bluuebunny/crossref_metadata_2025_split huggingface dataset:
Metal‐Mediated Synthesis of Multidomain Ligands—A New Strategy for Metallosupramolecular Chemistry
Authors: Constable Edwin C. , Thompson Alexander M. W. Cargill , Harveson Peter , Macko Ludwig , Zehnder Margareta 
Published: July 2007

AbstractThe bridging ligand bis{4′‐(2,2′:6′,2″‐terpyridinyl)}ether (1) can be prepared in 69% yield from the reaction of 4′‐chloro‐2,2′:6′,2″‐terpyridine (3) with 2,2′:6′,2″‐terpyridin‐4′(1′H)‐one (2) in Me2NCHO in the presence of KOH. More conveniently, complexes of 1 can be prepared in situ by the reaction of 2 with a ruthenium(II) complex of 3 in the presence of K2CO3. This methodology has been developed for the synthesis of a range of mono‐, di‐, tri‐ and hexanuclear complexes with a variety of Xtpy (Xtpy = 4′‐substituted 2,2′:6′,2″‐terpyridine) terminator ligands. The molecular structure of 1 (a = 9.623(2), b = 11.241(1), c = 11.828(1) Å; space group P1; α = 93.064(9), β = 107.072(14), γ = 99.088(14)°; Z = 2, R = 0.0450, Rw = 0.0577) has been determined. The generality of the methodology may ultimately be limited by the sensitivity of the ether‐linkage in 1 to attack by nucleophiles.
Potential Linear‐Chain Organic Ferromagnets
Authors: Yoshizawa Kazunari , Hoffmann Roald 
Published: July 2007

AbstractThe electronic structures of conjugated polymers containing methyl radicals, carbenes, and nitrogen‐based radicals coupled in various ways through benzene rings are examined, employing band‐structure calculations in the extended Hückel approximation. The structural and electronic properties of polymers with a para‐phenylene or meta‐phenylene coupling unit are compared. In the polymer with methyl radicals coupled through a para‐phenylene unit, a pairing or Peierls distortion occurs to remove the degeneracy at the Fermi level. The resulting bandgap is nevertheless relatively small; we conclude that such polymers are likely to exhibit high electrical conductivity upon doping, very much like polyacetylene. On the other hand, in the polymers with a meta‐phenylene coupling unit, striking symmetry‐determined, halfoccupied narrow bands appear at the Fermi level and contribute to the stability of the ferromagnetic state. The relation of a potential ferromagnetic state to metallic, CDW, and SDW states is discussed from the viewpoint of orbital interactions in extended systems. We suggest novel 3‐ and 4‐fold helical structures for the meta‐phenylene‐coupled polymers.
An Enantioselective Synthesis of cis‐4‐tert‐Butoxycarbamoyl‐1‐methoxycarbonyl‐2‐cyclopentene—A Useful, General Building Block
Authors: Trost Barry M. , Stenkamp Dirk , Pulley Shon R. 
Published: July 2007

AbstractThe amino acid derivative in the title represents an important building block for the synthesis of a number of biologically important targets such as the antiviral carbanucleosides and amidinomycin. By using asymmetric palladium‐catalyzed desymmetrization of meso‐2‐ene‐1,4‐diols, cis‐1,4‐dibenzoyloxy‐2‐cyclopentene can be converted to the enantiomerically pure title compound in only four steps. Chemoselective ester reduction allows entry into the domain of carbanucleosides, whereas double‐bond reduction provides the precursor for amidinomycin. In an ancillary study, a facile diastereoselective cis‐hydroxylation provides aminocyclopentitols, compounds that have proven to be potent glycosidase inhibitors.
Intramolecular Long‐Range Exchange Coupling in Dinuclear Copper(II) Complexes with Cu…︁Cu Separations Greater than 10Å
Authors: Bürger Kai‐Silke , Chaudhuri Phalguni , Wieghardt Karl , Nuber Bernhard 
Published: July 2007

AbstractMagnetic long‐range exchange coupling between two unpaired electrons of two copper(II) ions in a square‐pyramidal ligand environment (d)1, which are rigidly separated by > 10 Å by an organic spacer ligand (e.g., a μ‐dicarboxylato ligand) has been systematically studied. The following complexes have been synthesized: [{LCu(OH2)}2(μ‐terephthalato)](ClO4)2 (I), [LCuII(OH2)(benzoato)](CIO4) (2), [{LCuII(NCS)}2(μ‐terephthalato)]‐2 CH3OH (3), [{LCuII(NCS)}2 (μ‐4,4′‐oxybisbenzoato)] · 3 CH3OH (4), [{LCuII(OH2)}2 (μ‐tetrafluoroterephthalato)](CIO4)2 (5), [{LCuII(OH2)}2{LCuII}2 (μ‐tetrafluorobenzoato)3](CIO4)2 · 6 H2O (6), [{LCuII(OH2)}2 (μ‐biphenyl‐4,4′‐dicarboxylato)](CIO4)2 (7), [LCuII(μ‐biphenyl‐4,4′‐dicarboxylato)]n (HZO)2n (8), [{LCuII(NCS)}2 (μ‐biphenyl‐4.4′‐dicarboxylato)] (9), [{LCu(OH2)}2, (μ‐trans,trans‐2,4‐hexadien1,6‐dicarboxylato)] (CIO4)2 (10) (L = 1,4,7‐trimethyl‐1,4,7‐triazacyclononane). Complexes 1, 2, 3, 4, and 6 have been characterized by X‐ray crystallography; the structure of 1 has been described previously.[15] Temperature‐dependent magnetic susceptibility data for all complexes have been measured over the range 2.0–298 K. In complexes 2, 3, 4, 6, 8, and 9 only very weak, if any, intermolecular antiferromagnetic or, in the case of 6, ferromagnetic coupling has been observed (|J|⩽2cm−1; H= −2JS1 S2 S1 = S2 = 1/2). In contrast, in complexes 5 (2J=−80cm−1), 7(2J=−62cm−1), and 10 (2J = −100 cm−1) relatively strong intramolecular antiferromagnetic exchange coupling has been detected. The intramolecular Cu…︁Cu separation is 11.3Å in 5, 15.6 Å in 7, and approximately 12.0Å in 10. These data demonstrate that intramolecular antiferromagnetic exchange coupling of considerable magnitude is possible over distances > 15 Å, provided that the effective magnetic orbitals are favorably aligned.
Isolation of Selected Exchange Processes in Nuclear Magnetic Resonance
Authors: Zwahlen Catherine , Vincent Sébastien J. F. , Schwager Martina , Bodenhausen Geoffrey 
Published: July 2007

AbstractWe have shown that it is possible to inhibit the transfer of magnetization in a system with several exchanging sites in dynamic equilibrium, as in a mixture of cis‐ and trans‐ZrCl4L2 with excess free ligand L (L = (CH3O)3PO). The forward and backward reaction rates involving two selected sites can be studied while the effect of competing exchange processes is “quenched”. This can be achieved either by selective inversion of the magnetization of the two chosen sites in the course of the reaction interval, or alternatively by inversion of the magnetization of all other sites in the exchange network. The rate of exchange from the free to the cis site was determined to be kcis←free = 0.018 s−1. In the usual methods, this process would tend to be overshadowed by the almost two hundred times faster competing exchange process from the cis to the trans site (Ktrans←cis = 3.32 s−1).
Alkenyl Fischer Carbene Complexes and α,β‐Unsaturated Imine Derivatives: Synthesis of Azepines and Mechanistic NMR Studies
Authors: Barluenga José , Tomás Miguel , Ballesteros Alfredo , Santamaría Javier , Carbajo Rodrigo J. , López‐Ortiz Fernando , García‐Granda Santiago , Pertierra Pilar 
Published: July 2007

Abstract4‐Amino‐1‐azadienes 1 react with α,β‐unsaturated Fischer carbene complexes at −40°C to give stereoselectively a variety of substituted 3H‐4,5‐dihydroazepines 3; similarly, 1‐hydroxy‐1‐azadienes (α,β‐unsaturated oximes) 6 afforded the corresponding azepine derivatives 7. Chiral, nonracemic carbene complexes 11 gave azepines 12–13 (d.e. = 40–44%) upon reaction with oxime 6a; the major isomers were obtained in a diastereomerically and enantiomerically pure form (45–50% overall yield) after crystallization. An X‐ray structure of 12a allowed assignment of the absolute stereochemistry. The acid hydrolysis of azepines synthesized provided racemic and enantiomerically pure 1,6‐dicarbonyl compounds (±)‐5, (±)‐9, and (–)‐14, as well as diol (–)‐15. The mechanism of the reaction of 1 and 2 was investigated by multinuclear (1H, 13C, 15N, and 183W) NMR characterization of four intermediates (A, B, C, and D) at low temperature. The experimental sequence of events involves: i) 1,2‐nucleophilic addition of the unsubstituted imine nitrogen of 1 to the metal carbene function (zwitterion A, −60°C), ii) cyclization to the seven‐membered ring with 1,2‐migration of the pentacarbonyl metal (zwitterion B, −40°C), iii) reductive elimination and coordination of the metal to the amine nitrogen (intermediate C, −40°C), and iv) thermal decomplexation and tautomerization (intermediate D and compound 3, above −20°C).
Resolution of the Absorbance and CD Spectra and Formation Constants of the Complexes between Human Serum Albumin and Methyl Orange
Authors: Ambrosetti R. , Bianchini R. , Fisichella S. , Fichera M. , Zandomeneghi M. 
Published: July 2007

AbstractDifference absorbance and circular dichroism techniques show that two complexes are formed between human serum albumin (HSA) and Methyl Orange (MO). The stoichiometries of the two HSA‐MO complexes (1:1 (C1) and 1:2 (C2)), their association constants (K1, 1 = 2.32 (0.18) × 105 M−1 and K1, 2 = 1.12 (0.15) × 1011 M−2), and both absorbance and dichroic spectra have been determined by a computational approach. Nearly 900 experimental points, consisting of absorbance and CD measurements registered in the 340–550 nm interval and over a wide range of concentrations of protein and ligand, have been included in a unique fitting procedure. The Scatchard plot indicates the existence of a unique binding site which can accommodate up to two molecules of MO in a positive co‐operative process. Calculation of the CD spectrum for the C2 complex according to the DeVoe method reproduces the fitted dichroic spectrum for the same complex. The shapes of the fitted absorbance and dichroic spectra, as well as the influence of concentrated NaCl or ethylene glycol on the absorbances of both free MO and HSA‐MO mixtures are consistent with the presence of dominant electrostatic interactions in C1. The C2 complex can be envisaged as a unique chromophore, consisting of two MO units associated in a stacking process into the same binding site of HSA, leading to a well‐defined chirality. The general validity of this multitechnique, multiwavelength approach in the investigation of protein‐ligand complexes is discussed.
In Situ 13C Solid‐State NMR and Ex Situ GC–MS Analysis of the Products of tert‐Butyl Alcohol Dehydration on H‐ZSM‐5 Zeolite Catalyst
Authors: Stepanov Alexander G. , Sidelnikov Vladimir N. , Zamaraev Kirill I. 
Published: July 2007

AbstractThe hydrocarbon products that are formed upon dehydration at 296–673 K of tert‐butyl alcohol (tBuOH), adsorbed on H‐ZSM‐5 zeolite in concentrations equal to that to active AlOHSi sites in the catalyst, have been analyzed by 13C solid‐state MAS NMR and GC–MS. To facilitate 13C NMR analysis, the alcohol selectively labeled with 13C isotope in the COH group was used. It was found that tBuOH transforms to the adsorbed C8 butene dimers plus a trace amount of alkanes at 296 K. Butene dimers exist inside HZSM‐5 pores in the form of interconverting adsorbed octene, octyl silyl ether, and octyl carbenium ion; octyl silyl ether is the main adsorption state. Fluxionality of the carbenium ion form provides a pathway for isomerization of the highly branched hydrocarbon skeleton of the intial alcohol to the predominantly linear one in the adsorbed butene dimer. The driving force for the isomerization into the linear structure is the shape selectivity induced by the small size of the zeolite channels. At 373 K the adsorbed butene dimers further crack into species that contain an average of about 6.5 carbon atoms, in addition to further alkanes. At 448 K the adsorbed C3–C7+ paraffins become the predominant hydrocarbon products observed with both in situ 13C NMR and ex situ GC–MS. Simultaneously, a mixture of adsorbed polyenes is formed. According to 13C CP/MAS NMR, polyenes exist in the zeolite pores in the form of rather stable cyclopentenyl cations. At 573–673 K adsorbed cyclopentenyl cations further transform into a mixture of condensed and simple aromatics and then into xylenes and toluene. Simultaneously, paraffins crack further to give mainly C3–C4 paraffinic species at 573 K and propane at 673 K.
Effect of the Position and Number of Chiral Carbons on Ferroelectric Liquid Crystals from Multichain Mononuclear ortho‐Palladated Complexes
Authors: Thompson Neil J. , Serrano José Luis , Baena Maria Jesús , Espinet Pablo 
Published: July 2007

AbstractA series of twelve mononuclear ortho‐palladated complexes incorporating a Schiff base and a β‐diketone ligand have been synthesised. These compounds have four nonequivalent terminal chain positions and differ in the number and position(s) of chiral chains [(R)‐2‐methylheptyl] in the structure. A study of the ferroelectric properties of the compounds has been performed. It is found that the spontaneous polarisation is highly dependent not only on the number of chiral carbons present, but also on the position within the molecular core.
Polysaccharides of Intracrystalline Glycoproteins Modulate Calcite Crystal Growth In Vitro
Authors: Albeck Shira , Weiner Steve , Addadi Lia 
Published: July 2007

AbstractAssemblies of glycoproteins from within the mineralized tissues of sea urchins and mollusks both interact in vitro in a similar manner with growing calcite crystals. A protein‐rich fraction, a polysaccharide‐rich fraction, and a fraction composed of densely glycosylated peptide cores were obtained by chemical and enzymatic treatment of the glycoproteins from sea‐urchin spines. Each fraction was partially purified and characterized (amino acid composition, FTIR and NMR spectroscopy). A comparison of the interactions of these fractions with growing calcite crystals in vitro shows that the polysaccharide moieties of these glycoproteins are intimately involved in the interaction with growing calcite crystals on planes approximately parallel to the c crystallographic axis. Presumably the polysaccharides in the mollusk‐shell glycoproteins are likewise responsible for the similar interactions of these macromolecules with calcite. We suggest that structured polysaccharide moieties of glycoproteins are important in controlling aspects of crystal growth in vivo as well.
Mechanisms of Water Exchange between Lanthanide(III) Aqua Ions [Ln(H2O)n]3+ and bulk water: A Molecular Dynamics Simulation Approach Including High‐Pressure Effects
Authors: Kowall Thomas , Foglia François , Helm Lothar , Merbach André E. 
Published: July 2007

AbstractWe studied the microscopic mechanisms of the water exchange reaction between the hydration shells of lanthanide(III) ions (Ln = Nd, Sm, Yb) and bulk water by means of molecular dynamics simulations. In contrast to the residence time of a water molecule in the first hydration shell (τres (1st shell) = 1577, 170 and 410 ps for Nd3+, Sm3+ and Yb3+, respectively), that in the second hydration shell is nearly independent of the type of the cation and amounts to 12–18 ps. Along the lanthanide series a change in the coordination number from 9 to 8 is coupled to a changeover in the water exchange mechanism. The observed water exchange events on the [Nd(H2O)9]3+ aqua ion follow a dissociatively activated Id mechanism via an eightfold‐coordinated transition state of square antiprismatic geometry. The lifetime of the transitory square antiprism varies between virtually 0 and 10 ps. The assignment of an Id mechanism (instead of a limiting D mechanism) is supported by the existence of a preferential arrangement between the exchanging water molecules (1800) and by the fact that the calculated average activation volume ΔV≠ = + 4.5 cm3 mol−1 is clearly smaller than the estimated activation volume ΔV≠lim ≈ΔV0 = + 7.2 cm3 mol−1 for a limiting D process. In the case of Sm3+ a ninth water molecule exchanges frequently between the first hydration shell and the bulk and maintains the coordination equilibrium between a [Sm(H2O)8]3+ and a [Sm(H2O)9]3+ aqua ion. The resulting trajectory pattern of incoming and leaving water molecules is an alternation of elimination and addition reactions and cannot be classified into the scheme of D, I or A mechanisms for substitution processes. The reaction volume ΔV0 for the coordination equilibrium [Sm(H2O)8]3+ + H2O → [Sm(H2O)9]3+ can be evaluated consistently both by a thermodynamic and a geometric approach. The observed exchange events for [Yb(H2O)8]3+ exhibit the characteristics of an Ia mechanism. The water exchange takes place via a transition‐state geometry close to that of a tricapped trigonal prism and involves a slightly negative activation volume.
Vanadium(II)‐ and Niobium(III)‐Induced, Diastereoselective Pinacol Coupling of Peptide Aldehydes to Give a C2‐Symmetrical HIV Protease Inhibitor
Authors: Kammermeier Bernhard , Beck Gerhard , Holla Wolfgang , Jacobi Detlev , Napierski Bernd , Jendralla Heiner 
Published: July 2007

AbstractPeptide aldehydes 15 a–c are prepared without epimerization from enantiomerically pure (S)‐α‐amino acids (Scheme 3). Reductive pinacol homocoupling of 15 a–c, induced by vanadium complex 11 or niobium complex 16 in refluxing THF, yields C2‐symmetrical (S,R,R,S)‐configurated 6a, 6b and 2, respectively, with moderate to high stereoselectivity (Scheme 4). In a novel protocol for the preparation and utilization of THF solutions of 11, the isolation of air‐sensitive intermediates can be avoided and the potent HIV protease inhibitor 2 prepared in enantio‐ and diastereomerically pure form on a kilogram scale without chromatographic purification. The (S,R,R,S) selectivity of the pinacol homocouplings is confirmed by means of an independent, stereochemically unequivocal synthesis of 6 a and 2 from D‐mannitol 4 (Scheme 1).
Structural Principles of the Coordination Number Eight: WF, ReF, and XeF
Authors: Adam Steffi , Ellern Arkady , Seppelt Konrad 
Published: July 2007

AbstractWF6, ReFn (n = 6 and 7), and XeF6 combined with NOF to give (NO+2) WF, (NO+)2ReF, and (NO+)2XeF2–8, respectively. Also NO2F reacted with ReF6 to form (No2+)ReF2–8. Cs2XeF8·4BrF5 crystallized from a solution of Cs2XeF8 in BrF5. These five compounds were subjected to X‐ray structure determinations. The structure of [NO(NOF)2]+IF–8 was used as a standard for comparison. All anions exhibit square‐antiprismatic geometry, independent of their electronic configurations. Bond lenghts in ReF2–8 and XeF2–8 are larger than in WF2–8 and IF–8, owing to the presence of nonbonding electrons. Deviations from the ideal structure in XeF2–8 are attributed to cation‐anion interactions.
Stabilization of Octahedral Thorium Clusters by Interstitial Hydrogen
Authors: Braun Thomas P. , Simon Arndt 
Published: July 2007

AbstractThe synthesis, structure, and characterization of the cluster compounds Th6HxBr15 (x = 5 and 7) are reported. The compounds can be prepared by a reversible hydrogenation/dehydrogenation process; their structures have been determined by X‐ray and neutron diffraction methods. Both compounds form a body‐centered cubic structure of octahedral cluster units with five or seven hydrogen atoms in eight disordered positions above the faces of the octahedron. The chemical bonding in these diamagnetic compounds has been analyzed: the simple counting rules for octahedral cluster compounds are fulfilled, and MO and band structure calculations confirm the hydridic nature of the bonding.
Synthetic Cyclic Oligosaccharides–Syntheses and Structural Properties of a Cyclo[(1 → 4)‐α‐L‐rhamnopyranosyl‐(1 → 4)‐α‐D‐mannopyranosyl]trioside and ‐tetraoside
Authors: Ashton Peter R. , Brown Christopher L. , Menzer Stephan , Nepogodiev Sergey A. , Stoddart J. Fraser , Williams David J. 
Published: July 2007

AbstractAn efficient polycondensation–cyclization approach to the synthesis of cyclodextrin analogues is demonstrated by the preparation of cyclohexaoside 1 and cyclooctaoside 2. The key intermediate, disaccharide 3, bearing the cyanoethylidene group as a glycosyl donor function and the trityloxy group as a glycosyl acceptor function was prepared in 15 steps starting from L‐rhamnose and D‐mannose. The crucial cyclooligomerization of the disaccharide monomer 3 was carried out in the presence of TrClO4 as a promoter with the use of ultra‐dry conditions at normal concentrations. This reaction led to formation of the cyclic oligosaccharides 28 and 29 (in 34 and 31% yield, respectively), which were deprotected to afford 1 and 2, respectively. The X‐ray crystal structural analysis of the cyclooctaoside 2 reveals a cylindrical shape for the cyclic oligosaccharide with C4 symmetry. Individual molecules of 2 are arranged perfectly in stacks that form nanotubes in the solid state.
Me3TTF–PO3H2, a Redox Phosphonic Acid and Its Monoanilinium Salt [PhNH+3][Me3TTF–PO(OH)O‐], the Electrocrystallized Neutral (Zwitterionic) π Radical [Me3TTF–PO(OH)O‐]·+, and Their Associated Lamellar Constructions in the Solid State
Authors: Dolbecq Anne , Fourmigué Marc , Krebs Frederik C. , Batail Patrick , Canadell Enric , Clérac Rodolphe , Coulon Claude 
Published: July 2007

AbstractThe reaction of Me3TTFLi (TTF = tetrathiafulvalene) with ClP(O)‐(OEt)2followed by hydrolysis with Me3SiBr affords the novel π‐donor molecule trimethyltetrathiafulvalenylphosphonic acid (Me3TTF–PO3H2) in a partially oxidized form. Subsequent reduction and neutralization with aniline gives the corresponding phosphonate monoanilinium salt. A unique hydrogen‐bonded hexagonal net is identified within the lamellar structure of [PhNH+3][Me3TTF–PO(OH)O‐], which is described by analogy with the anti‐CaSi2structure type. Electrocrystallization of the former salt yields single crystals of a neutral (zwitterionic) π radical, formulated as [Me3TTF–PO(OH)O‐]·+. Their structure reveals the presence of hydrogen‐bonded molecular ribbons whose association creates a novel layered architecture similar to that obtained within radical cation salts of π‐donor molecules of larger spatial extension. The analysis of the calculated HO‐MO–HOMO intermolecular interaction energies demonstrates that these slabs contain strong π‐π intermolecular interactions despite the nonexistence of any 2D network of short S ⃛S contacts. The spin susceptibility of [Me3TTF–PO‐(OH)O‐]·+, determined by single‐crystal ESR measurements, is characteristic of triplet excitons, the origin of which may be understood from the electronic structure of the compound.
Kinetic Analysis of the Stepwise Platination of Single‐ and Double‐Stranded GG Oligonucleotides with Cisplatin and cis‐[PtCl(H2O)(NH3)2]+
Authors: Sadler Peter J. , Barnham Kevin J. , Berners‐Price Susan J. , Frey Urban 
Published: July 2007

AbstractWe report the first direct comparison of the kinetics of platination of defined single‐ and double‐stranded DNA with the anticancer drug cisplatin. The courses of the reactions of the 14‐mer duplex d(A‐T‐A‐C‐A‐T‐G‐G‐T‐A‐C‐A‐T‐A)·d(T‐A‐T‐G‐T‐A‐C‐C‐A‐T‐G‐T‐A‐T) with [15N]cisplatin and cis‐[PtCl(H2O)‐(15NH3)2]+ and of each of the single strands with [15N]cisplatin have been studied at 298 K, pH 6, by [1H, 15N] NMR spectroscopy. As expected the reactions of cisplatin proceed via cis‐[PtCl(H2O)(NH3)2]+, and lead to two monofunctional adducts on the duplex and two on the GG single strand. In both the GG single strand and the duplex, one of the two G's is platinated faster than the other (by a factor of ca. 4). Remarkably, ring closure on the duplex to form the GG chelate occurs about an order of magnitude faster for one monofunctional adduct than for the other. The latter monofunctional adduct has distinctive 1H and 15N NMR chemical shifts for Pt‐NH3, and is very long‐lived (persists for >5 d). The Pt‐Cl bond in this monofunctional adduct is protected from hydrolysis by the duplex. In contrast, the two monofunctional adducts on the GG single strand undergo ring closure at about the same rate. Equilibria between kinked and distorted forms of the GG platinated duplex, the platination of G's on the complementary strand, and the potential biological significance of long‐lived monofunctional adducts of platinum drugs with DNA are discussed.