -phenylalanine anhydrideACTA CRYSTALLOGRAPHICA SECTION C, Issue 4 2000Hitoshi Kanazawa The molecules of the title compound, 4-benzyl-1,3-oxazolidene-2,5-dione, C10H9NO3, are linked by intermolecular hydrogen bonds between the imino group of the five-membered ring and an adjacent carbonyl O-atom, along the c axis. The benzyl groups are stacked in a layer and the five-membered rings are arranged in another layer sandwiched by the benzyl group layer. This sandwich structure should explain the high polymerizability of the title compound in the solid state. [source] Nickel(II) Complexes Bearing NH2CH2CH2CH2NH2 and o -C6H4(NH2)2 Ligands: Synthesis, Structures and Their Ethylene Polymerization BehaviorCHINESE JOURNAL OF CHEMISTRY, Issue 2 2009Xiaoming LU Abstract Two nickel complexes [Ni(NH2CH2CH2CH2NH2)3]Cl2 (1) and [Ni(C6H4N2H4)2Cl2] (2) have been obtained and characterized by IR and single crystal X-ray diffraction analysis. In complex 1, the nickel atom was in a chiral pseudo-octahedral [NiN6] geometry environment, which was coordinated by three 1,3-propylenediamine to form three six-membered rings. However in complex 2, besides connected with two o -phenylenediamine through four Ni,N bonds to form two five-membered rings, the nickel center was coordinated by two Cl to form trans -Ni-Cl2, which was different from general reported diimine nickel complexes. Activated with MAO, MMAO or Et2AlCl, these nickel complexes exhibited considerable good catalytic activity [up to 3.59×106 g·mol,1·h,1 for complex 2] for ethylene polymerization with main products as dimers or trimers of ethylene. [source] | |||||||||||||||||||||||||