bluuebunny/crossref_metadata_2025 huggingface dataset:
Estimation of Ultrafilter Performance Based on Characterization Data
Authors: Peterson Reid A. , Geeting John G. H. , Daniel Richard C.
Published: July 2007

Because of limited availability of test data with actual Hanford tank waste samples, a method was developed to estimate expected filtration performance based on physical characterization data for the Hanford Waste Treatment and Immobilization Plant in Richland, Washington. A test with simulated waste was analyzed to demonstrate that filtration of this class of waste is consistent with a concentration polarization model. Subsequently, filtration data from actual waste samples were analyzed to demonstrate that centrifuged solids concentrations provide a reasonable estimate of the limiting concentration for filtration.
Calcium‐Ion Removal from Peat by a Mechanical Filter Cake Washing Process
Authors: Ruslim F. , Nirschl H. , Mezhibor A. , Rikhvanov L.
Published: July 2007

This contribution aims to bridge two different fields of science, viz., geoecology and mechanical process engineering. The study reports on the application of mechanical washing processes especially filter cake washing, on calcium‐ion removal from peat, which is a natural material that is used in different fields of application such as agriculture, medicine, cosmetics, etc. The interesting properties of peat such as its porous structure and the sorption behavior influence the distribution of liquid inside the bulk and the liquid flow behavior through the porous structure. Experimental results are obtained from filtration tests using differential gas pressure in a filter nutsche. The washing efficiency is determined for different pressures and specific amounts of applied wash liquor. It is found that the water repellent phenomena that occurs when peat has been dried, affects the washing efficiency in a very negative way. The results obtained are compared to the conventional filter cake washing process.
Catenated Cyclodextrins
Authors: Armspach Dominique , Ashton Peter R. , Ballardini Roberto , Balzani Vincenzo , Godi Anna , Moore Christopher P. , Prodi Luca , Spencer Neil , Stoddart J. Fraser , Tolley Malcolm S. , Wear Trevor J. , Williams David J. , Stoddart J. F.
Published: July 2007

A novel synthetic approach is described for the construction of catenanes in aqueous solution from a partially methylated cyclodextrin (CD)‐namely, heptakis(2,6‐di‐O‐methyl‐β‐cyclodextrin) (DM‐β‐CD)‐and a range of substrate molecules that contain a hydrophobic central core in the form of a 4,4′‐disubstituted biphenyl unit (usually bitolyl) carrying two hydrophilic polyether side chains terminated by primary amine functions. In water, the amphiphilic catenane precursors form 1:1 complexes with β‐CD and DM‐β‐CD and 2:1 (guest: host) complexes with the larger γ‐CD. Macrocyclizations of the biphenyl‐containing substrates with aromatic diacid chlorides in aqueous solution and in the presence of DM‐β‐CD under Schotten‐Baumann conditions afforded‐in low yields‐a range of [2]‐ and [3]catenanes. When a consitutionally asymmetrical diamine was employed as the substrate, orientational isomers of a [2]catenane were obtained. A [3]catenene incorporating a macrocyclic tetralactam was found to exist as a mixture of head‐to‐head and head‐to‐tail isomers, which could be separated by high pressure liquid chromatography and identified unambiguously by nuclear magnetic resonance spectroscopy. One of the [2]catenanes afforded good single crystals from which the solid state structure was determined by X‐ray crystallography. Other techniques which aided the characterization of these novel compounds included ultraviolet/visible and luminescence spectroscopy, dynamic nuclear magnetic resonance spectroscopy and fast atom bombardment mass spectrometry. Generally speaking, the catenated cyclodextrins are soluble in halogenated and aromatic hydrocarbons as well as in hydroxylic solvents. The existence of these new compounds gives us a unique insight into the nature of the noncovalent bonding interactions that cyclodextrins employ in binding substrate molecules.
Completely Spirocyclopropanated Macrocyclic Oligodiacetylenes: The Family of “Exploding” [n]Rotanes
Authors: de Meijere Armin , Kozhushkov Sergei , Haumann Thomas , Boese Roland , Puls Carsten , Cooney Mark J. , Scott Lawrence T.
Published: July 2007

A general approach to the family of completely spirocyclopropanated macrocyclic polydiacetylenes, that is, cyclic dehydrooligomers of 1,1‐diethynylcyclopropane 4, is reported. The characterized examples of these “exploding” [n]rotanes are for n = 5, 6, 7, 8, 9, 10, and 12. X‐ray crystal structure analyses for the hydrocarbons with n = 5, 6, 7, and 8 disclose a strong electronic interaction between the cyclopropane and the acetylene units leading to a significant shortening of the distal and lengthening of the proximal cyclopropane bonds. While the five‐sided compound 18 can occur as a planar or envelope‐shaped molecule, depending on the solvent from which crystals are grown, the six‐ (19), seven‐ (20), and eight‐sided (21) molecules all have chair conformations. While the butadiyne units in 18 and 19 are bent slightly outwards, those in the seven‐ and eight‐sided molecules 20 and 21, respectively, are bent distinctly inward. All these compounds are extremely high‐energy molecules: when struck with a spatula or a pestle, they go off with a puff to yield black soot.
Asymmetric Control of 1,3‐Dipolar Cycloaddition Reactions with Azomethine Ylides by Means of Proline Esters as Chiral Auxiliary Groups
Authors: Waldmann Herbert , Bläser Edwin , Jansen Martin , Letschert Hans‐Peter
Published: July 2007

Upon treatment with triethylamine or DBU in the presence of LiBr, aromatic and aliphatic imines of amino acid esters are converted to N‐metalated azomethine ylides. These 1,3‐dipoles undergo highly stereoselective cycloadditions with N‐acryloyl‐(S)‐proline esters in THF at −78 to −40°C to afford highly substituted pyrrolidines with complete regiocontrol and good to excellent diastereomeric ratios. The chiral auxiliary groups can readily be removed from the cycloadducts by simple acid hydrolysis. To rationalize the observed stereoselectivity a transition‐state model is proposed in which the lithium cation is coordinated to both the 1,3‐dipole and the dipolarophile.
A Dual‐Mode Molecular Switching Device: Bisphenolic Diarylethenes with Integrated Photochromic and Electrochromic Properties
Authors: Kawai Stephen H. , Gilat Sylvain L. , Ponsinet Rachel , Lehn Jean‐Marie
Published: July 2007

The bisphenolic dithienylethene molecules 1a and 1b were synthesized in overall yields of 45% from 4‐bromoanisole and 44% from 2.6‐di‐tert‐butyl‐4‐iodophenol, respectively. The corresponding extended quinones 3a and 3b were also prepared. Photochemical studies showed that compounds 1 are photochromic; the open forms 1 could be converted with UV light of 312nm to the closed coloured forms 2 with photostationary states lying at essentially complete conversion (> 98%). The 1a‐2a system was found to exhibit good resistance to photofatigue and thermal stability for both photoisomers. Cyclic voltammetry studies involving the 2/3 couples showed that whereas 2b undergoes irreversible oxidation at + 0.85 V (vs. SCE in THF), the hydroquinone 2a is reversibly oxidized at an E1/2 of + 0.72V (in MeCN, quasi‐reversibly in THF at + 0.81 V); this reflects the differences in deprotonation behaviour of the generated QH2/2+ species. The large difference in oxidation potential between 1a and 2a allows the photochemical switching of redox properties. In a complementary fashion, redox switching of the photochromic properties within the 2a‐3a pair is possible since 3a is stable to visible light. Owing to this unique behaviour, the triad consisting of 1‐3a represents a novel molecular device with mutually regulating photo‐ and electrochromic behaviour. In addition, the ability to interconvert between the three stable states makes the system well‐suited as the basis for an optical memory system with multiple storage and nondestructive readout capacity through a write‐lock‐read‐unlock‐erase cycle.
Total Synthesis of Rapamycin
Authors: Nicolaou K. C. , Piscopio Anthony D. , Bertinato Peter , Chakraborty Tushar K. , Minowa Nobuto , Koide Kazunori
Published: July 2007

Details of the total synthesis of rapamycin (1) are reported. The synthesis required the preparation of intermediates 4 – 9 in nonracemic form; key coupling reactions included a chromium‐mediated addition of vinyl iodide 8 to aldehyde 7 and an Evans aldol reaction to couple fragments 62 and 9. Intermediates 4 and 6 were joined through an amide bond formation to afford advanced intermediate 71. Swern oxidation of the diol in 71 was followed by a selective removal of the TES groups and a second Swern oxidation. Finally, removal of the remaining silyl protecting groups provided fully deprotected, penultimate intermediate 2 in which all carbons were in their proper oxidation state. Macrocyclization was achieved through a tandem inter/intramolecular palladium‐mediated Stille coupling reaction between distannylethene 3 and bis(vinyl iodide) 2. This latter process accomplished in one step the installation of the remaining two carbons of the natural product and the completion of its total synthesis.
Mono‐ and Bis(pentaisopropylcyclopentadienyl) Cobalt and Rhodium Sandwich Complexes and Other Decabranched Cyclopentadienyl Complexes
Authors: Buchholz Dirk , Gloaguen Bruno , Fillaut Jean‐Luc , Cotrait Michel , Astruc Didier
Published: July 2007

The acidity of methyl protons in the 18‐electron cationic complexes [MCp*Cp]1PF and [MCp]+PF (M = Co or Rh, Cp = η5‐C5H5, Cp* = η5‐ C5Me5) has been used for novel syntheses of starburst organometallic complexes. [CoCp*Cp]+PF was deprotonated at −30 °C with Si2Me6NK (1 equiv) in THF to give the fulvene complex [CoICp(η4‐C5Me4CH2)]. This was not stable at 20°C, but was characterized by 1H and 13C NMR at −30 °C. The complexes [MCp*Cp]+PF (M = Co or Rh) reacted with excess base (KOH or tBuOK) and alkyl halides (RX = CH3I, C2H5I, CH2CHCH2Br, or PhCH2Br) to give decasubstituted complexes (i.e., two hydrogens of each methyl group are replaced by two R groups). Distinct endo and exo alkyl groups are observed in the new complexes by NMR at room temperature. Coalescence occurs at higher temperatures; this indicates that the bulky alkyl groups are rotating. The calculated activation parameters (ΔG≠) are 71.3±0.8, 70.3±0.8, and 81.0±0.8 kJmol−1 for [Co(C5iPr5)Cp]+PF, [Rh(C5iPr5)‐Cp]+PF (in C6D5NO2), and [Co{C5‐(CHEt2)5}Cp]+PF6 (in o‐C6H4Cl2), respectively. The single directionality of these ligands is confirmed by the X‐ray crystal structure of [Co(C5iPr5)Cp]+PF6. This structure clearly shows the “paddle wheel” conformation adopted in the C5iPr5 ligand, due to steric hindrance between adjacent isopropyl groups and the staggered conformation of the two rings. The relative E° values measured by cyclic voltammetry show that the electron‐donating properties of the new C5(CHR2)5 ligand are between those of C5H5 and C5Me5‐ With the decamethylcobalticinium salt [CoCp]+PF, the permethylation reactions with base and CH3 I did not go to completion, even under forcing conditions and with repeated reaction (the maximum number of methyl groups introduced was 14). On the other hand, [RhCp]+PF, in which the distance between rings is larger than in the Co analogue, reacted with excess KOH and CH3I to give [Rh(C5iPr5)2]+PF6 in 55% yield; only one diastereoisomer was detected by NMR (ΔG≠ = 85.5±0.8 kJ mol−1 in C6D5NO2). The C5iPr5 sandwich complexes are extremely robust, even at the 19‐electron (CoII) stage. The 20‐electron K+ salt of the CoI anion was regiospecifically protonated by H2O at the Cp ligand to give [Co(C5iPr5)(η4‐C5H6)] and was decomplexed at 190°C to K+(C5iPr5)−.
Ab initio Study of the Coordination Modes of the Tetrahydroborato Ligand: Structure of the [Cu(BH4)(PH3)n] (n = 1, 2, 3) Complexes
Authors: Jarid A. , Lledos A. , Jean Y. , Volatron F.
Published: July 2007

All‐electron ab initio calculations (MP4/MP2 level) have been performed on [Cu(BH4)(PH3)n] (n = 1, 2, 3) complexes. Full‐geometry optimizations were carried out in each case, and the stationary points were characterized by the diagonalization of the analytically calculated Hessian matrix. The η2 coordination mode, with a tetrahedral arrangement around the copper atom, is the most stable structure for n = 2, while for n = 3 a strongly nonlinear η1 coordination mode is preferred. These results are in agreement with the experimental data available on related complexes. For n = 1, for which there is no experimental data, the η3 structure turns out to be the most stable. The energy differences associated with some changes in the coordination mode (η1 → η2 for n = 3 and η3 → η2 for n = 1) are small. Finally, a mechanism for the exchange between terminal and bridging hydrogen atoms is proposed for each complex under study.
The Alkenyl Mechanism for Fischer‐Tropsch Surface Methylene Polymerisation; the Reactions of Vinylic Probes with CO/H2 over Rhodium Catalyst
Authors: Turner Michael L. , Long Helen C. , Shenton Adele , Byers Peter K. , Maitlis Peter M.
Published: July 2007

Results consistent with the participation of vinyls in the initiation and of alkenyl species in the propagation steps of the Fischer–Tropsch reaction are reported. Substantial incorporation of 13C2 into the alkene and alkane (C3–C7) hydrocarbon products occurred when doubly labelled vinyls (13C2H3Br. (13C2H3)4Si, or 13C2H4) were added as molecular probes to the hydrogenation of carbon monoxide over rhodium/ceria/silica catalysts (1 atm, 220°C). There was, by contrast, no significant incorporation of 13C1 into any of the organic products; thus cleavage of the C2 probe did not occur. The degree of 13C2 incorporation decreased with increasing molecular mass of the hydrocarbon; this indicates that the probe molecule initiated but did not propagate. A mathematical model based on polymerisation of surface methylenes initiated by a vinyl, propagated by alkenyls and terminated by reaction with a surface hydrogen or by coupling has been developed to explain the 13C2 incorporation data. Under the conditions of the experiments, the relative ability of the probes to initiate is: vinyl bromide (60%)>tetravinylsilane (30%)>ethene (15%). Substantial formation of 13C4 products also occurred when vinyl bromide or tetravinylsilane were used as probes; this arises from a dimerisation of the vinyl on the surface, a process which has been modelled in homogeneous systems and also by other workers in studies on single crystal surfaces. There was no significant 13C incorporation into the oxygenates (methanol, ethanol, acetaldehyde); these products are formed by a different path.
The Solution Structure of Oxidized HiPIP I from Ectothiorhodospira halophila; Can NMR Spectroscopy Be Used to Probe Rearrangements Associated with Electron Transfer Processes?
Authors: Bertini Ivano , Eltis Lindsay D. , Felli Isabella C. , Kastrau Dieter H. W. , Luchinat Claudio , Piccioli Mario
Published: July 2007

In the 1H NMR spectrum of the oxidized form of the high‐potential iron‐sulfur protein (HiPIPI) from Ectothiorhodospira halophila, 91% of the total proton resonances and 100% of the residues have been assigned. The standard COSY, NOESY, and TOCSY sequences have been optimized for the paramagnetism of the molecule. Extensive assignment of the 15N NMR spectrum has been obtained through HMQC spectra. With 1437 dipolar connectivities, of which about 10% involved fast‐relaxing protons, a family of 18 structures was generated with an RMSD of 0.65 Å by using the programs developed by Wüthrich. The family of structures was further refined by various calculation steps; the final RMSD was 0.48 Å. The structures appear to be very similar but not equal to the structures of the reduced protein. Despite the similarity in structure, significant variations in the chemical shifts are observed. A similar behavior was observed for the homologous protein from Chromatium vinosum. It is concluded that NMR is a sensitive tool to monitor differences between oxidized and reduced proteins; however, the detailed structural variations should be evaluated with caution at the present level of resolution, which roughly corresponds to a resolution of 2.5 Å in an X‐ray structure.
Ab Initio ECP/DFT Calculation and Interpretation of Carbon and Oxygen NMR Chemical Shift Tensors in Transition‐Metal Carbonyl Complexes
Authors: Kaupp Martin , Malkin Vladimir G. , Malkina Olga L. , Salahub Dennis R.
Published: July 2007

Carbon and oxygen NMR chemical shift tensors for Group 6 hexacarbonyl complexes M(CO)6 (M  Cr, Mo, W) have been calculated by using a combination of quasirelativistic metal effective‐core potentials and density‐functional theory. Comparison with high‐resolution solid‐state shift tensors indicates excellent agreement between theory and experiment. The sensitivity of the shifts to the W‐C distance in W(CO)6 is discussed. A breakdown of the shielding tensor components into contributions from localized molecular orbitals allows the detailed interpretation of the trends on going down Group 6, and of differences to free CO. Group trends in the carbon shielding tensors are related largely to contributions from M‐C σ‐bonding orbitals. The presence of occupied metal (n–1)p and (n–1)d orbitals is partly responsible for the changes on going from free to metalbound CO. The origin of the less pronounced trends in the oxygen shielding tensors is more complicated. The influence of scalar relativistic effects on the shift tensors has been studied for W(CO)6 and is found to be relatively small, in spite of considerable changes in the W‐C distance.
Bis[2]catenanes and a bis[2]rotaxane–Model Compounds for Polymers with Mechanically Interlocked Components
Authors: Ashton Peter R. , Huff Jürgen , Menzer Stephan , Parsons Ian W. , Preece Jon A. , Stoddart J. Fraser , Tolley Malcolm S. , White Andrew J. P. , Williams David J.
Published: July 2007

The self‐assembly of three bis[2]catenanes and a bis[2]rotaxane, by two complementary strategies, is reported. A synthetic route to derivatives of bis‐para‐phenylene[34]crown‐10 (BPP34C10) and 1,5‐naphtho‐para‐phenylene[36]‐crown‐10 (1/5NPP36C10) containing a fused five‐membered ring with a secondary amine function is described. These intermediate N‐allylimido macrocyclic polyethers undergo template‐directed reactions with 1,1′‐[1,4‐phenylenebis‐(methylene)]bis‐4,4′‐bipyridinium bis‐(hexafluorophosphate) and 1,4‐bis(bromo‐methyl)benzene to produce [2]catenanes containing an N‐allyl functionality. The N‐allylimido macrocyclic polyethers have also been reduced and deprotected to afford macrocycles possessing a free NH group, which are then linked through a 4,4′‐biphenyldicarbonyl spacer to produce bis(crown ether)s, in which each crown ether moiety has two recognition sites. These ditopic BPP34C10 and 1/5NPP36C10 derivatives are capable of sustaining self‐assembly reactions at both recognition sites to yield bis[2]catenanes. The self‐assembly of a complementary bis[2]catenane, in which two tetracationic cyclophanes are linked together with a flexible hexyl chain, has also been achieved by treating 1,1′‐[1,4‐phenylenebis(methylene)]bis‐4,4′‐bipyridinium bis‐(hexafluorophosphate) with a compound containing two linked 1,4‐bis(bromomethyl)benzene units in the presence of BPP34C10. Replacing BPP34C10 with a dumbbell‐shaped compound containing a linear polyether unit intercepted by a naphthalene residue and terminated by two bulky adamantoyl groups has led to the self‐assembly of a bis[2]rotaxane. The X‐ray crystal structures of one of the catenanes and its associated crown ether component are reported, together with solution state dynamic 1H NMR spectroscopic studies, showing that there is substantial degree of order characterizing the molecular structure of the catenanes.
Fluorescent Sensors for Transition Metals Based on Electron‐Transfer and Energy‐Transfer Mechanisms
Authors: Fabbrizzi Luigi , Licchelli Maurizio , Pallavicini Piersandro , Perotti Angelo , Taglietti Angelo , Sacchi Donata
Published: July 2007

Fluorescent sensors for 3d divalent metal ions have been designed by means of a supramolecular approach: an anthracene fragment (the signalling subunit) has been linked to either a cyclic or a noncyclic quadridentate ligand (the receptor). Occurrence of the metal‐receptor interaction is signalled through the quenching of anthracene fluorescence. When the receptor (i.e., the dioxotetramine subunit of sensors 2 and 3) is able to promote the one‐electron oxidation of the metal, quenching takes place through a photoinduced metal‐to‐fluorophore electron‐transfer mechanism. In the case of sensors containing a tetraamine binding subunit (4 and 5), quenching proceeds by an energy‐transfer process. Selective metal binding and recognition can be achieved by varying the pH, and metal ions can be distinguished (e.g., CuII from NiII) by spectrofluorimetric titration experiments in buffered solutions. Whereas systems 2, 3 and 5 show reversible metal binding behaviour, the cyclam‐containing system 4 irreversibly incorporates transition metals (due to the kinetic macrocyclic effect) and cannot work properly as a sensor.
Substrate Uptake and Activation by Dimolybdenum and Ditungsten Hexaalkoxides–Factors Influencing the Cleavage of C–X Multiple Bonds in the Reactions Between [Mo2(OR)6] (R  tBu or CH2tBu) and Ar2CS, Et2NCN, and P(nBu)3
Authors: Budzichowski T. A. , Chisholm M. H. , Folting K.
Published: July 2007

The reaction between diarylthiones and [Mo2(OCH2tBu)6] in hydrocarbon solvents yielded [Mo2(OCH2tBu)6‐(μ‐S)(CAr2)] by cleavage of the CS double bond. The Lewis base adduct [Mo2 (OCH2tBu)6(μ‐S) (CPh2) (PMe3)] (2) has been crystallographically characterized; it contains six‐ and five‐coordinate Mo atoms linked through μ‐S and μ‐OR groups. The rate of the cleavage of the CS bond has been studied by variable‐temperature 1H NMR in [D8]toluene. A Hammett plot shows that both electron‐donating and electron‐releasing substituents in the aryl groups enhance the rate relative to Ph2CS. The activation parameters for cleavage of the CS bonds in Ph2CS, (p‐MeOC6H4)2CS, and (m‐CF3C6H4)2CS exhibit essentially identical values for ΔS‡, while ΔH‡ is somewhat smaller for the thiones bearing electron‐donating or ‐withdrawing substituents. We also studied the reversible binding of Et2NCN to [Mo2(OtBu)6] as a model system wherein the CN bond is reduced but not cleaved and the (MM)6+ center oxidized. Finally, the reversible binding of P(nBu)3 to [Mo2(OCH2tBu)6], wherein the product exhibits no net oxidation of the metal centers, has been similarly studied. The activation parameters for these three classes of reactions are compared with previous studies of the reversible binding of CN− to [M2(OR)6] compounds and the reaction 2[W2(OiPr)6] → [W4(OiPr)12]. We propose that the activation parameters associated with CS cleavage are associated with the formation of a μ‐η1,η2‐SCAr2 reactive complex similar to others described elsewhere,[23] and that they result from a) the bimolecular nature of the reaction and b) reorganizational energy. The greater facility of CX multiple‐bond cleavage in reactions involving [W2(OR)6] compounds relative to [Mo2(OR)6] compounds, the substituent effects, and the dependence on R are discussed in terms of the orbital energy match between the MM and CX π and π* orbitals, and an analogy is made with neutral Diels‐Alder cycloaddition reactions.
1,2,3‐Triphosphetenes, First Examples of a New Class of Phosphorus Heterocycles
Authors: Pucknat Heike , Grobe Joseph , Le Van Duc , Broschk Burkhardt , Hegemann Marianne , Krebs Bernt , Läge Mechtild
Published: July 2007

Hitherto unknown 1,2,3‐triphosphetenes RCPPCF3PCF3 (3a–d) are formed as main products in reactions of phosphaalkynes RCP [R = iPr2N (1a), tBu (1b), Me2EtC (1c), 1‐methylcyclohexyl (1d)] with the cyclotetraphosphane (PCF3)4 (2). According to NMR results the CF3 groups in 3a–d have a trans disposition; an X‐ray diffraction study of 3a confirms this structure. The PP bond lengths in 3a are equal [2.201 (2) and 2.204 (2) Å] and correspond to single bonds. A considerable shortening is observed for the sp2‐C–N bond (1.336 Å) which, together with the elongation of the PC bond (1.746 Å), indicates effective π donation of the lone pair on nitrogen. Surprisingly, 3a can be prepared in quantitative yields by reaction of the PP ylide Me3PPCF3 (6) with 1 a (molar ratio: 2:1). In contrast, the corresponding reactions of 6 with the alkyl‐substituted phosphaalkynes 1b–d lead to the novel phosphorus ylides Me3PC(R)PPCF3PCF3PCF3 [R = tBu (10a), Me2EtC (10b), 1‐methylcyclohexyl (10c)] in good yields. In their molecular groundstate structures, determined by X‐ray diffraction, the lone pair on the phosphano P atom prefers the syn position with respect to the ylidic PC bond. An unusual lengthening of the sp2‐C–C bond [1.553 (4) (10a), 1.543 (6) (10b), 1.551 (4) Å (10c)] to values typical for sp3‐C/sp3‐C distances is observed.
Zintl Anions as Starting Compounds for the Synthesis of Polynuclear Transition Metal Complexes
Authors: Ahlrichs Reinhart , Fenske Dieter , Fromm Katharina , Krautscheid Harald , Krautscheid Ulrike , Treutler Oliver
Published: July 2007

The Zintl anion P3−7 reacts with complexes of transition metal halides to form multinuclear metal phosphorus clusters. Reaction of Li3P7·3 DME with [FeCp(CO)2Br] or [NiCl2(PBu3)2] leads to the formation of [P7{FeCp(CO)2}3] (1) and [{Ni(PBu3)2}4P14] (2), respectively. X‐ray structure determinations show that in 1 the P7 cage of Li3P7 remains intact but in 2 a P14 framework is formed by linkage of two norbornadiene‐like P7 units. The P14 skeleton coordinates to four Ni(PBu3)2 groups. LiCp* and CoCl2 react with P7(SiMe3)3 to give [(Cp*Co)3‐(P2)3] (3) with three Cp*Co groups bridged by P2 units. Reaction of FeCl2 and LiCp* with P7(SiMe3)3 yields [(Cp*Fe)3P6][FeCl3(thf)] (4) or [(Cp*Fe)3{n3‐P3)Fe}P6] (5), depending on the reaction conditions. In 4 the structure of the Fe3P6 core of the [(Cp*Fe)3P6]+ cation is comparable to closo‐B9H92−, while in 5 the Fe4P6 core does not obey the Wade rules. The unusual Co‐As cluster [Co6As12(PEt2Ph)6] (6), prepared from K, As, and [CoCl2(PEt2‐Ph)2], can be described as a Co6As6 heteroicosahedron linked to two Co3As3 octahedra by common Co3 faces. A theoretical treatment within the density functional approximation reproduces the experimental structures of 2 and 6 and allows an interpretation of molecular electronic structures. In 2 one finds P‐P double bonds that are delocalized to some extent into vacant Ni AOs. For the cage compound 6 the Co 3 d AOs participate in cage bonding and 38 electrons can be assigned to cage bonds; this is in accord with the (2N + 2) rule for 18‐atom cages but not with the usual electron counting rules.
Thianthrene 5‐Oxide as a Mechanistic Probe in Oxygen Transfer Reactions: The Case of Carbonyl Oxides versus Dioxiranes Revisited
Authors: Adam Waldemar , Golsch Dieter , Görth Felix Christian
Published: July 2007

Thianthrene 5‐oxide (SSO) constitutes a useful mechanistic tool for the assessment of the electronic character of oxygen transfer agents by means of their Xso values, which reflect the extent of nucleophilic oxidation at the SO site in SSO. Treatment of dioxiranes 1 a–d with the SSO probe confirms that these are electrophilic oxidants (Xso < 0.15). Dioxirane sulfoxidation is sensitive to protic solvents and acids, which implies a polar mechanism (SN2) with nucleophilic attack of the sulfide electron pair on the dioxirane peroxide bond. In contrast, the carbonyl oxides 2 a and b, diethyl persulfoxide (3), and adamantylideneadamantane perepoxide (4) are nucleophilic oxidants (Xso > 0.85). However, the cyclopentadienone carbonyl oxides 2 c and d show low Xso values typical for electrophilic oxygen transfer agents. For these carbonyl oxides, photoisomerization to the respective dioxiranes is proposed under the photooxidation conditions of the diazoalkanes. Additionally, the trans/cis ratio of bissulfoxides (SOSO) formed provides valuable information on the stereochemical course of the oxygen‐transfer process due to the steric requirements of the oxidant. Thus, the electrophilic dioxiranes preferentially attack the axial lone pair of SSO because of repulsion by the peri hydrogen atoms to afford trans‐SO‐SO.
Regioselectivity in the Gas‐Phase Nucleophilic Attack on O‐Protonated 3‐Methyl‐2‐cyclohexen‐1‐ol and 1‐Methyl‐2‐cyclohexen‐1‐ol
Authors: Dezi Emanuela , Lombardozzi Antonietta , Renzi Garbriele , Pizzabiocca Adriano , Speranza Maurizio
Published: July 2007

Our radiolytic study of the occurrence in the gas phase of concerted SN2′ reactions on several open‐chain allylic oxonium ions generated in the gas phase from the attack of gaseous GA+ acids (GA+CnH+5 (n = 1, 2), iC3H+7, and (CH3)2F+) on suitable substrates is now continued with cyclic allylic alcohols, such as 3‐methyl‐2‐cyclohexen‐1‐ol (1) and 1‐methyl‐2‐cyclohexen‐1‐ol (2), with both MeOH and NMe3 as neutral nucleophiles. With MeOH as the nucleophile, the substitution reaction exclusively takes place on 1 as the starting compounds, whereas when the substrate is 2 it is accompanied by extensive elimination. With NMe3, only the elimination reaction is observed in the same systems. The analysis of the isomeric distribution of the substitution and elimination products allows definition of the corresponding reaction patterns. As for open‐chain oxonium ions, the nucleophilic attack on O‐protonated 1 and 2 is preceded by significant intramolecular interconversion. Partial unimolecular dissociation of the same ionic intermediates also takes place. After careful evaluation of the extent of these side processes, it is demonstrated that the O‐protonated 1 undergoes the concerted SN2 process with MeOH almost exclusively (≥ 99%). With O‐protonated 2, how ever, the concerted SN2′ pathway (84–95%) prevails over the classical SN2 one (6–17%). Concomitant [1,2] (E2) and [1,4] elimination (E2′) pathways involve attack of the selected nucleophiles on the oxonium ions from 1 and 2. Their relative extent (E2′/E2:1.78–1.96 (1); 1.43‐1 70 (2)) appears only slightly dependent on the nature of the ionic substrate, the nucleophile (whether MeOH or NMe3), and the leaving group (whether H2O or MeOH). The effects of both intrinsic structural factors and experimental conditions in determining the SN2′/SN2 and E2′/E2 branchings in the selected oxonium ions is discussed and compared with related gas‐phase data.
Unexpected Basicity of a Hexacoordinate Silicon Compound, {2,6‐Bis[ (dimethylamino)methyl]phenyl}bis(1,2‐benzenediolato)silicate
Authors: Chuit Claude , Corriu Robert J. P. , Mehdi Ahmad , Reyé Catherine
Published: July 2007

The high basicity of {2,6‐bis‐[(dimethylamino) methyl]phenyl} bis (1,2‐benzenediolato)silicate (1) was demonstrated by its quantitative protonation in methanol to give the zwitterion 2. It was found that 1 is much more basic (pKa in CH2Cl2 = 16.7) than the “proton sponge” 1,8‐bis(dimethylamino)naphthalene (3) and also much more basic than {2,6‐bis‐[(dimethylamino) methyl]phenyl} bis (1,2‐benzenediolato)phosphorane (4). The stability of the zwitterion 2, the geometry of which corresponds to a more perfect octahedron than 1, can explain the high basicity of 1. Dynamic NMR studies of 2 in solution show that at low temperature the hydrogen‐bonding interaction with one oxygen atom observed in the solid state is maintained. On raising the temperature, firstly the hydrogen bond breaks, then dynamic coordination occurs, in which the NMe2 groups displace each other in conjunction with transprotonation.
A 2,3‐Connected Tellurium Net and the Cs3Te22Phase
Authors: Liu Qiang , Goldberg Norman , Hoffmann Roald
Published: July 2007

The bonding in the recently reported Cs3Te22phase, which contains both Te8rings and remarkable Te6sheets, is studied by approximate molecular orbital theory. Our focus is on the geometric and electronic features of the unique 2,3‐connected Te net found as a substructure in this phase. The calculations show that both the linear and T‐shaped Te geometries in the 2,3‐connected Te net of Cs3Te22are determined by their particular electron count. Both types of tellurium atoms are hypervalent; we make connections to other well known hypervalent molecules, such as XeF2, I3−, and BrF3. Several possible variations and distortions of this net are discussed, all of which are found to be less stable. The discrete crown‐shaped Te8units that appear in the phase show normal covalent bonding and should occur in smaller molecular entities, too. According to our computations, Cs3Te22should be metallic. Two structurally related phases, CsTe7and Cs2Te15, are suggested.
1,3‐Alternate Calix[4]arenecrown‐5 Conformers: New Synthetic Ionophores with Better K+/Na+ Selectivity than Valinomycin
Authors: Casnati Alessandro , Pochini Andrea , Ungaro Rocco , Bocchi Carlo , Ugozzoli Franco , Egberink Richard J. M. , Struijk Helma , Lugtenberg Ronny , De Jong Feike , Reinhoudt David N.
Published: July 2007

New 25,27‐dialkoxycalix‐[4]arenecrown‐5 conformers 8, 10, and 11 have been synthesized and studied. The compounds 8a and 8b, fixed in 1,3‐alternate structure, have been obtained in 57 and 40% yield, respectively, by reaction of the corresponding 25,27‐dialkoxycalix[4]arenes 7a–b with tetraethylene glycol di‐p‐toluenesulfonate in the presence of Cs2CO3. The cone 10a and 10b and the partial cone 11 conformers were obtained by selective demethylation of the 25,27‐dimethoxycalix[4]arenecrown‐5 (6a) and subsequent dialkylation with NaH/DMF and KOtBu/THF, respectively. In the solid state (X‐ray), compound 6a adopts a flattened cone conformation, which is also found to be most abundant in CD3CN and CD3OD solution. Upon complexation with potassium picrate compound 6a was converted quantitatively into the 1,3‐alternate conformation. All new ligands synthesized were used in the extraction of alkali metal cations from H2O into CHCl3, and as active components in supported liquid membranes and in chemically modified field effect transistors. Results were compared to those obtained with with the natural antibiotic valinomycin 1. All ligands showed high selectivity for potassium. Ligand 8a, fixed in the 1,3‐alternate conformation, is more selective than valinomycin and shows the highest K+/Na+ selectivity known so far.
Redox Properties of the Diatomic Bare Iron Chalcogenides FeO and FeS in the Gas Phase
Authors: Harvey Jeremy N. , Heinemann Christoph , Fiedler Andreas , Schröder Detlef , Schwarz Helmut
Published: July 2007

The electron‐transfer behavior of the binary iron chalcogenides FeO and FeS has been examined by means of mass spectrometry and ab initio calculations using the averaged coupled‐pair functional (ACPF) method. The experimental and theoretical results are in good agreement with each other and also with previous studies. The ionization energies (IE) of the diatomic species are found to be IE(FeO) = 8.8±0.2 eV, IE(FeO+) = 17.9±0.4 eV, IE(FeS) = 8.3±0.3 eV, and IE(FeS+) = 16.3±0.5 eV. Two new diatomic dications, FeS2+ and FeO2+, are shown to exist as metastable minima on the corresponding potential‐energy surfaces. The data enable an evaluation of the intrinsic gas‐phase redox properties of FeS and FeO, and the comparison demonstrates that iron sulfide is more prone to undergo facile reduction and oxidation than iron oxide.
Processing of Blue Boron Nitride Thin Films with a Solid–Gas Reaction
Authors: Mokhtari Mohamed , Park Hyung S. , Roesky Herbert W. , Johnson Stephen E. , Bolse Wolfgang , Conrad Jens , Plass Winfried
Published: July 2007

For the first time, solid–gas reaction techniques have been used for the synthesis and processing of thin films of boron nitride. Clear to intensely colored blue BN films were grown on Si(100) substrates by the transformation of borazine (B3N3H6) with a titanium complex as initiator under flowing nitrogen gas. The thickness of the films ranged from 70 to 100 nm, as determined by Rutherford backscattering (RBS) and atomic force microscopy (AFM) analyses. The intensity of the blue color of the thin film can be correlated to its thickness. The composition of the film determined by RBS studies corresponds to the stoichiometric formula B0.49N0.45O0.06, and N/B and O/B ratios are found to be 0.92 and 0.12, respectively. Nitrogen contents determined by nuclear reaction analysis agree well with the RBS results. Moreover, Auger electron spectroscopy (AES) measurements show that no titanium is present in the films and confirm the composition determined by RBS studies. X‐ray photoelectron spectroscopy (XPS) shows the presence of boron and nitrogen in the blue BN film. Electron spin resonance (ESR) experiments at 293 and 12 K indicate a single broad signal with a g value (g = 2.005) close to that of a free electron. This synthetic approach provides opportunities for the preparation of new thin‐film materials and for the fundamental study of solid‐gas reactions.
Synthesis, Crystal Structure, Magnetism, and Magnetic Anisotropy of Cyclic Clusters Comprising six Iron(III) Ions and Entrapping Alkaline Ions
Authors: Caneschi Andrea , Cornia Andrea , Fabretti Antonio C. , Foner Simon , Gatteschi Dante , Grandi Romano , Schenetti Luisa
Published: July 2007

The reaction of ferric chloride and β‐diketones (HL) in alkaline methanol solution represents a good synthetic route to hexairon(III) clusters [MFe6‐(OCH3)12(L)6]+ (M = Na, Li), which exhibit an unusual sixfold molecular symmetry. Single‐crystal X‐ray diffraction showed that the six octahedrally coordinated iron(III) ions define a ring and are linked by twelve bridging methoxide ligands. The resulting [Fe6(OCH3)12] skeleton has the remarkable property of acting as a host for an alkali‐metal ion both in the solid state and in organic solution, as demonstrated by 23Na and 7Li NMR experiments. The magnetic behavior of these systems is consistent with the presence of a nonmagnetic S = 0 ground state and of antiferromagnetic exchange interactions between the high‐spin ferric ions. The energy of the excited states was studied in detail by high‐field DC and pulsed‐field differential magnetization experiments at 0.7 and 1.5 K. Single‐crystal susceptibility measurements at variable temperature revealed a sizeable magnetic anisotropy, which has been successfully analyzed in terms of single‐ion and dipolar contributions. The results are relevant to research into the origin of superparamagnetic‐type behavior in transition‐metal clusters.
Photoswitched and Functionalized Oligothiophenes: Synthesis and Photochemical and Electrochemical Properties
Authors: Tsivgoulis Gerasimos M. , Lehn Jean‐Marie
Published: July 2007

The extended dithienylethene compounds 1–3 have been synthesized. They undergo photochromic and electrochemical changes by photoinduced interconversion between open and closed forms of type A and B. In addition, both forms of the N‐methylated derivatives 1b and 2b have absorption bands in which excitation results in very large differences in fluorescence between the two forms with very little effect on the opening/closing state, a feature of interest for optical memory data systems. The compounds 2 and 3 contain six and eight conjugated thiophene units in the closed forms, respectively, and thus represent oligothiophenes endowed with a photoactivated switch; they are of special interest in this respect in view of the potential use of switched oligothiophenes in molecular electronic devices.
The [Fe(etz)6](BF4)2 Spin‐Crossover System—Part Two: Hysteresis in the LIESST Regime
Authors: Hauser Andreas , Hinek Roland , Spiering Hartmut , Gütlich Philipp
Published: July 2007

In the [Fe(etz)6](BF4)2 spincrossover system the iron(II) complexes occupy two nonequivalent lattice sites, sites A and B. Complexes on site A show a thermal high‐spin (HS) → low‐spin (LS) transition at 105 K, whereas complexes on site B remain in the HS state down to 10 K. Complexes on both sites exhibit light‐induced spin state conversions (LIESST) at 20 K: LS → HS on site A with λ = 514.5 nm, and HS → LS on site B with λ = 820 nm. The relaxation processes subsequent to the HS ⇌ LS conversion on site B reveal a light‐induced HS⇌LS bistability for the complexes on site B at 70 K. The bistability as well as the absence of a thermal spin transition on site B are attributed to a thermal hysteresis for the B‐site complexes with a critical temperature T↑c≈︁77 K on heating. This hysteresis can be interpreted in terms of strong cooperative effects of elastic origin, which, in addition, cause characteristic deviations of the relaxation on site B from first‐order kinetics (self‐acceleration). In contrast, the HS → LS relaxation at 60 K on site A after irradiation with λ = 514.5 nm shows an unusual self‐retardation.
New Ruthenium(II) Complexes of Functionalized Monoanionic Aryldiamine N,C,N'‐Terdentate Ligands: Syntheses of [RuII{2,6‐(Me2NCH2)2‐4‐R‐C6H2}‐(terpy)]+Cl‐; X‐ray Structure of a Dimeric Organolithium Compound, [Li{2,6‐(Me2NCH2)2‐4‐Ph‐C6H2}]2
Authors: Steenwinkel Pablo , James Stuart L. , Grove David M. , Veldman Nora , Spek Anthony L. , van Koten Gerard
Published: July 2007

The new anionic functionalized aryldiamine ligands [2,6‐(Me2NCH2)2‐4‐R‐C6H2]‐ (R = Me3SiCC, C6H5, Me3‐Si), formally derived from [2,6‐(Me2‐NCH2)2C6H3]‐, have been prepared as their lithium compounds. The compound [Li{2,6‐(Me2NCH2)2‐4‐Ph‐C6H2}]2 crystallizes in the monoclinic space group C2/c (no. 15) with a = 13.1225(5), b = 13.5844(7), c = 18.9859(12) Å, β = 105.329(5)°, V = 3264.0(3) Å3. Z = 4. The structure refinement converged to R1 = 0.0374 for 2037 observed reflections [Fo>4σ(Fo)] and wR2 = 0.0922 for 2560 unique data. The organolithium compounds have been used in transmetalation reactions to give the corresponding functionalized organoruthenium(II) complexes [RuII{2,6‐(Me2NCH2)2‐4‐R‐C6H2}‐(terpy)]+Cl‐ (terpy = 2,2′;6′,2′‐terpyridine). The RuII species with R = HC°C has also been synthesized.
Studies Towards the Total Synthesis of Epothilones: Asymmetric Synthesis of the Key Fragments
Authors: Schinzer Dieter , Limberg Anja , Böhm Oliver M.
Published: July 2007

Members of a new class of macrolide—the so‐called epothilones (1)—showing a taxol‐like biological activity have recently been isolated. A convergent approach to1is presented, and the asymmetric syntheses of the three key intermediates3, 4, and8are reported.
Ruthenium(II)‐Catalyzed Oppenauer‐Type Oxidation of Secondary Alcohols
Authors: Almeida Maria L. S. , Beller Marion , Wang Guo‐Z. , Bäckvall Jan‐E.
Published: July 2007

Highly efficient ruthenium‐catalyzed Oppenauer‐type oxidations of secondary alcohols to ketones have been developed. The catalytic system consists of [(PPh3)3RuCl2] (1) and K2CO3 or [(C4Ph4COHOCC4Ph4)(μ‐H)(CO)4Ru2] (2) in refluxing acetone. The catalytic reaction is of high efficiency and permits a catalyst: substrate ratio of 1:1000 at 56 °C. In some cases the initial turnover rate exceeds 1500 h‐1. The reaction was found to be general and compatible with double bonds and oxidation‐sensitive aromatics. With some allylic alcohols, isomerization to saturated ketones took place. The reaction proceeds by ruthenium‐catalyzed dehydrogenation of the alcohol and subsequent hydrogen transfer to acetone. No primary kinetic isotope effect was observed for the catalytic reaction when α‐deuterated 1‐phenylethanol was employed as substrate (kH/kD=1.1); this shows that β‐hydride elimination from a ruthenium alkoxide intermediate is not the rate‐limiting step.
Ab Initio Quantum Mechanical Model Calculations on the Catalytic Mechanism of Aspartylglucosaminidase (AGA): A Serine Protease‐Like Mechanism with an N‐terminal Threonine and Substrate‐Assisted Catalysis
Authors: Peräkylä Mikael , Rouvinen Juha
Published: July 2007

Ab initio quantum mechanical model calculations were used in studying the acylation and deacylation steps of the catalytic mechanism of aspartylglucosaminidase (AGA). AGA catalyses the hydrolysis of an amide linkage between oligosaccharide and asparagine by utilising an N‐terminal threonine as a catalytic amino acid. Results are reported for the model enzyme reaction at the MP2/6–31+G*//HF/6–31 + G* + ΔZPE level. Contribution of aqueous solvation to the relative energies was estimated by using the continuum solvation model of Tomasi. The serine protease‐like catalytic mechanism was found to be feasible for AGA. The protonated α‐amino group of the substrate of AGA was suggested to enhance the catalysis by stabilising the anionic oxygen of the substrate, which is formed in the reaction, and by lowering the pKa of the nucleophilic oxygen of the N‐terminal threonine. Finally, the similarities in the catalytic mechanisms of AGA and other amidohydrolases were discussed.
The Role of Water Exchange in Attaining Maximum Relaxivities for Dendrimeric MRI Contrast Agents
Authors: Tóth Éva , Pubanz Dirk , Vauthey Sylvain , Helm Lothar , Merbach André E.
Published: July 2007

Macrocyclic GdIII complexes attached to dendrimers represent a new class of potential MRI contrast agents. They have an extended lifetime in the blood pool, which is indispensable for their application in magnetic resonance angiography, and high relaxivities, which reduce the dose required to produce quality images. We performed a variable‐temperature and ‐pressure 17O NMR study in aqueous solution and at 14.1, 9.4, and 1.4 T on the water exchange and rotational dynamics of three macrocyclic GdIII complexes based on polyamidoamine dendrimers, as well as on the GdIII complex of the monomer unit with the linker group. The water exchange rates k298ex for generation 5 [G5(N{CS}N‐bz‐Gd‐{DO3A}{H2O})52], generation 4 [G4(N‐{CS}N‐bz‐Gd{DO3A}{H2O})30], generation 3 [G3(N{CS}N‐bz‐Gd{DO3A}‐{H2O})23], and the monomer [Gd(DO3A‐bz‐NO2)(H2O)] complexes are 1.5±0.1, 1.3±0.1, 1.0±0.1, and 1.6±0.1 × 106 s‐1, respectively, and the activation volumes ΔV≢ of water exchange on the latter two compounds are + 3.1±0.2 and + 7.7±0.5 cm3 mol‐1, indicating dissociatively activated exchange reactions ({CS}N‐bz‐{DO3A}=1‐(4‐isothiocyanatobenzyl)amido‐4,7,10‐tri(acetic acid)tetraazacyclododecane). The rotational correlation times for the dendrimers are 4 to 8 times longer than for monomeric or dimeric GdIII poly(amino carboxylates). As a consequence of the slow rotation, the proton relaxivities of these dendrimer complexes are considerably higher than those of smaller complexes. However, the low water exchange rates prevent the dendrimer proton relaxivities from attaining the values expected from the increase in the rotational correlation times. Modifications of the chelating ligand may result in a faster water exchange and thus allow the full benefit of slow rotation to be achieved.
Interaction of Acetonitrile with Olefins and Alcohols in Zeolite H‐ZSM‐5: In situ Solid‐state NMR Characterization of the Reaction Products
Authors: Stepanov Alexander G. , Luzgin Mikhail V.
Published: July 2007

The reaction products and intermediates from the interaction of acetonitrile with olefins (oct‐1‐ene) or alcohols (tert‐butyl alcohol) in zeolite H‐ZSM‐5 at 296 K have been characterized with 13C and 15N solid‐state NMR. It has been shown that coadsorption of acetonitrile and olefin on H‐ZSM‐5 gives rise to the intermediate N‐alkylnitrilium cation represents a persistent species inside a zeolite under anhydrous conditions. Upon admittance of water to the pores of the zeolite, the N‐alkylnitrilium cation slowly converts into N‐alkylamide in accordance with the classic Ritter reaction. In the case of acetonitrile and alcohol, just after coadsorption both the intermediate N‐alkylnitrilium cation and the final N‐alkyl‐amide are identified simultaneously; the former slowly disappears over a few days. Thus, 1) it has been shown that the Ritter reaction can occur not only in liquid acidic media but also on a solid acid catalyst, zeolite H‐ZSM‐5; 2) N‐alkylnitrilium cations have been detected and characterized with solid‐state NMR as persistent intermediates in the Ritter reaction for the first time while the reaction proceeds.